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

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 similar to 680 degrees C at 7.0 similar to 7. 6kbar for the M-1 stage, 680 similar to 790 degrees C at 9.5 similar to 10. 7kbar for the metamorphic peak M-2 stage and 580 similar to 720 degrees C at 6. 5 similar to 7. 6kbar for the retrograde M-3 stage, respectively. Two stages of metamorphic assemblages are preserved in the metapelitic gneisses, the prograde metamorphic assemblages (M-1) and the peak metamorphic assemblages (M-2). Calculated metamorphic P-T conditions of these two stages are 620 similar to 710 degrees C at 4. 9 similar to 5. 6kbar and 710 similar to 760 degrees C at 7. 3 similar to 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 amplabolitic gneisses yielded metamorphic ages ranging from 1938Ma to 1967Ma, ca. 100Ma earlier than the ubiquitous similar to 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.