The Palaeoproterozoic evolution of the Litchfield Province, western Pine Creek Orogen, northern Australia: Insight from SHRIMP U–Pb zircon and in situ monazite geochronology

Abstract

The Litchfield Province, located on the western margin of the Palaeoproterozoic Pine Creek Orogen, northern Australia, comprises variably metamorphosed pelitic and psammitic sediments, extensively intruded by voluminous granitoids and mafic rocks. SHRIMP U–Pb in situ monazite geochronology of undeformed granulite-facies migmatitic Hermit Creek Metamorphics (ca. 730 °C, ca. 5 kb) and mid-amphibolite-facies Fog Bay Metamorphics (ca. 600 °C, ca. 4 kb) indicates mid- to high-grade metamorphism affected these rocks at ca. 1855 Ma. SHRIMP U–Pb analysis on low Th/U metamorphic zircon rims from the Fog Bay Metamorphics also indicate metamorphism at ca. 1855 Ma. The undeformed nature of the Hermit Creek and Fog Bay Metamorphics suggest that metamorphism was not accompanied by significant penetrative strain. These observations suggest that the elevated geotherm at this time may have been due to conductive and advective thermal input within a thinned lithosphere during intra-crustal extension and rifting. Metamorphism at ca. 1855 Ma in the Litchfield Province is similar in age to that reported from the Hooper Orogen and the Western and Central Zones of the Halls Creek Orogen, but not with the Eastern Zone. Furthermore, there appears no evidence for the ca. 1845–1835 Ma Halls Creek Orogeny in the Litchfield Province, suggesting that the inferred suture in the Halls Creek Orogen between the Kimberley Craton and the proto North Australian Craton, may not extend into the Litchfield Province and that the Litchfield Province may have been contiguous with, or forms part of, the Kimberley Craton. The upper-amphibolite-facies Welltree Metamorphics (ca. 700 °C, ca. 4 kb), in contrast, preserve monazite grains that are aligned with a penetrative biotite fabric and are dated at 1813 ± 3 Ma, thus recording a previously unrecognised tectono-thermal event in the Pine Creek Orogen.