Mafic and ultramafic inclusion suites from the Rio Grande rift (New Mexico) and their bearing on the composition and thermal state of the lithosphere

Abstract

Mafic and ultramafic inclusions from Abiquiu, from Cieneguilla, and from Elephant Butte in the northern and central Rio Grande rift in New Mexico have been examined in detail. The type and variety of inclusions differ greatly at each site: at Abiquiu, the inclusions consist of pyroxenite and megacrysts of plagioclase and orthopyroxene; at Cieneguilla, primarily harzburgite, minor granulite, and megacrysts of magnetite and clinopyroxene are present; and at Elephant Butte spinel lherzolite, granulite, clinopyroxenite, and megacrysts of spinel, magnetite, clinopyroxene, olivine, and plagioclase occur. Mineral chemistry and textures indicate that the lithic inclusions (excluding lherzolite and harzburgite) represent fragments of several separate intrusive bodies of different bulk composition located at different depths in the lithosphere. Inclusions with igneous textures were derived from mafic intrusions in the lower crust or upper mantle. Such intrusions may be part of a wedge of mafic rocks emplaced into the lithosphere above a mantle diapir associated with rifting. Granulite with metamorphic texture was probably derived from the uppermost mantle and may have been part of the pre-existing lithosphere, or possibly was an early component of the mafic wedge which was subsequently recrystallized. Spinel-bearing lherzolite and harzburgite are probably accidental fragments of mantle country rock. Megacrysts have compositions similar to the same phases in associated pyroxenite and granulite. Their origin is related to high-pressure crystallization of mafic magmas, but whether as phenocrysts from their host or as (now disaggregated) pegmatites from magmas of the same eruptive episode, cannot in general be determined. ( P,T) conditions at depth can best be estimated for the Elephant Butte suite of inclusions. Spinel lherzolite indicates equilibration temperatures of 935–1030°C at pressures of 14–20 kbar; spinel-two-pyroxene granulite yields 900–980°C at 6–13 kb. A best fit geothermal gradient passes through 1000°C at 50 km, which is significantly higher than the oceanic geotherm. When compared to gradients calculated from surface heat flow data, these data suggest that the geotherm beneath the rift is non-steady state and that the high observed surface heat flow results from shallow (< 50 km) crustal intrusions.