Petrogenesis of early Paleozoic granitoids in the Dunhuang Block, NW China: Insights into magma evolution and crustal thickening

Abstract

The occurrence of adakitic rocks can provide insight into the evolution of continental crust and deep geodynamic processes in subduction zones. In this paper, we investigated zircon U-Pb ages and Hf isotope, whole-rock major and trace elements, and whole-rock Sr-Nd-Pb isotopes of the Lianghu quartz diorites and Shigong monzogranites in the Dunhuang Block, to understand their petrogenesis and tectonic setting as well as the tectonic evolution of the Dunhuang Block in the early Paleozoic. LA-ICP-MS zircon U-Pb dating results suggest that the Lianghu quartz diorites and Shigong monzogranites were emplaced at ca. 455 ± 3 Ma and 431 ± 3 Ma, respectively. The Ordovician quartz diorites exhibit relatively high Sr (734–904 ppm) contents, negative-to-negligible Eu anomalies (Eu/Eu* = 0.87–0.99), high Sr/Y (52.5–124) ratios, but low Yb (0.68–1.32 ppm) and Y (7.27–14.0 ppm) contents. Moreover, they exhibit relatively high Mg# (52.0–54.7) values, Cr (23.1–88.0 ppm) and Ni (15.0–53.3 ppm) contents, similar to those of adakitic rocks originating from subducted oceanic crust. The quartz diorites display low initial 87Sr/86Sr (0.703464–0.704783), negative εNd(t) of −3.2 to −1.3, and positive εHf(t) of +3.8 to +8.0, with two-stage model ages for the Nd and Hf systems of 1.12 to 1.09 Ga and 1.17 to 0.91 Ga, respectively, indicative of a heterogeneous magma sources. They have relatively high radiogenic 206Pb/204Pb(t) (18.219–19.066), 207Pb/204Pb(t) (15.616–15.669), and 208Pb/204Pb(t) (39.042–39.198), signifying that some subduction-related materials (e.g., ocean island basalts) may be involved as sources. These characteristics indicate that the Lianghu quartz diorites were derived from the partial melting of subducted oceanic slabs (sediments + fluids), and that they underwent subsequent interaction with peridotite in the mantle wedge, with limited assimilation of crustal materials during magma ascent. The Silurian monzogranites contain high Sr content (655–674 ppm) but very low contents of Y (2.49–4.19 ppm) and Yb (0.24–0.37 ppm), yielding high Sr/Y ratios (161–263); those values, together with low MgO (0.30–0.41 wt.%), Mg# (40.6–41.4), Cr (2.39–3.28 ppm), and Ni (2.19–2.95 ppm) contents, resemble those of thickened lower crust-derived adakitic rocks. They show εHf(t) and εNd(t) values of −2.5 to +3.0 and −3.6, with the respective two-stage model ages of 1.55 to 1.20 Ga and 1.32 Ga. Their low Pb values, 206Pb/204Pb(t) = 18.184, 207Pb/204Pb(t) = 15.595, and 208Pb/204Pb(t) = 37.717, also suggest addition of minor mantle-like melts. These geochemical features indicate that the parent magmas of the monzogranites originated either from the melting of heterogeneous crustal sources, or through mixing of thickened juvenile crust, Mesoproterozoic crust materials, and minor mantel-derived materials in the stability field of garnet-bearing amphibolite. Given the coeval regional magmatism and tectonic setting, we propose that the continental crust thickening of the Dunhuang Block began roughly in the late Ordovician (~35 km) and culminated in the early Silurian (~50–55 km), as evidenced by the La/Yb ratios of quartz diorites (12.2–25.2) and monzogranites (57.6–60.9). The corresponding transition of the tectonic regime from subduction to collisional setting occurred at ~440 Ma. The Dunhuang Block could undergo thickening processes associated with underplating of mantle-derived magmas during subduction of the southern Paleo-Asian Ocean, and subsequently collided with the Shibanshan terrane (a Paleozoic continental arc) along the southern Beishan orogen at ca. 440–430 Ma. Our results show that the Dunhuang Block represents a continental arc in the early Paleozoic, and was involved in the Paleozoic orogenesis of the Central Asian Orogenic Belt.