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This paper describes the Ordovician stratigraphy of the Sibumasu Block, which formed part of equatorial peri-Gondwana during the early Paleozoic, and summarizes the palaeoenvironmental conditions that prevailed during that time. Upper Cambrian sequences are commonly dominated by coarse-grained clastic rocks of shallow-marine origin. The lithology of the Lower Ordovician varies from region to region, with fine-grained clastic rocks in the Shan Plateau, shallow-marine clastic rocks in northern to western Thailand and shallow-marine carbonate rocks in southern Thailand to northwestern Malaysia. Middle Ordovician strata consist entirely of shallow- to deeper-water carbonate rocks, with the exception of turbidites in midwestern Malaysia. Upper Ordovician rocks show an overall deeper environment, and the uppermost Ordovician consists of pelagic and deeper, fine-grained clastic rocks. On the basis of palaeobiogeographical data, the Sibumasu Block was adjacent to Western Australia throughout the early Paleozoic. Upper Cambrian rhyolitic rocks in the Shan Plateau and Lower–Middle Ordovician volcanics in midwestern Malaysia may have formed as a result of tectonomagmatism associated with the subduction of the Proto-Tethys Ocean plate into the northern Gondwanan margin. The close similarity of Late Ordovician fossil assemblages from Sibumasu and South China may be due to a sea-level rise during this time.

Incremental accretion of continental fragments from East Gondwana to Eurasia resulted in the growth of Asia and rise of the Tibetan Plateau, yet its detailed evolution remains uncertain. Argoland, a continental fragment that rifted from NW Australia during the Late Jurassic, played a key role in the initial opening of the Indian Ocean and the evolution of eastern Tethys. However, its present identity remains elusive, with East Java-West Sulawesi currently assumed to be the most likely option. To constrain the missing Argoland and its role in India-Asia convergence, we report new detrital zircon data from Sulawesi, Indonesia, and West Burma, Myanmar, and synthesize literature results from relevant regions in Southeast Asia, which (>15,000) reveal age profiles of West Sulawesi, the central Sulawesi metamorphic belt, and southeast Borneo comparable to that of Bird's Head, New Guinea, whereas age patterns of West Burma and East/West Java are similar to those of NW Australia. Notably, the most dominant age populations in NW Australia are rarely detected in Sulawesi and Borneo. These observations, combined with previous geological records and recent paleomagnetic data, suggest that West Burma is the mysterious Argoland, opposing the currently favored East Java-West Sulawesi model, with East Java and West Sulawesi probably having originated from NW Australia and Bird's Head, respectively. We estimate an average northward motion of ∼6–8 cm/yr for West Burma, which split from NW Australia to approach the equator during ca. 155–95 Ma, shedding new light on the reconstruction and breakup of northern East Gondwana, progressive building of Southeast Asia, and India-Asia convergence.

Before the India–Asia collision, Neotethyan subduction gave rise to an Andean-type convergent margin on the southern margin of Asia. To investigate the spatial and temporal distribution of the subduction-related magmatism, we undertook a combined determination of zircon U–Pb ages and Hf isotopes of Mesozoic to Paleogene intrusive and volcanic rocks from southern Tibet to Myanmar to characterize the two parallel magmatic belts that have previously been considered separately. One belt extends from the Gangdese Batholith in the Southern Lhasa sub-terrane to the Lohit Batholith, the Sodon Pluton and the Popa–Loimye Arc in the West Burma Block, and the other from the Central Lhasa Plutonic Belt to the Bomi–Chayu Batholith, the Dianxi Batholith and the Shan Scarps Batholith in central Myanmar. The Gangdese belt, as the main arc component, consists typically of I-type granitoids that contain magmatic zircons showing positive eHf(t) values. In contrast, the Central Lhasa Plutonic Belt belt is dominated by S-type granites in which most zircons show negative eHf(t) values suggesting the involvement of older continental crust in their petrogenesis. The distinct geochemical characteristics indicate not only distinct tectonic settings of their genesis but also the diverse nature of the crust forming the exotic continental ribbons amalgamated to Asia. Supplementary material: Details of sample locations and analytical results are available at: https://doi.org/10.6084/m9.figshare.c.4311485

