EIGHT MILLION YEARS OF FOCUSED MAGMATISM: THE STRUCTURAL SETTING AND INTRUSIVE HISTORY OF THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

Geologic mapping and dating have shown that intrusive activity migrated eastward across the central Sierran arc between about 217 and 80 Ma and concluded with the growth of at least four large nested intrusive suites along the eastern margin of the Sierra Nevada batholith between 92 and 83 Ma. This eastward progression of magmatism is not as apparent within these suites, however, where young relatively felsic members typically intrude the centers of older, more mafic ones. This suggests that local structures focused intrusion within the long-lived magmatic systems that produced the suites.

The two most southerly suites—John Muir and Mount Whitney—appear to be localized at extensional stepovers along the coeval proto-Kern Canyon—Sierra Crest dextral shear system. In the Mount Whitney Intrusive Suite (MWIS) northeast-trending dikes and fracture zones display sinistral offsets and internal shearing consistent with their development as antithetic fractures in an extensional duplex. It is inferred that similar structures at depth channeled magmas rising from the lower or middle crust to the exposed level (perhaps 6-8 km paleodepth) where these magmas then spread laterally to build elongate laccolithic intrusions parallel to the NNW trend of the batholith.

Geologic mapping and a growing body of U/Pb zircon dates indicate that most of the MWIS grew during three intrusive events at 90.6-88.9 Ma, 87.6-86.4 Ma, and 85.4-83.5 Ma. Samples from a longitudinal transect of the middle member indicate that this pluton crystallized during two distinct events at about 87.6 and 86.4 Ma, however, and a prominent contact within the oldest member suggests it also had an strongly episodic growth history. Northeast-trending granite porphyry dikes that intrude the suite’s youngest member lie on the same linear trend of mafic mineral abundance versus age that tracks the long-term compositional evolution of the suite’s major members and suggest that magmatic activity in the MWIS persisted until at least 82.6 Ma.