Permian marine basalts (the Dashibao Formation) in the SongpanGanzi Terrane to the west of the Yangtze Block, SW China, yield a SHRIMP zircon U-Pb weighted mean age of 263 ؎ 2 Ma. The Dashibao basalts are characterized by high TiO 2 contents (1.73-4.65 wt. %) and Ti/Y ratios with a mean of 577, and OIB-like rare earth element (REE) and incompatible element patterns. Geochemical variation within the basalt succession allows division into two groups; Group 1 with an alkaline composition is distinguished by higher TiO 2 and P 2 O 5 contents, along with higher Ti/Y and Sm/Yb ratios than the underlying Group 2 that consists predominantly of tholeiitic lavas. Both groups possess weakly to moderately positive Nd (t) values (0.82 to 5.28), but the Group 2 tholeiitic basalts show relatively depleted signatures (most Nd (t) >2.5) when compared to their Group 1 counterparts ( Nd (t) <2.5). REE modeling is consistent with variable degrees of melting of primitive mantle within the garnet stability field, and reveals that the Group 1 alkaline basalts could have been generated by lower degrees of melting (5-11%) than the Group 2 tholeiites (up to 19%). The initial Nd isotope discrepancy is interpreted in terms of depleted asthenospheric involvement in the early stage Group 2 tholeiitic magma. Combined geochronology, petrography and geochemistry for the Dashibao Formation confirms that it was temporally and genetically associated with the Emeishan basalts, and is therefore an integral part of the Emeishan large igneous province. The new zircon U-Pb dating supports the view that the Emeishan volcanism could be a boundary event occurring at or around the Middle-Late Permian (the Guadalupian-Lopingian) transition, and thereby confirms the validity of a causal connection with the end-Guadalupian mass extinction.

Chang’E-5 is the first lunar sample return mission of China. The spacecraft was landed in the northwest of the Procellarum KREEP Terrane (43.0576°N, 308.0839°E) on 1 December 2020 and returned 1731 g samples from a previously unvisited region. The landing area has been proposed as one of the youngest mare basalt units of the Moon and holds important information of lunar thermal evolution and chronology. However, the absolute model ages estimated in previous studies are quite different, ranging from 2.07 Ga to 1.21 Ga. Such significant difference may be caused by (1) different crater counting areas, (2) different crater diameter ranges, (3) effects of secondary craters, and (4) biases in crater identification. Moreover, the accurate landing site was unknown and the ages were estimated over the Eratosthenian-aged mare unit (Em4) instead. In light of the above unsatisfactory conditions, this study seeks to establish a standard crater size-frequency distribution of the CE-5 landing site. We derived the concentrations of FeO and TiO2 to map out the pure basaltic areas where external ejecta deposits are negligible and thus secondary craters are rare. Based on the geochemistry of basaltic ejecta excavated by fresh craters in the mare unit, the FeO concentration threshold for mapping pure basaltic areas is 17.2 wt.%. The morphologically flat subunits in the pure basaltic areas were selected for crater identification and age dating to exclude the contamination of external ejecta to the best as we could. In the Chang’E-5 sampling site subunit, we detected 313 craters with a diameter greater than 100 m and derived the absolute model age as 1.49−0.084+0.084 Ga. The craters identified in all pure basaltic subunits of Em4 gave the model age of 1.41−0.028+0.027 Ga. As least affected by secondary craters, the crater size-frequency distribution of the sample-collected pure basaltic subunit can provide important constraints for lunar cratering chronology function in combination with isotopic age of returned samples.