Day 2: The Fine Gold Intrusive Suite— Records of the nascent Cretaceous arc
Published:January 01, 2014
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Jade Star Lackey, Callie L. Sendek, Jane L. Eisenberg, 2014. "Day 2: The Fine Gold Intrusive Suite— Records of the nascent Cretaceous arc", Formation of the Sierra Nevada Batholith, Vali Memeti, Scott R. Paterson, Keith D. Putirka
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Today’s trip introduces Field Forum participants to the earliest stages of magmatism in the Cretaceous section of the Sierran arc. Despite the paucity of fresh rock exposed in the western foothills, we have determined that the Fine Gold Intrusive Suite is a massive (>3100 km 2 ) and long-lived (ca. 19 million years, Lackey et al., 2012) intrusive complex, preserving a record of magmatism that provides considerable insight into the evolution of arc magma systems intruding across major crustal terrane boundaries. The primary units of the Fine Gold Intrusive Suite that will be viewed during the field excursion include the Bass Lake Tonalite, Ward Mountain Trondhjemite, and Knowles Granodiorite ( Fig. 2-1 ). The low-K2O, 87Sr/86Sr, high-Sr/Y, high- ?18O, and peraluminous composition of the Fine Gold Intrusive Suite rocks are geochemically distinct from the younger rocks of the central and eastern Sierra (Fig. 2-2 )…
Today’s trip introduces Field Forum participants to the earliest stages of magmatism in the Cretaceous section of the Sier- ran arc. Despite the paucity of fresh rock exposed in the western foothills, we have determined that the Fine Gold Intrusive Suite is a massive (>3100 km2) and long-lived (ca. 19 million years, Lackeyet al., 2012) intrusive complex, preserving a record of magmatism that provides considerable insight into the evolution of arc magma systems intruding across major crustal ter- rane boundaries. The primary units of the Fine Gold Intrusive Suite that will be viewed during the field excursion include the Bass Lake Tonalite, Ward Mountain Trondhjemite, and Knowles Granodiorite (Fig. 2-1). The low-K2O, 87Sr/86Sr, high-Sr/Y, high- δ180, and peraluminous composition of the Fine Gold Intrusive Suite rocks are geochemically distinct from the younger rocks of the central and eastern Sierra (Fig. 2-2); likewise, they have several mineralogical traits not found elsewhere in the Sierra. Despite these differences, many of the processes forming the Fine Gold Intrusive Suite, e.g., incremental emplacement and styles of magma mingling, mirror those that are common throughout the batholith. Moreover, the changes in the Fine Gold Intrusive Suite record important spatial and/or temporal shifts in magma composition relating to the establishment of the characteristic Sierran magmatic system.
This guide describes the major and minor plutonic units that we will see in the Fine Gold Intrusive Suite. The stops are not proportional to the magma volumes, or we would spend most of our trip in the Bass Lake Tonalite. The route follows a broad loop from Oakhurst to the southwest, across roof pendants in the suite that provide a sense of the wall rock encountered and its response to the emplacement of Fine Gold Intrusive Suite magmas (Fig. 2-1).
The Fine Gold Intrusive Suite lies at the transition between the Cretaceous Sierra Nevada proper and 140–115 Ma granitoid batholith rocks that have recently been recognized in accreted basement terranes beneath the San Joaquin Valley (Saleeby, 2007; Saleeby et al., 2010). Additional exposures of pre–Fine Gold Intrusive Suite Cretaceous intrusive rocks in the region include those as young as 153 Ma in the Guadalupe Intrusive Complex to the north (Putirka et al., this volume) and the 134 Ma Tonalite of Millerton Lake (Bateman, 1992). The Fine Gold Intrusive Suite was emplaced during orthogonal subduction of the Farallon plate beneath western North America (Engebretson et al., 1985). Seismic studies show one of the thickest sections of arc crust (>45 km) in the Sierra Nevada Batholith to be preserved beneath parts of the Fine Gold Intrusive Suite (Fliedner et al., 2000; Frassetto et al., 2011). Al-in-hornblende barometry estimates suggest that erosion has exposed the Fine Gold Intrusive Suite to depths between 10 and 20 km (Ague and Brimhall, 1988).
Original barometry determinations are restricted to the eastern Fine Gold Intrusive Suite and are not temperature corrected using plagioclase feldspar compositions (e.g., Anderson, 1996). Additional barometry, including metamorphic phase equilibria studies, is needed to determine paleodepths throughout the Fine Gold Intrusive Suite.
The Fine Gold Intrusive Suite truncates several continuous belts of metamorphic wall rocks on its northeastern margin (Fig. 2-1). The major formations within the southeast-striking metamorphic belt include Jura-Triassic arc terranes, and the Shoo Fly and Calaveras Complexes (Fig. 2-1). Continuous exposures of these rocks occur to the N-NW in areas around the Guadalupe Intrusive Complex (Putirka et al., this volume). These terranes comprise accreted arc lithologies that include ophiolitic mélange, slate, quartzite, argillite, and marble (Paterson et al., 1989; Snow and Scherer, 2006; Putirka et al., this volume). Western belt rocks are dominantly Upper Triassic to Lower Jurassic volcanogenic rocks deposited on Paleozoic ophiolitic mélange and mafic basement generated during abyssal magmatism during the Early Ordovician and Permo-Carboniferous (Saleeby, 2011). The Shoo Fly Complex, northeast of the Fine Gold Intrusive Suite (Fig. 2-1), is composed of Late Paleozoic quartzite, pelite, marble and calc-silicate rocks, and is intruded by scattered Devonian orthogneiss bodies (Schweickert et al., 1988). The Calaveras Complex is defined by a Permian to Upper Triassic subduction mélange chert-argillite sequence containing blocks of limestone, amphibolite, greenschist/stone, and basalt (Ernst et al., 2008). Sedimentary rocks comprising the Shoo Fly and Calaveras complexes show a preponderance of aged continental detritus based on both Sri values and detrital zircon populations (Grasse et al., 2001; Ernst et al., 2008). Preliminary detrital zircon studies of wall rocks in the immediate field area show grains as young as Jurassic in Calaveras Complex rocks on the east side of the Coarsegold septum (Paterson, 2012, written commun.).
