Discussion on Wang et al. (2000) 'Chemistry of Hf-rich zircons from the Laoshan I- and A-type granites, Eastern China', Mineralogical Magazine, 64, 867-877

摘要

Wang et al. (2000) presented interesting new data on the occurrence of Hf-rich zircon in peralkaline granitic rocks. It is shown that the knowledge of the Hf distribution in zircon is important for the understanding of Zr/Hf ratios defined by whole-rock analysis. However, the processes leading to the unusual Zr/Hf fractionation are still poorly constrained. Some aspects are discussed further here. Analysis of available data demonstrates that extremely low Zr/Hf ratios found in topaz- and rare-metal bearing granites may be either a primary signature of some evolved granitic melts or, alternatively, they are produced during albitization of the rocks. Topaz- and rare-metal bearing granites may be P-rich and contain zircon moderately enriched in Hf (2-9 wt. percent HfO_2). Another group is P-poor and the Hf content in accessory zircon is high (up to 35 wt. percent HfO_2). Both types of intrusions may occur within a single orogen as demonstrated by the example of the Hercynian magmatism in the Erzgebirge/Krusne hory region (Germany/Czech Republic).