The implications of Sr-isotope disequilibrium for rates of prograde metamorphism and melt extraction in anatectic terrains

Abstract

Abstract The preservation of chemical disequilibrium during anatexis can provide constrains on the rates of high-temperature processes provided that, firstly, the appropriate diffusion coefficients are known, and secondly, isotopes in the protolith have not been reset by dynamic recrystallization. In one example of an anatectic migmatite from the western Himalaya, isotopic homogenization has been approached between the leucosome, mesosome and melanosome at c. 20 Ma, although apatite, with a low Sr diffusivity, remained in strong disequilibrium at that time. Intrusive granites are more likely to preserve evidence of isotopic disequilibrium provided a rapid rate of prograde metamorphism culminates in a rapid rate of melt extraction. Evaluation of the available Sr-isotope database from the Miocene Himalayan granites and their metasedimentary protoliths suggests that any increase of 87 Sr/ 86 Sr in the melt relative to its source is less than 0.002. Mass-balance calculations indicate that isotopic exchange between plagioclase and mica in the protolith, and plagioclase and melt prior to extraction, must have occurred over a period in excess of 100–200 ka. Since rapid melt extraction (within <50 ka) is indicated by accessory phase dissolution rates, a comparatively slow rate of prograde metamorphism is indicated by the lack of Sr-isotope disequilibrium. This supports heating from internal heat production in thickened crust rather than from thermal advection or dissipative heating.