The initiation of pluton formation is rarely preserved as the rock record is typically overprinted by younger intruding pulses. An exception is the 80 km2 Kuna Crest lobe, which marks the initiation of the 95–85 Ma, 1100 km2 Tuolumne Intrusive Complex in the Sierra Nevada, California, USA. We present a detailed map of the lithologies and structure of the Kuna Crest lobe, associated sheeted complex and satellite plutons, and their host rocks, using chemical abrasion–isotope dilution–thermal ionization mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon geochronology, element and isotope geochemistry, and Al-in-hornblende thermobarometry to conclude the following: (a) The 94.91 ± 0.53 Ma to 92.75 ± 0.11 Ma Kuna Crest lobe and its marginal sheeted complex preserved the oldest intrusive pulses and most mantle-like compositions of the entire Tuolumne Intrusive Complex. (b) Emplacement began with magma wedging of low volume magma pulses resulting in a sheeted complex that is compositionally heterogeneous at outcrop scales, but isotopically homogeneous. (c) These early magmas established a pre-heated pathway within just a few hundreds of thousands of years that gave way to the formation of the ~1.5 million-year-long active, compositionally more homogeneous but isotopically more heterogeneous magma mush across the Kuna Crest lobe. The host rocks and previously intruded magma were displaced largely vertically through downward flow. (d) The melt-interconnected mush zone in the lobe allowed for magma mixing and crystal-liquid separation at the emplacement level. We interpret this lobe to represent an upper- to mid-crustal, vertical magma transfer zone that likely fed shallower plutons and potentially volcanic eruptions. We propose a filter pressing mechanism driven by vertical magma transport through the lobe resulting in margin-parallel fabrics, plagioclase-rich crystal cumulates, inward draining and upward loss (of up to 40%) of interstitial melts. Some inward drained melts hybridized with later intruding Half Dome magmas at the transition to the main Tuolumne Intrusive Complex. Some of the lobe magmas, including fractionated melts, drained laterally into the strain shadow of the lobe to form the satellite plutons, further contributing to cumulate formation in the lobe.
This study documents that within only a few hundreds of thousands of years, arc magma plumbing systems are capable of establishing a focused magma pathway to build up to increasingly larger magma bodies that are capable of undergoing magma differentiation and feeding shallower plutons and volcanic eruptions.
Understanding the magmatic assembly of an intrusive complex and the simultaneous displacement of host rocks is marked by the lack of agreement on how to best interpret growth of these bodies using structural, geochemical, and geochronologic data sets (e.g., Bateman and Chappell, 1979; Paterson and Vernon, 1995; Coleman et al., 2004; Bartley et al., 2006; Schoene et al., 2012; Brown, 2013; Coint et al., 2013; Cao et al., 2016; Alasino et al., 2017; Hines et al., 2018). Controversy is further increased when plutonic studies focus interpretations on the magmatic history alone without integrating multiple data sets from both magmatic and surrounding host rocks. Furthermore, typically more than one emplacement mechanism plays a role during the construction of an intrusion, with their importance changing over time (Buddington, 1959; Paterson and Fowler, 1993; Paterson and Vernon, 1995; Dietl et al., 1999; Miller and Paterson, 2001; Cruden et al., 2017). This task of understanding the growth of magma bodies becomes even more challenging as their size and longevity increase, which for large plutonic systems can be well over hundreds of km3 in size and have durations of magmatic activity over several millions of years (Matzel et al., 2006a; Miller et al., 2007; Walker et al., 2007; Memeti et al., 2010a; Schoene et al., 2012). If so, the challenge of understanding the temporal evolution increases because the initial stages of pluton growth are often overprinted by subsequent, commonly more voluminous, magmatism. Thus, the preserved history within and around an intrusion is information about the magmatic evolution and host rock adjustments integrated over the entire time of magmatic activity (e.g., Miller and Paterson, 2001; Matzel et al., 2006a; Memeti et al., 2010a; Schoene et al., 2012; Paterson et al., 2016), and is likely dominated by the last stages of pluton activity, which may be quite distinct from how the body initiated.
One example of where the initiation of a large and long-lived intrusive complex has been preserved is the Kuna Crest (KC) lobe of the ca. 95–85 Ma Tuolumne Intrusive Complex (TIC) in the central Sierra Nevada, California, USA. The Kuna Crest lobe protrudes into the host rock southeast of the main intrusive body, and it is both the oldest and the most mafic part of the entire TIC (Memeti et al., 2010a, 2014; see chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb single zircon geochronology and whole rock compositional data below). Whole rock geochemistry presented in this paper indicates that the magmas arriving in the KC lobe were also the most mantle-like in the entire TIC. Because the KC lobe formed away from the main locus of magmatism, the initiation of the intrusive complex was not overprinted or recycled into the subsequently intruding younger, more voluminous magmas (Memeti et al., 2010a; Paterson et al., 2016). The KC lobe provides the unique opportunity to examine: (1) the internal and host rock structures to determine initial emplacement mechanisms that produced the lobe and its continued growth, (2) the time scales of early magma emplacement, (3) the initial composition of the magmas that were emplaced at this level of the crust before they interacted with slightly younger, more voluminous and more evolved magmas in the main TIC body, and (4) the extent of physical and chemical interaction between magmas and host rocks during the first ~1–2 m.y. of magmatic activity in the TIC, when magmatism appears to have begun by the emplacement of compositionally heterogeneous, relatively low volume pulses, but soon gave way to the formation of a more voluminous magma body.
After a brief introduction to the geology of the TIC, we follow with a detailed description of the field geology, petrography, and structures of the KC lobe, its nearby satellite bodies and host rocks. We present new detailed mapping and CA-ID-TIMS and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) single zircon U-Pb geochronology to put both pluton growth and host rock displacement into temporally well-constrained events and durations. We follow with a presentation of new whole rock element and isotope geochemistry of the KC units in the lobe and nearby satellite bodies and Al-in-hornblende thermobarometry. These are compared to the same data sets in the main intrusive body to ascertain the magmatic processes responsible for the compositional and structural variations during construction of the KC lobe and the subsequent magmatic evolution of the main intrusive complex. We will conclude by proposing a model for how the KC lobe and thus the TIC were initially constructed, as compared to physical and chemical processes that became more prominent during its subsequent evolution, and then discuss implications for the internal and external evolution of large plutonic systems.
THE TUOLUMNE INTRUSIVE COMPLEX AND ITS HOST ROCKS
The 1100 km2, Cretaceous TIC (Fig. 1) is one of four intrusive complexes (previously referred to as intrusive series and then intrusive suites, but we prefer intrusive complex due to the isotopic variations and multi-pulse nature of the plutonic units) that occupy a large portion of the eastern Sierra Nevada crest today (Kistler and Fleck, 1994). These intrusions were emplaced during the 100–85 Ma magma flare-up of the Sierra Nevada batholith (Coleman and Glazner, 1997; Ducea, 2001; Paterson and Ducea, 2015) at ~6–10 km paleodepths (Ague and Brimhall, 1988). All four intrusions are similar in outcrop area (~1000 km2), and they are composed of mostly granodioritic and granitic compositions involving only minor intermediate to mafic units.
The TIC is emplaced into greenschist to amphibolite facies metamorphosed sedimentary, volcanic, and older intrusive host rocks. On its west side, the metamorphic host rock package is composed of Precambrian to Paleozoic quartzites, metapelites, and marble of the Snow Lake block (Lahren et al., 1990; Memeti et al., 2010b) that is intruded by older, Jurassic and largely Cretaceous granitoid plutons (Huber et al., 1989). North, south, and east of the TIC, the exposed host rock consists of—from west to east—Cretaceous metavolcanics of the Minarets sequence, Jurassic metavolcanics intercalated with marine metasediments of the Sawmill Canyon sequence, the Triassic metavolcanics and metasediments of the Koip sequence, and Paleozoic marine metasediments (Greene, 1995; Greene and Schweickert, 1995; Schweickert and Lahren, 2006; Attia et al., 2018; Ardill et al., 2020a), all of which are intruded by Mesozoic granitoids. The arc was actively deforming under a dextral transpressive regime (Tikoff and Greene, 1997; Schweickert and Lahren, 1993; Sharp et al., 2000; Tobisch et al., 2000; Cao et al., 2015, 2016) during the construction of Late Cretaceous plutons, resulting in NW-SE host rock structures and NW-SE oriented dextral strike slip faults, accompanied by significant vertical ductile thickening (Paterson and Memeti, 2014; Cao et al., 2015, 2016).
Tuolumne Intrusive Complex
The TIC is a calc-alkaline, metaluminous, magnetite series, composite intrusion, which exhibits an inward and crudely NE migrating, nested pattern of three major units with older, more mafic phases at the margins of the intrusion, and progressively younger, more felsic units toward the center and northeastward region of the complex (Fig. 1; Bateman, 1992; Kistler and Fleck, 1994; Memeti et al., 2010a, 2014; Paterson et al., 2016). The three major TIC units are separated by sharp to gradational contacts and are easily distinguished in the field: (1) The outer ca. 94–91 Ma (Kistler and Fleck, 1994; Coleman et al., 2004; Memeti et al. 2010a, 2014) granodiorite of Kuna Crest to the east and its equivalents along the western and southern margins (quartz-diorite of May Lake, tonalites of Glacier Point and Grayling Lake, after Bateman and Chappell, 1979) are fairly heterogeneous in texture, consisting of mostly fine- to medium-grained diorites, quartz-monzodiorites, granodiorites, and tonalites, to local gabbroic and also granitic compositions in the KC lobe as reported here in more detail. Most KC and associated rock units exhibit moderately intense magmatic and local subsolidus fabrics and shear zones. (2) The ca. 92–88 Ma (Kistler and Fleck, 1994; Coleman et al., 2004; Matzel et al., 2005, 2006a; Memeti et al., 2010a, 2014) Half Dome Granodiorite (HD) is subdivided into the outer equigranular unit (eHD), which is characterized by conspicuous euhedral hornblende (≤2 cm length), biotite books (≤1 cm in diameter), and titanite (≤1 cm length), and the inner porphyritic unit (pHD), which contains K-feldspar phenocrysts as much as ~3 cm long (characteristically crowded with fine inclusions of hornblende, biotite, and plagioclase) and fewer mafic minerals (Bateman and Chappell, 1979). (3) The ca. 88–84.5 Ma (Kistler and Fleck, 1994; Coleman et al. 2004; Matzel et al., 2005, 2006a; Memeti et al., 2010a, 2014) porphyritic Cathedral Peak Granodiorite (CP) includes characteristic, up to 12-cm-long K-feldspar phenocrysts and 1-cm-large quartz aggregates in a medium-grained matrix (Bateman and Chappell, 1979). The ca. 87 Ma (Bracciali et al., 2008) Johnson Granite Porphyry (JP) is a fine-grained, equigranular leucogranite with antecrystic K-feldspar megacrysts and biotite entrained from older units (Memeti et al., 2014). The Johnson Granite Porphyry was originally regarded as the single youngest unit in the southern center of the TIC (Bateman and Chappell, 1979), but new mapping and geochronology reveal that it is only one of a number of sizeable leucogranitic bodies that are dispersed throughout the HD and CP (Fig. 1), and that the youngest dated leucogranitic TIC body (ca. 84.5 Ma) is in the northern Cathedral Peak lobe (Memeti et al., 2010a, 2014; Paterson et al., 2016). The Sentinel granodiorite, which crops out to the southwest of the TIC, was originally included as part of the Tuolumne intrusion by Calkins (1930), however, geochemical data revealed that the Sentinel granodiorite is compositionally quite different from the rest of the intrusive complex (Bateman and Chappell, 1979). This makes the marginal KC unit and its equivalents the oldest parts of the TIC, the largest area of which is exposed in a lobe in the southeastern portion of the TIC. The Sentinel Granodiorite and other pre-TIC intrusions are now considered part of a regional magma focusing system that culminated in the formation of the TIC (Ardill et al., 2018).
