NEW U-PB ZIRCON AGES OF THE GRANITES OF DINKEY DOME: REEXAMINING THE ORIGINS OF THE SHAVER INTRUSIVE SUITE

The Shaver Intrusive Suite of the Central Sierra Nevada, CA, includes a group of small felsic plutons surrounded by or adjacent to the 102 ± 1 Ma (Tobisch et al. 1993, J. of Struct. Geol.) Dinkey Creek Granodiorite (DCG); their absolute ages were previously undetermined. We report 13 new U-Pb zircon ages, obtained by LA-MC-ICPMS, for 9 of the plutons and 2 associated diorites. Ages were determined from a survey of 25 or more individual zircons from 13 samples using 25-35 µm spots. Samples yield reasonable information that is concordant. Most plutons range in age from 99–102 Ma, including the granites of Mushroom Rock (100.4 ±2.8 Ma), Lower Bear Creek (100.8 ± 3.1 Ma), and north of Snow Corral Meadow (101.3 ±1.8 Ma), as well as the diorites of Mud Lakes (99.6 ±1.6 Ma) and Rainbow Mine (102.2 ± 2.1 Ma). The northern lobe of the granite of Sheepthief Creek (STC) dates to 101.6 ± 2.8 Ma; the granite of Dinkey Lakes—mapped as part of the granite of Dinkey Dome—yields ages of 101.2 ± 3.1 Ma and 102.1 ± 2.1 Ma.

Two plutons are approximately 105 Ma: the granite of Ordinance Creek (104.8 ± 2.8 Ma), which is intruded by the Mushroom Rock pluton, and the southern lobe of the granite of STC (104.8 ± 2.3 Ma). The east and west sides of the garnet-bearing granite of Dinkey Dome are 119.7 ± 5.2 Ma and 119.8 ± 2.0 Ma, respectively. The granite of Shorthair Creek yields the youngest age of this group (96.8 ± 2.0 Ma). A tenth felsic pluton at Coyote Creek was analyzed but sparse zircons provided unreasonable age information.

These results show that plutons formerly grouped together represent 4 pulses of magmatism. The namesake for this group, the Dinkey Dome pluton (Bateman 1992, USGS prof. paper), is considerably older than any pluton in the area and documents magmatism inboard of the 119 locus of Sierran magmatism. In addition, the STC plutons are two distinct stages of magmatism, and the Shorthair Creek pluton is temporally related to the 96 ± 3 Ma (Tobisch and Cruden 1998, Geology) leucogranite of Bald Mountain. The preponderance of 99-102 Ma ages indicates a late stage felsic magmatic event that is transitional with or shortly post dates, evidenced by general absence of foliations in these plutons, emplacement of the 102 ± 1 Ma DCG.