Petrogenesis of Early Cretaceous High Ba-Sr Granitoids in the Jiaodong Peninsula, East China: Insights into Regional Tectonic Transition

Abstract

Element geochemistry, Sr and Nd isotope, and LA-ICP-MS zircon U-Pb isotope data have been obtained for the granitoids of Dazesan pluton in the Jiaodong Peninsula, East China, and their intermediate microgranular dark enclaves so as to reveal their petrogenesis and tectonic implications. These granitoids have high SiO2 (68.25–71.56 wt.%), K2O (3.44–5.50 wt.%), total alkalis (K2O + Na2O = 7.29–9.00 wt.%), Sr (451–638 ppm), Ba (1157–2842 ppm) and light rare earth elements (LREEs) (131.57–210.08), with strong depletion both in heavy rare earth element (HREE) and high field strength element (HFSE) concentrations as well as unclearly Eu anomalies, showing typical signatures of high Ba-Sr granitoids. They possess high (La/Yb)N (32–50) and Sr/Y (50–79) values and low MgO (0.76–1.11 wt.%), Cr (9.9–19.6 ppm) and Ni (4.51–7.04 ppm) concentrations. All the above geochemical compositions are similar to those of late Early Cretaceous granitoids, in combination with zircon LA-ICP-MS U-Pb ages of 119.6 ± 1.3 to 120 ± 1 Ma for these granitoids obtained in this study, indicating c. 120 Ma probably represents the lower limit of ages when late Early Cretaceous granitoids emplaced in the Jiaodong Peninsula. The microgranular dark enclaves, forming a linear trend with their host granitoids on the oxide against SiO2 plots, display higher MgO contents of 3.05–4.39 wt.% at lower SiO2 concentrations of 54.25–56.84 wt.% and possess a zircon LA-ICP-MS U-Pb age of 119 ± 2 Ma, identical to those of these granitoids, indicating the acid magma and intermediate magma were coeval. Furthermore, dark enclaves and their host granitoids have indistinguishable (87Sr/86Sr)i values of 0.709523–0.70972 and 0.709361–0.709858, respectively, and plot within a two-liquid immiscible field on the Greig pseudoternary phase diagram. In addition, they have markedly parallel REE patterns, with the dark enclaves having much greater REE and HFSE abundances than those of their host granitoids. Therefore, it is suggested that liquid immiscibility is a viable model to explain the chemical compositional variations between the Dazeshan granitoids and their dark enclaves. Based on the element geochemistry, geochronology and Sr- and Nd-isotope of the Dazeshan granitoids and their dark enclaves, it is envisaged the crust-derived acid melts due to partial melting of ancient continental lower crust in the Jiaodong Peninsula (mainly Neoarchean-Palaeoproterozoic basement in the Jiaobei terrane) containing a subduction-related material, resulting from the addition of the enriched subcontinental lithospheric mantle-derived melts, assimilated the lithospheric mantle-derived basic melts and formed the homogeneous magma chamber at the crust base, then split into two immiscible liquids, with one granitic liquid producing the Dazeshan granitoids and the other intermediate one forming the dark enclave during its ascent. Combined with previous studies, the identification of a lithospheric mantle-derived material in the Dazeshan granitoids suggests a catastrophic lithospheric thinning at c. 120 Ma, reflecting an abrupt change in the direction of Palaeo-Pacific plate subducting and the corresponding regional tectonic transition from E–W extension to NW–SE extension.