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Abstract

To expand our evaluation of the Mesozoic Sierran arc, we need to consider the volcanic section that overlies the intrusive parts of the arc seen in Days 1–4. While doing so, we can discuss evidence for links between the magmatic and volcanic components of the arc, the nature of regional sedimentation and tectonism that accompanied active volcanism, and the challenges of estimating volume rates of volcanic activity in ancient arc systems. The most complete exposures of the volcanic section of the Mesozoic arc in the central Sierra occur in several pendants near the eastern topographic divide of the Sierra Nevada Mountains. These volcanic sections are exposed in the Saddlebag and Ritter Range pendants. We have completed new 1:24,000–1:10,000 scale mapping in four areas within these eastern pendants, which from north to south are: (1) Eagle Creek pendant around Twin Lakes, (2) the Virginia Canyon area, (3) the Saddlebag Lake pendant, and (4) the northern Ritter Range pendant…

Introduction

To expand our evaluation of the Mesozoic Sierran arc, we need to consider the volcanic section that overlies the intrusive parts of the arc seen in Days 1-4. While doing so, we can discuss evidence for links between the magmatic and volcanic components of the arc, the nature of regional sedimentation and tecto- nism that accompanied active volcanism, and the challenges of estimating volume rates of volcanic activity in ancient arc systems. The most complete exposures of the volcanic section of the Mesozoic arc in the central Sierra occur in several pendants near the eastern topographic divide of the Sierra Nevada Mountains. These volcanic sections are exposed in the Saddlebag and Ritter Range pendants. We have completed new 1:24,000-1:10,000 scale mapping in four areas within these eastern pendants, which from north to south are: (1) Eagle Creek pendant around Twin Lakes, (2) the Virginia Canyon area, (3) the Saddlebag Lake pendant, and (4) the northern Ritter Range pendant. We will share posters displaying maps, ages, structures, and chemistry from each area during the field trip, although we focus on the Saddlebag Lake section below. In this pendant, we will walk through a steeply dipping and westward-younging section that starts in Paleozoic sediments below the arc passes upward through Triassic, Jurassic, and local Cretaceous sections, and finishes in the eastern edge of the Tuolumne intrusive complex, where we will look at magmatic recycling and mingling (Fig. 5-1).

Figure 5-1.

Map of the Sawmill Canyon area. Red dots—trip stops. Mapping contributions from University of Southern California Undergraduate Team Research program, Jiri Žák, Sean Hartman, Vali Memeti, and Scott Paterson.

Figure 5-1.

Map of the Sawmill Canyon area. Red dots—trip stops. Mapping contributions from University of Southern California Undergraduate Team Research program, Jiri Žák, Sean Hartman, Vali Memeti, and Scott Paterson.

Previous work in the Saddlebag Lake pendant includes studies by Brook (1977), Brook et al. (1974), Keith and Seitz (1981), Kistler and Swanson (1981), Kistler (1993), Bateman et al. (1983) and Schweickert and Lahren (1993a, 1999, 2006). A particularly well exposed and varied section in this pendant occurs in the east-west–trending Sawmill Canyon area (Figs. 5-1 and 4-2). Here, the arc units are bound to the east by older deep- and shallow-water Paleozoic units and to the west by the Tuolumne Intrusive Complex (Fig. 5-1). From east to west, these units typically consist of (1) Paleozoic, chert-bearing, quartzite-dominated, metasediments interpreted by Schweickert and Lahren (2006) as parts of the Roberts Mountain allochthon; (2) probable Permian units including mafic metavolcanic units with local pillows, conglomerates (called Diablo Formation by Schweickert and Lahren, 2006), and mixed metasiltstones and metasandstones (called Candelaria Formation by Schweickert and Lahren, 2006); (3) a tectonically reactivated unconformity previously examined by Brook et al. (1974); (4) the Cooney Lake conglomerate and associated metasandstone and metavolcanic units; (5) the Koip sequence, a complex pile of andesitic to rhyolitic metavolcanic units with local volcaniclastic and metasedimentary beds; (6) a large, dextral-transpressive ductile shear zone, herein named the Steelhead Lake shear zone, which evolved to a complex brittle fault system, herein named the Steelhead Lake brittle fault system; (7) a package of moderately to thinly bedded metasedimentary rocks including quartzites, phyllites, calc-silicate rocks, metaconglomerates, and local marble and volcanic units (Sawmill Canyon sequence of Schweickert and Lahren, 2006); and (8) a Cretaceous sequence of less deformed, typically dacitic to rhyolitic, clastic to phenocryst-bearing metavolcanic rocks, which are exposed immediately along the margin of the Tuolumne Intrusive Complex. Here, the Tuolumne Intrusive Complex consists of the Kuna Crest and Half Dome phases that are truncated by the Cathedral Peak unit of the batholith. For a detailed description of the complicated relationships between these batholithic units in the Sawmill Canyon area, see Paterson et al. (2008).

Barth et al. (2011) report four sensitive high-resolution ion microprobe (SHRIMP) U-Pb single-zircon ages in this region, a 221 Ma age from a dioritic body near Odell Lake (just north of Saddlebag Lake), and three ages from rhyolitic units in the Koip sequence just south of Sawmill Canyon that range from 232 Ma (near base of section) to 219 Ma (at top of section just east of the Steelhead Lake brittle fault). We also have obtained a series of new ages as follows: (1) an 88.5 Ma chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon age from the Cathedral Peak phase of the Tuolumne Intrusive Complex near Steelhead Lake; (2) a 221 Ma laser ablation–inductively coupled plasma–mass spectrometry (LA- ICP-MS) age from a diorite body at Saddlebag Lake; (3) LA- ICP-MS zircon ages from four metavolcanic samples, two from the Koip sequence, which have ages of 216 and 227 Ma, and two from the western belt of metavolcanic rocks, which have ages of 95 and 113 Ma; (4) LA-ICP-MS ages from detrital zircons in four samples from the Sawmill Canyon sequence just west of the Steelhead Lake brittle fault that have minimum zircon age peaks between 172 and 189 Ma and older Paleozoic and Precambrian zircon age peaks; and (5) LA-ICP-MS ages from detrital zircons of one sample from a metasandstone associated with the Cooney Lake conglomerate that has a continuous spread of zircon ages from 238 to 271 Ma and two older zircon ages ca. 405 Ma. Our new ages agree with those reported by Barth et al. (2011) for the Koip sequence and associated ca. 221 Ma plutons. They also match well with our new ages obtained in the Eagle Creek, Virginia Canyon, and northern Ritter Range areas.

