Paleoproterozoic metamorphic P-T-t path and tectonic significance of the Luoning metamorphic complex at the southern terminal of the Trans-North China Orogen, Henan Province
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The NW-SE-striking Taihua metamorphic complex crop out discretely along the southern margin of the Trans-North China Orogen (TNCO), among which the Luoning metamorphic terrane is mainly composed of TTG-like gneisses, amphibolitic gneisses and metapelitic gneisses. Garnets in amphibolitic gneisses are commonly embayed and surrounded by amphibole+plagioclase+quartz symplectites. Three generations of mineral assemblages have been recognized in the amphibolites: the prograde assemblage (M1) is represented by plagioclase+amphibole±biotite±epidote+quartz enclosed in garnet porphyroblasts, the peak metamorphic assemblage (M2) is dominantly composed of garnet porphyroblasts and the matrix minerals plagioclase+amphibole+biotite+quartz, and the retrograde metamorphic assemblage (M3) is represented by the symplectitic intergrowth of amphibole+plagioclase+quartz rimming garnet porphyroblasts. Thermodynamic computation shows that the metamorphic conditions of the plagioclase-amphibole gneisses are 600~680℃ at 7.0~7.6kbar for the M1 stage, 680~790℃ at 9.5~10.7kbar for the metamorphic peak M2 stage and 580~720℃ at 6.5~7.6kbar for the retrograde M3 stage, respectively. Two stages of metamorphic assemblages are preserved in the metapelitic gneisses, the prograde metamorphic assemblages (M1) and the peak metamorphic assemblages (M2). Calculated metamorphic P-T conditions of these two stages are 620~710℃ at 4.9~5.6kbar and 710~760℃ at 7.3~8.3kbar, respectively. The Taihua metamorphic complex is deciphered to have undergone metamorphism characterized by clockwise P-T paths including the nearly isothermal decompression (ITD) segments, which are inferred to be related to the final amalgamation of North China Craton (NCC) along the TNCO. SIMS and ICP-MS U-Pb dating of metamorphic zircons extracted from amphibolitic gneisses yielded metamorphic ages ranging from 1938Ma to 1967Ma, ca. 100Ma earlier than the ubiquitous ~1850Ma metamorphic event recorded in most of the metamorphic terrains in the TNCO. The amalgamation of NCC along TNCO was a long and somewhat different and complex process.Keywords:
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The Trans-North China Belt (TNCB) is a Paleoproterozoic collisional orogen (ca.1.9-1.8Ga) responsible for the amalgamation of the North China Craton.Detail field works in Lüliangshan, Hengshan, Wutaishan and Fuping massifs where the belt is well exposed, allow us to draw new tectonic map and crustal-scale cross sections.The available petrologic, radiometric, geochronologic data are integrated in a geodynamic evolution scheme for this orogen.The Low Grade Mafic Unit (LGMU) is interpreted as an ophiolitic nappe rooted in a suture zone located in the western part of the Lüliangshan.This ophiolitic nappe overthrusts to the SE upon the Orthogneiss-Volcanites Unit (OVU) that consists of a bimodal volcanic-sedimentary series metamorphosed under amphibolite facies conditions intruded by calcalkaline orthogneiss.The OVU is a composite Neoarchean-Paleoproterozoic magmatic arc developed during two stages (ca.2500 and 2100 Ma) upon a continental basement corresponding to the western extension of the Neoarchean Fuping massif.The OVU overthrusts to the SE the Fuping massif along the Longquanguan shear zone.This stack of nappe, coeval with an amphibolite facies metamorphism, is dated at ca 1880 Ma.Subsequently, the metamorphic series experienced a widespread migmatization at 1850 Ma and was intruded by post-orogenic plutons dated at 1800 Ma.The weakly to unmetamorphosed Hutuo Supergroup unconformably overlies the metamorphosed and ductilely deformed units (OVU and LGMU), but it is also involved in a second tectonic phase developed in subsurface conditions.These structural features lead us to question the ca 2090 Ma age attributed to the Hutuo supergroup.Moreover, in the Fuping massif, several structural and magmatic lines of evidence argue for an earlier orogenic event at ca 2100 Ma that we relate to an older west-directed subduction below the Fuping Block.The Taihangshan Fault might be the location of a possible suture zone between the Fuping Block and an eastern one.A geodynamic model, at variance with previous ones, is proposed to account for the formation of the TNCB.In this scheme, three Archean continents, namely from West to East, the Ordos, Fuping and Eastern Blocks are separated by the Lüliang and Taihang Oceans.The closure of the Taihang Ocean at ca 2100 Ma by westward subduction below the Fuping Block accounts for the arc magmatism and the 2100 Ma orogeny.The second collision at 1900-1880 Ma between the Fuping and Ordos blocks is responsible for the main structural, metamorphic and magmatic features of the Trans-North China Belt.
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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 + quartz 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 in 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).
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