Thermal evolution of Archean basement rocks from the eastern part of the North China craton and its bearing on tectonic setting

Abstract

The basement rocks in the eastern zone of the North China craton are composed predominantly of pretectonic tonalitic-trondhjemitic-granodioritic gneisses and syntectonic granitoids, with rafts of supracrustal rocks consisting of ultramafic to felsic volcanic and sedimentary rocks, metamorphosed over a range of conditions from greenschist to granulite facies. Most mafic granulites, amphibolites, and some pelitic gneisses and schists preserve the prograde, peak, and post-peak textures. The prograde metamorphic stage is indicated by mineral inclusions within minerals of the peak stage, represented by the assemblages of hornblende + plagioclase + quartz ± biotite in mafic granulites, chlorite + actinolite + epidote + plagioclase + quartz in amphibolites, and biotite + plagioclase + quartz in pelitic gneisses. The peak stage is shown by assemblages of orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the mafic granulites, hornblende + plagioclase + quartz + garnet in garnetiferous amphibolites, and garnet + sillimanite + plagioclase + quartz + biotite in pelitic gneisses. The post-peak stage is represented by garnet + quartaz symplectic coronas in mafic granulites, actinolite + garnet retrogressive rims around garnet or hornblende grains in amphibolites, and kyanite replacing sillimanite or staurolite replacing sillimanite + garnet pelitic gneisses. These textural relations and their P-T estimates define counterclockwise P-T paths. The P-T-t(ime) paths of metamorphic rocks from the eastern zone reflect an origin related to the intrusion and underplating of large amounts of mantle-derived magmas that not only provided heat for the metamorphism but also added a large volume of mostly mafic material to the base of the crust. Crustal growth therefore was dominated by vertical addition of mantle-derived magmas to the crust. Combined with lithologic, structural, metamorphic, and geochronologic considerations, these large volumes of underplated magma are considered to be related to upwelling mantle plumes (hotspots).