Two phases of early Palaeozoic magmatism at the Wude area in the western Bainaimiao arc (Inner Mongolia, China): Petrogenesis and tectonic implications

Abstract

Arc magmatism plays a crucial role in constraining the tectonic evolution of orogenic belts. This study focuses on two phases (Phase I and II) of the Early Palaeozoic intrusive rocks in the Wude area, located in the western Bainaimiao arc terrane (BAT), southeastern Central Asian Orogenic Belt. We performed whole-rock geochemistry, zircon U–Pb chronology, and Hf isotope, amphibole geochemical analyses. Phase I comprises granites and quartz diorites with zircon U–Pb age range from 464 to 457 Ma. These rocks are medium- to high-K calc-alkaline, fractionated I-type granites exhibiting an arc-like geochemical signature, such as enrichments in Rb, Ba, Th, U, Pb, and Sr and depletions in Nb, Ta, Ti, and P. Phase II intrusive rocks consist of congeneric hornblendites, diorites, plagioclase porphyrite, and granite porphyry, with zircons U–Pb ages from 446 to 442 Ma and positive ε_Hf(t) values (+ 1.50 to + 13.39). These rocks display island arc magma compositions characterized by enrichments in large-ion lithophile elements (e.g., Ba, U, Pb, and Sr) and depletions in high-field-strength elements (e.g., Nb, Ta, Zr, Hf, and Ti). Amphiboles from the hornblendites and diorites in the Wude area are classified as magnesio-hornblendes, indicating crystallization temperatures of 702.5–763.6 °C, depths of 13.3–19.1 km (based on TiO₂ contents), and REDOX conditions of ΔNNO + 0.52 to 1.55. The positive zircon ε_Hf(t) values and calculated crystallization pressures of 3.09–4.95 Kbar (13.3–19.1 km) support the formation of cumulate hornblendites and diorites in a juvenile island arc setting within the middle crust. These findings provide insights into the tectonic and paleogeographic evolution of the western Bainaimiao Arc during the Early Palaeozoic. Geochronological and geochemical data suggest a tectonic transition occurred between Phase I and Phase II. Based on the age of the ophiolitic mélange and previous works, this transition may reflect a shift from a south-dipping subduction of the Paleo-Asian Ocean to a north-dipping subduction of the southern Bainaimiao Ocean.