The Metamorphic Peridotites From Within The Hongqiyingzi Group In Chicheng, North Hebei Provinve: A Highly Dismembered Ophiolite?

Metamorphic peridotites from the Chicheng, North Hebei Province, China occur as separate tectonic lenses or lumps within the biotite-plagioclase gneisses of the Late Paleozoic Hongqiyingzi Group. In space, the metamorphic peridotites are often closely associated with retrograded eclogites. The olivines'components are relative constantly. They contain more w(MgO), but less w(FeO), w(CaO), w(MnO). Their average w(NiO2) is near to the mantle peridotites'(0.4%;Sato ,1977). En'components are also relative constantly, and w(MgO) in En is higher while w(FeO) and w(CaO) are lower. Cr/(Cr+Al) in the metamorphic peridotites'chromites ranges from 0.647 to 0.865, which is significantly higher than deep sea peridotite corresponding value in the chromite, equivalent of Dick and Bullen (1984) classⅢin the spinel peridotite and the composition features are compatible with the pro-arc or oceanic crust transition zone. In Cr/(Cr+Al) vs. Mg/(Mg+Fe2+) digram, with Cr/(Cr+Al) increased Mg/(Mg+Fe2+) reduced accordingly, and this trend is similar to the rest of the world ophiolitic metamorphic. These results suggest that the study area metamorphic peridotites are from ophiolite metamorphic peridotite and they should come from the highly depleted mantle, and has a certain affinity to the island arc.The metamorphic peridotites of harzburgitic composition, contain more MgO, but less CaO, Al2O3 and FeO*, and their average w(TiO2) is 0.02 %, which is lower than the mid-ocean ridge mantle TiO2 content, but is corresponding to Subduction zone ophiolite rock TiO2 content Their total REE is less than the primitive pyrolite and the PM (primitive mantle)-normalized TME (Transitional metal element) distribution patterns are anisomerous W-type, and have remarkably negative Ti and Cu anomaly. Geochemical characteristics of metamorphic peridotites show they are similar to SSZ-type pyrolite that came from upper mantle under subduction zone and might represent the depleted pyrolite which underwent higher partial melting.Retrograded eclogites'protolith belongs to the basic rock, their w(SiO2) content ranges from 44.57% to 54.03%. Retrograded eclogites contain more w(MgO) (6.15%-11.31%) and Mg# ranges from 48.68 to 65.34. Their w(TiO2) ranges from 0.92% to 1.70%, the average is 1.25%, which between in a typical mid-ocean ridge tholeiite (1.50%) and active continental margin of island arc tholeiite (0.83%). In TiO2-MnO-P2O5 vs. FeO*/MgO-TiO2 diagram, samples fall into the MORB or IAT field. With TiO2 increased Al2O3/TiO2 and CaO/TiO2 reduced accordingly, and this trend is consistent to the evolution of the ocean ridge basalt. In this areas , retrograded eclogites with three different REE distribution patterns. The first one is LREE-depleted, which is MORB-like ophiolitic basalts; The second one is flat REE pattern, which is similar to T-type Middle Ocean Ridge Basalts; The last one is LREE enriched pattern, which is distinguish from N-type MORB,E-type MORB and Ocean Island Basalts. But the last pattern can't represent retrograded eclogites'protolith nature, because it must be contaminate by continental crust. Compatible high field strength elements (HFSE) in retrograded eclogites are similar to MORB, their fSm/Nd ranges from 0.08 to 0.39, the average is 0.23 andεNd(when t= 438 Ma) ranges from1.37 to 9.94, the average is 6.34. Geochemistry of retrograded eclogites shows that, they come from depleted mantle and their protolith have the properties of both mid-ocean ridge basalts and island arc tholeiite.In this area, metamorphic peridotites and retrograded eclogites in close intergrow in space, they have some genetic connection. Metamorphic peridotites belong to ophiolitic peridotites and they formed in an island arc environment, from the subduction zone under the mantle. Retrograded eclogites formed in a back-arc basin, which developed on the basis of island arc. So, metamorphic peridotites and retrograded eclogites'protolith-tholeiite are the highly dismembered ophiolite, which should be formed in an island arc environment. They once underwent the subduction then emplaced into the Hongqiyingzi Group by the faulting mainly.