Geochronology, geochemistry and Nd–Hf isotopes of the Xiaokouzi granite from the Helanshan complex: Constraints on the Paleoproterozoic evolution of the Khondalite Belt, North China Craton

Abstract

The Khondalite Belt is one of the three Paleoproterozoic mobile belts in the North China Craton and was formed by a continental collision between the Yinshan and Ordos Blocks at ca. 1.95 Ga. Although the metamorphism, geochemistry and geochronology of the Khondalite Belt have been extensively studied, the provenance and depositional setting of the khondalite protoliths remain controversial. In this study, U–Pb zircon ages and geochemistry of the Xiaokouzi granite in the southern Helanshan Complex, part of the Khondalite Belt, are investigated. Zircon U–Pb dating indicates that the Xiaokouzi granite was formed between 2023 ± 14 Ma and 2034 ± 16 Ma. The granite shows variable SiO2 (64.33–74.14 wt%), moderate Al2O3 (14.75–17.23 wt%) and A/CNK values (1.09–1.66), low CaO (0.33–2.35 wt%), TiO2 (0.30–0.66 wt%), TFe2O3 (2.58–5.76 wt%), MgO (0.82–2.32 wt%) and K2O (2.57–4.96 wt%) contents. These features indicate that the Xiaokouzi granite is a peraluminous S-type granite. The Xiaokouzi granite has Cr and Ni contents of 32.7–84.5 ppm and 10.7–19.3 ppm, respectively. The granite is enriched in Cs, K, Pb, and Zr, and depleted in Nb, Ta, P, and Ti. The granite displays variable ɛNd (t) values (–1.7 to +3.5) with a corresponding two-stage Nd model age of 2.2–2.5 Ga. In addition, the granite has high initial 176Hf/177Hf ratios (0.281569–0.281789) and positive ɛHf (t) values (+1.1 to +8.1) with a two-stage Hf model age of 2.2–2.6 Ga suggesting a juvenile source. The crystallization age of the Xiaokouzi granite is synchronous with the peak in the detrital zircon age population from metasedimentary rocks in the Khondalite Belt. In addition, Hf isotope values from the Xiaokouzi granite are consistent with the detrital zircons from metasedimentary rocks. Together with other Paleoproterozoic igneous rocks in the Khondalite Belt, it is inferred that Paleoproterozoic magmatic rocks in the Khondalite belt may be the potential provenance for khondalite protoliths. Combined with previous geological data from the Khondalite Belt, we propose that the Helanshan khondalite protoliths were most likely deposited in an active continental margin near the Ordos Block.