EXAMINING THE EPISODIC EVOLUTION OF THE SIERRA NEVADA ARC THROUGH TH/U RATIOS IN GREAT VALLEY GROUP DETRITAL ZIRCON

The Sierra Nevada magmatic arc of California developed in three pulses of voluminous magmatism (250-210 Ma, 180-145 Ma, 125-85 Ma) separated by two lulls of reduced magmatism (210-180 Ma, 145-125 Ma). Previous studies of zircon in the arc and detrital zircon in the retro-arc region revealed lower Th/U ratios during magmatic lulls, suggesting differences in average melt compositions, and therefore differences in the conditions of magma genesis during periods of low versus high magmatic volumes. We compiled Th/U from 1,965 detrital zircon grains from the Great Valley Group forearc basin that were previously analyzed for U-Pb ages. Our results indicate that Th/U recorded by 250 Ma through 85 Ma detrital zircon on the west side of the arc has an average value of 0.49 ± 0.23. This average Th/U value does not vary significantly through magmatic pulses and lulls or with geographic location in the basin. These Th/U data do show slight variations in standard deviation, with averages from the two lull magmatic states showing smaller standard deviations (0.16 and 0.13, respectively) when compared to the standard deviations of the pulse averages (0.38, 0.21, and 0.19). These results may indicate differences in arc magmatism between the sources of sediment in the retro-arc and forearc regions; alternatively, these results may indicate that Th/U ratios in zircon are not sensitive indicators of the conditions of magma genesis within an overall continental arc setting, and other trace element ratios in zircon may prove to be more effective means to discriminate the average melt compositions through the evolution of the Sierra Nevada magmatic arc.