Figure 5-2.

Panorama of Sawmill Canyon area in the Saddlebag Lake pendant. Viewpoint is from ridge to the south of Sawmill looking north along the pendant. Red star marks location of Stop 5-1 overview point. White dashed line—steeply dipping unconformity between Mesozoic arc (to left) and Paleozoic sedimentary rock (to right). Yellow lines—rhyolite tuff boundaries: R2—rhyolite tuff of Saddlebag Lake (222 Ma), R3—rhyodacite tuff of Greenstone Lake (U-Pb 219 Ma). Pink line—eastern edge of Tuolumne Intrusive Complex (TIC).

Figure 5-2.

Panorama of Sawmill Canyon area in the Saddlebag Lake pendant. Viewpoint is from ridge to the south of Sawmill looking north along the pendant. Red star marks location of Stop 5-1 overview point. White dashed line—steeply dipping unconformity between Mesozoic arc (to left) and Paleozoic sedimentary rock (to right). Yellow lines—rhyolite tuff boundaries: R2—rhyolite tuff of Saddlebag Lake (222 Ma), R3—rhyodacite tuff of Greenstone Lake (U-Pb 219 Ma). Pink line—eastern edge of Tuolumne Intrusive Complex (TIC).

Stratigraphic units in the Saddlebag pendant region generally have steeply west-dipping bedding and contain a steeply west-dipping, mostly bedding-parallel foliation axial planar to both rare, large-scale, moderately plunging folds and more common, small-scale folds with variable plunges in which bedding and sometimes an older mineral fabric are transposed. Mineral lineations are widespread and outside of shear zones typically plunge steeply. Fabric intensities and associated strains measured from clastic objects are heterogeneous but the latter increase up to >85% shortening perpendicular to foliation and >175% extension parallel to lineation. The western Cretaceous metavolcanics tend to have lower average strain and moderately to steeply dipping bedding implying that an unconformity and/or fault exist between these units and the underlying Jurassic section.

Triassic, rare Jurassic, and widespread Cretaceous plutons occur in the broader Saddlebag pendant area and intrude the already steeply dipping volcanic and sedimentary rocks described above. Barth et al. (2011) describe the Triassic plutons just to the east of this pendant called the granite of Lee Vining Canyon (218–220 Ma) parts of the Scheelite Intrusive Suite. We have dated and begun to examine two Jurassic plutons north of here called the Green Lakes pluton (165 Ma; Mundil et al., 2004) and the Buckeye pluton (174 Ma, W. Cao, 2013, personal commun.). Widespread Cretaceous plutons also occur with our new ages establishing that an increasing number of 95-97 Ma bodies occur along the margins of the Tuolumne Intrusive Complex. The Tuolumne Intrusive Complex seen in Day 4 is by far the largest Cretaceous pluton and intrudes out the western edge of the Saddlebag Lake pendant. In the Sawmill area, the Half Dome, Kuna Crest, and Cathedral Peak intrusives are compositionally similar to the same rocks we report for other regions of the Tuolumne Intrusive Complex (Paterson et al., 2008). East of the Steelhead Lake shear zone, we have one analysis of the Triassic diorite intrusion occupying the southeast end of Saddlebag Lake. At 58.1 wt% SiO2, it is a quartz diorite, but with unusually high Al2O3 (19.2 wt%), a feature shared with the Triassic metavolcanic rocks in this area. Otherwise, this diorite body is calc-alkaline, metaluminous, and high K in composition. This one analysis is more mafic than the 62 to 77 wt% silica range reported for the Triassic Scheelite Intrusive Suite by Barth et al. (2011), but otherwise has similarities in being both rather calcic and potassic for its level of SiO2.

The ~1–3-km-wide Steelhead Lake shear zone deforms the western portion of the Koip sequence, all of the Sawmill Canyon sequence and the outermost margin of the Tuolumne Intrusive Complex. Within this zone, dextral shear indicators become common in approximately subhorizontal surfaces, the intensity of strain increases, and the mineral lineation tends to rotate toward more southeast or northwest orientations with moderate to steep plunges. In this shear zone, and located at the immediate contact between the Koip and Sawmill Canyon sequences, we have discovered a variety of features, including fault scarps, breccias, brittlely deformed and truncated leucogranite dikes of Cathedral Peak granite, and local pseudotachylites (Whitesides et al., 2010). Brittle kinematic indicators are locally variable but in general also indicate oblique dextral slip on the main NW-striking faults. Thus, our present interpretation is that this brittle fault system represents continued motion during cooling of the formerly wider ductile shear zone. We have obtained two 40Ar/39Ar cooling ages of 84.4 ± 0.2 Ma and 83.7 ± 0.3 Ma (P. Renne, 2012, personal commun.) from coarse and fine biotite, respectively, in a sample from ductilely sheared rocks a few meters from the main brittle fault near Steelhead Lake, indicating that the transition from ductile to brittle motion occurred ca. 80–84 Ma.

We have new chemical data for the Triassic and Cretaceous metavolcanic rocks, which are similar and range from basalt (Triassic only) to rhyolite. Interestingly, the Triassic suite includes both tholeiitic (basalt to andesite) and calc-alkaline (rhyolite) members, whereas the Cretaceous volcanics (andesite to rhyolite) are all calc-alkaline in composition.