The steeply dipping Melones fault zone (Paterson and Wainger, 1991) bounds the Jura-Triassic belt (Foothills Terrane) and Calaveras Complex as well as scattered roof pendants and septa in the Fine Gold Intrusive Suite (Fig. 2-1). Structural patterns show that this zone continued to the south before emplacement of the Fine Gold Intrusive Suite; however, the precise location of the projected trace of the fault zone remains unconstrained. Ongoing studies, including detrital zircon studies, may resolve the location of the Melones fault zone, especially toward the southern margin of the Fine Gold Intrusive Suite, where it abuts poorly exposed wall rocks of the Kings River Ophiolite and Kings Sequence (Saleeby, 2011).
Plutonic Rocks Of The Fine Gold Intrusive Suite
The ~3100 km2 Fine Gold Intrusive Suite was formally named for exposures of the Bass Lake Tonalite and Ward Mountain Trondhjemite along Fine Gold Creek. Despite incomplete exposure, U.S. Geological Survey (USGS) mapping (cf., Bateman, 1992) in the region showed large-scale patterns of foliation and trends in modal and compositional makeup that were used to define the extent of the Bass Lake Tonalite, the largest (~2700 km2) and most diverse unit of the Fine Gold Intrusive Suite. The various smaller intrusions of the suite, including the Knowles Granodiorite, Ward Mountain Trondhjemite, and stocks of the Granodiorite of Arch Rock (Fig. 2-1) are included within the suite according to their being intruded or crosscut by the Bass Lake Tonalite. Bateman (1992) noted that the group of rocks as a whole is remarkable for very low contents of potassium and alkali feldspar.
The Bass Lake Tonalite
Covering over 2700 km2, the Bass Lake Tonalite comprises 86% of the Fine Gold Intrusive Suite (Fig. 2-1). The Bass Lake Tonalite has abundant plagioclase feldspar, a variable (2–40) but high average color index (CI; 18), and is commonly deformed. Although the entirety of the unit is named tonalite, the Bass Lake Tonalite varies from diorite to granodiorite (50–73 wt% SiO2, Fig. 2-2). There is a distinct anti-correlation of biotite and hornblende modes, with euhedral biotite and hornblende regularly attaining large (1–3 cm) size (Figs. 2-3A and 2-3B). Honey-brown titanite is locally up to 1% of the mode. The Bass Lake Tonalite displays magma-mingling textures that are similar to those described elsewhere in the Sierra (Barbarin, 1991; Bateman, 1992). It commonly contains mafic enclaves, which were used as the basis for mapping foliations within the Tonalite (Fig. 2-3C). Continuity of foliations over many kilometers led Bateman (1992) to designate the Bass Lake Tonalite as a single unit, although breaks in foliation were proposed to indicate possible internal contacts between magma bodies. The large area of the Bass Lake Tonalite was clearly paradoxical. Bateman and colleagues employed isotopic and magnetic susceptibility methods to try to recognize cryptic internal contacts in the Bass Lake Tonalite (Bateman et al., 1991). They also sought to understand how emplacement of the Ward Mountain Trondhjemite magma, hypothesized to be a mushy, semi-solid magma, caused apparent deformation in older parts of the Bass Lake Tonalite (Bateman et al., 1983). Although sparingly cited in the literature, both studies show some of the earliest deliberations on the complexities of pluton assembly in the Sierra.
Ward Mountain Trondhjemite
This is the oldest unit of the suite (Lackey et al., 2012), forming two subcircular domes outcropping over 185 km2 (Fig. 2-1). The trondhjemite is distinctively gneissic (Fig. 2-3D), has a white color (CI average = 7), and a fine- to medium-grained equigranular texture. It typically contains <15% (average 10%) biotite, and sparse (<5%) alkali feldspar, occurring as “stringers” in the gneiss and in subequant grains where more abundant. A few percent muscovite and trace amounts of garnet (Lackey et al., 2006) reflect its peraluminous (A/CNK = 1–1.1) bulk composition (Fig. 2-2J). The outcrop at Stop 3 has fibrolite sillimanite after muscovite in shear bands. Near the edges of the domes, detailed mapping shows thin screens of metamorphic wall rock separating the trondhjemite from the Bass Lake Tonalite (Bateman and Busacca, 1982). Elsewhere at the margin of the trondhjemite, xenoliths of metamorphic wall rock are concordant with foliation and variably disaggregated to form schlieren. Xenocrystic zircon cores dated at ca. 130–140 Ma are also common (Lackey et al., 2012), confirming field evidence for a relatively “dirty” margin of the Ward Mountain Trondhjemite.
Granodiorite of Arch Rock
These small stocks are commonly associated with pendants in the Fine Gold Intrusive Suite, and are generally fine- to coarse-grained biotite-granite to granodiorite, although hornblende-tonalite to trondhjemite compositions occur locally. They cover 121 km2 as a whole. Most rocks are light in color (CI’s of 4–12, e.g., Hensley Lake granite in Fig. 2-3E). Bateman (1992) noted that their variations suggest they may not all be from the same intrusive event and hypothesized that some were shallowly intruded, based on the occurrence of miarolitic cavities. The Arch Rock type locality is at the west entrance of Yosemite National Park. The granodiorite east of Hensley Lake that we will see on the field trip is peraluminous (Fig. 2-2J) and contains magmatic muscovite, conspicuous deep red garnets, and local pegmatitic domains with tourmaline. The Hensley Lake outcrop also contains magmatic cordierite that has altered to pinnite, and is the only occurrence of magmatic cordierite known in the Sierra.
The single pluton of Knowles Granodiorite is 124 km2. The rock is typically a light-gray, medium-grained, equigranular, muscovite-bearing biotite granodiorite (Fig. 2-3F) with a color index of 10 (5–15, Bateman 1992). The lack of foliation and equigranular nature of the granodiorite has led to extensive quarrying for use as a building stone in California, especially in San Francisco after the 1906 earthquake. It is widely used as a memorial stone, including as the base of John Muir’s headstone in Martinez, California (Black “Granite” from the Academy Intrusion at the southwestern margin of the Fine Gold Intrusive Suite is used for Muir’s headstone). The granodiorite continues to be quarried in Raymond and environs, being marketed globally by the Cold Springs Granite Company as “Sierra White.” The Knowles Granodiorite exhibits the same distinct peraluminous character as the other small-volume plutons in the Fine Gold Intrusive Suite, with similar Harker trends (Fig. 2-2).
