New Mexico Geological Society Annual Spring Meeting — Abstracts

The 1.45 Ga Evergreen Valley Plutonic Complex (EVPC) is a bimodal layered plutonic complex exposed in the Las Vegas Range of the southern Sangre de Cristo Mountains of New Mexico. The complex is an east-west trending 2 km x 1 km elliptical body that cross-cuts a 1.7 Ga garnet tonalite and is in unconformable contact with Paleozoic sedimentary rocks at its north and south contact. The EVPC consists of two main units:  a massive coarse subhedral granular granite that comprises the eastern two-thirds of the complex and a quartz monzogabbro unit that encompasses the western third. The granite contains major microcline, quartz, plagioclase, biotite and accessory magnetite, epidote, and zircon. The granite is homogeneous in mineralogy and isotropic in texture. The quartz monzogabbro contains major but variable amounts of plagioclase, hornblende, biotite, quartz, microcline, and epidote and accessory titanite and magnetite. The quartz monzogabbro is separated from the main granite by an ~ 30m zone of magma mingling. This commingling zone contains several 0.5-1.0-meter thick alternating southwest-dipping mafic and felsic layers. The complex also presents field relations such as silicic pipes and load ball structures, which are consistent with an origin as a mafic-silicic layered intrusion. Contacts between the mafic and felsic units are sharp but interdigitating indicating that the mafic-felsic units were comagmatic. While the EVPC demonstrates magmatic layering, it lacks an internal plutonic foliation and other features characteristic of circa 1.4 Ga synorogenic plutons in the U.S. southwest.