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Myanmar, the third largest global tin supplier, is an important component of the Southeast Asian tin province. We have conducted laser ablation-inductively coupled plasma-mass spectrometry U-Pb dating of cassiterite, wolframite, and zircon and Re-Os dating of molybdenite from six primary and two placer Sn deposits in Myanmar. A combination of our geochronological data with previous studies revealed that three episodes of Sn mineralization in the Western tin belt of Southeast Asia formed during the closure of multiple Tethys oceans, namely the Late Triassic (~218 Ma) mineralization in a collisional setting after closure of the Paleo-Tethys, the Early Cretaceous (~124–107 Ma) mineralization during subduction of the Meso-Tethys, and the Late Cretaceous to Eocene (~90–42 Ma) mineralization related to the Neo-Tethys subduction. Recurrent Sn mineralization is recorded not only in the Western tin belt but also in the Central and Eastern tin belts in Southeast Asia. Compilation of currently available cassiterite U-Pb ages from all over the world revealed that durations of regional Sn mineralization events are typically in the range of ~5–30 m.y., whereas the Neo-Tethys subduction in Southeast Asia generated prolonged Sn mineralization lasting up to ~50 m.y. The Southeast Asian tin province, as a whole, has the longest cumulative episodes of mineralization, compared to other Sn provinces. The Sn mineralization ceased in the late Eocene when the tectonic setting changed from Neo-Tethys subduction to dextral motion along a series of strike-slip faults and extrusion of the Indochina block in Southeast Asia.

Ultrahigh-temperature (UHT) metamorphism is critical for understanding the most extreme thermal evolution of continental crust. However, UHT metamorphism predominantly occurred in the Precambrian and is rarely observed in the modern Earth. Here, we report the discovery of ∼25 Ma UHT granulites from the Mogok metamorphic belt (MMB) in Myanmar via a combined study of petrology and geochronology. The studied pelitic granulites well preserve a peak mineral assemblage of garnet + sillimanite + plagioclase (antiperthite) + K-feldspar + quartz + Ti-rich biotite + rutile + ilmenite. Pressure (P)-temperature (T) pseudosections and conventional geothermobarometry data only constrain the P-T conditions of the peak stage to 900°C. In situ SIMS and LA-ICP-MS U-Pb dating and trace element analyses show that both metamorphic zircon cores and rims have flat heavy rare earth element (HREE) patterns with negative Eu anomalies. The metamorphic zircon rims show the lowest HREE contents and yield 206Pb/238U ages of 24.9±0.5 and 25.4±0.6 Ma, respectively, representing the timing of UHT metamorphism. Our results indicate that the central MMB underwent ∼25 Ma UHT metamorphism, which is possibly induced by continental rifting along the thinned orogenic lithosphere. Our data, as well as reported Cenozoic UHT events, further suggest that UHT metamorphism can be produced in the modern plate tectonic regime by lithospheric extension.

The tectonic evolution of the Sibumasu Block during the Permian remains controversial, and Permian faunas and their paleobiogeographic affinities provide some insight into its paleogeographic and tectonic evolutionary histories. In this paper, a new brachiopod fauna dominated bySpinomartinia prolificaWaterhouse, 1981 is described from the uppermost part of the Taungnyo Group in the Zwekabin Range, eastern Myanmar. This brachiopod fauna includes 23 species and its age is well constrained as late Kungurian by the associated conodonts, i.e.,Vjalovognathus nicolliYuan et al., 2016 andMesogondolella idahoensis(Youngquist, Hawley, and Miller, 1951), contrary to the late Sakmarian age given to the same brachiopod faunas previously reported from southern Thailand and Malaysia. Based on comprehensive comparisons of the Cisuralian brachiopod faunas and other data in different parts of the Sibumasu Block, we consider that they are better subdivided into two independent stratigraphic assemblages, i.e., the lower (earlier)Bandoproductus monticulus-Spirelytha petaliformisAssemblage of a Sakmarian to probably early Artinskian age, and the upper (younger)Spinomartinia prolifica-Retimarginifera alataAssemblage of a late Kungurian age. The former assemblage is a typical cold-water fauna, mainly composed of Gondwanan-type genera, e.g.,BandoproductusJin and Sun, 1981,SpirelythaFredericks, 1924, andSulciplicaWaterhouse, 1968. The latter assemblage is strongly characterized by an admixture of both Cathaysian and Gondwanan elements, as well as some genera restricted to the Cimmerian continents. Notably, the spatial distribution pattern of these two separate brachiopod assemblages varies distinctly. The Sakmarian cold-water brachiopod faunas have been found in association with glacial-marine diamictites throughout the Sibumasu Block including both the Irrawaddy and Sibuma blocks. In contrast, the Kungurian biogeographically mixed brachiopod faunas are only recorded in the Irrawaddy Block, unlike the Sibuma Block that contains a contemporaneous paleotropical Tethyan fusuline fauna. Thus, it appears likely that by the end of Cisuralian (early Permian), the Sibumasu Block comprised the Irrawaddy Block in the south with cool climatic conditions, and the Sibuma Block in the north with a temperate to warm-water environment, separated by the incipient Thai-Myanmar Mesotethys.

Despite decades of research, the mechanisms and processes of subduction initiation remain obscure, including the tectonic settings where subduction initiation begins and how magmatism responds. The Cretaceous Mawgyi Volcanics represent the earliest volcanic succession in the Wuntho-Popa arc of western Myanmar. This volcanic unit consists of an exceptionally diverse range of contemporaneously magmatic compositions which are spatially juxtaposed. Our new geochemical data show that the Mawgyi Volcanics comprise massive mid-oceanic ridge basalt (MORB)-like lavas and dikes, and subordinate island arc tholeiite and calc-alkaline lavas. The Mawgyi MORB-like rocks exhibit flat rare earth elements (REEs) patterns and are depleted in REEs, high field strength elements (except for Th) and TiO2 concentrations relative to those of MORBs, resembling the Izu-Bonin-Mariana protoarc basalts. Our geochronological results indicate that the Mawgyi Volcanics formed between 105 and 93 Ma, coincident with formation of many Neotethyan supra-subduction zone ophiolites and intraoceanic arcs along orogenic strike in the eastern Mediterranean, Middle East, Pakistan, and Southeast Asia. Combined with its near-equatorial paleo-latitudes constrained by previous paleomagnetic data, the Wuntho-Popa arc is interpreted as a segment of the north-dipping trans-Neotethyan subduction system during the mid-Cretaceous. Importantly, our restoration with available data provides new evidence supporting the hypothesis of a mid-Cretaceous initiation of this >8000-km-long subduction system formed by inversion of the ∼E-W–trending Neotethyan oceanic spreading ridges, and that this was contemporaneous with the final breakup of Gondwana and an abrupt global plate reorganization.