Bateman (1992) adopted a “best” average age of 114 Ma for the Bass Lake Tonalite based on patterns of discordance from 12 individual bulk U-Pb zircon age determinations that ranged from 105 to 124 Ma. Stern et al. (1981) suggested that younger ages (108 and 105 Ma) recorded in the Bass Lake Tonalite east of the Coarsegold septum (Fig. 2-1) were from a younger zone of the Bass Lake Tonalite, which they called the Oakhurst pluton. Additionally, the Ward Mountain Trondhjemite and Knowles Granodiorite were dated at 115 and 112 Ma, respectively, and one of the stocks of the Granodiorite of Arch Rock yielded a U-Pb age of 116 Ma (Bateman, 1992).
New laser ablation–multicollector–inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) ages presented by Lackey et al. (2012) show a range of ages (121 to 105 Ma) for the Bass Lake Tonalite (Fig. 2-4), from which they define three domains that show a broad pattern of eastward younging (Fig. 2-4C), although older ages on the eastern margin of the Fine Gold Intrusive Suite suggest some inward younging of the suite as well. The ages indicate an incremental emplacement style similar to the Tuolumne Intrusive Complex (Coleman et al., 2004; Memeti et al., 2010; Memeti et al., this volume) and John Muir Intrusive Suite (Davis et al., 2012). Besides new ages in the Bass Lake Tonalite, the revised age of 124 Ma for the Ward Mountain Trondhjemite suggests that it was deformed and domed upward by younger members of the Fine Gold Intrusive Suite. In addition, a revised age of ca. 114 Ma for the Knowles Granodiorite is in accord with field relations that it crosscuts the Western Domain of the Bass Lake Tonalite.
In a given sample from the Fine Gold Intrusive Suite, the spectrum of ages from populations of zircon usually shows a well-defined plateau of magmatic ages, with two to five xeno- crysts (e.g., Fig. 2-4B). In the Bass Lake Tonalite, these older xenocrysts appear to match the age of preceding magmatic pulses (i.e., Central Domain rocks contain Western Domain xenocrysts). Some samples contain a suite of inherited cores that range from 130 to 145 Ma and are mantled by magmatic overgrowths. The xenocrystic cores are intriguing in that they match the ages of plutonic rocks from Saleeby et al.’s (2010) Great Valley Batholith but correspond to a prominent lull in magmatism in the Sierran arc shown in dated plutonic rocks (Irwin and Wooden, 2001) as well as detrital zircons (Paterson, 2012, written commun.). The survival and overgrowth of cores in these two intrusions are not surprising given their peraluminous composition, which promotes only partial resorption of inherited zircon from melting crustal sources (e.g., Williams, 1992; Miller et al., 2003). Sier- ran zircons rarely retain inherited cores (Fu et al., 2008) given that most batholith magmas are chemically and thermally poised to resorb inherited zircon (Miller et al., 2007).
To summarize age relations (Fig. 2-4), intrusion of the Ward Mountain Trondhjemite was followed closely by the emplacement of isolated stocks of granites and trondhjemites assigned to the Granodiorite of Arch Rock; additional dating of these stocks is needed to fully describe the tempo and mode of this smallvolume style of magmatism. The emplacement of the Western Domain Bass Lake Tonalite is synchronous with some of these stocks, as well as the Academy Intrusive Complex and other mafic ring-dike complexes to the south (Clemens-Knott and Saleeby, 1999), but it is typically the case that early intrusion of smallvolume tonalitic to granite stocks were followed by an engulfing intrusion of the Bass Lake Tonalite. The youngest (ca. 105 Ma) belt of Bass Lake Tonalite rocks indicates an eastward and potentially inward migration of magmatism in the Suite.
With the addition of our investigations to those of Truschel (1996) and Bateman (1992), many samples of the Fine Gold Intrusive Suite have been analyzed for whole-rock geochemistry, despite challenges of discontinuous outcrops, and the extensive alteration of rocks in the foothills. Lackey et al. (2012) contoured these data in the Fine Gold Intrusive Suite and noted that in general, indicators of magmatic source change from west to east and correspond to age domains. They generally show that older Western Domain magmas have lower Rb and K2O, an indication of the incorporation of greater proportions of altered arc rocks instead of aged North American crust. Setting aside the effects of magma mixing and fractional crystallization on Harker trends, the Bass Lake Tonalite and other Fine Gold Intrusive Suite units show general overlap of some major elements (FeO*, Mg#, CaO, Na2O; Fig. 2-2A-2-2D) with those of the younger, Middle and Late Cretaceous Sierran Arc. However, the Fine Gold Intrusive Suite has distinctly lower K2O, Na2O + K2O, somewhat elevated Al2O3, and low Rb at a given SiO2 (Figs. 2-2E-2-2H). Also, a fair number of the Bass Lake Tonalite samples, and nearly all of the Ward Mountain and Knowles samples are peraluminous (Fig. 2-2J). An overall deduction from whole-rock trends is that the same processes of differentiation and crystallization are likely at play throughout the emplacement of the Cretaceous, but the more primitive arc sources of the Fine Gold Intrusive Suite are distinct. For instance, Sr/Y versus Rb/Sr values show that the Ward Mountain Trondhjemite is a high-Sr/Y (values approaching 100) granitoid rock (Fig. 2I). Comparison of Sr/Y in such rocks shows that the high ratios are linked to low Y (typically <10 ppm; Lackey et al., 2012) rather than high Sr. The conventional wisdom of California arc studies is that this is the signature of melts contributed from garnet-bearing ocean crust residues but not necessarily melting of subducting slab (e.g., Tulloch and Kimbrough, 2003). Garnet-bearing residues are recognized from xenolith suites in the Sierra, but stabilization of garnet may occur during expulsion of these rocks beneath the arc after melting (e.g., Ducea, 2002; Saleeby et al., 2003), explaining why most Sierran granitoid rocks are not high Sr/Y (Fig. 2-2I). The implication of the high-Sr/Y ratios is that the Ward Mountain Trond- hjemite is derived from a deeper source on average than later Fine Gold Intrusive Suite magmas.
