CRUSTAL RECYCLING AND ISOTOPIC DIVERSITY IN THE SHAVER INTRUSIVE SUITE, CENTRAL SIERRA NEVADA, CA

The Sierra Nevada batholith features many classic, well-studied intrusive suites, particularly those emplaced along the Sierran crest during a period of voluminous magmatism from 98–83 Ma. Those suites are notable for their zonation, often ranging gradationally from tonalite or granodiorite margins to granodiorite or granite cores. Nearly all of these suites are oldest at their margins and youngest in their cores, with timespans in individual suites up to 10 Ma.

The Shaver Intrusive Suite, located west of the Sierran crest in the central Sierra Nevada, is notable for its contrasts to the zoned intrusive suites. Whereas the Shaver suite was constructed over 9 Ma (106-97 Ma), similar to timescales of other Sierran suites, it does not preserve the same large scale intrusive relationships of those suites, such as concentric zoning or gradational contacts. Instead, the Shaver Intrusive Suite is dominated by the large Dinkey Creek Granodiorite (DCG), with over a dozen smaller, discrete plutons (mostly granite, with minor diorite) intruded before, during and after emplacement of the DCG.

Here we present new data, including O isotopes in zircon, whole rock major, trace and REE compositions, and whole rock Sr and Nd isotopic data, that demonstrate the Shaver Intrusive Suite was the locus of notable crustal recycling and is also one of the most heterogeneous suites in the central Sierra Nevada batholith. Major and trace element data indicate some of the granite plutons are highly peraluminous (some exposures contain magmatic garnet, muscovite, and aluminosilicates), whereas the enclave-rich DCG is metaluminous. Six granitic whole rock samples preserve εNd_t from -4 to -8, and initial ⁸⁷Sr/⁸⁶Sr from 0.7054 to 0.7087. The Sr isotopic data mostly correlate with δ¹⁸O, which indicate that supracrustal input into the Shaver suite was generally high, but variable: δ¹⁸O ranges from 6.60-8.02‰. Two contemporaneous diorite plutons are isotopically (O, Sr, Nd) indistinguishable from the much more felsic members of the suite. We suggest that these data reflect a highly variable source(s) with a significant crustal component; the heterogeneities in the suite may have been preserved by discrete delivery of small batches of melt to shallow levels, preventing large scale mixing and homogenization both between and within plutons.