Late Devonian A-Type Granites from the Beishan, Southern Central Asia Orogenic Belt: Implications for Closure of the Paleo-Asia Ocean

Abstract

The closing time of the Paleo-Asian Ocean (PAO) has long been in the focus of research as well as of controversial debates. The Paleozoic A-type granites distributed in the Beishan Orogenic Belt (BOB) at the southern margin of the Central Asian Orogenic Belt (CAOB) provide pivotal clues to constrain the closure of the PAO. In this paper, the newly recognized Duhongshan A-type granites from the middle Huaniushan arc in the BOB (zircon LA-ICP-MS U-Pb ages of ca. 376–374 Ma) are thoroughly studied. The rocks have high SiO2, K2O contents with peralkaline character, and display high Zr + Nb + Ce + Y contents (354–543 ppm), 10,000 × Ga/Al (4.1–4.9), Y/Nb (3.2–5.3), Rb/Nb ratios (8.5–14.1), and a zircon saturation temperature in the range of 877–950 °C, indicative of A2-type granites affinities. The Duhongshan granites display enriched in Th and U; depleted in Ba, Sr, and Ti; with slightly positive whole-rock εNd(t) values (+1.86 to +2.21), indicating an origin related to partial melting of lower crustal material in post-collision extension settings. Combined with previous reported results, we conclude that the granitoids in the middle Huaniushan arc were mostly formed around 424–367 Ma and can be divided into two types based on petrochemistry: (a) A-type granites, which generally have high SiO2 and K2O, derived from the relatively shallow crustal source in post-collision tectonic settings; and (b) adakite and I-type granites, which display high Sr/Y ratios as well as Nb, Ta, and Ti depletion, likely generated from the melting of juvenile crust in active continental margin arcs. Integrating the previous regional investigations, we propose that the Hongliuhe–Niujuanzi–Xichangjing Ocean (HNXO) of the PAO was closed and transformed in the post-collision extensional tectonic stage during the Late Devonian and formed as post-collision magmatism, while the arc magmatism may be related to the subduction of the Liuyuan Ocean, which is located in the Southern HNXO.