Geochronological and geochemical constraints on the Lüliang Group in the Lüliang Complex: Implications for the tectonic evolution of the Trans-North China Orogen

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 late Neoarchean to Paleoproterozoic granitic plutons and meta-supracrustal rocks, of which the latter are subdivided into the Jiehekou, Lüliang, Yejishan, and Heichashan/Lanhe Groups. The Lüliang Group is composed mainly of siliciclastic rocks in the lower part and volcanic rocks in the upper part, all of which have been metamorphosed from greenschist- to amphibolite-facies. U–Pb ages of detrital zircons from metasedimentary rocks in the Lüliang Group yield four populations of 3061–2920 Ma, 2790–2600 Ma, 2599–2280 Ma and 2211–2202 Ma, suggesting that their provenances were derived mainly from the Lüliang and Taihua Complexes in the Trans-North China Orogen. The meta-volcanic rocks give igneous zircon U–Pb ages of 2209–2178 Ma and εHf(t) values of −0.8 to +3.6. Geochemically, they are subalkaline basalts and andesites that can be classified into two groups, of which one group has flat LREEs, weak enrichment in LREEs relative to HREEs and flat HREEs and displays E-MORB-like spider diagrams with the exception of insignificantly negative Nb–Ta, Zr–Hf and Ti anomalies. In contrast, the other group displays stronger LREE fractionation, stronger enrichment in LREEs relative to HREEs and mildly fractionated HREEs and is characterized by “spiky” spider diagrams similar to arc volcanic rocks. The first group is interpreted to have originated from a Neoarchean E-MORB source, whereas the second group shows inputs of newly subduction-derived fluids in the mantle source. These geochronological and geochemical signatures indicate the existence of an 2.2–2.1 Ga continental back-arc system in the Lüliang Complex of the Trans-North China Orogen, which is consistent with the model that the collision between the Eastern and Western Blocks to form the coherent basement of the North China Craton in the Paleoproterozoic, not at the end of Neoarchean.