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), 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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