USING DETRITAL ZIRCON GEOCHEMISTRY TO STUDY DEEP ARC PROCESSES

We explore the hypothesis that trace element (TE) compositions of detrital zircon from arc-adjacent basins provide a record of arc magmatism that is different from, but complementary to zircon from exposed plutons of the Sierra Nevada arc. To do so, we evaluate the TE geochemistry of 312 Mesozoic (256-97 Ma) detrital zircon grains separated from sandstones deposited ca. 145-80 Ma along the length of the Great Valley forearc basin and ca. 140 Ma within the Goldstein Peak intra-arc basin. TE analyses reveal four groups of dominantly Late Jurassic zircon having anomalous trace element compositions that identify processes of differentiation occurring within the deep arc lithosphere. REE-Sc-Nb-Hf-U-Th covariations define three populations of REE-rich grains that are interpreted as crystallizing from (1) differentiates produced by extensive fractional crystallization; (2) compositionally transient magmas generated by hornblende resorption during decompression; and (3) arc basalts modified by assimilation-fractional crystallization involving partial melts of deep continental crust. In each case, zircon crystallization likely preceded significant hornblende crystallization/re-crystallization, as this phase rapidly diminishes magmatic REE abundances. A fourth, low-REE population of Middle Jurassic to Early Cretaceous zircon having high Gd/Yb ratios is interpreted as having crystallized from partial melts of arc rocks in the presence of garnet, documenting a recycled arc component. Taken together, the anomalous compositions of these four zircon groups relative to “typical” Mesozoic arc zircon imply that these zircon (and their host magmas) do not usually reach the upper arc crust, where eruption and/or erosion could release the anomalous zircon cargo to arc-adjacent basins. We connect the Late Jurassic peak age of these geochemically anomalous detrital zircon to plutonic suites indicative of a ca. 148 Ma arc extensional event, including the 600-km-long Independence dike swarm and a suite of southern Sierra Nevada gabbros that record a ca. 40-epsilon-Hf-unit zircon “pull up”. We propose that an abrupt extensional event ripped open the arc, evacuating geochemically anomalous, zircon-bearing magmas from the deep crust and permitting depleted-mantle-derived magmas to intrude the shallow arc crust.