The Kuntabin Sn-W deposit, located in southern Myanmar, is characterized by abundant greisen-type and quartz vein-type cassiterite and wolframite mineralization. We have conducted multiple geochronological methods and isotope and trace element analyses to reveal the age and evolution of the Kuntabin magmatichydrothermal system.Zircon U-Pb dating of the two-mica granite yielded a weighted mean 206Pb/238U age of 90.1 ± 0.7 Ma. Cassiterite U-Pb dating provided a lower intercept age of 88.1 ± 1.9 Ma in the Tera-Wasserburg U-Pb concordia diagram. Molybdenite Re-Os dating returned a weighted mean model age of 87.7 ± 0.5 Ma and an isochron age of 88.7 ± 2.7 Ma. These ages indicate a genetic relationship between granite and Sn-W mineralization in the Kuntabin deposit and record the earliest magmatism and Sn-W mineralization in the Sibumasu and Tengchong terranes related to subduction of the Neo-Tethys oceanic slab. Three generations of cassiterite have been identified with distinctive cathodoluminescence textures and trace element patterns, indicating the episodic input of ore-forming fluids and distinctive changes in the physical-chemical conditions of the Kuntabin magmatichydrothermal system. Sudden changes of fluid pressure, temperature, pH, etc., may have facilitated the deposition of Sn and W. Rhenium contents of molybdenite from the Kuntabin deposit and many other Sn-W deposits in Myanmar are characteristically low compared to porphyry Cu-Mo-(Au) deposits worldwide. In combination with zircon Hf isotope signatures, we infer that granites associated with Sn-W deposits in Myanmar were predominantly derived by melting of ancient continental crust and contain minimal mantle contribution.Subduction of the Neo-Tethys oceanic slab from west of the West Burma terrane reached beneath the Sibumasu terrane and led to magmatism and Sn-W mineralization at ~90 Ma when the Kuntabin deposit was formed. The Paleoproterozoic Sibumasu crust was activated during the subduction-related magmatism to form predominantly crust derived melts. After a high degree of fractional crystallization and fluid exsolution, physical-chemical changes of the hydrothermal fluid resulted in Sn and W precipitation to form the Kuntabin Sn-W deposit.

Indian continental subduction can explain Cenozoic crustal deformation, magmatic activity and uplift of the Tibetan Plateau following the India-Asia collision. In the western Himalayan syntaxis and central Himalaya, subduction or underthrusting of the Indian Plate beneath the Eurasian Plate is well known from seismological studies. However, because information on the deep structure of the eastern Himalayan syntaxis is lacking, the nature of the Indian subduction slab beneath Myanmar and the related tectonic regime remain unclear. Here, we use receiver function common conversion point imaging from a densely spaced seismic array to detect direct structural evidence of present-day Indian continental subduction beneath Asia. The entire subducting Indian crust has an average crustal thickness of ~30 km, dips at an angle of ~19°, and extends to a depth of 100 km under central Myanmar. These results reveal a unique continental subduction regime as a result of Indian-Eurasian continental collision and lateral extrusion., Indian continental subduction can explain crustal deformation, magmatic activity and uplift of the Tibetan Plateau following collision, however, the nature of the Indian subducting slab beneath Myanmar and the related tectonic regime remain unclear. Here, the authors present direct structural evidence of present-day Indian continental subduction beneath Asia.

Cretaceous mafic–felsic intrusions are extensively distributed in the West Burma terrane (Myanmar), but their petrogenesis and tectonic setting still remain unclear. In this study, whole-rock geochemical and Sr–Nd as well as zircon U–Pb and Hf isotopic data for the Cretaceous mafic–felsic intrusions from the Banmauk–Kawlin area (northern Myanmar) are presented. Precise zircon U–Pb dating results indicate that they emplaced at Cretaceous (~ 110–90 Ma) and likely belong to an eastward extension of the coeval Gangdese magmatic belt in the southern Lhasa terrane (Tibet). The studied Cretaceous intrusions (SiO2 = 41.97 to 74.54 wt%) mostly have calc-alkaline and Na-rich characteristics, and strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb and Ta on primitive mantle-normalized diagrams, consistent with geochemical characteristics of arc-type magmas. Most of Cretaceous gabbroic and dioritic intrusions with relatively depleted mantle Sr–Nd–Hf isotopic compositions (87Sr/86Sri = 0.7041–0.7048, eNd(t) = 0.2–6.8, and eHf(t) = 4.3–15.1) and high Ba/La ratios were possibly derived from partial melting of mantle wedge metasomatized by slab-derived fluids and underwent a certain degree of fractional crystallization. Whereas Cretaceous granodiorite and granite probably formed by partial melting of juvenile arc lower crust on the basis of positive eNd(t)–eHf(t) values (− 2.4 to 6.6 and − 2.8 to 15.3) and low initial Sr isotopic ratios (87Sr/86Sri = 0.7045–0.7063). A wide range of Th/Nb ratios and Sr–Nd–Hf isotopic values in these intrusions suggests that their juvenile arc lower crust possibly formed by melting of slab-derived fluids and/or sediment melts metasomatized mantle. Overall, Cretaceous (~ 110–90 Ma) mafic–felsic intrusions in the West Burma terrane may have formed in an arc setting during subduction of the Neo-Tethyan oceanic lithosphere, suggesting a prolonged Neo-Tethyan magmatic arc system from southern Tibet to Southeast Asia.

