The Jackass Lakes pluton (JLP), located in the central Sierra Nevada batholith, is a 98 Ma composite intrusion that preserves field, structural, and petrologic evidence of how incrementally emplaced plutons grow and evolve both spatially and temporally. In contrast to many other Sierra Nevada batholith intrusions, the compositional and textural diversity found within the JLP allows individual increments to be easily discerned in the field. Previous work has resulted in two different incremental emplacement models for the JLP. The first states that the JLP was emplaced and assembled via vertical diking or sheeting, some downward return flow along the margins of the pluton, and local stoping. The second involves replenishment by mafic sheets that may represent originally subhorizontal floors of an evolving magma chamber. We present new data and a model suggesting that the JLP (1) contains multiple, irregularly shaped intrusions of both felsic and mafic material that do not represent dikes or paleofloors; (2) magma increments were extensively mingled, both between and within intrusions; (3) records evidence of magma mixing locally and possibly at the intrusion scale; (4) has not been tilted; (5) has magmatic mineral fabrics that record superimposed regional strain, not emplacement-related strain; (6) preserves large metavolcanic pendants representing subhorizontal roof contacts; and (7) was emplaced by ductile deformation of its host rocks, return flow, and widespread stoping of older host rock and its internal increments. This model, based on field, structural, strain, thermobarometric, and petrologic analyses, elucidates that the JLP construction is considerably more complex spatially and temporally than previous models suggest, and highlights processes involved during incremental emplacement of plutons.
Plutons are integral parts of most orogenic belts and represent the plumbing system by which mantle melts are fractionated and crust is formed. Thus, understanding their physical and chemical evolution is a critical step in understanding the evolution of orogens and the growth of continents. Large magmatic arcs like those that make up the North American Cordillera are characterized by long and complicated deformational histories punctuated by short periods of voluminous magmatism (Ducea, 2001; Coleman and Glazner, 1997). The emplacement of large volumes of magma in a short period of time, such as occurred in the Sierra Nevada batholith, serves to transfer enormous amounts of mass and heat within the crust, and influences the tectonic evolution of the entire orogenic belt.
Recently two hypotheses regarding pluton construction have received a great deal of attention. Both models raise issues about the types of processes that occur during pluton construction (e.g., diking, diapirism, magma mixing, mingling, stoping), rates of pluton construction, sizes and durations of magma chambers, evolution of magma source regions, magma–host rock material transfer processes and the interpretation of internal magmatic structures. The first involves construction of magma chambers and/or plutons via multiple pulses of magma, rather than large (pluton sized), single pulses (Buddington, 1959; Cobbing and Pitcher, 1972; Pitcher, 1978; Kistler et al., 1986; Frost and Mahood, 1987; Coleman et al., 1995; McNulty et al., 2000; Glazner et al., 2004). Although this concept has existed since the mid-twentieth century, recent controversies have arisen regarding the compositions, textures, geometries, sizes, numbers, and ages of increments that make up these systems (e.g., Paterson and Vernon, 1995; McNulty et al., 1996; Petford, 1996; Johnson et al., 2003; Coleman et al., 2004). The second model, championed by Wiebe and colleagues (Wiebe, 1974, 1993, 1994; Chapman and Rhodes, 1992; Wiebe and Collins, 1998), has been motivated by studies of subhorizontal mafic and felsic layering in plutons in Maine. Although this model also involves multiple pulses of mafic magma into a felsic chamber, Wiebe and Collins (1998) suggested that layers form subhorizontally, and thus many plutons represent large cumulate piles with stratigraphic information preserved by the different layers. This hypothesis has recently received much attention and its criteria have been used to evaluate other composite intrusions in other orogens, even where layering is not subhorizontal (e.g., Brown and McClelland, 2000; McNulty et al., 2000; Wiebe, 1999, 2000; Cruden and McCaffrey, 2001; Miller and Miller, 2002).
The Jackass Lakes pluton (JLP), located in the central Sierra Nevada (Figs. 1 and 2), is an excellent locality to examine models of incremental pluton growth because (1) it has previously been described as a composite intrusion (McNulty et al., 1996; Peck, 1980); (2) the two aforementioned models for incremental construction have been proposed for this pluton (McNulty et al., 1996; Wiebe, 1999, 2000); and (3) it is very well exposed with good vertical relief (>1200 m), allowing for detailed examination of the compositions, textures, geometries, sizes, numbers, and ages of increments, the interactions within and between increments, and pluton–host rock relationships. Most important, increments of magma emplaced into the JLP are easily distinguished in the field, on the basis of compositional and textural diversity, whereas many Sierran plutons and intrusive suites do not show such dramatic variability.
In the following are descriptions of the two previous models proposed for JLP construction, with new data: field observations of the compositions, textures, geometries, sizes, numbers, and relative ages of JLP increments, an evaluation of internal structural elements, initial studies of host rock structures, preliminary petrographic analyses, and thermobarometric data.
We discuss how these data relate to previous models and propose a new model that states that the JLP (1) contains multiple irregularly shaped, both felsic and mafic intrusions that formed a magma chamber(s) and that do not represent paleofloors or dikes; (2) preserves magma increments that were extensively mingled, both between and within intrusions; (3) records evidence of local magma mixing; (4) has not been tilted during or after emplacement; (5) has magmatic mineral fabrics that mainly record superimposed regional strain, not emplacement-related strain; (6) preserves large metavolcanic pendants representing subhorizontal roof contacts; and (7) was emplaced by ductile deformation, return flow (Tobisch et al., 2000; McNulty et al., 1996), and by widespread stoping of both the older host rock and earlier intrusions. We discuss the general implications of our hybrid model for the JLP and other magmatic systems.
In an effort to be perfectly clear regarding the description and interpretation presented below, we offer a few definitions, which have often been interpreted in a variety of ways. We do not suggest that the entire community accepts the definitions, merely that these are how the terms are used in this paper.
Magma herein is a mixture of melt, volatiles, and crystals; in regard to crystallinity, magmas as defined in this study may have high crystal content (50%–99%) and rarely involve melt-dominated mixtures (e.g., >70% melt).
Magma chamber is the region of a pluton that contains interconnected melt; there is no requirement for a magma chamber to be the same size as the final pluton and multiple magma chambers are possible within one pluton.
Emplacement is the act of magma intrusion and coupled host rock displacement during ascent and local enlargement (emplacement) of magmatic systems.
Magmatic stoping is the process of completely disconnecting and surrounding a piece of host rock (xenolith) by magma, resulting in movement of the block; stoping may occur along magma chamber roofs, sides, and floors at any crustal level via processes such as diking and fracturing.
Stoped block is a host rock xenolith, disconnected and surrounded by magma that has subsequently been rotated and/or translated in the magma; stoped blocks may be displaced downward (sinking) or upward (floating), depending on the density contrast between block and magma, and may commonly show evidence of disaggregation and sometimes melting.
Raft is a host rock xenolith that is surrounded in two dimensions by plutonic material where no discernable rotation and/or translation (relative to in situ host rock) of that xenolith can be demonstrated; however, formation of multiple rafts in one location requires stoping between rafts and/or translation of previously formed rafts.
Pendant is a large (>500 m) exposure of host rock completely surrounded in two dimensions by plutonic material that may or may not be in place. Roof pendants occur above plutonic material and may still be attached to host rock.
Dike is an intrusion formed by transport of magma via a propagating crack (purely elastic) driven by buoyancy and/or magma pressure with a length/width ratio >100.
Sheet is an intrusion with a length/width ratio between 10 and 100 formed by processes that include elements of dike-like behavior as well as ductile deformation (e.g., Miller and Paterson, 2001).
Diapirs and diapirism refer to viscoelastic diapirs (not Hot-Stokes diapirs), bodies consisting of one or more batches of magma rising together, with length/width ratios <10, surrounded by host rock deforming by brittle and ductile processes, and for which ascent is driven by buoyancy plus regional stress (see Miller and Paterson, 1999); this often leads to intrusions with irregular geometries and contacts.
Magmatic fabric refers to foliations and lineations formed during magma crystallization, with sufficient melt present to permit rotation of the framework grains without accumulation of crystal-plastic strain (Paterson et al., 1998).
