Zircon U–Pb and trace element data from rocks of the Huai’an Complex: New insights into the late Paleoproterozoic collision between the Eastern and Western Blocks of the North China Craton

Abstract

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 has not been precisely constrained yet. Six samples, including five high-grade metamorphic rocks and a pegmatite dike, from the Huai’an Complex in the northern segment of the TNCO were selected for zircon U–Pb dating and trace element analysis. A tonalite-trondhjemite-granodiorite (TTG) gneiss sample and a high-pressure (HP) mafic granulite sample contain magmatic zircon cores, yielding protolith formation ages of 2503 ± 17 and 1964 ± 38 Ma, respectively. Detrital zircons of magmatic origin in two metasedimentary rocks show concordant or moderately discordant U–Pb ages of 2683 ± 22 to 1964 ± 13 Ma, indicating that they were mainly derived from the TNCO with some possibly derived from the Archean continental rocks of the North China Craton. Metamorphic zircons in the TTG gneiss and two metasedimentary rocks exhibit low HREE contents, flat HREE patterns and negative Eu anomalies, implying their formation coeval with garnet and feldspar under grannulite-facies conditions. Metamorphic zircons in an HP mafic granulite and a sample of leucosome from a TTG gneiss also have depleted HREE contents, flat HREE patterns, but only weak negative Eu anomalies, which may also indicate formation under granulite-facies conditions, whereas the weak Eu anomalies may be due to the whole-rock composition. All these metamorphic zircons have formation temperatures of 702–749 °C, approaching the peak metamorphic temperatures of HP granulite rocks in this area. They have identical ages of ca. 1845 Ma within analytical errors, which is the best 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 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 retrograde metamorphism. 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. These may indicate either a protracted event or at least three ‘peaks’ of metamorphism. A pegmatite dike 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 in the Huai’an area.