A newly recognised 1860–1840 Ma tectono-magmatic domain in the North Australia Craton: Insights from the Tennant Region, East Tennant area, and the Murphy Inlier

Abstract

New U-Pb monazite ages from amphibolite-facies metasedimentary rocks from the Tennant Region, Murphy Inlier and intervening East Tennant area, together with existing data, reveal the presence of an 1860–1840 Ma tectono-magmatic domain over 600 km long towards the centre of the North Australian Craton. In-situ ion probe U-Pb dating of biotite-hosted monazite in amphibolite-facies schist in the Tennant Region yielded an age of 1858 ± 7 Ma, which is attributed to north–south shortening (D1) at 1860–1855 Ma. Existing data indicate that D1 was associated with east–west trending, upright folds and mostly low-grade, regional metamorphism (M1) in the Tennant Region and the Murphy Inlier. The D1 event preceded voluminous and widespread felsic magmatism between 1855 and 1845 Ma. This included the emplacement of the Tennant Creek Supersuite, as well as the Yungkulungu Formation and equivalent stratigraphy, in the Tennant Region and in East Tennant, and the Nicholson Granite and Cliffdale Volcanics in the Murphy Inlier. Newly determined monazite ages from amphibolite-facies schist from the East Tennant area and the Murphy Inlier constrain a second episode of deformation and metamorphism (D2/M2) to ∼ 1845 Ma, coincident with the cessation of widespread magmatism. D2 is characterised by regional southeast to northeast trending shear zones. Phase equilibria modelling reveals that peak pressure–temperature (P–T) conditions during M2 in the East Tennant area were 2.8–3.3 kbar and 655–680 °C, indicating an extremely high apparent geothermal gradient (>190 °C/kbar) that was likely influenced by the preceding magmatism. Existing data indicate that D2 also affected the Tennant Region, where it coincided with significant Cu-Au-Bi mineralisation, albeit at significantly lower P–T conditions (sub-greenschist facies) than in the East Tennant area. The development of the 1860–1840 Ma tectono-magmatic domain, extending from west of the Tennant Region to east of the Murphy Inlier, marks an intermediate step in the migration of tectonism in the North Australia Craton, from the Arnhem Province in the north at 1880–1860 Ma to the Aileron and Tanami provinces in the south by ca. 1830 Ma.