GEOCHEMISTRY OF MINOR PHASES AND IMPLICATIONS FOR THERMOMETRY AND MELT EVOLUTION IN CALC-ALKALIC ROCKS

In calc-alkalic granitic suites, major phase exchange thermometry and minor phase trace element abundances and derived trace element thermometry may be useful for evaluating melt evolution. The relevance of these minor phases for petrologic studies depends on the extent of equilibrium co-crystallization in the final assemblage, as well as useful activity models or at least constraints on activities. At pressures consistent with emplacement at shallow to intermediate crustal depths, cation exchange and trace element thermometers indicate most crystallization and melt evolution of these suites occurred between 800 and 660°C, consistent with experimental phase relations in oxidized and hydrous systems. Pl-hbl rim pairs from granodiorite yield cation exchange temperatures broadly consistent with the hydrous solidus. Coupled isotopic and trace element analyses of coexisting euhedral zircon and titanite provide tests of the application of the trace element thermometers, and allow us to evaluate SiO2 and TiO2 activities necessary for precise application of the existing calibrations of these thermometers. In our samples, zircon is typically polygenetic, with abundant preserved premagmatic zircon, indicating that it is quite resistant to dissolution over the time scales of these magma systems, and arguing for full characterization and caution in interpretation of trace element abundances. Strongly zoned magmatic zircon typically yields T<800°C and rim temperatures overlap with those calculated from pl-hbl exchange equilibria, consistent with co-crystallization of hbl+pl+zrc. Textural evidence suggests titanite crystallized with quartz and hornblende rims, part of a late assemblage succeeding hbl+pl+mt+ilm, indicating crystallization with rising SiO2 activity but not directly indicating co-crystallization with zircon. Titanites are also strongly zoned, and yield an apparent range of crystallization temperatures broadly similar to zircon. Both zircon and titanite yield consistent compositional ranges for many trace elements across many crystals, suggesting crystallization was controlled by partition coefficients within the range of published values. If so, then order of crystallization can be deduced and reasonable constraints placed on activities for hydrous calc-alkalic melts.