We present thermodynamic estimates of pressures, temperatures, and volatile activities in variably deformed, gabbroic to granitic, Cretaceous (115--100 Ma) batholithic and framework rocks of the Tehachapi Mountains, southernmost Sierra Nevada, California. Al 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-hornblende-plagioclase-quartz and garnet-biotite-plagioclase-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¿ to 790¿C. 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 (≥690¿C). Veinlike paragenesis of garnet in the metaigneous rocks suggest formation related to the presence of a fluid phase.

Thermodynamic estimates of volatile activities in these garnet-bearing assemblages suggest variable, mostly CO2-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 ocurred at these great depth 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 high-pressure/high-temperature Cretaceous batholithic rocks in Salinia and the San Gabriel Mountains, but direct correlation is not warranted. 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, suggestive of a weak, ductile middle crust susceptible to prolonged deformation and possible delamination. ¿American Geophysical Union 1993