Zircon U-Pb SHRIMP isotopic dating of granitic garnet-biotite gneiss of the Aojiao Group at Aojiao Village of Dongshan County, Fujian Province, and its geological significance

Abstract：In this paper the authors conducted zircon U-Pb SHRIMP isotopic dating of the granitic garnet-biotite gneiss of the Aojiao Group at Aojiao Village of Dongshan County in Fujian Province. The Th/U ratios in the range of 0.31-1.01 (0.48 on average) show that the zircon is a typical magmatic mineral. The weighted mean 206Pb/238U age is 152.8±1.6 Ma（n=17,MSWD=1.12）, which is considered to be the formation age of the gneiss. Some conclusions have been reached：（1）The granitic garnet-biotite gneiss of the Aojiao Group was formed in late Jurassic rather than in pre-Devonian；（2）the Qinyingshan Formation of the Aojiao Group should be divided into metamorphic intrusions and metamorphic sedimentary strata.