Study Of The P-T Path Chronology Of High-pressure Metamorphic Rocks From Chengde And Xuanhua,North Hebei Province

The North China Craton is one of the oldest cratons in the world.The formation and evolution of NCC can be traced back to 3800 million years ago.It recorded many tectonic events on the earth.In the past two decades,a lot of research on petrology,geochronology,geochemistry,and structural geology in the North China Craton has been done by geologists from different countries to resolve the above disputes.Then some tectonic evolution models have been put forward.This paper concerns chiefly with high-pressure metamorphic rocks in the northern margin of the North China Craton.Based on the detailed petrographic observation,electron probe testing and zircon U-Pb chronology,the metamorphic stages of high-pressure granulite and plagioclase amphibolite in different regions are divided,the temperature and pressure are calculated and the P-T path is restored.The experimental results show that the basic granulite get in Xiwang mountain is mainly composed of garnet,plagioclase,clinopyroxene,orthopyroxene,magnetite,ilmenite and amphibolite.It underwent four metamorphic stages:pre-peak metamorphism granulites facies mineral assemblage consists of garnet+plagioclase+quartz+clinopyroxene;high-pressure granulite facies mineral assemblage comprises garnet,clinopyroxene and plagioclase;medium-pressure granulite facies mineral assemblage contains plagioclase,orthopyroxene,magnetite and ilmenite;retrogressive metamorphic amphibolite facies mineral assemblage is characterized by plagioclase and hornblende.Geological thermobarometries showed that the temperature-pressure conditions of different metamorphic stages are calculated roughly to be 627?669?,8.2?8.7 kbar;728?878?,10?15 kbar;771?849?,7?9 kbar;632?637?,4.2?4.5 kbar,respectively,indicating a clockwise P-T evolutional path of isothermal decompression.Combined with information available,we propose that the high-pressure granulites are formed from continental collison orogenics and undergo a rapid crustal uplift during the pressure decrease history.The basic granulite in the Luanping area,Sanggan structure zone,mainly consists of six minerals:garnet,hornblende,plagioclase,clinopyroxene,orthopyroxene,and quartz.Petrography characteristics reflect that it underwent three metamorphic stages:the peak high-pressure granulite-facies assemblage(M₁)is defined by garnet+clinopyroxene+quartz;the medium-pressure granulite-facies metamorphism(M₂)is composed of plagioclase+orthopyroxene;the amphibolite facies stage(M₃)is characterized by plagioclase and hornblende.Geothermobarometers dates suggest that the temperature and pressure conditions of different metamorphic stages are estimated roughly to be 636-719?,7.9-11 kbar;775-861?,5.6-7.1 kbar,649-653?,4.2-4.3 kbar,respectively.Zircon U-Pb geochronology suggests that the age of magmatic emplacement is about 2500 Ma,and the metamorphic age of high-pressure granulite facies is about 1.85 Ga.Combined with petrography characteristics,a clockwise P-T path characterized by decompression with a temperature decrease can be reconstructed.In addition,we propose that the two-pyroxene granulite form this region formed during collisional orogeny.The amphibolite from south Pingquanqigou is mainly composed of garnet,plagioclase,clinopyroxene and hornblende.The only high-pressure granulite-facies assemblage remained in this area.The traditional thermobarometer showed that the temperature-pressure conditions of this facie is 636-719?,7.9-11 kbar.It has been demonstrated that chemical composition from the core to mantle has no significant change,Ca element decrease and Mg,Fe increase from the mantle to edge on EPMA elemental mapping,corresponding to the early stage of exhumation.Zircon U-Pb analysis suggests that the metamorphic age of high-pressure granulite-facies is about 1.85 Ga.