RARE-EARTH PATTERNS IN ZIRCONS FROM THE MANASLU GRANITE AND TIBETAN SLAB MIGMATITES (HIMALAYA) - INSIGHTS IN THE ORIGIN AND EVOLUTION OF A CRUSTALLY-DERIVED GRANITE MAGMA

The Tibetan Slab gneisses are currently considered as the source for t he High-Himalayan leucogranites. The rare-earth element distributions in zircons from the Tibetan Slab migmatites and the Manaslu leucograni te (Nepal Himalaya), were investigated by in situ ion probe analysis. These data combined with textural information have been used to elucid ate the zircon growth conditions and, indirectly, the processes involv ed in incipient anatexis and evolution of granitic magmas. In the migm atites, the zircons from gneisses and melanosomes have rounded shapes and variable REE patterns with low Yb contents (145-700 ppm) and chond rite-normalized (Yb/Sm)(N) ratios (less than or equal to 81). Zircons from low-Zr tonalitic leucosomes are morphologically and chemically in distinguishable from those of the gneisses and melanosomes. Zircons fr om the high-Zr tonalitic leucosomes and granitic leucosomes are euhedr al and show higher Yb contents (409-2820 ppm) and (Yb/Sm), ratios (gre ater than or equal to 145) than those of the gneisses and melanosomes. The euhedral shapes and distinctive REE patterns of zircons from the high-Zr leucosomes and granitic leucosomes are consistent with crystal lization from a melt, whereas the morphological and chemical similarit ies of zircons from the low-Zr leucosomes with those from the gneisses and melanosomes suggest inheritance without significant chemical chan ge. In the Manaslu granite, zircons have rounded cores with REE patter ns distinct from those of the rims (e.g., 250 ppm less than or equal t o Yb less than or equal to 710 ppm in the core, 965 ppm less than or e qual to Yb less than or equal to 2775 ppm in the rim) but comparable t o those from the Tibetan Slab gneisses suggesting inheritance. The rim compositions, however, are distinct from those of either zircon types of the Tibetan Slab leucosomes, indicating that the leucosomes cannot be the unsegregated equivalent to the Manaslu granite parental magma, Comparison of the rim compositions with fractional crystallization mo dels suggests that the range in zircon Sm and Yb contents are consiste nt with zircon crystallization from a monazite-saturated, xenotime-und ersaturated melt. The Yb contents in the different zircons studied, an d their variation within a single zircon, further suggest boundary-lay er effects and magma compositional heterogeneity, in agreement with pr evious models which considered that the Manaslu granite resulted from the aggregation of magma batches.