Experimental and field based investigations into the behavior of zircon in hydrothermal and deep-tectonic environments during mountain-building and crustal-evolution events.

Abstract

The mineral zircon (ZrSiO4) is a particularly robust geochronometer and arguably one of the most important minerals available to study the evolution of the Earth’s crust. Despite a high preservation potential, zircon grains extracted from high-grade metamorphic rocks associated with mountain building display evidence of alteration, recrystallization, or new-growth, during which new geochronological information is generated. Geochemical and textural evidence suggests these zircon reactions are catalyzed by hydrous fluids generated during the metamorphic evolution of their host rock. Because these metamorphic rocks are witnesses to the most extreme tectonic and metamorphic processes, linking the geochemistry and geochronology of zircon grains to their metamorphic environment remains one of the most vital yet unresolved barriers to understanding crustal geodynamics.

This study focuses on unraveling the response of zircon to mountain building using geological samples from the uplifted and exposed roots of the Dabie Mountains in Central East China, and experimentally measuring zircon/fluid element partition coefficients using quartz-saturated hydrothermal fluids.

Results suggest that when fluids are present, zircon is susceptible to recrystallization and alteration involving loss of radiogenic Pb and rejection of a non-ideal thorite (ThSiO4) solid solution component. Detailed exploration of zircon geochemical and isotopic characteristics can distinguish specific mechanisms of zircon recrystallization and growth, which can be used to date events during the evolution of mountain belts. The results of our zircon/fluid element partitioning study show zircon to incorporate the Rare Earth Elements (i.e. lanthanides) with an increasing preference with decreasing ionic radii (i.e. Dzircon/fluid La << Dzircon/fluid Lu), similar to zircon/melt element partitioning. Overall, partition coefficients are low, not exceeding ~10 for the most compatible elements (i.e. Yb, Lu, Y, Sc, Th, and U). Ratios of Dzircon/fluid Th/U range from 0.3 to 2.7, suggesting that zircon Th/U ratios of 10-2 to 10-3 for grains from metamorphic rocks result from loss of a thorite component during metamorphism.