Day 1: Guadalupe Igneous Complex
Published:January 01, 2014
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Keith D. Putirka, Joe Canchola, Megan McNaughton, Oscar Smith, Gerardo Torrez, Scott R. Paterson, Mihai Ducea, 2014. "Day 1: Guadalupe Igneous Complex", Formation of the Sierra Nevada Batholith, Vali Memeti, Scott R. Paterson, Keith D. Putirka
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We begin our journey through the Mesozoic Sierran arc with an examination of the Guadalupe Igneous Complex, a layered Jurassic pluton that intrudes into largely oceanic materials in the Foothills Terrane of the Western Metamorphic belt (Figs. 1-1 and 1-2). The Guadalupe Igneous Complex is an intuitively pleasing target to begin with because of its outboard (western) location and because it consists of some of the most mafic (>8% MgO gabbros) and felsic (high-silica and high-K2O granophyres and rhyolites) igneous units that we will see on this trip and thus raises some longstanding petrologic questions about the connections between mafic and felsic granitoids in arcs. It is also an exciting objective because of the preservation of its likely feeder zone (the Hornitos pluton), internal layering, and capping volcanics (Fig. 1-2)…
We begin our journey through the Mesozoic Sierran arc with an examination of the Guadalupe Igneous Complex, a layered Jurassic pluton that intrudes into largely oceanic materials in the Foothills Terrane of the Western Metamorphic belt (Figs. 1-1 and 1-2). The Guadalupe Igneous Complex is an intuitively pleasing target to begin with because of its outboard (western) location and because it consists of some of the most mafic (>8% MgO gabbros) and felsic (high-silica and high-K2O granophyres and rhyolites) igneous units that we will see on this trip and thus raises some longstanding petrologic questions about the connections between mafic and felsic granitoids in arcs. It is also an exciting objective because of the preservation of its likely feeder zone (the Hornitos pluton), internal layering, and capping volca- nics (Fig. 1-2).
The Guadalupe Igneous Complex entirely intrudes the highly deformed units of the Foothills Terrane. Regional (Clark, 1964; Saleeby, 1982; Schweickert et al., 1984) and local (Paterson et al., 1991; Haeussler and Paterson, 1993) geologic studies imply that the Guadalupe Igneous Complex and Hornitos were emplaced at very shallow depths within a crust at least 15 km thick, consisting of oceanic basement slivers, overlain by Jura-Triassic arc-related volcanics (ca. 200 Ma Peñon Blanco and ca. 160 Ma Logtown–Gopher Ridge) and sedimentary (ca. 155 Ma Mariposa Formation) rocks (see also Bogen, 1985; Snow, 2007), all deformed in a SW-vergent fold and thrust belt and metamorphosed to at least lower greenschist facies. These stratigraphic units vary from east to west across the Jura- Cretaceous Bear Mountains shear zone. Tobisch et al. (1987), Vernon et al. (1989), and Paterson et al. (1991) all concluded that the Guadalupe Igneous Complex represents a structurally intact pluton, deformed along its western margin by the Bear Mountains fault zone, a listric, reverse (with a small oblique dextral component) ductile shear zone.
Saleeby et al. (1989) obtained U-Pb bulk zircon ages of 150 ± 1 Ma from one sample of the Hornitos pluton and 151 ± 1 Ma of three different phases of the Guadalupe Igneous Complex (a two-pyroxene gabbro, a granophyric granite, and a migmatitic leucosome). Ernst et al. (2009) examined single zircons in one Guadalupe Igneous Complex sample under cathodoluminescence (CL) and noted growth zoning but no obvious inherited cores. They obtained single-zircon, core and rim, sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb ages from 24 zircons most of which clustered on a concordia and gave an average age of 153 ± 2 Ma. Several discordant ages were older and could be interpreted as possible xenoor antecrystic zircons. Preliminary laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) single-zircon ages from the granophyre and overlying rhyolitic volcanics give identical ca. 152 Ma ages. Field observations indicate that these units are all comagmatic with other units in the Guadalupe Igneous Complex.
