Zircon U–Pb geochronology, Hf–O isotopes and whole-rock geochemistry of A-type granitoids in Halajun, South Tienshan, NW China: Implications for subdivision, petrogenesis and tectonic settings

Abstract

Abundant A-type granitic intrusions are located within the South Tienshan Orogenic Belt (STOB) and the adjacent area to the Tarim Block. Most of these intrusions share the analogical epoch of the Tarim Large Igneous Province (TLIP), however display diverse alkalinity and geochemical signature. There is controversy in deciphering the subdivision, petrogenesis and geodynamic setting of these intrusions based on their varied geochemical compositions. In this contribution, we conduct integrated research of whole-rock major and trace element compositions, zircon U–Pb geochronology, and in-situ zircon Hf–O isotopic characteristics of several Halajun intrusions in the STOB, to unravel their subdivision, petrogenesis and geodynamics. The Halajun IV quartz-syenite, Kaladuowei quartz-monzonite and tourmaline-rich granite show Permian crystallization ages of 283.4 ± 1.1 Ma, 280.2 ± 1.3 Ma and 270.7 ± 1.5 Ma, respectively. These intrusions exhibit high Zr + Nb + Ce + Y concentrations (449–1592 ppm), 10,000 × Ga/Al values (3.31–4.47), Na2O + K2O (6.9–10.9 wt%), (K2O + Na2O)/CaO (3.5–19.2) and FeOT/(FeOT + MgO) (0.67–0.95), which are similar to those of typical A-type granitoids. These samples show various degrees of enriched Rb, Th, U, Zr, Hf and Pb, and depleted Ba, Sr and Eu. Decoupling of Nb and Ta in late granitic phases infers that the Nb–Ta differentiation occurred during the magmatic evolution. The in-situ zircon Hf and O isotopes of these intrusions show wide ranges of εHf(t) (−1.2 to +4.7) and δ18O (+6.2‰ to +8.2‰), indicating that crust- and mantle-derived materials were both involved in the quartz-syenite, quartz-monzonite and tourmaline-rich granite. End-member mixing model shows that the mantle contribution is ∼80% for quartz-syenite and ∼60–65% for quartz-monzonite and tourmaline-rich granite, respectively. With the increasing crustal materials of the parental magma and the subsequent fractionation of ilmenite and biotite, they exhibit a gradual tendency from A1-type to A2-type granitoids. This suggests that A-subtypes may evolve during the magmatic process. Considering the same tectonic setting and a common parental source, the quartz-syenite, quartz-monzonite and tourmaline-rich granite should be A1-type. The Halajun plutons were emplaced between the STOB and Tarim Block, and their generation might be related to the contemporaneous Permian Tarim mantle plume. The primitive magmas, derived from mantle plume, was mixed with the ancient Proterozoic crust and formed the Halajun A1-type granitoids.These alkaline intrusions share two tectonic regimes, i.e., the southern margin of the Tienshan Orogenic Belt and the northern margin of TLIP. The regional tectonic setting was in the transitional period from a compressive regime to an extensional or post-collisional one in the Late Paleozoic. The rising plume generated the ultramafic pipes, and provided dynamic energy to the extensional motion. The collapse of the Tienshan Orogenic Belt and the contemporary heavy eruption of TLIP not only afforded channels for the upwelling mantle magma, but also cycled the lithosphere or crustal materials into the mantle, which might have been involved in the formation of the A2-type granitoids in the STOB. The A1-type granitoids were mainly derived from the mantle source along the terminal suture.