Multiple alternating forearc- and backarc-ward migration of magmatism in the Indo-Myanmar Orogenic Belt since the Jurassic: Documentation of the orogenic architecture of eastern Neotethys in SE Asia

Abstract

We integrated our new data with a review of published work on the Indo-Myanmar Orogenic Belt (IMOB) of the eastern Neotethys in Myanmar, western Yunnan (China) and Naga Hills (India), in order to better understand subduction-accretion processes of the Neotethys Ocean in SE Asia, and interactive relationships between the Gondwana and Laurasia supercontinents. From east to west, the IMOB is divided into three geographic units, the Shan-Thai Plateau, Myanmar Central Basin and Indo-Burma Range; the Mogok Metamorphic and Slate Belt is within the Shan-Thai Plateau.The Shan-Thai Plateau and eastern Myanmar Central Basin contain Paleozoic sedimentary successions, low-grade to amphibolitic facies metamorphic rocks, and the Luxi ultramafic and Myitkyina-Mogok ultramafic-diorite belts. Unmetamorphosed and metamorphosed sediments have similar detrital zircon age peaks of 1000 Ma, and 700 Ma or 500 Ma, and they are separated by the Luxi ultramafic belt, whose εNd(0) and 87Sr/86Sr(i) values are similar to surrounding granite, diorite and migmatite and the γOs = −4.8 ~ −8.8 is indicative of old sub-continental lithospheric mantle, rather than an oceanic suture zone. These data and relations suggest that these regions belong to the Cimmerian continental sliver, derived from the Gondwana supercontinent.The Myitkyina-Mogok ultramafic-diorite belt, located along the eastern margin of the Myanmar Central Basin that was associated with the continental sliver, contains peridotite, andesite, hornblende gabbro, diorite, granodiorite and plagiogranite, all with arc geochemical signatures and ages of 177–166 Ma. The source region of some of these igneous rocks contains a component of EM1, which is close to low 87Sr/86Sr(i) ratio of a migmatite in the Cimmerian continental sliver as shown by SrNd isotopes values, suggesting this belt developed as a Jurassic continental margin arc. The Mogok Metamorphic and Slate Belt contains granodiorite, diorite, tonalite, granite and migmatite extending from the Gongshan, Tengchong and Luxi areas in western Yunnan (China) to Mandalay, Kalaw, Padatchaung and Moulmein in Myanmar. These rocks have arc geochemical character and yield ages of 185–170 Ma, 128–112 Ma, 91 Ma and 75–45 Ma, suggesting development of four episodes of arc magmatism. Along the central axis of the Myanmar Central Basin there are magmatic arc-related 106–94 Ma diorite, granodiorite and tonalite, as well as Eocene and Neogene-Present (38–0 Ma) andesite, dacite, trachyte and rhyolite.Accretion took place along the central axis of the Myanmar Central Basin towards the Indo-Burma Range, and westward to the present trench axis on the margin of the Indian Ocean, as shown by westward younging of accretionary complexes and trench-slope basins. A Jurassic accretionary complex, located in the Hpakan area in the north of the Myanmar Central Basin, contains 189–147 Ma high-pressure metamorphic rocks such as jadeitite, garnet-mica schist and glaucophane schist. In the eastern belt of the Indo-Burma Range Early Cretaceous (119–115 Ma) accretionary complexes consist of upper plate Triassic unmetamorphosed and low-grade metamorphic sediments, and lower plate amphibolites, high-pressure metamorphic rocks and Jurassic-Early Cretaceous oceanic plate stratigraphy containing MORB- and OIB-type basalts and thin-bedded radiolarian cherts. Late Early Cretaceous (~100 Ma) and Late Cretaceous to Middle Eocene (75–45 Ma) trench-slope basin deposits, containing abundant fossil plants and shallow-water sedimentary structures, cover most of the Indo-Burma Range, where sediments rest with an angular unconformity on older accretionary complexes.The presence of earlier magmatic arcs, accretionary complexes and trench-slope basins suggests that the present trench on the margin of the Indian Ocean has evolved by continuous eastward subduction since the Jurassic. The magmatic arc axis migrated eastward (landward) during some time periods (Jurassic to Early Cretaceous, and Late Cretaceous to Middle Eocene) and westward (seaward) migration during others (Early Cretaceous to late Early Cretaceous, and Middle Eocene to Present), during continuous east-dipping subduction.