Geochronological constraints on the timing of migmatization in the Dabie Shan, East-central China

Abstract

Combined isotopic dating techniques reveal a complicated tectonothermal history for the migmatites at Fenghuangguan and Zongluzui in the Northern Dabie Unit (NDU). The zircon U-Pb data indicate that the rocks were migmatized about 131.7 ± 1.1 Ma ago. Two whole-rocks (leucosome and melanosome) and three mineral separates yield an internal Rb-Sr isochron age of 115 ± 2 Ma. The 40Ar/39Ar step-heating analyses for four leucosomes give isochron age of 114 ± 1 Ma and 111 ± 1 Ma for hornblende, 107 ± 0.5 Ma for biotite and 95 ± 2 Ma for K-feldspar. These data indicate that the NDU was uplifted after the migmatization at a cooling rate of 10-12°C/Ma. Biotite 40Ar/39Ar step-heating analyses for a felsic granulite at Huangtuling give an isochron age of 194 ± 2 Ma and a plateau age of 195 ± 2 Ma, suggesting that the granulite facies metamorphism predated the regional migmatization. Decoupling between the Rb-Sr age for biotite and 40Ar/39Ar ages for hornblende is interpreted as recrystallization induced by a fluid action. The migmatization is considered as one result of the extensive thermal event in east China, also resulting in the Yanshanian magmatic activities. The previously published isotopic ages of about 130 Ma for the orthogneisses in the NDU may simply record this thermal event. The Nd and Sr isotopic data indicate the existence of pre-Cretaceous, or even Precambrian rocks in the NDU.