Wang et al
Abstract:The Yanliao rift zone along the northern margin of the North China Craton exposes a suite of volcanic rocks comprising trachybasalts and trachytes interlayered with dolostones in the Tuanshanzi Formation of the Changcheng Group. The overlying Dahongyu Formation is composed of olivine basalts, trachybasalts, trachyandesites, trachytes, and minor sub-alkaline basalts, intercalated with a lower sandstone and an upper dolostone sequence. LA-ICP-MS zircon U-Pb age data from the volcanics reveal that the Tuanshanzi Formation was deposited at ca. 1670 Ma, and the Dahongyu Formation at ca. 1625–1664 Ma. Combined with previous studies, these new data indicate that the lowest boundary age of the Changcheng Group, marking the timing of initiation of the Yanliao rift, is ca. 1680 Ma. Except for the sub-alkaline basalts, the other alkaline volcanic rocks of the two formations are characterized by high Nb/Y ratios (1–3), and possess SiO2 contents of 43.3-60.5 wt.%, K2O of 0.72-15.69 wt.%, MgO of 0.10-8.56 wt.%, and with Mg# of 2–64. These rocks also display strongly fractionated REE patterns and generally positive Nb and Ta anomalies, with high (Nb/La)PM and (Nb/Th)PM values of 0.85-1.39 and 0.96-2.05, respectively. They show positive zircon εHf(t) values (+ 0.1 to + 1.8 for the trachybasalt sample 11PG31-2) and whole-rock εNd(t) values (+ 0.83 to + 2.25). These geochemical features, together with trace element modeling, suggest that the alkaline volcanic rocks were generated by fractional crystallization of magma derived from low-degree partial melting of an OIB-like depleted asthenospheric mantle source, with only minor involvement of melts generated from the overlying enriched lithospheric mantle source. In contrast, the sub-alkaline basalts are characterized by low Nb/Y ratios of 0.09-0.19, and show fractionated REE patterns and pronounced negative Nb-Ta anomalies with low (Nb/La)PM and (Nb/Th)PM values of 0.07-0.21 and 0.10-0.33, respectively. In combination with the positive zircon εHf(t) values (+ 0.8- + 4.3), we infer that these rocks were derived from the partial melting of a depleted lithospheric mantle source previously metasomatized by fluids from subducted slab. Combined with previous studies on the 1680–1780 Ma magmatic rocks along the northern margin of the NCC, our study identifies a late Paleoproterozoic asthenospheric upwelling in this region. The generation of the Yanliao rift and the eruption of these volcanic rocks were possibly triggered by the delamination of the continental lithosphere in a post-orogenic setting following the final amalgamation of the North China Craton within the Columbia supercontinent.
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