Geochemistry and Petrology of Dikes and Enclaves of the Mesoproterozoic Enchanted Rock Batholith, Llano Uplift, Central Texas
The Enchanted Rock Batholith (ERB) is a Mesoproterozoic-aged undeformed intrusive igneous body forcefully emplaced into the south-central portion of the Llano Uplift, Central Texas as the product of a Grenville-aged orogenic event. It is a metaluminous to marginally peraluminous fractionated A-type granite hosting ferroan to marginally magnesian, calc-alkalic to marginally alkali-calcic, peraluminous enclaves elongated parallel to the ERB's margins and a grouping of ferroan, calc-alkalic, peraluminous dikes, termed Marschall Creek Dike Complex, in the central-western portion of the batholith. Field, petrographic, and geochemical analysis suggests that the Marschall Creek Dike Complex and the enclaves of the ERB evolved through recharge fractional crystallization with limited magma mixing from ERB, undergoing similar processes during evolution evident by disequilibrium textures present in nearly every sample and correspondingly negative trends of all major oxide vs. SiO2 plots, apart from K2O vs. SiO2, consistent with the fractionation of calcic plagioclase and biotite and, to a lesser extent, zircon, apatite, and titanite. This is supported by the negative trends in the compatible trace elements (i.e., Sr, Ba, Co, V, Zn, and Zr), the partition of LREE and HREE, and negative europium and cerium anomalies of REE. Biotite-rich margins, entrained host material, rapakivi texture, deformation of the enclaves, and undulated surfaces on the dikes are consistent with emplacement into a partially molten host. The doming and zonation of the ERB and orientation of the enclaves parallel to the structure's perimeter suggests that the batholith was emplaced by ballooning.