Petrogenesis and Tectonic Implications of the Late Silurian–Early Devonian Bimodal Intrusive Rocks in the Central Beishan Orogenic Belt, NW China: Constraints by Petrology, Geochemistry and Hf Isotope

A large number of Late Silurian–Early Devonian intrusive rocks are distributed in the central Beishan orogenic belt（BOB）. Tectonic setting of these intrusive rocks is of great significance to the study of the subduction and accretion of the Paleo-Asian Ocean. Previous studies show that most of the intrusive rocks in this region are S-type or A-type granitoids. In this study, we firstly reported the Late Silurian–Early Devoniandia bases, granodiorites on the southside of the Baiyunshan ophiolitic mélanges belt, as a part of Hongliuhe-Xichangjing ophiolitic mélanges belt（HXOMB）. Zircon LA-ICP-MS U-Pb dating yields emplacement ages between 418 and 397 Ma, REE distribution patterns exhibit enriched LREE and flat HREE in the diabases, the discriminant diagrams show that the diabases have geochemical characteristics of intraplate basalt. The granodiorites in this paper present more like S-and A-type granitoids reported, showing the geochemical characteristics of syn/post-collision granites. Actually, the bimodal magmatic rocks are developed during Late Silurian–Early Devonian on both sides of the HXOMB, which are related to the tectonic background of the post orogeny extension. The diabases are tholeiitic with relative strong depleted εHf（t）(+8.1 to +13.0), which are mainly from relative depleted mantle. The granodiorites are calc-alkaline with relative slightly depleted εHf（t）(+0.7 to +5.6) and the lower Mg# and MgO contents（34.6 – 36.9, 0.50 wt.% – 1.19 wt.% respectively）, reflecting the source characteristics of meta-basalt. Therefore, the remelting of juvenile crust may be the main way of continental crust accretion during Late Silurian–Early Devonian in the central BOB.