PETROGENESIS OF THE RATTLESNAKE MOUNTAIN SILL, TRANS-PECOS, TEXAS

The Trans-Pecos magmatic province (TPMP) is a large igneous province with alkaline igneous activity spanning compressive and tensional stress regimes. Within the TPMP, Rattlesnake Mountain sill (RMS) is an internally differentiated analcite-bearing syenodiorite intrusion located on the western border of Big Bend National Park, Texas. RMS (~26 Ma) is characteristic of a group of about ten intrusions emplaced during the early tensional period of magmatic activity in the TPMP. The aim of this study on RMS is to determine source material, and temperature, pressure and fluid conditions at the locus of melting, as well as to better delineate processes of differentiation prior to emplacement of the intrusion. This data will shed light on petrogenesis of some alkaline magmas and on the tectonic history of the TPMP. Data from previous studies indicate that the RMS magma likely has a mantle source with an OIB-type reservoir and has undergone fractional crystallization within the crust. This study tests that hypothesis with geochemical data and and piston-cylinder experiments.