Studies by Hanson et al. (1993) and Sorensen et al. (1998) on metavolcanic rocks in the Ritter Range, correlative with those seen here, established that in addition to low-grade regional metamorphism, three metamorphic events affected these rocks: (1) widespread K-Na metasomatism caused by interactions with seawater in Jurassic and older units; (2) 200–400 °C mild alkali Na-Ca alteration caused by meteoric fluids; and (3) ~400 °C local alteration by fluids around Cretaceous plutons, typically represented by veins of quartz, chlorite, epidote, actinolite, and chalcopyrite. Our initial geochemistry on the metavolcanic rocks in the Saddlebag Lake pendant shows considerable spread in K and Na suggestive of alteration and element mobility, as noted by these authors. For example, a 222 Ma metarhyolite has whole- rock and quartz δ180 values that are elevated, 11.6%c and 12.6%e, respectively (Lackey et al., 2008), but zircon (6.2%e), shows a magmatic δ180 typical for Triassic volcanic rocks (Jade Star Lackey, 2012, personal commun.). Metamorphic garnet has high δ180 (~9.0%e) as well. Thus, hydrothermal alteration by seawater is inferred to have elevated whole-rock and quartz phenocryst values, not affecting zircon, which is resistant to oxygen isotope exchange. This alteration occurred before burial and contact metamorphism, at moderate temperature (150–300 °C), which is consistent with findings by Hanson et al. (1993) and Sorensen et al. (1998) for the metavolcanic rocks to the south.

For the nearby ca. 88 Ma Cathedral Peak phase of the Tuolumne Intrusive Complex, we determined by Al-in-horn blende-plagioclase thermobarometry (Anderson and Smith, 1995; Anderson 1996; Holland and Blundy, 1994) emplacement at 1.9 ± 0.3 kbar and 745 ± 29 °C. However, we also found that several hornblende and plagioclase pairs yielded high-temperature, subsolidus results with estimated temperatures ranging from 630 to 670 °C. These likely reflect conditions during shearing in the Steelhead Lake ductile shear zone and cooling of the Tuolumne Intrusive Complex.

Road Log

Mileage Directions

0. 0 Mammoth Inn at Mammoth Ski resort.

4.0 Turn left (east) onto Main Street.

7.8 Go under Highway, turn left, and head north on Highway 395.

31.8 Turn left onto Highway 120 and head toward Yosemite National Park.

41.4 Turn right onto Saddlebag Lake road.

41.5 Keep right.

43.0 Sawmill Canyon trailhead.

43.5 Stop 1.

STOP 1: Overview Sawmill Canyon

Pull-out to left for overview of Sawmill Canyon area. You are standing on the Paleozoic Palmetto Formation (this location = red star in Fig. 5-2), looking across the tilted Mesozoic volcanic arc section, and seeing the eastern edge of the Tuolumne Intrusive Complex (Figs. 5-1 and 5-2). We will hike up the ridge just south of Sawmill Canyon through this arc section. Figure 5-2 provides an overview of this region from a southern position looking north along the Saddlebag Lake pendant.

Turn around and drive back to Sawmill Canyon trailhead and park. Take your lunch, water, and plenty of sun protection for hiking in the Sawmill Canyon area for the remainder of the day.

Trail Log

[All UTMs are in Nad27 Conus and zone 11S.]

[Protolith rather than metamorphic rock names are used whenever possible.]

Hike west from the Sawmill campground down to Lee Vining Creek: good shallow river crossing at (0300436, 4203423).

STOP 2: Deep Marine Sediments below Arc

(0300419, 4203312)

Here we will examine folded and steeply dipping cherts, shales, siltstones, and sandstones, interpreted by Schweickert and Lahren (2006) to be part of the Ordovician–Devonian Palmetto Formation. Up to four periods of deformation occur in this mélange-like unit. In one nearby sample, we were able to obtain only a few detrital zircon ages from this unit, which range from ca. 360 Ma to 2.8 Ga. In a second sample near Tioga Pass, we dated 94 zircons ranging from 426 Ma to 3.7 Ga.

STOP 3: Base of Mesozoic Arc (Koip Sequence)

(0300379, 4203269)

We have now walked across the tectonized, angular unconformity (Fig. 5-3), separating Paleozoic sediments and units at the base of the Mesozoic arc (Brook et al., 1974). Locally, metavolcanic rocks occur immediately above (west) of this unconformity, such as the Black Mountain tuff: Barth et al. (2011) obtained an age of 232 Ma from this tuff. But more often sandstones, local volcanic rocks, and conglomerates of the Cooney Lake polymict conglomerate occur above the unconformity (Fig. 5-3). Schweickert and Lahren (2006) provide an excellent description of these conglomerates and sandstones. From one of the sandstone layers just south of this stop, we have obtained detrital zircon ages that define a single broad peak with ages ranging between 238 and ca. 270 Ma (Fig. 5-4). This limited age range supports a restricted local arc source and is compatible with a rock age close to the 232 Ma age reported from nearby volcanic rocks by Barth et al. (2011), although drawing attention to the 6 m.y. gap between the youngest detrital zircon age and likely rock age. Note also the cross-bedding and grading, both of which indicate younging up (west) in these steeply dipping units. We have also dated zircons in four other Cooney Lake samples . collected along strike from Tioga Pass to near Twin Lakes: zircon ages range continuously from 216 Ma to 260 Ma.

Figure 5-3.

Photo (looking south) of steeply dipping, angular unconformity (just to right of map board). Paleozoic deep-water sediments to left and Mesozoic arc volcanics and sedimentary units to the right.

Figure 5-3.

Photo (looking south) of steeply dipping, angular unconformity (just to right of map board). Paleozoic deep-water sediments to left and Mesozoic arc volcanics and sedimentary units to the right.

Figure 5-4.

Laser ablation–inductively coupled plasma mass spectrometry U-Pb detrital zircon ages from a sandstone sample in the Cooney Lake conglomerate. Length of vertical lines shows errors on individual age. MSWD— mean square of weighted deviates

Figure 5-4.

Laser ablation–inductively coupled plasma mass spectrometry U-Pb detrital zircon ages from a sandstone sample in the Cooney Lake conglomerate. Length of vertical lines shows errors on individual age. MSWD— mean square of weighted deviates

(0300311, 4203079): Now walking across Triassic, fining up (west) sequence of polymict conglomerates overlain by flow and air-fall volcanic deposits. Compositions of volcanics vary from dacite to andesite.

STOP 4: Eastern Edge of Steelhead Lake Shear Zone

(0299822, 4203030)

Figure 5-5.

Metasediments with dextral shear-sense indicators including asymmetrically boudinaged veins.

Figure 5-5.

Metasediments with dextral shear-sense indicators including asymmetrically boudinaged veins.