Haeussler and Paterson (1993) completed a paleomagnetic study of the Guadalupe Igneous Complex and overlying volcanics, and concluded that the Complex was rotated by 28° (westside up) by motion on the listric Bear Mountain fault zone in agreement with the ~30° east-dipping contacts between internal units and with bedding in overlying volcanic units; the deepest part of the Guadalupe Igneous Complex on the west side is probably not the base of the chamber, but exposes the deep western side of the chamber. Paterson et al. (1991) completed the only study to date of the nearby Hornitos pluton and based on the identical compositions, age, structural continuity of sheeting in the H and NW corner of the Guadalupe Igneous Complex, interrupted only by a region of poor exposure, concluded that it was likely a lower slightly uplifted, vertically sheeted portion of the Guadalupe Igneous Complex. The Hornitos pluton is now interpreted to represent deeper levels of the Guadalupe Igneous Complex moved up and slightly north by the listric motion on the Bear Mountain fault zone. Thus our recent syntheses of all of these studies suggest that the Hornitos–Guadalupe Igneous Complex volcanics represent a synintrusive, vertically stratified system, as approximately displayed in Figure 1-2.
Best (1963) was the first to map the Guadalupe Igneous Complex (Fig. 1-3) and examine its mineralogy and whole-rock compositions. His work established that the Guadalupe Igneous Complex is a layered pluton with compositions ranging from hornblende-, olivine-, and pyroxene-bearing gabbros near the western base, to highly evolved granitic compositions near the structural top (east), with a limited range of intermediate compositions in between. Best and Mercy (1967) followed with a study of mineral compositions. Four pyroxene pairs from that work can be used to yield T estimates of 696–886 °C using the Brey and Kohler (1990) thermometer and 821–939 °C using models of Putirka (2008) (assuming an emplacement depth of 1 kbar; see Haeussler and Paterson, 1993), which overlap with the 800 °C estimate from Haeussler and Paterson (1993). Initial whole-rock O isotope studies by Matt Paige (2010, personal commun.) yield magmatic values (δ180 = 5.5–6.5) for much of the chamber, indicating that at least some magmas did not interact with intruded host rocks.
ROAD LOG (MILEAGE IS CUMULATIVE)
0.0 (UTM 11S 0264664, 4134954) Junction Highways 41 and 49 in Oakhurst. In Finegold Intrusive Suite (focus of Day 2). Drive north on Highway 49 toward Mariposa.
9.8–10.4 Finegold Intrusive Suite with metavolcanic rafts and blocks.
13.2 Finegold Intrusive Suite outcrops tonalite to variable compositions.
20.7 Bootjack, Darrah Road.
21.8 Layered metavolcanics.
22.8 Metavolcanics, steeply dipping.
23.5 Finger of Finegold Intrusive Suite in metavolcanics.
24.8 Old Highway junction. Continue on Highway 49. Melones fault zone on ridge to left.
26.0 (11S 0237948, 4152630) Junction 49 and 140. Turn right and pass through the town of Mariposa. This is an old mining town, established around 1850 by John C. Fremont. The town is largely underlain by metavolcanics.
26.8 Turn left on Highway 49.
29.1 Crossing meta-ultramafic sliver along Melones fault zone. As we cross the fault, we drive into the Late Jurassic Mariposa Formation, which largely consists of slates and graywackes.
30.9 Mount Bullion, in Mariposa Formation, Melones fault zone, and then Late Jurassic metavolcanics to right (east).
31.5 Turn left onto Toll Road toward the town of Hornitos. Here we are still in eastern belt of the Mariposa Formation. This eastern belt of the Mariposa Formation is characterized by well-preserved, thin-bedded sediments containing Late Jurassic buccia fossils, and displaying low strain and low metamorphic grade. Detrital zircon ages ranging from ca. 153 Ma to Precambrian have been reported from this unit by Snow and Ernst (2008).
34.7 Exposures of the Peñon Blanco metavolcanic breccias.
34.9 Stop 1.
40.0 High-grade cordierite ± sillimanite paragneisses to north along Mount Gaines Road in the roof of a plu- ton. Migmatitic paragneisses and schists to south (left) on Slate Gulch Road along NW border of layered gab- bros in the Guadalupe Igneous Complex.
42.4 Indian Gulch Road junction on left. Keep straight toward Hornitos. Hornitos pluton to the north (right). The Hornitos pluton intrudes the central belt mélange.