The Mogok continental foreland region and Katha-Gangaw range (KGR) are located in the north-central section of the Myanmar plate, which is a component of the Eurasian plate. The origin of KGR, exposed along northern Myanmar (SE Asia), is still up for argument, despite numerous prior studies. Based on the petrography, geochemistry, and detrital zircon U-Pb geochronology of metamorphic rock samples, the current study focuses on the tectonic evolution of the KGR. The study also emphasizes the phenomenon of microcontinents rifting from the Gondwanan supercontinent and their subsequent amalgamation with Asia. Detrital zircon ages from four samples in the southern region of KGR peak at 634 Ma, 525 Ma, 290 Ma, and 248 Ma, and two samples yielded > 40% of the grains of younger than 400 Ma. Similar results were obtained from three samples (out of six) from the central region of the KGR. All of the samples from the northern part of KGR are older than 400 Ma, with the exception of MT-02A, which contains nearly all of the younger grains. These younger peaks are identical to the zircon U-Pb ages of the Indochina block, the Sibumasu block, and the Pane Chaung Formation of the Myanmar plate, as well as the Langjiexue Formation (southeastern Tibet). This similarity raises the possibility of either these units being a source region of strata in northern Myanmar or sharing a similar source. The geochemistry of metamorphic rocks samples from KGR revealed loss-on-ignition (LOI) values of 0.29–4.18 wt%, emphasizing the modest to moderate alteration. The samples are enriched in large-ion lithophile elements (LILEs), and depleted in high-field strength elements (HFSEs). All metamorphic samples are peraluminous, indicating the linkage with collisional orogenies. This result is most comparable to upper continental crustal provenance. Hence, the metamorphic rocks in KGR regions must be associated with the crustal materials.

The forearc basin in Myanmar is significant in understanding the development of continental forearc basins. We present stratigraphic, sandstone petrographic, and U-Pb detrital data from Upper Cretaceous–Eocene strata of Chindwin and Minbu sub-basins in the Central Myanmar Depression. The Upper Cretaceous lower Kabaw Formation consists of turbiditic conglomerate, sandstone, and mudstone in the Minbu sub-basin. The composition of conglomerates are mainly schist and subordinate quartz. Prominent detrital zircon age probability peaks are between 260 and 223 Ma, similar with that of Upper Triassic Pane Chaung turbidites and Kanpetlet schist on the West Burma plate. In the upper Kabaw Formation, turbiditic volcanic-rich sandstones have major age populations ranging from 103 to 70 Ma in both Minbu and Chindwin sub-basins. The Paleocene slope environment Paunggyi Formation, which overlies the Kabaw Formation, mainly consists of conglomerate, sandstone, mudstone, and tuff beds in the Minbu sub-basin. In contrast, the Paunggyi Formation in the Chindwin sub-basin is composed of sandstone and mudstone; major detrital zircon age populations from the Paunggyi Formation are between 100 and 60 Ma. Eocene strata in both basins are composed mainly of shallow marine to delta sandstone and mudstone. Major detrital zircon age populations are 100–36 Ma and 600–500 Ma. The Late Cretaceous–Eocene ages from Upper Cretaceous–Eocene strata overlap with igneous crystallization ages from the Western Myanmar Arc. We propose that the Chindwin and Minbu sub-basins developed as parts of a forearc basin along the west flank of Western Myanmar Arc (present coordinate). The forearc basin initiated in Albian time atop the continental West Burma plate due to the formation of a structural high along the western margin of West Burma plate.

Ruby (red corundum) is one of the most prominent colored gemstones in the world. The highest-quality ruby (“pigeon blood” ruby) comes from marbles of the Mogok Stone Tract in central Myanmar. Although Mogok ruby has been exploited since the 6th century AD, the formation time of this gemstone is ambiguous and controversial. In this paper, we describe a mineralogical, geochemical, and geochronological study of ruby and titanite in ruby-bearing marbles obtained from an outcrop in the Mogok Stone Tract, central Myanmar. Petrographic observations have shown that titanite generally occurs in the marble matrix or occurs as inclusions in ruby. These two types of titanite exhibit identical chemical compositions. In situ secondary ion mass spectrometer (SIMS) U–Pb dating of the separated titanite from two representative samples of ruby-bearing marbles yielded lower intercept ages of 25.15 ± 0.24 Ma (MSWD = 0.26) and 25.06 ± 0.22 Ma (MSWD = 0.15), respectively. Because the closure temperature of the U–Pb system in titanite is close to the temperature of ruby growth, the obtained U–Pb ages (~25 Ma) are suggested to represent the timing of the studied ruby formation in Mogok. The acquired ages are in agreement with the timing of post-collisional extension in the Himalaya related to the migration of the eastern Himalayan syntaxis. Combining our dating results with previous geochronological data from the Mogok Stone Tract, we suggest that the formation of the studied ruby is most likely related to the high-temperature metamorphic event in the marbles during the India–Asia collision. Our study not only confirms that texturally constrained titanite could be a precise geochronometer to date the mineralization of different types of ruby, but also provides important geochronological information linking gemstone formation to the India–Asia collision.

The active tectonics in Myanmar is governed by the ongoing northward indentation and obliquely-eastward subduction of India into Eurasia. So far, detailed seismic structure of the crust and uppermost mantle at the eastern flank of the India-Eurasia collision zone remains highly debated. With seismic waveforms recorded at 79 broadband stations in Myanmar, we build a regional shear velocity model in the depth range of 0–80 km by joint inversion of ambient noise derived Rayleigh wave dispersion and P-wave receiver functions. Common conversion point stacking was performed along two representative profiles. We observe clear variations in the seismic velocity and discontinuity structures beneath this region. 1) A sedimentary layer covers the eastern fore-arc trough of the Central Myanmar Basin, with shear velocity less than 2.5 km/s and thickness increasing from ~8 km at 22°N to ~18 km at 23°N. The fore-arc Chindwin basin is evidently thicker than the back-arc Shwebo basin, an abrupt drop in sediment thickness towards the east appears immediately below the Wuntho-Popa magmatic arc. 2) Crustal low-velocity (LV) anomalies (

Linking the India‐Tibet collision to the north and the Andaman oceanic subduction to the south, Myanmar occupies a crucial position in the India‐Eurasia convergence system. Various seismological studies have indicated that the Indian plate is obliquely subducted along the Burma arc. However, the depth extent and continuity of the subducted slab remain enigmatic. With seismic recordings collected from 114 recently deployed seismic stations, we map the topographies of the mantle transition zone (MTZ) boundaries, that is, the 410‐ and 660‐km discontinuities, beneath Myanmar using receiver functions. Regional 3‐D velocity models were adopted to account for the lateral velocity heterogeneity. The 410‐km discontinuity is uplifted by over 15 km within 95°E‐97°E and 21°N‐24°N beneath Myanmar. This feature correlates well with the east‐dipping high‐velocity anomaly in the tomographic models, with a velocity increase of 0.9%–1.2% at the 410‐km discontinuity depth, suggesting that the subducted slab has reached the MTZ. The uplift of the 410‐km discontinuity terminates to the south at ∼21°N, indicating a distinct change in slab geometry. Our results also reveal a depressed 660‐km discontinuity, which is spatially offset to the southwest of the uplifted 410‐km discontinuity. We propose that the offset between the 410‐km discontinuity uplift and the 660‐km discontinuity depression could indicate a slab break‐off and tearing beneath Myanmar, which was triggered by the northward motion of the Indian plate during the eastward subduction. We further speculate that the slab tear could mark the transition from oceanic to continental plate subduction.