We have been walking through andesitic to dacitic, often clast-rich metavolcanic rocks. At this location, we begin to see structural evidence of oblique dextral shear in the Steelhead Lake shear zone (Fig. 5-5), the likely northern continuation of the Sierra Crest shear zone (Tikoff and Greene, 1997). We also begin to see an increase in the number and thickness of quartz veins toward the Tuolumne Intrusive Complex.

(0299709, 4202966): Excellent shear-sense indicators including large shear bands.

STOP 5: Contact between Triassic and Jurassic Packages

(0299448, 4202856)

We have now reached a major contact between the Triassic volcanic rocks (Koip sequence) and a poorly studied sequence of Jurassic, well-bedded marine sediments and local volcanic layers (Sawmill Canyon sequence). The boundary is marked by both dextral ductile and brittle faulting, with the latter marked by large quartz veins and breccias. We have discovered and begun to map out a very complex and laterally extensive network of brittle faulting and will briefly present what we have so far learned about this fault system (see also Stop 5-6). We have now dated detrital zircons from a number of metasandstone samples in these units, all with minimum age peaks at 170–175 Ma and both older, Paleozoic and Precambrian ages (Fig. 5-6).

(0299253, 4202665): Here, we are walking over a zone of folded metasediments, epidote-rich zones, and local quartz-rich vugs often associated with the quartz veins and brittle faults. Inspection of the vugs suggests that high fluid pressures caused cavities to remain open in quartz veins, forming vugs with free inward growth of minerals.

STOP 6: Jurassic Units and Pseudotachylite

(0299253, 4202619)

From Stop 5 to here, we have been walking across mixed metasedimentary and metavolcanic units deformed by both, earlier ductile shear and slightly younger brittle faulting, the latter associated with widespread brecciation and quartz veins. The metasedimentary units include sandstones (locally cross-bedded), shales, rare conglomerates, calc-silicate units, and volcaniclastics. Volcanic units here are typically thinner bedded and fine grained and presumably represent more distal deposits. At this location, we also find ultracataclasite and/or potential pseudotachylite, which suggests dry fault-slip events during paleoearth- quakes in contrast to the huge fluid volumes suggested by the large quartz veins (Fig. 5-7). The transition between ductile to brittle dextral shear is also intriguing and presently the focus of further study. Near Steelhead Lake, just north of here, we have obtained 40Ar/39Ar biotite cooling ages of ca. 83–84 Ma from ductilely sheared rocks adjacent to the brittle fault, suggesting that the transition between ductile and brittle faulting occurred at ca. 80-83 Ma. This transition was driven by the shutting down of active magmatism and thus widespread cooling of the arc, plus slow exhumation of the Sierra Nevada. The transition is associated with an evolving fluid-flow history documented in four different vein sets: (1) early tourmaline-bearing leucogranite veins from the ca. 88 Ma Cathedral Peak granodiorite that are synductile deformation but cut by the brittle faulting; (2) tourmaline-rich veins; (3) thin chlorite-epidote-bearing veins typically associated with leaching halos; and (4) quartz veins associated with brecciation and brittle faulting.

Figure 5-6.

Detrital zircon age distribution (normalized probability) plots of all zircon ages obtained from five different samples collected from the Jurassic sedimentary sequence (Sawmill Canyon sequence). Large inset shows enlargement of Phanerozoic peaks. Close inspection of minimum zircon age peaks indicates a maximum rock age of ca. 175 Ma. Also note the presence of Precambrian zircons in these samples, which precludes a volcanic origin.

Figure 5-6.

Detrital zircon age distribution (normalized probability) plots of all zircon ages obtained from five different samples collected from the Jurassic sedimentary sequence (Sawmill Canyon sequence). Large inset shows enlargement of Phanerozoic peaks. Close inspection of minimum zircon age peaks indicates a maximum rock age of ca. 175 Ma. Also note the presence of Precambrian zircons in these samples, which precludes a volcanic origin.

Figure 5-7.

Quartz vein breccias. These are widespread features in this part of the Sawmill Canyon area and are always associated with brittle faults.

Figure 5-7.

Quartz vein breccias. These are widespread features in this part of the Sawmill Canyon area and are always associated with brittle faults.

Cretaceous Volcanics on Ridge to West

These tend to consist of more massive, thicker bedded units, with less ductile strain and shallower bedding-plane dips than the Jurassic units. The contact between the Jurassic and Cretaceous units here must have been an unconformity but is now also deformed. We have obtained two LA-ICP-MS zircon ages from these volcanics at ca. 113 and ca. 95 Ma. Our working hypothesis is that these volcanics are related to similar Cretaceous Minarets Caldera volcanics in the Ritter Range area studied by Tobisch et al. (2000).

(0298908, 4202988): Boudinaged quartz draining from leucogranite vein.

Here, we are still in mixed metasediments and volcanic rocks. An example of quartz-rich material draining off of a boudinaged leucogranitic dike to form thin quartz veins. Cretaceous volcanics can be seen in the hills across the lake to the southwest.

(0298659, 4203077): Increased diking in metasediments near the Tuolumne Intrusive Complex contact.

Near the Tuolumne Intrusive Complex, we find an increasing number of dikes, here from the hornblende-bearing Half Dome granodiorite phase of the Tuolumne Intrusive Complex. Migmatitic terranes with thin leucosomes also begin to appear in some domains raising questions about in situ melting and potential assimilation as the Tuolumne Intrusive Complex is approached.

STOP 7: Margin of the Tuolumne Intrusive Complex and Internal Sheeting

(0298652, 4203464)

We are now at the eastern margin of the Tuolumne Intrusive Complex (Fig. 5-8). Note the highly irregular and stepped margins (often ~90° changes in orientation), dikes, and local migmatites. In the intrusive complex, we are standing in a very complex “sheeting” or mingling zone described by Paterson et al. (2008). Here, we find a number of these zones formed along ~E-W cracks cutting through the Kuna Crest and Half Dome units (Fig. 5-8B). The largest of these forms a ~70° bend and strikes N-S along the Tuolumne Intrusive Complex margin where we are presently standing. Note that the N-S-trending Kuna and Half Dome units are truncated at this sheeted zone. We find two domains of Half Dome granodiorite north of this sheeted zone both surrounded by the Cathedral Peak granodiorite (Fig. 5-8A). In this larger sheeted zone, we will examine an impressive array of magmatic structures, often identical to those we talked about on Day 4. Our present interpretation of this zone is that large pieces of older magma mush to solid phases of the Tuolumne Intrusive Complex were broken off by an internal stoping-like process during which new Cathedral Peak magma interacted with still hypersolidus Half Dome magmas and intruded into regions where blocks were being displaced. Some intriguing connections with the synemplacement ductile shearing in the nearby host rocks indicates that the regional dextral transpression likely played a role in driving the magmatic stoping process. The magmatic structures we will examine reflect the dynamic, unstable environment during internal stoping and magma mixing and pulsing.