44.0 Turn right on High Street into town of Hornitos. Hornitos means “little ovens” in Spanish; it was originally a quiet Mexican town in 1848 before the quiet was disrupted by the Gold Rush; it is rather quiet again now. The post office opened in 1856. D. Ghirardelli and Co. (of chocolate fame) opened a dry goods store here in 1859. Forty thousand dollars in gold was shipped out of the town on a daily basis. From a population of ~15,000 in 1879, it dwindled down by 1932 to ~60 people (http://abclocal.go.com/kfsn/ story?id=9181341, accessed 9 December 2013).
44.1 Turn right on St. Catherine Road (toward church), and stay left past dump area to outcrops of Hornitos pluton.
Stop 1: Overview of Foothills Terrane and Host Rocks of the Guadalupe Igneous Complex (10S 0755789, 4154594)
Outcrops of ca. 200 Ma breccia-dominated Peñon Blanco metavolcanics in hinge of large, SW-vergent anticlinal fold (Fig. 1-1). Hunter Valley metavolcanics and Jurassic intrusives are exposed in the core of an anticline to the NW. Mariposa Formation lies in the western and eastern limbs. Volcanic units tend toward finer grained sizes toward the limbs of the fold. The view west from the turnout provides an overview of the northeastern margin of the Guadalupe Igneous Complex and the central Foothills Terrane.
38.0 Intersection with Hornitos Road. Turn right toward the town of Hornitos. Leaving Peñon Blanco volcanics and entering the central belt of Mariposa Formation. This central belt is much more mélange-like on a regional scale with large blocks and/or slices of metavolcanics; the rocks are highly variable but typically exhibit higher strains and metamorphism, and less well-preserved bedding. Higher-grade metamorphic rocks occur as blocks in this zone (Miller and Paterson, 2001). Disruption may be due to early motion on Bear Mountain fault zone, which marks the western edge of this central belt.
Stop 2a: Vertically Sheeted Hornitos Pluton
Here, we are located at the western edge of the internally sheeted and compositionally heterogeneous, ca. 151 Ma Hornitos pluton. This pluton has not been carefully mapped or studied by anyone, although Paterson et al. (1991) discuss its general features. The pluton consists of many vertical mafic to felsic sheets with widths from tens of centimeters to tens of meters that trend toward the base of the Guadalupe Igneous Complex. The eastern half of the pluton is dominated by gabbros that are compositionally identical to those seen in the Guadalupe Igneous Complex, whereas the western half has a greater proportion of felsic sheets intermixed with gabbros. The western margin (see Stop 2b) is highly deformed in the Bear Mountain fault zone and sometimes displays both magmatic and high-temperature subsolidus fabrics and folds. Mingling occurs in some domains. At present our working hypothesis is that this pluton is the vertically sheeted feeder zone to the Guadalupe Igneous Complex (Fig. 1-2). Thus the vertical gabbro sheets here would feed into the more gently dipping gabbro sheets in the Guadalupe Igneous Complex. We are not yet certain whether the more felsic sheets at the top of the Hornitos are the feeder systems to more felsic units higher up in the Guadalupe Igneous Complex or a deeper level of felsic magma stagnation, which may have inhibited flow of mafic magmas from the Hornitos pluton to the Guadalupe Igneous Complex.
Optional Stop 2b: Mingled and Mylonitized Hornitos Pluton
Return to High Street, turn right, and merge right onto Bear Valley Road. In ~1.1 miles, pull off on dirt road to the left at a major road bend (10S 0743761, 4155891) and park. At this stop we can look at the highly mingled and high-temperature mylonitic NW corner of Hornitos pluton (great outcrops in dry stream to NE), strongly deformed by the Bear Mountain fault zone, which here separates this pluton and central Foothills belt from a western belt of 160 Ma metavolcanic rocks called the Logtown Ridge Formation (low hills to west). At least four intrusive phases, sometimes folded and often displaying a steeply plunging mylonitic lineation, are visible (Paterson et al., 1991). Saleeby et al. (1989) report a bulk zircon age of 150 ± 1 and a 40Ar/39Ar hornblende cooling age of 140 Ma from this location. Tobisch et al. (1987) present changing bulk compositions and O isotopic values from metamorphic rocks deformed in the Bear Mountain fault zone near this locality. Return south through the town of Hornitos.
