Hafnian zircon from the apical part of the Suzhou granite, China

Hafnian zircon generally is found in granitic pegmatite, but much more rarely in granite. Here, we describe an occurrence in the apical part of the Suzhou granite (unit II); in China. We distinguish three evolved facies: from the apical region downward, these are the albite-enriched, the topaz-bearing, and the biotite-bearing facies, which are characterized by significant enrichment in Nb, Ta, Zr, Hf, Th and the rare earths. In the biotite-bearing facies, the zircon contains less than 10 wt.% HfO2. Levels of HfO2 greater than 25%, and even attaining 35%, characterize the hafnian zircon in the albite-enriched facies. This degree of enrichment is the greatest known so far in hafnian zircon. The level of Hf in zircon of the topaz-bearing facies is intermediate. The striking decrease in Zr/Hf with increasing degree of evolution of granite reflects an important decoupling in these two elements. The Suzhou granite thus displays a pegmatitic character. Where the zircon is associated with niobo-tantalates, we document a close correlation between Hf/(Hf + Zr) in the zircon and Ta/(Nb + Ta) in the niobo-tantatates; this affinity between Hf and Ta seems more dependent on geochemical than crystal-chemical factors.