Tectono-thermal evolution of the Palaeoproterozoic Granites–Tanami Orogen, North Australian Craton: Implications from hornblende and biotite 40Ar/39Ar geochronology
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The Palaeoproterozoic Granites–Tanami Orogen (GTO) hosts a number of gold deposits located in the southern margin of the North Australian Craton. The major stratigraphic succession is the Palaeoproterozoic Tanami Group which is subdivided into the Dead Bullock Formation and conformably overlying Killi Killi Formation. New geochemical data for the ca. 1864 Coora and Groundrush dolerite sills in the Dead Bullock Formation suggests that they have the same characteristics with the enriched back-arc basin basaltic rocks from the former Stubbins Formation, such as tholeiitic affinity, high TiO2 contents (0.94 to 1.24 wt.%) and low Mg# (41–45), slightly enriched LILE (Rb, Th, U, and K), weakly depleted HFSE (Nb, Ta), and relatively flat REE patterns. Their magma was generated by high degree decompressional melting (5–15%) of the asthenosphere source with an input of 3–4% subduction-relatedmaterial. The petrological and geochemical similarities of igneous rocks providenew evidence for the assignment of the ca. 1864 Ma former Stubbins Formation and the Mount Charles Formations in the Dead Bullock Formation of the Tanami Group. These conclusions confirmed that the extensive Palaeoproterozoic Tanami Group was deposited in a back-arc basin environment.Hornblende and biotite 40Ar/39Ar geochronological study identified three major tectono-thermal events in the GTO since the deposition of the Dead Bullock Formation. The ca. 1840 Ma 40Ar/39Ar cooling age of metamorphic hornblende from the Coora and Groundrush dolerite sills in the Dead Bullock Formation provided precise age constraint for the first Palaeoproterozoic tectono-thermal event during the evolution of the Granites–Tanami back-arc basin. This age is highly consistent with the ca. 1850–1840 Ma subduction and peak metamorphism events in the North Australian Craton (NAC) associated with the Halls Creek Orogeny in the Halls Creek Orogen, and the Tennant Orogeny in the Tennant Creek Inlier. The 40Ar/39Ar age of 1753 ± 8 Ma for the early biotite (Bib) from the Dead Bullock Formation is a cooling age record of the peak greenschist facies metamorphism during the ca. 1795 Ma Tanami Orogeny, which is interpreted as the continental–continental collisional event between the GTO and the Arunta Orogen in the Central Australian Craton to the south. The approximately 40 million year slow cooling is a combined result of exhumation, heat released from the upwelling asthenospheric mantle, regional magmatism and long-term crustal residence. This tectono-thermal event was associated with gold mineralisation in the GTO and the Proterozoic NAC was finally cratonised during this period. The late biotite (Big) is a recrystallization product of hydrothermal alteration. Its 40Ar/39Ar age of 1718 ± 8 Ma revealed the last tectono-thermal overprint to the GTO and Arunta Orogen. This age reflected the distal thermal influence by the ca. 1723 Ma Strangways Event in the Arunta Orogen.
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