SMALL VOLUME PERALUMINOUS GRANITES AS WINDOWS INTO ANATECTIC CONVERSION OF ACCRETED TERRANES TO CRUST IN CONTINENTAL ARCS

Small-volume peraluminous granitoid rocks from the 124–105 Ma Fine Gold Intrusive Suite preserve important records of magma source and mixing processes during initiation of the Sierran magmatic arc. In particular, they record abundant inheritance of Early Cretaceous zircon cores (130–143 Ma) that are overgrown by at least two younger (124–118 Ma) domains. Such temporal and textural information is typically absent from zircons in the Sierra, presumably because inherited zircon is dissolved in the largely metaluminous magma compositions. Using in situ O (SIMS) and Hf (LA-ICP-MS) isotope analyses of the different textural domains in these zircon grains, we document the following patterns: (1) Inherited zircon cores have high and variable εHf_(t) (3–13) and low to moderately elevated δ¹⁸O (5.0–6.3‰), reflecting derivation from sources that were mantle-like with variable crustal input; 2) First-stage overgrowths show high and variable δ¹⁸O (8.0–13.4‰), and more restricted eHf_(t) = 3–7 than cores. The δ¹⁸O values are the highest yet reported for zircon in the Sierra Nevada; (3) Rims on zircons have lower and less variable δ¹⁸O (7.0–8.0‰) and lower but still variable εHf_(t) (1–7.5) values. Overall, the isotopic variations in these zircons are interpreted as recording partial melting of hydrothermally altered accretionary complex rock (volcanic?) because older cores are overgrown by domains crystallized from melts with high δ¹⁸O, but minimally changed εHf_(t). Lower and less variable δ¹⁸O values, and slight lowering of εHf in the last stages of grain growth is taken as evidence of homogenization within the overall magma, averaging contributions from mixtures of small-volume partial melts. With a span of at most 10 million years between original deposition of these rocks, melting, and homogenization of the melts, the peraluminous granites of the Fine Gold Suite suggest efficient recycling of accreted terranes into granitic crust.