Previous Emplacement Studies of the Tuolumne Intrusive Complex
Bateman and Chappell (1979) published one of the first studies of the entire TIC but did not focus on host rock emplacement mechanisms. However, their figures and statements in the paper imply that early diapiric emplacement shouldered aside host rock and later pulses eroded and discordantly cut across earlier magma. Paterson and Vernon (1995) argued that the TIC grew from multiple nested diapirs emplaced by multiple (both brittle and ductile) host rock displacement processes largely resulting in downward movement of older units. Tobisch et al. (2000) concluded that in the Ritter Range area “downward flow” of the host rocks occurred in the plutonic aureoles during emplacement. Several authors (e.g., Bateman and Chappell, 1979; Žák and Paterson, 2005, 2010; Paterson et al. 2016; Ardill et al., 2020b) present evidence of internal emplacement processes of one pulse into another and document evidence of erosion and recycling of older units into younger pulses and downward flow of magma.
In a recent collection of papers (e.g., Glazner et al., 2004; Coleman et al., 2012; Bartley et al., 2006, 2018), the authors suggested that the entire TIC formed from multiple dike-shaped pulses resulting in either a vertically sheeted complex or a folded-stacked-sill complex, neither of which evolved to form a large magma chamber in which processes modified magma compositions at the emplacement level. These latter emplacement models rest on the following hypotheses: (a) U-Pb zircon geochronology (Coleman et al., 2004) implies that large magma chambers are not viable at any time during the 10 m.y. of TIC evolution, (b) no evidence exists for magma mixing or convective movement of magmas at the emplacement site, (c) the magmatically folded sills should result in steep dips and northerly strikes of contacts and foliations at the pluton margins and shallow dips at the center as well as a new axial planar foliation, (d) several granodiorite to leucogranite solidifications fronts in the equigranular Half Dome granodiorite area between May Lake and Tenaya Lake represent stacks of sills (Coleman et al., 2012) or dikes (Bartley et al., 2018), and (e) TIC rocks mostly consist of minerals, rocks, or structures that formed during late-magmatic to subsolidus recrystallization, which annealed and thus erased evidence of dike contacts formed during emplacement (Bartley et al., 2006; Glazner et al., 2008; Coleman et al., 2012; Glazner and Johnson, 2013). We evaluate these hypotheses in the discussion section.
Standard Geological Methods
Detailed field mapping was completed at 1:10,000–1:24,000 scale in both the KC lobe and adjoining host rocks, while using previously mapped geologic quadrangle maps by Huber and Rinehart (1965), Kistler (1966), and Huber et al. (1989) for reference. Macroscopic mineralogical and structural data (foliations and lineations) were collected in both rock types. The focus for petrography was placed on the KC lobe units, for each of which multiple thin sections were examined.
U-Pb Zircon Geochronology
Zircon crystals were separated using conventional methods of crushing and grinding of grapefruit-sized (~1 kg) samples, followed by heavy liquids and a magnetic separation. Individual zircons were picked and photographed for CA-ID-TIMS analysis of single zircons.
Analytical methods for ID-TIMS of chemically abraded single zircon crystals follow those described in Mundil et al. (2004) and Memeti et al. (2010a). Results regarding tracer calibration are reported in Irmis et al. (2011) and Griffis et al. (2018). Uncertainty on individual 206Pb/238U ages ranges from 0.1% to 2% and is largely a function of crystal size and resulting low abundance of U and radiogenic Pb. The accuracy of individual ages from samples yielding only small zircon crystals (such as C49 and C63) is therefore somewhat compromised as the correction for common Pb contributes considerably to the uncertainty, despite low procedural blanks for common Pb (typically at the 0.5–1.0 pg level). Individual 206Pb/238U ages for each sample (Table 1) are dispersed (except where uncertainties are large), which makes the data interpretation somewhat subjective when weighted mean ages are calculated. We selected the youngest subset of individual 206Pb/238U ages with a permissible probability of fit (>0.05) resulting in uncertainties on the weighted mean of 0.1–0.5%.
Cathodoluminescence Imaging of Zircons and LA-MC-ICP-MS Method
For LA-ICP-MS analysis, 50–100 grains were selected from the separates and incorporated into a 1-inch epoxy mount together with fragments of the Sri Lanka zircon standard at the Arizona Laserchron center, University of Arizona, Tucson, Arizona, USA. The mounts were sanded down ~20 microns into the zircon grains, polished, imaged, and cleaned prior to isotopic analysis. Cathodoluminescence (CL) imaging of zircons was performed at the Arizona Laserchron center on a Hitachi 3400N SEM equipped with a Gatan Chroma CL system.
U-Pb zircon geochronology using laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) was conducted using methods typically followed in the Arizona Laserchron Center and described in Gehrels et al. (2008). The analyses involved ablation of zircon with a New Wave UP193HE Excimer laser using a spot diameter of 30 microns and a Nu high resolution ICP-MS, which is equipped with a flight tube of sufficient width that U, Th, and Pb isotopes are measured simultaneously. All measurements are made in static mode, using Faraday detectors. Concentrations of U and Th are calibrated relative to the Sri Lanka zircon standard. The analytical data are reported in Supplemental Table S21. Uncertainties shown in the table are at the 1-sigma level, and include only measurement errors. Analyses that are >20% discordant (by comparison of 206Pb/238U and 206Pb/207Pb ages) or >5% reverse discordant were not considered further.
Whole Rock XRF Major Oxide and Solution ICP-MS Trace Element Analyses
Grapefruit- to melon-sized (and larger if coarser or porphyritic texture) whole rock samples were analyzed for major and trace elements by X-ray fluorescence (XRF) and inductively coupled plasma–mass spectrometry (ICP-MS) at the Geoanalytical Laboratory at Washington State University, Pullman, Washington, USA. Data are reported in Table S3 (see footnote 1 for all supplemental tables) and procedures explained in Knaack (2003) and Johnson et al. (1999). Plotting and interpretation of the geochemical data and the calculation of CIPW norms for classification purposes was done with the Geochemical Data Toolkit (GCDkit) developed by Janoušek et al. (2006).
ID-TIMS Whole Rock Sr and Nd Isotope Analyses
Whole rock isotopic ratios 87Sr/86Sr and 143Nd/144Nd as well as trace element concentrations of Rb, Sr, Sm, and Nd were determined by isotope dilution thermal ionization mass spectrometry at the University of Arizona on an automated VG Sector multicollector instrument fitted with adjustable Faraday collectors and a Daly photomultiplier. The analytical procedures used for the isotope analyses are described in Otamendi et al. (2009) and Drew et al. (2009). Data are summarized in Table S4 (see footnote 4).
Electron Microprobe Mineral Analyses for Plagioclase-Hornblende Thermobarometry
To determine major oxide compositions of plagioclase and hornblende for plagioclase and Al-in-hornblende thermobarometry using calibrations of Holland and Blundy (1994), and Anderson and Smith (1995) and Anderson et al. (2008), electron microprobe analysis was conducted at University of California, Los Angeles using a JEOL JXA-8200 Superprobe equipped with five wavelength dispersive X-ray spectrometers. The operating conditions included a 15 nA beam current, 15 kV accelerating voltage, and 10 micron defocused beam to avoid Na and K loss by diffusivity. Counting time on the peak ranged from 10 s to 20 s. Analyses were completed from core to rims to determine range of composition on juxtaposed plagioclase-hornblende pairs, but the plagioclase-hornblende thermobarometry results are based on rim compositions.
Lithologies and Structure of the Host Rocks to the Kuna Crest Lobe
The host rocks to the KC lobe magmas are part of the Ritter Range pendant, which consists of the most extensive and well-exposed section of Mesozoic metavolcanic arc rocks in the Sierra Nevada batholith. Our recent mapping largely supports previous studies by Tobisch et al. (1977, 2000) and Fiske and Tobisch (1978, 1994), which document that this pendant consists of a series of fault-bounded, internally thinned tectono-stratigraphic packages of metavolcanic and rare metasedimentary sequences, within which stratigraphy has been both cut out and repeated. These stratigraphic packages contain well-preserved bedding that dips steeply and youngs to the southwest (Fiske and Tobisch, 1978; Tobisch et al., 1986, 2000; Paterson and Memeti, 2014). Metavolcanic units consist of lava and ashflow tuffs and volcaniclastic deposits of andesitic to rhyolitic compositions with minor intercalated fine-grained quartzite (Fig. 2).
Most metavolcanic rocks in this area reached upper greenschist to locally amphibolite grade metamorphic conditions based on plagioclase and amphibole compositions. Accessory metamorphic epidote occurs; lower greenschist minerals, such as chlorite, are notably absent near plutons and increase in abundance with distance away from pluton contacts. Secondary albite and actinolite occur, but are rare. Hornblendes range from 5 to over 12 wt% Al2O3 and the plagioclase are as calcic as An79. Clearly, some of these mineral compositions are relict igneous. Our estimates of metamorphic conditions near the TIC based on hornblende-plagioclase compositions for these units are all above 595 °C, and mostly above 640 °C (Memeti et al., 2014).
Tobisch et al. (2000) defined five tectono-stratigraphic packages. Using multigrain U-Pb zircon dating, they established that the three eastern packages of volcanic and sedimentary units range in age from Late Triassic to Jurassic (Fiske and Tobisch, 1978; Stern et al. 1981; Tobisch et al., 2000; Paterson and Memeti, 2014; Attia et al., 2018). The fourth package consists of the Minarets Caldera with ages ranging from ca. 102 to 95 Ma (Tomek et al., 2017). The fifth package is well west of the area discussed in this paper.
Host Rock Structures
The metavolcanic and metasedimentary sequences around the KC lobe define four, fault-bounded packages that match well those described immediately to the south by Fiske and Tobisch (1978, 1994) and Tobisch et al. (1977, 2000). Here, the tectonic packages of steeply dipping metavolcanic rocks are bordered to the east by older Paleozoic rocks called the Rush Creek and Palmetto sequences studied by Greene (1995) and on the west by the Minarets caldera units that form package #4. The eastern boundary between the Paleozoic and Mesozoic units is a late Paleozoic unconformity subsequently reactivated in the Cretaceous by the ~2–3-km-wide, oblique, dextral, transpressive Gem Lake shear zone (Brook et al., 1974; Greene and Schweickert, 1995). All Triassic to middle Cretaceous stratigraphic units dip steeply and typically strike northwest: Late Cretaceous units in the Minarets caldera dip moderately to the SW with variable strikes. All units are internally strained and typically preserve a bedding-parallel cleavage and steeply plunging mineral lineation, although the amount of strain associated with these structures varies tremendously as a function of both rock type and location (e.g., Tobisch et al., 1977; Cao et al., 2015). On average, strains are highest in package #1, particularly in the Gem Lake shear zone, where values of >60% shortening are common (Horsman et al., 2008). These shortening values decrease to ~40% average shortening in packages #2 and #3 and to typically <30% shortening in package #4 (Minarets caldera). Tectono-stratigraphic package bounding faults are largely bedding parallel and fairly cryptic, although we have locally found ductile, greenschist-grade mineral lineations, breccias, and local pseudotachylites associated with these faults. The structural contact aureole (the area of host rock around a pluton deformed due to pluton emplacement) around the KC lobe varies dramatically. Along the eastern margin no contact aureole is obvious, in large part due to parallelism of this contact and older regional structures and also due to overprinting by movement on the dextral transpressive Gem Lake shear zone (Greene and Schweickert, 1995). But both previous mapping by Greene (1995) and mapping by Cao et al. (2016) and our own indicates that major stratigraphic host rock units and faults are not deflected and strain generally does not increase toward the TIC margin, but instead is mostly truncated by the pluton contact along this margin (Fig. 2). Only local, map scale evidence exists that suggests host rock units are “pushed aside” or displaced laterally during growth of the lobe (see discussion).
