ZIRCON CRYSTAL MORPHOLOGY, TRACE ELEMENT SIGNATURES AND HF ISOTOPE COMPOSITION AS A TOOL FOR PETROGENETIC MODELLING: EXAMPLES FROM EASTERN AUSTRALIAN GRANITOIDS

Abstract

In situ laser ablation inductively coupled plasma mass spectrometry analysis of trace elements, U–Pb ages and Hf isotopic compositions of magmatic zircon from I- and S-type granitoids from the Lachlan Fold Belt (Berridale adamellite and Kosciusko tonalite) and New England Fold Belt (Dundee rhyodacite ignimbrite), Eastern Australia, is combined with detailed studies of crystal morphology to model petrogenetic processes. The presented examples demonstrate that changes in zircon morphology, within single grains and between populations, generally correlate with changes in trace element and Hf-isotope signatures, reflecting the mixing of magmas and changes in the composition of the magma through mingling processes and progressive crystallization. The zircon data show that the I-type Kosciusko tonalite was derived from a single source of crustal origin, whereas the S-type Berridale adamellite had two distinct sources including a significant I-type magma contribution. Complex morphology and Hf isotope variations in zircon grains indicate a moderate contribution from a crustal component in the genesis of the I-type Dundee rhyodacite. The integration of data on morphology, trace elements and Hf isotope variations in zircon populations provides a tool for the detailed analysis of the evolution of individual igneous rocks; it offers new insights into the contributions of different source rocks and the importance of magma mixing in granite petrogenesis. Such information is rarely obtainable from the analysis of bulk rocks.