Some Remarks on Melting and Extreme Metamorphism of Crustal Rocks

Abstract

Typically melting occurs during decompression in ultra-high-pressureterranes, along the evolution from peak pressure to peak temperature inmedium-temperature eclogite-high-pressure granulite terranes and bysimple prograde heating in granulite facies and ultrahigh-temperature(UHT) metamorphic terranes. The source of heat must be due to One ormore processes among thickening and radiogenic heating, viscousdissipation, and heat from asthenospheric mantle. Melt-bearing rocksbecome porous at a few vol% melt initiating an advective flow regime.As the melt volume approaches and exceeds the melt connectivitytransition ( 7 vol% melt), melt may be lost from the system in thefirst of several melt-loss events. In migmatites and residualgranulites, a variety of microstructures indicates the former presenceof melt at the grain scale and leucosome networks at outcrop scalerecord melt extraction pathways. This evidence supports a model offocused melt flow by dilatant shear failure at low melt volume as thecrust weakens with increasing melt production. Melt ascent is initiatedas ductile fractures but continues in dykes that propagate as brittlefractures. Crustal rocks undergo melting via a sequence of reactionsbeginning with minimal melt production at the wet solidus (generally < 1vol% melt, unless there is influx of H2O-rich fluid). The major phaseof melt production is related to hydrate-breakdown melting (perhaps > 50vol% melt, depending on the fertility of the protolith composition andthe intensive variables). At temperatures above the stability of thehydrate, assuming significant melt loss by this point, low-volume meltproduction continues by consumption of feldspar(s) and quartz at UHTconditions (generally < 10 vol% melt at peak UHTM conditions).Significant melt loss is a contributory factor to achieving UHTs becausedehydration of the system limits the progress of heat-consuming meltingreactions among the residual phases in the source.