Grenville magmatism in west Texas: Petrology and geochemistry of the red bluff granitic suite

The Red Bluff granitic suite. (RBGS) underlies part of the Franklin Mountains in west Texas. It was emplaced into a Middle Proterozoic shelf sequence at 1120±35 Ma. The suite is predominantly metaluminous; it consists of early granitic to quartz syenitic sills, a main mass of alkali feldspar granite, small bodies of alkali feldspar syenite, leucogranitic dikes, and late-stage peralkaline arfvedsonite granite. Late-stage, mildly alkaline (transitional) ferrobasaltic dikes are also present. The suite shows continuous compositional variation from quartz syenite to leucogranite. It is characterized by an iron-rich mineral assemblage, estimated f(O₂) near FMQ initial emplacement temperatures near 1050°C, and high concentrations of high field strength and rare earth elements. These features serve to classify the suite as A-type; trace element discrimination diagrams identify it as 'within plate'. When plotted in multielement diagrams, trace element patterns of both the granitic rocks and the ferrobasalts are generally subparallel to those of ocean island basalts. No elemental evidence for a subduction-related origin is present among the granitic rocks. Major element mass-balance models that use observed phases are consistent with an origin by fractional crystallization from a transitional ferrobasaltic parent. However, trace element tests of these models indicate that trapped liquid remained in the crystalline residue ( 'in situ crystallization'). Partial melting models that use known basement rocks as parental compositions generally fail to reproduce the observed trace element patterns, and those that fit individual rock compositions cannot explain the continuous compositional vanation of the suite. Derivation from basaltic parental magmas indicates that the RBGS represents addition of juvenile material to the crust during Grenville time. The mildly alkaline nature of the granitic suite and its lack of subductionrelated geochemical features are consistent with an origin in a Zone of regional extension, as suggested by recent geophysical and petrological studies.