GENERAL GEOLOGIC SETTING
In map view the Cretaceous JLP (Figs. 1 and 2; note that the full-size version of the Fig. 2 map is available as Plate 1 [see footnote 1]) is an ∼13 × ∼17 km rectangular body separated into four lobes by slightly older metavolcanic and plutonic pendants (Peck, 1980). The U-Pb zircon geochronology provides a ca. 98 Ma age (Stern et al., 1981; McNulty et al., 1996). The pluton is crosscut to the south by the ca. 90 Ma Mount Givens pluton (McNulty et al., 2000), and to the north by the 95 ± 2 Ma Red Devil Lake pluton (Tobisch et al., 1995), the Turner Lake pluton, and the ca. 88 Ma megacrystic facies of the Half Dome granodiorite, which is part of the Tuolumne batholith (Fleck and Kistler, 1994; Gray, 2003). The JLP intrudes the 99 ± 1 Ma Illilouette Creek pluton to the west (Tobisch et al., 1995) and Early Cretaceous metavolcanic rocks of the Ritter Range pendant to the east (Fiske and Tobisch, 1994; McNulty et al., 1996). The eastern and western margins of the pluton strike north-south, and it is likely that the JLP extended further to the north and south prior to intrusion of the Tuolumne batholith and Mount Givens granodiorite (McNulty et al., 1996). A previous barometric study using the Al-in-hornblende barometer yielded an emplacement pressure estimate of 4.5 kbar for the JLP (Ague and Brimhall, 1988), a pressure that is revised to a lower value in this study.
The JLP is a composite intrusion made up of numerous internal, often sheet-like intrusions (McNulty et al., 1996). Several intrusions of diorite and/or quartz diorite form meter- to hundreds of meters–scale bodies throughout the JLP (Peck, 1980). Associated with sheet-like mafic intrusions are numerous enclave swarms in more felsic units. Enclave swarms typically strike northwest-southeast and dip steeply.
Abundant xenoliths of host rock are found throughout the JLP and vary in size from kilometer-scale pendants to centimeter scale, and are generally elongate parallel to the mineral and enclave foliation in the pluton (McNulty et al., 1996; Peck, 1980). The largest pendants are metavolcanic with lithologies similar to those in the Minarets Caldera sequence exposed in the Ritter Range pendant to the east (Figs. 1 and 2; McNulty et al., 1996). Some pendants also consist of a porphyritic leucogranite with miarolitic cavities, called the Post Peak porphyry, which could be a slightly earlier, subvolcanic phase of the JLP (Peck, 1980). In its type locality the Post Peak porphyry contains abundant mafic enclaves of variable size, geometry, and composition, making it a distinctive unit. Smaller xenoliths are particularly common near the pendants, but occur throughout the pluton.
The JLP was interpreted by Peck (1980) to be a resurgent magmatic body that intruded approximately coeval volcanic and subvolcanic plutonic rocks of the ca. 98–101 Ma Minarets Caldera sequence (Stern et al., 1981; Fiske and Tobisch, 1978, 1994). In fact, it has been suggested that the JLP volcanic system represents an unusual example where good exposures of both intrusive and extrusive rocks of the same magmatic system are preserved and can be studied in some detail (e.g., Peck, 1980; Lipman, 1984).
PREVIOUS WORK ON THE JLP
There are currently two models proposed for the emplacement and temporal evolution of the JLP. The first (McNulty et al., 1996) states that the JLP was assembled via vertical diking or sheeting, is not tilted, and that pluton growth was accommodated by some downward return flow along the margins of the pluton, and local stoping. The second (Wiebe, 1999, 2000) states that the present subvertical mafic sheets represent multiple, formerly gently dipping floors formed in a magma chamber that are inferred to be rotated to their present steep dips. We describe these two models and their application to the JLP.
Dike Construction of JLP
McNulty et al. (1996) conducted a study of the distribution of petrologic phases and structures in the JLP wall-rock system. They concluded that the JLP represents a shallow-level magma chamber formed by injection of many sheet-like magma pulses. The linear eastern and western contacts of the pluton (Fig. 1), a predominance of subvertical, north-striking petrologic units including dioritic and felsic dikes within the pluton and wall rocks, meter- to kilometer-scale pluton–wall-rock interfingering, north-south–striking enclave swarms and magmatic foliation within the pluton, and solid-state foliation in the wall rocks led McNulty et al. (1996) to propose a dike assembly model. They suggested that dike assembly is supported by an east to west fluctuation in enclave density and in magmatic fabric intensity. Furthermore, extensive mafic and felsic magma mingling and back intrusion between units along with geochronologic data suggest rapid construction.
In the McNulty et al. (1996) model the pluton formed essentially in its present orientation, with sheet-like bodies emplaced via dike injection along fractures. Volcanic bodies (septa) in the pluton were displaced downward along dike walls. They noted that internal contacts are not obvious in the JLP, and suggested that dike contacts are cryptic due to rapid magma injection rates and the subsequent coalescence of dikes to form a magma chamber. Magmatic fabrics were interpreted to reflect flow in the dikes and microgranitoid enclave swarms were interpreted as disaggregated synplutonic dikes. Magnetic lineations determined using the anisotropy of magnetic susceptibility technique were interpreted to record flow directions in dikes (subhorizontal lineations) or the location of feeder conduits (steep lineations).
Displacement of host rock during emplacement involved multiple mechanisms of host rock removal (McNulty et al., 1996). Fracture propagation and dilation were estimated to account for ∼40% of the required space, and this dilation may have been associated with strain partitioning in the arc during oblique convergence. Preliminary strain analyses (six samples) of wall rock led McNulty et al. (1996) to estimate that an additional ∼25% of the space required for emplacement was produced by ductile wall-rock shortening and return flow of elongate wall-rock septa. The remaining 35% of space is accounted for by formation of the overlying Minarets Caldera and associated roof collapse, and/or stoping in the subvolcanic magma chamber.
Stratified Chamber Construction and Tilting
In the following model, construction of a stratified, composite pluton requires an existing felsic magma chamber in a relatively crystal poor state that undergoes crystal fractionation, resulting in the accumulation of minerals forming a partially solidified, aggrading floor at the base of the chamber (Wiebe, 1974, 1993; Wiebe and Collins, 1998). Mafic magmas enter these chambers via dikes and spread laterally at the interface between the partially solidified floor and the overlying crystal-poor chamber (Wiebe, 1993; Wiebe and Collins, 1998). Subsequent mafic intrusions record the position of the subhorizontal chamber floor through time (Wiebe, 1993; Wiebe and Collins, 1998). The general observations that lead to the development of this model include depositional structures preserved in mafic layers, cumulate textures observed in felsic plutonic material, and geochemical and petrologic evidence of crystal fractionation (Wiebe and Collins, 1998).
Wiebe (1999, 2000) proposed that the JLP was constructed by the scenario just described. The observed subvertical, northwest-southeast–trending mafic layers within the JLP were interpreted to accumulate initially along subhorizontal surfaces within a dominantly silicic chamber. The layers cooled quickly and mixed with the magmas, forming layering, hybrid magmas, and enclaves. Multiple mafic intrusions thus represent stratigraphic markers within the JLP. According to Wiebe (1999, 2000), younging criteria indicate deposition on 10°–30° dipping surfaces, with eastward younging of the pluton toward the Ritter Range and a western base of the pluton at the contact with the Illilouette Creek pluton (Fig. 1). This model for the JLP requires tilting of the initially subhorizontal layers to their present steep dips. Tilting can be accommodated by a synemplacement to postemplacement >60° rotation about a subhorizontal, northwest-southeast axis to achieve the sequence of top to the east younging or by an asymmetric, synmagmatic, floor downdropping mechanism (Cruden, 1998; Wiebe and Collins, 1998).
It is important to note that after additional work in the JLP, R.A. Wiebe (2002, 2005, personal communs.) agrees that the simple scenario of stratified construction outlined above may not be appropriate for the JLP, but does suggest that information regarding younging particularly from the Madera Creek area is still present. However, we include this model for completeness.
DESCRIPTION OF THE JACKASS LAKES PLUTON
General Field Observations
The many increments found in the JLP vary in composition, texture, geometry, size, number, and relative age. Distinguishing individual intrusions is often straightforward when modal composition and/or texture differ; however, some internal contacts are cryptic and difficult to follow. Contact relations range from sharp to gradational, sharp contacts being the most common (e.g., Fig. 3; note that a full size version of Fig. 3 map is available as Plate 2 [see footnote 2]). Contacts are typically steep throughout the pluton. Sharp contacts are irregular at the outcrop through map scale (Fig. 4). Local examples of mutually crosscutting relationships between adjacent increments are observed. Gradational contacts, where observed, often show a smooth transition between units on the meter to tens of meter scale. Sometimes gradational contacts have distinct hybrid magmas separating the two end-member units (Fig. 5). Crosscutting relationships, inclusions, and mingling structures reveal a complex temporal evolution of magma pulsing throughout the pluton (Fig. 3).
