Petrogenesis and tectonic implications of the iron-rich tholeiitic basalts in the Hutuo Group of the Wutai Mountains, Central Trans-North China Orogen
The Hutuo Group, located in the Wutai Mountains area, is characteristic of Paleoproterozoic strata in the North China Craton, and plays an important role in establishing evolutionary processes in the Trans-North China Orogen. We present petrological observations along with geochemical and Nd isotopic data for basalts in the Qingshicun and Hebiancun Formations of the Hutuo Group. These basalts are enriched in TFeO (11.97−18.01 wt.%), consistent with occurrences of iron-rich basalt world-wide. They also display relatively high MgO (4.95−12.25 wt.%), Ni (79−121 ppm) and Cr (37−101 ppm) and low in SiO2 (41.37−51.95 wt.%). Therefore, the iron-rich character of the Hutuo Group basalts originated with their parental magma rather than via shallow crustal fractionation at low oxygen fugacity, as inferred for many high iron basalts. The Hutuo Group basalts exhibit weak to moderate differentiation of light REE over heavy REE ((La/Lu)cn = 2.61−6.79) with minor or slight Eu anomalies (Eu/Eu* = 0.77−1.41), and also show the Nb and Ti troughs without obvious Zr and Hf anomalies. The ɛNd(t) values of the basalts range from −1.23 to +0.81 with TDM of 2489−2664 Ma. The basalts most plausibly originated from the fertile subcontinental lithosphere mantle and, in the case of the Qingshicun Formation basalts, also underwent the crustal contamination. Basalts in the Hutuo Group also have the high Zr/Hf ratios, and present high Ti/V values of 24−55 and Zr/Y ratios ranging from 4.6 to 13.6, suggesting that they were emplaced in a within-plate setting. Considering the sedimentary associations and detrital fragments in the Hutuo and Gantaohe Groups, and bimodal magmatism along the Trans-North China Orogen, we propose that 2.2−2.0 Ga geological events within the orogen were rift-related, but did not result from subduction or island arc processes.