A Grenville-age layered intrusion in the subsurface of west Texas: petrology, petrography, and possible tectonic setting

The Nellie intrusion is a thick (more than 4420 m) mafic to ultramafic layered intrusion with a radiometric age of ~1163 Ma. Rock types change abruptly with stratigraphic height and include norite, pyroxenite, gabbronorite, hornblende gabbro, gabbro, anorthosite, harzburgite, and lherzolite. The general order of crystallization was olivine, orthopyroxene, plagioclase + clinopyroxene, and hornblende. Eleven cyclic units were identified on the basis of evidence for injection of basaltic magma; these can be grouped into three-megacyclic units. Although the intrusion is approximately coeval with Keweenawan magmatism and with emplacement of diabasic dikes in western North America, it is dissimilar in detail to both suites of rocks. Nevertheless, its composition and geophysical setting are consistent with emplacement in an extensional tectonic environment. -from Authors