Petrogenesis of the Late Triassic Biluoxueshan granitic pluton, SW China: Implications for the tectonic evolution of the Paleo-Tethys Sanjiang Orogen

Abstract

The Sanjiang Orogen is important for us to better understand the tectonic evolution of the Paleo-Tethys Ocean. However, the petrogenesis and tectonic setting of the Late Triassic igneous rocks in the central part of the orogen are poorly understood. In this study, we present new whole-rock elemental and isotopic compositions for the Biluoxueshan Pluton, which consists of quartz diorite, granodiorite, and alkali-feldspar granite. Zircon U-Pb dating yields crystallisation ages of 231 ± 1 Ma for the quartz diorite, 227 ± 1 Ma for the granodiorite, and 227 ± 2, 224 ± 2, 222 ± 1, and 217 ± 3 Ma for alkali-feldspar granites, indicating a prolonged emplacement of the pluton. Quartz diorites are metaluminous, high-potassium calc-alkaline, and magnesian in composition. Granodiorites are characterized by peraluminous composition and the existence of normative-corundum, resembling S-type granite. They have negative zircon εHf(t) values (−13 to −10.8) and old Hf crustal model ages (2060–1920 Ma), indicating a Paleoproterozoic crustal source. Alkali-feldspar granites are metaluminous to weakly peraluminous, have higher SiO2 content, FeOT/MgO ratio and differentiation index (DI) than other samples, and pronouncedly negative Eu anomalies, which are characteristics of highly fractionated granites. They have narrow εHf(t) values of −2.7 to +2.6 with corresponding Hf crustal model ages of 1410–1060 Ma, indicating a Mesoproterozoic juvenile crustal source modified by mantle-derived magma. By combining previous and the new data, we proposed a new tectonic evolution model of a Late Triassic northward scissor-like post-collisional extension during ca. 235–223 Ma in the Changning-Menglian Suture Zone.