GREAT VALLEY GROUP DETRITAL ZIRCON TRACE ELEMENT SIGNATURES: A PARTIAL RECORD OF SIERRA NEVADA ARC MAGMATISM

Although numerous studies document episodic magmatism in the Sierra Nevada arc, specific magmatic processes that resulted in periods of voluminous magmatism or relative lulls remain poorly understood. Trace element analysis of detrital zircon from the Great Valley Group (GVG) complements analysis of in situ Sierran zircon to provide a more complete record of arc magmatism. We measured trace elements in 213 previously dated Mesozoic detrital zircon grains from the GVG.

Most GVG zircon trace element compositions suggest an enriched or continental arc provenance. Although variable, GVG zircon compositions do not show systematic differences during times of voluminous magmatism or magmatic lulls. Most GVG zircons show heavy rare earth element enrichment and a moderate negative Eu anomaly, typical of continental arc zircon that crystallized within an amphibole-rich system. Ti concentrations are largely less than 20 ppm, characteristic of low crystallization temperatures in arc igneous rocks. GVG zircon crystallized at relatively lower temperatures during Triassic and Early Jurassic time, with relatively higher temperatures in Middle Jurassic through Cretaceous time. Middle and Late Jurassic zircons show the greatest geochemical variation.

GVG results generally overlap in situ Sierran zircon geochemistry, but some key differences in our results suggest that the western side of the arc may have been geochemically distinct from the central and eastern arc. Low Ce/Gd and Sc/Yb more typical of marginal or oceanic arc systems characterize some Jurassic and Cretaceous GVG zircons. Triassic GVG zircons show less variability in trace element ratios than central and eastern Sierran zircons, while Jurassic GVG zircons show greater variability. GVG results also partially overlap with proximal retro-arc detrital zircon (McCoy Mountains Formation), but GVG zircons lack geochemical separation between Jurassic and Triassic zircons, and GVG Triassic zircons have higher U/Yb and lower Th/U than retro-arc zircons. Low Th/U ratios in GVG zircons indicate a lack of input from Mojave (crustal) sources. Thus, GVG zircons may record western, outboard arc magmatism that is not represented in zircon geochemistry from either the central and eastern Sierran zircons or the proximal southern retro-arc detrital zircons.