Abstract
AbstractThe well-characterized Sierra Nevada magmatic arc offers an unparalleled opportunity to improve our understanding of continental arc magmatism, but present bedrock exposure provides an incomplete record that is dominated by Cretaceous plutons, making it challenging to decipher details of older magmatism and the dynamic interplay between plutonism and volcanism. Moreover, the forearc detrital record includes abundant zircon formed during apparent magmatic lulls, suggesting that understanding the long-term history of arc magmatism requires integrating plutonic, volcanic, and detrital records. We present trace-element geochemistry of detrital zircon grains from the Great Valley forearc basin to survey Sierra Nevadan arc magmatism through Mesozoic time. We analyzed 257 previously dated detrital zircon grains from seven sandstone samples of volcanogenic, arkosic, and mixed compositions deposited ca. 145–80 Ma along the length of the forearc basin. Detrital zircon trace-element geochemistry is largely consistent with continental arc derivation and shows similar geochemical ranges between samples, regardless of location along strike of the forearc basin, depositional age, or sandstone composition. Comparison of zircon trace-element data from the forearc, arc, and retroarc regions revealed geochemical asymmetry across the arc that was persistent through time and demonstrated that forearc and retroarc basins sampled different parts of the arc and therefore recorded different magmatic histories. In addition, we identified a minor group of Jurassic detrital zircon grains with oceanic geochemical signatures that may have provenance in the Coast Range ophiolite. Taken together, these results suggest that the forearc detrital zircon data set reveals information different from that gleaned from the arc itself and that zircon compositions can help to identify and differentiate geochemically distinct parts of continental arc systems. Our results highlight the importance of integrating multiple proxies to fully document arc magmatism, demonstrating that detrital zircon geochemical data can enhance understanding of a well-characterized arc, and these data may prove an effective means by which to survey an arc that is inaccessible and therefore poorly characterized.
Highlights
The well-exposed and intensely studied Sierra Nevada magmatic arc of California offers an unparalleled opportunity to understand continental arc magmatism
All Great Valley Group detrital zircon plot within the field of continental zircon defined by Grimes et al (2007) for U/Yb and Hf, several grains plot on or close to the upper limit of U/Yb defined for unambiguously oceanic zircon (Fig. 4)
Eleven Great Valley Group detrital zircon grains plot in the “oceanic” abundant “continental arc” zircon in the forearc setting, we suggest that field on all three discrimination diagrams of Grimes et al (2015), and the absence of at least a few “oceanic” grains in the western Sacramento they were not considered further in comparisons of forearc and Valley samples, combined with their presence in the southern Gravelly arc zircon (Fig. 5)
Summary
The well-exposed and intensely studied Sierra Nevada magmatic arc of California offers an unparalleled opportunity to understand continental arc magmatism. Bedrock zircon and intra-arc detrital zircon studies have indicated three episodes of high-flux magmatism in the Sierra Nevada, with magmatic pulses in Triassic, Middle and Late Jurassic, and mid-Cretaceous time, separated by two periods of low magmatic flux in Early Jurassic and Early Cretaceous time (Fig. 1; e.g., Paterson and Ducea, 2015) This chronologic record of Sierran magmatism preserved in modern exposures is necessarily partial, given that much of the associated volcanic cover has been eroded, while the deeper, unexhumed portions of the arc remain inaccessible; more than 80% of the surface area of batholithic rock is Cretaceous in age (Saleeby and Dunne, 2015). Modern surface exposure of the Sierra Nevada magmatic arc is not representative of the full magmatic history of the arc, making it challenging to decipher details of older magmatism as well as the dynamic interplay between plutonism and volcanism
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