RB-SR ISOTOPE CHARACTERIZATION OF PEGMATITES FROM THE TUOLUMNE INTRUSIVE SUITE, CALIFORNIA, USA

Pegmatite dikes are common in granitic plutons and reflect crystallization of the fluid-rich last increments of the system. We measured ⁸⁷Sr/⁸⁶Sr ratios for several pegmatites from the Tuolumne Intrusive Suite (TIS) in the Sierra Nevada batholith, California to examine the possible role of mass transfer between wall rocks and the plutons. The wall rocks have significantly higher ⁸⁷Sr/⁸⁶Sr than the granitoids, so this system can provide a sensitive test of fluid-rock interaction.

Meter-scale pegmatite dikes along the margins of the TIS cut both wall rocks and granitoids. Initial ⁸⁷Sr/⁸⁶Sr ratios were determined for pegmatites and host rocks (Cretaceous granitoids and metasediments) of the TIS by thermal ionization mass spectrometry (TIMS), and concentrations of Rb and Sr were measured by X-ray fluorescence. All values below are age-corrected. Granitoids along this traverse range from 0.7056 to 0.7066 (Kistler et al, 1986; Gray, 2003). Wall rocks along the margins of the suite display variable ⁸⁷Sr/⁸⁶Sr, ranging from 0.710 to 0.774 (Mills, 2007) and are significantly more radiogenic than the igneous materials. Pegmatites, with ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.706 to 0.709, are more radiogenic than the granitoids from which they were derived.

Pegmatites have higher ⁸⁷Sr/⁸⁶Sr ratios than TIS granitoids, but lower than the metasedimentary wall rocks. We interpret these data as indicating interaction between aqueous fluids from late-stage granite crystallization and wall rocks. Pegmatites in the TIS represent the final stages of pluton development, and hydrothermal circulation could drive fluid paths that traverse wall rocks and the crystallizing pluton, leading to ⁸⁷Sr/⁸⁶Sr ratios between those of the two end members.