The territory of Myanmar, situated at the eastern flank of the India-Asia collision zone, is characterized by complex tectonic structure and high seismicity. From west to east, this region consists of three nearly NS-trending tectonic units: the Indo-Burma Ranges, the Central Basin and the Shan Plateau. Detailed structure of the crust and uppermost mantle beneath Myanmar can provide crucial constraints on regional tectonics, subduction dynamics as well as seismic hazard assessment. Yet seismic velocity structure beneath this region is poorly determined due to sparse regional seismic networks.In this study, we utilize seismic data recorded at 80 broadband stations in Myanmar, among which 70 stations were deployed in 2016 under the project of China-Myanmar Geophysical Survey in the Myanmar Orogen (CMGSMO), 9 stations are operated by IRIS and the remaining one is from GEOFON. We measured the Rayleigh-wave phase velocity dispersion from the ambient noise cross-correlations at periods between 5 s and 40 s by using the automatic frequency-time analysis (AFTAN). A fast marching surface wave tomography (FMST) approach was then adopted to invert the 2-D phase velocity maps in the study region. Our preliminary results show variable crustal structure across central Myanmar, with a strong low-velocity zone north of 22°N in the Indo-Burma Ranges. Since Rayleigh-wave dispersion is more sensitive to absolute velocity speed than to velocity contrasts, the ongoing study jointly inverts the dispersion data with P-wave receiver functions to better determine the velocity discontinuities and thus provides tighter constraints on the shear-velocity structure beneath central Myanmar.

It is well known that there are many Sn–W–Mo and Cu–Au metal resources in Myanmar. However, the absence of precise mineralization ages for these deposits significantly hinders to understand the close genetic relationship among metallogeny, ore-bearing intrusions, and tectonic evolution. In this study, two groups of consistent molybdenite Re–Os and zircon U–Pb ages indicate that the Dapingba Mo–W deposit in northern Myanmar (Tengchong terrane) formed at Early Cretaceous (∼119 Ma and ∼114 Ma). The Dapingba ore-bearing granitic magmas were mainly derived from melting of ancient Tengchong crust on the basis of zircon Hf isotopic compositions (eHf(t) = −9.3 to 2.5). Moreover, the first reported cassiterite U–Pb ages of ∼60 Ma for the Bawapin and Kalonta Sn–W deposits in southern Myanmar (western Sibumasu terrane) suggest that the two deposits formed at Paleocene. Whereas the precise molybdenite Re–Os age for the Shangalon porphyry Cu–Au deposit in the West Burma terrane indicates that this deposit formed at Eocene (∼39 Ma). Importantly, combined with previous studies on tectonic evolution and magmatic petrogenesis in Myanmar and southwest China, a comprehensive Mesozoic-Cenozoic tectono-magmatic and metallogenic model are proposed. Two periods of Early Cretaceous (∼120–114 Ma) and Late Cretaceous (∼75–70 Ma) crust-derived felsic melts and related Mo–Sn–W deposits from the Tengchong and western Sibumasu terranes likely formed in a continental arc during subduction of the Meso- and Neo-Tethys oceanic lithosphere, respectively. Subsequently, Paleocene-Eocene (∼60–50 Ma) Sn–W deposits and coeval ore-bearing granites in the western Sibumasu terrane possibly formed during roll-back of the Neo-Tethys oceanic slab. After ∼50 Ma India–Asia collision, Eocene (∼41–39 Ma) porphyry Cu–Au (West Burma terrane) and Sn–W deposits (western Sibumasu terrane) likely formed during the Neo-Tethys oceanic slab tear and break-off in a collisional setting. Whereas Miocene (∼19–14 Ma) high sulfidation epithermal Cu deposit in the West Burma terrane possibly formed in an arc setting during oblique subduction of the Indian oceanic lithosphere.

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 eNd(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 Sr Nd 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.

The Indian plate descends obliquely eastward beneath the Eurasian plate along the Burmese arc. Previous tomographic results revealed a high-velocity structure that plunges eastward into the deep mantle beneath Myanmar. However, the shallow structure beneath Myanmar remains unclear due to the lack of local seismic observations. Based on the local seismic data recorded by a newly deployed dense array in Myanmar, we obtain a three-dimensional velocity structure of the crust and lithospheric mantle above 100 km by employing the double-difference tomography method. Our imaging results support the existence of the Indian continental lithosphere to a depth of at least 100 km with a dip angle of ∼25° beneath the Indo-Burma Ranges and the Central Myanmar Basin. At the deep end of the continental slab, the high V P , high Vs and rather low average V P / V S ratio indicate that the lowermost portion of the crust from 80 to 120 km may have experienced partially metamorphic eclogitization. The imaging results also provide direct seismic evidence for the origin of the last Monywa volcanic activity in the Holocene. This subduction-related volcanism is characterized by a prominent low Vs anomaly in the lithospheric mantle, which indicates continental lithosphere dehydration and partial melting in mantle wedge. These findings provide new seismic constraints to understand the continental collision system between India and Eurasia under Myanmar and the related magmatic activities of the Monywa volcano.

