Abstract

The Peñon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Peñon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ∼900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Peñon Blanco Formation, which is made up of ∼700 m of crystal-lithic basaltic tuff, 1–3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Peñon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Peñon Blanco arc against the inboard Calaveras Complex.

New geochemical data from the Peñon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Peñon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30–60 km depth by low to moderate amounts of partial melting (∼3%–5% for Jasper Point and 5%–7.5% for Peñon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Peñon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America.

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