2.2 Ga magnesian andesites, Nb-enriched basalt-andesites, and adakitic rocks in the Lüliang Complex: Evidence for early Paleoproterozoic subduction in the North China Craton

Abstract

The Lüliang Complex is located at the western margin of the middle segment of the Trans-North China Orogen, along which the Western and Eastern Blocks amalgamated to form the basement of the North China Craton. The complex consists of the Paleoproterozoic granitic plutons and meta-supracrustal rocks, of which the latter are subdivided into the Jiehekou, Lüliang, Yejishan, and Heichashan/Lanhe Groups. A meta-volcanic rock from the Yejishan Group gives a magmatic zircon U–Pb age of 2188 ± 48 Ma and εHf(t) values of + 0.06 to + 5.42. Based on the geochemical characteristics, four different ~ 2.2 Ga igneous rock suits have been identified in the Lüliang Complex. The calc-alkaline basalts and andesites show the geochemical features of typical ‘normal’ arc volcanics with pronounced LREE enrichments relative to HREE, negative Nb, Ta, P and Ti anomalies and εNd(t) values of − 2.69 to + 2.14. The adakitic rocks are characterized by high Na2O/K2O ratios, weak to positive Eu anomalies, positive Sr and Ba but negative Nb and Ti anomalies, and relatively clustered and near to zero εNd(t) values. The Nb-enriched basalts and andesites have higher Nb, Zr and TiO2 contents, higher Nb/Ta (14.4–19.3) and Nb/U (20.6–23.2) ratios than the majority of arc basalts and andesites and relatively variable εNd(t) values (− 1.54 to + 3.02). The magnesian andesites are characterized by anomalously high MgO, Cr and Ni contents and have εNd(t) values of − 3.90 to + 1.17. The occurrence of adakitic rocks, Nb-enriched basalts and andesites and magnesian andesites has been described from many modern arcs featuring subduction of oceanic slab. Therefore, we suggest that similar mechanism may have played an important role in the production of the ~ 2.2 Ga igneous rocks in the Lüliang Complex and thus the final collision between the Eastern and Western Blocks along the TNCO must have happened at some time after ~ 2.2 Ga.