Kuna Crest Lobe and Associated Magmatic Units
The KC lobe was originally mapped as undivided granodiorite of Kuna Crest by Huber and Rinehart (1965), who noted the variable composition and texture of Kuna Crest rocks and mapped the satellite plutons between Waugh Lake and Garnet Lake to the southeast, and by Kistler (1966), who was the first to recognize the “sheared granodiorite of Koip Crest” on the east side of the lobe and first to map the Rush Creek pluton and quartz monzonite of Billy Lake. These studies were followed by Bateman and Chappell (1979) and Huber et al. (1989).
The KC lobe can be subdivided into four different magmatic domains with distinct compositional and structural characteristics (Figs. 1 and 2): (1) a marginal sheeted complex (KCSC), (2) the main body of the KC lobe (KC unit I through unit III), (3) the interior Kuna Crest-Half Dome hybrid unit (KC-HD) representing the transition into the equigranular Half Dome in the main body of the TIC, and (4) the Waugh Lake, Thousand Island Lake, and Island Pass satellite plutons at the tip of the KC lobe. We also explored the geologic relationship of the Rush Creek pluton to the KC system as it is at least spatially related to the KC lobe bodies in this southeastern region of the TIC (Fig. 2). Here, we introduce the individual magmatic units of the Kuna Crest lobe sensu lato in more detail.
Kuna Crest Sheeted Complex
The sheeted complex of the KC lobe is composed of hundreds of subvertical, parallel sheets of medium- to locally coarse-grained gabbro, diorite, and granodiorite that are typically one to tens of meters thick, but locally as thick as >100 m (Figs. 2–5). The occurrence of the different compositions and their grain sizes do not follow any particular pattern across the sheeted complex. These sheets are locally intercalated with metavolcanic and metasedimentary slivers of the host rocks described by Fiske and Tobisch (1994) (Fig. 4). Some of the “host rocks” to the magmatic sheets—a few of which are clearly igneous and could be only slightly older KC magmas or magmas unrelated to the KC—are migmatitic where they are sandwiched between undeformed magmatic sheets and along the contact to the main body of the KC lobe (Figs. 2 and 4A–4C). These host rock (injection) migmatites display centimeter- to decimeter- scale veins of mostly the same magmatic composition as the larger sheets, although locally these veins are leucogranitic in composition (Fig. 4). No obvious restitic material was found to suggest that the veins formed from in situ melting of the host rock. The contacts between the different magmatic rock compositions and the host rocks are typically sharp and steep. The sheeted complex has a total width of 0.2–1 km and occurs in segments along the curved eastern, southern, and southwestern margins of the main body of the KC lobe (Fig. 2).
Kuna Crest Lobe Sensu Stricto
The main body of the greater KC lobe is exposed over ~80 km2 and made of semi-concentrically arranged units varying in texture, grain size, and composition (Fig. 2). Compositions range from minor monzogabbro, monzodiorite to dominantly medium-grained quartz-monzodiorite, and granodiorite (Figs. 3 and 5, and 6). We distinguish three major, concentric units from lobe margin to the interior of the lobe: KC unit I is composed of a dark gray, coarse- to medium-grained monzodiorite to quartz-monzodiorite (Figs. 3B, 3C, and 5D) that grades into unit II. The latter is mainly composed of a finer, medium-grained, gray quartz-monzodiorite and locally granodiorite (Fig. 6A). Both units contain cm-wide, mafic, and leucocratic schlieren and up to several meter large hornblende + biotite + plagioclase + oxide + titanite enriched cumulates (Fig. 7A). Typically 1-cm diameter large biotite-hornblende-Fe-oxide clots (glomerocrysts) are abundant in these two outermost units of the KC lobe (Figs. 3C, 5A, and 5B) and less common in the two inner units. Metavolcanic host rock xenoliths up to several meters across are most common at the margin of the lobe. KC unit II grades into medium- to locally coarse-grained KC quartz-mozondiorite to granodiorite with markedly lower color index (unit III; Figs. 3D, 3E, and 6B). Rare euhedral books of biotite are more abundant toward the contact with the transitional Kuna Crest–Half Dome granodiorite (KC-HD), which extends from the interior of the lobe toward the NW, where the lobe joins the main intrusive complex (Fig. 6F). The KC-HD transitional unit ranges in composition from a quartz-monzodiorite to a granodiorite and includes euhedral books of biotite, up to 1 cm euhedral hornblende, and up to 1 cm long titanite crystals, all of which are characteristic of the eHD granodiorite (Figs. 2 and 6F). However, KC-HD is distinct from the eHD and also shows characteristics of KC lithologies (subhedral to anhedral hornblende and biotite), which is why we refer to this unit as the transitional KC–HD granodiorite. All of the KC units contain quartz-diorite enclaves and enclave swarms as well as quartz-diorite sheets or irregular bodies from which some enclaves have formed (Fig. 7B). Where the lobe joins the main TIC, KC unit I and KC unit II are truncated by KC unit III and the KC-HD hybrid unit near Ireland Lake (Fig. 2). At Potter Point, where Half Dome intruded into the KC-HD unit, large, stoped, cognate blocks of the KC-HD unit are surrounded by eHD mineral cumulates and enclaves (Fig. 7C; see Žák and Paterson, 2010).
Satellite Plutons at Waugh and Thousand Island Lakes
To the southeast of the lobe, KC unit III granodiorite grades into a ~2 × 2 km body of fine- to medium-grained, biotite-porphyritic to equigranular diorite to granodiorite near Waugh Lake—originally mapped as part of the Kuna Crest (Huber and Rinehart, 1965) but herein referred to as the Waugh Lake pluton (Fig. 6C). The Waugh Lake pluton locally contains up to 50 vol% of metavolcanic host rock blocks (Fig. 7D), some of which are map scale size (Fig. 2). The Waugh Lake pluton has highly angular, stepped contacts with the surrounding host rocks (Fig. 2).
The Waugh Lake pluton has two sheet-like extensions to the southeast toward Thousand Island Lake, where it links to a 1.5 × 2 km granodiorite body (Fig. 2). NW of Thousand Island Lake, it is mostly composed of medium- to fine-grained granodiorite to granite with euhedral hornblende laths and biotite (Fig. 6D). To the southeast of the lake toward Garnet Lake, the granodiorite to granite transitions gradually into a very fine-grained leucogranite (Figs. 2 and 6E). We refer to this unit as the Thousand Island Lake pluton. In contrast to the Waugh Lake pluton, the Thousand Island Lake pluton exposes only rare host rock xenoliths.
The Rush Creek Pluton
The Rush Creek pluton is a zoned intrusive body with three mapped units located on the SE end of the KC lobe between Waugh and Gem lakes (Fig. 2), originally mapped as the granodiorite of Rush Creek and quartz monzonite of Billy Lake (Kistler, 1966). The main rock type is a medium-grained, hornblende-biotite granodiorite to granite containing mafic enclaves. The outer, marginal unit is finer grained and darker in color and dominantly composed of a quartz monzonite to diorite. The small unit mapped in its center is a hornblende-plagioclase-oxide cumulate body. On the east side, the Rush Creek pluton is largely overprinted by strong solid state, mylonitic deformation from the Gem Lake shear zone. The Rush Creek pluton generally resembles KC type rock units, but with only poor age constraints, it has been unclear if this pluton is part of the same magma system. Kistler and Swanson (1981) originally dated the “Billy Lake-Rush Creek pluton” using the Rb/Sr whole rock method and obtained a 99.9 ± 2.2 Ma from the main hornblende-biotite granodiorite unit.
Structure in the Kuna Crest Lobe and Associated Magmatic Units
All of the KC lobe units are separated by 50–200 m wide gradational contacts with the exception of the southeast portion of the medium-grained, lighter-colored KC lobe unit III, which forms a sharp contact with marginal units I and II as it cuts across them and strikes toward the Waugh Lake diorite-granodiorite body at the lobe tip (Figs. 2 and 6C). The sheeted margins of the KC lobe typically have sharp intrusive contacts between sheets and to the host rock. The contact between KC lobe unit I and the sheeted margins is also sharp and locally separated by thin septa of host rock (e.g., at Marie Lakes; Figs. 2, 4A, 4D, and 4E).
The southern tip of the lobe clearly intrudes across a number of undeflected stratigraphic units (Cao et al., 2016), although outcrop-scale examples of this discordance are rare due to the presence of the slightly younger Waugh Lake pluton. The satellite plutons intrude across stratigraphic units in their host rocks and locally across the tectono-stratigraphic package bounding faults as defined by Tobisch et al. (2000) (Fig. 2). The Waugh Lake pluton is a highly discordant body with host rock rafts and stoped blocks and a highly stepped margin at tens of meters scale (Figs. 2 and 7D). The Thousand Island Lake and Island Pass plutons are also characteristic of discordant pluton-host rock boundaries, but stoped blocks are not as abundant as in the Waugh Lake pluton. In addition, the Thousand Island Lake pluton extends in an elongate, increasingly narrowing intrusion in a southeast direction.
Magmatic and Solid-State Fabrics
Magmatic fabrics in the KC lobe are defined by the magmatic alignment of hornblende, biotite, and plagioclase, varying in intensity from weak to moderately well aligned. The dominant magmatic fabric in the KC lobe is a steeply plunging lineation and a steeply dipping magmatic foliation that is roughly margin parallel across most of the lobe, but gradually changes to a regional NW-SE and ESE-WNW striking, steeply dipping magmatic foliation in the center (Fig. 2; Žák et al., 2007; Memeti et al., 2010a). The mineral fabrics overprint internal contacts, which is particularly evident where the fabrics and the contacts are at an angle to one another (Fig. 2). Local magmatic flow structures concentrating mafic minerals and accessories, such as irregularly shaped schlieren layers and schlieren layers forming tubes and troughs display mineral alignments parallel to the layers (Paterson, 2009). Several centimeter-scale leucogranitic schlieren are observed across all KC units and, like the magmatic foliation, are typically oriented margin/contact parallel (Figs. 8A). Locally, these leucogranite schlieren and aplite dikes, as wide as 1 m, are magmatically folded with the prominent magmatic foliation oriented parallel to axial planes (Žák et al., 2007, their Fig. 6D). Similar strain is locally preserved in fine-grained quartz-diorite sheets that are normally mingled with host KC lithologies as cm- to dm- large enclaves. Locally, these sheets are folded in the magmatic state with the magmatic foliation oriented parallel to the axial plane of the magmatic fold (Fig. 8D). The sheets can also be boudinaged with boudin necks, forming low-strain zones, often filled with leucogranite that drained from the surrounding magma mush (Fig. 8B).
In the sheeted complex, the host rock and magmatic sheets are internally foliated (magmatic and subsolidus) parallel to lithologic contacts. Local magmatic veins are oriented mostly parallel to the mineral foliation and contacts, but are also found to be locally at an angle to lithologic contacts and folded (Fig. 4). Individual sheets in the sheeted unit are generally oriented parallel to the margin of the KC lobe and follow the steeply dipping host rock foliation. At the NE margin the sheets are oriented parallel to the regional NW-SE structural grain. Along the SE margin, the sheet orientations and internal foliations strike NE and dip steeply. This part of the sheeted complex intrudes discordantly across host rock units and internal contacts of the Rush Creek pluton (Fig. 2). Host rock foliations are also deflected from NW to NE orientations over a distance of ~1 km from the lobe contact (the lobe is ~10 km across: so this deflection is 1/10 of the lobe diameter). In the Marie Lakes complex, the sheets and foliations are oriented WNW-ESE and farther NW these structures align again with the regional structural orientations.