The isotopic studies of the 1970s–1980s are summarized by Bateman (1992), who chronicles early Sr isotope investigations (Kistler and Peterman, 1973; Kistler, 1990; Bateman et al., 1991) of the location of the boundary between accreted arc terranes and North American continental crust (i.e., the initial 87Sr/86Sr (Sri) = 0.706 isopleth). Nd and O isotope analyses of the original Kistler and Peterman samples and some new samples were analyzed by DePaolo (1981) and Masi et al. (1981). These included analyses of wall-rock composites from the Jura-Triassic arc rocks (e.g., Mariposa Formation) and the Calaveras Complex. The unpublished Master’s thesis of Truschel (1996) employed a battery of isotopes (O, Sr, Nd, and Pb) to consider the Fine Gold Intrusive Suite as a whole, and more recently, modern O isotope analyses of zircon, other minerals, and whole rocks have been undertaken in detail (Lackey et al., 2006, 2008, 2012). Lackey et al. (2012) report 26 Hf isotope analyses on zircons from the Fine Gold Intrusive Suite as well as the Dinkey Creek Granodiorite (2), Academy intrusion (1), and Shuteye Peak Granite (1).
In general, Sr values are low (0.704) in western parts of the Fine Gold Intrusive Suite, and increase eastward, only reaching Sri >0.706 in two of the easternmost samples. Nd and Pb isotope ratios correlate with Sri indicating a mixture of Proterozoic continental crust and low-Sri (0.704), high-εNd (+8) reservoirs interpreted to be mixtures of mantle melts and young (Paleozoic-Mesozoic) sedimentary rocks. Western volcaniclastic rocks have lower Sr values as well (0.703–0.706; Kistler and Peterman, 1973; DePaolo, 1981); those in the Calaveras Complex are higher (0.707–0.715), reflecting derivation from sediments that contain greater proportions of North American crustal detritus. All formations have high whole-rock δ18Ό (10-23%e; Boehlke and Kistler, 1986; Bateman et al., 1991), with the highest values coming from chert-argillite mélange protoliths (e.g., Lackey et al., 2006). The unusually high δ (whole rock >10%e) of the Fine Gold Intrusive Suite (Masi et al., 1981; Lackey et al., 2008), but correspondingly low Sri values led Lackey et al. (2008) to propose that the Fine Gold Intrusive Suite is a mixture of mantle-derived magmas and Paleozoic to Mesozoic ocean crust or equivalent age volcanogenic sedimentary rocks that were altered to high δ18O before melting in the Cretaceous. Negative correlation of Sri and δ18Ό in the Bass Lake Tonalite east of the Melones fault was attributed to greater proportions of Calaveras Complex rock in magma sources or as a contaminant.
New Hf isotope data have changed our understanding of the Fine Gold Intrusive Suite. These data show that rocks in the Eastern Domain defined by Lackey et al. (2012) show substantially lower values (Fig. 2-5), and a distinctly different O-Hf array compared to the Western Domain, which projects toward values inferred for North American crust or metasedimentary wall rocks derived from such crust. Because the older magmas in this domain have higher εHf(t), it can be deduced that the onset of a vigorous magmatic system that was capable of greater crustal reworking occurred in the third stage of Fine Gold Intrusive Suite magmatism. Despite this indication of greater crustal mobilization, the Fine Gold Intrusive Suite still only has two samples known to have Sri values >0.706, with most being <0.7055. Thus, the Sri = 0.706 line essentially does not cut through the Fine Gold Intrusive Suite; it is more likely that the final stage of magmatism in the Fine Gold Intrusive Suite led to higher Sri values within the younger suites to the east (e.g., the Shaver and Yosemite Intrusive Complexes). In effect the 0.706 line is the eastern margin of the Fine Gold Intrusive Suite, coinciding with the Calaveras–Shoo Fly break.
Unlike Sri, εHf(t) shows a step at the projected location of the Melones fault zone (Fig. 2-5). This step is manifested as a gradual eastward decrease of εHf(t) in the majority of samples west of the Melones fault zone. Older (>113 Ma) samples of Bass Lake Tonalite east of the fault zone define their own trend but are offset downwards by ~2 epsilon units (Fig. 2-5). Younger samples of the Eastern Domain parallel the trend of the older samples, but they are offset still lower by an additional ~2 epsilon units. The overall break in trends of Hf isotopes suggests a fundamental change in source composition that is recorded in the oldest Bass Lake Tonalite rocks. Coincidence of the Hf step with projected location of the Melones fault zone at the Coarsegold septum is intriguing, suggesting a terrane break; however, it may not reflect the influence of the wall-rock configurations, which are not likely to be trans-crustal features. Because Sri isotopes are decoupled from Hf and don’t match the step, Hf isotopes may simply show greater input of aged rocks to sources east of the step, potentially documenting a break in lower mafic crust or lithospheric mantle given that it is decoupled from Sri isotopes, which trace the location of the “sialic” crust (e.g., Kistler and Peterman, 1973). The present coverage by Nd and Sri isotopes is not sufficient to compare with Hf in order to test the hypotheses that Hf traces a feature of the lower crust or mantle, or that high Sri values are linked to tapping of upper crustal sources.
Those rocks that contain significant xenocrystic components of ca. 130–140 Ma zircons are excellent targets for investigations of magmatism in the pre-Fine Gold Intrusive Suite portion of the arc. For instance, new single-zircon O and Hf isotope analyses (Lackey et al., 2013) show that the Ward Mountain Trondhjemite preserves a large range of isotopic variation (δ18 5-10%e; εHf = +5.5-13) recording the progressive re-working of juvenile arc rocks into granitic crust in a span of 10 million years. Because the δ18Ό values of some cores are up to 10%e, these rocks also show that some intermediate-stage (130-140) magmas were the products of melting of mantle-derived rocks that had been hydrothermally altered at Earth’s surface. As discussed by Jeon et al. (2012) for the New England batholith of Australia, this implies a rapid “magma to mud to magma” transformation. In the case of the Ward Mountain Trondhjemite, convergence of zircon rim domains to an average δ18O value of ~7.2%c indicates mixing of magmatic sources in the 124 Ma magmatic episode.
