USING MAJOR AND TRACE ELEMENTS TO TRACK THE TRANSITION FROM SUBDUCTION TO RIFTING IN THE SANTA ROSALIA BASIN, GULF OF CALIFORNIA, BAJA CALIFORNIA SUR, MEXICO
Based on K/Ar geochronology and geochemical work by Conly (2005)1, 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. As part of the Baja Basins REU, we are mapping the volcanic rocks in detail and determining their major and trace element geochemistry, in preparation for detailed U-Pb zircon and 40 Ar/ 39 Ar geochronological work as well as isotopic analysis. Our preliminary study of the trace element geochemistry of 20 samples from 18 separate map units confirms Conly’s (2005) hypothesis that the subduction-rift transition is present, although our distinction of map units and division of subduction vs. transitional units differs.
Our sample compositions include basalt, basaltic trachyandesite, basaltic andesite, andesite, trachyandesite, dacite, and rhyolite and include lavas, lava domes, intrusions, block-and-ash flow tuffs, ignimbrites, and pyroclastic fall deposits. Two groups emerge when plotted on a La/Sm vs. La plot, indicating a higher degree of partial melting for the group depleted in LREE. This implies that the two groups represent two distinct magmatic systems. Additionally, when plotted on normalized Rare Earth Element (REE) diagrams the same grouping appears. The first group presents an enrichment of light REE (LREE) elements and depletion in heavy REE (HREE), while the second group shows depletion in LREE and enrichment in HREE. Those two patterns are consistent with arc and MORB-like rift signatures, respectively.
1 A.G. Conly et al., 2005, Journal Volcanology and Geothermal Research 142, 303-341.