44.3 Return to High Street, turn left, and return to the main road (10S 0744186, 4153946). Turn left on Hornitos Road.
45.7 Turn right on Indian Gulch Road.
49.8 Junction with Slate Gulch Road; continue straight.
51.3 Bridge across Bear Creek. As you walk upstream (east), you can pass through an amphibolite-trondhjemite migmatite complex up to 1 km wide, exposed along the western contact of the Guadalupe Igneous Complex, from which an age of a leucosome of 151 Ma ± 1 was reported in Saleeby et al. (1989). The migmatitic rocks are emplaced along the Bear Mountain fault zone; these rocks may represent either partial melts of older Jurassic metavolcanics, or they may represent a younger dike swarm, related to the emplacement of the Guadalupe Igneous Complex. Saleeby et al. (1989) also report 40Ar/39Ar hornblende and biotite cooling ages of ca. 146 to 141 Ma near here.
53.1 Junction with Highway 140. Turn right.
53.3 Junction with Old Highway. Turn left.
Alternatively, keep straight on Highway 140 for 1.1 miles and stop at large road cuts of western margin of Guadalupe Igneous Complex.
Optional Stop 3a
Gabbros and anorthosites and highly deformed western margin of the Guadalupe Igneous Complex. The eastern end of this road cut displays largely magmatic layering and microstructures in gabbros and anorthosites. As you walk west, the amount of subsolidus deformation increases, eventually resulting in high-temperature mylonites associated with fluid flow in the Bear Mountain fault zone (Vernon et al., 1989; Paterson et al., 1991; Lafrance and Vernon, 1993). The contact between the Guadalupe Igneous Complex and host rocks occurs at the western end of the road cut, although it is now difficult to identify because of the intense deformation in both units. Host rocks initially consist of thinly banded gneisses and schists with local refolded folds that formed in the Bear Mountain fault zone. Metavolcanic and exotic blocks locally occur in the shear zone. Farther west along Highway 140, the intensity of shearing and temperature of metamorphism decreases as the western edge of the Bear Mountain fault zone is crossed, and good sections of folded slates and metasandstones in the Mariposa Formation can be observed (Best, 1963).
53.9 Stop 3b.
Stop 3b: Layered Gabbros and Fractionates in Road Cuts and Stream (10S 0755018, 4144149)
Here we see what Best (1963) described as “layered gab- bros”; only here, unlike many classic layered intrusions, these are very fine grained; some even have basalt-like textures in thin section. Most whole-rock compositions thus very likely represent liquids, rather than crystal cumulates. Visits to the area with Bob Wiebe suggest to him that these gabbros are intruded within the lower levels of a felsic magma chamber, as layered mafic intramagmatic flows (intrusions of mafic magma into a felsic magma chamber), as seen in other places in the Sierra Nevada (Wiebe et al., 2002). Here, however, the felsic host is completely absent; so in this view, the host magma must represent prior intrusions of similarly mafic magmas. Best (1963) suggested that these mafic rocks are the parent magmas for all felsic Guadalupe Igneous Complex samples (i.e., that the entire Complex formed by closed-system fractionation). Interestingly, all but two Guadalupe Igneous Complex units fall on an iso- chron (Fig. 1-3) that yields an age (152 ± 6.8 Ma) that is similar to that obtained by single-crystal zircon studies (153 ± 2 Ma; Ernst et al., 2009); the two units that fall off of this isochron are samples from the rhyolite unit that caps the Guadalupe Igneous Complex, and a fine-grained sample of granophyre that lies just below the rhyolite, and may well represent a hypabyssal phase of the rhyolite.
The mafic rocks at this outcrop are also notable with regard to their compositions. The Tuolumne Intrusive Complex and Bass Lake Tonalite have compositions that are typical for the Sierra Nevada batholith; the Bass Lake Tonalite and Tuolumne Intrusive Complex are mostly intermediate (although the Tuolumne Intrusive Complex ranges to highly silicic compositions) (Fig. 1-4B), and these intermediate compositions are likely a result of magma mixing.
