Petrology of the Mount Whitney Intrusive Suite, eastern Sierra Nevada, California: Implications for the emplacement and differentiation of composite felsic intrusions

Abstract

The Mount Whitney Intrusive Suite is a nested intrusion composed of three granitic plutons that were emplaced along the eastern margin of the Sierra Nevada batholith between 88 and 83 Ma. Geologic relations indicate the suite was formed by magmas that spread laterally at a depth of ~10 km after initially rising along fractures developed at an extensional stepover between the proto–Kern Canyon fault and the Sierra Crest shear zone. Cenozoic uplift and erosion have exposed the interior of the suite, making it possible to study textural and compositional variations within the body that yield insights into the complementary roles of source variability and crystal fractionation in producing zoned intrusions. Although isotopic data indicate that contributions to the Mount Whitney Intrusive Suite from preexisting crustal rocks increased over time, major differences in the compositional and textural zoning of its members also refl ect gradual warming of the upper crust. The oldest pluton is isotopically and compositionally diverse, and consists of distinct phases that apparently underwent limited mixing and fractionation following their emplacement into cool host rocks. Conversely, the younger members are more isotopically uniform, and the youngest displays regular pluton-scale variations in composition and texture consistent with individual inputs having mixed, fractionated, and undergone textural coarsening as it cooled slowly within the older members or wall rocks warmed by them. A thermal model of the suite indicates that the intervals during which successive magma inputs were likely to hybridize and undergo subsequent in situ differentiation increased as the intrusion grew. For recharge at an interval intermediate between those required for 50% crystallization in different parts of the suite, model results suggest that the outer part would have developed as a composite body in which individual phases underwent limited differentiation, whereas the inner part would have developed as a more homogeneous body capable of large-scale differentiation. These predictions match trends among the suite’s members and suggest that warming of the upper crust may account for much of its textural and compositional diversity. Local differentiation also occurred in the upper parts of the younger members, where rhyolitic melts were segregated as a result of solidifi cation front instability.