STOP 8: “Double Stoping,” Tubes, Troughs, and Mingling

(0298564, 4203412)

As we move into the “sheeted zone,” we will cross an impressive collection of different styles of magmatic layering and troughs, tubes, magmatic folds and faults, enclaves, dikes, cognate inclusions, and xenoliths (Figs. 5-9, 5-10, and 5-11; see also Paterson, 2009). Cognate inclusions of Kuna Crest (Fig. 5-11) and both porphyritic Half Dome and Cathedral Peak magmas and their differentiates all occur in this zone.

STOP 9: Stepped Pluton Margin, Migmatites, and Sideways Diapir

(0298561, 4203309)

We have returned to a stepped part of the eastern contact of the Tuolumne Intrusive Complex and Jurassic host rocks (Fig. 5-8). Syndeformational migmatitic zones in the host rock are well displayed here. Comparison of boudinaged magmatic sheets and host rock shows clear examples of rheological reversal (magma becomes stronger) during emplacement. Also note gorgeous magmatic diapir of Kuna Crest magma moving sideways from chamber wall along with displaced fragments of host rock (Fig. 5-12). Here, we can continue our discussion about the implications of a “sideways moving” diapir that we started yesterday at Stop 4-4.

Figure 5-8.

Map of complex sheeted and recycling zones in the margin of the Tuolumne Intrusive Complex (TIC) at the western end of Sawmill Canyon (modified from Paterson et al., 2008). Note the truncation of the Kuna Crest and Half Dome units in this area. Also note the stepped margin of the TIC. Numbers in “B”—field trip stops.

Figure 5-8.

Map of complex sheeted and recycling zones in the margin of the Tuolumne Intrusive Complex (TIC) at the western end of Sawmill Canyon (modified from Paterson et al., 2008). Note the truncation of the Kuna Crest and Half Dome units in this area. Also note the stepped margin of the TIC. Numbers in “B”—field trip stops.

STOP 10: Edge of Truncation Zone, Schlieren Troughs, K-Feldspar Clusters, and Magmatic Faulting

(0298387, 4203289)

We will now look at two related exposures right at the southern edge of the truncation zone (Fig. 5-8). Here, we are looking at schlieren-bounded troughs, local faulting, K-feldspar accumulations, some deformed troughs, local magmatic faults, and late Cathedral Peak dikes (Fig. 5-13). The top of this exposure is the northern end of the Kuna Crest granodiorite. To the west and slightly uphill of this outcrop is another exposure (the famous “George Bergantz outcrop”) displaying spectacular troughs, magmatic folds and faults, and younging directions (Fig. 5-14). Normal Half Dome granodiorite occurs to the south of this outcrop. This exposure also marks the western end of the sheeted zone (Fig. 5-8) where it is intruded out to the west by the Cathedral Peak granodiorite.

Figure 5-9.

Magmatic folding and faulting (at ruler) of sheeted zone.

Figure 5-9.

Magmatic folding and faulting (at ruler) of sheeted zone.

Figure 5-10.

Block of metamorphic host rock intruded by Kuna Crest granodiorite, now surrounded by Half Dome and Cathedral Peak granodiorites. We interpret this example as a host-rock block stoped into Kuna Crest granodiorite, which was re-stoped into the Half Dome intrusion and thus became a “double stoped block.”

Figure 5-10.

Block of metamorphic host rock intruded by Kuna Crest granodiorite, now surrounded by Half Dome and Cathedral Peak granodiorites. We interpret this example as a host-rock block stoped into Kuna Crest granodiorite, which was re-stoped into the Half Dome intrusion and thus became a “double stoped block.”

Figure 5-11.

Steeply plunging stationary tube of porphyritic Half Dome magma, which is now locally intruded by Cathedral Peak granodiorite and pegmatitic dikes. Tube diameter decreases with time. See Paterson et al. (2008) for further description.

Figure 5-11.

Steeply plunging stationary tube of porphyritic Half Dome magma, which is now locally intruded by Cathedral Peak granodiorite and pegmatitic dikes. Tube diameter decreases with time. See Paterson et al. (2008) for further description.

Figure 5-12.

Gorgeous magmatic diapir of Kuna Crest granodiorite moving away from chamber wall along with displaced fragments of host rock. Note mushroom head and narrow tail plus folding of hostrock layering under mushroom cap, all consistent with structures seen in other diapirs.

Figure 5-12.

Gorgeous magmatic diapir of Kuna Crest granodiorite moving away from chamber wall along with displaced fragments of host rock. Note mushroom head and narrow tail plus folding of hostrock layering under mushroom cap, all consistent with structures seen in other diapirs.

Figure 5-13

Edge of sheeted zone (looking south) exposed at Stop 10 with main phase of the Kuna Crest granodiorite at the top of the photo. Both Half Dome and Cathedral Peak phases occur in this exposure below the truncated Kuna Crest unit. Late leucogranite dikes from Cathedral Peak. Note trough truncations indicate younging in sheeted zone is up or toward the older margin.

Figure 5-13

Edge of sheeted zone (looking south) exposed at Stop 10 with main phase of the Kuna Crest granodiorite at the top of the photo. Both Half Dome and Cathedral Peak phases occur in this exposure below the truncated Kuna Crest unit. Late leucogranite dikes from Cathedral Peak. Note trough truncations indicate younging in sheeted zone is up or toward the older margin.

Figure 5-14.