Within the Fine Gold Intrusive Suite, in particular the Bass Lake Tonalite, dark, fine-grained mafic inclusions (e.g., Figs. 2-3C and 2-6A) are common and are regularly distributed across the pluton in the form of dikes, enclaves, and schlieren. The even distribution of the inclusions was argued by Bateman (1992) as evidence that they were mingled into the host magma at early stages. The rounded to double-convex discoid shape of the inclusions and the presence of schlieren (dark, streaky layers visible near the margins of intrusions) indicate that the intrusions were magmatic when they joined the host magma. This is further supported by isotopic and elemental data that show similar compositions between the host rock and the inclusions, including changes from place to place in the pluton (Bateman, 1992). Mafic enclaves often show evidence of reaction with the former tonalite magma (Figs. 2-6B and 2-6C) on the basis of diffuse boundaries that contain concentrations of large (1–3-cm-long) euhedral hornblende crystals (Fig. 2-6C).
These observations suggest fluid exchange during mingling of the magmas, causing hornblende coarsening. Because the geochemical composition of the inclusions has been largely overprinted by that of the host rock, it is difficult to determine an origin; however, disequilibrium textures such as sodic rims on plagioclase grains within enclaves show disequilibrium (thermal and chemical) between the enclaves and hosts (e.g., Barbarin, 1991). Trough and scour structures like those found in younger parts of the Sierra Nevada Batholith (e.g., Žák et al., 2007; Paterson, 2009) are occasionally observed. We will see one of the mafic dikes that are widely distributed within the Bass Lake Tonalite.
Summary: Comparison With Younger Cretaceous Magmatism
The Fine Gold Intrusive Suite yields an extensive history of the beginning of the Cretaceous Sierran Arc. Similarities to central and eastern Sierran intrusive suites include the following: (1) magma volumes and intrusive rates; (2) a complex construction history like that seen in the Tuolumne Intrusive Complex (Memeti et al., this volume); and (3) outcrop and micro-textures. These similarities indicate a consistent set of governing processes of emplacement, crystallization, and deformation in the middle to upper crust of the arc. Among the main differences are: (1) mineralogical diversity with an extreme range of compositions from gabbro to peraluminous granite; (2) greater heterogeneity of isotopes reflecting diverse sources as well as the influence of terrane breaks; and (3) older, small-volume plutons and stocks surrounded by later, large-volume plutons. Few instances of intermingled older and younger plutons occur in the eastern Sierra, where coherent intrusive complexes appear to reflect the well-established magma production of the eastern Sierra. These differences in magmatic emplacement records certainly beg the question of how tectonic configuration, like the change from orthogonal to oblique convergence of the subducting plate midway through construction of the arc (e.g., Glazner, 1991; Tobisch et al., 1993, 1995; Tikoff and de Saint Blanquat, 1997), controls how early-stage magmas are preserved or eroded by later magmas, and how the pre-arc batholithic crust is reworked or shuffled by processes such as retroarc thrusting (e.g., Ducea and Barton, 2007; DeCelles et al., 2009).
As we visit outcrops, the major units, important field relations, and supplementary data collected on the rocks at particular stops will be discussed. Limited exposure and private property restrictions limit us mostly to roadside exposures; however, there is access to some of the freshest rock of the Fine Gold Intrusive Suite. The trip departs from Oakhurst and heads southwest, first climbing through a mixture of California live oaks and associated chaparral scrublands, then descending into oak savannah landscapes, and ultimately to open grasslands at the lowest elevations. The geology has profound effects on the types of vegetation in the region, as you will see when we cross into the western metamorphic belt rocks. East of Oakhurst lie densely forested, middle-slope areas that have long been logged for their stands of pine and fir. Distances are logged in miles (approximate values given), and UTM coordinates of stops are projected according to the WGS84 ellipsoid.
00.0 (11S 0264664 4134954): Junction Highways 41 and 49 in Oakhurst. The trip starts in the 108–105 Ma Oakhurst Pluton of Bateman (1992), the youngest domain within the Fine Gold Intrusive Suite. Drive west on Highway 41 toward Madera. As the road begins to climb the large ridge west of town, weathered Bass Lake Tonalite outcrops along the road.
Stop 1: Bass Lake Tonalite of the Oakhurst Pluton
00.8 (11S 0263970 4133636): Pull out along the shoulder on the north side of the road and proceed uphill in small groups to the first outcrops as the road begins a broad turn to the south. Here are typical outcrops of the Oakhurst pluton. Pay attention to the size and abundance of hornblende and biotite as well as any fabrics or structures in the tonalite. It was in these outcrops and those farther up the grade that Bateman et al. (1991) conducted their detailed 20-sample traverse of O, Sr, and magnetic susceptibility in the western margin of the Oakhurst pluton. The question driving their detailed work was: How does one recognize internal contacts in a pluton when textural cues or limited outcrop don’t provide straightforward answers, but geochronology indicates a break in age? In essence, this was one of the first locations where Sierran geologists recognized incremental emplacement of a pluton. Here, the Bass Lake Tonalite Sri is 0.70560 and δ18 (whole rock) is 9.6%e (Bateman et al., 1991).
Return to vehicle and continue west on Highway 41.
01.4: Look east past the Oakhurst sign toward the Sierra. The large ridge on the skyline ~15 km away is Shuteye Ridge and marks the eastern limit of the Fine Gold Intrusive Suite. The ridge is composed of the Granite of Shuteye Peak (102 Ma in Bateman  but as old as 114 Ma [Lackey et al., 2012]) as well as the Granodiorite of Whisky Ridge (103 Ma; Bateman, 1992). From this same location, the suite is exposed 30 km to the westsouthwest, out to the edge of the Great Valley, and similar distances to the south-southeast and north-northwest as we saw on Day 1 of the Field Forum (Fig. 2-1). Granite domes exposed on the ridge show some of the classic exfoliation weathering that is more common in the higher elevations of the Sierra. Bass Lake is concealed from view but lies at the base of Shuteye Ridge along this view. As you’ll note from the Oakhurst sign, the town celebrated its centennial in 2012, being founded previously as Fresno Flats. Timber, such as that seen in the Shuteye Ridge area and to the east, was and remains a major commodity of the area, and sales of supplies to gold miners served as a major economic stimulus in the midto late 1800s (Fresno Flats Historical Museum, www.fresnofl atsmuseum.org/, accessed August 21, 2012).
