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

Granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain-boundary melt, melt segregation into a vein network, ascent of the melt through the network and, finally, crystallisation of the melt. Experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the High Himalaya allow the timescales required for each of these processes to be assessed. Discordant temperatures determined from monazite and zircon thermometry for Himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in LREE implying that melts have probably been extracted 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 mechanisms are important in extracting these melts from their source. The transport distances of Himalayan granitic melts of ∼10 km may be achieved by the ascent of magma through dykes in about 1 day. At such rates even the largest granite could theoretically be emplaced in ∼10 years. Crystallisation of Himalayan melts involves much longer periods. If emplaced as thin sheets (∼100 m wide) a timescale of >500 years is required compared with >30 ka for single stage intrusion of the larger laccoliths. For composite sheet complexes magma crystallisation, rather than melt ascent, comprises the rate-determining step on the emplacement of the intrusion. The overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka, and possibly less than 1 ka. In contrast, the timescale required for prograde heating of the protolith is ≫1 Ma. Since the melt production rate is determined by heat flow into the protolith, and not by reaction kinetics (for any geologically significant period) we conclude that heat flow, determined by both the mechanism of heating and the thermal diffusivities of crustal rocks, provides the overall rate-determining step of crustal anatexis.