TWO distinct basaltic suites characterize ~1165-to 1120-Ma magmatism in west Tbxas. The ~1 1 6 3 Ma Pecos mafic intrusive complex consists of layered mafic and ultramafic rocks. Megacyclic units record repeated influx and mixing of tholeiitic magmas to yield a predominantly noritic body. Basaltic magmatism also accompanied emplacement of the 1120 f 35-Ma Red Bluff granitic suite in the Franklin Mountains. In terms of their major elements, these basaltic rocks are transitional between tholeiitic and alkaline, but their low concentrations of Nb suggest a subduction-related origin. Except for their low Nb contents, the Franklin Mountains basaltic rocks are similar to coeval basaltic intrusive and volcanic rocks in the southwestern U.S.A. Of the numerous proposed tectonic settings, an extensional environment is preferred. Possible extensional associations include splays of the Midcontinent Rift, rifting caused by collision of a continental fragment during the Grenville orogeny, and extension related to collapse of over-thickened crust during the Grenville event. The low Nb contents of basalts associated with Franklin Mountains basaltic rocks are best explained as an artifact of older subduction, which modified the mantle source of the basalts hundreds of millions of years prior to their emplacement.

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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. Norite is most abundant, but gabbro and hornblende gabbro are locally abundant. Rare olivine-rich layers are also present. The general order of crystallization was olivine, orthopyroxene, plagioclase + clinopyroxene, and hornblende. Mg#'s, expressed as 100 Mg/(Mg + Fe), range from 76.3 to 85.8 for olivine, 56.7 to 84.9 for orthopyroxene, 62.5 to 90.3 for clinopyroxene, and 52.4 to 82.8 for amphibole. Mg#'s vary with height and display abrupt reversals, which indicate open-system addition of new mafic magma. Eleven cyclic units were identified on the basis of evidence for injection of basaltic magma; these can be grouped into three megacyclic units. The abundance of orthopyroxene, and mineral compositional evidence for Fe enrichment within cyclic units, indicates that parental magmas were subalkaline and tholeiitic. Plagioclase in equilibrium with olivine ranges from An65 to An46, which precludes an arc-related magma source. 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.

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