ARCHITECTURE OF EXTENSION-RELATED PROTEROZOIC UPPER CRUST IN THE GRANITE-RHYOLITE PROVINCE, WEST TEXAS AND ADJACENT NEW MEXICO

The 1.39-1.34Ga granite-rhyolite province forms much of the Texas panhandle and adjacent New Mexico basement; it is locally overlain by the Debaca terrane (a metasedimentary and metavolcanic unit intruded by gabbro). Seismic data for this area show the broadly layered nature of the basement, although the nature of the reflectors has until recently been uncertain. Numerous wells penetrate several hundred meters into basement and provide a 3-D view of the upper crustal architecture. Hornblende-bearing quartz monzonite is the oldest dated rock type (~1380Ma, U-Pb SHRIMP on zircon); it was intruded and overlain by ~1360 Ma granite and comagmatic ignimbritic rhyolite, respectively. Some of the 1360 Ma granites are sill-like. Younger quartz syenite (~1340Ma) represents the final felsic phase of magmatism. All of the quartz syenite is ultrapotossic, whereas the older felsic rocks have high-K chemical affinities. All have magnesian biotite compositions, which suggests relatively oxidizing conditions. Fe-rich, mildly alkaline, OIB-like gabbroic sills intrude the granite-rhyolite pile and locally form extensive sheets >200m thick. Nd model ages for the gabbros are in the 1440Ma range, which implies that they were contemporaneous with the felsic rocks. This bimodal suite thus consists of massive granitic bodies, and also of voluminous rhyolitic sheets, granitic sills, and gabbroic sills. The thickness and lateral extent of these units can explain many of the prominent seismic reflections imaged within the basement of west Texas.

The tectonic setting of the suite is interpreted to be extensional on the basis of (1) its bimodal nature, (2) the alkaline character of both the mafic and felsic samples, and (3) Nd isotope evidence for a crustal source of the felsic magmas. Features characteristic of a subduction setting are absent. Instead the data agree with previous models in which decompression melting of the mantle results in large volumes of mafic magma that trigger extensive crustal anatexis.