Three sets of amazing schlieren-bounded troughs young- ing from right to left in photo and consisting of the porphyritic phase of the Half Dome. The early set (to right) is magmatically folded. The second (middle) set shows a magmatic fault. These troughs are cut by the main phase of the Cathedral Peak just a short distance to the right of this photo. The main truncated phase of the Half Dome is to the south (behind) this photo. Late leucogranitic dike cutting across all troughs is derived from the Cathedral Peak unit. This exposure is informally called the “George Bergantz” outcrop.

Figure 5-14.

Three sets of amazing schlieren-bounded troughs young- ing from right to left in photo and consisting of the porphyritic phase of the Half Dome. The early set (to right) is magmatically folded. The second (middle) set shows a magmatic fault. These troughs are cut by the main phase of the Cathedral Peak just a short distance to the right of this photo. The main truncated phase of the Half Dome is to the south (behind) this photo. Late leucogranitic dike cutting across all troughs is derived from the Cathedral Peak unit. This exposure is informally called the “George Bergantz” outcrop.

STOP 11: K-Feldspar Megacryst-Rich Pipe and Bounding Tubes

(0298569, 4203542)

After walking back through the sheeted zone, we will examine a large, steeply plunging K-feldspar megacryst-rich pipe and bounding migrating tubes in which the pipe intruded through older sheeting. Also note small host-rock pieces in the pipe. We do not have chemistry from these units and are thus uncertain whether the pipe is an accumulation from porphyritic Half Dome or Cathedral Peak magmas.

Descend back down (east) Sawmill Canyon on a good trail to Sawmill Canyon campground and car parking. We will pass back through both the Jurassic and Triassic sections of the arc.

References Cited

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315, p.
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,
J.L.
Smith
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D.R.
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The American Mineralogist
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80
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Barth
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Wooden
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Riggs
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Schweickert
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R.A.
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2011
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Early Mesozoic pluton ism and volcanism in the east-central Sierra Nevada of California: Geosphere
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4
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877
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P.C.
Kistler
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R.W.
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A.J.
,
1983
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Geologic map of the Tuolumne Meadows Quadrangle, Yosemite National Park, California
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 :
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,
500
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,
C.A.
,
1977
,
Stratigraphy and structure of the Saddlebag Lake roof pendant, Sierra Nevada, California:
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C.A.
Nokleberg
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W.J.
Kistler
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R.W.
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1974
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Nature of the angular unconformity between the Paleozoic metasedimentary rocks and the Mesozoic metavolcanic rocks in the eastern Sierra Nevada, California:
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571
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Hanson
,
R.B.
Sorensen
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S.S.
Barton
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Fiske
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R.S.
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1993
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Blundy
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J.
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W.J.
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J.F.
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1993
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, 489-10, 501, doi: 10.1029/JB086iB11p10489.
Lackey
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Valley
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2008
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The oxygen isotope record: Journal of Petrology
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Mundil
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2004
,
Geochronological constraints (40Ar/39Ar and U/Pb) on the thermal history of the Tuolumne Intrusive Suite (Sierra Nevada, California):
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2009
,
Magmatic tubes, troughs, pipes, and diapirs: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne Batholith, Sierra Nevada, California: Geosphere
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Zak
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Janousek
,
V.
,
2008
,
Growth of complex sheeted zones during recycling of older magmatic units into younger: Sawmill Canyon area, Tuolumne Batholith, Sierra Nevada, California:
Journal of Volcanology and Geothermal Research
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177
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2
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M.M.
,
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Lahren
,
M.M.
Trexler
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J.H.
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Mcdougall
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Acknowledgments

Paterson acknowledges support from National Science Foundation grants EAR-0537892 and EAR-0073943 and three years of financial student support through the geologic mapping education component (EDMAP) of the U.S. Geological Survey National Cooperative Geologic Mapping Program. We thank numerous University of Southern California (USC) graduate and undergraduate students, many associated with USC’s Undergraduate Team Research program, for their assistance in constructing excellent maps and obtaining LA-ICP-MS zircon ages in the Sawmill Canyon area. Yosemite National Park Rangers are gratefully acknowledged for their constant support and interest in our work.

Figures & Tables

Figure 5-1.

Map of the Sawmill Canyon area. Red dots—trip stops. Mapping contributions from University of Southern California Undergraduate Team Research program, Jiri Žák, Sean Hartman, Vali Memeti, and Scott Paterson.

Figure 5-1.

Map of the Sawmill Canyon area. Red dots—trip stops. Mapping contributions from University of Southern California Undergraduate Team Research program, Jiri Žák, Sean Hartman, Vali Memeti, and Scott Paterson.

Figure 5-2.

Panorama of Sawmill Canyon area in the Saddlebag Lake pendant. Viewpoint is from ridge to the south of Sawmill looking north along the pendant. Red star marks location of Stop 5-1 overview point. White dashed line—steeply dipping unconformity between Mesozoic arc (to left) and Paleozoic sedimentary rock (to right). Yellow lines—rhyolite tuff boundaries: R2—rhyolite tuff of Saddlebag Lake (222 Ma), R3—rhyodacite tuff of Greenstone Lake (U-Pb 219 Ma). Pink line—eastern edge of Tuolumne Intrusive Complex (TIC).

Figure 5-2.

Panorama of Sawmill Canyon area in the Saddlebag Lake pendant. Viewpoint is from ridge to the south of Sawmill looking north along the pendant. Red star marks location of Stop 5-1 overview point. White dashed line—steeply dipping unconformity between Mesozoic arc (to left) and Paleozoic sedimentary rock (to right). Yellow lines—rhyolite tuff boundaries: R2—rhyolite tuff of Saddlebag Lake (222 Ma), R3—rhyodacite tuff of Greenstone Lake (U-Pb 219 Ma). Pink line—eastern edge of Tuolumne Intrusive Complex (TIC).

Figure 5-3.

Photo (looking south) of steeply dipping, angular unconformity (just to right of map board). Paleozoic deep-water sediments to left and Mesozoic arc volcanics and sedimentary units to the right.

Figure 5-3.

Photo (looking south) of steeply dipping, angular unconformity (just to right of map board). Paleozoic deep-water sediments to left and Mesozoic arc volcanics and sedimentary units to the right.

Figure 5-4.

