Petrology, Geochemistry And Petrogenesis Of Petrogenesis Of The Supracrustal Rock From Yunzhou Within The Chicheng,North Hebei Province, China

The Supracrustal rock from Yunzhou in the Chicheng County, northern Hebei Province, China is a series of sedimentary-metamorphic rock with the porphyroblastic texture and the mediun-fine grained lepido-granoblastic groundmass and gneissic structure. Some blastopsammitic texture are preserved locally. It mainly consists of plagioclase (28%～32%), quartz (35%～40%) and biotite (18%～25%) with the presence of minor garnet (2%～8%) and graphite (1%～5%). It mainly consists of plagioclase, quartz and biotite, with the presence of minor garnet and graphite. The electron microprobe analyzes show that plagioclase in the garnet biotite plagiogneiss is higher, mainly An=36.5-44.3andesine;Biotite chemical composition strongly alterated. It can be divided into high-iron biotite and high-magnesium biotite with some irregular changes from the core to the edge of the grains.Garnet in the Supracrustal rock of Yunzhou is mainly composed of almandine, pyrope and grossular come second. There are certain differences in chemical composition in garnets of different areas. It shows a obvious ringlike characteristic from thecore to the edge of the garnets from Lv Hebu, Qi Lidun and Dong Goulou, and with decreasing grossular and pyrope and increasing almandine and spessartine which indicating a process of cooling down and depressurization in metamorphic evolution. Garnet-biotite mineral thermometer result (sample050303) shows the core temperature reaches780℃, while the edge temperature is only585℃which suggesting garnet in The Supracrustal rock from Yunzhou might experience a granulite facies to amphibolite facies regression from the corn to the edge. All the Supracrustal rock from Yunzhou samples have wider variation in major components. The total REE is not very high and with that of49.45×10～6140.10×10-6, displaying LREE-enriched and HREE-flatted patterns with chondrite-normalized (La/Yb)CN of5.07～8.70and LREE/HREE of5.23～9.16. Most of these samples show middling negative Eu anomaly and slight negative Ce anomaly, the chondrite-normalized Eu/Eu*and Ce/Ce*are separately0.63～0.81and0.74～1.01, and have lower contents of Rb、Sr、Cs、Ba、Zr、Hf、Nb、Ta、Th and U relative to those of upper crust but with higher Sr/Ba and Th/U value.The DF values of The garnet biotite plagiogneiss are negative (-0.35to-5.10,-3.11in average), indicating that the protolith of the garnet biotite plagiogneiss should be sedimentary; the geochemical characteristics and restore illustration of the protolith of the metamorphic rock show the protolith should be the politic (or pelitic-bearing) sandstone which is in accordance with thecommon graphite mineralization and locally preserved blastopsammitic texture in the biotite plagiogneiss.The Al/Al+Fe+Mn of samples of the study area ranged from0.51to0.61, all greater than0.5,with MnO/TiO2of0.10to0.16, indicating that the protolith is continental margin sedimentary rock mainly composed of terrigenous fragments.The Supracrustal rock from Yunzhou (Ce/Ce*)CN value is0.74～1.01(average0.91), between the ocean basin (0.55) and the continental margin (0.90～1.30), and more close to the latter’s rare-earth elements characteristics. Therefore, the protolith tectonic environment similar to continental island arc.Therefore, the Supracrustal rock from Yunzhou in the study area should be the politic (or pelitic-bearing) sandstone, a set of terrigenous fragments formed in the late Paleozoic in a continental island arc environment maybe and then occured a granulite facies-amphibolite facies alteration with its inner denegerated eclogite and other compositions.