The dominant composition in the JLP is a medium- to coarse-grained, equigranular, biotite-hornblende granodiorite with abundant dispersed mafic microgranitoid enclaves (Fig. 4B), although compositions vary from granite (sensu stricto) to diorite. Compositional and textural variability exists even within the dominant granodiorite both within and between intrusions (Fig. 3). Diorite and quartz diorite intrusions are scattered throughout the pluton and also show variability in composition and texture (Fig. 2). Mafic intrusions are usually not in contact with each other, but are isolated by more felsic units. Hybrid units are often associated with the mafic units and macroscopically have compositions and textures that are between typical JLP granodiorite and the diorite and/or quartz diorite intrusion.
The geometry and size of intrusions found within the JLP range from sheets to irregular intrusions that are often equidimensional (length/width ratios <10) in map section with steeply dipping contacts. The following general pattern of intrusion exists from east to west within the pluton. Dikes are observed primarily in the metavolcanic host rock along the eastern margin of the pluton (Fig. 6). The dikes in this domain vary from centimeter to the tens of meters scale (Fig. 6). The eastern margin of the pluton is made up of a mix of sheets and irregular intrusions, while in the west intrusions with irregular geometries dominate. Sizes of irregularly shaped intrusions also increase from east to west; the easternmost pluton has intrusions that range from tens to hundreds of meters in diameter while the central-eastern to western JLP has intrusions that are hundreds of meters to kilometers in diameter.
A similar east-to-west variation exists in the number of intrusions present within JLP. In the eastern domain, there are thousands of dikes that intrude the metavolcanic host rock (Fig. 2). As intrusion size increases outside the easternmost domain, the number of intrusions decreases. Nevertheless, excluding the eastern metavolcanic margin, the number of intrusions that make up the JLP is conservatively estimated to be in the hundreds to perhaps in the range of 1000. For example, in the detailed map area near Madera Creek (Fig. 3), eight distinct JLP units were identified (eight if the Post Peak pluton is considered part of the JLP; Peck, 1980) that were >∼8 km2.
Age relationships between JLP increments can often be determined using crosscutting relationships and inclusions. Crosscutting relationships and inclusions indicate that there is a general westward younging of intrusive age. Sheets and dikes within the eastern JLP and its metavolcanic host rock are truncated by irregular intrusions in the eastern domain of the pluton. Some xenoliths of eastern margin metavolcanics contain dikes similar to the intact host rock to the east that are truncated at xenolith margins. Farther west, age relationships between intrusions are more difficult to decipher due to variability in geometry and size. Detailed mapping in the Madera Creek area has shown that, for the most part, relative ages are easily determined in this area and JLP phases can be placed in an intrusive order (see Fig. 3 and map legend).
Internal Characteristics of Intrusions
In addition to overall heterogeneity within the JLP, many increments show considerable internal variability. Individual intrusions contain a variety of magmatic structures, including magmatic foliation and lineation, mafic microgranitoid enclaves and enclave swarms, mafic-felsic interactions along mafic sheet margins, similar mafic-felsic magma interaction along non-sheet-like mafic intrusion margins, schlieren layering, and schlieren-bound magma tubes.
The JLP has a ubiquitous mineral fabric defined by the aligned subhedral to euhedral feldspars (plagioclase and potassium feldspar), biotite, and hornblende (Fig. 7). The magmatic foliation defined by both minerals and enclaves is quite consistent with average orientations of 339°/90° and 336°/89°, respectively (Figs. 8A, 8F). JLP magmatic foliation is approximately parallel to lithologic layering and foliation in the metavolcanic host rock to the east and some of the pendants in the JLP (Figs. 2 and 8). One exception is in the eastern pluton, where abundant host rock blocks are present and where foliation strikes are variable but dips remain steep (Fig. 2). Magmatic foliation intensity is moderate to strong throughout the pluton. Spatial analysis of intensity based on a qualitative foliation intensity scale (scale ranges from 0 to 5 where 0 = no fabric, 3 = strong fabric, and 5 = hypersolidus mylonite; see Miller and Paterson, 2001; Paterson et al., 1998) shows no pluton-wide variation. Throughout the JLP, magmatic foliation overprints internal contacts at all scales (Figs. 3, and 7D). A moderate to strong mineral lineation that is steeply plunging nearly everywhere in the pluton (Fig. 2) and that is defined by alignment of feldspar and hornblende is on foliation planes. The average trend and plunge of the mineral lineation is 344°/86° (Fig. 8G).
Nearly all of the intrusions that make up the JLP contain mafic microgranitoid enclaves and enclave swarms (Fig. 4B). Enclave compositions range from diorite to granodiorite and are usually fine grained. Within typical JLP granodiorite, enclaves are evenly distributed with local enclave swarms. In hybrid units (e.g., Fig. 3), enclave and enclave swarm density sharply increases, often accounting for ∼20%–40% of exposed surface area. Enclaves, both single and those that make up enclave swarms, are often elongate in map section and in three dimensions have oblate shapes. In most cases mafic microgranitoid enclave orientations mimic the orientation of the mineral foliation (Figs. 7A and 8F); only a few examples of foliation are defined by enclaves slightly oblique to the mineral foliation (Fig. 7C). Enclave swarms commonly have sharp boundaries. Specifically, within a swarm enclave density is greater than in surrounding pluton (50%–80% enclave) and enclave swarm matrix is often texturally and/or modally different than the host magma. The shapes of swarms vary from circular to elliptical to irregular in map section. Enclave size is moderately variable, but is dominantly in the centimeter to tens of centimeters range, while enclave swarms vary in size from tens to hundreds of meters in diameter. Both individual elongate enclaves and elongate enclave swarms normally mimic the orientation of the magmatic foliation pattern (Figs. 2 and 3). The Post Peak phase is a notable exception in that it contains close to 50% enclaves in its type area (central JLP below Post Peak; Fig. 2). Furthermore, Post Peak mafic microgranitoid enclaves vary dramatically in composition, texture, geometry, size, and orientation at the outcrop scale. For example, in the type locality 7 different enclave types were observed at one outcrop. Shapes are considerably variable and commonly have irregular blob shapes as opposed to the elliptical and/or oblate shapes observed elsewhere in the JLP. Sizes range from millimeter to meter scale, and there is usually no preferred orientation to enclaves.
Interaction of mafic and felsic magma is observed within many JLP intrusions (Fig. 9). Mafic magmas often form irregular intrusions with associated sheeting in more felsic intrusions of JLP. With the exception of the eastern margin of the JLP, sheeting is normally localized within hybrid units in domains proximal to larger mafic intrusions in the pluton (Figs. 2 and 3). The orientation of mafic sheets is dominantly parallel to the structural grain in the area, that is, northwest-southeast and steeply dipping. Structures formed by mafic-felsic interactions include chilled margins, felsic cuspate-mafic lobate patterns, and rarely felsic pipes, all of which may indicate local younging and/or growth directions (Figs. 9A–9C). Our detailed mapping in the Madera Creek area (Fig. 3) indicates a general lack of consistency among these structures at many exposures of mafic-felsic margins (Fig. 9B). Some exposures have a consistent younging direction at the outcrop scale, but younging directions are not consistent between localities (e.g., Figs. 9A, 9C). A rose diagram of younging directions from exposures with consistent younging illustrates this relationship (Fig. 8J).
Other structures such as schlieren layering and schlieren-bound magma tubes are observed in the JLP. Orientation of schlieren layering is variable but is often subparallel to mineral foliation (Fig. 8H).Schlieren troughs associated with schlieren layering (similar to cross-bedding in sedimentary rocks) provide crosscutting relationships and yield inconsistent younging directions in the pluton. Furthermore, regardless of the orientation of layering, the mineral fabric within the layers is almost exclusively parallel to the mineral fabric observed outside the layering. Schlieren-bound magma tubes (e.g., Weinberg et al., 2001), although relatively rare in the JLP, have axes that are consistently vertical to subvertical in orientation (Figs. 8I and 9D).
Host Rock Description and Structure
Host rocks surrounding and within the JLP are dominantly metavolcanic units interpreted to be associated with the Minarets Caldera (Peck, 1980) and were described extensively by McNulty et al. (1996), Lowe (1996), Fiske and Tobisch (1994), and Tobisch et al. (1977) (Figs. 1, 2, and 3). Compositions of the metavolcanic host rock units range from rhyolite to basaltic andesite, and the most abundant units are massive welded tuffs and lithic tuffs (e.g., Fig. 10A) with minor occurrences of lapilli tuff and ash-flow tuff.
