Compositional modification and trace element decoupling in rutile: Insight from the Capricorn Orogen, Western Australia

Abstract

The trace element composition of rutile is widely used to constrain the temperature and timing of geological events, the provenance of detrital grains and as a potential indicator of proximal ore mineralisation. Understanding the processes of trace element mobility in rutile is therefore paramount to the successful application of rutile geochemistry to geological research. Using a combination of electron backscattered diffraction and laser-ablation split-stream ICP-MS, we studied the microtextures, trace-element contents and U–Pb isotopes of rutile from the siliciclastic rocks of the Mesoproterozoic Edmund Group, Capricorn Orogen (Western Australia). The Capricorn Orogen experienced multiple tectonothermal events during the Proterozoic under conditions spanning from low- to high-grade metamorphism. Rutile textures, U–Pb ages, trace-element variations and calculated Zr-in-rutile temperatures reveal different generations of rutile, including detrital or metamorphic-hydrothermal grains.U–Pb age data of detrital rutile indicate a wide span of ages (ca. 2.7–1.7 Ga) of the source rocks and Cr-Nb compositions point to metamafic, metapelitic and granitic origins for this rutile. The youngest detrital ages of 1.68 Ga constrain the maximum depositional age of the sedimentary package. The comparison of age spectra across the stratigraphy indicates a change in provenance, with sourcing from circa 1.7 Ga rocks during early stages of deposition, followed by unroofing of older basement during deposition of the youngest units of the Edmund Group. Partial resetting of detrital rutile is observed in the increase of 207Pb/206Pb ratios, accompanied by a decrease of Zr-in-rutile calculated temperatures. Lower-temperature metamorphic-hydrothermal rutile yielded imprecise Mesoproterozoic ages (1.3–1.1 Ga). This rutile has significant variations of Zr/Hf and Nb/Ta extending to values both lower and higher than primitive mantle, which may record crystallisation from, or interaction with hydrothermal fluids. Partial remobilisation of trace elements in metamorphic-hydrothermal rutile is demonstrated by sub-grain domains with sharp geochemical variations and by evidence of element diffusion at grain margins. Orders of magnitude differences in trace element concentrations (including Nb, Cr and Zr) in metamorphic-hydrothermal rutile grains suggest thorough investigation of rutile textures needs to be carried out prior to using the trace element concentrations in rutile for provenance and Zr-in-rutile temperature studies.