Correlation of late Mesozoic mafic and ophiolitic rocks suggests that in the late Lower Cretaceous a northeasterly-facing mafic arc was emplaced onto southeast Borneo, western Sumatra, western Myanmar (Burma) then 1100 km south of its present position, the Mogok belt of Myanmar and the Denqen–Bangong Co ophiolite zone in Tibet, all then situated on the southwestern margin of Asia. Reversal in tectonic polarity and eastward subduction generated a late Cretaceous magmatic arc in western Myanmar and Sumatra, and its continuation through the Mogok belt into the Gandise batholith of Tibet. Magmatism, interrupted in the latest Cretaceous, probably resumed in the western Myanmar–Tibet and Sumatra arc segments in the early Palaeogene. It was again interrupted in the Mid-Eocene when an east-vergent ophiolite nappe overrode the Indo-Burman Ranges, deforming Lower Eocene turbidites derived from the magmatic arc to the east. Following renewed northward and eastward subduction of ocean floor in the latest Eocene, India collided with northern Myanmar, and with Tibet along a suture south of the Jurassic–Triassic flysch and adjacent Yarlung ophiolite. In the Mogok segment of the late Cretaceous arc, crust thickened in the early Cretaceous orogeny underwent extension to form a metamorphic core complex as Myanmar, west of the Sagaing Fault moved northward. More than half of this northward movement preceded the well-documented 450 km of post-early Miocene dextral displacement on the Sagaing Fault and related spreading in the Andaman Sea. Generation of late Cretaceous to early Eocene I- and S-type granites in and east of the Mogok belt was related either to crustal shortening, or more probably, to subsequent extension in the Mogok belt.

[1] The Mogok metamorphic belt (MMB) extends for over 1500 km along the western margin of the ShanThai block, from the Andaman Sea north to the eastern Himalayan syntaxis. Previous geochronology has suggested that a long-lasting Jurassic-early Cretaceous subduction-related event resulted in emplacement of granodiorites and orthogneisses (171-120 Ma) and a poorly constrained Tertiary metamorphic event. On the basis of new U-Pb isotope dilution thermal ionization mass spectrometry and U-Th-Pb laser ablationmulticollector-inductively coupled plasma mass spectrometer geochronology presented here, we propose two Tertiary metamorphic events affected the MMB in Burma. The first was a Paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at $59 Ma. A second metamorphic event spanned late Eocene to Oligocene (at least from 37, possibly 47, to 29 Ma). This resulted in the growth of metamorphic monazite at sillimanite grade, growth of zircon rims at 47-43 Ma, sillimanite + muscovite replacing older andalusite, and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45.5 ± 0.6 Ma and 24.5 ± 0.7 Ma. These data imply high-temperature sillimanite + muscovite metamorphism peaking at 680°C and 4.9 kbar between 45 and 33 Ma, to around 606-656°C and 4.4-4.8 kbar at 29.3 ± 0.5 Ma. The later metamorphic event is older than 24.5 ± 0.3 Ma, the age of leucogranites that crosscut all earlier fabrics. Our structural and geochronological data suggest that the MMB links north to the unexposed middle or lower crust rocks of the Lhasa terrane, south Tibet, and east to high-grade metamorphic core complexes in northwest Thailand.

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Recently available geological data on Tibet, Yunnan and Burma facilitate recognition of Phanerozoic subduction systems and two continental fragments of Gondwanaland which extend through mainland SE Asia. A Cambrian subduction system and 5 Mesozoic-early Caenozoic collision belts are identified in the region. Indochina, eastern Thailand and Central Tibet accreted to China in the early Triassic; western SE Asia and S Tibet separated from Gondwanaland in the Permian or early Triassic and collided with Asia in the late Triassic, and the western Burma island arc system collided with Asia in the Jurassic. Upper Triassic flysch and schist in the eastern Indoburman Ranges were accreted to western Burma in the Jurassic-early Cretaceous during eastward subduction and formation of a back-arc thrust belt. Following collision of Greater India with arc complexes to the N in the Cretaceous, and with Tibet in the early Eocene, western Burma moved northwards about 450 km along the Sagaing-Namyin ridge-thrust transform fault. Formation of the 3 major belts of tin granites and the porphyry copper deposits in the region took place in specific tectonic settings within the subduction systems and collision belts.