PETROGENESIS OF MIDPROTEROZOIC GRANITIC MAGMAS - EXAMPLES FROM CENTRAL AND WEST TEXAS

Circa 1.1 Ga granitic magmatism in Texas was manifested as two composi tional groups: (1) the 1.12 Ga Red Bluff granitic suite in west Texas; and (2) 1.12-1.07 Ga granites of the Llano uplift of central Texas. B oth suites share some characteristics typical of 'anorogenic' granites (e.g. potassium-and iron-rich bulk compositions, Fe-rich hydrous sili cates, emplacement conditions involving low oxygen fugacities and wate r contents) and exhibit similar isotopic characteristics. However, roc k associations, mineral chemistries, and trace element compositions of the two suites are distinct and no single petrogenetic model for the two suites is possible. The Red Bluff granitic suite includes cogeneti c syenites, quartz syenites and granites; transitional ferrobasaltic d ikes are also present. In contrast, syenitic and mafic rocks are not a ssociated with the Llano granites. The Llano granites contain biotite and calcic amphibole with lower Fe/(Fe+Mg) ratios compared to those oc curring in the Red Bluff rocks. Alkali amphiboles (e.g. arfvedsonite) occur in the Red Bluff granites but not in the Llano granites. The Red Bluff granitoids are characterized by high FeOT/MgO ratios, high (Na2 O + K2O), high concentrations of HFSE and rare earth elements (REE), a nd other features typical of A-type, 'within-plate' granites [e.g. the Pikes Peak batholith (PPB)]. The Llano granites are geochemically dis tinct with generally higher P2O5 and Sr, lower Na2O, FeOT/MgO, Zr, Y a nd REE, and much lower Ta and Nb. Nd isotopic data overlap between the two granite suites and have 'juvenile' signatures. However, trace ele ment data suggest different petrogeneses for the two suites. The Red B luff suite is interpreted as having a direct derivation from mantle so urces via extended fractional crystallization of basaltic parental mag mas, with minor crustal assimilation. The Llano granites appear to rep resent anatectic melts derived from slightly older, juvenile crustal s ources; some melts underwent fractional crystallization controlled by feldspar and accessory minerals. The petrology and geochemistry of sim ilar to 1.1 Ga granites in Texas indicate that they should not be cons idered as part of a single 'anorogenic' magmatic event. The Red Bluff granitic suite was emplaced into a shelf sequence, north of the Grenvi lle Front, within a broad zone characterized by mild extension. In con trast, Llano granites are late-stage intrusions emplaced into multiply deformed and metamorphosed crust, south of the Grenville Front, durin g or after waning stages of Grenville orogenesis. (C) 1997 Elsevier Sc ience B.V.