Abstract

In the Foothills terrane, western metamorphic belt, California, the Bear Mountains fault zone (BMFZ)—a zone of complex, multiple deformation—has deformed composite maficfelsic granitoids of the Guadalupe igneous complex (GIC) and the Hornitos pluton at magmatic to upper amphibolite facies temperatures. Evidence for high-temperature deformation includes the occurrence of orthopyroxene, clinopyroxene, and brown hornblende in syndeformationally recrystallized aggregates in deformed rocks of the GIC and the occurrence of melt leucosomes in the axial surfaces of folds in cordierite—K-feldspar migmatites in contact metamorphic country rocks in the BMFZ. Away from the BMFZ, composite granitoids of the GIC show no evidence of magmatic flow, so that deformation of the GIC granitoids in the BMFZ was entirely by solid-state flow. This suggests that the GIC was intruded before this mylonitic deformation in the BMFZ. However, other evidence suggests some deformation in the BMFZ before intrusion of the pluton. In the Hornitos pluton components of composite granitoids showing relatively weak solid-state deformation are strongly elongate, suggesting a component of magmatic flow, although later subsolidus deformation also occurred in most places. Supporting evidence of magmatic flow in the Hornitos pluton also includes dimensional alignment of plagioclase laths and prismatic hornblende grains, as well as preferred orientation of twins in recrystallized aggregates of elongate grains of plagioclase. Away from the BMFZ, the regional metamorphic grade is low, suggesting that heat for the high-temperature deformation and metamorphism was supplied by the intrusions.

U/Pb zircon ages are consistent with emplacement of the GIC and the Hornitos pluton, as well as their deformation in the BMFZ, around 150 Ma. 40Ar/39Ar ages of hornblende in foliations in the deformed granitoids and of biotite in foliations in the contact aureole rocks suggest that the plutons and the country rocks in the BMFZ remained hot enough for amphibolite to upper greenschist facies conditions to persist for 5 to 15 Ma after emplacement of the plutons.

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