Laser ablation–inductively coupled plasma mass spectrometry U-Pb detrital zircon ages from a sandstone sample in the Cooney Lake conglomerate. Length of vertical lines shows errors on individual age. MSWD— mean square of weighted deviates

Figure 5-4.

Laser ablation–inductively coupled plasma mass spectrometry U-Pb detrital zircon ages from a sandstone sample in the Cooney Lake conglomerate. Length of vertical lines shows errors on individual age. MSWD— mean square of weighted deviates

Figure 5-5.

Metasediments with dextral shear-sense indicators including asymmetrically boudinaged veins.

Figure 5-5.

Metasediments with dextral shear-sense indicators including asymmetrically boudinaged veins.

Figure 5-6.

Detrital zircon age distribution (normalized probability) plots of all zircon ages obtained from five different samples collected from the Jurassic sedimentary sequence (Sawmill Canyon sequence). Large inset shows enlargement of Phanerozoic peaks. Close inspection of minimum zircon age peaks indicates a maximum rock age of ca. 175 Ma. Also note the presence of Precambrian zircons in these samples, which precludes a volcanic origin.

Figure 5-6.

Detrital zircon age distribution (normalized probability) plots of all zircon ages obtained from five different samples collected from the Jurassic sedimentary sequence (Sawmill Canyon sequence). Large inset shows enlargement of Phanerozoic peaks. Close inspection of minimum zircon age peaks indicates a maximum rock age of ca. 175 Ma. Also note the presence of Precambrian zircons in these samples, which precludes a volcanic origin.

Figure 5-7.

Quartz vein breccias. These are widespread features in this part of the Sawmill Canyon area and are always associated with brittle faults.

Figure 5-7.

Quartz vein breccias. These are widespread features in this part of the Sawmill Canyon area and are always associated with brittle faults.

Figure 5-8.

Map of complex sheeted and recycling zones in the margin of the Tuolumne Intrusive Complex (TIC) at the western end of Sawmill Canyon (modified from Paterson et al., 2008). Note the truncation of the Kuna Crest and Half Dome units in this area. Also note the stepped margin of the TIC. Numbers in “B”—field trip stops.

Figure 5-8.

Map of complex sheeted and recycling zones in the margin of the Tuolumne Intrusive Complex (TIC) at the western end of Sawmill Canyon (modified from Paterson et al., 2008). Note the truncation of the Kuna Crest and Half Dome units in this area. Also note the stepped margin of the TIC. Numbers in “B”—field trip stops.

Figure 5-9.

Magmatic folding and faulting (at ruler) of sheeted zone.

Figure 5-9.

Magmatic folding and faulting (at ruler) of sheeted zone.

Figure 5-10.

Block of metamorphic host rock intruded by Kuna Crest granodiorite, now surrounded by Half Dome and Cathedral Peak granodiorites. We interpret this example as a host-rock block stoped into Kuna Crest granodiorite, which was re-stoped into the Half Dome intrusion and thus became a “double stoped block.”

Figure 5-10.

Block of metamorphic host rock intruded by Kuna Crest granodiorite, now surrounded by Half Dome and Cathedral Peak granodiorites. We interpret this example as a host-rock block stoped into Kuna Crest granodiorite, which was re-stoped into the Half Dome intrusion and thus became a “double stoped block.”

Figure 5-11.

Steeply plunging stationary tube of porphyritic Half Dome magma, which is now locally intruded by Cathedral Peak granodiorite and pegmatitic dikes. Tube diameter decreases with time. See Paterson et al. (2008) for further description.

Figure 5-11.

Steeply plunging stationary tube of porphyritic Half Dome magma, which is now locally intruded by Cathedral Peak granodiorite and pegmatitic dikes. Tube diameter decreases with time. See Paterson et al. (2008) for further description.

Figure 5-12.

Gorgeous magmatic diapir of Kuna Crest granodiorite moving away from chamber wall along with displaced fragments of host rock. Note mushroom head and narrow tail plus folding of hostrock layering under mushroom cap, all consistent with structures seen in other diapirs.

Figure 5-12.

Gorgeous magmatic diapir of Kuna Crest granodiorite moving away from chamber wall along with displaced fragments of host rock. Note mushroom head and narrow tail plus folding of hostrock layering under mushroom cap, all consistent with structures seen in other diapirs.

Figure 5-13

Edge of sheeted zone (looking south) exposed at Stop 10 with main phase of the Kuna Crest granodiorite at the top of the photo. Both Half Dome and Cathedral Peak phases occur in this exposure below the truncated Kuna Crest unit. Late leucogranite dikes from Cathedral Peak. Note trough truncations indicate younging in sheeted zone is up or toward the older margin.

Figure 5-13

Edge of sheeted zone (looking south) exposed at Stop 10 with main phase of the Kuna Crest granodiorite at the top of the photo. Both Half Dome and Cathedral Peak phases occur in this exposure below the truncated Kuna Crest unit. Late leucogranite dikes from Cathedral Peak. Note trough truncations indicate younging in sheeted zone is up or toward the older margin.

Figure 5-14.

Three sets of amazing schlieren-bounded troughs young- ing from right to left in photo and consisting of the porphyritic phase of the Half Dome. The early set (to right) is magmatically folded. The second (middle) set shows a magmatic fault. These troughs are cut by the main phase of the Cathedral Peak just a short distance to the right of this photo. The main truncated phase of the Half Dome is to the south (behind) this photo. Late leucogranitic dike cutting across all troughs is derived from the Cathedral Peak unit. This exposure is informally called the “George Bergantz” outcrop.

Figure 5-14.

Three sets of amazing schlieren-bounded troughs young- ing from right to left in photo and consisting of the porphyritic phase of the Half Dome. The early set (to right) is magmatically folded. The second (middle) set shows a magmatic fault. These troughs are cut by the main phase of the Cathedral Peak just a short distance to the right of this photo. The main truncated phase of the Half Dome is to the south (behind) this photo. Late leucogranitic dike cutting across all troughs is derived from the Cathedral Peak unit. This exposure is informally called the “George Bergantz” outcrop.