Metavolcanic host rocks to the east of the pluton consist of a sequence of steeply tilted to slightly overturned units that generally trend northwest-southeast, parallel to the eastern margin of the pluton, although considerable variability in the orientation of lithologic layering is noted (Figs. 2 and 8L; McNulty et al., 1996). Metavolcanic host rock has a continuous cleavage defined by both aligned minerals and elongate lithic clasts. Cleavage generally strikes northwest-southeast with an average orientation of 329°/69° and is parallel to both lithologic layering in the host rock and the foliation in the JLP (Fig. 8O). The lineation observed is characterized as a mineral and stretching lineation defined by aligned minerals and elongate lithic clasts and quartz aggregates. Mineral lineation in this domain is variable, but typically moderate to steeply plunging with average orientation of 040°/70° (Fig. 8P).
The northwest-southeast–striking eastern margin has kilometer-scale steps along it (e.g., Figs. 1 and 2) as well as abundant smaller steps on a scale of hundreds of meters (Figs. 10B, 10C). Cleavage and lithologic layering in the metavolcanics are commonly truncated at the contact, and metavolcanic blocks are abundant (Fig. 10C). Locally, tight to isoclinal folds are noted with axial planes parallel to the cleavage in the metavolcanics and/or the contact with the pluton. Rarely, cleavage is gently folded with steep axes and axial planes parallel to the pluton foliation and the general trend of the contact (Fig. 10C). No major faults or shear zones were observed in the eastern margin metavolcanics. The closest major shear zone is the Bench Canyon shear zone, which is >1 km east of the margin of the pluton (McNulty, 1995; Tobisch et al., 2000). Smaller shear zones and faults with orientations similar to the cleavage in the metavolcanics are observed locally, and show reverse-sense, pluton-side-up kinematics and minor (submeter) displacements. However, overall kinematics in the eastern domain are predominantly symmetric both in lineation parallel and perpendicular sections. Minor brittle fractures and faults overprint higher temperature, ductile structures.
There are a number of kilometer-scale pendants found within and along the perimeter of the JLP, including the Post Peak, Merced Peak, Sing Peak, Granite Creek, and Madera Creek (Figs. 1, 2, 3, and 11). These pendants are often topographically higher than the majority of the exposed JLP, and consist primarily of metavolcanic units similar to those found along the eastern margin of the pluton (Fig. 2). Most pendants have gently dipping but irregular lower contacts with the JLP. Centimeter- to decimeter-scale host rock blocks with lithologies similar to those of the pendants, typically composing 25%–75% outcrop area, are observed within 100 m of pendant contacts (Fig. 11). Often, porphyritic leucogranites (e.g., Post Peak pluton; Fig. 2) are found intruding the lower margins of pendants and are in turn intruded by the JLP. Dikes of JLP are common in pendants and some (e.g., Granite Creek, Madera Creek, and southern Merced Peak) are pervasively intruded by dikes (Fig. 2). Pendants with abundant dikes often grade into the JLP via an intrusive breccia. Specifically, there is a gradation from pendant with dikes to pluton with xenoliths over several tens to hundreds of meters where dikes increase until it is a mix of JLP with blocks and rafts of host rock and finally into JLP with fewer host rock xenoliths. Orientations of dikes in pendants are extremely variable (e.g., Fig. 8K).
Some pendants are elongate, with long dimensions roughly parallel to the foliation in the JLP and the eastern margin host rock (Fig. 1). Cleavage in pendants is typically parallel to the magmatic foliation in the JLP (Figs. 1, 2, and 3). Cleavage is defined by alignment of minerals and elongate clasts, similar to the eastern margin host rock. Lineation, which is a stretching lineation, defined by aligned minerals and elongate lithic clasts and quartz aggregates, is subvertical. A notable exception is the Post Peak pendant, which has lithologic units with east-west orientations, a steeply dipping reverse shear zone that strikes east-west, and cleavage that is variable but with a dominant east-west–trending component (Figs. 2 and 8Q). Similar east-west fabric orientations also occur in host rock in the southernmost JLP (Fig. 1; McNulty et al., 1996; Peck, 1980).
With any incrementally constructed pluton, early increments of magma become host rock for later magma batches. Plutonic host rock for the JLP is thus either the slightly older Illilouette Creek pluton to the west or any JLP increment present prior to intrusion of a later pulse (Figs. 2 and 3). The lithologic characteristics of these plutonic host rocks were outlined in the preceding; more detailed petrography is presented in the following section. Here we describe some general features of host rock xenoliths, including patterns of deformation noted in both types of host rock.
Both metavolcanic and plutonic blocks and rafts are common throughout the JLP. The metavolcanic xenoliths have compositions identical to those of the perimeter host rock and pendants, and frequently can be correlated to nearby larger blocks, pendants, or host rock (Figs. 1, 2, 3, 10C, and 11). Plutonic xenoliths, or in some cases cognate inclusions, consist of internal phases of the JLP and porphyritic leucogranites (e.g., Post Peak pluton) and are more common along intrusive contacts (Fig. 3). Blocks of internal phases often can be correlated to nearby intrusive phases and thus used in determining the relative timing of JLP intrusions (Fig. 3). Multiple block lithologies sometimes occur at a single location, including metavolcanic blocks, plutonic blocks of earlier phases, and porphyritic leucogranites (Figs. 3 and 12A). Furthermore, it is not uncommon to find single composite blocks within JLP. There are two different types of composite blocks observed; the first are blocks that are made up of multiple compositions (e.g., Fig. 12B), and the second are blocks of one composition fully enclosed within a block of another composition. Composite blocks have constituents that include combinations of internal JLP phases, metavolcanics, and/or leucogranite units.
Varying degrees of deformation surrounding and within blocks are noted in the JLP. Generally magmatic foliation orientation in plutonic rock that surrounds xenoliths does not change. Only rarely do blocks disrupt internal magmatic structures such as foliation or layering (Fig. 12C). Internal block fabrics (solid state in the metavolcanics and magmatic in plutonic) are predominantly parallel to fabrics found in the enclosing JLP. Synmagmatic structures are frequently found in both metavolcanic and plutonic blocks. Dikes and veins of JLP granodiorite that intrude blocks are sometimes folded or boudinaged and have magmatic foliations that are parallel to internal block cleavage and/or surrounding JLP magmatic foliation regardless of their orientation (Fig. 12D).
In order to quantify the deformation within metavolcanic units, strain analyses were completed in 21 localities. The data come from various locations, including the eastern margin metavolcanics, the Post Peak and Madera Creek pendants, and numerous large metavolcanic blocks throughout the pluton (Table 1). Measurements were taken in the field and laboratory analyses were done on samples and oriented field photographs primarily on metavolcanic units with lithic clasts. The techniques of Shimamoto and Ikeda (1976) and Miller and Oertel (1979) were used to calculate three-dimensional fabric ellipsoids for each locality. An average of ∼55% shortening along the Z-axis of the fabric ellipsoid was determined for the samples with a range in shortening from 27%–82% (Table 1). Both ellipsoid shape and strain intensity calculations are also quite variable throughout the pluton and within domains.
We collected 25 samples from the Madera Creek area of the JLP for petrographic and ongoing geochemical analyses. The mineralogy and petrography of each unit were determined from standard and polished thin sections and stained slabs. Of these samples, 11 are from units assigned in the field as JLP granodiorite (Kj in Fig. 3), 4 are from the Madera Creek quartz diorite (Kmc in Fig. 3), and 9 samples were regarded as hybrids of either JLP granodiorite or the Madera Creek quartz diorite. Three samples were considered JLP hybrids and six were considered quartz diorite hybrids. Note that samples deemed to be hybrids often do not reflect the character of entire hybrid units mapped in Figure 3; rather, they represent hybrid domains found within heterogeneous map units or along contacts between map units of different composition (e.g., Fig. 5). One sample of Long Creek quartz diorite (LCQD in Fig. 2) was included in the analyses because this unit has textures similar to those of some of the hybrid units in Madera Creek. The Madera Creek area illustrates how composition, texture, geometry, size, and relative ages all vary within the JLP; we present here the petrologic characteristics of the intrusions found in this area as a proxy for the entire JLP.
