GEOLOGIC AND PETROGRAPHIC ANALYSIS OF ARC-RIFT TRANSITION VOLCANIC ROCKS OF THE SANTA ROSALIA BASIN, GULF OF CALIFORNIA RIFT, BAJA CALIFORNIA SUR, MEXICO

This study examines the geology & petrography of arc to rift transition volcanic rocks of the Santa Rosalia (SR) rift basin in the Gulf of California (Mexico), as part of the Baja Basins NSF-REU (Research Experience for Undergraduates) project. The SR basin is a rift basin that formed by ca. 7 Ma and is being actively mined for stratabound CU-MN-CO of the Boleo Formation. It is floored by Miocene volcanic rocks inferred by Conly (2005) to record a transition from arc to rift volcanism at ca. 12-11 Ma. The current study is the first to provide a detailed map of the basin’s major volcanic units, cross sections, visual estimates of mineral abundances and photomicrographs of minerals and textures.

Based on geochemistry, map units were divided into a subduction and a transitional rift assemblage. The subduction assemblage includes basaltic andesite, trachyandesite and andesite lavas; a trachyandesite lava dome; an andesite peperite intrusion; and a silicic ignimbrite that is overlain by subduction assemblage lava. The lavas and lava dome have plagioclase, clinopyroxene (cpx) and altered hornblende phenocrysts and glomerocrysts in a flow-banded plagioclase microlithic matrix. The peperite intrusion has plagioclase phenocrysts in a groundmass that varies from glassy to microlitic, and has accidental volcanic rock fragments. The silicic ignimbrite consists of pumice with plagioclase and cpx phenocrysts and altered volcanic glass shards. The transitional rift assemblage includes basalt, basaltic-andesite, and andesite lavas, as well as two silicic ignimbrites that interfinger with and overlie transitional rift lavas. The lavas have phenocrystic and groundmass plagioclase, and phenocrysts of cpx and hornblende, including the basalt, in which the hornblende phenocrysts must be zenocrystic. One of the two transitional rift silicic ignimbrites, the non-welded tuff of El Morro, was previously dated by K/Ar on matrix at ca. 8-9 Ma by Conly (2005); its pumices are trachydacite in composition. It is overlain by a previously unrecognized silicic ignimbrite we informally refer to as the purple welded ignimbrite, a rhyolite ignimbrite. Both ignimbrites have rare hornblende and cpx.

This study will be used to select samples for geochronological, isotopic and mineral chemistry studies.

Conley et al., 2005, JVGR 142, 303-341.