Centimeter-size intrusions of gabbros and plagiogranites occur in mantle peridotites of the Myitkyina ophiolite, Myanmar. The gabbros mainly consist of plagioclase and clinopyroxene, whereas orthopyroxene occasionally occurs. The plagiogranites are mainly composed of plagioclase, quartz and amphibole, with small amount of accessory minerals, such as zircon, apatite and rutile. Plagioclase in the gabbros varies from andesine to anorthite (An37–91), whereas plagioclase in the plagiogranites is less calcic (An1–40). Clinopyroxene in the gabbros is pervasively altered to hornblende. The gabbros contain 42.97–52.88 wt% SiO2, which show negative correlations with Al2O3, CaO and MgO, but positive correlations with Na2O, P2O5 and TiO2. Microtextural relations reveal the crystallization of clinopyroxene prior to plagioclase in the Myitkyina gabbros. This suggests that the gabbros were crystallized from hydrous melts, which is also supported by the occurrence of orthopyroxene and anorthitic plagioclase in some gabbros. The gabbros have slightly enriched Sr-Nd isotopes, with initial 87Sr/86Sr ratios of 0.703938–0.706609 and eNd(t) values of + 2.4–+7.2, and relatively variable Hf isotopes, with eHf(t) values of + 13.4–+24.9. A subduction component is required to explain the decoupled Nd-Hf isotopes of the gabbros. Binary mixing suggests that addition of ca 2% subducted sediments to a depleted mantle can account for the Nd-Hf decoupling. Therefore, both petrological and geochemical data of the gabbros support that the Myitkyina ophiolite was originated in a supra-subduction zone setting. The plagiogranites have compositions of tonalites and trondhjemites, containing 56.93–77.93 wt% SiO2, 1.27–10.79 wt% Na2O and 0.05–0.71 wt% K2O. They are slightly enriched in LREE over HREE and display positive anomalies in Eu, Zr, Hf but negative Nb anomalies. Very low TiO2 contents (0.03–0.2 wt%) of the plagiogranites suggest that they were not products of fractional crystallization of MORB but were generated by hydrous melting of gabbros. This process is also supported by the nearly constant contents of both La and Yb with increasing SiO2 of the plagiogranites. The Sr-Nd-Hf isotopes of the plagiogranites indicate the involvement of crustal component in their genesis. We proposed that the Myitkyina plagiogranites were generated by partial melting of the mafic protoliths in the subducted slab.

The Indo-Burma Range (IBR) of Myanmar, the eastern extension of the Yarlung-Tsangpo Neotethyan belt of Tibet in China, contains m,langes with serpentinite, greenschist facies basalt, chert, sericite schist, silty slate and unmetamorphosed Triassic sandstone, mudstone and siltstone interbedded with chert in the east, and farther north high-pressure blueschist and eclogite blocks in the Naga Hills m,lange. Our detailed mapping of the Mindat and Magwe sections in the middle IBR revealed a major similar to 18 km antiformal isocline in a m,lange in which greenschist facies rocks in the core decrease in grade eastwards and westwards symmetrically 'outwards' to lower grade sericite schist and silty slate, and at the margins to unmetamorphosed sediments, and these metamorphic rocks are structurally repeated in small-scale imbricated thrust stacks. In the Mindat section the lower western boundary of the isoclinal m,lange is a thrust on which the metamorphic rocks have been transported over unmetamorphosed sediments of the Triassic Pane Chaung Group, and the upper eastern boundary is a normal fault. These relations demonstrate that the IBR metamorphic rocks were exhumed by wedge extrusion in a subduction-generated accretionary complex. Along strike to the north in the Naga Hills is a comparable isoclinal m,lange in which central eclogite lenses are succeeded 'outwards' by layers of glaucophane schist and glaucophanite, and to lower grade greenschist facies sericite schist and slate towards the margins. In the Natchaung area (from west to east) unmetamorphosed Triassic sediments overlie quartzites, sericite schists, actinolite schists and meta-volcanic amphibolites derived from MORB-type basalt, which are in fault contact with peridotite. Olivine in the peridotite has undulatory extinction suggesting deformation at 600-700 A degrees C, similar to the peak temperature of the amphibolite; these relations suggest generation in a metamorphic sole. The amphibolites have U/Pb zircon ages of 119 +/- 3 Ma and 115 Ma, which are close to the zircon ages of nearby calc-alkaline granite and diorite, which belong to an active continental margin arc that extends along the western side of the Shan-Thai block. The IBR accretionary complex and the active continental margin arc were generated during Early Cretaceous (115-128 Ma) subduction of the Neotethys Ocean.

The provenance of Sibumasu terrane sedimentary rocks and their tectonic relationships with surrounding terranes exposed in Southeast Asia record separation and accretion of Gondwana-derived terranes during Late Paleozoic and Mesozoic time. This paper reports sandstone petrographic and U–Pb detrital zircon geochronologic data from Ordovician to Lower Jurassic strata within the Sibumasu terrane in Shan State, Myanmar. The Ordovician strata are composed of limestone and siltstone. The Lower Silurian Linwe and Upper Silurian Namhism Formations are comprised of limestone, silty sandstone, conglomerate and sandstone, respectively. Sandstones from both Ordovician and Silurian strata are dominated by 567–470 Ma and 982–917 Ma detrital zircons that are interpreted to be sourced from the eastern Gondwana supercontinent. The Carboniferous unit is composed of metasedimentary rocks (phyllite, slate, quartzite, and meta-marl). Sandstones from Carboniferous units show a strong 1165–1070 Ma detrital zircon age peak that is not present in the Ordovician and Silurian strata. These zircon grains were most likely derived from both the Albany–Fraser Province in Southwest Australia and Maud Province in Antarctic. The Upper Triassic to Lower Jurassic, shallow marine Loi-an Group consists of thin- to medium-bedded sandstone and mudstone that unconformably overly the Permian to Middle Triassic Plateau Limestone Group. Sandstones from the Loi-an Group contain abundant Permian to Triassic detrital zircons that are interpreted to have been derived from the Sukhothai Arc of the western Indochina terrane. Formation of this arc is attributed to eastward (present coordinates) subduction of Paleo-Tethyan oceanic lithosphere beneath Indochina. Therefore, we propose that the Sibumasu terrane was juxtaposed against northwestern Australia as part of the Gondwana supercontinent during Paleozoic to Early Permian time. During the Late Triassic and Early Jurassic, Sibumasu strata record an abrupt influx of Permian and Triassic zircon grains, signifying a change in provenance from Gondwana to the Sukhothai Arc. These data are consistent with a tectonic model involving rifting of the Sibumasu terrane from Gondwana and subsequent docking with the Indochina/Simao terranes during Mesozoic time.

