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.% HfO2). Another group is P-poor and the Hf content in accessory zircon is high (up to 35 wt.% HfO2). Both types of intrusions may occur within a single orogen as demonstrated by the example of the Hercynian magmatism in the Erzgebirge/Krušné hory region (Germany/Czech Republic).

Zirconium and Hf are nearly identical geochemically, and therefore most of the crust maintains near-chondritic Zr/Hf ratios of ∼35–40. By contrast, many high-silica rhyolites and granites have anomalously low Zr/Hf (15–30). As zircon is the primary reservoir for both Zr and Hf and preferentially incorporates Zr, crystallization of zircon controls Zr/Hf, imprinting low Zr/Hf on coexisting melt. Thus, low Zr/Hf is a unique fingerprint of effective magmatic fractionation in the crust. Age and compositional zonation in zircons themselves provide a record of the thermal and compositional histories of magmatic systems. High Hf (low Zr/Hf) in zircon zones demonstrates growth from fractionated melt, and Ti provides an estimate of temperature of crystallization (TTiZ) (Watson and Harrison, 2005). Whole-rock Zr/Hf and zircon zonation in the Spirit Mountain batholith, Nevada, document repeated fractionation and thermal fluctuations. Ratios of Zr/Hf are ∼307–40 for cumulates and 18–30 for high-SiO2 granites. In zircons, Hf (and U) are inversely correlated with Ti, and concentrations indicate large fluctuations in melt composition and TTiZ (>100°C) for individual zircons. Such variations are consistent with field relations and ion-probe zircon geochronology that indicate a >1 million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO2 zone.

Zircon commonly occurs as one of important accessory HFSE-bearing minerals in A-type granite. A detailed electron microprobe study was carried out on zircon from the Laoshan complex, Eastern China, which is composed of I- and A-type granites. Zircon from the I-type rocks is relatively poor in trace elements (HfO2<2 wt.%, UO2, ThO2 and Y2O3 <1 wt.%), but that from the A-type rocks is richer in Hf, U, Th and Y. Hafnian zircon with a HfO2 content of up to 12.37 wt.% was found in the arfvedsonite granite, which is considered the most evolved facies in the A-type suite. Enrichment in Hf is generally observed at the rims of zircon crystals relative to the cores. The Hf enrichment in zircon, and the association of exotic REE- and HFSE-bearing minerals are linked to hydrothermal activity, suggesting that during the last stage of crystallization of the A-type magma, fluids enriched in REE, HFSE, F−, CO32− and PO43− were released.