But mixing of what? The Guadalupe Igneous Complex provides clues in its bimodal nature. If mixing is the cause of intermediate compositions elsewhere in the Sierra Nevada Batholith, mixing was in any case clearly inhibited at the Guadalupe Igneous Complex and so mafic and felsic end-member magmas are preserved. Moreover, due to the tilt of the pluton, we can see how these magmas were emplaced and how they interacted with one another. The mafic rocks here at Stop 3b have between 7 wt% and 9 wt% MgO, and so are mantlederived liquids (initial 87Sr/86Sr ratios are 0.7033; Fig. 1-4A), thus providing a window into the mantle processes that created the Sierra Nevada Batholith. Although some rare outcrops elsewhere in the Sierra have similarly high MgO contents (e.g., Sisson et al., 1996), none that we are aware of reveal as clear a geologic context with regard to pluton emplacement and the evolution of felsic magmas.
Also notable at this stop are what Best (1963) referred to as “felsic segregations.” These segregations form small dikes that are mostly less than a meter in length, and just a few cm in width, and are often surrounded by vesicular aureoles of host gabbro. They are almost certainly not far-traveled and so most likely derived by in situ fractionation of the gabbros. The gabbros themselves show fine (cm-scale) layering in some areas (most evident in the stream-washed arroyo below) and yield slightly fractionated derivatives. The layers alternate between high MgO–low-SiO2 assemblages of mostly clinopyroxene + plagioclase + hornblende ± orthopyroxene with slightly coarser layers of clinopyroxene + plagioclase (notably lacking hornblende), with lower MgO and variable SiO2 (Fig. 1-4B). Compositional variations among these layers can be described by a two-step fractional crystallization process (when F = 5%) of observed phases (Best and Mercy, 1967).
These felsic segregations are quite similar to the granitoids that occur at higher structural levels (although not identical, as we show later, having lower TiO2). Our fractional crystallization model, although possibly applicable to the segregations themselves, might not apply to all Guadalupe Igneous Complex granitoids, given the observations of Table 1, which expresses the areal coverage of Guadalupe Igneous Complex map units (Fig. 1-3).
The granites alone comprise more than 20% of the outcrop area (Table 1), and the mingled zone is in most places >10% granitoid host rock. If outcrop area is anywhere near to an estimate of volume, then granitoids comprise 25%–30% of the total igneous mass of the Guadalupe Igneous Complex; this amount of granitoid melt appears too great to be derived by closedsystem differentiation of observed gabbroic materials. However, it should also be noted that the base of the Guadalupe Igneous Complex is not exposed, and so our estimates of granite fraction are maximum values. Sisson et al. (2005) indicate that as much as 25% of granitic magma can be derived by hornblende + plagioclase fractionation of a mafic precursor magma, and we cannot discount such a model.
|Map unit||Percentage of outcrop area|
|Mingled zone (agmatite)||32.1|
|Map unit||Percentage of outcrop area|
|Mingled zone (agmatite)||32.1|
The felsic dikes at this locality are by all appearances in situ differentiates; they are coarse grained (pegmatitic) with biotite and hornblende accompanying quartz and plagioclase feldspar. They are “granitic” with respect to SiO2, and the few samples (n = 5) that have been analyzed exhibit a wide range in K2O, from <1 wt% to nearly 7 wt%. It is quite possible that such melts might feed into the overlying granitic units, an issue that we will consider in later stops.
Perhaps most fascinating is the discontinuous nature of the fractionation process that forms the felsic segregates. Extensive analyses of rocks from this outcrop reveal no intermediate compositions between the mafic gabbros and the felsic dikes. The compositions of the felsic dikes are most easily explained (Fig. 1-5) by fractional crystallization, using the most evolved of the hornblende-bearing gabbroic layers as a parent magma, and observed phase compositions (Best and Mercy, 1967). Batch equilibrium crystallization models can only reproduce the felsic magmas (again using the most evolved of the mafic hornblende-bearing layers as a parent magma), if distribution coefficients applicable to felsic liquids are used. But the lack of intermediate liquid compositions is problematic for both mass-balance models. A more detailed study of mineral compositions may better reveal a viable mass-balance mechanism.
