THE TRANS-PECOS MAGMATIC PROVINCE, TEXAS: AN EXAMPLE OF MAGMATISM IN A TRANSITIONAL TECTONIC SETTING

The Trans-Pecos Magmatic Province (TPMP) is a large, alkalic magmatic province that formed largely between 48 and 27 Ma, bracketed between the end of the Laramide Orogeny (ca. 50 Ma) and the start of the Basin and Range Orogeny (ca. 24 Ma) in far west Texas. The northwestern sector of the TPMP, which includes the Davis Mountains volcanic field (DMVF), overlies Laurentian basement; the southeastern sector, which includes Big Bend National Park (BBNP), overlies sedimentary and metasedimentary rocks accreted to Laurentia at the end of the Paleozoic Era. Metaluminous to peralkaline quartz-trachyte and rhyolite comprise the dominant volume of volcanic rocks throughout the TPMP; nepheline-trachyte and phonolite occur as a NW-SE trending chain of intrusive bodies along the eastern margin of the province. The tectonic setting of the TPMP has been alternately described as orogenic (a segment of a discontinuous far-inland volcanic arc that extended north to Montana) and anorogenic (one of several magmatic provinces proximal to Río Grande rift basins). Based on comparison with modern environments the TPMP can be thought to have formed in a transitional tectonic environment consisting of four main phases during which the stress field shifted from compressional to neutral to extensional and magma sources shifted from more lithospheric to more asthenospheric.: (1) The collision (Laramide) phase (100-45 Ma) that resulted in lithosphere stacking and orogenic growth, and was less favorable for magma ascent. (2) The post-collision phase (45-36 Ma), initiated during roll-back or break-off of the subducting slab which resulted in astheospheric upwelling, partial melting of the (slab fluid-metasomatized) lithospheric mantle, partial melting of the crust due to the intrusion of basaltic magmas, and widespread and voluminous volcanism. (3) The post-orogenic phase (36-27 Ma), characterized by uplift and an increase in bimodal and peralkaline volcanism resulting from delamination of the lithosphere. (4) The relaxation (Basin and Range) phase (27 to 16 Ma), characterized by normal faulting, a lack of silicic magmatism, and an overall and rapid decline in magmatism.