The Petrological Characteristics And The Protolith Of High-pressure Granulite Of Chicheng And Xuanhua, North Hebei Province

High-pressure granulite as an important part orogenic belts crust, it clearly records the evolution of the lower crust and the mantle boundary environment. This paper chooses the high-pressure granulites of the north of Hebei as research subjects,which is located in Chicheng Womakeng County and Xuanhua Dadonggou area. Through studying the characters of petrography, mineralogy, geochemistry and geochronology of high-pressure granulite, we determine the structural environment of high pressure granulites and its original rock, so it provides new evidence for the study of North China craton high pressure granulite.The high-pressure granulite of the Chicheng County of the north of Hebei are mainly in the form of lenticle and block mass with outputting in surrounding rocks in the wild, and could see the tectonic movement forming the visible "strip" and " S " deformation. Country rocks include garnet biotite amphibolite gneiss and garnet amphibolite gneiss.Petrographic shows Chicheng high pressure granulite metamorphism has gone through three stages of metamorphic evolution: the first stage is the formative stages of the high pressure granulite(metamorphism stage), and mineral assemblage is Cpx+Pl+Q; the second stage is the peak phase of high-pressure granulite facies,and mineral assemblage is Grt+Cpx+Q+Pl; the third stage is the representative of retrograde metamorphism of amphibolite facies stage, mineral assemblage Amp+Pl±Grt. Among them, the garnet is aluminum-based garnet, and mainly constitutes by almandine(Alm), pyrope(Prp), calcium aluminum garnet(Grs) and small amounts of manganese aluminum garnet(Sps). Since garnet particle core portion to the edge portion, Ca O content is between 6.326% ~ 8.106%, with an average of 6.959%, and Mg O content is between 3.063% ~ 6.409%, with an average of 4.60%. As the content of pyrope(Pyr) increases, and almandine(Alm) component content slightly increased first and then clearly decreased, mainly performance is reduced. Plagioclase in rocks includes three types: "red circle" of plagioclase, symbiosis with monoclinic pyroxene plagioclase and symbiosis with the substrate of plagioclase, this three kinds of plagioclase is neutral feldspar(An = 28.5 ~ 38.8). The rock monoclinic pyroxene is diopside. Using Grt-Cpx thermometer and Grt-Cpx-Q-Pl pressure gauge calculated thepressure of high pressure granulite facies is 1.037 Ga ~ 1.196 Ga, and the temperature is 619.7 ?- 658.6 ?. Trace element Sr and Ba in the sample average is higher than the earth's crust, the rest of the Rb and Cs, Ta, Nb, Hf, Zr and Th/U ratios are below the average of the earth's crust. Distribution patterns of rare earth elements is divided into two categories: one is slightly tilted right type and flat type, the overall characteristics of light rare earth elements concentration and heavy rare earth elements flat. High pressure granulite of X basic value is plus(3.87 ~ 6.56),which shows that the original rock is igneous rock. Its metamorphic protolith restoration diagram and tectonic environment discrimination diagram show that the original rock is the tholeiite series and forming environment is island arc environment, the characters of chronology shows a magmathermal event happened in 1904 ma.The high-pressure granulite of the Xuanhua of the north of Hebei are mainly in the form of lenticle and block mass with outputting in surrounding rocks in the wild. Due to the effect of tectonic movement, lenticle "X" cleavage and quartz vein "Z" shape. Surrounding rock is biotite plagioclase gneiss. Petrography shows that high pressure granulite Xuanhua mainly experienced five period of metamorphic evolution stages: the first stage is the formation of high pressure granulite phase(phase prograde), and mineral assemblage is Cpx+Pl+Q; the second stage is the peak stagehigh pressure granulite facies stage, mineral combination Grt+Cpx+Q+Pl; the third stage is retrograde metamorphism in the early stages of pressure granulite facies, and mineral assemblage is Grt+Opx+Cpx+Pl; the fourth stage is the low-pressure granulite facies,and mineral assemblages is Cpx+Opx+Pl; the fifth stage is amphibolite facies stage,and mineral assemblage is Amp+Pl±Grt. Among them, the garnet is aluminum-based garnet, since the particle core portion to the edge position, Ca O content is between 5.503% ~ 8.224%, with an average of 6.754%, Mg O content is between 4.542%- 6.69% with an average of 5.737%.As the pyrope(Pyr) increases, almandine(Alm) component content increased first, and then decreased. Plagioclase in rocks include three types: plagioclase in "red circle", symbiosis with monoclinic pyroxene plagioclase and symbiosis with the substrate of plagioclase, plagioclase in the "red eye" which includes andesine, labradorite, celsian and the other two forms of labradorite. The rock monoclinic pyroxene is diopside. Using Grt-Cpx thermometer and Grt-Cpx-Q-Pl pressure gauge calculated thepressure of high pressure granulite facies is 0.901 Ga ~ 1.173 Ga, and the temperature is 662.6 ?- 801.7 ?. Trace element Sr and Ba in the sample average is higher than the earth's crust, the Rb and Cs, Ta, Nb, Hf, Zr and Th/U ratios are below the average of the earth's crust. Distribution patterns of rare earth elements is divided into two categories: one is slightly tilted right type, the overall characteristics of light rare earth elements concentration and heavy rare earthelements flat.High pressure granulite of X basic value is plus(4.43 ~ 6.57),which shows that the original rock is igneous rock. Its metamorphic protolith restoration diagram and tectonic environment discrimination diagram show that the original rock is the tholeiite series and forming environment is island arc environment.From what has been discussed above, Chicheng and Xuanhua high pressure granulite field occurrence is lenticle and block mass produced in the surrounding rock. But because of the difference of the contact of the surrounding rock and experienced tectonic movement, it resulted in the difference of the mineral composition, retrograde metamorphism mineral and metamorphic period time. Retrograde metamorphism Chicheng area is uplift faster than Xuanhua area. In terms of mineral composition, the content of Ca O of garnet of Chicheng is slightlyabove the Xuanhua district and the content of Ca O of garnet of Chicheng is below the Xuanhua district, respectively, the types of plagioclase in the "red eye" is simpler than Xuanhua region. The rock monoclinic pyroxene is diopside. The temperature of Chicheng high pressure granulite was lower than Xuanhua region, and the pressure was higher than Xuanhua region.Two areas of trace element Sr and Ba average is higher than the earth's crust, and the rest of the Rb, Cs, Ta, Nb, Hf, Zr and Th/U ratios are average below the earth's crust, and xuanhua region is higher than the Chicheng(except U and Th). Rare earth elememts distribution patterns in two regions are maintain tilted right type,and the characteristics of light rare earth elements concentration and heavy rare earthelements flat., but Chicheng region has a type of flat, light and heavy rare earth differential degree is not clear than Xuanhua region. Back two regions of the original rock and tectonic environment, two areas of high pressure granulite protolith is tholeiite series and forming environment is island arc environment, but a magmathermal event happened in 1904 ma in Chicheng region.