Genesis of the Late Paleozoic Dongwanzi pluton, eastern Hebei
冀东晚古生代东湾子岩体的岩石成因研究

The Late Paleozoic Dongwanzi pluton is made up of hornblendite, pyroxenite and gabbro. Cumulate origin of these rocks is suggested by the cumulative textures, the convex-upward REE patterns of whole rock and mafic minerals (diopside and hornblende), and the low and large variation of compatible elements such as V in hornblendites. Calculated REE patterns of the parental magma in equilibrium with the pyroxenite show strong fractionation between LREE and HREE. The enrichment in LILE (e. g. Sr, Ba, K) and depletion in HFSE (e. g. Nb, Ta, Ti), along with the compositions of diopside and hornblende, suggest their arc magmatic affinity. High Ca-diopside, magmatic hornblende and the reactions between diopside and melt to form amphibole all can be attributed to the high content of water in the parental magma. It seems that the parental magma with H2O content >3%, was originated from partial melting of a metasomatized mantle wedge which was dominated by amphibole-bearing spinel peridotite. The metasomatism by fluids released from the down-going Paleo-Asian oceanic slab led to the formation of the enriched mantle. Based on these data, we believe that the Dongwanzi mafic pluton was formed as a consequence of subduction of the Paleo-Asian Ocean beneath the northern margin of the North China craton.