THE CHARACTERISTICS AND AGE OF THE ZHOUJIAWAN ROCK MASS IN THE DABIE MOUNTAINS AREA,HUBEI PROVINCE

The Zhoujiawan gneissose granite mass in the Dabie Mountains area, Hubei province, was in the past wrongly determined to be an early Yanshanian intrusion. This paper presents the authors’ recent preliminary research results in respects of the occurrence characteristics, petrochemical composition and isotopic age of this rock mass, and points out that the Zhoujiawan rock mass is actually a Proterozoic intrusion. It has the following characteristics: the rocks belong to the cale-alkaline series, the gneissose structure is well developed, part of plagioclase exhibits a distinct phenomenon of potassium metasomatism, the mafic content and REE abundance are slightly high, and the accessory mineral zircon is almost of a single colour-light purplish-red. Nine zircon samples collected from the rock mass yield an average U-Pb apparent age of 1973m. y. and a Pb-Pb isochron age of 1973±89 m.y.; the upper intercept age on the Witherill concordia plot is 2032±173 m.y. The rock mass cuts the metamorphic rock series of the Hongan Group （early Proterozoic） but is in turn unconformably overlain by the middle-upper Proterozoic dolomite marble, so the time of its intrusion should be equivalent to the Wutai Stage. The Zhoujiawan rock mass may be regarded as the representive of the Precambrian granitoids in the Dabie Mountains area, and its characteristics are obviously different from those of the granitoid intrusions formed since the Paleozoic.