At and near the lobe-host rock contact, the sheeted complex and outermost lobe unit I reveal strong, solid-state, and steep foliation and lineation with flattening fabrics (S > L). Local cm-dm long, discrete subsolidus shears occur closer to the lobe margin, some of which are intruded by undeformed to solid-state deformed leucogranite to granodiorite several cm thick (Figs. 2 and 8C). The shears are dominantly steeply dipping, although few, moderately dipping and subhorizontal shears are also present. The shears have inconsistent kinematics and often form a conjugate system with the acute angle oriented parallel to the host rock margins, indicative of flattening strain. These solid state foliations, lineations, and shears occur at their widest extent at the eastern margin of the host rock-KC lobe contact (1 km) and were observed over only tens of meters to 100 m at the northeastern, southeastern, and southwestern margins (extent shown with white line in Fig. 2). Foliations and “leucosomes” in the injection migmatites generally follow the host rock structure.
The Waugh Lake pluton displays well-developed, regional NW-striking and steeply dipping magmatic foliations and steeply plunging magmatic lineations that are statistically parallel to regional NW-striking host rock structures. The Thousand Island Lake pluton fabrics, however, are characteristic of well-developed WNW-ESE orientations and steep lineations. Both plutons commonly have weak to moderate solid-state overprints near their margins and expose local injection migmatites in their host rock aureoles. The Rush Creek pluton fabrics are weak to moderate with a pre-dominantly NW-SE striking magmatic foliation, which is increasingly overprinted by subsolidus deformation and is mylonitic in nature in the Gem Lake shear zone.
CA-ID-TIMS U-Pb Zircon Ages of the Kuna Crest Lobe Units
Four new CA-ID-TIMS U-Pb single zircon ages and two ages reported in Memeti et al. (2010a) provide information on the order of crystallization of the different KC lobe units and associated magmatic bodies (Fig. 9). The individual zircon ages obtained amongst all samples discussed here range from ca. 96.42 Ma (05183) to 92.68 Ma (KCL-536, Table 1; Fig. 9). The zircon ages scattered toward older values are interpreted as either xenocrystic or antecrystic zircons, that is zircons that are inherited from the host rock or crystallized in earlier batches of magmas, respectively (definition from Miller et al., 2007; see also TIC related zircon age discussion in Memeti et al., 2010a, 2014, and Paterson et al., 2016). The younger zircon ages are used to calculate the end of zircon crystallization and therefore the final solidification of the rocks (Fig. 9).
The oldest ages of the lobe complex, and for the entire TIC, are found in the Marie Lakes marginal sheeted complex in the southern part of the KC lobe (KCSC) yielding 94.91 ± 0.53 Ma (C63, n = 6) and 94.79 ± 0.13 Ma (C49, n = 10). Zircon crystallization subsequently moved into the margin of the KC lobe (KC unit I) producing an age of 94.44 ± 0.15 Ma (KCL428, n = 6; Memeti et al., 2010a). The interior of the lobe at the transition to the Half Dome granodiorite in the main Tuolumne body (KC-HD), however, did not crystallize until 92.75 ± 0.11 Ma (KCL536, n = 6), ~1.7 m.y. after the lobe margin crystallized and 2.2 m.y. after the sheeted complex solidified. The Waugh Lake pluton and the Thousand Island Lake pluton at the tip of the KC lobe crystallized at 93.51 ± 0.47 Ma (05183, Memeti et al., 2010a) and 93.66 ± 0.36 Ma, respectively—at least 1 m.y. after the KCSC and KC lobe unit I crystallized, but before the crystallization of the lobe magmas that transitioned into the main Tuolumne body (KC-HD; Fig. 9).
The CL images generally show two forms of zircons; a longer, prismatic form with no to simple zoning, and a second, more equidimensional, stubby form with oscillatory zoning (Fig. 10). The zircons generally include small grains of apatite, although inclusion-free zircons were picked for the geochronology analyses. While the Waugh Lake pluton (05-183) contains only the prismatic form and the marginal KC unit I (KCL-428) contains only stubby zircons, one of the samples from the Marie Lakes sheeted complex (C-49) and the KC-HD transitional unit (KCL-536) contain one larger and one smaller, stubby population of zircons. In C-49, the larger zircon type is prismatic with no obvious zoning and the smaller form is oscillatory zoned. In KCL-536, both the larger and the smaller forms are stubby and oscillatory zoned with especially the larger zircons indicating darker, higher U cores and interior zones. For the most part, the larger zircons in both samples (C49L and KCL536L) yielded ages a few hundred thousand years older (some are the same age within uncertainty) and the smaller zircons yielded younger ages (C49S and KCL536S) (Table 1).
LA-MC-ICP-MS Zircon Ages of the Kuna Crest Sheeted Margin near Gem Lake, Migmatites, the Rush Creek Pluton, and the Island Pass Hornblende Gabbro
Four additional units were dated with LA-ICP-MS U-Pb zircon geochronology to help put the different igneous units in temporal relationship to one another: the Gem Lake Kuna Crest sheeted margin (A-23), the migmatite (sample C-55) between Marie Lakes and Waugh Lake, the Rush Creek pluton (D-108), and the Island Pass hornblende gabbro (sample B-90). The weighted mean age of the Gem Lake Kuna Crest sheeted margin yielded a zircon crystallization age of 94.22 ± 0.94 Ma, the migmatite sample produced an age of 94.73 ± 0.64 Ma, the Rush Creek pluton yielded an age of 97.6 ± 1.5 Ma, and the Island Pass hornblende gabbro generated an age of 89.6 ± 1.1 Ma (Fig. 11; Table S2). The migmatite and gabbro samples both yielded minor antecrystic or xenocrystic zircons at the upper age range, but for the most part, the individual zircon ages overlap within uncertainty. The youngest ages below the weighted mean age could have undergone Pb-loss due to the lack of chemical abrasion of these zircons (which was done with the CA-ID-TIMS dated samples).
Petrology of the Kuna Crest Lobe Units
Mineral characteristics in all KC units and associated bodies are generally similar. KC unit I through unit III monzogabbro-diorite to quartz-monzodiorite and granodiorite units (Figs. 2 and 3, 5, and 6) are composed of varying amounts of plagioclase, hornblende, and biotite and lack or have lesser amounts of quartz and K-feldspar. Titanite (both primary and secondary), apatite, zircon, rutile, magnetite, Fe-Ti-oxides (ilmenite) are accessory minerals. Minor secondary epidote is present also. The subhedral to anhedral, sometimes poikilitic, light olive green to light brown pleochroic hornblendes show patchy zoning, typically have a noticeable abundance of apatite inclusions, and many have relict clinopyroxene cores associated with inclusions of quartz. Locally, hornblende has retrograded to biotite. Hornblende is rarely euhedral and occurs in euhedral form in more appreciable abundance in the transition zone from KC lobe to Half Dome (KC-HD). Biotite shows red-brown to light brown pleochroism and is often associated with Fe-Ti oxides and rutile. Hornblende, biotite, and opaque phases (magnetite and ilmenite) locally occur as glomerocrysts (clots). Plagioclase is euhedral to subhedral, often characterized by oscillatory zoning and typical albite twinning. The Michel-Levy albite twinning extinction angle method reveals an anorthite content in the KC rocks ranging from labradorite to albite (the latter are likely cuts through rim compositions). Some plagioclase in the outer margins of the KC lobe (unit I and sheeted margins) have sieve texture cores that are rimmed by oscillatory zones and are juxtaposed to plagioclase with only oscillatory zoning and smaller grain size, indicating a minimum of two different populations of plagioclase (Figs. 5E and 5F). Larger plagioclases with disequilibrium texture may show albite overgrowth (Fig. 5E). Abundant plagioclase-plagioclase contacts suggest a degree of crystal accumulation or melt loss (Fig. 5F). A few grains of plagioclase include small, seemingly euhedral epidote, but given results from thermobarometric estimates and their occurrence in plagioclase cores, they are likely replacing more calcic plagioclase cores and thus are secondary in nature. Where quartz occurs, it was clearly deformed under high temperatures showing rectangular subgrains, i.e., chess board pattern extinction. Alkali feldspar (herein referred to as K-feldspar) is mostly poikilitic in nature indicating late interstitial crystallization in a mostly crystallized magma. It is generally perthitic through albite exsolution and locally microclinic as revealed through tartan twinning. The quartz-monzodiorites to granodiorites of the central KC-HD unit and the Thousand Island Lake granodiorites to granites are composed of a similar mineral assemblage, but with higher concentrations of quartz and K-feldspar and less mafic minerals, and higher variability of texture and grain size. Biotite is partially chloritized in the KC-HD unit. Hornblende-and-biotite intergrowths almost exclusively occur in the Waugh Lake diorite.
A total of 31 samples were analyzed for major and trace element analyses from nine distinguished units of the greater KC lobe area including the sheeted margins of the lobe and associated satellite bodies (Table S3). These analyses are compared to 24 geochemical analyses of the KC unit and associated rocks in the main part of the TIC from previous studies (Bateman and Chappell, 1979; Burgess, 2006; Burgess and Miller, 2008; Gray, 2003; Gray et al., 2008; Kistler et al., 1986; Paterson et al., 2008; Solgadi and Sawyer, 2008). Table S3 also includes five samples from the Rush Creek pluton.
The KC lobe units are compositionally quite varied (Fig. 12): Using CIPW norm calculations from whole rock compositions and plotting in the QAP Streckeisen (1978) diagram as well as using the Middlemost (1994) rock classification, they include monzogabbros (with > An50 plagioclase in KCSC and KC lobe unit I; Island Pass hornblende monzogabbro), monzodiorites and quartz-monzodiorites (KCSC, KC lobe I and II), granodiorite (KC lobe unit III, KC-HD, Waugh Lake pluton, Thousand Island Lake pluton) to sparse granite and leucogranite compositions (Thousand Island Lake pluton, KC-HD). SiO2 varies from 49 to 71 wt%, and goes up to 77 wt% when aplite dikes are considered (Table S3). There is an apparent compositional gap between ~63 and 67.5 wt% SiO2, which is only partially present in the KC from the main Tuolumne body. The largest changes in major oxide content in the KC rocks are in FeOt (2–9 wt%) and CaO (2–10 wt%). CaO shows a scattered linear decrease and K2O a scattered linear increase with increasing silica. Other major oxide (Al2O3, FeOt, MgO, Na2O, TiO2, P2O5) variations are broadly negatively correlated with silica, but have significant scatter and subtrends, especially at low silica contents (Fig. 12). MgO, FeOt, and TiO2 have a bifurcate pattern at <58 wt% silica, which could be related to cumulus mafic phases. In addition, Al2O3 and P2O5 have a steeper decrease with silica below 58 wt%; phosphate is likely indicating apatite as a cumulus phase. Important to note is that no simple distribution pattern is apparent along the trends of analyses from the various KC lobe units. With the exception of the Thousand Island Lake pluton, which plots as a granite at the highest SiO2, data points from each unit are roughly spread over the extent of the whole major oxide distribution. The Island Pass gabbro mostly plots separately from all other Kuna Crest rocks. The Waugh Lake pluton plots with KC lobe unit III samples (Fig. 12).
