GRENVILLE MAGMATISM IN WEST TEXAS - PETROLOGY AND GEOCHEMISTRY OF THERED BLUFF GRANITIC SUITE

The Red Bluff granitic suite (RBGS) underlies part of the Franklin Mou ntains in west Texas. It was emplaced into a Middle Proterozoic shelf sequence at 1120 +/- 35 Ma. The suite is predominantly metaluminous; i t consists of early granitic to quartz syenitic sills, a main mass of alkali feldspar granite, small bodies of alkali feldspar syenite, leuc ogranitic dikes, and late-stage peralkaline arfvedsonite granite. Late -stage, mildly alkaline (transitional) ferrobasaltic dikes are also pr esent. The suite shows continuous compositional variation from quartz syenite to leucogranite. It is characterized by an iron-rich mineral a ssemblage, estimated f(O-2) near FMQ, initial emplacement temperatures near 1050 degrees C, and high concentrations of high field strength a nd rare earth elements. These features serve to classify the suite as A-type; trace element discrimination diagrams identify it as 'within p late'. When plotted in multielement diagrams, trace element patterns o f both the granitic rocks and the ferrobasalts are generally subparall el to those of ocean island basalts. No elemental evidence for a subdu ction-related origin is present among the granitic rocks. Major elemen t mass-balance models that use observed phase are consistent with an o rigin by fractional crystallization from a transitional ferrobasaltic parent. However, trace element tests of these models indicate that tra pped liquid remained in the crystalline residue ('in situ crystallizat ion'). Partial melting models that use known basement rocks as parenta l compositions generally fail to reproduce the observed trace element patterns, and those that fit individual rock compositions cannot expla in the continuous compositional variation of the suite. Derivation fro m basaltic parental magmas indicates that the RBGS represents addition of juvenile material to the crust during Grenville time. The mildly a lkaline nature of the granitic suite and its lack of subduction-relate d geochemical features are consistent with an origin in a Zone of regi onal extension, as suggested by recent geophysical and petrological st udies.