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Post-arc Plio-Pleistocene volcanism in the Tahoe-Truckee volcanic field, northern Sierra Nevada, California

Post-arc Plio-Pleistocene volcanism in the Tahoe-Truckee volcanic field, northern Sierra Nevada, California

Abstracts with Programs - Geological Society of America 32(7): 150

Subsequent to passage of the southern margin of the Gorda Plate and the cessation of Mio-Pliocene arc volcanism, the Lake Tahoe-Truckee River region continued to experience significant volcanic activity between 2.9 and 1.2 Ma. Whereas widespread Mio-Pliocene rocks consist of porphyritic plagioclase, pyroxene and hornblende andesites (e.g., Squaw Peak), small volume Plio-Pleistocene centers are dominantly olivine-pyroxene-bearing basalts through dacites. All Plio-Pleistocene compositional lava groups follow calc-alkaline fractionation trends, like the Mio-Pliocene volcanic rocks, but are commonly more K (sub 2) O-rich and have higher REE abundances. All compositional groups have similar primitive mantle-normalized REE patterns, except for two groups which are less LREE-enriched like the Mio-Pliocene volcanic rocks. Pb (206/204 = 18.85-19.15), Sr (0.7038-0.7062) and Nd (0.51248-0.51277) isotope ratios are variable within each lava group, but groups overlap isotopically with each other and with Mio-Pliocene volcanic rocks. Isotope ratios are intermediate between extension-related basalt of the Western Great Basin and arc rocks of the modern south Cascade Range. We interpret the overlap in isotopic composition with Mio-Pliocene arc rocks to indicate that Plio-Pleistocene magmas were also derived from subduction-modified mantle wedge that was previously fluxed by fluids derived from the Gorda Plate during the late Pliocene. Mantle decompression melting may have continued until 1.2 Ma due to Sierran uplift and a thermal slab window effect. The more alkaline nature and distinct mineralogy of the Plio-Pleistocene lavas reflect lower degrees of mantle melting and different magma emplacement histories compared to Mio-Pliocene magmas.

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