- UW Department of Geology and Geophysics
Two distinct basaltic suites characterize ∼1165- to 1120-Ma magmatism in west Texas. The ∼1163 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 ± 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.