From sediment to granite: timescales of anatexis in the upper crust

Granite formation is the culmination of a sequence of events initiated by p rograde heating of the protolith and followed by formation of a grain-bound ary melt, melt segregation into a vein network, ascent of the melt through the network and, finally, crystallisation of the melt. Experimental constra ints on the formation of a crustal melt from the incongruent melting of mus covite combined with geochemical studies of anatexis in the High Himalaya a llow the timescales required for each of these processes to be assessed. Di scordant temperatures determined from monazite and zircon thermometry for H imalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in LREE implying that melts have probably been extr acted in less than 10 ka. Experimental studies suggest that some Himalayan melts are also undersaturated in Zr, implying segregation may have occurred within 100 years. Such short timescales confirm that deformation-driven me chanisms are important in extracting these melts from their source. The tra nsport distances of Himalayan granitic melts of similar to 10 km may be ach ieved by the ascent of magma through dykes in about I day. At such rates ev en the largest granite could theoretically be emplaced in similar to 10 yea rs. Crystallisation of Himalayan melts involves much longer periods. If emp laced as thin sheets (similar to 100 m wide) a timescale of > 500 years is required compared with > 30 ka for single stage intrusion of the larger lac coliths. For composite sheet complexes magma crystallisation, rather than m elt ascent, comprises the rate-determining step on the emplacement of the i ntrusion. The overall timescales of melt segregation and emplacement for ma ny orogenic granites are therefore less than 10 ka, and possibly less than 1 ka. In contrast, the timescale required for prograde heating of the proto lith is much greater than I Ma. Since the melt production rate is determine d by heat flow into the protolith, and not by reaction kinetics (for any ge ologically significant period) we conclude that heat flow, determined by bo th the mechanism of heating and the thermal diffusivities of crustal rocks, provides the overall rate-determining step of crustal anatexis. (C) 2000 E lsevier Science B.V. All rights reserved.