TWIN OF THE TUOLUMNE: NEW GEOCHRONOLOGY FROM THE MONO PASS INTRUSIVE SUITE

The Late Cretaceous Sierra Nevada batholith is characterized by several normally-zoned suites of nested plutons. Although the Tuolumne Intrusive Suite (TIS) in Yosemite National Park has been studied extensively, a remarkably similar suite centered approximately 65 km to the south at Mono Pass has received comparatively little attention. Like the TIS, the Mono Pass intrusive suite (MPIS) consists of a megacrystic core displaying gradational contacts with more mafic equigranular marginal units. New U-Pb geochronology suggests that in addition to the parallels in petrography and contact relationships, the MPIS was also emplaced in the same extended manner and during the same time interval as the TIS.

Zircons from four units of the MPIS were dated by U-Pb TIMS methods, and high-precision weighted mean 206Pb/238U ages were obtained for three of these. Zircons from the Mono Creek Granite, representing the megacrystic core of the suite, yielded an age of 87.0 ± 0.1, whereas zircons from the Round Valley Peak Granodiorite, the eastern equigranular marginal unit, yielded an age of 88.8 ± 0.2 Ma. Zircons from the Lake Edison Granodiorite, the western equigranular marginal unit, did not yield overlapping dates but suggest a crystallization age between 90.7 and 91.1 Ma. Finally, sparse zircons from the Rock Creek gabbro, a small mafic mass representing a possible mafic end member of the suite, yielded an age of 95.3 ± 0.3 Ma.

The large spread of ages over eight million years strongly suggests that like the Tuolumne intrusive suite, the MPIS was formed by the episodic emplacement of multiple batches of magma, precluding the existence of a single large fractionating parent body of magma in the upper crust. The data also demonstrate that the MPIS and TIS were constructed concurrently. However, since the MPIS has not yet been subjected to a detailed petrologic investigation, it remains to be seen whether the two suites are products of the same sources.