POSSIBLE CONSTRAINTS ON ANATECTIC MELT RESIDENCE TIMES FROM ACCESSORYMINERAL DISSOLUTION RATES - AN EXAMPLE FROM HIMALAYAN LEUKOGRANITES

The concentrations of LREE and Zr in a granitic melt formed by anatexi s of a metapelitic protolith will be buffered by the stability of mona zite and zircon respectively. The rate at which equilibrium is reached between dissolving monazite and zircon and a static melt is limited b y the rate at which Zr and LREE can diffuse away from dissolution site s. If melt extraction rates exceed the rates at which the LREE and Zr in the melt become homogenized by diffusion, extracted melts will be u ndersaturated with respect to these elements. Evidence from accessory phase thermometry suggests that for many Himalayan leucogranites gener ated by crustal anatexis, the melts equilibrated with restitic monazit e and zircon prior to extraction. In contrast, discordant temperatures determined from accessory phase thermometry suggest that tourmaline l eucogranites from the Zanskar region of NW India did not equilibrate p rior to extraction. Quantitative interpretation of this discordance as sumes that the melt was static prior to extraction, and that accessory phase inheritance was minimal. Modelling of the time-dependant homoge nization process suggests that tourmaline leucogranites generated at 7 00 degrees C probably remained in contact with restitic monazite in th e protolith for less than 7 ka and certainly less than 50 ka. Such rap id extraction rates suggest that deformation-driven mechanisms were im portant in removing these melts from their source.