Late Palaeoproterozoic post-collisional magmatism in the North China Craton: geochemistry, zircon U-Pb geochronology, and Hf isotope of the pyroxenite-gabbro-diorite suite from Xinghe, Inner Mongoha

摘要

The North China Craton (NCC) witnessed a prolonged subduction-accretion history from the early to late Palaeoproterozoic, culminating with final collision at ca. 1.85 Ga and assembling the continental blocks into the cratonic framework. Subsequently, widespread post-collisional magmatism occurred, particularly along the Trans-North China Orogen (TNCO) that sutures the Eastern and Western blocks of the NCC. Here we present petrological, geochemical, and zircon U-Pb geochronological and Lu-Hf data from a pyroxenite (websterite)-gabbro-diorite suite at Xinghe in Inner Mongolia along the northern segment of the TNCO. The internal structures and high Th/U values of the zircons from the gabbro-diorite suite suggest magmatic crystallization. LA-ICP-MS U-Pb age data on three gabbros and one diorite from the suite yield emplacement ages of 1786.1 ± 4.8, 1783 ± 15 ,1754 ± 16 and 1767 ± 13 Ma, respectively. The ε_(Hf)(t) shows mostly positive values (up to 5.8), with the lowest value at -4.2, suggesting that the magma was derived from dominantly juvenile sources. The generally low SiO_2 and high MgO values, and other trace element features of the Xinghe suite are consistent with fractionation from a mantle-derived magma with a broadly E-MORB affinity, with no significant crustal contamination. Recent studies clearly establish that the major magmatic pulse associated with rifting of the NCC within the Columbia supercontinent occurred in the late Mesoproterozoic at ca. 1.3-1.2 Ga associated with mantle plume activity. This, together with the lack of robust geochemical imprints of rift-related magmatism in the Xinghe suite, prompts us to suggest a tectonic model that envisages magma genesis associated with post-collisional extension during slab break-off, following the westward subduction of the Eastern Block and its collision with the Western Block. The resulting asthenospheric upwelling and heat input might have triggered the magma generation from a heterogeneous, subduction-modified sub-lithospheric mantle source for the Xinghe rocks, as well as for similar late Palaeoproterozoic suites in the TNCO.