Early Silurian adakitic high-Mg diorite from the Longshan area: Implication for melting of mantle lithosphere in the south-eastern Qilian Orogenic Belt

摘要

The Longshan area is located in the junction zone between the Qinling and eastern Qilian orogenic belts. Early Palaeozoic igneous rocks in this area can provide important insights into the evolution of Shangdan Ocean. This paper reports new petrology, chronology, major and trace element geochemistry, Sr-Nd isotope, and zircon Lu-Hf isotopic compositions for the Early Silurian pyroxene diorite in the Longshan area. The pyroxene diorite has a zircon U-Pb age of 432 +/- 1 Ma (MSWD = 0.25, n = 27). The pyroxene diorite intrusion displays low SiO2 (53.34 to 56.32 wt.%), high Na2O (3.11% to 3.79 wt.%) and Sr (1112 to 1234 ppm) contents, and moderate Mg# (52.70 to 55.00) values, high Sr/Y (67 to 84) ratios, depleted in Y and HREE; these features are identical with the low-Si adakite or sanukitoids that derived from melting of metasomatized mantle wedge. The pyroxene diorite intrusion has evolved Sr-Nd isotopic compositions, (Sr-87/Sr-86) i = 0.705286 to 0.705310, epsilon(Nd)(t) = -3.4 to -3.9. Zircons from the pyroxene diorite display variable epsilon(Hf)(t) values of -3.6 to +0.1. These features suggest that the pyroxene diorite intrusion was derived from a hydrous melting metasomatized mantle wedge which has been modified by slab melts. The occurrence of pyroxene diorite indicates an Early Silurian active continental margin setting in the south-eastern Qilian Orogenic Belt.