Abstract

New U-Pb zircon and 40Ar/39Ar geochronological data for the Mesoproterozoic rocks at Van Horn, far west Texas, indicate that metarhyolite, quartzite, phyllite, and metabasalt of the Carrizo Mountain Group were formed ca. 1380–1330 Ma and are probably a southern part of the Southern Granite–Rhyolite 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 Tumbledown Formations and the younger Hazel Conglomerate (ca. 1035 Ma). The Castner Marble and Mundy Breccia of the Franklin Mountains near El Paso are coeval, and probably correlative, with the Allamoore-Tumbledown sequence. The ca. 1120 Ma granite and rhyolite boulders in the Hazel Formation are coeval with the Red Bluff Granite Suite and rhyolite of the Thunderbird Formation, 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 events of eastern North America and the Llano uplift of central Texas, they contrast starkly in their much lower (lowest greenschist) metamorphic grade and degree of deformation. It is likely that the Allamoore-Tumbledown-Castner-Mundy sequences, with their shallow-water sedimentary rocks and bimodal volcanic assemblages, accumulated ca. 1250 Ma in rift basins that formed on the southern margin of Laurentia during Grenville orogenesis. Renewed or continued extensional tectonism is implied by the presence of ca. 1120 Ma alkaline granitic and/or basaltic rocks in the Franklin Mountains, at Pajarito Mountain in New Mexico, in the Apache Group of southern Arizona, and in the Pahrump Group of the Death Valley region of southeastern California; all of these occurrences have been attributed to accumulation in rift basins. The striking alignment of these occurrences suggests that they were formed along a zone of transcurrent faulting that extended along the southern margin of Laurentia during Grenvillian (Mesoproterozoic) time.

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