THE MONAZITE RECORD OF MELTING AND DEFORMATION IN OROGENIC CRUST

Accurate dates for the occurrence of melting and deformation are crucial for constraining the impacts of orogenesis on continental crust. Melt weakening of the crust during orogeny is particularly effective at promoting middle to lower crustal flow. Additionally, shear zone development and propagation is critical for accommodating plate-scale deformation in orogenic plateaus. In both examples, syn-kinematic accessory phases can provide important constraints on the timing of these processes. Monazite is a common geochronometer in Ca-poor and peraluminous bulk compositions that is sensitive to both melting +/- fluids and strain. Two decades of research on the monazite + melt system has demonstrated that monazite Y- and Th-zoning, in association with peritectic garnet, are some of the best tools for dating melt weakening in migmatites and felsic granulites during orogenesis. Th-rich, Y-depleted monazite domains in leucosome and as inclusions in peritectic garnet represent accurate constraints on melt weakening since they date the fluid-absent melt reaction that produces garnet + melt. During the same past two decades, significant progress has been made at identifying and dating fabric-defining syn-kinematic monazite in shear zones at all length-scales: m- to hundreds of km-scale. Many of these examples involve dissolution precipitation creep and/or fluid-mediated dissolution reprecipitation resulting in monazite grains with either sigmoidal or delta-type geometries with sense-of-shear identical to that observed in thin-section and outcrop. These grains provide direct age constraints on episodes of shear zone displacement during orogenesis. Future research is needed to constrain and balance the metasomatic and metamorphic reactions that lead to the production of syn-kinematic monazite in both fluid- and melt-dominated systems.