Geology and geochronology of Grenville-age rocks in the Van Horn and Franklin Mountains area, west Texas: Implications for the tectonic evolution of Laurentia during the Grenville

dNew U-Pb zircon and Ar-40/Ar-39 geochronological data for the Mesoproteroz oic rocks at Van Horn, far west Texas, indicate that metarhyolite, quartzit e, phyllite, and meta-basalt of the Carrizo Mountain Group were formed ca, 1380-1330 Ma and are probably a southern part of the Southern Granite-Rhyol ite province of W.R. Van Schmus and coworkers. These rocks were thrust over the ca, 1250 Ma carbonate-basalt-rhyolite sequence of the Allamoore and Tu mbledown Formations and the younger Hazel Conglomerate (ca, 1035 Ma), The C astner Marble and Mundy Breccia of the Franklin Mountains near EI Paso are coeval, and probably correlative, with the Allamoore-Tumbledown sequence. T he ca, 1120 Ma granite and rhyolite boulders in the Hazel Formation are coe val with the Red Bluff Granite Suite and rhyolite of the Thunderbird Format ion, both occurring in the Franklin Mountains, indicating that formation of granite and rhyolite was widespread in far west Texas at this time. Although these rocks are temporally related to the classic Grenvillian even ts of eastern North America and the Llano uplift of central Texas, they con trast starkly in their much lower (lowest greenschist) metamorphic grade an d degree of deformation. It is likely that the Allamoore-Tumbledown-Castner -Mundy sequences, with their shallow-water sedimentary rocks and bimodal vo lcanic as semblages, accumulated ca, 1250 Ma in rift basins that formed on the southern margin of Laurentia during Grenville orogenesis. Renewed or co ntinued extensional tectonism is implied by the presence of ca, 1120 Ma alk aline granitic and/or basaltic rocks in the Franklin Mountains, at Pajarito Mountain in New Mexico, in the Apache Group of southern Arizona, and in th e Pahrump Group of the Death Valley region of southeastern California; all of these occurrences have been attributed to accumulation in rift basins. T he striking alignment of these occurrences suggests that they were formed a long a zone of transcurrent faulting that extended along the southern margi n of Laurentia during Grenvillian (Mesoproterozoic) time.