Thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the Sierra Nevada Batholith, Tehachapi Mountains, California

Abstract

We present thermodynamic estimates of pressures, temperatures, and volatile activities in vailably deformed, gabbroic to granitic, Cretaceous (115-100 Ma) batholithic and framework rocks of the Tehachapi Mountains, southernmost Sierra Nevada, California. A1 contents of hornblende in granitoids imply igneous emplacement at --8 kbar in the southernmost Tehachapi Mountains, with lower pressures (3-7 kbar) to the north. Metamorphic pressures and temperatures for garnet-bearing paragneisses and metaigneous rocks were estimated on the basis of garnet-homblende-plagioclase-quartz and garnet-biotiteplagioclase-quartz thermobarometers. Disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite-based thermobarometers to rocks of contrasting composition and mineralogy. Preferred pressures cluster at 7.1-9.4 and 3.6-4.3 kbar. Incomplete knowledge of reaction histories, however, limits our interpretation of the lower pressures because they are minimum estimates. The --4-kbar samples are all from a small area and, if our interpretation is correct, they imply a local, more shallow event superimposed on crust once residing at deeper structural levels. Garnet-hornblende and garnet-biotite temperatures are less coherent, likely owing to retrograde Fe-Mg exchange, and range from 570 ø to 790øC. The majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling; they are not granulites. Country rock paragneisses are typically migmatized at "peak" metamorphic conditions near that of the wet granite solidus (>690øC). Veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase. Thermodynamic estimates of volatile activities in these garnet-bearing assemblages suggest variable, mostly CO 2-rich fluid compositions, in the absence of any pervasive fluid flux. The igneous rocks of the Tehachapi Mountains were thus intruded at depths of --30 km, making them the deepest known exposed components of the Cretaceous Sierra Nevada batholith. Metamorphism occurred at these great depths and, perhaps, locally after --15 km of uplift before --87 Ma, implying an uplift rate of 1.2 mm/yr. (A minimum uplift rate is 0.6 mm/yr.) This original uplift and possible subsequent uplift events may have been related to underthrusting of a block of Rand Schist from what is now the southeast, with concomitant widespread ductile deformation. The deduced pressure-temperature and uplift history is similar to those of highpressure/high-temperature Cretaceous batholithic rocks in Salinia and the San Gabriel Mountains, but direct correlation is not wan-anted. When compared with higher-level intrusive rocks from analogous portions of the Sierra Nevada batholith to the north, the Tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average, but displaying a similar level of isotopic hybridization involving mantle and crustal sources. The batholith is quartz-rich at these levels, ...

Melt evolution and residence in extending crust: Thermal modeling of the crust and crustal magmas

“…The assumptions of localized tectonic extension and repeated diking events likely contribute to heat preservation and construction of mush regions. There are relatively limited geophysical constraints on the expanse and residence time for lower crustal melt that focus on: 1) petrologic observations on exhumed lower crustal terrains that include for example Ivrea, North Italy (e.g., Sinigoi et al, 1994;Voshage et al, 1990;Wedepohl, 1995); Kohistan, Pakistan (e.g., Burg, 2011;Dhuime et al, 2009;Jagoutz et al, 2007;Jagoutz and Schmidt, 2012); Fraser and Musgrave Ranges, Australia (e.g., Fletcher et al, 1991;White et al, 1999); Sierra Nevada and Peninsular Ranges, California (e.g., DePaolo, 1981a;Pickett and Saleeby, 1993); Fiordland, New Zealand (e.g., Klepeis et al, 2003) and 2) seismic studies exemplified by Main Ethiopian Rift (e.g., Keranen et al, 2004) and Western United States (e.g., McCarthy and Thompson, 1988). However, while useful, these observations provide a limited picture of the temporal history of lower crustal magmas.…”

Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

“…The bounding structure is the Rand fault, a structurally modified subduction megathrust flat that the schist protolith assemblage was originally subducted along. Metamorphic equilibration pressures in the Rand schist are $9 kb [Pickett and Saleeby, 1993;Jacobson, 1995]. Equilibration pressures in proximal upper plate rocks range from 7 to 11 kb.…”

Late Cretaceous-Paleocene Extensional Collapse and Disaggregation of the Southernmost Sierra Nevada Batholith

“…Exhumation of the batholith commonly has been assumed to have occurred by erosion, with much of the removed material deposited immediately to the west of the batholith as forearc basin sediments that today are exposed along the western side of the San Joaquin and Sacramento valleys (Mansfield, 1979;Bateman, 1992). The southernmost part of the batholith south of Lake Isabella, in contrast, has been exhumed to much deeper crustal lev els of ~15 to 30 km (Sharry, 1981;Ague and Brimhall, 1988;Pickett and Saleeby, 1993), which means that more of the batholithic crust in the south has been removed and transported elsewhere. Malin et al (1995) suggested that crustal thickening related to emplacement of Rand Schist beneath the southernmost Sierra Nevada batholith in Late Cretaceous time may have led to the subsequent collapse of the crust and exhumation of the deep-level rocks in the region along normal faults.…”