Latest Paleoproterozoic (ca. 1.8-1.6 Ga) extensional tectonic setting in the Dunhuang terrane, NW China: Evidence from geochronological and geochemical investigations on A-type granite and metamafic rock

Latest Paleoproterozoic (ca. 1.8-1.6 Ga) magmatic rocks outcrop in the Dunhuang terrane, represented by A-type granites and mafic (basaltic) rocks that have metamorphosed into amphibolites. The A-type granites, emplaced at ca. 1.79-1.77 Ga, are geochemically characterized by high Na2O + K2O, Fe2O3 T, Zr, Nb, and Ce contents, as well as high Fe2O3T/(Fe2O3T + MgO) and Ga/Al ratios. Furthermore, they have Nb/Ta, Y/Nb, Rb/Nb, and Sc/Nb ratios of 12.10-15.56, 1.45-1.79, 3.52-6.51, and 0.11-0.19, respectively, showing affinity to A(2)-type granite. The A-type granites have negative epsilon(Nd)(t) values (-5.4 to -4.8) with Neoarchean depleted mantle (T-DM2) ages (2591-2494 Ma), corresponding to coupling between E, f (t) values (-4.85 to -0.92) and T-DM2 ages (2817-2556 Ma) of zircons. Therefore, the A-type granite pluton was mostly generated by partial melting of Neoarchean tonalitic to granodioritic basement rocks of the Dunhuang Complex in a postcollisional tectonic setting following a late Paleoproterozoic continent-continent collisional event. The metamafic rocks have a protolith age of 1605 +/- 45 Ma and metamorphic age of 317 +/- 20 Ma, indicating a Paleozoic tectonic event. The metamafic rock samples are geochemically characterized by relatively high alkali (Na2O + K2O = 4.39-4.81 wt%) contents and low Nb/Y (0.63-0.66) ratios, and they show steep rare earth element (REE) patterns with light REE enrichment and insignificant Eu anomalies and Nb-Ta, Zr-Hf, and Ti anomalies, resembling subalkaline oceanicisland basalt affinity. They have positive epsilon(Nd)(t) values (+0.8 to +1.8) close to the chondrite evolutionary line and variable epsilon(Nf)(t) values (-1.09 to +9.06) of zircons. Hence, the protolith of the metamafic rocks may have been produced by magma mixing processes between a depleted mantle source and a metasomatized lithospheric mantle source during the initial rifting stage in an extensional setting, completing the formation of the Precambrian Dunhuang Complex. Considering the ca. 1.85-1.80 Ga regional metamorphism in the Dunhuang terrane, the latest Paleoproterozoic (ca. 1.8-1.6 Ga) A(2)-type granitic magmatism and mafic magmatism documented the postorogenic to initial rifting processes following the global-scale late Paleoproterozoic collisional event, which is comparable with ca. 1.80-1.67 Ga postcollisional and ca. 1.60-1.53 Ga anorogenic magmatism in the North China craton, but different from that of the Tarim craton.