THE TERTIARY TRANSITION FROM "SUBDUCTION-RELATED" TO "RIFT-RELATED" MAGMATISM IN SOUTHERN SONORA, MEXICO: A FIELD, PETROLOGIC, AND GEOCHEMICAL STUDY

The coastal province in Sonora, Mexico is characterized by multiple NNW-SSE trending fault-bounded ranges (Sierra El Aguaje, Sierra Liebre, and Sierra Santa Ursula) that expose >1 km thick 20-9 Ma volcanic sequences, which range in composition from basalt to rhyolite. Plate tectonic reconstructions suggest these volcanic rocks were erupted during the transition from subduction to transtension/rifting in the Gulf of California. Major and trace element geochemistry serves as a fingerprint for both tectonic environments as well as the transition itself. Synsubduction volcanic rocks (>12.5 Ma) have typical arc affinities, including a depletion in high field strength elements (including the characteristic Nb-Ta troughs) and an enrichment in large-ion lithophile elements. They also have significantly higher potassium content and a light rare earth element enrichment, relative to younger samples, which may indicate significant crustal contamination. The syntranstensional volcanic rocks (<12.5 Ma) exhibit a more variable trace element geochemistry, which overall resembles an ocean island basalt (OIB) signature. Previous studies in Baja California and Sierra Santa Ursula suggest there was a dramatic transition from synsubduction calc-alkaline volcanism to syntranstension suboridinate, tholeiitic volcanism, which our study does not show. Aside from these trace element variations, this study shows both groups have fundamentally similar major element geochemisty—they are both calc-alkaline volcanic rocks.