U-Pb chronology of the Northampton Complex, Western Australia – evidence for Grenvillian sedimentation, metamorphism and deformation and geodynamic implications

Abstract

Conventional and SHRIMP U-Pb analyses of zircon, monazite, titanite and apatite from the high grade rocks of the Northampton Complex in Western Australia provide constraints on the timing of metamorphic processes and deformation events in the northern Darling Mobile Belt (western margin of the Archean Yilgarn Craton). Paragneisses and mafic volcanics and/or intrusions have undergone granulite facies metamorphism in a probable extensional tectonic setting prior to formation of W- to NW-verging folds and thrusts cut by normal shears (interpreted as late collapse structures) during the main deformation event (D₁). These structures are folded by open to tight folds with NW-striking axial surfaces developed in a second, NE-SW contractional event (D₂). Zircons from a mafic granulite provide an age of 1079 ± 3 Ma attributed to new zircon growth prior to, or at the peak of regional granulite facies metamorphism. Metamorphic monazites extracted from a paragneiss yield an identical age of 1083 ± 3 Ma. The similarity of ages between zircons from the mafic granulite (1079 ± 3 Ma) and monazites from the paragneiss (1083 ± 3 Ma) is interpreted to reflect fast cooling and/or rapid uplift, which is consistent with thrusting of the gneissic units during the first deformation event (D₁) associated with the onset of retrograde metamorphism. Granitic activity at 1068 ± 13 Ma was followed by intrusion of post-D₂ pegmatite (989 ± 2 Ma), which constrains the end of metamorphism and associated deformation. Cooling of the complex to about 500 °C is timed by the apatite age of 921 ± 23 Ma. SHRIMP U-Pb ages of detrital zircons from a paragneiss sample yield a maximum age of 2043 Ma, with no evidence of an Archean Yilgarn signature. A majority of ages between 1.6 and 1.9 Ga are consistent with derivation from the Capricorn Orogen on the northern margin of the Yilgarn Craton. Younger detrital zircons with 1150–1450 Ma ages, however, indicate an additional source that had undergone early Grenvillian igneous or metamorphic event(s) and also places a maximum age constraint upon deposition. The source of this clastic material may have been from within the southern Darling Mobile Belt or from Greater India (adjacent to the Northampton Complex in Rodinia reconstructions). This study documents an extended Grenvillian history, with basin formation, sedimentation, granulite facies metamorphism, contractional tectonics (two periods with orthogonal directions of shortening) and late pegmatite emplacement taking place between 1150–989 Ma on the western margin of the Yilgarn Craton. Ages recorded in this study indicate that the proposed global distribution of Grenvillian belts during assembly of the Rodinia supercontinent should be reassessed to include the Darling Mobile Belt.