01.5: Outcrops (to 3.8) of the Oakhurst pluton up the grade and down into flats.
03.9: First outcrop of the Coarsegold septum on south side of Highway 41.
04.2: Turnout on north side of road shows good exposures of rusty-red biotite phyllite of the Coarsegold septum (Fig. 2-1). Detrital zircon grains as young as Lower Jurassic (Paterson, 2012, written commun.) are found in this outcrop, suggesting that the rocks are on the western edge of the Calaveras Complex.
04.9: Pale-green, andesitic (~51% SiO2) metavolcanic rocks designated as part of the Bullion Mountain Formation (Bateman, 1992); contact metamorphosed to hornblende-hornfels facies.
05.5: Entering town of Coarsegold, founded in 1899, but first inhabited in 1850 by Texas miners who found gold in the local creek. In late October, the town has a Tarantula Festival to celebrate the mating season of the many eight-legged inhabitants of the area.
07.0: Coarsegold center and historic village. The town lies on a solitary intrusion mapped as Bass Lake Tonalite, which intrudes the Coarsegold septum in this area (Fig. 2-1). One of the strongly reduced samples of Ishihara and Sasaki (1989) was collected in the small pluton here and cited as evidence of the strong effect of wallrock composition on magma fO2.
07.3: Cross back into Bass Lake Tonalite (to 08.1), which continues along Highway 41 in outcrop until contact (covered) with Ward Mountain Trondhjemite. Throughout this stretch of road, the landscape is typically oak savannah with scattered outcrops of the Fine Gold Intrusive Suite.
08.1: Ward Mountain Trondhjemite. This medium-grained, equigranular trondhjemite (seen at Stop 2) is foliated and forms low outcrops along Highway 41 throughout this interval (to 14.5). Some road cuts permit inspection by a handful of individuals, but large groups should avoid these outcrops.
13.5: The Ward Mountain Trondhjemite here dated at ca. 124 Ma by Lackey et al. (2012).
14.6: Bass Lake Tonalite outcrops along Highway 41.
16.1: Enter southern body of Ward Mountain Trondhjemite.
16.8: Country Road 200. Turn left onto 200 and proceed SE 4.3miles.
Stop 2 (Optional): Mafic Metavolcanic Septum
21.1 (11S 0262385 4114246): Park on south side of road and carefully cross road to large road cut. This relatively fresh outcrop is part of an unnamed pendant and shows a number of features typical of a serpentine-rich foothills belt pendant. The west end of the outcrop is mostly serpentinite. Note the near-vertical dip in the rhythmically bedded sandstones in the middle of the outcrop, although folding and random distribution of blocks of different lithologies on the east side of the outcrop show the chaotic style of deformation in these kinds of rocks. Subvertical veins of anthophyllite crosscut the east side of the outcrop.
Return to vehicle and continue east on Road 200.
Stop 3: Ward Mountain Trondhjemite
21.2 (11S 0263450 4114999): Park on south side of Road 200. Proceed across road to outcrops to examine mineralogy and foliation in the Ward Mountain Trondhjemite. This exposure is at the SE flank of the dome that was recently traversed. Fine Gold Creek, the suite’s namesake, can be crossed a couple miles down the road. The evidence of solid-state deformation in the trondhjemite was previously thought to be a result of syndeformational doming of the body that creates a concentric foliation that dips away from the center of the intrusion (Fig. 2-1). The revised age of 124 Ma allows a scenario wherein apparent doming may be related to deformation caused by later intrusions. Development of fibrolitic sillimanite after muscovite (Fig. 2-7) in discrete shear bands within this outcrop suggests deformation is subsolidus and accompanied by reheating.
Turn around and retrace route to intersection of Road 200 and Highway 41.
27.2: Intersection of Road 200 and Highway 41. Turn left on 41. Outcrops of Ward Mountain Trondhjemite continue. Watch for the sign for the San Joaquin Experimental Range on the south side of the road; it is an indicator that the turn for County Road 406 is coming up (~0.35 miles).
29.4: Poorly marked turn onto Road 406; take a right and head N, descending; the road becomes unpaved. Ranches in the area raise mixed livestock, and the fencing of private land is pervasive. Sparse outcrops in this area are the southern Knowles Granodiorite pluton, which is locally poorly exposed.
Stop 4: Knowles Granodiorite
32.0 (11S 0251258 4109255): A broad, sloping outcrop of Knowles Granodiorite in the southern Knowles pluton. The even-grained texture of this granodiorite has made it valued as a building stone. The conspicuous lack of foliation compared to other intrusions was used as evidence that this is the youngest intrusion in the Western Domain of the Bass Lake Tonalite. New dating confirms this since both the Knowles and Bass Lake Tonalite ages in the area are older than expected (Fig. 2-4). Like the Ward Mountain Trondhjemite, the Knowles Granodiorite contains xenocrystic zircon cores of 130–140 m.y. (Lackey et al., 2012). Thus, the pluton may have tapped the same source as the Trondhjemite, albeit ~10 million years later.
Return to vehicle and continue NW on Road 406.
32.9: Junction with Road 209 (paved); note abandoned quarry in Knowles Granodiorite.
35.2: All exposures of granitoids disappear, indicating entry into biotite-schists (Fig. 2-1) thought to be Jurassic based on correlations with foothills belt rocks (Bateman et al., 1982). These wall rocks continue ~6 miles to the south as the Adobe Hill pendant, and vary from metavolcanic to metapsammite (Bateman et al., 1982). Note the transition from oak savannah to open grasslands. Granitic dikes weather in relief.
36.5: A cottonwood grove indicates groundwater close to the surface. Quartz veins are abundant in the area, and inspection of areal photographs shows that they have high density in the schists near the contact with the Bass Lake Tonalite and disappear ~700 m into the schists.
38.0: Junction with Road 400. Take a right and head north.
