New Mexico Geological Society Annual Spring Meeting — Abstracts

Latest Mesozoic to mid-Cenozoic age igneous rocks in the southern United States and northern Mexican Cordillera show variable enrichment in alkali elements, with a notable enrichment in alkalinity from west to east. The cause of this pattern is still debated, particularly in the Trans-Pecos region of southern New Mexico, west Texas, and northern Chihuahua where magmatism resulted from multiple tectonic processes. One end-member model suggests that incipient extension associated with the Rio Grande Rift produced decompression melting and low-degrees of mantle melting to generate the alkaline magmas. This model predicts a symmetrical spatial pattern with less alkalic (more metaluminous) magmatism in the core of the rift and more alkalic magmatism on the rift flanks. Another end-member model suggests that magmatic activity is predominantly subduction-related and that the west-to-east increase in alkalinity is due to a shallowing subduction angle and the progressive dehydration of the subducting slab. The decreasing amounts of available fluids may result in decreasing melt-fractions toward the east. This model predicts an asymmetric spatial pattern with alkalinity steadily increasing toward the east. This study presents new and compiled geochemical data and new geochronological data to help evaluate spatial and temporal trends in alkalinity in the Trans-Pecos province. A primary goal of the research is to produce an updated isopleth map showing how alkalinity changes across the region.