In the Madera Creek area, units range from coarse- to fine-grained diorite and quartz monzodiorite through granodiorite and granite (Fig. 13). The JLP granodiorites are dominated by plagioclase and biotite with subordinate alkali feldspar, hornblende, sphene, minor iron-titanium oxides consisting primarily of magnetite, and apatite. In thin section, plagioclase forms euhedral to subhedral crystals that exhibit oscillatory and normal zoning (An29–31). Cores of plagioclase crystals have inclusions of biotite, sphene, apatite (prismatic and acicular), zircon, and/or iron-titanium oxides. Mafic minerals are dominated by biotite ± hornblende. Biotite and hornblende contain inclusions of zircon, epidote, apatite, sphene, magnetite, and rarely plagioclase. Sphene is primarily subhedral with wedge-shaped morphology, but irregular anhedral grains were also observed, indicating both primary and secondary growth. Alkali feldspar is present in moderate amounts as anhedral crystals and is sometimes observed as an interstitial phase. Quartz is also anhedral and interstitial or forms secondary intergrowths into sodic plagioclase (myrmekite). Other minor constituents include anhedral iron-titanium oxides, mostly magnetite, and euhedral zircon that is often included in biotite. Chlorite along with epidote and sericite occur as alteration products on grain boundaries, cleavage faces, and fractures.
The Madera Creek quartz diorite (Kmc) is a fine- to medium-grained phase located in the southwestern zone of the Madera Creek area (see Fig. 3). Rock compositions range from monzodiorite to quartz monzodiorite (Fig. 13). This unit is dominated by plagioclase, hornblende, and biotite with lesser amounts of accessory minerals (sphene, apatite, iron-titanium oxides), alkali feldspar, and quartz. Oscillatory zoned and often sericitized, plagioclase forms euhedral to subhedral crystals with inclusions of hornblende and biotite. Hornblende is abundant and typically forms subhedral crystals, with inclusions of biotite, apatite, sphene, and magnetite. Biotite is commonly intergrown with hornblende and contains inclusions of zircon, apatite, sphene, opaques, and rarely plagioclase. Sphene occurs as irregular anhedral crystals (secondary), but some subhedral prismatic crystals (magmatic) were also observed. Quartz is less abundant in this unit compared to JLP units and occurs as anhedral crystals. Alkali feldspar is observed in lesser amounts. Other minerals observed in trace amounts are zircon and secondary epidote.
Hybrids of JLP granodiorite and Madera Creek quartz diorite are found throughout the Madera Creek area, exhibiting contact zones ranging from sharp to gradational with gradational contact exhibiting mixing and mingling relationships (Figs. 4 and 5). Based on grain size, mafic mineral content, and relative proximity to end-member units, most were determined in the field to be closer to the typical JLP granodiorite composition, while others were closer to the Madera Creek quartz diorite composition and termed quartz diorite hybrids. In all hybrid units varying proportions of plagioclase, quartz, alkali feldspar, biotite, and hornblende are the dominant minerals, with accessory sphene, apatite, zircon, epidote (secondary), and magnetite. Xenocrysts of perthitic orthoclase occur in quartz diorite hybrids. Biotite, the dominant mafic mineral, is intergrown with and replaces amphibole.
Thermobarometry of Igneous Crystallization
Mineral compositions and backscattered electron images for five samples collected across the JLP for thermobarometry were obtained with a Cameca SX-100 microprobe at New Mexico Institute of Mining and Technology, equipped with three wavelength-dispersive spectrometers, plus secondary electron and high-speed backscattered electron detectors. Samples were specifically collected from units not deemed to be hybrids. Plagioclase was analyzed by a defocused electron beam to prevent alkali loss; other minerals were analyzed with a focused beam. Selected points from core to rim were analyzed within hornblende and plagioclase in contact with quartz. Core and interior points were analyzed to determine the extent of chemical zoning. Rim points were analyzed to maximize the probability that crystals are at equilibrium with the full assemblage.
Hornblende is present within all units and is primarily euhedral, having well-defined contacts with quartz and plagioclase with little textural variation observable in thin sections. Textural relationships revealed formation of hornblende during magmatic crystallization, including near-solidus conditions. For each sample, with the exception of GP117, ∼40 analyses, consisting of rim and core points, were completed on 3–4 grains. For sample GP117, 10 analyses of 1 grain were conducted as this was the only unaltered hornblende grain that was in contact with quartz. It was also the only sample that yielded a subsolidus pressure-temperature solution. Compositions of amphibole were found to be gradational, with increasing Fe, Na, and K toward the edenite end member. Mg/(Mg + Fe) ratios for these hornblendes ranged from 0.50 to 0.58 and total Al content ranged from 0.9 to 1.8. Backscattered imagery (high gain) revealed patchy zoning within several grains.
Points analyzed in the patchy zones exhibit little chemical variation and plot within the same hornblende-type field, indicating that slight compositional variations within the grain will not influence barometric calculations. Selection of homogeneous zones within crystals was performed during microprobe analysis to minimize any effects of alteration on the data set. Furthermore, comparing M2 site (Alvi) to T site (Aliv) distinguishes between primary and secondary formation of hornblende crystals. This comparison revealed that the majority of JLP hornblendes were of primary igneous origin; outliers were not used in thermobarometric calculations to negate the effects of retrogression on pressure-temperature estimates.
Plagioclase selected for analysis was in contact with quartz and hornblende and exhibited little to no compositional zoning. Most points analyzed were from the rims of grains to assure equilibrium with the near solidus phase assemblage. Plagioclase compositions from JLP units are relatively consistent, with an average of An19 on rims and An21 on cores. Compositions for plagioclase taken from the Long Creek Quartz Diorite unit are slightly higher, with average of An31 on the rims and An34.5 in the cores.
Pressure and temperature estimates show minimal variation from northwest to southeast within the JLP and the neighboring Illilouette Creek pluton, as shown in Figure 2, with pressure estimates ranging from 2.8 to 3.1 kbar and temperature estimates ranging from 677 to 708 °C. Thermobarometry results for the five samples collected are detailed in Table 2.
Most of the pressure and temperature estimates plot above the granodiorite wet solidus with the noted exception of sample GP117. Although its solidus pressure estimate of 2.8 ± 1.0 kbar is within the range of other samples, its temperature estimate of 641 ± 21 yields a pressure-temperature solution that is marginally subsolidus and substantially different from other samples from the pluton. Possible causes for this could be the small data set used in the thermobarometric calculations. Also, sample GP117 contained only one hornblende grain that met the requirements necessary for implementation of thermobarometry. Excluding sample GP117, average pressure of emplacement for JLP is 3.0 ± 0.5 kbar, with an average temperature of 678 ± 17 °C. This emplacement pressure is distinct from the 4.5 kbar reported by Ague and Brimhall (1988) that lacked temperature correction.
Evaluation of Previous Pluton Construction Models
Two models for incremental growth and emplacement of plutons have been proposed for the JLP that must be reconciled using previous and new data. Both models and their criteria are very testable in a magmatic system like the JLP because of the excellent exposure of the pluton and its metavolcanic host rock. It is important to reiterate here that R.A. Wiebe (2002, 2005, personal communs.) agrees that a simple scenario of stratified construction does not work for the JLP, but does suggest that information regarding younging, particularly from the Madera Creek area, is still present. We include an evaluation of this model here for completeness and because prior to and during field work we were excited about the possibility that it was applicable.
This incremental construction model suggests dikes were primarily responsible for JLP construction (McNulty et al., 1996). Bulk emplacement mechanisms involved during construction include expansion of dikes, ductile shortening and downward flow of metavolcanic host rock along the eastern margin of the pluton, stoping, and caldera formation (McNulty et al., 1996). Note that we agree with most observations and some interpretations regarding the construction of the JLP made by McNulty et al. (1996). The main point of contention with their model is the interpretation that the JLP was constructed mainly by dikes and that the magmatic fabrics reflect flow in these dikes.
We agree with the McNulty et al.’s (1996) interpretation that the eastern margin metavolcanics represent an injection zone that is dominated by diking. Thus it is clear that dikes were involved in initial stages of JLP construction. However, the majority of the intrusions to the west of the injection zone and within the pluton that truncate the injection zone dikes do not suggest construction by diking. Most of the interior portions of the JLP are dominated by intrusions with irregular sizes and geometries, do not have large length/width ratios, and even when elongate do not match dike characteristics (e.g., Paterson and Miller, 1998a).
We also disagree with the interpretation that dikes in metavolcanic pendants in the JLP have orientations similar to those of the dikes in the eastern domain (McNulty et al., 1996). The metavolcanic pendants are pervasively intruded by dikes. However, they have variable orientations (Fig. 8K) and grade into injection migmatites, which are continuous with larger intrusions found along pendant margins. Thus we suggest that as larger intrusions were emplaced, particularly late during chamber construction, dikes emanating from the larger intrusions were injected into the pendants.
