IGNEOUS ZIRCON: TRACE ELEMENT COMPOSITION AS AN INDICATOR OF SOURCE ROCK TYPE

Abstract

Trace element abundances in igneous zircons, as determined by electron microprobe and laser-ablation microprobe ICPMS analysis, are shown to be sensitive to source rock type and crystallisation environment. The concentrations of 26 trace elements have been determined for zircons from a wide range of different rock types and reveal distinctive elemental abundances and chondrite-normalised trace element patterns for specific rock types. There is a general trend of increasing trace element abundance in zircons from ultramafic through mafic to granitic rocks. The average content of REE is typically less than 50 ppm in kimberlitic zircons, up to 600-700 ppm in carbonatitic and lamproitic zircons and 2,000 ppm in zircons from mafic rocks, and can reach per cent levels in zircons from granitoids and pegmatites. Relatively flat chondrite-normalised REE patterns with chondrite-normalised Yb/Sm ratios from 3 to 30 characterise zircons from kimberlites and carbonatites, but Yb/Sm is commonly over 100 in zircons from pegmatites. Th/U ratios typically range from 0.1 to 1, but can be 100-1000 in zircons from some carbonatites and nepheline syenite pegmatites. The geochemical signatures characteristic of zircon from some rock types can be recognised in bivariate discriminant diagrams, but multivariate statistical analysis is essential for the discrimination of zircons from most rock types. Classification trees based on recursive partitioning techniques provide a rapid means of relating parent rock type to zircon trace element analysis; zircons from many rock types can be discriminated at confidence levels of 75% or more. These trees allow recognition of the provenance of detrital zircons from heavy mineral concentrates, and significantly enhance the usefulness of zircon in regional crustal studies and as an indicator mineral in mineral exploration.