Fluid-enhanced melting during prograde metamorphism

Abstract

Anatexis is a commonly recognized feature of high-grade metamorphism, but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path. A suite of small-volume, deformed, two-mica leucogranites has been recognized within the High Himalayan Crystalline Series of the Garhwal Himalaya. These granites are consistently more siliceous than minimum-melt granite compositions and are characterized by low Rb/Sr ratios, high Ba, low abundances of HFS elements and positive Eu anomalies. Such trace-element characteristics contrast strongly with the geochemistry of the well-studied Early Miocene leucogranites of the High Himalaya, derived from fluid-absent melting. Sm–Nd garnet dating of one deformed granite indicates a crystallization age of 39±3 Ma, c. 15 Ma before the emplacement of the more voluminous High Himalayan leucogranites. Whilst some entrainment of restitic phases cannot be excluded, trace element signatures suggest a low temperature (<650°C) crustal melt formed under conditions of high H 2 O activity. Positive Eu anomalies and unusually low Rb/Sr ratios are indicative of rapid, disequilibrium melting. Fluid-enhanced melting may be a common feature of prograde upper amphibolite-facies metamorphism of orogenic belts, predating peak metamorphism by at least 15 Ma. These melts will only crystallize within this period if they segregate from their protoliths. Subsequent dating of long-lived melts would indicate erroneously young ages for the prograde melting events. However, melts formed in this way may be recognized by their distinctive trace-element chemistry. The persistence of early formed melts within an orogen provides insights into the prograde heating path, and may be critical in controlling the rheology of the middle crust, and hence its deformational history.