Geochemistry and petrogenesis of Mesoproterozoic (~ 1.1 Ga) magmatic enclaves in granites of the eastern Llano Uplift, central Texas, USA

Abstract

Mesoproterozoic (~1.1Ga) plutons of the eastern Llano Uplift, central Texas, USA contain two types of magmatic enclaves (<1% by vol.). Although volumetrically insignificant, the enclaves contain important petrogenetic information. Type I enclaves are felsic in composition (70–75wt.% SiO2), with mineral assemblages and chemical compositions comparable with the host granites, but typically display a finer grained texture. They are interpreted as partly chilled disrupted material from the margins and roof of the plutons. Type II enclaves are intermediate in composition (~56–69wt.% SiO2), with many elements defining trends continuous with the host granites. Both types of enclaves display sharp borders in contact with the host granite suggesting magma quenching with little or no physical exchange between host granite and enclave magma.Type II enclaves contained within the Marble Falls (MF) and Lone Grove (LG) plutons exhibit enrichments in Y, Nb, and Zr relative to their respective host granites. Enrichments in these incompatible trace elements at low SiO2, renders unlikely the possibility that the MF and LG Type II enclaves are the result of partial melting (anatexis) of mafic crustal rocks. Numerical modeling of fractional crystallization and simple mixing fails to explain the observed trace element trends.Because no coeval mafic to intermediate rocks are exposed in the uplift, characteristics of Type II enclave source magma(s) is uncertain. However, assuming source magmas similar to primitive continental arc basaltic andesite, trace-element trends (i.e., incompatible element enrichment and compatible element depletion) can be adequately replicated by a replenishment fractional crystallization (RFC) model. Chemistry of the MF and LG Type II enclaves suggest repeated replenishment of primitive magmas with only limited interaction with the host granitic magmas; the more primitive enclave magmas evolving in near chemical isolation by RFC processes. However, evidence from Type II enclaves in two other plutons in the Llano Uplift (Kingsland and Enchanted Rock) suggest that the isolation was non-ideal; i.e., some limited mixing may have occurred. Rapid quenching likely limited the potential for physical and chemical exchange between Type II enclaves and their host granite magmas.