Magmatic fabric patterns, including anisotropy of magnetic susceptibility data, within the JLP were interpreted by McNulty et al. (1996) to record flow during emplacement of dikes. If this were the case, magmatic foliation is expected to be roughly parallel to the margins of the dikes and to possibly preserve crystal tiling and other kinematic indicators in cross section (Philpotts and Asher, 1994; Vernon, 2000). However, magmatic foliations observed in dikes, sheets, and irregular intrusions throughout the JLP and its metavolcanic host rock are typically parallel to the regional structural grain (northwest-southeast and steep) regardless of the shape, size, or orientation of the intrusion. Furthermore, the observation that foliation overprints internal contacts in the JLP indicates that fabric formation occurred after chamber growth and late in the crystallization history of pulses in the pluton (Paterson et al., 1998).
Stratified Chamber Construction and Tilting
Stratified chamber construction of the JLP requires that laterally extensive mafic sills formed in an already existing felsic chamber. A few diorite and quartz diorite sheets occur adjacent to larger mafic intrusions (e.g., the Madera Creek area). Even these sheets are often not well developed, are not laterally continuous, and do not occur in much of the pluton. Coupled with the observation that the bulk of the JLP consists of hundreds of more irregular intrusions of predominantly felsic material suggests that a simple scenario of numerous sheet-like intrusions of mafic magma input into a more silicic magma chamber is unlikely.
Deriving younging and/or growth directions in sheeted plutons requires consistency of the criteria, at least within the sheeted domains of the pluton. This consistency was clearly established by Wiebe and coworkers for other plutons (Wiebe and Collins, 1998; Wiebe, 1993, 1994). However, our studies in the JLP indicate that there is a lack of consistency of younging directions obtained along mafic-felsic boundaries. The rose diagram of magma younging directions in Figure 8, which are only those outcrops or domains that were internally consistent with respect to younging direction, yield directions that (1) are not consistent between domains, (2) do not indicate that younging is consistently eastward in the JLP, and (3) are not compatible with the currently steep orientations of all magmatic tubes in the JLP. We suggest that many of these features can form by other processes in addition to those presented in the Wiebe model (e.g., Pignotta et al., 2005).
Despite the domainal nature of the mafic sheeting, and the lack of consistency of younging directions, tilting of the pluton still remains possible. If the pluton was tilted about an approximately north-south axis to the east, with the base of the pluton to the west and the roof to the east and bringing mafic sheets to their present steep dips, there should be an ∼13 km or ∼4 kbar pressure differential across the pluton, with lowest pressures in the east and highest pressures in the west. Thermobarometric determinations completed along an approximately east-west transect across the pluton yield near uniform pressure estimates (Fig. 2; Table 2). Emplacement pressures range from 2.8 to 3.1 kbar and, within the limits of precision, are indistinguishable and thus preclude the possibility of any major postemplacement tilt.
Additional observations suggest a lack of large tilting of the JLP: (1) the lack of accommodation faults and/or shear zones of JLP age that would be required for tilting; these would be approximately east-west structures located at the northern and southern margins of the pluton; (2) the gently dipping contacts of metavolcanic pendants, which are interpreted to represent roof contacts for the JLP (Fig. 11); (3) magmatic features like magma tubes that have subvertical axes (Figs. 8I and 9D) that are interpreted to result from gravitational instabilities and represent paleovertical in magma chambers (Weinberg et al., 2001); (4) host rock blocks that disrupt magmatic layering that are also interpreted to indicate paleovertical (Fig. 12C; Paterson and Miller, 1998); and (5) the magnitude and direction of tilting required to get sheets from horizontal to near vertical is unrecognized in similar aged plutons from this part of the Sierra Nevada (Frei et al., 1984; Gilder and McNulty, 1999).
The lack of evidence for tilting in the JLP combined with the inconsistent younging directions suggests that the mafic sheets observed in the JLP were not formed as gently dipping layers, but instead formed in their present steep orientation. We suggest that instead of vertical younging and buoyancy, the structures observed along the mafic-felsic contacts formed as a consequence of magma mingling and the rheologic contrast between the two magmas (Perugini and Poli, 2005; Petford, 2003; Bergantz, 2000; Hallot et al., 1996; Frost and Mahood, 1987). For example, Hallot et al. (1996) showed that the interaction of two non-Newtonian magmas will produce pipes, synplutonic dikes (or in this case sheets), and dendritic structures via viscous fingering identical to structures observed in the JLP. Paterson et al. (2004) discussed similar viscosity and rheology relationships between mafic microgranitoid enclaves and their host magma. Alternatively, the structures along mafic-felsic contacts could represent cuspate-lobate folds formed as the more competent mafic sheets are buckled in the less-competent granodiorite host (e.g., Ramsay and Huber, 1987).
New Incremental Emplacement Model for the Jackass Lakes Pluton
We present a new model for incremental construction highlighted by (1) emplacement of numerous magma increments that vary in composition, texture, geometry, size, and age resulting in a complex spatial and temporal evolution of the JLP, (2) no tilting of the chamber, (3) extensive mingling and some mixing between magma pulses at the emplacement level, (4) multiple material transfer processes that removed metavolcanic host rock and older plutonic phases throughout pluton construction, and (5) the formation of magmatic mineral fabrics by an increment of regional strain during final crystallization.
Pluton Construction via Incremental Magma Emplacement
Our model for the incremental construction of the JLP involves several emplacement mechanisms that change through time and space in an overall contractional tectonic regime (see following structural history discussion). Initial magma emplacement involved sheeted intrusion, preserved in eastern margin metavolcanics and the easternmost JLP (Figs. 6 and 14). Thus, during the earliest stages of construction, when metavolcanic host rocks were relatively cool, dikes played an important role. However, the majority of the JLP does not exhibit the same clear-cut diking or sheeting observed in the east or described in other sheeted plutons (e.g., the McDoogal pluton, Mahan et al., 2003; Entiat and Cardinal Peak plutons, Miller and Paterson, 1995; Main Donegal granite, Hutton, 1982; Pitcher and Berger, 1972). Early intrusions serve to heat and mechanically weaken host rock and become host rock for later increments. Overall, host rock behavior becomes increasingly viscoelastic. Subsequent magma increments that cut the early sheeted eastern domain are larger, more equidimensional, and have more irregular contact relations with other increments. This suggests that magma flux into the emplacement level increases and pulses of magma rise, at least in part, as viscoelastic diapirs or sheets (e.g., Miller and Paterson, 2001, 1999) (Fig. 14; see following discussion of material transfer processes).
As larger pulses of predominantly felsic magma rose to the emplacement level, greater amounts of heat and mass allowed for the growth of a magma chamber (kilometer-scale active chambers). We suggest that increments achieved high crystal content (∼30%–60%) based on the observations that most contacts are steep and sharp, indicating high effective viscosity during subsequent intrusion (e.g., Bergantz, 2000). However some contacts do show gradations or mixing relationships suggestive of lower crystal content. Stoped blocks and rafts preserved along the contacts also indicate high effective magma viscosity both in JLP units that make up the blocks and those that entrap them (Paterson and Miller, 1998b). In general, magma flux migrated and increased from the sheeted eastern margin to the west to form what is now the entire pluton.
Both field and geochronologic data suggest that magma input was rapid and that one or more multikilometer-scale magma chambers formed in the JLP. The size of the JLP magma chambers may have been quite large during the evolution of the system due to relatively rapid addition of both felsic and mafic magma. For example, in the Madera Creek area we interpret that the majority of the domain was magmatic at the same time (see following discussion of mingling and structural history). Large regions within the JLP (several cubic kilometers) may have existed in hypersolidus conditions, and it is possible that several regions (like the Madera Creek area) within the pluton may have been magma chambers synchronously. Furthermore, the complex internal makeup of the JLP suggests that, along with rapid input into the system, removal of magma from the system either to higher level magma chambers or directly to the surface is likely. Detailed geochronology, geochemistry, and thermal modeling are required to resolve these issues.
