Genesis of the Hengling magmatic belt in the North China Craton: Implications for Paleoproterozoic tectonics

Abstract

The 2200–1880Ma igneous rocks in the central and eastern parts of the North China Craton (NCC) constitute a new Hengling magmatic belt (HMB), which includes the ~2147Ma Hengling mafic sill/dyke swarm, the ~2060Ma Yixingzhai mafic dyke swarm, and the ~1973Ma Xiwangshan mafic dyke swarm. The three swarms are contiguous and have experienced variable degrees of metamorphism from greenschist to low amphibolite facies (Hengling), medium granulite facies (Yixingzhai), and medium/high-pressure granulite facies (Xiwangshan). They are all tholeiitic in composition typically with 47–52wt.% SiO2 and 4–10wt.% MgO, and all show light rare earth element enrichments and Nb- and Ta-depletion. Their Nd TDM ages are in the range of 2.5–3.0Ga. Specifically, the Hengling and Yixingzhai dykes/sills are depleted in Th, U, Zr, Hf and Ti, whereas the Xiwangshan dykes are enriched in U and weakly depleted in other elements. Variable Sr-anomalies indicate significant feldspar accumulation (positive anomalies) or fractionation. The εNd(t) values of the three swarms are: −3.2–+3.0 (Hengling), −1.7–+1.8 (Yixingzhai) and −1.4–+1.0 (Xiwangshan).These mafic representatives of the HMB originated from the >2.5Ga sub-continental lithospheric mantle of the NCC, and with A-type granites and other igneous associations in this belt they likely evolved in an intra-continental rift. The progressive changing compositions of the three swarms are interpreted in terms of their source regions at different depths, i.e., shallower and shallower through time. And the decrease in scale and size of the intrusions and their magma volumes indicate the progressive weakening of magmatism in this rift. The rocks in this belt are different chronologically, petrologically and chemically from those in the Xuwujia magmatic belt (XMB). We propose that the two magmatic belts represent two different magmatic systems in different blocks of the NCC, i.e., an eastern block (with the HMB) and a western block (with the XMB). Terminal collision was possibly a result of ridge subduction between the two blocks, which led to exhumation of the igneous rocks in the two belts from different crustal levels, distinguishable by their different grades of metamorphism.