International audience; The geological history of the Burmese subduction margin, where India obliquely subducts below Indochina, remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader Tibetan-Himalayan orogen. Various scenarios for the evolution of the orogen have been proposed, including a collision of India with Myanmar in the Paleogene, a significant extrusion of Myanmar and Indochina from the India-Asia collision zone, or very little change in paleogeography and subduction regime since the India-Asia collision. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene – Oligocene times to reconstruct the evolution of the Burmese margin during the early stages of the India-Asia collision. We report on sedimentological, geochemical, petrographical, and geochronological data from the Chindwin Basin - the northern part of the Burmese forearc - and integrate these results with previous data from other basins in central Myanmar. Our results show that the Burmese margin acted as a regular Andean-type subduction margin until the late Middle Eocene, with a forearc basin open to the trench and fed by the denudation of the Andean volcanic arc to the east. We show that the modern tectonic configuration of central Myanmar was formed 39-37 million years ago, when the Burmese margin shifted from an Andean-type margin to a hyper-oblique margin. The forearc basin was quickly partitioned into individual pull-apart basins, bounded to the west by a quickly emerged accretionary prism, and to the east by synchronously exhumed basement rocks, including coeval high grade metamorphics. We interpret this shift as resulting from the onset of strike-slip deformation on the subduction margin leading to the formation of a paleo-sliver plate, with a paleo fault system in the accretionary prism, pull apart basins in the forearc, and another paleo fault system in the backarc. This evolution implies that hyper-oblique convergence below the Burmese margin is at least twice older than previously thought. Our results reject any India-Asia convergence scenario involving an early Paleogene collision of India with Myanmar. In contrast, our results validate conservative geodynamic models arguing for a close-to-modern precollisional paleogeography for the Indochina Peninsula, and indicate that any postcollisional rotation of Indochina, if it occurred at all, must have been achieved by the late Middle Eocene.

The Main Range and Eastern Provinces are two major granite belts in Southeast Asia. These granite belts extend southward from the southeastern Tibetan Plateau to Myanmar, and through Thailand into Peninsular Malaysia. They are interpreted to represent the magmatic expression of the closure of the Paleo-Tethys from the Permian to the Triassic. Myanmar lies in the heart of these granite belts. The Kyaing Tong and Tachileik granites in the far east of Myanmar are important components of the granite belts of Southeast Asia; however, due to the lack of reliable geochronology within eastern Myanmar, delineation of the Main Range and Eastern Province belts in this region is very poorly constrained. Here we present new zircon U–Pb age, whole-rock composition, and Sr–Nd isotope data from the Kyaing Tong and Tachileik granites from eastern Myanmar to address this geological problem. Measured ages of 207–216 Ma from the Kyaing Tong granites imply that they are a northern extension of the Main Range Province, whereas the Tachileik granites yield ages of 246–250 Ma, which suggests that they are the northern extension of the Eastern Province granite belt. Both belts in eastern Myanmar comprise biotite monzogranites and granodiorites and show similar geochemical features, such as having a high aluminum saturation index and an unfractionated composition. The granites from both provinces show enrichment in light rare earth elements (REE) and negative Eu anomalies. All samples demonstrate characteristic negative Ba, Nb, Ta, Sr and Ti anomalies, and a positive Pb anomaly, when plotted on spidergrams. The Kyaing Tong granites have high and variable initial 87Sr/86Sr ratios (0.717735–0.731271), negative eNd (t = 215 Ma) values (−14.2 to −10.4), and old TDM2 ages. Similarly, the Tachileik granites have high and variable initial 87Sr/86Sr ratios (0.715336–0.722712), negative eNd (t = 250 Ma) values (−12.4 to −11.3), and old TDM2 ages. Sr–Nd isotope values show that these granites may be generated by mixing of two end-member lithologies: amphibolite and schist of the Lancang Group, which represents the lower crust of the Indochina block. We consider that both the Kyaing Tong and Tachileik granites are of I-type affinity. They were derived from partial melting of the amphibolite and underwent assimilation of schist. Our petrogenetic and zircon U–Pb age data support models that relate the Eastern Province granites to continental arc during the Permian and syn-collisional magmatism in the Early Triassic, and the Main Range Province granites to post-collisional magmatism during the Middle to Late Triassic.

Ophiolites within Myanmar have been commonly divided into two belts, i.e., the Eastern Belt and Western Belt. The Kalaymyo ophiolite from the Western Belt crops out at the eastern margin of the Indo-Burma Range and was formed during the Early Cretaceous, whereas the Myitkyina ophiolite from the Eastern Belt was formed during the Middle Jurassic. Both ophiolites are characterized by the occurrence of big massifs of mantle peridotites. Mantle peridotites of the Kalaymyo ophiolite are mainly composed of spinel lherzolites and harzburgites, with minor plagioclase peridotites. Mantle peridotites of the Myitkyina ophiolite mainly consist of spinel harzburgites, with minor dunites. Spinel lherzolites from the Kalaymyo ophiolite have relatively fertile compositions, with 40.12–45.19 wt.% MgO, 1.1–2.74 wt.% Al 2 O 3 and 0.67–2.67 wt.% CaO. Their spinels have Cr# values of 0.12–0.4, yielding fractional melting degrees of 3–15%. In comparison, spinel harzburgites from the Kalaymyo ophiolite are more refractory, with 42.08–48.73 wt.% MgO, 0.09–0.99 wt.% Al 2 O 3 and 0.07–0.8 wt.% CaO. Their spinels have Cr# values of 0.3–0.73, giving 12–21% degrees of fractional melting. Plagioclase peridotites from the Kalaymyo ophiolite have compositions intermediate between spinel lherzolites and harzburgites. Compared to the spinel peridotites, spinels in the plagioclase peridotites have relatively higher TiO 2 contents. Harzburgites from the Myitkyina ophiolite, containing 40.88–48.16 wt.% MgO, 0.13–1.65 wt.% Al 2 O 3 and 0.1–1.68 wt.% CaO, have refractory compositions similar to the Kalaymyo harzburgites. Spinels in the Myitkyina harzburgites with low TiO 2 contents (i.e.