Most trace and rare earth element plots show large scatter across the range of SiO2 represented in these rocks, especially at lower SiO2 contents, which is typical of cumulate rocks (Fig. 13, see also rare earth element (REE) data). One exception is the more linear trend of Sr plotted against SiO2 and Sr versus Rb. The range of selected trace elements for all KC lobe rocks combined is as follows: Sr ~260–858 ppm, Ba ~352–1572 ppm, Rb ~63–241 ppm, Zr ~52–318 ppm, La ~10–48 ppm, Ce ~20–84 ppm, Nb ~3–20 ppm, Y ~7–27 ppm (Fig. 13, Ce and Nb are not shown).
The REE pattern is characterized by enriched light REE (LREE; 30–150 times chondrite) compared to the heavy REE (HREE; 4–20 times chondrite) with most analyses indicating a slightly negative Eu anomaly, except for the Thousand Island Lake pluton, which displays a larger Eu anomaly (Fig. 13). When REE are compared to the average REE composition of KC lobe rocks, the majority of the samples display a “flat” REE pattern (with very small positive or negative Eu anomalies) and plot at around EuN/EuN* = 1 and LaN/SmN = 1 (Fig. 13). Several low-SiO2 samples from KCSC and KC lobe units I–III as well as the Island Pass gabbro, however, exhibit dome-shaped REE pattern with typically positive Eu anomalies (EuN/EuN* > 1) and LaN/SmN < 1, which is consistent with accumulated plagioclase and/or hornblende. KC-HD, Thousand Island Lake and a few other more evolved KC lobe unit I–III samples show negative Eu anomalies (EuN/EuN* < 1) and scoop-shaped REE pattern consistent with plagioclase and titanite fractionation. The fairly linear decrease of Sr with increasing SiO2 is consistent with plagioclase fractionation and the increase of LaN/SmN with hornblende fractionation to produce the more evolved compositions (Fig. 13). Sample KCL-386 from KC lobe unit III, which plots at higher SiO2, high-Sr, high EuN/EuN*, and low LaN/SmN, is interpreted to represent a melt-depleted granodiorite/granite residue (likely representing a separate stage of accumulation compared to the low SiO2 rocks).
Overall, the major oxide and trace element variations of the KC units from the KC lobe show somewhat similar plots to analyses conducted on KC rocks in the main part of the intrusive complex, however, the data from the KC lobe display considerably more spread and more positive Eu anomalies (Figs. 12 and 13).
Sr and Nd Isotope Signatures
The greater KC lobe rocks (including satellite plutons) mostly plot between −2 and −3 εNd and 0.7054 and 0.7058 87Sr/86Sr(i). These values are the most primitive and most homogeneous isotope values in the entire TIC (Fig. 14; Table S4). Sample C32 from the Rush Creek pluton yielded Sr97.6 Ma = 0.704861, εNd97.6 Ma = 1.12, which is significantly more primitive compared to any KC compositions.
We used Al-in-hornblende-plagioclase thermobarometry (calibrations of Holland and Blundy, 1994, and Anderson and Smith, 1995; Anderson et al., 2008) to estimate pressures and temperatures of hornblende and plagioclase rims in textural equilibrium with one another. Our results from the KC lobe, Waugh Lake pluton, Thousand Island Lake pluton, and Rush Creek pluton in this region show, within uncertainty, similar pressures and temperature estimates for depth of magma crystallization: Crystallization conditions are calculated at 1.8 ± 0.3 kbar and 777 ± 19 °C for the Waugh Lake pluton, 2.3 ± 0.3 kbar at 727 ± 36 °C for the Thousand Island Lake pluton, 683 ± 23 °C at 2.8 ± 0.4 kbar for the KC-HD unit, and 2.0 ± 0.3 kbar at 734 ± 18 °C for the Rush Creek pluton (Fig. 15; Table S1; see footnote 1).
DISCUSSION: FORMATION OF THE KUNA CREST LOBE AND IMPLICATIONS FOR THE FORMATION OF THE TUOLUMNE INTRUSIVE COMPLEX
Age Relationships and Durations of Magmatism in the Kuna Crest Lobe
U-Pb zircon geochronology documents that the KC lobe and associated bodies were constructed over ~2 m.y. The oldest parts of the lobe are found in the sheeted complex preserved in segments around the entire KC lobe with CA-ID-TIMS ages from two sheets at Marie Lakes yielding 94.91 ± 0.53 Ma (C63) and 94.79 ± 0.13 Ma (C49) and the lobe margin northwest of Gem Lake a 94.22 ± 0.94 Ma LA-ICP-MS age (Figs. 2 and 9, and 11). Although the uncertainties for two of these samples are a little high (due to small grain size and relatively low U yielding low radiogenic Pb, and the lack of pre-treatment of zircons to remove Pb loss effects for the LA-ICP-MS age)—the ages from the three sheeted margin samples are the same within uncertainty, and together with the observed field characteristics, suggest that crystallization (and likely the emplacement due to the small intrusive body size) of these small magma increments occurred relatively rapidly over a short duration. The sharp contacts with other igneous sheets and sheet-margin-parallel foliations suggest that emplacement and crystallization was rapid enough to limit magma interaction (such as mixing, mingling) between sheets.
The marginal KC unit I at 94.44 ± 0.14 Ma (KCL428, Memeti et al., 2010a) is fairly close in age to the sheeted margin (Fig. 9) and suggests that magma increments might have been emplaced rapidly enough to start the construction of a larger, more interconnected magma body, ultimately producing what is mapped as the KC lobe sensu stricto. A ~1.5-m.y.-long zircon crystallization history and gradational contacts from the margins of the lobe leading up to the time of final crystallization of the central KC-HD transitional unit at 92.75 ± 0.11 Ma suggests that the KC lobe was magmatically active more or less continuously over ~1.5 m.y. This does not necessarily mean that the KC lobe was statically crystallizing from margin to interior over this long time span. 2-D finite difference thermal modeling from our earlier study, which considered temperature effects on thermal diffusivity, estimated that the KC lobe body would have taken 500,000–600,000 years to fully crystallize (Memeti et al., 2010a). 3-D modeling would decrease this time scale due to the faster cooling rates to be expected with the increase of surface area. Thus, one implication from our geochronology results is that the exposed KC lobe represents a cross section of a magma transfer zone through which magmas passed for a prolonged period of time, presumably to feed shallower intrusions and/or volcanic eruptions. Some magma might have been lost to downward flow during the rise of new magmas. Either way, the preserved KC lobe rocks are only some fraction of the magmas that passed through that region. At this point we cannot account for magma that was lost to other crustal levels without more constraints.
The 94.73 ± 0.64 Ma migmatite zircon crystallization date indicates that the migmatites at the southern margin of the KC lobe formed during crystallization of the sheeted complex and the KC lobe sensu stricto, confirming that the migmatites are part of KC lobe magmatism. Migmatites are also found between purely igneous sheets in the sheeted complex (Fig. 4) and are locally crosscut by larger sheets and irregularly shaped magmatic bodies. Based on the absence of restitic material and the mostly magmatic nature of the small “leucosome” veins, which look like typical KC granodiorite magmas, we consider these “leucosomes” to be just smaller scale magma injections (Fig. 4). Hence, we interpret that the KC lobe migmatites are injection migmatites forming at the edge or the tip of larger magmatic sheets.
The 93.51 ± 0.47 Ma Waugh Lake pluton and 93.66 ± 0.36 Thousand Island Lake pluton at the tip of the KC lobe were also emplaced and crystallized within the time span of the crystallization of the KC lobe sensu stricto. These age relationships, the sharp contact that is related to KC unit III protruding across KC units I and II toward the tip of the lobe as well as geochemical characteristics, support the hypothesis that KC unit III transitions and is physically and chemically related to the Waugh Lake pluton (Figs. 2 and 12, and 13). It is likely that the magmas that formed the Waugh Lake pluton and Thousand Island Lake pluton were lobe magmas that extruded into the strain shadow of an already crystallizing KC lobe. With a fairly thick crystallized lobe margin (KC lobe units I and II) at the time KC unit III was still mobile, the KC lobe would have formed a fairly rheologically strong body compared to the surrounding host rock. The younging of the ages from the Waugh Lake pluton to the Thousand Island Lake pluton as well as the grading to more evolved magmas in the southeast sheet of the Thousand Island Lake pluton indicate that the magmas migrated to the southeast during satellite pluton emplacement.
Our U-Pb zircon geochronology study suggests the main, interior unit of the Rush Creek pluton crystallized ca. 95–97.5 Ma (Fig. 11), which is younger than the original, ca. 100 Ma Rb/Sr age of Swanson and Kistler (1981). Although we have calculated a weighted mean age of ca. 97.6 ± 1.5 Ma (LA-ICP-MS age) using the greatest number of overlapping individual zircon ages and their uncertainties from one sample (D-108; Fig. 2), it is possible that the Rush Creek pluton age is as young as ca. 95 Ma, if only the youngest ages (autocrysts?) are used to calculate a minimum crystallization age. However, it is unclear if these younger zircons yielded younger ages due to Pb loss since they do not form a consistent age cluster. Cross-cutting field relationships require the Rush Creek pluton to be older than the Gem Lake sheeted margin dated at 94.2 ± 0.94 Ma (A-23; Fig. 2). Based on the field relationships and the geochronology alone, it is inconclusive whether the Rush Creek pluton is part of the KC system. However, the significantly more primitive Sri and εNd values compared to the isotopically more evolved and homogeneous KC units imply it is not directly part of the KC system and thus it is not further examined here (see Swanson and Kistler, 1981, for a more detailed description of its field relations and petrology). It is interesting to note, however, that given the spatial and overall timing relationship, the isotopically more primitive, zoned Rush Creek pluton preceded the intrusion of the KC system and used a similar magma ascent pathway during its construction. Such early, more mafic intrusions are interpreted to have contributed to the warming of the crust, leading to magma focusing as discussed by Ardill et al. (2018).
The LA-ICP-MS 89.6 ± 1.1 Ma Island Pass hornblende gabbro weighted mean age, which includes most of the zircon ages from the sample (Fig. 11), is ~4 m.y. younger than any other intrusive body in the entire lobe and nearby regions. We speculate that this age may be too young (due to Pb loss in zircons crystallizing from high U magmas) and the <0.2 km2 Island Pass hornblende gabbro body is related to the rest of the KC system. Alternatively, the Island Pass gabbro may be younger, although it is difficult to conceive that such a small body with cumulate textures is the only evidence remaining from younger magmatism in this area. Careful CA-ID-TIMS geochronology might be able to resolve this issue.
The 94.91 Ma to 92.75 Ma zircon crystallization ages of the KC lobe region include the oldest ages that have been thus far reported in the 1100 km2 TIC magmatic system, including the KC and its equivalents on the west side of the TIC. The few ages that are available for the KC unit and its equivalents in the main TIC body range between 93.5 ± 0.7 Ma and 92.8 ± 0.4 Ma (Coleman et al., 2004; Memeti et al., 2010a). This renders the sheeted complex and the margin of the KC lobe sensu stricto as a seldom preserved record of the first stages of TIC emplacement—a physical and petrologic record that is typically erased in many intrusions.