Contents

References

References Cited

Anderson
,
J.L.
,
1996
, Status of thermobarometry in granitic batholiths, in
Brown
,
M.
Candela
,
P.A.
Peck
,
D.L.
Stephens
,
W.E.
Walker
,
R.J.
Zen
,
E.
, eds.,
The Third Hutton Symposium on the Origin of Granites and Related Rocks: Geological Society of America Special Paper
315, p.
125
138
.
Anderson
,
J.L.
Smith
,
D.R.
,
1995
,
The effect of temperature and oxygen fugacity on Al-in-hornblende barometry:
The American Mineralogist
 , v.
80
, p.
549
559
.
Barth
,
A.P.
Walker
,
J.D.
Wooden
,
J.L.
Riggs
,
N.R.
Schweickert
,
R.A.
,
2011
,
Birth of the Sierra Nevada magmatic arc:
Early Mesozoic pluton ism and volcanism in the east-central Sierra Nevada of California: Geosphere
 , v.
7
, no.
4
, p.
877
897
, doi: 10.1130/GES00661.1.
Bateman
,
P.C.
Kistler
,
R.W.
Peck
,
D.L.
Busacca
,
A.J.
,
1983
,
Geologic map of the Tuolumne Meadows Quadrangle, Yosemite National Park, California
: U.S. Geological Survey Geologic Quadrangle Map GQ-1570, scale 1
 :
62
,
500
.
Brook
,
C.A.
,
1977
,
Stratigraphy and structure of the Saddlebag Lake roof pendant, Sierra Nevada, California:
Geological Society of America Bulletin
 , v.
88
, p.
321
334
, doi:10.1130/0016-7606(1977)88<321:SAS0TS>2.0. C0;2.
Brook
,
C.A.
Nokleberg
,
W.J.
Kistler
,
R.W.
,
1974
,
Nature of the angular unconformity between the Paleozoic metasedimentary rocks and the Mesozoic metavolcanic rocks in the eastern Sierra Nevada, California:
Geological Society of America Bulletin
 , v. 85 p.
571
576
, doi: 10.1130/0016-7606(1974)85<571: NOTAUB>2.0.CO;2.
Hanson
,
R.B.
Sorensen
,
S.S.
Barton
,
M.D.
Fiske
,
R.S.
,
1993
,
Long-term evolution of fluid-rock interactions in magmatic arcs:
Evidence from the Ritter Range pendant, Sierra Nevada, California, and numerical modeling: Journal of Petrology
 , v.
34
, p.
23
62
, doi: 10.1093/petrology/34.1.23.
Holland
,
T.
Blundy
,
J.
,
1994
,
Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry:
Contributions to Mineralogy and Petrology
 , v.
116
, p.
433
447
, doi:10.1007 /BF00310910.
Keith
,
W.J.
Seitz
,
J.F.
,
1981
,
Geologic map of the Hoover Wilderness and adjacent study area, Mono and Tuolumne counties, California
: U.S. Geological Survey Miscellaneous Field Studies Map MF1101-A, scale 1
 :
62
,
500
.
Kistler
,
R.W.
,
1993
, Mesozoic intrabatholithic faulting, Sierra Nevada, California, in
Dunne
,
G.
Mcdougall
,
K.
, eds.,
Mesozoic Paleography of the Western United States—II: Pacific Section, SEPM (Society for Sedimentary Geology), Book 71
, p.
247
262
.
Kistler
,
R.W.
Swanson
,
S.E.
,
1981
,
Petrology and geochronology of metamorphosed volcanic rocks and a Middle Cretaceous volcanic neck in the east-central Sierra Nevada, California:
Journal of Geophysical Research
 , v.
86
, no. B11, p.
10
, 489-10, 501, doi: 10.1029/JB086iB11p10489.
Lackey
,
J.S.
Valley
,
J.W.
Chen
,
J.H.
Stockli
,
D.F.
,
2008
,
Evolving magma systems, crustal recycling, and alteration in the central Sierra Nevada batholith:
The oxygen isotope record: Journal of Petrology
 , v.
49
, p.
1397
1426
.
Mundil
,
R.
Nomade
,
S.
Paterson
,
S.
Renne
,
P.R.
,
2004
,
Geochronological constraints (40Ar/39Ar and U/Pb) on the thermal history of the Tuolumne Intrusive Suite (Sierra Nevada, California):
Eos (Transactions, American Geophysical Union)
 , v.
85
, p.
47
, abs. V53A-0616.
Paterson
,
S.R.
,
2009
,
Magmatic tubes, troughs, pipes, and diapirs: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne Batholith, Sierra Nevada, California: Geosphere
, v.
5
, no.
6
, p.
496
527
, doi:10.1130 /GES00214.1.
Paterson
,
S.R.
Zak
,
J.
Janousek
,
V.
,
2008
,
Growth of complex sheeted zones during recycling of older magmatic units into younger: Sawmill Canyon area, Tuolumne Batholith, Sierra Nevada, California:
Journal of Volcanology and Geothermal Research
 , v.
177
, no.
2
, p.
457
484
, doi: 10.1016/j.jvolgeores.2008.06.024.
Schweickert
,
R.A.
Lahren
,
M.M.
,
1993a
, Tectonics of the east-central Sierra Nevada–Saddlebag Lake and northern Ritter Range pendants, in
Lahren
,
M.M.
Trexler
,
J.H.
Jr
Spinosa
,
C.
, eds.,
1993
,
Crustal Evolution of the Great Basin and Sierra Nevada: Cordilleran/Rocky Mountains Section: Reno, Department of Geological Sciences, University of Nevada, Geological Society of America Guidebook
, p.
313
351
.
Schweickert
,
R.A.
Lahren
,
M.M.
,
1993b
, Triassic–Jurassic magmatic arc in eastern California and western Nevada: Arc evolution, cryptic tectonic breaks, and significance of the Mojave–Snow Lake fault, in
Dunn
,
G.
Mcdougall
,
K.
, eds.,
1993
,
Mesozoic
Paleogeography
of the Western United States—II: Pacific Section, SEPM (Society for Sedimentary Geology) Book 71
, p.
227
246
.
Schweickert
,
R.A.
Lahren
,
M.M.
,
1999
,
Triassic caldera at Tioga Pass, Yosemite National Park, California: Structural relationships and significance
:
Geological Society of America Bulletin
 , v.
111
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