Geochemical Analysis of Arc to Rift Transitional Volcanism of the Boleo Copper District: Santa Rosalia, Baja


Jonathan Rice
BS Candidate
Advisor: Dr. Dave Kimbrough

Thursday, May 5th, 2016
CSL 422 – 10:30am

This study examines major and trace element volcanic geochemistry of
the subduction to rift transition in volcanic rocks of the Santa
Rosalia basin, adjacent to the Gulf of California (Mexico), as part of
the Baja Basins NSF-REU (National Science Foundation-Research
Experience for Undergraduates) project.

Based on K/Ar geochronology and geochemical work by Conly (2005), this
transition is recorded in volcanic rocks less than ca. 12 Ma in the
Santa Rosalia basin, thus raising the possibility that rift volcanism
played a role in the generation of the ca. <7 Ma Boleo-type
stratabound Cu-Mn-Co deposits that are being actively mined from
sedimentary rocks of the basin.

Our sample compositions include basalt, basaltic trachyandesite,
basaltic andesite, andesite, trachyandesite, dacite, and rhyolite from
lavas, lava domes, intrusions, block-and-ash flow tuffs, ignimbrites,
and pyroclastic fall deposits. Although all samples show calc-alkaline
trends consistent with subduction regimes, three groups emerge based
on trace element plots and Eu anomalies in normalized rare earth
element diagrams. These indicate multiple magmatic sources and
variable degrees of partial melting indicated by La/Sm vs La plot, and
presence of adakite-like signatures in Sr/Y vs Y plot. This is
consistent with partial melting of very young, relatively hot, oceanic
crust rather than basic dehydration processes associated with normal