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Carboniferous Alaskan-type complex along the Sino–Mongolian boundary, southern margin of the Central Asian Orogenic Belt

Abstract:

We present zircon ages and geochemical data for the Hongshishan Carboniferous Alaskan-type mafic–ultramafic complex exposed in the Beishan area along the Sino–Mongolian boundary, southern margin of the Central Asian Orogenic Belt. This complex mainly consists of dunite, harzburgite, lherzolite, wehrlite, and gabbro, which intrudes Early Carboniferous volcanic rocks and reveals a zoned structure. Zircons of a gabbro sample yielded a 206Pb/238U age of 357 ± 4 Ma, reflecting the time of Early Carboniferous magmatism. Zircon ages were also obtained for an andesite (322 ± 3 Ma) and a basaltic andesite (304 ± 2 Ma). High initial Nd isotope whole-rock values suggest that the Hongshishan gabbro [εNd(t) = +9.6 − +10.2] and basalt [εNd(t) = +10.0 − +10.8] were derived from a depleted mantle source. Slightly lower εNd(t) values for the ultramafic rocks [εNd(t) = +8.5 − +8.7] suggest some interaction of the parental magma with the continental crust. In contrast, the Late Carboniferous Quershan samples in this area represent subduction-related arc volcanic rocks with Adakite-like compositions. The early Carboniferous Hongshishan Alaskan-type complex was interpreted to represent the remnants of a magma chamber that crystallized at the base of a mature island arc, whereas the Quershan island arc volcanic rocks suggest the resurrection of the subduction process after arc-continent collision and uplift of the roots of the arc.