Rb-Sr isotopic characterization of pegmatite dikes 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. I measured 87Sr/86Sr ratios for several pegmatites from the Tuolumne Intrusive Suite in the Sierra Nevada batholith, California to examine the possible role of mass transfer between wall rocks and intruding magma. Variation in isotopic composition between wall rocks and plutons provides a sensitive test of fluid-rock interaction. Meter-scale pegmatite dikes along the margins of the intrusive suite cut both wall rocks and granitoids (including tonalite, granodiorite, and granite). Initial 87Sr/86Sr ratios in granitoids along an east-west traverse range from 0.7057 to 0.7066 (Kistler et al, 1986; Bateman et al., 1988; Gray et al., 2008). Wall rocks along the margins of the suite display variable 87Sr/86Sr, ranging from 0.710 to 0.774 (Mills et al., 2009), significantly more radiogenic than the igneous materials. Age-corrected pegmatite 87Sr/86Sr ratios range from 0.706 to 0.709. Pegmatites have higher 87Sr/86Sr ratios than granitoids, but lower than the metasedimentary wall rocks. I interpret these data as indicating interaction between hydrous fluids from late-stage granite crystallization and wall rocks. Pegmatites in the Tuolumne Intrusive Suite represent the final stages of pluton development, and hydrothermal circulation could drive fluid paths that traverse wall rocks and the crystallizing plutons, resulting in 87Sr/86Sr ratios between those of the two end members.Bachelor of Scienc