The batch equilibrium model in Figure 1-5 is shown to illustrate a mechanism potentially analogous to that described by Bachmann and Bergantz (2004), which can yield compositional gaps. However, the lack of intermediate-composition liquids may instead indicate the operation of nonequilibrium processes (e.g., Lesher and Walker, 1988).
55.9 Cross Schoolhouse Road, and enter western edge of large mingling zone (agmatite of Best, 1963).
56.4 Big Valley Road with exposures of mingled diorites and granites.
57.0 Stop 4
Stop 4: Best’s (1963) Agmatite (What We Term the Mingled Zone) (10S 0759930, 4143848)
Here we examine some of the massive granitic host rocks into which the mafi c rocks of the Guadalupe Igneous Complex were intruded. In the mingled zone generally, mafi c intrusions often occur as rounded blobs, and a rare few have chilled margins, indicating that both the mafic rocks and the granitic host were at least partially molten at the time of contact. Outcrops such as these were called “Agmatite” by Best (1963), using a term originated by Sederholm (1923) to describe “older rocks cemented by granite.” Given the genetic connotations of the term, and the fact that the mafic rocks are the same age as the granites into which they intrude, we prefer the name “mingled zone.”
Figure 1-6A compares compositions of gabbros from Stop 3 (open circles) as well as felsic dikes from that stop (yellowfilled circles) to granitoid host rocks (sampled from this level of the pluton and upwards; blue diamonds) and mafic dikes from the mingled zone (mostly from Stop 6; brown squares). The gabbros at this stop are nearly identical in composition to those rocks seen at Stop 3, having the same SiO2 and MgO; their only major element contrast is here the mafic rocks are displaced to slightly higher K2O, compared to the layered gabbros at Stop 3 (all other oxides are effectively identical). As we will discuss at a later stop (Stop 6), the mafic intrusions and host rocks interact almost not at all.
The granites into which the gabbros intrude separate into two groups, with high and low K2O (Fig. 1-6A). The lack of rocks in the center of the triangular array in SiO2-K2O space (Fig. 1-6A) suggests that only end members (rather than mixtures of such) mix with one another; at least one of these end members (we surmise that it is the low-K2O granitoid magma, which we shall see later at Stop 5), is not available for mixing when the gabbro and high-K2O granite magmas are blended.
The low-K2O granitoids are of special interest because they may represent truly juvenile additions of granitic material to the crust, as seen at Stop 3. A key question is whether low-K2O granitoids from within the mingled zone, or within the higher structural levels of the Guadalupe Igneous Complex formed by delivery of low-K2O fractionates from the lower gabbro units. Figure 1-6B, however, shows that this is unlikely. Five felsic dikes from the gabbro layers viewed at Stop 3 all have TiO2 contents that are lower than that for any of the granitic samples from the mingled zone or granites from higher structural levels. These low-TiO2, low-K2O melts thus appear to be restricted to the gabbro layer, and the granitoids at higher structural levels are formed by some process other than fractionation of Guadalupe Igneous Complex gabbroic magmas. At present, we favor the model of Sisson et al. (2005), i.e., that high K2O granitoids are partial melts of a hydrated, lower mafic crust, but we cannot disallow that the granitoids formed by fractionation of parent magmas that are not exposed. Although awaiting isotopic data to test these models, major oxide data show that the Guadalupe Igneous Complex granitoids are generally not very high with respect to A/CNK ratios, and none of our extensive analyses of the surrounding Mariposa Formation yield more than one or two samples that might be appropriate as bulk assimilants, so as to reproduce Guadalupe Igneous Complex granite compositions. This does not disallow partial melting of such materials, but the moderate A/CNK ratios of the Guadalupe Igneous Complex seem to argue against these Guadalupe Igneous Complex granitics as being S-type granites. In contrast, however, experimental partial melts of hydrated basalts from Ratajeski et al. (2005) and Sisson et al. (2005) match Guadalupe Igneous Complex granitic compositions precisely with respect to all major oxides.