Stop 5 (Optional): Magma–Wall-Rock Interactions
38.8 (11S 0243944 4109130): The road cuts through a low but resistant outcrop here. There are a few things to notice in the cut on the north side of the road. Two different former magmas are evident: (1) a finegrained equigranular biotite, garnet-bearing leucogranite. Chemically it is high-Na2O (6-7 wt%) SiO2 (72-75 wt%) trondhjemite. (2) A very fine-grained granitic aplite that crosscuts the trondhjemite is higher SiO2 (~77 wt%) but lower Na2O (4-5 wt%). Comparison of these two rock types to other Fine Gold Intrusive Suite rocks shows that the older one (1) has affinities with the Hensley Lake melts to the NE, perhaps being a differentiated aplitic melt. The younger (2) is one of the unassigned intrusives in the area that is strongly lineated adjacent to wall rocks and may be considerably older than some of the rocks we are seeing today. Obviously the outcrop has domains of biotite schist, which coarsens near contacts with the granitic domains. Within granite, you’ll find examples of various xenoliths of the biotite-schist wall rock, some displaying sharp contacts with the host leucogranite, while others are wispier, showing evidence of partial melting into the magma. Sulfides in the granite are locally concentrated suggesting they are xenocrystic (e.g., Clarke et al., 2009).
Return to vehicle and continue NE on Road 400.
39.5: Road 603 intersection. Return to this intersection after the next stop.
40.2: Begin outcrops of granodiorite of Hensley Lake.
Stop 6: Garnet-Bearing Granodiorite of Hensley Lake
40.5 (11S 245160 4110909): The garnet-bearing biotite granodiorite of Hensley Lake is included within the granodiorites of Arch Rock. The pull-out is on the edge of the Hensley Lake Recreation area that was built by the Army Corps of Engineers, who used part of this intrusion for construction of the recreation area. Boulders in the parking lot can be examined, but no hammers, please; across the road is plenty of outcrop, but don’t cross the fence; it’s private land. The granodiorite has megacrystic (to 3 cm) K-feldspar here, and contains deep-red garnet phenocrysts. Muscovite is more abundant in aplitic and pegmatitic domains, and tourmaline is also found therein. A U-Pb zircon age is pending at the time of this report; however, Bateman et al. (1982) mapped a zone of deformation at the margin of the Hensley Lake intrusion suggesting it was deformed by both the Knowles Grano- diorite and the Bass Lake Tonalite, and therefore predates both. Analyses of δ18Ό of garnet and zircon in the Hensley Lake body showed both minerals to have δ18ϋ values of ~8.0%e (Lackey et al., 2006). The high values are indicative of supracrustal residence of the source, in keeping with its peraluminous character (A/CNK ~1.1; Fig. 2-2J). If you are interested, outcrops continue ~0.5 mile farther; at the turnoff to the Vista Point in the Army Corps recreation area, the Bass Lake Tonalite is seen to be intruding metamorphic wall rocks.
Return to vehicle and re-trace route back to intersection with Road 603.
41.5: Turn right onto Route 603 and drive north; we’ll mostly be in Bass Lake Tonalite along the way, passing the main entrance into the Hensley Lake Recreation area.
44.9: Intersection with Route 600. Turn right and head NE.
45.0: Good view back toward Sierra and Shuteye Ridge, if conditions are good. Passing through road cuts and knobs of Western Metamorphic belt schists.
46.9: Back into Bass Lake Tonalite (Fig. 2-1).
50.9: Intersection with Road 606 (Knowles Road); stay straight on 600, but note that following the Knowles Road for 2.6 miles takes you to the Raymond Quarry in the Knowles Granodiorite. The quarry is operational, and one can see some of the workings from the road.
51.9: Raymond town center. Raymond was founded in 1856; it was originally called Wildcat Station because of an instance when local wildcats were consumed during a shortage of meat. It was on the Southern Pacific rail line, and at one point served as a gateway into Yosemite. In 1903, President Theodore Roosevelt and John Muir disembarked from the train here after traveling from San Francisco. They boarded a stagecoach to Yosemite, where they would discuss adding protections to Yosemite that would eventually lead to its designation as a National Park (Raymond Museum [www. southyosemitemuseums.org/rm/], 2012).
52.7: Leaving town you see younger Knowles Granodiorite crosscutting the Bass Lake Tonalite in weathered outcrops on north side of the road.
Stop 7: Bass Lake Tonalite Hornblende Logjam: Flowing and Growing
53.3 (11S 0242907 4124322): Park on the north side of the road and carefully cross over to the road cut. Toward the east end of the outcrop is a steeply dipping, finegrained, mafic (SiO2 = 56 wt%) dike. The dike’s bulk chemistry compares well with that of other mafic dikes in the tonalite (Barbarin, 1991; Bateman, 1992), and in fact only minor and trace element ratios (e.g., Ti/ Nb, Sc, Fig. 2-8) distinguish the dike from a low-silica equivalent of the tonalite. Such similarity is in contrast to the Dinkey Creek Granodiorite in the Shaver Intrusive Suite, which is intruded by high-K2O dikes that appear unrelated to the main Dinkey Creek Granodiorite magma (e.g., Dorais et al., 1990), but clearly the dike is a late-stage feature. Note that the geochemistry of the dike is distinct from that of mafic enclaves in the Bass Lake Tonalite, which we will discuss at the next stop. The relationship of the enclaves and mafic dikes is uncertain. The spectacular hornblende logjam adjacent to the dike suggests comagmatic exchange between the two domains, but the hornblende “logjam” is offset ~25 cm from the margin of the dike. The coarse-grained hornblende and tonalite suggest growth in the presence of an aqueous fluid in addition to melt. Could this feature also be coarsening related to thermal cycling (e.g., Higgins, 1999)? Our interpretation of the texture is that fluid exchange between the dike and host magma caused Mg and Fe to diffuse out of the dike and into the surrounding magma where hornblende crystallized. Within the dike, xenocrysts(?) show evidence of disequilibrium suggesting that the dike has sampled parts of the tonalite during transport and emplacement. Examples of this same dike and hornblende feature are found elsewhere in the Bass Lake Tonalite (Fig. 2-6), some show more extensive reaction and dispersal of dike materials into the tonalite magma (e.g., Fig. 2-6), and they show magma mingling features that are similar to those described in other intrusive complexes that we will visit (e.g., Miller et al., this volume; Memeti et al., this volume).
53.6: Ahwahnee turnoff to Road 415. Turn right and proceed 0.5 mile.