Size, Shape, and Physical Processes within the Kuna Crest Lobe and Satellite Units
The size and shapes of the compositionally distinct KC units drastically vary from thin, cm-scale, margin parallel sheets at the lobe margin to km-wide gradational zones arranged in a concentric pattern in the KC lobe. The satellite plutons vary in size from 0.2 km2 (Island Pass gabbro) to 8 km2 (Thousand Island Lake) and are more compositionally homogeneous. The sharp sheet contacts in the sheeted complex and the satellite plutons suggest that these were separate intrusive bodies of cm- to km-scale size (Fig. 2). The gradational zones with overprinting magmatic fabrics in the KC lobe (Fig. 2; Žák et al., 2007) suggest that these units behaved as a melt interconnected magmatic body with at least some magma mixing that occurred at the emplacement level. The interconnected magmas formed magma mush zones that included at least adjacent units, but potentially encompassed much of the lobe at some time.
Where contacts along plutonic-host units are sharp, stoping of external host rock into KC magmas (Fig. 7D) and internal erosion of older KC units into younger is abundant, preserving examples of magmatic erosion/recycling during incremental growth (Žák and Paterson, 2010; Paterson et al., 2016). Sharp and stepped contacts as well as stoped blocks are a good indicator that the host rock (either metamorphic or older plutonic material) was rheologically strong and showing brittle behavior because of large temperature differences between intruding and intruded magma and/or high strain rates during magma emplacement.
In contrast, magma mingling is apparent across the entire KC lobe through an abundance of dispersed quartz-diorite enclaves, enclave swarms, and now frozen, magmatically disintegrating, fine- to medium-grained quartz-diorite dikes and/or sheets. Locally, the host magma was clearly intruded by tabular sheets of quartz-diorite with straight sharp contacts and mingled at the emplacement level. In most other cases, it is unclear where magma mingling occurred to form individual enclaves and enclave swarms of especially mixed compositions (during ascent or at the emplacement level?; Figs. 7C and 7D).
Tens to hundreds of meter-scale, plagioclase-bearing cumulates enriched in hornblende-clinopyroxene-biotite-magnetite-ilmenite-apatite are found at the sheeted margins and in the interior of the KC lobe. The glomerocrysts of the same mineralogy in the KC lobe units I and II range from cm- and rarely dm-scale in size. They could have formed through the disintegration of such larger bodies. Alternatively, they may represent metastable pyroxene crystals that were transported from deeper levels of the magma plumbing and retrograded to hornblende. In either case, the mafic mineral and accessory accumulations suggest at least local crystal-liquid fractionation took place.
The Significance of the Compositional Variation in the Kuna Crest Lobe
Isotope Compositions of the KC Lobe
With the exception of six samples, the samples from the KC lobe and satellite bodies cluster at more primitive εNd and 87Sr/86Sri isotope compositions than KC rocks exposed in the main TIC with almost no overlap. The other six samples from the KC lobe are more evolved, resembling isotopic compositions of the Half Dome and even the Cathedral Peak units (Fig. 14). To explain these outliers, it is important to consider the location of these samples.
KCL-759 is located near the southern KC lobe margin (Fig. 2), an area locally contaminated with host-rock blocks. Local partial melting and assimilation of some of these host rock blocks would easily explain the shift to more crustal values for this sample. Samples KCL-202 and KCL-221 are both from the northwestern end of the KC lobe where the outer two KC lobe units are truncated by KC lobe unit III first and subsequently by the KC-HD hybrid unit at the transition to the main intrusive complex (Fig. 2). These samples plot either between Kuna Crest and Half Dome isotope values (KCL-202), or are similar to equigranular Half Dome compositions (KCL221). The hybrid to equigranular Half Dome isotope compositions are also found in two other KC samples at the transition to the equigranular Half Dome in the main TIC body: Sample DWTC-6.1 from the westernmost sheet of the Gaylor Ridge KC sheeted complex north of Hwy 120 at Tioga Pass, and sample DWTC-8 from the Kuna Crest Ridge west of Spillway Lake (samples are from north of the map area in Fig. 2). Due to the proximity of all four of these samples to the Half Dome unit, these samples likely acquired their final isotopic signature during interactions with the intruding Half Dome granodiorite magma. However, the textural and mineralogical characteristics of these samples are most similar to KC magmas, which is why they were mapped as KC units. We conclude from these observations that subtle influx of Half Dome magma caused the shift in isotope compositions at the transition of these KC units to Half Dome while still preserving some KC characteristics. This might have occurred through fine scale percolation of interstitial melt in an already fairly rigid KC magma mush (see also Cashman et al., 2017), but evolved interstitial melt would have quite low Sr and Nd (low Nd if they reached titanite saturation) concentrations, which would require a lot of infiltration to change the whole rock isotopic values. In contrast, sample KCL-536 from the transitional KC-HD unit underwent thorough magma mixing to form a true crystal-melt mixture between KC and HD magmas. It has mineralogical and textural characteristics of both KC and HD granodiorite rocks and was mapped as, and is interpreted to be, a true hybrid magma between the two (Fig. 14). This is documented, for example, in KCL-202, which shows petrographically and chemically at least two different plagioclase and two different K-feldspar populations (see below; Krause et al., 2010). Barnes et al. (2016) revealed at least two chemically distinct hornblende populations in the KC-HD unit that can be tracked back to both adjacent KC and HD units. In addition, the gradational contacts between the KC lobe and the KC-HD unit, and the KC-HD unit and the eHD also support these interpretations.
Isotopic Pattern of the Kuna Crest Lobe in Relation to the Main Tuolumne Body
The fact that 1–2 m.y. younger KC magmas from the main TIC body (Memeti et al., 2010a; Paterson et al., 2016) cluster at slightly more radiogenic isotope compositions compared to the majority of the KC lobe isotopes indicates that even the KC magmas became more radiogenic over time, following the overall trend of the younger Half Dome and Cathedral Peak units (Fig. 14). The isotopically fairly homogeneous, but more evolved compositions of the KC rocks in the main TIC could have been established in a continuously evolving magma source, or due to increased mixing of the KC magmas with crust-derived materials in the source (Kistler et al., 1986). The subsequent dramatic increase of the scatter in the isotopic values of Half Dome and Cathedral Peak magmas (Fig. 14) suggests that the relative isotopic homogeneity of the KC magma compositions was increasingly obscured due to recycling and mixing processes during ascent and/or at the emplacement level (Paterson et al., 2016) as well as a greater heterogeneity derived from the source at the peak of TIC magmatism (Kistler et al., 1986).
Remarkably, the 94.91 Ma to 92.75 Ma KC lobe reveals as much compositional variation as the entire 95–84.5 Ma TIC (Memeti et al., 2014) with rock types ranging from gabbro to leucogranite (quartz monzodiorites and granodiorites form the majority of the rock units) despite the relative isotopic homogeneity in the KC lobe (Fig. 12). Regardless of the units, the whole rock major oxide and trace element compositions are compatible with our petrographic observations: decrease in hornblende and relative increase of biotite toward higher silica content, the presence of clinopyroxene in hornblende cores of more mafic rock types and their absence in felsic compositions, the decrease of anorthite content in plagioclase and higher abundance of K-feldspar with increased whole rock SiO2, and the varied distribution of accessory minerals titanite, apatite, and Fe-Ti-oxides. The REE distribution with elevated LREE's compared to HREE's is attributed to plagioclase ± hornblende ± titanite fractionation (Frey et al., 1978) (Fig. 12). The scatter in Y and La (also Ce; Fig. 13) was likely caused by local variations of titanite in the rocks. Thus, although the isotopic compositions and overall mineralogy of KC lobe and associated units is similar, the large variation in modal mineral abundances from sample to sample, even within one unit, suggests fractional crystallization processes at local to more regional scales.
Geochemical Pattern in the Sheeted Complex Segments and Kuna Crest Lobe Units I–III
Samples from the Marie Lakes and the Gem Lake sheeted zones have mostly low silica (53–59 wt%) and high compatible oxides while MgO is relatively low (~2 wt%) compared to the interior KC lobe units (Fig. 12). In contrast, the compositions for KC lobe units I, II, and III show a large spread in SiO2 (54–63 wt%) composition, the other major oxides, and trace and rare earth elements, especially at low SiO2 content. This large geochemical scatter is attributed to the high degree of mineralogical heterogeneity of the KC rocks, which is also observed in thin section.
Geochemical scatter is naturally expected for the KC sheeted margins given rock types change across sharp sheet contacts at the meter to tens of meter scale. The broad distribution of major oxides and trace elements along and across trends (where trends are present) for samples from within each KC lobe unit I, II, and III, which grade into one another, is surprising, however, as each unit was mapped separately based on internally shared mineralogy, texture, color index, and grain size characteristics. Yet elemental geochemistry doesn't follow map-scale patterns and each unit is characterized by large spreads along and across elemental arrays. We attribute this large elemental heterogeneity in each unit to crystal-liquid separation yielding large variations especially in liquidus phases like plagioclase, but also hornblende, titanite, apatite, Fe-oxides, and glomerocrysts, all of which modified the final geochemical signatures.
Crystal Accumulations due to Interstitial Melt Loss in the Kuna Crest Lobe
Field, petrographic, and geochemical observations suggest that rocks in these plutonic units at least partially formed local and more regional crystal accumulations. Local hornblende-plagioclase-oxide cumulates (Fig. 7) are found across the KC lobe. Most plagioclase-rich KC rocks (quartz-diorites of KCSC and KC lobe units I and II) show enrichment in plagioclase and abundant mutual plagioclase-plagioclase contacts as is typical in cumulate textures (Fig. 5F). This implies that interstitial melt escaped from at least the earlier crystallizing lobe margins into the longer-lived interior of the lobe (Fig. 16), northwestward toward the main batholith body, southward to form satellite bodies and/or upward and away from the level of exposure, concurrently with the inward migration of the solidification front as supported by geochronology (Figs. 2 and 9, and 11). The migration of interstitial melts from a magma mush is also indicated by the preservation of abundant mm- to cm-thick leucogranitic segregations of granitic melts (Figs. 8A, 8B, and 8D) that are generally oriented parallel to the steeply dipping magmatic foliation and lithologic contacts. These are found in both KC lobe sensu stricto and the sheeted complex.
Where the leucogranitic segregations are at an angle to the magmatic foliation, they are typically folded tightly within the sheeted margins and KC lobe unit I, where local subsolidus deformation (subsolidus foliation and steep lineations; magmatic and subsolidus shears with inconsistent kinematics as is typical for flatting strain) is present. The extent of this area is shown with the white line on the geologic map (Fig. 2). The increase of flattening strain toward the lobe margins and in the sheeted complex, the concentric and gradual lithological changes following the lobe margin, parallel and steeply dipping magmatic foliations and steeply plunging lineations, tightly magmatically folded leucogranitic segregations and mafic dikes, suggest that the magma mush was additionally compacted against the crystallizing margin from within the lobe interior, most likely due to continued magma flow in a vertical magma transfer zone (Fig. 16). This process would cause filter pressing of an already fairly crystal-rich magma mush, aiding the escape of interstitial melt to mix into magmas passing through or crystallizing within the interior of the lobe. To determine how much melt was lost from these mushes, Barnes et al. (2019) used hornblende element compositions to calculate melt compositions needed for zircon saturation. They compared the results to bulk-rock compositions and concluded that up to 40%–50% melt was lost from the Kuna Crest rocks. This crystal-liquid fractionation in the lobe contributed to local and lobe-wide compositional variations. Gradually fractionating packages of KC magma mush that lost melt to and mixed with an intruding more evolved Half Dome granodiorite magma, such as parts of the KC lobe unit III and the KC-HD transitional unit (>60% wt% SiO2), is the preferred explanation for the hybrid nature of the central lobe unit.