60.4 Stop 5.
Stop 5: Granophyres (10S 0763907, 4143833): Compositional Significance of Massive Low-K2O Granitoids
At this stop we see massive low-K2O granitoids (Fig. 1-6A), but from here upwards, massive low-K2O granitoids are increasingly abundant and intermixed with high-K2O rocks. Only two compositional patterns are evident: (1) K2O and Na2O are inversely correlated (Fig. 1-7). (2) Amphibole contents tend to increase in the direction of higher Na2O and lower K2O (Fig. 1-7). Thin-section analyses show no relationship between K2O content and either grain size or the amount of granophyric texture (present throughout the granitic part of the pluton but generally more abundant at the top of the pluton). Other major oxides also appear to preclude any simple fractionation model that involves the addition or removal of feldspars, or other silicate phases, since both high- and low-K2O granites span equivalent ranges of CaO, Al2O3, and SiO2 (and all other oxides, besides Na2O).
The only model that appears to be viable is diffusion based. Acosta-Vigil et al. (2005) have performed experiments that show Na diffusing from anhydrous glass toward hydrated granitic melt, while K in the same experiments diffuses toward anhydrous glass. Acosta-Vigil et al. (2005) see no change in Al/(Na + K) in their compositional profiles, and Si and Al are only affected by dilution in the hydrated melts. Their model is that Al has a preference to be charge balanced by Na under hydrous conditions. Here in the Guadalupe Igneous Complex, the observed increase in hornblende contents in Na-rich granites may well support this style of diffusion, to generate high- and low-K2O granites in the upper part of the Guadalupe Igneous Complex.
60.7 Continued exposures of granophyres from last stop.
62.0 Hornblende-plagioclase, low-K “granophyre.” But probably Bass Lake tonalite.
62.4 White Rock Road. Continue straight.
63.9 Turn left onto Yanqui Gulch Road. Entering Mariposa Formation (marine slates and graywackes).
67.1 Turn left on Highway 140. Mariposa Formation.
67.7 Highly altered Mariposa Formation.
67.8 Great exposures of bedded and folded Mariposa Formation.
67.9 Rock shop to left.
69.0 Entering bedded volcanics at the top of the Guadalupe Igneous Complex.
71.2 Stop 6: Mingled Zone (10S 0758727, 4151051). Good pull-out for parking on right
Mingling and Mixing in the Guadalupe Igneous Complex
At this stop we see one of the most spectacular examples of mafic-felsic magma interaction in the Sierra Nevada. Except, here, the degree of mixing between mafic intrusives and felsic host is almost nil. Figure 1-8A shows Na2O versus SiO2 for these and other samples, where it is clear that both the mafic and felsic end members trend not toward one another but toward an intermediate composition, which matches the most evolved compositions within the meladiorite (excluding highly felsic host magmas there); the meladiorite lies near the center of the Guadalupe Igneous Complex, between the layered gabbros below and the mingled zone above.
The most mafic rocks from this mingled zone outcrop range to MgO and SiO2 contents identical to the layered gabbros of Stop 3, except in having slightly higher, but still very low K2O (Fig. 1-6A). However, unlike the layered gabbros, the mafic rocks here are much more varied in composition, forming two (Fig. 1-8B; CaO versus SiO2) or even three (TiO2 versus SiO2; not shown) distinct groups, each of which can be described by fractionation of a single mafic parent, but involving slightly differing amounts of clinopyroxene, plagioclase, hornblende, and magnetite. We suggest that variations within the mafic intrusive rocks here represent intrusions from the meladiorite layer, which contain a range of mafic magmas formed from a similar parent magma, but differentiated to different extents under slightly different conditions (varying temperature and H2O), and then internally, partially homogenized prior to intrusion. The meladiorite zone is thus a separate zone of mixing (mostly between mafic rocks and their differentiates), whereas the mingled zone is a zone of intrusion but little actual magma mixing.
No less interesting are variations in felsic samples at this outcrop. Detailed analyses of mafic enclaves and felsic hosts reveal no compositional relationships relative to physical proximity to enclave/host interfaces (e.g., enclaves themselves are compositionally unzoned), nor enclave size. That felsic samples trend not to the mean of the mafic compositions but to a mafic end member that is at best expressed by just a few of the mafic dikes indicates that variations in the granitic host pre-dated mafic magma intrusion. We cannot disallow that the compositional trend of the granitic magmas might also represent fractionation from a mafic parent, rather than mixing, but as noted earlier, it is unlikely that the granites here formed by closed-system fractionation from observed Guadalupe Igneous Complex mafic rocks, and Na-rich plagioclase cumulates are not evident. A mixing model, though, removes what would in any case be an otherwise strange coincidence that felsic host rocks compositionally intersect and/or emanate from an end member of the mafic suite (e.g., evolved meladiorite).
