The Geochemical Characteristics And Metamorphism Of Jianping Group Granulite In North-east Of Nuluerhu Uplift

It is important for comprehensive understanding the distribution of the Jianpi-ng group high-pressure granulitic metamorphic rock zone and North China Platform high-pressure granulitic metamorphic rock zone,that Yanjiazhangzi hypersthene gran-ulite and Lujiadi garnet-plagioclase pyroxenite in the north-east of Nuluerhu uplift in North China Platform as new places are found in this study,which outcropes Jianping group high-pressure granulitic metamorphic rock.With studying of metam-orphism to reveal the forming and the Evolution Mechanism.On bases of collecting sufficient previous material, and taking field surveys and c-omprehensive laboratory test, this paper states the observation for field geological chara-cterist-ics for granulitic metamorphic rock of Jianping group in Nuluerhu uplift of n-orthernmargin of the north China platform, and the study of petrography, mineral, lith-ogeoch-emistry, metamorphism and recovering original rock for collected samples.By field survey, granulitic metamorphic rock occurrences in supracrustal encla-ves of archean gneiss complexes. Supracrustal enclaves are composed of amphibo-lite, in which granulitic metamorphic rock presents banted-like structure, lenticular structure and stratoid structure. The research of indoor petrographical shows that mineral association of hypersthene granulite is composed of hypersthene+plagio-clase+biotite+quartz; and major ingredient of garnet-biotite-amphibol-plagioclasep-yroxenite are garnet+clinopyroxene+plagioclase+hornblende+biotite+quartz Thus, it can been seen that weakly-gneissose strucrure,massive structure; granuloblast-ic texture,poikilitic blastic texture,blastoporphyritic texture and retrograde metamor-phism texture etc.The research of petrochemistry and petrological geochemistry of granulite sh-ows that major ingredient feature of granulitic metamorphic rock is located in dist-ribution range of terrain granulites of the world, which is similar to chemical co-mposition of lower crust in North China Platform.The totel REE of hypersthene granulite are lower, light and heavy REE show obvious fractionation and δEu is positive anomaly, slightly right-oblique REE distribution patterns.The totel REE of garnet-plagioclase-pyroxenite are high, light and heavy REE fractionation is not cl-ear and δEu is negative anomaly, slightly right-oblique REE distribution patterns. Siderophile elements,high field strength elements lost and lithophile element richm-ent. In the spider diagram of trace element of granulite, it shows the obvious fea-tures of riching in Ba and lost of U and Th.This maybe also presents original c-omposition feature of the Archean lower crust of north China craton; Comparing with the average of similar granulites in other parts of world, the granulite has a high ratio of Sm/Nd and low ratio of LaN/YbN. That shows that differentiation degree of granulite of north China craton are generally low, also presents that co-mposition of the Archean lower crust have low differential degree.By studying major elements and rare earth lelment and trace element of the si-ngle mineral, it is concluded that orthorhombic pyroxene is metamorphogenetic hypers-thene, which is monomineral with low REE, but very rich tombar thite, both have stron-g negative anomaly of δEu, tombar thite distribution curve incline steeply to left.Lithop-hile element and high field strength elements in hypersthene lost obviously,siderophi-le elements such as V and Co are rich and Ni and Cr comparingly lost.Orthopyroxen-e are mainly composed of metamorphogenetic diopsite.Garnet has relatively weak growi-ng zone, with low REE, but riches tombar thite, main for negative anomaly of δEu. Dis-tribution curve of tombar thite inclines to left slowly, and curve of rich tombar thite wit-h weak8Eu negative anomaly.The total of garnet of trace elements is low, compare-ng with primitive mantle, major for lost, and strong lost for lithophile element Sr. Am-phibole is composed of metamorphogenetic tschermakitic hornblende, with high REE a-nd rich tombar thite and light and heavy REE fractionation is clear, with negative anomaly of δEu.It is significant that Lithophile element and high field strength elem-ents Nb and Ta in Amphibole are very rich.Plagioclase is oligoclase, comparing ha-ving rich tombar thite, with obvious fractionafion and strong δEu positive anomaly. Lithophile element are rich obviously, especially Sr and Pb, high field strength el-ements,siderophile elements are main for lost.Biotite is iron-rich magnesia mica, with low REE and medium positive anomaly positive anomaly of δEu, light REE enrichment that is not obvious fractionafion. It is rich with lithophile element,esp-ecially Rb and Ba, but Sr lost, high field strength elements Nb and Ta obviously rich.The Orthopyroxene and garnet in garnet-plagioclase pyroxenite and Amphibole and biotite which is coexistence with them are attained granulitic.so, garnet-plagio-clase pyroxenite should be granulitic metamorphic rock.The peak metamorphic temp-erature higher in hypersthene granulite that is800-868℃; There are2period of the metamorphic temperature in garnet-plagioclase pyroxenite.The first metamorphic te-mperature could reach650-750℃, the pressure ranged from0.86to1.2Gpa and then, the retrometamorphism could be about500℃.Through the calculation of forming te-mperature of the garnet-plagioclase pyroxenite,it could reach the temperature-Pressure condition in granulitic metamorphic,with a clockwise nearly isothermal decompressi-onal evolvement trend of P-T-t.The research in the protolith recovery and the tectonic setting of hypersthene granulite and garnet-plagioclase pyroxenite shows,the Protolith restoration is interm-ediate volcanic rocks-andesite which is formd in oceanic island arc tectonic setting, and transformed into High pressure neutral granuliet during continent-continent coll-ision.