ZIRCON CRYSTALLIZATION HISTORY OF THE NORTHERN MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

The Mount Whitney Intrusive Suite (MWIS) is the southernmost of four concentrically-nested granitic intrusions that were emplaced along the eastern margin of the Sierra Nevada batholith during a period of dextral shear along the proto-Kern Canyon fault and Sierra Crest shear zone. The MWIS is centered at an extensional stepover between these structures, suggesting that the ascent of the magmas that built the suite was facilitated by local extensional structures formed in response to regional dextral shear.

Published U/Pb ages indicate that (1) zircon crystallization in the three granodioritic plutons that comprise the bulk of the MWIS spanned the interval 90.6-83.4 Ma and (2) age ranges for the individual plutons are consistent with their relative ages inferred from field relations. In an effort to better define the zircon crystallization histories of the suite’s three major plutons and learn if compositional and textural discontinuities within these bodies mark temporal breaks between intrusive phases we have used laser-ablation ICP-MS to obtain zircon ages that have typical 2s errors of ±0.65% for 16 new samples.

Combined with published TIMS ages, our new data indicate that zircon crystallization in the suite’s oldest member, the Granodiorite of Sugarloaf-Lone Pine Creek, occurred between 90.6-88.9 Ma. In the northern part of the middle member, the Paradise Granodiorite, zircon crystallization occurred during two periods: ca. 87.6 Ma in the northernmost part of the pluton and ca. 86.4 Ma in the central part. The boundary between these age domains corresponds with an apparent step in microdiorite enclave abundance that may mark an internal contact between distinct intrusive phases. Finally, throughout most of the northern part of the suite’s youngest member, the Whitney Granodiorite, zircon crystallization occurred at ca. 84.7 Ma. A sample from the northernmost part of the pluton is slightly older (85.4 Ma) and two from the central part are younger (83.9 and 83.4 Ma), but no clear correlations have yet been observed between zircon ages and the textural and compositional differences within the pluton.

Although the volumes of individual intrusive phases are still poorly known these new ages provide a first step towards constraining the magmatic fluxes and thus the thermal history of this long-lived intrusive suite.