Geochemistry of Proterozoic granitic plutons from New Mexico, U.S.A.

Abstract

Thirty-eight Proterozoic granitic plutons from New Mexico have been analyzed for major and trace elements. The results indicate a four-fold classification into high-Ca, high-Si, high-K, and trondhjemite groups. The high-Si and high-K groups are further divided into low-REE and high-REE subgroups. Trondhjemites have been found only in the extreme north-central part of the state while members of the high-Ca group dominate in the northern part of the state. The high-Si and high-K groups characterize the central and southern parts of the state. Normative data suggest that the last two groups are initial melts or highly fractionated melts emplaced at low water pressures. Geochemical model studies favor an origin for the high-Ca group by about 50% partial melting of siliceous granulite in the lower crust. The results of the high-Si and high-K groups are most consistent with models for their origin involving 65–75% shallow fractional crystallization of high-Ca group magmas. Removal of variable but small amounts of REE-rich accessory minerals are required for the production of some of these magmas. The origin of the trondhjemites appears to necessitate at least a two-stage model. Extensive regional heating and partial melting of the lower crust in New Mexico giving rise directly (or indirectly) to the granitic magmas may have been caused by rising mantle plumes between 1.8 and 1.4 b.y. ago.