Remarkable Eu Anomalies Research Articles

Large-scale removal of lithosphere underneath the North China Craton in the Early Cretaceous: Geochemical constraints from volcanic lavas in the Bohai Bay Basin

Cratons are generally considered as the most stable tectonic units on the Earth. Rare magmatism, seismic activity, and intracrustal ductile deformation occur in them. However, several cratons experienced entirely different fates, including the North China Craton (NCC), and were subsequently destroyed. Geodynamic mechanisms and timing of the cratonic destruction are strongly debated. In this paper, we investigate a suite of Mesozoic intermediate to felsic volcanic rocks which are collected from boreholes in the Liaohe Depression of the Bohai Bay Basin the eastern NCC. These volcanic rocks have Precambrian basement-like Sr–Nd isotopic characteristics, consistent with derivation from the lower continental crust underneath the NCC. The Late Jurassic (~165Ma) intermediate volcanic rocks don't exhibit markedly negative Eu anomalies, which require a source beyond the plagioclase stability field. And the low heavy rare earth elements (HREEs) contents of these samples indicate that their source has garnet as residue. The Early Cretaceous (~122Ma) felsic volcanic rocks are depleted in HREEs but with remarkable Eu anomalies, suggesting that their source have both garnet and plagioclase. The crust thicknesses, estimated from the geochemistry of the intermediate and felsic rocks, are ≥50km at ~165Ma and 30–50km at ~122Ma, respectively. The crustal thinning is attributed to lithospheric delamination beneath the NCC. Our results combined with previous studies imply that the large-scale lithospheric removal occurred in the Early Cretaceous, between 140 and 120Ma.

Petrogenesis of the Kuangshancun and Hongshan intrusive complexes from the Handan–Xingtai district: Implications for iron mineralization associated with Mesozoic magmatism in the North China Craton

The Handan–Xingtai district in eastern China exposes numerous late Mesozoic intrusions. Among these, the Kuangshancun intrusive complex is one of the major ore-related intrusions whereas the Hongshan complex is barren, although both intrusions display similar geochemical characteristics. The Kuangshancun complex consists of diorite and monzonite, with zircon LA-ICP-MS U–Pb age of 133.7±1Ma. The Hongshan complex mainly consists of syenite and shows zircon U–Pb age of 134.5±1Ma. The mineral chemistry of plagioclase from both complexes reveals normal zoning, consistent with the fractional crystallization process. Rocks of the Kuangshancun complex show SiO2 in the range of 58.92–63.84wt.%, Na2O of 4.63–8.81wt.%, and Al2O3 of 16.14–18.18wt.%, together high Sr/Y (14–54) and high LaN/YbN (8.30–16.18) ratios. They show enrichment in LREE and depletion in HREE and HFSE, with no remarkable Eu anomalies, similar to the features of adakites. The whole rock initial 87Sr/86Sr ratios range from 0.706661 to 0.706722 and εNd (t=134Ma) between −15.26 and −15.12, which are consistent with zircon 176Hf/177Hf ratios of 0.281940–0.282059 and εHf (t) values of −27.0 to −22.7. Samples of the Hongshan complex show SiO2 in the range of 56.57–68.16wt.%, high Sr/Y (19–112) and high LaN/YbN (11.39–16.82) ratios. The zircon εHf (t) values are in the range of −15.9 to −12.8 and εNd (t=134Ma) is between −9.82 and −8.62. The Kuangshancun complex was derived through partial melting of an enriched lithospheric mantle contaminated by lower continental crust components. The Hongshan complex was also derived from the EM I-like mantle source. However, the calculated zircon Ce4+/Ce3+ and Eu/Eu∗ ratios indicate that the source magma of the Kuangshancun complex were characterized by higher oxygen fugacity as compared to that the Hongshan complex. The high oxidation states and high water contents are considered as possible key factors that led to the iron mineralization in the Kuangshancun and other intrusions in the Handan–Xingtai district.

Geochemical Constraints of Sediments on the Provenance, Depositional Environment and Tectonic Setting of the Songliao Prototype Basin

Abstract The geochemistry of sediments is primarily controlled by their provenances, and different tectonic settings have distinctive provenance characteristics and sedimentary processes. So, it is possible to discriminate provenances, depositional environments and tectonic settings in the development of a sedimentary basin with the geochemistry of the clastic rocks. The analytical results of the present paper demonstrate that sediments in the Songliao prototype basin are enriched in silica (SiO2=66.48–80.51%), and their ΣREE are 30–130 times of that of chondrite with remarkable Eu anomalies. In discriminating diagrams of Eu/Eu* vs ΣREE and (La/Yb)N vs ΣREE, most samples locate above the line Eu/ Eu*=1, on the right of the line Eu/Eu*/ΣREE=1 and under the line La/Yb)N/ΣREE=1/8, which indicates that the depositional environment of sediments in the basin was oxidizing. In addition, variations of MgO, TiO2, Al2O3, FeO+Fe2O3, Na2O and CaO vs SiO2 reflect a tendency of increasing mineral maturity of sediments in the basin. Analyses also show that the protolytes of the sediments in the basin were felsic igneous rocks, and the Songliao prototype basin was in a tectonic setting of an island arc on an active continental margin, with violent sinistral strike‐slip.