Zircon behaviour during crustal anatexis. Evidence from the Tibetan Slab migmatites (Nepal)

Abstract

The morphology and internal structures of zircon populations from three distinct types of migmatites from Central Nepal (low-Zr and high-Zr in situ migmatites and injection migmatite) were studied to examine the behaviour of zircon during anatexis. The zircons from mesosomes and melanosomes were all residual zircons characterized by rounded morphologies with little overgrowth and a small elongation ratio, suggestive of a detrital or metamorphic origin. Zircons from the leucosomes showed distinctive features. The low-Zr tonalitic leucosomes we characterized by zircon populations with the same features as those from mesosomes and melanosomes, indicative of no or very few transformations during migmatization. The high-Zr leucosome contained euhedral crystals with morphological features typical of magmatic zircons found in crustal peraluminous magmas; internal zoning patterns suggested a complex evolution involving dissolution and growth. The injected granitic leucosome contained well-developed magmatic zircons. The role of subsolidus differentiation, partial melting, and melt composition on the contrasted behaviour of zircons during anatexis is discussed. The zircon populations from the Tibetan Slab migmatites are distinct from those of the Manaslu granite, therefore suggesting that the migmatite leucosomes are not the unsegregated equivalents to the melts which gave raise to the Manaslu-type granites.