Abstract

Proterozoic granites in the Llano Uplift, central Texas, intruded metamorphic rocks in a southern segment of the Grenville orogen. Three compositional groups of granite are present; two of these have not been previously recognized. The most widespread group occurs in three emplacement styles: (1)irregular plutons (synkinematic) were emplaced at 1119–1116 Ma during low-pressure Grenville metamorphism and deformation; (2) associated with these, there are volumetrically minor rhyolite dikes (1098–1093 Ma) and hypersolvus granite; and (3) large 1093–1072 Ma postkinematic circular to ovoid plutons are the most abundant of this compositional group, and many show rapakivi textures. A second, more felsic group has much higher light (L) to heavy (H) rare earth element (REE) ratios. The third group is highly fractionated, contains topaz locally, is enriched in Rb, Nb, and HREEs, and is depleted in Sr, Zr, and LREEs. No other igneous rocks were emplaced at the same time as the granites at the current level of exposure. Llano granites cannot be considered anorogenic or A-type on the basis of their tectonic setting, timing, or compositions. Sr, Nd, and Pb isotopic data indicate that these granites were derived from Proterozoic mantle and crust. The granites intruded thick continental crust at one margin of Laurentia and completed the consolidation of one segment of the Grenville orogen.

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