Late Miocene calc-alkalic volcanism in northwestern Mexico: an expression of rift or subduction-related magmatism?

Magmatism in NW Mexico records a Late Miocene transformation from convergence to extension in the Gulf of California rift system. Miocene calc-alkalic rocks in the Baja California peninsula are related to the final subduction of the Farallon plate system, but the heterogeneous nature of volcanism younger than 12.5 Ma has led to conflicting tectonic interpretations. Neogene volcanic rocks in the Sierra Santa Ursula, Sonora, were emplaced in three magma pulses, according to mapping, K–Ar geochronology, and geochemistry. From 23.5 to 15 and 14 to 11.4 Ma, calc-alkalic rocks show an arc-like signature. The 12–11 Ma calc-alkalic dacites, however, are characterized by higher K, Rb, ⁸⁷Sr/⁸⁶Sr, and light REE abundances than are the older rocks. The timing, petrography, and geochemistry of the 12–11 Ma rocks are interpreted to reflect postsubduction magmatism. A change in magma chemistry from predominantly calc-alkalic to tholeiitic rocks at 10.3 Ma corresponds to orthogonal extension during early Gulf of California evolution. Sr, Nd, and Pb radiogenic isotope signatures show minor changes over time. The volcanic record for 20–12.5 Ma at Sierra Santa Ursula and adjacent areas is consistent with the reconstructed history of the Guadalupe microplate. The interval of magmatism produced from 12 to 11 Ma appears to reflect changes in plate geometry during the transition from subduction to rifting.