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 the High-Himalayan leucogranites. The rare-earth element distributions in zircons from the Tibetan Slab migmatites and the Manaslu leucogranite (Nepal Himalaya), were investigated by in situ ion probe analysis. These data combined with textural information have been used to elucidate the zircon growth conditions and, indirectly, the processes involved in incipient anatexis and evolution of granitic magmas. In the migmatites, the zircons from gneisses and melanosomes have rounded shapes and variable REE patterns with low Yb contents (145-700 ppm) and chondrite-normalized (Yb/Sm)(N) ratios (less than or equal to 81). Zircons from low-Zr tonalitic leucosomes are morphologically and chemically indistinguishable from those of the gneisses and melanosomes. Zircons from the high-Zr tonalitic leucosomes and granitic leucosomes are euhedral and show higher Yb contents (409-2820 ppm) and (Yb/Sm), ratios (greater 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 crystallization from a melt, whereas the morphological and chemical similarities of zircons from the low-Zr leucosomes with those from the gneisses and melanosomes suggest inheritance without significant chemical change. In the Manaslu granite, zircons have rounded cores with REE patterns distinct from those of the rims (e.g., 250 ppm less than or equal to Yb less than or equal to 710 ppm in the core, 965 ppm less than or equal to Yb less than or equal to 2775 ppm in the rim) but comparable to 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 models suggests that the range in zircon Sm and Yb contents are consistent with zircon crystallization from a monazite-saturated, xenotime-undersaturated melt. The Yb contents in the different zircons studied, and their variation within a single zircon, further suggest boundary-layer effects and magma compositional heterogeneity, in agreement with previous models which considered that the Manaslu granite resulted from the aggregation of magma batches.