Possible Constraints on Anatectic Melt Residence Times from Accessory Mineral Dissolution Rates: An Example from Himalayan Leucogranites

Michael Ayres, Nigel Harris and Derek Vance
Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK
Institut für Isotopengeologie, ETH-Zentrum, 8092 Zürich, Switzerland

Abstract: The concentrations of LREE and Zr in a granitic melt formed by anatexis of a metapelitic protolith will be buffered by the stability of monazite and zircon respectively. The rate at which equilibrium is reached between dissolving monazite and zircon and a static melt is limited by the rate at which Zr and LREE can diffuse away from dissolution sites. If melt extraction rates exceed the rates at which the LREE and Zr in the melt become homogenized by diffusion, extracted melts will be undersaturated with respect to these elements. Evidence from accessory phase thermometry suggests that for many Himalayan leucogranites generated by crustal anatexis, the melts equilibrated with restitic monazite and zircon prior to extraction. In contrast, discordant temperatures determined from accessory phase thermometry suggest that tourmaline leucogranites from the Zanskar region of NW India did not equilibrate prior to extraction. Quantitative interpretation of this discordance assumes that the melt was static prior to extraction, and that accessory phase inheritance was minimal. Modelling of the time-dependant homogenization process suggests that tourmaline leucogranites generated at 700°C probably remained in contact with restitic monazite in the protolith for less than 7 ka and certainly less than 50 ka. Such rapid extraction rates suggest that deformation-driven mechanisms were important in removing these melts from their source.

Keywords: anatexis • melt residence time • leucogranites • Himalayas

Mineralogical Magazine; February 1997 v. 61; no. 404; p. 29-36; DOI: 10.1180/minmag.1997.061.404.04
© 1997, The Mineralogical Society
Mineralogical Society (www.minersoc.org)