Zircon and monazite U-Pb dating of the Doi inthanon core complex, northern Thailand: Implications for extension within the Indosinian Orogen

The Doi Inthanon metamorphic core complex occupies a prominent central position within the northwestern Thai gneiss belt which has been inferred to be as old as Precambrian and as young as Permo-Triassic. U-Pb dating of zircon from core orthogneisses of the complex suggests that the orthogneisses were derived from granitic protoliths with essentially the same Late Triassic-Early Jurassic age as the Mae Cham pluton (205 + 3/- 4 Ma) to the west of the Doi Inthanon complex. Monazite ages from the orthogneisses indicate that low-pressure and high-temperature metamorphism and migmatitization occurred in the Late Cretaceous (84-72 Ma); limited published radiometric age data from elsewhere in the migmatitic gneiss belt suggest that widespread, minor plutonism accompanied this thermal event, A younger U-Pb date of 26.8 +/- 0.5 Ma for zircon and monazite from the Mae Klang granite which intruded mylonitic gneisses on the eastern side of the Doi Inthanon complex indicates that there was also significant plutonism in the late Oligocene. These U-Pb ages constrain the development of the complex, and hence of the migmatitic gneiss belt as a whole, to an interval between Late Cretaceous and Early Miocene. Ductile shearing, uplift, and tectonic denudation via detachment faulting chronologically overlap the initiation of extensional basin development to the east and southeast of the gneiss belt and imply a genetic connection. Thus Oligocene-Early Miocene thermal rifting from the Gulf of Thailand northward into mainland Thailand may have been the direct product of crustal extension rather than the transtensional product of regional strike-slip faulting.