In combination, these compositional patterns and textures indicate that the Guadalupe Igneous Complex was at one time a large, heterogeneous magma chamber that extended at least as deep as the base of the meladiorite zone (i.e., the top of the layered gabbros) up to the top of the granitic layer (Fig. 1-9). In our tentative model, mafic magmas at the top of the layered gabbro pond below a granitic magma cap, and fractionate and mix to form at least two strains of meladiorite-like liquids. These fractionated liquids then form the main boundary between gabbroic liquids below and granitic liquids above. At the base of the granitic layer, mixing occurs between granitoid magmas and the fractionated gabbroic products (e.g., meladiorite), but not the main phase of gabbroic magmas below, perhaps because these latter were either too dense or too deep, or both. Later, forceful injection of magmas from the heterogeneous gabbroic and meladiorite layer intrude far into the granitic Guadalupe Igneous Complex magmas, forming the “agmatite” or mingled zone, and enclaves and mafic dikes at higher structural levels; this event occurs mostly after convection has been halted within the granitic magma mush. The high amounts of hornblende in the meladiorites relative to underlying gabbros may indicate that this phase of injection of mafic products into overlying granitic magmas may be triggered when such magmas have decreased density as they approach water saturation. Figure 1-9 shows our preliminary model that explains the major element compositions of the main igneous units of the Guadalupe Igneous Complex (Fig. 1-3). A provocative implication of this model is that the mafic-felsic/liquid-liquid interactions occur at the pluton scale, not just locally within a magma body.
71.2 Turn around and head east on Highway 140.
72.8 Entering granophyres.
73.5 Stop 7: Aplite in Granophyre on Road Cut to North of Road (10S 0761333, 4151778). We can walk along road to next stop. These units contain a mix of high and low-K2O granophyre.
73.7 Stop 8: Layered Volcanics (10S 0761644, 4151823).Bedding is tilted 28° based on both dips and paleomagnetic studies of Haeussler and Paterson (1993). Poorly studied. New CA-LA-ICP-MS age of 152 that overlaps with all Guadalupe Igneous Complex ages.
74.5 Leave volcanics and drive back into structurally overlying Mariposa Formation. Dip changes.
78.6 Crossing the Melones fault zone and leaving the Foothills Terrane.
79.5 Turn right onto Highway 49 and return to Oakhurst.
Keith Putirka thanks the students enrolled in Igneous and Metamorphic Petrology at Fresno State in the years 2006 and 2012; these students contributed greatly to the initial reconnaissance phases of geochemical studies of the Guadalupe Igneous Complex.
Figures & Tables
Formation of the Sierra Nevada Batholith
This comprehensive field guide takes you on a six-day, west-to-east geologic journey across the Mesozoic magmatic arc of the central Sierra Nevada in California. It contains a summary of field, structural, geochemical, and geochronologic data collected by a number of researchers on individual intrusions (Guadalupe Igneous Complex and nearby Hornitos pluton, Fine Gold Intrusive Suite, Yosemite Valley Intrusive Suite, Tuolumne Intrusive Complex). This guide also includes data on the basement terranes intruded by these intrusions, Mesozoic volcanic-sedimentary sections, and data derived of several Sierra Nevada-wide data sets (plutonic, volcanic and sedimentary geochronology, strain analyses, structures, and geochemistry). Syntheses of these data sets at the end of the guide focus on magmatic processes from the mineral to the arc scale, as well as contemporaneous tectonics, and the tempos controlling magmatism, deformation, exhumation, and erosion/redeposition in the Sierran arc.
- absolute age
- Central California
- felsic composition
- field trips
- greenschist facies
- high temperature
- ICP mass spectra
- igneous rocks
- mafic composition
- mass spectra
- metamorphic rocks
- mineral assemblages
- partial melting
- plutonic rocks
- road log
- United States
- Mariposa Formation
- Guadalupe Complex
- Hornitos Pluton