Metamorphic grade of Paro Formation, western Bhutan and its implications

In western Bhutan, a package of rocks known as the Paro Formation (PF) consists of garnetiferous mica schist, quartzite, marble, calc-silicate, and slivers of orthogneiss. It is ca. 5.5 km thick and is exposed as a tectonic window. The base of PF is a staurolite-kyanite bearing schist that transitions into muscovite-biotite-garnet bearing quartzite upsection. The quartzite is fine-grained in the lower section and coarser at the upper section and contains three marble bands and slivers of orthogneiss and calc-silicates. The PF contains fibrolite that is localized near 460 Ma granite intrusions. The alignment of micas and grain shape preferred orientation of quartz crystals are observed in the basal schist and these fabrics become more pervasive upsection, indicating these rocks have been ductiley deformed with the intensity of deformation increasing toward the upper contact with Greater Himalayan (GH) rocks. The GH rocks are structurally above, and completely surround the PF. Our mapping of the PF- GH contact indicates it is a circular window and that the western, southern, and eastern contacts are the same folded thrust. While the GH rocks at the western, southern, and eastern contacts have muscovite-biotite-garnet-kyanite-fibrolite together with partial melt, the GH rocks at the northern contact have attained second sillimanite isograd. Mineral assemblage indicate that Paro rocks in general have attained garnet to kyanite zone metamorphism with sillimanite zone metamorphism locally. This grade of metamorphism is higher than what is observed in the eastern Bhutan from the same structural level i.e. immediately below the MCT where rocks have attained garnet zone metamorphism. Our mapping indicates that the grade of metamorphism within GH rocks, as well as the thickness of the metamorphic zone below the MCT, vary significantly from east to west. Based on metamorphic grade and ductile fabrics, Paro rocks have been previously considered to be GH rocks. However, the detrital zircon spectra of different Paro Formation samples vary widely from sample to sample and collectively contain DZ peaks at 0.5, 0.8, 1.2, 1.4, 1.7, 1.8 and 2.5 Ga zircons. A strong detrital zircon signal at ~800 Ma in several samples requires the PF must be younger than Neoproterozoic in age. However, the presence of 500 Ma detrital zircon grains in one sample suggests that PF is Cambrian or younger. 460 Ma and 462 Ma granite intrusions bracket the youngest deposition age of PF as Ordovician.