Abstract
The Chijianling-Guandishan pluton is distributed in the western and southeastern parts of the Lüliang Complex, which is located in the middle segment of the Trans-North China Orogen, the North China Craton. The main rock types of the pluton are medium to fine-grained dioritic and granitic gneisses metamorphosed to the lower amphibolite facies. Most dioritic and granitic samples display similar zircon U–Pb concordia ages of 2216 ± 5 Ma to 2110 ± 23 Ma, but only one dioritic sample at the western margin gives a younger age of 2069 ± 5 Ma. Zircon ɛHf(t) values of the older and younger diorites vary from −0.27 to +4.11 and + 5.35 to +7.99, respectively. The granites exhibit a large range of SiO2 contents (65.5–76.0 wt%), relatively high Fe-number (0.69–0.96), and metaluminous to weakly peraluminous character. Two-thirds of the granites show typical geochemical affinities of A-type granitoids (Group I), such as high FeOt contents (2.0–7.4 wt%), MALI (Na2O + K2O – CaO) indexes (4.9–10), 10000*(Ga/Al) ratios (2.8–6.4) and Zr + Nb + Ce + Y contents (394 to 1154 ppm), whereas the remaining granites (Group II) display I-type granitic features. Also, Group I granites have zircon ɛHf(t) values of −1.08 to +3.15 and whole-rock 143Nd/144Nd initial ratios of 0.509823–0.509947, which are indistinguishable from those of the older diorites. However, some Group II granites have more enriched zircon ɛHf(t) values of −5.33 to −0.58 and whole-rock 143Nd/144Nd initial ratios of 0.509790 and 0.509793. Based on close association with contemporaneous mafic rocks with alkaline/peralkaline and ferroan affinities, continuously geochemically variations, and identical isotopic components, we infer that the A-type granites are the extreme differentiation (ca. 90%) products of the mafic rocks, which is also proved by trace element modeling. On the other way, Group II granitic magma was produced by the assimilation (up to 40%) of the ca. 2.50 Ga Yunzhongshan pluton with the A-type magmas. Together with the middle Paleoproterozoic transgressive deposit formations, mafic dykes, and A-type granites in the middle segment of the TNCO, our findings suggest a tectonic switching model similar to modern accretionary orogen.
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