Scheelite, which is an important ore of tungsten and colored gemstone, is well developed in the calc-silicate rocks from the Mogok metamorphic belt (MMB), Myanmar. In this study, the textural, mineralogical, and compositional characteristics of scheelite and its associated minerals were systematically investigated to constrain the petrogenesis of scheelite-bearing calc-silicate rocks and the tungsten transfer and mineralization mechanism in a hydrothermal–metasomatic system. The petrological evidence, bulk and mineral geochemical signatures, and mass-transfer calculations indicate that the calc-silicate rocks formed by local metasomatism of marble via the introduction of an externally derived Si–Al–Fe–W–F-bearing, H2O-rich fluid phase. The distinct compositional zonations [F, Fe, Ca, and heavy rare earth elements (HREEs)] of garnet in the calc-silicate rocks record a two-stage metasomatic process and significant compositional variation in the associated fluid. The late-stage metasomatic fluid that led to the formation of the F-rich garnet rims, scheelite, and most of the calc-silicate minerals has noticeably higher fluorine activity (aF−), oxygen fugacity (fo2), and HREE content than the early-stage metasomatic fluid responsible for the garnet cores. The MMB scheelite exhibits typical “skarn-type” compositional characteristics with a high LaN/YbN ratio (100–180), a negative Eu anomaly (δEu = 0.3–0.5), and a high Mo content (1100–1330 ppm). These geochemical signatures are primarily controlled by the protolith, metasomatic fluid, redox conditions, and coexisting mineral phases. The enrichment of rare earth elements (REEs) and high field strength elements (HFSEs) in the MMB scheelite was dominated by two substitution reactions: Ca2+ + W6+ = REE3+ + HFSE5+ and 3Ca2+ = 2REE3+ + □Ca (where □Ca is a Ca-site vacancy). Considerable amounts of F and OH in the metasomatic fluid substituted for O in the garnet via the substitute reaction 4(F, OH)− = 4O2− + Si4+, leading to a significant enrichment of F (up to 1.2 wt.%) and OH (up to 0.32 for nOH) and a negative correlation between F and Si in the garnet. Detailed petrographic observations show that the occurrence of scheelite in the MMB calc-silicate rocks is always associated with the growth of F-rich minerals such as garnet rims (0.8–1.2 wt.% F), vesuvianite (2.4–2.6 wt.% F), fluorite (48–49 wt.% F), apatite (3.9–4.1 wt.% F), and titanite (2.6–3.4 wt.% F). These textural characteristics, combined with the positive correlation of whole-rock F and W (as well as Sn, Mo) contents in the calc-silicate rocks, indicate that the elevated F contents increased the solubility of W in the infiltrating fluid, thereby allowing the W transfer in the hydrothermal–metasomatic system. The mineralization of scheelite was triggered by the crystallization of F-rich minerals during the formation of the calc-silicate rocks, which caused F depletion and consequent saturation of W in the metasomatic fluid. Our results suggest that, in the MMB metasomatic system, F rather than Cl is the key fluxing compound that facilitates the transfer of W and the mineralization of scheelite.

The Shangalon-Kyungalon Cu-Au (Mo) ore district is located on the south-eastern margin of the Kanzachaung batholith in the Wuntho-Popa Arc, Myanmar. Porphyry and epithermal Cu-Au mineralization are associated with the Late-Eocene magmatic rocks. In this study, zircon U-Pb dating of gabbroic diorite, diorite porphyry, tonalite porphyry, and granodiorite yielded ages of 38.0 ± 0.35 Ma, 37.8 ± 0.35 Ma, 37.4 ± 0.27 Ma, and 36.9 ± 0.26 Ma, respectively. These rocks show sub-alkaline characteristics, LREE-enriched patterns with weakly positive/negative Eu anomalies, enrichment of LILEs and depletion of HFSEs. They have obviously higher initial Sr ratios and lower eNd(t) and eHf(t) values (Isr = 0.7066; eNd(t) = −0.5 to −4.8; eHf(t) = 0.7–9.7) than the mid-Cretaceous Kanzachaung batholith (Isr = 0.7046; eNd(t) = 2.4–4.5; eHf(t) = 10.9–15.1), indicating crustal materials were most likely added to the arc magma source. In particular, the gabbroic diorite has higher initial Sr, more enriched Nd-Hf isotopic compositions (Isr = 0.7081, eNd(t) = −4.6 to −4.8, eHf(t) = 0.7–2.3) and lower SiO2 content relative to the other magmatic rocks studied (Isr = 0.7068–0.7059; eNd(t) = −0.5 to −2.8, eHf(t) = 5.0–9.7). The results suggest that the gabbroic diorite was derived from melting of a mantle wedge, metasomatized by subducted oceanic sediment. Underplating on juvenile lower crust followed, accompanied by MASH processes, which led to increased eNd(t) (and SiO2 content) and decreased initial Sr ratios in later formed rocks. Moreover, the adakite-like features (Sr/Y = 19.3–53.4, Y = 7.54–18.8,) suggest fractional crystallization of hornblende in a hydrous magma, potentially leading to formation of the Shangalon-Kyungalon porphyry-epithermal Cu-Au system. In summary, the Late-Eocene magmas in Shangalon-Kyungalon formed from a mantle source metasomatized by subducted sediments during Neo-Tethyan subduction beneath Western Burma terrane, with the involvement of juvenile crustal materials.

The magnitude and timing of deformation in the form of vertical-axis rotation in the eastern Tibetan Plateau and Southeast Asia during the Cenozoic period are the key to understanding the India-Asia collision and subsequent geological processes. Our new paleomagnetic and geochronological data from Myanmar show that the characteristic remanent magnetization (ChRM) mean directions of sedimentary tuffaceous layers (31 effective specimens) and rhyolites (11 paleomagnetic site-mean directions), which formed at ∼64.4 Ma and ∼26.6 Ma, were D±ΔD/I±ΔI = 50.8°±11.0°/33.1°±9.2° with α95 = 9.2° and D±ΔD/I±ΔI = 356.8°±6.5°/16.9°±6.2° with α95 = 6.2°, respectively. These data indicate that the West Burma Block (WBB) has experienced a significant clockwise rotation of 38.3°±6.9° since ∼64.4 Ma and a later counterclockwise rotation of 14.6 ± 4.2° since ∼26.6 Ma with respect to stable Asia. The clockwise rotation of ∼40° of the WBB since 64.4 Ma (early Paleocene) with respect to stable Asia is attributable to the India-Asia collision, and the later counterclockwise rotation of 14.6°±4.2° since 26.6 Ma (late Oligocene) is attributable to the dextral-slip faulting activity of the Sagaing Fault. These paleomagnetic analyses, combined with previously published geological research in SE Asia, are substantially consistent with the predictions of the extrusion model for the SE Asia realm.

