Late Paleoproterozoic Collision Between The Eastern And Western Blocks Of The North China Craton

The Trans-North China Orogen (TNCO) is interpreted as a continent-continent collisional belt along which the discrete Archean Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The timing of its formation is still inconclusive. Six samples, including five high-grade metamorphic rocks and a pegmatite dyke, from the Huai'an complex in the northern segment of the TNCO were selected for zircon U-Pb dating and trace element analysis. A TTG gneiss sample and a HP mafic granulite sample contain magmatic zircon cores, yielding protolith formation ages of 2503±17 and 1964±38 Ma, respectively. Detrital zircon of a magmatic origin in two metasedimentary rocks show concordant or moderately discordant U-Pb ages of 1964±13 to 2683±22 Ma, indicating that they were mainly derived from the TNCO with few from the Archean basement of the North China Craton. Metamorphic zircons in the TTG gneiss and two metasedimentary rocks exhibit low HREE contents, flat HREE pattern and a distinct Eu anomaly, implying their formation coeval with garnet and feldspar, and thus under grannulite-facies conditions. Metamorphic zircons in a HP mafic granulite and a leucosome sample also have depleted HREE contents, flat HREE patterns, but a slight negative Eu anomaly, which may also Were formed under granulite-facies condition, whereas the insignificant Eu anomaly may be due to the whole-rock composition. All these metamorphic zircons have formation temperatures of 702 to 749℃, approaching the peak metamorphic temperatures of HP granulite rocks in this area. They have identical～1845 Ma (2σ) within analytical errors, which is the best age estimate age for HP granulite-facies metamorphism of the Huai'an complex. These results confirm previous models proposing late Paleoproterozoic collision between the Eastern and Western Blocks of the North China Craton along the TNCO. On the other hand, some metamorphic zircons in the leucosome sample show planar or polygonal zoning, high Th, U contents and high Th/U ratios, high REE contents and a steep HREE patterns, indicating that they crystallized during partial melting with abundant garnet decomposition. They yielded a weighted mean age of 1819±13 Ma, which we interpret to register the time of exhumation. Combined with previously published data, there are three age peaks at 1876±6, 1849±2, and, 1814±4 Ma for metamorphic zircons from high-grade metamorphic rocks in the TNCO, which may correspond to prograde, peak, and retrograde metamorphism, respectively. The age of 1849±2 Ma may be regarded as the best estimate for peak metamorphism in the TNCO. A pegmatite dyke cutting the foliation of a mafic granulite boudin was dated at 1806±15 Ma, which signifies termination of collision between the Eastern and Western Blocks.