Petrogenesis of the Late Silurian–Early Devonian granites in the Longshoushan–Helishan area, Gansu Province, and its tectonic implications for the Early Paleozoic evolution of the southwestern Alxa Block
Abstract:The Alxa Block is located in between the Central Asian Orogenic Belt to the north and the Qilian Orogenic Belt to the southwest, but the tectonic setting of its southwestern margin during Early Paleozoic has long been in debate. Researchers who studied the Early Paleozoic plutons in eastern Beidashan believe the subduction of Paleo-Asian Ocean had reached the southwestern Alxa Block. But others proposed the post-collision extensional setting for this area based on Early Paleozoic magmatic rocks in eastern Longshoushan. It is noteworthy that there is no large fault in between the eastern Beidashan and Longshoushan, and the distances between the western Longshoushan and the two orogenic belts are the same as those for the eastern Beidashan and Longshoushan, indicating same tectonic setting. In order to discuss the Early Paleozoic tectonic setting of the western Longshoushan, the Dafosi alkali-feldspar granite, Wuba biotite granite and Zhangjiayao granodiorite were sampled for LA-ICP-MS zircon U–Pb dating and element and isotope geochemical analyses. The results indicate that the Dafosi alkali-feldspar granite was formed during Late Silusian (426 ± 2 Ma), and its major elements are characterized by high SiO2 (78.03%) and total alkali (7.96%) and low CaO (0.60%) and MgO (0.13%), with prominent V-type rare earth element pattern and negative Ba, Sr, and Eu anomalies, belonging to aluminous A2-type granite. The Wuba biotite granite and Zhangjiayao granodiorite were formed during Late Silurian (421 ± 2 Ma) and Early Devonian (406 ± 4 Ma), respectively, and both are high K calc-alkaline I-type granites. They have enriched LREEs (La/YbN = 10.69~13.79) and depleted Eu (δEu = 0.56~0.77), and exhibit apparently negative Nb-Ta anomalies and positive Zr-Hf anomalies at the same time. Moreover, all the three plutons have enriched Sr-Nd isotopic features and Mesoproterozoic two-stage Nd model ages. Therefore, they may be originated from the partial melting of Mesoproterozoic crust and indicate post-collisional tectonic setting. Besides, according to the statistic analysis and comparison, the tectonic evolution of southwestern Alxa Block could be divided into the subduction stage of Middle Ordovician–Early Silurian (ca. 461~441 Ma) and the post-collision stage of Middle Silurian–Early Devonian (ca. 432~397 Ma), and this process is comparable with the evolution of Qilian Orogenic Belt, indicating a genetic relationship.