We report on maar lakes and lacustrine carbonate varves in Myanmar. Carbonate varves are well developed in maar lake Twin Taung, the deepest of five maar lakes in Myanmar. Lamination consists of light and brown-colored laminate couplets in thin sections. A light- colored layer is composed mainly of calcite crystals with thickness 50-400 mm, and a dark-colored layer consists of other organic and siliceous matter. Climatic conditions in the study region are determined by the Indian monsoon and have a pronounced rainfall seasonality. The dry and hot season is between December and April, and warm and humid conditions prevail in summer and autumn. These characteristics are key to varve formation. In the dry and hot season, calcite crystals form with less precipitation and strong evaporation, whereas clastic and organic matter are deposited during the warm and humid season. An independent chronology derived from 137Cs and 210Pb shows good agreement with counted laminations. To compare with instrumental data, synchrotron radiation X-ray fluorescence measurement was used for extracting minor element datasets with high resolution. Based on principal component analysis and comparison between elemental variation and regional rainfall data, we interpret the elemental data in terms of climate variables such as precipitation and temperature. The first component includes lithogenic elements such as K, Ti, Fe, Cu, Zn, Rb and Zr, and might be controlled by various factors such as physical and chemical weathering processes, rainfall, and dust input. The second component, calcium and strontium, might be principally regulated by the balance between evaporation and precipitation. During drought periods, the calcium ion might supersaturate, causing carbonate calcite precipitation from the water column. Our preliminary result suggests that elements Ca and Sr in the annually laminated sediment might be useful indicators for studying paleoclimatic change at high resolution.

Subduction of the Indian continental lithosphere in the eastern Tibet-Himalaya orogenic belt remains unclear. Newly reported zircon U-Pb ages in this study indicate that the Shangalon intermediate-felsic magmatic rocks in northern Myanmar formed at Eocene-Oligocene (∼ 40–32 Ma). They have calc-alkaline to shoshonitic characteristics, LREE-enriched patterns, enrichments in LILE (e.g., Rb, Cs), and depletions in HFSE (e.g., Nb). Obviously, they show the higher initial Sr and lower Nd-Hf isotopic compositions (87Sr/86Sri = 0.7054–0.7082, eNd(t) = −5.3 to −0.4, and eHf(t) = −3.4 to 10.8) than Cretaceous arc-related mafic-felsic rocks in the West Burma terrane. A positive correlation between Nd isotopic compositions and SiO2 contents indicates that the Eocene Shangalon diorite and andesite with the lowest eNd(t) values (−5.3 to −4.0) likely derived from a mantle source contaminated by the subducted Indian continental lithosphere. Coupled with regional coeval OIB-like mafic melts and high temperature metamorphism, the Eocene-Oligocene Shangalon magma might have formed in the Neo-Tethyan oceanic slab break-off setting after the India-Asia collision. In addition, Eocene granodioritic rocks have the adakitic features, which possibly resulted by fractional crystallization of hornblende. The hornblende-dominated crystallization in the Shangalon ore-bearing granodioritic rocks is well consistent with magma evolution of H2O-rich melts, which play an important role in the formation of the Eocene Shangalon porphyry Cu-Au deposit.

The geochronology, petrogenesis and tectonic setting of granites associated with tin–tungsten mineralization in southern Myanmar remain unclear. This work presents whole-rock geochemical, Sr–Nd isotopic, and zircon U–Pb and Hf isotopic data for felsic intrusions in the Sibusima terrane. Zircon U–Pb dating indicates formation in the Late Cretaceous–Early Eocene (~84–48 Ma) and suggests that these intrusions likely represent a southward extension of the coeval magmatic belt in the Tengchong terrane. Primarily classified as high-K calc-alkaline, the granites show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, P, and Ti, and negative Ba and Sr anomalies on primitive mantle-normalized diagrams. P2O5, CaO, Al2O3, MgO, TiO2 contents, and Nb/Ta ratios decrease with increasing SiO2 contents, possibly consistent with evolution trends in fractionated I-type granitic magmas. In addition, these intrusions display a wide range of negative eNd(t) (−14.6–−5.5) and zircon eHf(t) values (–22.7–5.7), suggesting a dominant Sibusimacrustal source with a minor mantle contribution. Notably, the Eocene and a few Paleocene granites show more contribution of mantle material based on higher eNd(t) (−8.5–−5.5) and zircon eHf(t) (−11.0–−3.2) values. Large variations in zircon Hf isotopic compositions within an individual Eocene granitic intrusion (e.g., −15.2–5.7) and corresponding biotite-rich enclave (−17.8–0.2) likely indicate magma mixing. Considering the spatial-temporal distribution of magmatism in the West Burma and Sibusima terranes, the studied Late Cretaceous–Eocene felsic magmas likely formed in a continental arc setting during normal subduction (~100–60 Ma) and subsequent roll-back of the Neo-Tethyan oceanic lithosphere (~60–50 Ma). Additionally, most of granites have the elevated Sn with increasing Rb/Sr ratios, and decreasing TiO2 contents and Nb/Ta ratios, suggesting Sn enrichment is primarily controlled by magmatic fractionation. Meanwhile, Sn and Li depletions in some granites deviate from the magmatic evolution trend, likely as a result of fluid exsolution. The exsolved Sn-rich fluids could have made a genetic contribution to hydrothermal Sn mineralization in southern Myanmar.

We present new whole-rock major, trace, and platinum group element (PGE) and mineral chemistry data from the Kalaymyo peridotite massif in the central part of the Indo-Myanmar Ranges (western Myanmar) and discuss its mantle melt evolution. The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse-grained equigranular textures. They are composed of olivine (forsterite, Fo = 89.8–90.5), orthopyroxene (enstatite, En 86–91 , wollastonite, Wo 1–4 , ferrosilite, Fs 8–10 ; Mg# = 89.6–91.9), clinopyroxene (En 46–49 Wo 47–50 Fs 3–5 ; Mg# = 90.9–93.6), and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole-rock compositions with Mg#s of 90.1–90.8 and SiO 2 (41.5–43.65 wt%), Al 2 O 3 (1.66–2.66 wt%), and CaO (1.45–2.67 wt%) contents. They display light rare earth element (LREE)–depleted chondrite-normalized (CN) REE patterns with (La/Yb) CN = 0.04–0.21 and (Gd/Yb) CN = 0.40–0.84, and show a slight enrichment from Pr to La with (La/Pr) CN in the range of 0.98–2.36. The Kalaymyo peridotites are characterized by Pd-enriched chondrite-normalized PGE patterns with superchondritic (Pd/Ir) CN ratios (1.15–2.36). Their calculated oxygen fugacities range between the quartz-fayalite-magnetite (QFM) oxygen buffers, QFM–0.57 and QFM+0.90. These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5%–15%) of partial melting at a mid-oceanic ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched mid-oceanic ridge basalt–like melts percolating through these already depleted residual peridotites. The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges therefore represent mid-oceanic ridge–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward-migrating subduction-accretion system along the eastern margin of India.