S-type granite from the Gongpoquan arc in the Beishan Orogenic Collage, southern Altaids: Implications for the tectonic transition

Abstract

Voluminous Paleozoic intrusions occur in the Beishan Orogenic Collage (BOC) and their genesis and tectonic background are important to reconstruct the accretion-collision processes in the southernmost Altaids. Paleozoic is an important period for arc development in the BOC, where the Gongpoquan and Huaniushan arcs are located. There are two pulses of magmatism in the Huaniushan and Gongpoquan arcs, i.e., the ca. 470–423Ma I-type and ca. 424–395MaS- and A-type granitoids. In this study, we focus on two peraluminous granitic plutons in the Gongpoquan arc, i.e., the Baitoushan muscovite granite and Haergen two-mica granite, aiming at unraveling their petrogenesis and tectonic background. Zircon LA-ICP-MS U-Pb dating yields emplacement ages of ca. 409–395Ma and ca. 409Ma for the Baitoushan and Haergen plutons, respectively. Both the granitic plutons are strongly peraluminous with A/CNK ratios of 1.10–1.20, indicative of S-type affinities. The rocks are characterized by high SiO2 and K2O contents with high CaO/Na2O ratios. Moreover, the rocks possess low MgO contents, Rb/Sr and Rb/Ba ratios, together with their relatively high initial 87Sr/86Sr ratios (0.7139–0.7152) and less radiogenic εNd(t) values (−3.15 to −5.17), implying a clay-poor and plagioclase-rich crustal source.Compared with earlier pulse of arc-related magmatism (ca. 470–423Ma), the latter pulse of magmatism (ca. 424–395Ma) consists mainly of “normal granite” characterized by higher SiO2 (>66%) and K2O contents, weaker fractionated REE patterns and lower δEu values, and gabbroic to dioritic intrusions are only sporadic. Moreover, the granitoids of the latter pulse show variable but more crust-like Sr-Nd isotopic compositions ((87Sr/86Sr)0=0.7038–0.7327; εNd(t)=−6.70 to +0.33) than the earlier ones ((87Sr/86Sr)0=0.7024–0.7080; εNd(t)=−2.56 to +8.86), indicating that the Early Devonian (ca. 424–395Ma) experienced extensive crustal melting with minor involvement of mantle materials. Considering Early Devonian geological evidence, the transition from I- to S- and A-type magmatism probably reflects the Early Devonian amalgamation between the Gongpoquan and Huaniushan arcs, which caused not only regional unconformity and strong deformation-metamorphism, but also extensive melting of the accreted crustal materials in the BOC.