REE FRACTIONATION AND ND-ISOTOPE DISEQUILIBRIUM DURING CRUSTAL ANATEXIS - CONSTRAINTS FROM HIMALAYAN LEUKOGRANITES

The processes controlling rare earth element (REE) behaviour during cr ustal anatexis have profound implications not only for REE abundances of crustally derived granites but also for their model Nd ages. For ty pical protoliths of anatectic granites, such as are exposed in metapel ites from the High Himalayan Crystalline Series (HHCS), a high proport ion (>90%) of bulk-rock LREE and MREE are sited in the accessory phase s apatite and monazite, and of HREE in garnet and apatite. Mass-balanc e constraints indicate that observed REE abundances of Himalayan leuco granites cannot be generated from REE-deficient reactant phases in the ir metasedimentary protoliths (muscovite, biotite, plagioclase and qua rtz) but must result from the dissolution of accessory phases. REE and Th abundances in anatectic melts are modelled by combining empirical expressions for monazite, apatite and zircon solubilities (as a functi on of temperature, melt composition and melt water content) with analy sed accessory-phase REE abundances. This approach, which allows the co ntribution to the melt from each accessory phase to be identified, suc cessfully predicts Himalayan leucogranite trace-element abundances. Th e LREE and Th concentrations of a metapelite-derived melt are buffered by monazite stability, although apatite dissolution contributes up to 10% of LREE to the melt together with similar to 50% of the MREE and >90% of the HREE. Zircon contributes a negligible proportion of REE to the melt. During anatexis, dissolution of monazite and apatite in the melt results in: (1) increasing REE abundances and LREE/HREE ratios w ith increasing temperature of melting; (2) a negative Eu anomaly in th e melt; (3) high Sm/Nd ratios in the melt relative to its metasediment ary protolith; and (4) Nd-isotope disequilibrium between the melt and its protolith. All of these features are observed in the REE geochemis try of Himalayan leucogranites. The combined effects of Sm/Nd fraction ation and Nd-isotope disequilibrium during anatexis imply that model N d ages calculated for anatectic granites derived from apatite and mona zite bearing protoliths should be treated with considerable caution. ( C) 1997 Elsevier Science B.V.