Mineral-scale Trace Element and U-Th-Pb Age Constraints on Metamorphism and Melting during the Petermann Orogeny (Central Australia)

Abstract

High-pressure amphibolite-facies migmatitic orthogneisses from the Cockburn Shear Zone (CSZ), northern Musgrave Block in central Australia, were formed during the 580–520 Ma intraplate Petermann Orogeny. The shear-zone hosted orthogneisses are of an intermediate bulk composition that promoted the growth of rare earth element (REE)-bearing major phases (garne and hornblende), as well as numerous accessory phases (zircon, titanite, apatite, epidote and allanite), all of which are potential U–Th–Pb geochronometers and are involved in the distribution of REEs. We have integrated petrology and detailed in situ trace element analysis of major and accessory phases in samples collected outside and inside the CSZ to establish the relative timing of metamorphic mineral growth. This paper presents one of the first applications of newly developed in situ dating protocols on metamorphic allanite. Sensitive high-resolution ion microprobe geochronology on metamorphic zircon and allanite indicate that metamorphism and partial melting occurred between 559 ± 6 and 551 ± 6 Ma. Peak temperatures of 720–750°C, determined from rutile included in garnet, necessitate the presence of fluids to flux partial melting in the CSZ quartzofeldspathic rocks. Metamorphic zircon formed during cooling in the presence of melt near the granitic wet solidus at T ≤ 700°C. In contrast, allanite formed at different stages of the CSZ P–T path: (1) as a prograde sub-solidus phase (T < 650°C) formed in the presence of fluids, and (2) as melt-precipitated Th- and REE-rich overgrowths on pre-existing allanite. The ages of the two growth episodes are not isotopically resolvable by allanite dating. Trace element compositions indicate that in both melted and unmelted rocks, garnet and hornblende growth was primarily controlled by prograde sub-solidus hydration reactions that consumed feldspar below the metamorphic peak. REE compositions of the metamorphic zircon and allanite overgrowths that formed in the presence of melt also suggest disequilibrium with garnet. Thus, the major period of garnet and hornblende growth was not coeval with partial melting.