TEMPERATURE SPECTRA OF ZIRCON CRYSTALLIZATION IN PLUTONIC ROCKS

Abstract

The Ti-in-zircon thermometer is a potentially powerful new petrological tool, but conclusions drawn from such data are meaningful only to the extent that the underlying interpretational framework is sound. In the case of interpretation of detrital zircon crystallization temperature spectra, it has been assumed that comparisons can be made with potential host rocks based on calculated bulk zircon saturation thermometry. We show by calculation that most igneous rocks formed at high temperature (>750 °C) will yield Ti-in-zircon temperatures (TTizir) well above the wet granite solidus. This prediction is borne out by results from the Dala igneous complex, southeastern Tibet, which show a 300 °C range in TTizir beginning >100 °C above the calculated bulk zircon saturation temperature. Thus the dominant TTizir peak of >4 Ga zircons at ˜680 °C implies a wet, anatectic magma source rather than an origin in intermediate and mafic magmas. Given that preservation and analytical effects select against, or obscure recognition of, zircons formed at low temperature, the best explanation for the low Hadean TTizir peak is that it reflects derivation from rocks that experienced prograde melting conditions under near-water-saturated conditions.