Fluid-enhanced melting during prograde metamorphism

Anatexis is a commonly recognized feature of high-grade metamorphism, but s egregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path. A sui te of small-volume; deformed, two-mica leucogranites has been recognized wi thin the High Himalayan Crystalline Series of the Garhwal Himalaya. These g ranites are consistently more siliceous than minimum-melt granite compositi ons and are characterized by low Rb/Sr ratios, high Ba, low abundances of H FS elements and positive Eu anomalies. Such trace-element characteristics c ontrast strongly with the geochemistry of the well-studied Early Miocene le ucogranites 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 Himala yan leucogranites. Whilst some entrainment of restitic phases cannot be exc luded, trace element signatures suggest a low temperature (<650<degrees>C) crustal melt formed under conditions of high H2O activity. Positive Eu anom alies and unusually low Rb/Sr ratios are indicative of rapid, disequilibriu m melting. Fluid-enhanced melting may be a common feature of prograde upper amphibolit e-facies metamorphism of orogenic belts, predating peak metamorphism by at least 15 Ma. These melts will only crystallize within this period if they s egregate 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-eleme nt chemistry. The persistence of early formed melts within an orogen provid es insights into the prograde heating path, and may be critical in controll ing the rheology of the middle crust, and hence its deformational history.