Mingling and Mixing within and between JLP Increments
Rapid incremental growth of the JLP may explain some of the extensive magma mingling both between increments (e.g., Figs. 3 and 4A) and the local development of schlieren troughs and tubes within them. The tubes are evidence of local continued flow of magma and erosion of preexisting material occurring late in the crystallization history of the JLP (Weinberg et al., 2001; Zak and Paterson, 2005). The irregular nature of contacts between intrusions is interpreted to reflect mingling, at the emplacement level, of magma increments that had varying crystal contents. If all magma increments were crystal poor and emplaced rapidly, it is likely that the JLP would have homogenized (mixed) rather than preserved its compositional and textural diversity and typical sharp, mingled contacts. Mingling throughout the JLP suggests that large regions (kilometer scale) may have existed in hypersolidus conditions and thus were part of a magma chamber. Mutually intrusive relationships along many contacts further suggest that increments in contact with each other were not fully crystallized and that both had some melt fraction. Alternatively, it is possible that as new increments were added, melt was remobilized locally in the contact region, allowing for mutually intrusive relationships. The gradational nature of some contacts, and in particular single contacts that vary from gradational to sharp, suggests that melt percentages within and between magma increments vary spatially. With particular reference to mingling of mafic and felsic magmas, we interpret the mafic-felsic interaction structures used by Wiebe (1999, 2000) to reflect viscosity and rheologic differences between the magmas, rather than always reflecting gravitational settling indicating younging of the chamber.
Field observations (e.g., dispersed enclaves and general heterogeneity noted in some units) indicate that internal mingling also occurred within intrusions, but to a much greater degree than the localized mingling along contacts (Fig. 14). Our interpretation is that the majority of this mingling occurred at deeper crustal levels where magmas had lower viscosities, and intruded into the JLP chamber having already been mingled.
Field and petrographic evidence suggests that magma mixing occurred at a variety of scales in the JLP. Local mixing occurs infrequently along contacts, and a range of petrographic observations suggests that some magma mixing may have been responsible for larger, map-scale intrusions (Figs. 3 and 5), including the widespread occurrence in hybrid units of acicular apatite, cellular plagioclase, and rapakivi and antirapakivi feldspar mantling. Such textures were documented in undercooled mixing environments by Wyllie et al. (1962), Lofgren (1974), and Hibbard (1981). The composition of hybrid units sampled in Figure 5 (note the red outline symbols in Fig. 13) suggests that magma mixing occurred at this locality. Given that this detailed example of mixing is mimicked by other hybrid units sampled from the Madera Creek area, we suggest that some of the larger units termed hybrid in the field truly are hybrids resulting from mixing of end-member units found within the JLP. We interpret that the hybridization of larger units took place below, but near, the level of emplacement and intruded the JLP chamber because these units often have sharp contacts and mingling relationships with other JLP units rather than gradational contacts. Only locally is emplacement level mixing preserved (e.g., Fig. 5).
Multiple Material Transfer Processes Involved during Incremental Emplacement
The JLP is an example of a magmatic system that evolved from emplacement of magma pulses into preexisting metavolcanic host rock to emplacement of younger magma pulses into previously emplaced pulses (i.e., plutonic host rock; Fig. 14). We agree with the interpretations of McNulty et al. (1996) regarding this incremental growth; they suggested that material transfer of volcanic host rock is a combination of ductile deformation of host rock, downward flow of host rock, and stoping. Our calculations of strain (Table 1) as well as field and microstructural observations complement the data set presented by McNulty et al. (1996) and illustrate a heterogeneous strain pattern in JLP metavolcanic host rock. Synmagmatic deformation of metavolcanic host rock throughout the JLP is prevalent (e.g., Fig. 12D). We interpret that many host rock material transfer and deformation processes were operating synchronously during emplacement of JLP magmas. For example, in the eastern JLP stoped blocks of metavolcanic rocks with evidence of sheeting and ductile deformation are observed in the pluton, often with crosscutting dikes of JLP that are deformed (e.g., folded, boudinaged). Material transfer processes in older JLP increments also must be accounted for, because as new increments are added to the system they not only have the potential to mingle and mix with older increments, but processes such as diking, stoping and melt-present deformation of older plutonic host rock must take place. Input of new magma pulses into an active chamber will result in the interaction of pulses that may have varying degrees of crystallinity. Deformation of plutonic host rock that is above its solidus is possible, and filter pressing (Brown, 1994; Sawyer, 1994) may be an important process in these conditions. It is conceivable that melt can be mechanically pressed away from an incoming pulse as it is emplaced. This mechanical process may be further aided by the additional heat and/or volatiles associated with the incoming magma pulse. Remobilized melt can migrate by pervasive flow in the plutonic host magma and/or be focused into dikes (Brown and Solar, 1999; Weinberg, 1999). The complex spatial and temporal evolution of magma pulsing seen in the JLP suggests that at least some dikes result from input of new magma to the system and the subsequent remobilization of magma (see following), as opposed to dikes derived from JLP source regions. This melt remobilization may also be in part responsible for some of the compositional variability (hybridization) noted in the JLP. Large domains (kilometer scale) may have existed in hypersolidus conditions with mobile melt, and thus observed magmatic deformation could occur after intrusion and record increments of regional strain, as described in the following.
Stoping of earlier increments by younger is observed throughout the JLP and is most common near internal contacts (Fig. 3). This suggests that internal recycling occurred within the JLP that may have influenced the thermal history as well as the geochemistry of the pluton. It is also possible that stoped blocks of earlier intrusive units were not fully crystallized when incorporated into a new intrusion, allowing for melt migration and melt present deformation of blocks.
Structural History Recorded by the JLP
The magmatic fabrics in the JLP suggest that regional strain left an important imprint on fabrics during the final crystallization of the pluton. We interpret that the fabrics within the pluton record increments of regional strain late in the crystallization history of JLP magma chambers (Benn et al., 2001; Paterson et al., 1998; Pignotta and Benn, 1999). The overall plate tectonic setting for the Sierra Nevada batholith ca. 98 Ma reflects subduction that was taking place to the west and is interpreted to be a transpressive regime (Engebretson et al., 1985; Tikoff and Teyssier, 1992; Tobisch and Cruden, 1995; Tobisch et al., 2000). However, both contractional and extensional strain fields may exist within this regime (cf. Tobisch et al., 2000, and references therein). Regardless of the overall plate tectonic setting of the batholith, JLP magmatic fabrics record roughly northeast-southwest contraction throughout its latest crystallization that we interpret reflects the regional paleostrain field in the vicinity of the JLP during emplacement. The main magmatic foliation is typically parallel to regional structure recognized in the metavolcanic host rock (Tobisch et al., 2000; McNulty et al., 1996). Magmatic foliations that overprint internal contacts (Fig. 7C) regardless of contact orientation are interpreted to reflect this regional strain rather than emplacement-related strain. We have struggled to find an alternative interpretation that explains this observation, since deformation related to magma ascent and emplacement should form magmatic foliations that reflect margin-parallel flow. Furthermore, the steep magmatic lineation observed everywhere throughout the pluton indicates vertical extension along with the northeast-southwest contraction. Emplacement-related magma flow is often interpreted to produce more complex patterns than those observed in the JLP. For example, Cruden et al. (1999) described a complex lineation pattern in the Dinkey Creek and Bald Mountain plutons interpreted to reflect flow in feeder conduits and inflation of the plutons. Dikes of JLP within pendants and metavolcanic stoped blocks have magmatic fabrics that are parallel to the pluton and metavolcanic host rock fabrics regardless of their orientation. Fabrics that record flow of magma in the dikes should be parallel to the orientation of the dike or show a curved pattern, from margin parallel to perpendicular in the dike center (Philpotts and Asher, 1994; Paterson et al., 1998), but this is commonly not the case.
Similarly, metavolcanic and plutonic stoped blocks have subsolidus and magmatic fabrics, respectively, that are parallel to the regional trends observed in the JLP and its metavolcanic host rock, even when blocks are not proximal to any in-place host rock. Furthermore, synmagmatic structures (e.g., folding and boudinage of JLP dikes and veins) observed in stoped blocks have orientations that suggest that they formed in response to regional northeast-southwest shortening. We interpret these structures as a result of regional stresses transmitted through crystal-rich JLP magma (>80%) to trapped blocks.
Structures and strain observed within eastern metavolcanic host rock and the pendants are likely a complex combination of regional strain and pluton-related strain. Because lithologic layering and structural grain are parallel to the eastern margin of the JLP, determining the contributions of different processes to strain is difficult. Strain data (Table 1) do not show a consistent increase in strain intensity toward the margin of the pluton, as should be expected if pluton emplacement overprinted regional strain. The data collected illustrate the heterogeneous nature of strain in the JLP metavolcanic host rock. Strain heterogeneity is most likely due to competence contrasts between lithologic units and the thermal effects of JLP sheeting.
