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2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 6
Presentation Time: 8:00 AM-12:00 PM

SR AND ND ISOTOPIC EVOLUTION IN LATE CRETACEOUS LOWER TO UPPER CRUSTAL GRANITIC ROCKS FROM THE TRANSVERSE RANGES, SOUTHERN CALIFORNIA


WIEGAND, Bettina A.1, BARTH, Andrew P.2, WOODEN, Joseph L.3, PALMER, Emerson F.2, BROWN, Kenneth L.2 and NEEDY, Sarah K.2, (1)Geological Environmental Sciences, Stanford University, 367 Panama Street, Stanford, CA 94305, (2)Dept of Earth Sciences, Indiana Univ~Purdue Univ, Indianapolis, IN 46202, (3)U. S. Geol Survey, Menlo Park, CA 94025, bwiegand@pangea.stanford.edu

The Transverse Ranges in southern California expose a cross-section of the Mesozoic magmatic arc in the western North American Cordillera. Andean-type magmatism commenced in the Triassic and lasted through the Late Cretaceous. Calcalkaline arc plutons emplaced through continental crust show enriched isotopic signatures that are generally ascribed to crustal contamination or an enriched mantle lithosphere (e.g. Barth et al. 1995). Various plutonic rocks from lower to upper crustal intrusion depths were analyzed for Sr and Nd isotopic compositions to evaluate the evolution of those rocks. Lower crustal units include tonalite and diorite from the Waterman Mountain batholith, and Josephine Mountain intrusion in the San Gabriel Mountains (Central Transverse Ranges). Middle to upper crustal units are represented by granodioritic to granitic rocks of the structurally deeper sheeted plutonic complex, and shallower granites, both located in the Little San Bernardino, Pinto, and Eagle Mountains (Eastern Transverse Ranges). We compare a series of contemporaneous Late Cretaceous plutonic rocks with intrusion ages between 74 Ma and 83 Ma (U/Pb zircon SHRIMP ages). Initial Sri and εNd values range from 0.708-0.712 (–10 to –16) in upper crustal granites, and 0.708-0.714 (–11 to –16.5) in the sheeted plutonic complex. Lower crustal intrusive rocks are characterized by Sri and εNd values of 0.707-0.708 (-11 to –12) in the Waterman Mountain, and 0.709-0.710 (-10 to –11) in the Josephine Mountain intrusion. Data plot along a mixing trend indicative of an enriched mantle source for mafic rocks and minor but persistent assimilation of crustal rocks during ascent of mafic magmas into the upper crust. Sheeted middle crustal intrusive rocks and upper crustal granites show significant overlap in Sr and Nd isotopes, suggesting that magmas feeding the granites originate in mixing of tonalite and granite in the structurally lower sheeted complex.

Session No. 146--Booth# 145

T88. Geochemistry of Magmatic and Metamorphic Processes: In Honor of the Contributions of Joseph L. Wooden (Posters)
Tuesday, 30 October 2007: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 39, No. 6, p.407
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