GEOCHEMISTRY OF MIOCENE VOLCANIC ROCKS EXPOSED NEAR SAN LUIS RESERVOIR, CENTRAL CALIFORNIA COAST RANGES: MANTLE HETEROGENEITY IN A TRANSITIONAL SUBDUCTION-TRANSFORM SETTING

Miocene (7.4-9.5 MA) volcanic flows and pyroclastic rocks located in the San Jose 30’ x 60’Quadrangle near San Luis Reservoir (SLR) in the Central California Coast Ranges are part of a northwest-younging sequence of volcanic rocks inferred to be associated with passage of the Mendocino Triple Junction (MTJ) and formation of a slab window. Our SLR samples were collected at localities on opposite sides of the Ortigalita fault: Dinosaur Point (DP) to the west and Basalt Hill (BH) to the east of the fault. Major and trace element analyses for five samples collected at the BH locality (basalt characterized by a porphyritic texture with abundant olivine and clinopyroxene and sparse plagioclase phenocrysts) were acquired using XRF and ICP-MS at Washington State University and compared with analyses for five Dinosaur Point (DP) samples (basaltic andesite) reported in the literature. The BH and DP samples are subalkaline, medium-K calcalkaline rocks with nearly identical REE and MORB-normalized trace element patterns. The REE patterns show modest LREE enrichment and no Eu anomaly, suggesting minimal fractionation, and display arc-like enrichment in LILs and depletion in Nb and Ta, which is atypical of slab-window magmatism. The geochemical signatures of the SLR volcanic rocks are broadly compatible with infilling of the slab window by the relict supra-slab mantle wedge to the east. However, the BH and DP samples separate into two discrete groups on the basis of several trace element ratios (e.g. Zr/Hf, Nb/Ta, Ba/Ta), suggesting that they were derived from distinct mantle sources.

Future work will include geochemical and petrographic descriptions for four additional samples collected at Dinosaur Point and electron microprobe analysis to characterize mineral chemistry and zoning in the BH and DP samples. The SLR basalts and basaltic andesites will be compared to other volcanic rocks associated with passage of the MTJ, including basalt, andesite and dacite with an adakite-like chemical signature found as scattered dikes and plugs in the Diablo Range to the north of San Luis Reservoir and the Sonoma and Clear Lake volcanic fields located north of San Francisco.