OXYGEN ISOTOPE AND GEOCHEMICAL ANALYSIS OF VOLCANIC ROCKS FROM SOUTH-CENTRAL NEW MEXICO: INSIGHT ON CRUSTAL CONTAMINATION AND MAGMATIC EVOLUTION

Southern New Mexico's geological history is a record of complex tectonic and magmatic events between 45-24 Ma. Cenozoic tectonic events have left many questions about the magmatic sources and petrogenesis responsible for the volcanic activity distributed throughout New Mexico. Previous studies of the area propose continental arc magmatism (45-36 Ma) was triggered by slab “break-off” of the Farallon Plate, allowing inflow from the asthenosphere at around 28 Ma. Geochemical and isotopic signatures of volcanic rocks suggest crustal modification and hybridization by mafic magmas from the subducting Farallon Plate shifted source from a subduction-modified lithospheric mantle signature between 45-28 Ma to a more enriched or ocean island basalt (OIB)-like mantle signature around 28 Ma. Continental basalts and andesites derived with lithospheric mantle signatures are interpreted to have low concentrations of Nb and Ta and high concentrations in Ba, Sr, and Rb and low and variable ε_Nd (+2 to -8) compared to OIB's. However, athenospheric signatures from some upper Cenozoic basalts in the area suggest they are products from partial melting within the upwelling and decompressing asthenosphere. These magmas have high Nb and Ta concentrations and ε_Nd values between +7 and +4, which are similar to concentrations and values of North American OIB. Here we present isotopic data to evaluate the magmatic evolution of south-central New Mexico volcanic rocks through valuable, and formerly nonexistent, oxygen isotope data from the Uvas Basalts (28-24 Ma) and Rubio Peak Formation (45-36 Ma).These Oligocene and Eocene mafic to intermediate suites were selected based on their connection to the timing of shifts in magmatic composition in the area. New oxygen isotope data from silicate mineral separates and mass balanced models in combination with Sr and Nd isotopic data allows us to define the hybridization and modification of the crust during the transition from arc magmatism to Rio Grande rifting in southern New Mexico. We suggest that the Rubio Peak formation volcanic rocks represent a period of crustal contamination of large volumes of mantle basalts creating a mid-crustal hot zone. The Uvas basalts and basaltic andesites represent the first true mantle derived magmatism in the region and become progressively more hybridized over time.