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A west-to-east geologic transect across the Peninsular Ranges batholith, San Diego County, California: Zircon 176Hf/177Hf evidence for the mixing of crustal- and mantle-derived magmas, and comparisons with the Sierra Nevada batholith

By
Stirling E. Shaw
Stirling E. Shaw
Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW 2109, Australia
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Victoria R. Todd
Victoria R. Todd
1740 Clarion Drive, Williamsport, Pennsylvania 17701, USA
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David L. Kimbrough
David L. Kimbrough
Department of Geological Sciences, San Diego State University, San Diego, California 92182, USA
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Norman J. Pearson
Norman J. Pearson
Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW 2109, Australia
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Published:
January 01, 2014

Magma mixing was an important process in the genesis of plutonic suites of the Peninsular Ranges batholith, San Diego County transect. Contrary to expectations, minimum Hf arc mantle model ages (HfTAM) calculated from Lu-Hf spot analyses of zircon from 15 granite samples and one gabbro sample indicate a Neoproterozoic component in granites from the western zone of the batholith and even older crustal components, including a Paleoproterozoic component, in those from the eastern zone. The delineation between western and eastern zones in the San Diego County transect of the batholith corresponds closely with a rapidly formed suture zone marked by the western limit of Jurassic S- and transitional I-S-type granites, magnetic and gravity anomalies, and the δ1 8O gradient. Zircon U-Pb ages, many reported herein for the first time, indicate that Early Cretaceous I-type plutons were emplaced into the western zone of the batholith and stitched across both the suture zone and the central belt of deformed Jurassic S-type and I-S-type granites. I-type plutons that intruded east of the suture zone are mainly Late Cretaceous in age. Zircon U-Pb ages, measured as much as possible from the same grains used for 176Hf/177Hf analyses, not only provide a record of crystallization ages but also of the degree of zircon inheritance—of which there is little for Cretaceous western-zone I-type granites. The variation in 176Hf/177Hf (εHf(t)) values for the population of zircon grains from each plutonic sample is therefore interpreted to reflect the degree of magma mixing between crustal- and mantle-derived components between the time of melt generation and final pluton construction, a process that can only be reconciled with open-system chemical behavior. We consider the process of formation of the short-lived suture zone and the S-type granites of the Peninsular Ranges to be examples analogous to the short lived Bundarra Supersuite of the New England batholith (Jeon et al., 2012). The new Hf data of this study are compared to published Nd-Sm model age data for the Peninsular Ranges batholith and to new zircon Hf data for the Tuolumne intrusive suite of the Sierra Nevada batholith.

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GSA Memoirs

Peninsular Ranges Batholith, Baja California and Southern California

Douglas M. Morton
Douglas M. Morton
U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside, California 92521, USA
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Fred K. Miller
Fred K. Miller
U.S. Geological Survey, 904 West Riverside Ave., Spokane, Washington 99201, USA
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Geological Society of America
Volume
211
ISBN print:
9780813712116
Publication date:
January 01, 2014

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