Late Eocene-Oligocene granulite facies garnet-sillimanite migmatites from the Mogok Metamorphic belt, Myanmar, and implications for timing of slip along the Sagaing Fault

The Mogok Metamorphic Belt (MMB) in Myanmar is a polymetamorphic, mainly Paleogene granulite-uppermost amphibolite facies terrane consisting mainly of marbles and calc-silicates hosting spinel, ruby and sapphire. Jurassic charnockite-syenite intrusions, as well as Eocene-Miocene leucogranite intrusions are also present. Pelitic rocks are uncommon, and where present, have sillimanite, both as primary inclusions in garnet and as secondary Bt + Sil coronas around garnet. Core samples from the Kyi-Tauk-Pauk gold mine at Thabeikkiyin, north of Mandalay, are mostly Grt + Bt + Sill gneisses and migmatites with uncommon interbanded Opx + Grt + Bt gneisses. Pseudosection modelling suggests prograde garnet growth occurred by biotite-dehydration melting that reached peak P-T conditions of 870-970 degrees C and 0.9 GPa, and was followed by garnet breakdown forming coarse retrograde Bt + Sil coronas. U-Pb monazite data show an early high-grade granulite event at 43-32 Ma, and a later upper amphibolite sillimanite-grade event peaking at 23-20 Ma, with a change in monazite chemistry after c. 22 Ma that is consistent with fluid/(melt) interaction and garnet breakdown. Elevated Th/U ratios from-35 to 22 Ma, and at-18 Ma are compatible with melt influx at that time, timing that is similar to the age of the regional Kabaing leucogranite in the Mogok valley area. Our data show that peak granulite facies metamorphism along the Mogok Metamorphic belt was mainly Middle Eocene-Early Oligocene, with upper amphibolite facies metamorphism extending to earliest Miocene. The MMB is cut abruptly by the Sagaing fault, a largescale dextral fault that extends from the Andaman Sea north to the East Himalayan syntaxis. Our new U-Pb monazite data constrain the oldest age of initiation of the eastern branch of the cross-cutting Sagaing dextral strike slip fault at <20-18 Ma.

Automated summary

The Mogok Metamorphic Belt in Myanmar is a polymetamorphic terrane with mainly marbles and calc-silicates hosting spinel, ruby, and sapphire. The rocks show evidence of multiple metamorphic events, with core samples from the Kyi-Tauk-Pauk gold mine consisting mainly of garnet, biotite, and sillimanite gneisses. Monazite data suggest a high-grade granulite event followed by an upper amphibolite sillimanite-grade event, with a change in monazite chemistry indicating fluid/melt interaction and garnet breakdown.