Timing of Syenite‐Charnockite Magmatism and Ruby and Sapphire Metamorphism in the Mogok Valley Region, Myanmar

Abstract The Mogok metamorphic belt (MMB) extends for over 1,000 km along central Burma from the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle crustal metamorphic rocks of the Sibumasu plate. In the Mogok valley region, the MMB consists of regional high‐grade marbles containing calcite + phlogopite + spinel + apatite ± diopside ± olivine and hosts world class ruby and sapphire gemstones. The coarse‐grained marbles have been intruded by orthopyroxene‐ and clinopyroxene‐bearing charnockite‐syenite sheet‐like intrusions that have skarns around the margins. Syenites range from hornblende‐ to quartz‐bearing and frequently show layering that could be a primary igneous texture or a later metamorphic overprint. Calc‐silicate skarns contain both rubies and blue sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, occasional diopside, and blue apatite. Both marbles and syenites have been intruded by the Miocene Kabaing garnet‐muscovite‐biotite peraluminous leucogranite. New mapping and structural observations combined with U‐Th‐Pb zircon, monazite, and titanite geochronology from syenites, charnockites, leucogranites, meta‐rhyolite‐tuffs, and skarns have revealed a complex multiphase igneous and metamorphic history for the MMB. U‐Pb zircon ages of the charnockite‐syenites fall into three categories, Jurassic (170–168 Ma), latest Cretaceous to early Paleocene (~68‐63 Ma), and late Eocene–Oligocene (44–21 Ma). New ages from five samples suggest that metamorphism in the presence of garnet and melt occurred between ~45 and 24 Ma. U‐Pb titanite ages from the ruby marbles and meta‐skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma). U‐Th‐Pb dating shows that all the metamorphic ages are Late Cretaceous–early Miocene and related to the India‐Sibumasu collision.