The effectiveness of this filter-pressing process was likely dependent on the degree of crystallization of the lobe mush at different locations and thus its rheology during times of deformation as well as the intensity of deformation itself (Fig. 16). This interpretation agrees with Holness (2018), who suggested that interstitial rhyolite melt extraction through internal, buoyancy-driven processes (e.g., hindered settling, micro-settling, compaction) is rather inefficient and/or insignificant in silicic mush systems, thus emphasizing rejuvenation by magma replenishment, gas filter-pressing, or externally-imposed stress during regional deformation as more efficient processes. We speculate that the former two processes are more likely given the concentric compositional and structural patterns seen in the Kuna Crest lobe.
Although there is an overall gradual evolution of the KC magmas to higher SiO2 compositions, it is important to note that input of fairly mafic magmas did not cease after the formation of the KCSC and KC unit I. Quartz-diorite intrusions with irregular shapes continued to get emplaced in lesser volumes and mingle into KC lobe magma mush throughout its magmatic activity.
Lessons from Kuna Crest Lobe Minerals
Some minerals in the KC lobe units show clear evidence of multiple mineral populations. CL images of zircons, for example, reveal at least two different zircon populations in sheeted complex sample C49 and the KC-HD unit (KCL536) that are different in shape, size, and zoning pattern. The larger, prismatic zircons include more evolved (higher U) cores and interior zones and yielded slightly older ages (antecrysts) compared to the younger, smaller, stubby zircons (Fig. 10; Table 1). Textural and size differences identified in plagioclase petrographically (Fig. 5E) and with electron microprobe spot analyses and element maps on plagioclase from an earlier study on sample KCL-202 from the KC lobe-TIC transition (Krause et al., 2010; Oppenheim et al., 2021) also support the presence of at least two populations of plagioclase grown in different magmas juxtaposed through magma, and overgrown with rims from melt of a hybrid composition. The simplest explanation for more than one mineral population found in a thin section is to infer magma mixing during the emplacement of multiple magma increments. Looking at hornblende trace element analyses, Barnes et al. (2016) concluded that trace elements of hornblende from the different KC lobe units are unique suggesting the hornblendes from each unit grew from different magmas. The only exception is seen in the KC-HD hybrid unit where two (KC and HD) hornblende populations are found. The best explanation is that for most KC lobe units the hornblende crystallized relatively late and in equilibrium with rhyolitic and dacitic melts thus reflecting the composition of the final, post-mixing crystallizing melt, except for the KC-HD granodiorite unit, where hornblende crystallized before mixing took place (Barnes et al., 2016, 2019; Werts et al., 2020). This is consistent with the observation that there is no significant hornblende fractionation signature in the rocks.
An alternative model is that the hornblendes (and older plagioclase, zircons, etc.) grew from individual, larger magma pulses that composed the KC lobe units and were filter-pressed against the margin of the lobe. The interstitial melt migrated through adjacent mush areas and would have reacted with and modified already accumulated minerals in the mush. This caused the disequilibrium textures in the plagioclase and growth of albite rims over the old grains, or growth of new, euhedral, and Ab-rich plagioclase crystals. The frequently changing magma mush composition as new melts “soaked” the system might have locally allowed renewed zirconium saturation, and preserved the older antecrysts as inclusions in older, rock-forming minerals. Accumulations of minerals (or loss of rhyolitic melt) are also detected via melt compositions calculated from trace elements of hornblende (Werts et al., 2020) and plagioclase (Oppenheim et al., 2021), which, in the TIC, are more felsic in composition than the bulk rock composition of the rock the minerals came from. The latter model fits the open system behavior of the lobe mush as is supported by other observations discussed above (Fig. 16).
Compositions of the Satellite Plutons
The Thousand Island Lake pluton granodiorite to granite magmas are the most evolved of all KC lobe (sensu lato) at >67 wt% SiO2 and extend to leucogranites at the southeasternmost tip located just north of Garnet Lake. The Waugh Lake pluton data plot with KC lobe unit III, supporting the hypothesis that the Waugh Lake body was extruded from the KC lobe unit III resulting in a sharp contact with the outer lobe units I and II as magma extended toward the Waugh Lake body. This sharp contact also implies that the outer KC units I and II were fairly crystallized and rigid while the KC unit III was magmatically active. Crystal-liquid fractionation is mostly evident in the Thousand Island Lake pluton as suggested by the northwest, dominantly hornblende-biotite granodiorite body transitioning gradually into the leucogranitic and miarolitic cavity-bearing, southeastern sheet of the Thousand Island Lake pluton.
The Island Pass hornblende gabbro has cumulate texture and is low in silica and enriched in MgO, FeOT, and CaO, and was likely formed from the accumulation of hornblende and plagioclase from a more felsic magma.
Although the Rush Creek pluton is spatially associated with the Kuna Crest system, the contrasting isotope composition compared to the more homogeneous KC lobe sensu lato values precludes that it is part of the latter system. However, the oldest and isotopically most primitive magma in this area is represented by the Rush Creek diorite to granodiorite. It is relatively evolved at 61.3–69.7 wt% SiO2 content (Fig. 12; Table S3) and chemically not very diverse compared to the sheeted complex and the KC lobe units I–III. The major oxide and trace element variations are at least permissive of in situ fractional crystallization as the cause for its normal zonation, but a more detailed examination of the chemistry of this body, building on the study by Swanson and Kistler (1981), is needed to better understand its petrologic history.
Kuna Crest in the Lobe Versus in Interior TIC
The greater scatter in elemental data from the KC lobe sensu lato compared to KC data from the main TIC body (Figs. 12 and 13), the latter of which likely formed a much larger, interconnected magma chamber (Paterson et al., 2016), suggests that the magmas in the KC lobe differentiated at a smaller, more local scale and preserved the different stages of its complex magmatic evolution instead of being modified by younger magmatism as it was the case in the main TIC body (Paterson et al., 2016). Even though Bateman and Chappell (1979) and Bateman (1992), based on compositional and textural differences, gave distinct names (e.g., “tonalite of Glen Aulin,” “tonalite of Glacier Point,” and “granodiorite of Grayling Lake”) to KC-lobe-type magmas along the margins of the main TIC body, these workers also argued that these units belong to the same “Kuna Crest” intrusive event at the beginning of TIC emplacement and were just “torn apart” (Bateman, 1992) due to subsequent magmatism. Detailed geochemical analyses need to be done on these other KC units to fully examine their geochemical makeup and degree of heterogeneity compared to the KC lobe record, especially since they are slightly younger and isotopically more evolved than the KC lobe (Fig. 14). Given the chemical modification observed at the KC lobe-main TIC transition, it is possible that KC geochemistry in the main complex was modified during ascent and at the TIC emplacement levels and thus no longer represents original Kuna Crest type source magmas. This comparison makes it clear that the preservation of the less extensive but more variable history of the KC lobe is related to its early emplacement and peripheral location where the thermal gradient with the host rock was high.
Kuna Crest Lobe and Host Rock Structural Relationships
How was the Kuna Crest Lobe Sensu Lato Emplaced?
Our Al-in-hornblende pressure estimates indicate that the KC magmatic system intruded at ~2–2.5 kbar or 7.6–9.4 km depth (assuming a density of 2.7 g/cm3), agreeing with original estimates published for Sierra Nevada plutons by Ague and Brimhall (1988), pressure estimates from other parts of the Tuolumne (Memeti et al., 2009) and the compilation by Cao et al. (2016). Tobisch et al. (2000) and our own work document that subvertical beds, bedding-parallel cleavage, and the package-bounding faults in host rock around the KC lobe and satellite intrusions are regionally cut discordantly “cookie-cutter” style (Fig. 2). The host rock that once occupied the area of the KC lobe has been mostly removed from map view and only minor parts have been deflected around the lobe. Host rock removal by stoping is apparent with the highly stepped contacts of the Waugh Lake and Thousand Island Lake plutons at the tip of the KC lobe: Large amounts of rotated host-rock blocks are preserved in the Waugh Lake pluton especially. Except for local deflections at the two “corners” of the KC lobe, there is no evidence of lateral displacements of these units during pluton growth. Faults that would accommodate such lateral displacement are absent, and well-defined stratigraphic units are now entirely missing. Ductile deformation related to KC lobe emplacement is restricted to a several meters thick aureole around much of the lobe and up to 1 km on the southeast and southwest corners. This ductile deformation is apparent through the deflection of the host rock foliation from northwest striking regional orientations to northeast directions in the Gem Lake area, and to WNW orientations in the Marie Lakes area. Because the generally thin zone of ductile deformation and the subsolidus and magmatic shears with inconsistent kinematics at the margin of the lobe wrap around the lobe, we interpret this deformation to be related to the emplacement of the KC lobe, and not the Gem Lake shear zone located east of the area (Greene and Schweickert, 1995).
In contrast, the Waugh Lake and Thousand Island Lake plutons as well as the inner KC-HD units are dominated by regional magmatic fabrics with NW-SE to WNW-ESE striking and subvertical foliations and steep lineations (Fig. 2; Žák et al., 2007) indicating that these bodies show no record of lateral magmatic expansion or margin-parallel filter pressing.
Emplacement of the Sheeted Complexes
The sheets in the sheeted complexes intruded parallel to host rock anisotropy, typically steeply dipping bedding and bedding parallel cleavage, accomplished by magma wedging (Weinberg, 1999). The host rocks at the immediate contact with the magmatic sheets underwent synmagmatic ductile deformation resulting in horizontal shortening and vertical extension, evidence for magma wedging. This is recorded by a strong subsolidus foliation and sub-vertical lineation, and tightly folded magmatic veins and dikes as well as local shears with inconsistent kinematics (Fig. 4). Since the bedding and foliations in the sheeted complex are rotated away from the regional NW-SE striking fabric, they are interpreted to have been rotated during the emplacement of the KC lobe sensu stricto.
Emplacement of the Kuna Crest Lobe and Satellite Bodies
Tobisch et al. (2000) concluded that in the Ritter Range area “downward flow” of the host rocks occurred in plutonic aureoles during emplacement. We agree that either upward or downward movement of host rock occurred, although our mapping shows only local evidence that downward flow was localized in pluton aureoles. Instead, we think that in many cases downward flow during high-flux magmatism occurred as a regional event (Cao et al., 2016). Large sections of volcanic rocks that formed at or near the Earth's surface, including rocks of the Minarets caldera, which are just a few million years older than the lobe (Fiske and Tobisch, 1994), are now tilted, strained, and were intruded by plutons at ~7–9 km depths. Observations in the aureole around the KC lobe best fit an emplacement model of initial magma wedging of sheet-like bodies that evolved to amalgamated magma bodies formed during host rock downward flow (Weinberg, 1999; Miller and Paterson, 2001).
Once a mush-bearing KC lobe was formed, it is quite probable that downward flow of magma mush occurred within the lobe to accommodate intruding magma (Fig. 16). Above we also provided evidence for lost melts and magmas, which would have further helped accommodate newly arriving magma pulses.
It is interesting to note that both the Waugh Lake and Thousand Island Lake plutons intruded into host rocks just south of the tip of the lobe after KC lobe units I and II were at least mostly crystallized. We speculate that the lobe at that time was behaving as a strong body and formed a strain shadow, allowing the magmas to intrude into a low strain/stress site.
The emplacement of “dike-shaped” sheets of magma to form sheeted plutons that evolve to larger, irregular magmatic bodies has been described in several other studies (e.g., Pitcher and Berger, 1972; Hutton, 1992; McNulty et al., 1996). Miller and Paterson (2001) proposed a model of initial sheet-like mafic intrusions that successively evolved to larger, irregular shaped plutons in the Cascades core. They describe heterogeneous, more mafic, sheeted margins and tips of highly elongate plutons that were intruded by more homogeneous, tonalitic magma once a heated pathway was established. These magmas were emplaced at 20–25 km, much deeper levels than the KC lobe (7.6–9.4 km). Nonetheless, this model also applies to the observations in the KC lobe and associated bodies: initial magma wedging and lateral host rock shortening and vertical extension is followed by the amalgamation of multiple magma pulses into larger bodies that are then replaced by the intrusion of bigger, buoyant plutons whose emplacement is dominantly accommodated by downward flow.
