科研之友
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摘要
The late Carboniferous Dongwanzi Complex in the northern North China Craton is composed of intrusive pyroxenite, hornblendite, gabbro, and syenite. The mafic-ultramafic rocks of the complex exhibit typical cumulate textures, curved-upward REE patterns, and variable contents of compatible elements, suggesting a cumulate origin. The syenite shows Sr-Nd isotopic ratios similar to the mafic-ultramafic complex and positive Eu anomalies in the chondrite-normalized REE patterns, suggesting that the syenite may represent residual melt after significant fractional crystallization of mafic melt. The mafic-ultramafic cumulates have low HREE abundance and high (Tb/Yb)(N) (2.5-4.2) and Dy/Yb ratios (>2), indicating that they may have originated from melting of garnet peridotite in the mantle. The Dongwanzi Complex is characterized by a large variation in Sr-Nd isotopic composition, with I-Sr = 0.7035 to 0.7052 and epsilon(Nd)(t) = -4.0 to +5.2, which may be accounted for by mixing melts of depleted asthenospheric and enriched lithospheric sources. The radiogenic Os isotopic compositions of the complex ((Os-187/Os-188)(i) = 0.1344 to 0.3090) suggest slight contamination by mafic lower crust (<= 2.5% based on Os isotopic modelling). The Dongwanzi Complex exhibits arc-related whole-rock and mineral geochemical affinities, such as enrichment in LILE (e.g. Sr, Ba, K) and depletion in HFSE (e.g. Nb, Ta, Ti). The abundance of hornblende and high CaO contents (22-24 wt.%) of clinopyroxene suggest that the source was rich in H2O, probably due to the formation above a subduction zone. We conclude that the Dongwanzi Complex and the related crust-mantle interactions probably reflect formation in a back-arc extensional environment related to the subduction of the Palaeo-Asian Ocean beneath the northern margin of the North China Craton in late Palaeozoic time.
