Petrology and geochemistry of lavas from the rift basins in the Gulf of California

As a focus site of the recently concluded Rupturing of Continental Lithosphere and Birth of an Ocean initiative of the MARGINS program at NSF, the Gulf of California was the object of a number of investigations during the past several years. However, though studied extensively using geophysical methods, only a few basins have been studied petrologically (e.g. Alarcon, Guaymas). Here we report new major and trace element and Sr, Nd, and Pb isotope data for lavas sampled from select off-axis seamounts and outcrops of the ocean basin crust from the Lower Delfin basin in the north to the Pescadero basin in the south. Our samples provide petrologic data from locations previously unexplored. Lava samples from each location generally range from basalt to rhyolite, but rock type distribution is variable. In general, locations further south contain mostly basalts and some differentiated lavas whereas locations further north have fewer basalts and more differentiated lavas. The trace element characteristics of the basalts are highly variable, overlapping with those of the depleted mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) segments south of the Tamayo transform to the more enriched calc-alkaline basalts in the Guaymas Basin. The ⁸⁷Sr/⁸⁶Sr (0.70269-0.70328), ¹⁴³Nd/¹⁴⁴Nd (0.51299-0.51311), and ²⁰⁶Pb/²⁰⁴Pb (18.476-18.805), ²⁰⁷Pb/²⁰⁴Pb (15.528-15.583), and ²⁰⁸Pb/²⁰⁴Pb (38.048-38.449) ratios of these basalts also show variability and overlap with EPR MORB and the more variable samples cored during DSDP legs 63-65 in the Gulf. The more differentiated lavas trend away from MORB towards more crustal or radiogenic values. Our results combined with literature data suggest that magmatism in the Gulf of California varies widely over short length scales, most likely reflecting multiple sources and/or contamination by different lithospheric materials.