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 similar to 1163 Ma. Rock types change abruptly with stratigraphic height and include norite, py roxenite, gabbronorite, hornblende gabbro, gabbro, anorthosite, harzbu rgite, and Iherzolite. Norite is most abundant, but gabbro and hornble nde gabbro are locally abundant. Rare olivine-rich layers are also pre sent. The general order of crystallization was olivine, orthopyroxene, plagioclase + clinopyroxene, and hornblende. Mg#'s, expressed as 100M g/(Mg + Fe), range from 76.3 to 85.8 for olivine, 56.7 to 84.9 for ort hopyroxene, 62.5 to 90.3 for clinopyroxene, and 52.4 to 82.8 for amphi bole. Mg#'s vary with height and display abrupt reversals, which indic ate open-system addition of new mafic magma. Eleven cyclic units were identified on the basis of evidence for injection of basaltic magma; t hese can be grouped into three megacyclic units. The abundance of orth opyroxene, and mineral compositional evidence for Fe enrichment within cyclic units, indicates that parental magmas were subalkaline and tho leiitic. Plagioclase in equilibrium with olivine ranges from An(65) to An(46), which precludes an are-related magma source. Although the int rusion is approximately coeval with Keweenawan magmatism and with empl acement of diabasic dikes in western North America, it is dissimilar i n detail to both suites of rocks. Nevertheless, its composition and ge ophysical setting are consistent with emplacement in an extensional te ctonic environment.