Description
This study examines major and trace element volcanic geochemistry of the volcanic rocks of the Santa Rosalía basin, in the Gulf of California (Mexico), as part of the Baja Basins NSF-REU (National Science Foundation-Research Experience for Undergraduates) project. Recent work in the in the area by Conly (2005) proposes the presence of post-subduction volcanics that would record the transition from arc to rift volcanism, as Baja was wrenched away from mainland Mexico in the late Miocene. However, due to the unreliability of K/Ar dates and absence of sample locations, this study re-examines the major and trace element geochemistry to see if this transition was indeed present. METHODS: We mapped of the volcanic rocks in detail and determining their major and trace element geochemistry, analyzed at Washington State University, in preparation for (pending) detailed U-Pb zircon and 40Ar/39Ar geochronological work as well as isotopic analysis. RESULTS: Our preliminary study of major and trace element geochemistry of 22 samples from 18 separate map unit showed calc-alkaline trends of fractional crystallization, consistent with arc-related magmatism. 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. DISCUSSION: Two separate chemical groups formed within the data, which coincided with their geographic occurrence in either the north or south sub-basin. Rocks from the southern sub-basin were slightly more mafic and showed depletions in LREE and enrichments in HREE when samples are normalized to chondrites. When plotted on a La/Sm vs La plot, trends indicate variable degrees of partial melting from a similar magmatic source. Based on the similarities between the two groups and consistent arc signatures of Nb-Ta-Ti depletions, we feel that there is no chemical indication of rifting in these rocks. Since there are no direct stratigraphic controls between the two groups, there is no way to assume age relationships until new geochronology dates come in.
