THE 1.45 GA EVERGREEN VALLEY PLUTONIC COMPLEX: A CASE FOR ANOROGENIC MAGMATISM IN NORTHERN NEW MEXICO

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 a west-east trending 2 km x 1 km elliptical body that cross-cuts a lineated to foliated syntectonic 1.73 Ga garnet tonalite and is in unconformable contact with Paleozoic sedimentary rocks. The EVPC consists of two main units: a coarse subhedral granular granite that comprises the eastern two-thirds of the complex and a quartz gabbro-diorite unit that encompasses the western third. The granite contains major microcline, quartz, plagioclase and biotite and accessory magnetite, titanite, and zircon. The granite is homogeneous in mineralogy and isotropic in texture. The quartz gabbro-diorite contains major but variable amounts of plagioclase, hornblende, biotite, quartz, microcline, and epidote and accessory titanite and magnetite. The gabbro and diorite are separated from the main granite by an approximately 30 m zone of magma mingling and hybridization. This commingling zone contains several 0.5-2.0 m thick alternating southwest-dipping mafic and felsic layers. The mafic unit contains irregular patches of alkali feldspar, suggesting some degree of magma mixing. Contacts between the mafic and felsic units are sharp but interdigitating indicating that the mafic-felsic units were comagmatic. The commingling zone also contains a 10 m thick layer of tightly packed mafic pillows in a granitic matrix. These plutonic pillows are generally elongate (0.5-1.0 m x 0.25-0.5 m) with few having downward (northeastward) pointing protrusions. Field relationships suggest that the mafic pillows originated from tubular flow units and that the gabbro-diorite unit represents a mafic magma that intruded into and variably reacted with a silicic magma. While the EVPC demonstrates magmatic layering, it lacks a strong foliation and other features characteristic of circa 1.4 Ga synorogenic plutons in the U.S. southwest. Its bimodal character and cross-cutting structure suggest that the EVPC is an anorogenic complex that was emplaced before the onset of the regionally recognized 1.4 Ga orogenic episode.