Geochronology, geochemistry, and tectonic significance of a Paleoproterozoic diabase at southwestern margin of the NCC

  • The diabase and the granite porphyry formed at Paleo-proterozoic
  • The diabase and the granite porphyry are related to continental rifting
  • The diabase reflects the extensional activities of Helan aulacogen

The diabase and granite porphyry outcrop in the Baijiagou section within the Meso-Cenozoic Liupanshan fault zone at the southwestern margin of the North China Craton (NCC) porphyry, and they have been regarded previously as Meso-Cenozoic dykes according to their occurrence. Zircon U–Pb LA-ICP-MS age determining for the diabase and granite porphyry yield respectively 1804 ± 21 and 1792 ± 16 Ma, and they form a bimodal magmatic assemblage of Paleoproterozoic. The diabase can be classified into two major geochemical types, which are low-Ti (LT) and high-Ti (HT) in nature. The former is characterized by relatively high SiO2 (48.81–54.01 wt.%), P2O5, and Al2O3, and low TiO2 (1.41–1.54 wt.%), FeO, MnO, and CaO, and enriched whole-rock Sr and Nd isotopic composition ((87Sr/86Sr)i=0.703–0.708; εNd= -3.76 to -4.37) . The latter has lower SiO2(45.9–51.19 wt.%), P2O5, and Al2O3, and higher TiO2 (1.61–2.26 wt.%) and FeO than that of the former, and depleted Sr and Nd isotopic composition ((87Sr/86Sr)i=0.691–0.708; εNd= 3.27–5.03). Both types of diabase exhibit light rare earth element enrichment and flat middle–heavy rare earth element patterns. The LT diabase has negative Eu anomalies and higher total REE contents than the HT diabase. Th/Nb ratios of the LT and HT diabases are 0.28–0.29 and 0.08–0.10, respectively, which are similar to basalts formed in an intraplate spreading setting in association with a mantle plume. The regional geology suggests that intracontinental rifting occurred at the southwestern margin of the NCC in the Paleoproterozoic, and that the rifting was related to a mantle plume. This rifting event was part of the break-up of the Columbia supercontinent.

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