Karlstrom et al. (2017) suggested that the type of intrusion that is formed is controlled by the variable responses of host rocks to a given magma supply. Cold conditions in host rocks favor elastic host rock behavior and dike and sill emplacement. In a pre-heated magma pathway, host rocks are warm and will increasingly behave viscously forming an intrusion that is mostly controlled by the volume of the magma input. Warm crust thus favors storage of magmas and allows for larger, more equant plutons to form that are governed by a “reverse energy cascade.” The evolution of the KC lobe magmatic system fits this prediction by Karlstrom et al. (2017): The KC lobe initially formed a sheeted complex as smaller, heterogeneous volumes intruded into colder (but not cold) host rocks, but as soon as magmatism picked up and a preheated pathway was established, the area of interconnected magma mush increased dramatically culminating in the formation of a ~50 km2 mush system in the lobe, followed by waxing and waning stages of magmatism in portions of the 1100 km2 TIC (Memeti et al., 2014; Paterson et al., 2016; Oppenheim et al., 2021).
Comparisons with other Emplacement Models
We suggest that our emplacement/internal growth studies of both the lobe and main complex have implications for emplacement/internal growth of the entire TIC and explore these implications in this section.
Earlier we outlined a number of previous TIC emplacement studies and drew attention to a series of hypotheses arising from one group of papers (e.g., Glazner et al., 2004; Coleman et al., 2012; Bartley et al., 2006, 2018), in which the authors conclude that the entire TIC formed from multiple dike-shaped pulses resulting in either a vertically sheeted complex or a folded-stacked-sill complex. Here, we evaluate these hypotheses and inferred models.
(A) U-Pb zircon geochronology and large magma chambers. In the Kuna Crest lobe, we have documented that early sheeting evolved within a few hundred thousand years, to larger, interacting crystal mush systems that represent active magma chambers. When U/Pb ages from our more extensive geochronologic study for the entire TIC are contoured (Memeti et al. 2014; Paterson et al., 2016; Ardill et al., 2020b), broad regions (>hundreds of km2) have similar ages and gradational internal contacts between distinct units, implying that it is quite permissible for these to represent magma chambers, a conclusion supported by thermal modeling (Yoshinobu et al., 1998; Paterson et al. 2011; Karakas et al., 2017).
(B) Evidence of magma mixing or movement of magmas. In the lobe, we have documented an evolution from chemically non-interacting sheets to extensive crystal (zircon, plagioclase, amphibole, see also Barnes et al., 2016, 2021) and melt (hybrid zones, new rim growth) mixing and crystal-melt fractionation (gradational zones and lost melts) in the interior. Furthermore, we presented evidence of (1) mafic sheets intruding into host magma, mingling and enclave dispersion, (2) formation of schlieren-defined structures by crystal sorting, (3) evidence of internal erosion and recycling of units (see also Žák and Paterson, 2005, 2010), and (4) formation of steep gradational contacts and spatially continuous magmatic fabrics (see also Žák et al., 2007), all of which require active movement in magma mushes. A number of other studies have documented similar evidence for magma interaction across the TIC providing ample evidence of active mush systems. Petrologic relationships indicate mixing of compositionally distinct magmas and their mineral cargo at the whole rock and mineral scales as shown at the transition of the KC lobe to the TIC and other parts of the intrusion (Kistler et al., 1986; Wallace and Bergantz, 2002; Chambers et al., 2020; Oppenheim et al., 2021), internal magmatic erosion and recycling (Bateman and Chappell, 1979; Žák and Paterson, 2005, 2010; Paterson et al., 2016), widespread formation of schlieren-defined magmatic structures and fabrics (Reid et al., 1993; Žák et al., 2007; Paterson, 2009; Ardill et al. 2020b), and enclave formation and dispersal (Barbarin, 2005; Barnes et al. 2021).
(C) Vertical dike complex or magmatically folded sill stack. A vertical sheeted dike model requires regional E-W extension (Bartley et al., 2018), which is incompatible with regional magmatic structures preserved consistently throughout the TIC (Žák et al., 2007; Ardill et al., 2020b) and Cretaceous dextral transpression host rock structures preserved across the entire Sierra Nevada batholith (see summary in Cao et al., 2015, 2016, and references therein). An E-W emplacement model also requires E-W striking shear zones at both N- and S-ends of the TIC and associated offset of stratigraphy and other host rock markers, none of which have been recognized at any scale of mapping (e.g., Huber et al., 1989; Memeti et al., 2014). The Coleman et al. (2012) stacked-sill-model and subsequent E-W folding of the stack might be generally compatible with the regional transpressive regime. However, in contrast to the well-preserved vertical sheets at the Kuna Crest margin, no sill or dike contacts (excluding late aplite and pegmatite dikes) are preserved beyond the Kuna Crest margins that suggest a dike or sill emplacement mechanism. Regionally, a folded sill model should result in steep dips and northerly strikes of deformed units, contacts and foliations at the TIC margins, shallow dips of units and structures at the TIC center, and a N-S striking axial planar foliation. Not only are these features absent, measurements of internal contacts and fabrics mostly contradict these predictions (Žák et al., 2007; Paterson et al., 2016). The leucogranite bodies described in Coleman et al. (2012) are local, discontinuous pods and elongate bodies of granites that are observed in all TIC units and in many places have gradational relationships with the host magma (Memeti et al., 2014). We interpret these to represent late melts that drained out of magma mush and migrated short distances. They are likely marking local solidification fronts in the magma mush.
(D) Magmatic or subsolidus TIC minerals? From the early marginal sheets to the interior of the lobe, the vast majority of minerals record igneous textures and compositions. Subsolidus alterations are minor and easily avoided in petrographic and geochemical analyses. TIC minerals examined by all other studies indicate largely magmatic characteristics and records (e.g., Vernon and Paterson, 2008; Moore and Sisson, 2008; Solgadi and Sawyer, 2008; Memeti et al., 2014; Barnes et al., 2016, 2021; Paterson et al., 2016; Chambers et al., 2020; Werts et al., 2020; Oppenheim et al., 2021).
In conclusion, we do not see any record of TIC-wide diking (beyond late aplitic and pegmatitic dikes) during regional extension, folded sills, or wide-spread recrystallization of magmatic minerals and annealing of internal contacts that might have played a major role in the emplacement and the evolution of the TIC. Instead, we argue for initial emplacement through magma wedging as preserved at the Kuna Crest sheeted margins and subsequent incremental growth of magma bodies during downward host rock displacement and transpressive tectonics, resulting in more integrated magma chambers capable of undergoing magma processes, such as mixing, crystal-melt fractionation, i.e., mineral accumulation and melt-loss (Barnes et al., 2019; Werts et al., 2020; Oppenheim et al., 2021), and development of magmatic structures (Ardill et al., 2020b). It is likely that magmas and melts in these magma chambers rose upward to feed both shallower plutons and volcanic eruptions.
SUMMARY AND CONCLUSIONS
The 94.91–92.75 Ma KC lobe preserved a frozen magma transfer conduit formed at the initiation of the 95–84.5 Ma TIC. While most of the 1100 km2 large TIC is composed of large, gradually changing granitoid units, the ~80 km2 KC lobe system preserved a wide range of compositions at local scales derived from a relatively isotopically homogeneous source. It is composed of a sheeted complex of mafic to intermediate compositions at the lobe margin, a lobe body composed of concentrically arranged quartz monzodiorites and granodiorites that had interconnected melt between 94.44 Ma and 92.75 Ma, and satellite plutons composed of dioritic to leucogranitic units. Initial construction occurred as low volume, subparallel magma sheets that were emplaced via magma wedging. The sheets have widths of meters to tens of meters and hundreds of meters length. Compositions are more variable (from gabbro to leucogranite) and overall are more mafic but as isotopically primitive as the main KC lobe. By 94.44 Ma, the KC lobe evolved to a slightly more felsic, more equant, lobe size body that was formed by more frequently intruding and coalescing magma increments or larger magma batches during a stage of increased volume of magmatism. The KC lobe body underwent crystal-liquid separation (up to 40% melt loss) which resulted in crystal accumulation due to large-scale filter pressing and lateral shortening against the lobe margin possibly driven by the continued injection and transfer of magma through the interior of the lobe. Interstitial melt migrating through magma mush continued to modify mineral compositions and allowed new ones to nucleate. Satellite plutons formed through the extrusion of magmas from the lobe interior into the strain shadow of the increasingly strong KC lobe as it crystallized toward the interior. Half Dome magmas, isotopically distinct from the KC and forming the next inner unit of the TIC, intruded into the still crystallizing KC lobe interior and hybridized with KC magmas, continuing the locus of magmatism to the northwest during the early stages of magma focusing around the TIC (Paterson et al., 2016; Ardill et al., 2018).
We conclude by suggesting that perhaps many intrusions start off as more heterogeneous mafic sheeted complexes before increased magma flux swamps the area to form larger, more homogeneous plutons that have erased the smaller initial stages of magmatism. The reason why plutons, such as the ones in the Cascades of western North America, the Main Donegal Granite in Ireland, and the KC lobe sheeted complex of the TIC, might have preserved their (initial) sheeted complex stage record is because magmatism ceased at these locations and/or shifted its geographic locus to a different location before subsequent magmatism could swamp the geologic record. We also speculate that these more mafic (even if low volume) magmas are a critical record of the preheating of pathways for more silicic magmas during magma focusing, allowing younger pulses to ascend to higher levels in the plutonic plumbing and to feed volcanic eruptions in continental arcs (Ardill et al., 2018).
National Science Foundation (NSF) funding was provided through grants Division of Earth Sciences EAR-1550935 and EAR-1624854 to Memeti; EAR-1624847, EAR-0739651, and EAR-0537892 to Paterson; and EAR-0537717 to Mundil. The Arizona LaserChron Center is supported by NSF-EAR-1649254. Memeti and Paterson also acknowledge funding from the U.S. Geological Survey EDMAP program, which funded detailed mapping of the Kuna Crest lobe. Memeti thanks the California State University, Fullerton, College of Natural Sciences and Mathematics for providing funding for open access publication. The Berkeley Geochronology Center and Arizona Laserchron and their staff provided their laboratory facilities to obtain U-Pb zircon ages and help with the interpretation of the raw data. The University of Southern California, Los Angeles, undergraduate team research group 2010, including mentors Drs. Lawford Anderson and Wenrong Cao, are thanked for their help with mapping of the Kuna Crest satellite bodies and host rocks of the greater Waugh Lake area. We also want to especially thank Drs. Cal Barnes, Kevin Werts, Katie Ardill, and Louis Oppenheim, along with numerous other colleagues, for very fruitful discussions around the formation and evolution of the Tuolumne intrusion. Memeti is grateful to field assistant Dr. Irma Vejelyte for help during the mapping of the KC lobe. Graduate student Dustin Williams provided two samples for isotope analysis from the Gaylor Ridge area. Yosemite National Park kindly supported this research with research permits and fee waivers for park entrance fees and camping. We want to thank Dr. Tom Sisson and an anonymous reviewer for their very detailed and thoughtful reviews and discussions about the geochemistry and general geology of the Tuolumne Intrusive Complex, Sierra Nevada, California, USA. Thank you also to Drs. Rob Strachan (Science Editor) and Fu-Yuan Wu (Associate Editor) for the efficient editorial handling of the manuscript.