THERMAL EVOLUTION OF ARCHEAN BASEMENT ROCKS FROM THE EASTERN PART OF THE NORTH CHINA CARTON AND ITS BEARING ON TECTONIC SETTING

The basement rocks in the eastern zone of the North China craton are c omposed predominantly of pretectonic tonalitic-trondhjemitic-granodior itic gneisses and syntectonic granitoids, with rafts of supracrustal r ocks consisting of ultramafic to felsic volcanic and sedimentary rocks , metamorphosed over a range of conditions from greenschist to granuli te facies. Most mafic granulites, amphibolites, anti some pelitic gnei sses anti schists presence the prograde, peak, and post-peak textures. The prograde metamorphic stage is indicated hy mineral inclusions wit hin minerals of the peak stage, represented by the assemblages of horn blende + plagioclase + quartz +/- biotite in mafic granulites, chlorit e + actinolite + epidote + plagioclase + quartz in amphibolites. and b iotite + plagioclase + quartz in pelitic gneisses. The peak stage is s hown hv assemblages of orthopyroxene + clinopyroxene + garnet + plagio clase + quartz in the mafic granulites. hornblende + plagioclase + qua rtz + garnet in garnetiferous amphibolites, and garnet + sillimanite plagioclase + quartz + biotite in pelitic gneisses. The post-peak sta ge is represented by garnet + quartz symplectic coronas in mafic granu lites, actinolite + garnet retrogressive rims around garnet or hornble nde grains in amphibolites, and kyanite replacing sillimanite or staur olite replacing sillimanite + garnet in pelitic gneisses. These textur al relations and their P-T estimates define counterclockwise P-T paths . The P-T-t(ime) paths of metamorphic rocks from the eastern zone refl ect an origin related to the intrusion and underplating of large amoun ts of mantle-derived magmas that not only provided heat for the metamo rphism but also added a large volume of mostly mafic material to the b ase of the crust. Crustal growth therefore mas dominated by vertical a ddition of mantle-derived magmas to the crust, Combined with lithologi c, structural. metamorphic, and geochronologic considerations, these l arge volumes of underplated magma are considered to be related to upwe lling mantle plumes (hotspots).