No abstract

This paper presents a new detailed study including zircon U-Pb-Hf isotopic, whole-rock geochemical and Sr-Nd isotopic analyses of conglomerate and granitic pebbles from the molasse deposits in central Jilin Province, NE China. These data are used to better constrain the Late Permian–Triassic tectonic evolution regarding particularly the final closure of the Paleo-Asian Ocean (PAO) along the Changhun-Yanji suture (CYS). Zircon U-Pb data indicate that the granitic pebbles formed in the end-Permian (254–253 Ma). The youngest detrital zircon age of 231 Ma from the conglomerate, and presence of the overlying Upper Triassic Sihetun Formation suggests that the molasse deposits on the Jin′gui Island formed during the Late Triassic. The end-Permian granitic rocks display high SiO2 (66.07–74.77 wt %), with low MgO (0.55–2.05 wt %) and Mg# (31.61–43.64) values, together with depleted Hf and Nd isotopic values (εHf(t) = +1.61 to +11.62; εNd(t) = +3.3 to +4.2; (87Sr/86Sr)i = 0.706458–0.706842) and juvenile second-stage Hf model ages (1148–512 Ma), suggesting that they were probably generated by the partial melting of a Meso-Neoproterozoic juvenile metabasaltic lower crust. They are characterized by enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs), with affinities to igneous rocks forming in a subduction-related setting. This, combined with regional coeval subduction-related magmatic rocks, indicates that the PAO still existed along the CYS. In addition, the identification of Late Triassic molasse deposits on the Jin′gui Island in this study, coupled with occurrences of many Early Triassic syn-collisional granitoids along the CYS, indicates that the final closure of the PAO took place prior to the Late Triassic.

A suite of Paleogene mafic rocks was collected from boreholes in the Huimin Sag of the Jiyang Depression with the aim of investigating the petrogenesis and nature of mantle source for these rocks and further providing insights into the characteristic of related mantle plume. Whole-rock geochemical data indicate that the mafic rocks have relatively lower SiO2 (42.93%–48.57%) contents and similar characteristics to alkaline basalt and belong to transitional calc-alkaline series. These samples were clearly enriched in LREEs and depleted in HREEs and were also characterized by the enrichment of LILEs, incompatible elements, and HFSEs, similar to those of the Ocean Island Basalt (OIB). In addition, they exhibited Pb enrichment; Y, Pr, and Yb depletion; absence of Nb-Ta anomalies; high Hf and low Zr; and Rb/Yb ratios exceeding 1.0, indicating characteristics of intraplate rift-type alkaline basalt. The samples exhibited (Th/Ta)PM and (La/Nb)PM ratios less than 1 and plotted within the OIB, EMI, and EMII fields, indicating that crustal components had no role in the generation of the rocks. With the exception of individual samples that have a distinctive range of ε Nd values, the majority of samples have complex ε Nd values of -1.15 to 5.56, indicating a mixture of different sources, which was also apparent in the δ18O-87Sr/86Sr diagram, in which the samples plot close to the downward nonlinear curve. Based on the isotopic and trace elemental analyses, these igneous rocks are intraplate rift-type alkaline basalt and are of mantle plume origin. The variations in 87Sr/86Sr, 143Nd/144Nd, ε Nd values, LREEs, and HFSEs were probably due to the different locations of the mantle plume for different samples. The primary magma of the rocks likely originated from the melting of a mantle plume and the further metasomatism of lithospheric mantle, continental, or oceanic crust.