Orientations of lithologic layering and fabrics in the Post Peak pendant and metavolcanic host rock along the southern margin of the pluton are east-west; this has led previous workers to suggest that collapse of a caldera above the JLP was responsible for these orientations (McNulty et al., 1996; Fiske and Tobisch, 1994; Peck, 1980). A caldera collapse model would explain many of the features observed in the JLP, including orientation of structures in the metavolcanic host rock as well as its complex internal magmatic history, which most likely reflects removal of magma from the system or eruption. However, barometry from the pluton suggests that crystallization occurred at ∼10 km and thus a caldera collapse alone is not likely to bring material down to those depths (e.g., Lipman, 2007; Lipman et al., 1984). Similarly, the classic caldera collapse structures are not observed in the JLP, for example, ring faults and breccias (e.g., Lipman, 1984). Therefore, we envision bulk downward transport (which may include initial caldera collapse) of metavolcanics through systems above the JLP and with subsequent JLP intrusions forming the subhorizontal roof contacts now observed (e.g., Tobisch et al., 2000; Saleeby et al., 1990). Conversely, the barometry could be giving a false paleodepth and the JLP emplacement level was much shallower in the crust.
Our model does not require that fabrics and structures observed within the main JLP, dikes that intrude pendants and stoped blocks, the deformed stoped blocks, or the metavolcanic host rock to have formed synchronously. In particular, we envision that fabric and structural elements in the JLP must have been somewhat diachronous, as different parts of the JLP had different crystal content, and it is possible that multiple active magma chambers existed during pluton construction. Metavolcanic host rock certainly underwent preemplacement, synemplacement, and postemplacement strain that must have been somewhat variable over the time scale of pluton construction, in particular as magma and thus thermal inputs during construction varied. The general pattern of westward younging of intrusions may also hold to some degree for the timing of magmatic fabric formation. Certainly the magmatic fabrics formed in the dikes and/or sheets of the eastern margin metavolcanics may be younger, since the dikes and/or sheets were likely fully crystallized prior to truncation by interior JLP intrusions. The interior portions of the JLP likely crystallized last, and thus there is a time-transgressive nature to the magmatic fabrics observed in the JLP, suggesting that contractional strain was dominant in this vicinity for an extended period of time as the pluton crystallized. Additional and more detailed thermochronology may assist in determining timing relationships not only of magmatic inputs, but also of fabric formation and deformation of host rock, and may ultimately help characterize the regional paleostrain field further.
SUMMARY AND BROADER IMPLICATIONS
The JLP is an incrementally emplaced magmatic system that records a complex spatial and temporal history of pluton construction. We propose that the JLP (1) contains multiple, irregularly shaped intrusions of both felsic and mafic magma that do not represent paleofloors or dikes, but instead are interpreted as viscoelastic diapirs; (2) intrusions were extensively mingled below the level of emplacement with some mingling both between and within intrusions at the emplacement level; (3) records evidence of local magma mixing forming new phases at depth and locally preserved hybrid phases along internal contacts at the emplacement level; (4) has not been tilted largely during or after emplacement, as determined by application of both thermobarometry and using internal magmatic structures that provide younging directions and paleovertical; (5) has magmatic fabrics that record regional contraction and not emplacement-related strain; (6) preserves large metavolcanic pendants representing subhorizontal roof contacts; (7) was emplaced by ductile deformation of its metavolcanic host rocks, return flow, and widespread stoping; and (8) records evidence of material transfer processes (e.g., stoping) acting on pulses within this incrementally emplaced system and that early pulses in other incrementally emplaced systems should be considered host rock.
The style of short duration, incremental emplacement described here allows for multiple, large (<1 km- to kilometer-scale) magma chambers to be active for extended periods of time at or near the final emplacement level. The following are some implications.
Magmatic features such as mafic-felsic interaction structures and magma tubes can aid in determining viscosity of intrusions during construction, overall magma chamber rheology, and possibly paleovertical and/or younging directions.
Magma mixing occurs at or near the final emplacement level, forming hybrid intrusions and hybrid domains, which may explain at least some of the compositional and textural diversity exhibited by many plutons.
Within plutons melt remobilization is feasible through the addition of heat and by magmatic strain acting on existing partially crystallized increments. This process may explain late dikes of evolved compositions (e.g., aplite and pegmatite) and local cumulate textures.
During incremental emplacement both preemplacement country rock and intruded increments, that may or may not be completely crystallized, should be considered as host rock.
Fabric formation is possible over regions larger than individual increments since melt may be present in regions larger than any single increment, and fabric formation can occur late in the crystallization history. Specifically for the Sierra Nevada, the fabric pattern in the JLP suggests that during its construction and solidification (ca. 98 Ma), the regional strain field was contractional and spatially consistent at least at the scale of the pluton.
Limitations on the Hornblende-Plagioclase Thermobarometer
The total Al content of hornblende is a function of both pressure and temperature. (Anderson and Smith, 1995). Early empirical and experimental calibrations of the Al-in-hornblende barometer (Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1989; Schmidt, 1992, 1993) did not account for temperature dependence. This barometer potentially can record pressure and temperature (P, T) at or near magma solidification if hornblende is in equilibrium with quartz, potassium feldspar, biotite, and other late requisite solidus phases (Anderson, 1996).
In application for this study, temperature and pressure were determined using the Anderson and Smith (1995) calibration in concert with temperature based on the Holland and Blundy (1994) calibration for the equilibrium, edenite + albite = richterite + anorthite of the hornblende-plagioclase thermometer.
Given that the mineral reaction forming the basis of this thermobarometer has not been experimentally evaluated, it can only be used for granitic mineral assemblages and under a restricted range of hornblende composition (Anderson and Smith, 1995; Anderson, 1996). Fortunately, many arc-related granitoids have appropriate phase assemblages and phase compositions. One limitation of the hornblende-plagioclase thermobarometer results from complexities surrounding amphibole chemistry. Within selective sites, several cation substitutions can affect the total Al content of hornblende. The Tschermak substitution, which is favored by increases in pressure, decreases Si while subsequently increasing Al (iv and vi) cation substitutions in T and M2 sites, thereby increasing pressure estimates. In addition, increases in temperature favor the edenite substitution, whereby Na enters the A site while Al (VI) substitutes for Si in the T site, increasing total Al values (Deer et al., 1992; Hawthorne, 1981; Hammarstrom and Zen, 1986; Anderson and Smith, 1995). Another temperature-sensitive substitution involves Ti entering the M2 site for Mg. The incorporation of Ti into the M2 site is coupled with an increase in total Al (Hammarstrom and Zen, 1986; Anderson and Smith, 1995). To account for these changes in hornblende chemistry, all compositions were normalized to 13 cations, and all Fe was calculated by charge balance following the procedure of Holland and Blundy (1994).
Granitic magma mineral compositions are affected by changes in pressure, temperature, and to a large extent by oxygen fugacity. Arc-related granitoids crystallize within a limited range of fO2 (Anderson, 1996). Changes in fO2 affect valence states of iron, which in turn affect cation ratios and site vacancies within the hornblende mineral chemistry. These changes can directly affect calculated pressures. Crystallization under low fO2 conditions decreases Mg/Fe and Fe3+/Fe2+ ratios, thereby increasing Al substitution within the M2 site of hornblende independent of temperature and pressure. Under reducing magmatic conditions, increased Al values in hornblende can result in falsely high pressures (Anderson and Smith, 1995). Based on these results, Anderson and Smith (1995) recommended that the barometer be used for hornblende with Fe/(Fe + Mg) < 0.65. Hence, it is noted that all hornblende compositions utilized in this study appropriately have Fe/(Fe + Mg) ratios in the range 0.42–0.50.
We thank Jonathan Miller, Bob Wiebe, and Ricardo Presnell for thoughtful and thorough reviews of this manuscript, and Bob Wiebe for inviting us into the field in the Madera Creek area of the Jackass Lakes pluton (JLP) and the many discussions through the years regarding mafic-felsic magma interaction in the JLP and in general. We also thank Brendan McNulty for discussions regarding JLP geology and logistical advice. The staff and rangers at Yosemite National Park were very helpful during our field work in the park. Support for this work was provided to Pignotta by a Geological Society of America Student Research Grant, a Sigma Xi Grant in Aid of Research, and a Graduate Student Research Grant from the University of Southern California Department of Earth Sciences. Partial support was provided by the U.S. Geological Survey, National Cooperative Geologic Mapping Program Support through an EDMAP (Educational Institution Component of Geologic Mapping Program) grant to Paterson.