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

Abstract

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, Fe2O3T, 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 A2-type granite. The A-type granites have negative εNd(t) values (−5.4 to −4.8) with Neoarchean depleted mantle (TDM2) ages (2591–2494 Ma), corresponding to coupling between εHf(t) values (−4.85 to -0.92) and TDM2 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 oceanic-island basalt affinity. They have positive εNd(t) values (+0.8 to +1.8) close to the chondrite evolutionary line and variable εHf(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) A2-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.

Devonian Andean‐type convergence in the southern Dunhuang block (NW China): Petro‐structural, metamorphic P–T and geochronological constraints

“…Palaeozoic metamorphic reworking exemplified by the 437–317 Ma metamorphic zircon U–Pb ages is widely reported for numerous Neoarchean to Palaeoproterozoic magmatic and metamorphic rocks throughout the Dunhuang block (Wang et al, 2021; Zhao, Ao, Yan, et al, 2019; Zhao, Ao, Zhang, et al, 2019; Zong et al, 2013). Meanwhile, previous monazite U–Pb dating also yielded metamorphic ages of c. 366 Ma (Zhao et al, 2020), whereas titanite and apatite U–Pb dating and garnet Sm–Nd dating yielded metamorphic ages ranging from 409 to 237 Ma (Wang, Diwu, et al, 2021; Si et al, 2022).…”

Jurassic Evolution of the Dunhuang Basin and Its Implications for the Early History of the Altyn Tagh Fault, Northeast Tibet Plateau

“…(a–d) Detrital zircon ages in this study; (e) zircon age of magmatic and metamorphic rocks in the Dunhuang Basin (DHB) (L. M. Feng et al., 2020; Gan et al., 2021; Z. Y. He et al., 2014; Shi et al., 2020; Z. M. Wang et al., 2013, 2014; Z. D. Wang et al., 2021; H. Y. C. Wang, Chen, et al., 2017; H. Y. C. Wang, Wang, et al., 2017; J. X. Zhang et al., 2013; Y. Zhao et al., 2015, 2016, 2017, 2020; Y. Zhao, Ao, Yan, et al., 2019; Y. Zhao, Ao, Zhang, et al., 2019; Zong et al., 2013); (f) detrital zircon ages of Jurassic strata in the eastern DHB (Gehrels et al., 2003); (g) detrital zircon ages of modern river in the Bei Shan (W. T. Wang et al., 2016); (h) detrital zircon ages of Jurassic strata in the western Jiuxi Basin (K. X. An et al., 2018); (i) zircon U‐Pb age of magmatic rocks in the Qilian Shan (C. Wu et al., 2022); (j) zircon U‐Pb age in the North Altyn Tagh (X. D. Zhao et al., 2020); (k) detrital zircon ages of Middle Jurassic strata in the Lapeiquan area (X. D. Zhao et al., 2020); and (l) detrital zircon ages of Middle Jurassic strata in the Suganhu area (Cheng, Jolivet, et al., 2019; J. F. Zhao et al., 2020).…”

“…The basin basement is composed mainly of the Late Archeozoic—Paleoproterozoic Dunhuang complex (J. X. Zhang et al., 2013; Y. Zhang et al., 2020; Y. Zhao, Ao, Zhang, et al., 2019; Zong et al., 2013) and the Paleozoic sedimentary and volcanic rocks which undertook metamorphization (Z. Y. He et al., 2014; H. Y. C. Wang, Chen, et al., 2017; H. Y. C. Wang, Wang, et al., 2017; Zhao et al., 2015, 2016, 2017) (see Figure 2b).…”

“…He et al., 2014; H. Y. C. Wang, Chen, et al., 2017; H. Y. C. Wang, Wang, et al., 2017; Zhao et al., 2015, 2016, 2017) (see Figure 2b). The Dunhuang complex consists mainly of tonalite‐trondhjemite‐granodiorite gneisses (TTG), Paleoproterozoic supracrustal rocks and mafic‐acid magmatic rocks (J. X. Zhang et al., 2013; Y. Zhao, Ao, Yan, et al., 2019; Y. Zhao, Ao, Zhang, et al., 2019; Y. Zhao et al., 2015, 2020; Zong et al., 2013). The Paleozoic magmatic‐metamorphic rock consists mainly of ca.…”