Back-arc and post-collisional volcanism in the Palaeoproterozoic Granites-Tanami Orogen, Australia

Abstract

Pillow basalt, dolerite sills, and doleritic dykes are the major products of volcanism in the Granites-Tanami Orogen, and provide a window for understanding the magmatism and tectonic evolution of the orogen. The pillow basalt and dolerite sills from the Stubbins Formation have similar geochemical features, suggesting that they are genetically related. The pillow basalt and dolerite sills (ca. 1864Ma) have tholeiitic affinity, with high TiO2 contents (0.9–1.88wt.%), low Mg# (40–55), and positive ɛNd(t) values (+0.6 to +2.8). Enriched mid-ocean ridge basalt (E-MORB)-normalized REE and primitive mantle-normalized trace element patterns for these rocks show relatively flat REE distribution, slight enrichment of LILE (Rb, Th, U, and K) and weak negative anomalies of HFSE (Nb and P), which are the characteristics similar to those of the enriched back-arc basin basalt (E-BABB), but different from those of modern normal-MORB (N-MORB), E-MORB, oceanic island basalt (OIB), depleted back-arc basin basalt (D-BABB), and island arc basalt (IAB). The pillow basalt and dolerite sills are interpreted to be the results of a high-degree decompressional melting (5–15%) of the asthenosphere source in the spinel stability field. The mantle source of these rocks have primitive mantle signature with input of 3–4% subduction material. Convective flow operating during subduction, circulating the enriched material from wedge corner into the asthenospheric mantle beneath the Granites-Tanami back-arc basin and generated the mild enriched mafic magma that was emplaced as dolerite sills or erupted as basalt.“Late” (ca. 1800Ma) doleritic dykes in the region have high contents of MgO (7.15–14.0wt.%), Cr (262–757ppm), K2O (2.25–4.85, wt.%), with high Mg# (57–72), K2O/Na2O>2, and negative ɛNd(t) values range from −4.5 to 0.3. Their REE and trace element characteristics, such as enrichment of Rb, Ba, Th, U, K, LREE, and negative Nb–Ta anomalies, are similar to those of the upper (UCC) and middle continental crust (MCC), but more enriched than those of the lower continental crust (LCC). The interpreted continental–continental collision between the Granites-Tanami and Arunta orogens during the Tanami Orogeny (DGTO2; ca. 1800Ma) resulted in the thickening of the lower crust and lithospheric mantle root along the Willowra Lineament. The thickened lithospheric mantle root has higher density than the upper asthenospheric mantle leading to gravitational instability and resulting in convective removal. The detached denser lithospheric mantle root sank into asthenosphere triggered upwelling of the hot asthenospheric mantle material and extension. It is proposed here that melts derived from partial melting of the previously subduction-metasomatized lithospheric mantle with crustal assimilation generated the doleritic dykes, and modelling of geophysical data supports this interpretation. This post-collisional magmatism is approximately synchronously with the ca. 1800Ma gold mineralization and extensive granitic magmatism in the Granites-Tanami Orogen.