Stop 8 (Optional): Bass Lake Tonalite
54.1 (11S 0242907 4124322): Parking on the right and the left (small turnout). Here are a couple of small outcrops of the Bass Lake Tonalite on the east side of the road going down Road 415. These show nice alignment of subhedral hornblende and biotite phenocrysts in the foliation, and the outcrop has a high CI (28). Compared to the previous outcrop of the Bass Lake Tonalite, is this the same? Are we seeing a lithologic change suggesting an internal contact? No geochronology has been done on this and the previous outcrop, but it is planned.
Turn around and retrace your route down Road 415 to Road 600.
54.6: Intersection with Road 600, turn right.
56.7: Route climbs through a gabbro in Bass Lake Tonalite.
57.3: Climb out of the valley formed by Willow Creek, and cross onto a highland ridge. Chowchilla River valley is visible to the north. Road descends and then climbs again.
62.3: Road 600 crosses into the northern “tongue” of the strongly deformed Ward Mountain Trondhjemite (11S 0252605 4132803). The trondhjemite is coarser grained and shows a strongly developed foliation (to 62.9). Pullout across from a ranch entrance (#38620) can accommodate a couple of cars to make an impromptu stop.
63.4: Prospects on north side of road in metavolcanic rock of the Coarsegold septum (Fig. 2-1). Metavolcanic rocks dipping moderately east continue to 63.9.
64.8: Gabbro body in Bass Lake Tonalite.
Stop 9: Bass Lake Tonalite in Coarsegold Septum
65.3 (11S 0254660 4135695): A pullout on the north side of road is just a bit past the outcrop that contains mafic (CI of ~30) Bass Lake Tonalite. Closer inspection shows that the tonalite in the outcrop is filled with mafic enclaves that are subvertically elongated (10:1 aspect ratios) parallel to foliation in metamorphic wall rocks on the east side of the outcrop. The contact of the Bass Lake Tonal- ite and wall rocks unfortunately is covered. The tonalite is more mafic (SiO2 = 58.0-62.6 wt%; MgO = 3.5-3.7) than average tonalite but lies on major element trends (e.g., Fig. 2-8). The enclaves (SiO2 = 51.0-53.6 wt%; MgO = 4.8-5.2 wt%) lie roughly within a field defined by enclaves in the Bass Lake Tonalite (Barbarin, 1991). The pronounced stretching here suggests magmatic flow parallel to the contact with pendant rocks, and in general, this northwestern area of the Coarsegold septum is intruded and partially engulfed by the tonalite (Fig. 2-1). Examination of the phenocrysts in the mafic enclaves shows albitic overgrowths of plagioclase within the mafic enclaves, presumably a rim reequilibration reaction with a cooler host magma.
66.2: Calc-silicate domain within Coarsegold septum.
66.4: Gabbro body in Bass Lake Tonalite.
67.5: Outcrop of mafic metavolcanics, with near vertical bedding and boudinage structures. Metamorphic grade is pyroxene hornfels facies.
Stop 10 (Time Permitting): East Contact with Coarsegold Septum
67.7 (11S 0256840 4137312): Pull into the broad turnout on the south side of Road 600. Partial outcrop on the north side may be the contact, albeit concealed, of the Oakhurst pluton of the Bass Lake Tonalite against the Coarsegold pendant. Foliation in the wall rocks is again steeply dipping to the east. Examination of the Bass Lake Tonalite shows a low CI (~15), and few enclaves compared to the previous stop. Note the historical marker for the Poison Switch section house of the Sugar Pine lumber flume and the Enterprise gold mine. The Sugar Pine lumber flume was an engineering marvel. This v-shaped wooden flume was used to float rough lumber from Sugar Pine (~10 miles northeast of Oakhurst), down to finishing mills in Madera (~50 miles in all). Loggers also used the flume to get down to Madera on improvised “boats” (E Clampus Vitus, 2012; www.eclampusvitus.net/DBarnesFlume. html). If there is time, walk down to the outcrops in the Fresno River. Here, hard calc-silicate rocks in the pendant form excellent kettle and flute structures in the riverbed. Stringers of pink and green in the otherwise black rock are garnet + diopside. The prominent change of river course shows that it follows the contact of the Bass Lake Tonalite and Coarsegold septum for a fair distance before finally cutting across the septum.
Return to vehicle and continue east on Road 600.
69.1: Intersection with Highway 49; this is Ahwahnee. Muir and Roosevelt stayed here en route to Yosemite in 1903. Turn right and head south.
73.3: Intersection of Routes 49 and 41.
End Day 2 Log.
We thank E. Klemetti and R. Turnbull for reviews of this chapter, and K. Putirka for editorial handling. Much of the geochemical and geochronology data presented in the field guide and during the forum trip were made possible with help from past and present students in the Pomona Petrology Laboratory, including C. Windham, G. Romero, R. Frazer, G. Ruiz, A. Zilberfarb, and N. Raschick. Technicians L. Finley-Blasey and J. Harris maintain the Pomona X-ray fluorescence laboratory, which is partly supported by National Science Foundation grant DUE-CCLI 0942447 to J.S. Lackey. Research in the Fine Gold Intrusive Suite is supported by EAR-0948706 to J.S. Lackey and J. Miller.
Figures & Tables
Formation of the Sierra Nevada Batholith
This comprehensive field guide takes you on a six-day, west-to-east geologic journey across the Mesozoic magmatic arc of the central Sierra Nevada in California. It contains a summary of field, structural, geochemical, and geochronologic data collected by a number of researchers on individual intrusions (Guadalupe Igneous Complex and nearby Hornitos pluton, Fine Gold Intrusive Suite, Yosemite Valley Intrusive Suite, Tuolumne Intrusive Complex). This guide also includes data on the basement terranes intruded by these intrusions, Mesozoic volcanic-sedimentary sections, and data derived of several Sierra Nevada-wide data sets (plutonic, volcanic and sedimentary geochronology, strain analyses, structures, and geochemistry). Syntheses of these data sets at the end of the guide focus on magmatic processes from the mineral to the arc scale, as well as contemporaneous tectonics, and the tempos controlling magmatism, deformation, exhumation, and erosion/redeposition in the Sierran arc.
- absolute age
- alkaline earth metals
- Central California
- field trips
- geologic barometry
- isotope ratios
- metamorphic rocks
- road log
- stable isotopes
- United States
- whole rock
- zircon group
- Bass Lake Tonalite
- Fine Gold Intrusive Suite
- Knowles Granodiorite
- Ward Mountain Trondhjemite