U-Pb dating of high temperature metamorphic episodes in the Kon Turn Massif (Vietnam)

摘要

In Vietnam, recent studies demonstrated that the Kon Turn Massif was part of the Indosinian mountain system, together with the Truong Son Belt. One sector of the Kon Turn Massif (Song Ba valley) is a granulite facies terrain for which Ar-Ar and U-Pb ages cluster around 245-250 Ma. In the area of Song Bien river, also metamorphosed under granulite facies conditions, Ar-Ar ages of 410 Ma are displayed by biotite. In this study, U-Pb dating of monazite and zircon has been used to understand the reason for these two groups of ages, taking into account the metamorphic mineral assemblages related to granulite facies metamorphism in the two districts. For gran-ulites of Song Ba valley, ages of 245-250 Ma are confirmed from monazites. In the Song Bien district, ages of 465-470 Ma displayed by monazites are interpreted to correspond to the metamorphic climax. Zircons from one of these samples record the age of anatexis that has affected the granulitic rocks. The younger age recorded in the Song Ba valley is considered to reflect high temperature related to charnockitic intrusions emplaced near the end of the Indosinian orogeny, overprinting the earlier formed granulites and resetting the U-Pb and Ar-Ar ages. For the first time, two HT events are dated in the Kon Turn Massif, one occurring during the Ordovician and the other during the Permo-Triassic.