MIOCENE VOLCANISM IN THE SANTA ROSALIA REGION, GULF OF CALIFORNIA, BAJA CALIFORNIA: NATURE OF THE ARC-TO-RIFT TRANSITION AND POSSIBLE ROLE OF RIFT-TRIGGERED ADAKITE MAGMATISM IN GENERATING BOLEO STRATIFORM CU-CO-ZN SULFIDES

Miocene volcanic rocks in the Santa Rosalía region record the transition from arc magmatism, due to subduction of the Farallon plate, to rift magmatism, related to the opening of the Gulf of California (Conly et al., 2005)¹; these are estimated to be ~24-12 Ma and ~12 Ma to present (respectively), using published plate tectonic models. As part of the Baja Basins NSF-REU (Research Experience for Undergraduates) project, we describe detailed geologic maps of volcanic rock outcrops that surround and underlie the Santa Rosalía rift basin sedimentary rocks, the type locality of Boleo stratiform Cu-Co-Zn sulfides, currently being mined by Minera Boleo. We present new petrographic, geochemical and ⁴⁰Ar/³⁹Ar  geochronological data on the volcanic rocks (REU 2016), integrated with data gathered by the 2015 REU cohort.

We have now mapped 27 distinct volcanic map units, described the petrography of 58 samples in thin section, obtained and interpreted major and trace element geochemical data on 42 samples, and determined ⁴⁰Ar/³⁹Ar ages on 9 samples (3 more in progress). Lithofacies and compositions include: basalt to andesite lava; trachyandesite lava and lava dome; andesite peperite intrusion; silicic ignimbrite; andesite and dacite block-and-ash-flow tuff; and adakite lava and hypabyssal intrusion. We recognize a gradual transition from arc to rift signatures, spanning 13 Ma to 9.4 Ma, based on geochemistry integrated with 8 of our new ⁴⁰Ar/³⁹Ar dates. However, we have also discovered a younger magmatic event, consisting of adakite lava and hypabyssal intrusion, with an age of 6.11 ± 0.27 Ma (⁴⁰Ar/³⁹Ar plateau ages, hbl and groundmass). We follow Conly (et al., 2005)¹ in interpreting the adakites as the product of rift-related melting of a mantle previously metasomatized by subduction.

Our new age on the adakites is the same as the age of the Boleo stratiform Cu-Co-Zn sulfides, which Holt et al. (2000)² reported at 6.93–7.09 Ma (for the base of the Boleo Formation) to 6.14–6.27 Ma (for the top). We thus show that local adakite magmatism provided a heat engine, and possibly the hydrous fluids, for mineralization in the sedimentary basin.