To understand the geological control and tectonic forcing on the development of a major river system in SE Asia, new detrital zircon U-Pb and Hf isotopic data from the Salween drainage, Myanmar is reported. Three samples with a range of deposition ages corresponding to different tectonic periods of the Sibumasu Terrane reveal a dramatic change in the drainage system. Differing from the detrital compositions of modern sediments from the Salween river mouth, a Late Triassic sandstone sample shows zircon age distribution implying that the Salween drainage may have been connected at that time to the northeastern part of the Songpan-Ganzi Terrane and western part of the South China Block. U-Pb ages and ɛHf(T) isotopic data of detrital zircons from other major rivers in SE Asia, including the Yangtze, Red, and Mekong Rivers in the east and the Irrawaddy River and the Yalu-Tsangpo in the west are further synthesized. A marked discrepancy in zircons younger than Jurassic ages is identified. Similar to results from the Salween samples, these young zircons from the eastern rivers are overwhelmed by an old, recycled crustal component exhibiting negative ɛHf(T) values. In contrast, those from the western rivers are dominated by juvenile components that are characterized by high and positive ɛHf(T) values. The Salween drainage, therefore, lies in the frontier with its western outline broadly depicting the boundary between mature and juvenile crustal provinces in SE Asia.

Fig. A.5. Plots of εNd (T) values vs. initial Sr isotope ratios for the samples from Myanmar. For comparison, the data of the Tengchong block from Liu et al. (2009), Monywa and Popa volcanos from Yang (2008), and the eastern Transhimalayan batholith, SE Tibet from Lin et al. (2012) are also shown.

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 550 km long Shan Scarps segment of the sickle-shaped MMB. The oldest determination is 491 Ma 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 114 Ma protolith age. Our zircon U–Pb ages on undeformed intrusive rocks in the MMB range from Cretaceous to Miocene; the oldest (128 Ma), on a diorite near Yebokson, implies an earliest Cretaceous minimum age for regional metamorphism here. Younger ages on undeformed intrusions include 91 Ma on diorite at Mokpalin, 72 Ma on the weakly foliated garnet-bearing Nattaung granite, 44 and 48 Ma respectively on the MEC and Sedo granites, and 17–20 Ma on granite dykes. For the 128 Ma 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 105 Ma on diorite at Salingyi supports previous K–Ar ages (106–93 Ma) 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 103–80 Ma 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.

The Mogok metamorphic belt (MMB), over 1450 km long and up to 40 km wide, consists of regionally metamorphosed rocks including kyanite and sillimanite schists and granites lying along the Western margin of the Shan Plateau in central Myanmar and continuing northwards to the eastern Himalayan syntaxis. Exposures in quarries allow correlation of Palaeozoic meta-sedimentary, early Mesozoic meta-igneous and late Mesozoic intrusive rocks within a 230 km long northerly-trending segment of the MMB, from Tatkon to Kyanigan north of Mandalay, and with the Mogok gemstone district 100 km to the northeast. Relationships among the metamorphic and intrusive rocks, with sparse published radiometric age controls, indicate at least two metamorphic events, one before and one after the intrusion of Late Jurassic to early Cretaceous calc-alkaline rocks. These relationships can be explained by either of two possible tectonic histories. One, constrained by correlation of mid-Permian limestones across Myanmar, requires early Permian and early Jurassic regional metamorphic events, prior to an early Tertiary metamorphism, in the western part of but within a Shan-Thai – western Myanmar block. The second, not compatible with a single laterally continuous Permian limestone, requires pre-Upper Jurassic regional metamorphism and orogenic gold mineralization in the Mergui Group and western Myanmar, early Cretaceous collision of an east-facing Mergui-western Myanmar island arc with the Shan Plateau, and early Tertiary metamorphism in the MMB related to reversal in tectonic polarity following the arc-Plateau collision.

Permo-Triassic carbonates with minor basal clastics cover extensive tracts of the Shan Plateau in the eastern part of Myanmar. These carbonates are grouped into the Permian Thitsipin Limestone Formation and the Late Permian–Early Triassic Nwabangyi Dolomite Formation. The Thitsipin Formation occurs as isolated masses, in many places faulted against older rocks. It is now known that there is a regional unconformity between the Thitsipin Limestone Formation and the underlying silici-carbonates of Devonian age in Shan State, and between the Thitsipin Limestone Formation and Carboniferous meta-siliciclastics in Kayah State, Mon State, Kayin State and Tanintharyi Division. Some of the Thitsipin limestone is dolomitized and passes transitionally upward into the Nwabangyi Dolomite Formation. Permian shelf-carbonates occur within two different sequences: the silici-carbonate sequence of the Shan–Thai Block and the meta-siliciclastic sequence of the Mergui Terrane, suggesting that the Permian limestones were deposited after the emplacement of the Mergui Terrane onto the southern margin of the Shan–Thai Block. This event occurred when the Shan–Thai Block was in the southern hemisphere.