Zircon U–Pb ages in Myanmar: Magmatic–metamorphic events and the closure of a neo-Tethys ocean?

Abstract

Previous isotopic age studies have concluded that the main metamorphism in the Mogok Metamorphic belt (MMB) in Myanmar was Tertiary and caused by collision with or underthrusting by India. We present here results of zircon U–Pb age determinations on 18 samples from Myanmar, largely on intrusive rocks from the north-trending 550km long Shan Scarps segment of the sickle-shaped MMB. The oldest determination is 491Ma on orthogneiss north of Mandalay, interpreted as a protolith age and similar to the stratigraphic age of the Bawdwin Volcanics. Augen gneiss at Kyaukse has a 114Ma protolith age. Our zircon U–Pb ages on undeformed intrusive rocks in the MMB range from Cretaceous to Miocene; the oldest (128Ma), on a diorite near Yebokson, implies an earliest Cretaceous minimum age for regional metamorphism here. Younger ages on undeformed intrusions include 91Ma on diorite at Mokpalin, 72Ma on the weakly foliated garnet-bearing Nattaung granite, 44 and 48Ma respectively on the MEC and Sedo granites, and 17–20Ma on granite dykes. For the 128Ma diorite, Sr and Nd isotopes indicative of a continental crust component, and mapped continuity of metamorphic host rocks, suggest correlation with mid early Cretaceous plutons in the eastern Transhimalayas. Zircon U–Pb ages for the MEC, Sedo and other granites within the MMB and Slate belt help to define a late Cretaceous to early Eocene age range for mostly ilmenite series peraluminous granites within the Western Granite belt and more restricted Western tin belt of Myanmar and southern Thailand. In the Wuntho–Popa arc west of the dextral Sagaing Fault, our new U–Pb zircon age of 105Ma on diorite at Salingyi supports previous K–Ar ages (106–93Ma) for pre-Tertiary intrusions. In the Cretaceous this arc may have occupied the Andaman Sea and continued northwards through the Mokpalin diorites and west of the MMB to the 103–80Ma I-type plutons in the Gandise batholith of southern Tibet.We interpret the MMB and Slate belt as part of a Western Myanmar block separated in the Jurassic from the Shan Plateau to the east by the southwestward continuation of the Bangong–Nujiang–Luxi neo-Tethys I ocean. We relate most intrusive and metamorphic events in and near the MMB to westward subduction of this ocean and end-Jurassic collision of the Plateau with the overriding Western Myanmar-Slate belt block; to orogenic polarity reversal and early Cretaceous westward translation of the Plateau sequence over the suture zone; and to eastward subduction of the ancestral Indian Ocean or neo-Tethys II beneath Myanmar with generation of the Wuntho–Popa arc beginning before the late Cretaceous. The MMB includes meta-Plateau rocks near Mandalay but meta-Slate belt rocks to the north and south. Within the MMB, previous monazite–xenotime–thorite ages of Eocene–Oligocene, and Oligocene-early Miocene cooling ages on micas, with no deformation in Eocene granites, suggest a high-temperature thermal event related to intrusion of these and other granites during a previously proposed regional extension. Our data imply that the main fabric-forming metamorphic event in the MMB pre-dated the India–Asia collision.