Sibumasu Block Research Articles

Cassiterite and zircon U-Pb ages and compositions from ore-bearing and barren granites in Thailand: Constraints on the formation of tin deposits in Southeast Asia

The Southeast Asian Tin Province comprises western, central, and eastern belts and hosts significant granite-related Sn deposits. The genetic links between granites and Sn mineralization are still unclear. Most Sn deposits are in Thailand’s western and central belts, but their origin remains poorly elucidated due to the absence of direct dating of mineralization. Herein in-situ U-Pb age data of wolframite and cassiterite grains from nine representative Sn deposits in Thailand are obtained, which fall into two stages. Triassic deposits (224–210 Ma) are found in the Central belt, with Cretaceous deposits (78–67 Ma) in the Central belt being younger than those in the Western belt (84–74 Ma). However, ore-bearing granites, spanning two periods (227–205 Ma and 85–69 Ma), occur in central and western belts. Some Triassic ore-bearing granites exhibit significantly older ages than ore-forming ages. Newly identified ore-forming granites contain zircon grains with relatively low εHf(t) values (−29.5 to + 4.1; average = − 11.8), indicative of an origin from supracrustal sediments from the Sibumasu block. In contrast, barren granites have ages from 303–224 Ma and higher zircon εHf(t) values (−9.9 to + 13.2; average = +1.2), which suggests that they were derived from the juvenile mafic crust. Even after experiencing hydrothermal fluids exsolution, some low-fractionated ore-forming granites (D.I. < 90) still maintain remarkably high tin contents. Both ore-forming and barren granites crystallized under reducing conditions. Our study highlights the importance of Sn-rich sources of parental magmas in forming Sn deposits. The metasediment-rich basement of the Sibumasu block distributing along the continental margin is likely the Sn-rich source. These sources played a crucial role in forming two stages of tin deposits in distinct tectonic settings, that are syn-collisional crustal thickening in Paleo-Tethys and post-collisional extension-related settings in Neo-Tethys.

Spatio-temporal analysis of Permian-Cretaceous magmatic activities in the Tengchong block: Implications for tectono-magmatic evolution

Understanding the tectono-magmatic evolution history of the Tengchong block is crucial for elucidating the formation of the Eastern Tethys tectonic domain. However, the correlation and evolution of the Tengchong block with the Sibumasu and Lhasa blocks is controversial during the Permian and Cretaceous. This study explores the information contained within magmatic rocks using big data and spatio-temporal analysis, providing quantitative constraints for the discussion of the tectono-magmatic evolution of the Tengchong block. To more accurately assess true magma activities and reduce errors caused by preservation and sampling processes, we utilized local singularity analysis to obtain the singularity index time-series. Correlation analysis of zircon ages and εHf(t) (correlation coefficient ≥ 0.5) values indicates that the Tengchong block is more similar to the Sibumasu block. Results from time-lagged cross-correlation analysis indicate that the Tengchong block and Sibumasu block exhibit a shorter lag in magmatic activities (3 Myr). Wavelet analysis reveals similar periods of collision-related magmatic activities (57 Myr and 43 Myr). Integrating evidence from paleontology and ophiolite belts, we propose that the Tengchong block co-evolved more closely with the Sibumasu block than with the Lhasa block, suggesting similar tectonic processes during the Early Permian to Early Cretaceous. Approximately 250–236 Ma, in the western Tengchong block, partial melting of the lower crust occurs due to crustal thickening. Around 219–213 Ma and 198–180 Ma, after the Tengchong block collided with the Eurasian continent, the subduction of the Meso-Tethys Ocean commenced. Around 130–111 Ma, the overall tectonic feature was a scissor-like closure of the Meso-Tethys Ocean from north to south.

A new fusulinid species from the early Permian of northwestern Peninsular Malaysia: Evidence of the Artinskian warming event in the eastern Peri-Gondwanan Sibumasu Block

A new species of Crenulosepta, i.e., Crenulosepta perlisensis Vachard and Fassihi sp. nov., is described from a Cisuralian (Lower Permian) sandy limestones and calcareous sandstones in the Kubang Pasu Formation in northwestern Peninsular Malaysia, which forms part of the Peri-Gondwanan Sibumasu Block. Previous Malaysian authors reported the occurrence of Monodiexodina shiptoni and Monodiexodina sutschanica from the same area. The findings of this study provide a better overview of the Cisuralian fusulinid fauna found in this region. The presence of Crenulosepta confirms that the age of the uppermost part of the Kubang Pasu Formation is Artinskian, rather than extending into the Kungurian as previously recognized. The uppermost part of the Kubang Pasu Formation with its impoverished genus and species diversity is located just above the cold water brachiopods and diamictite beds. It indicates that during the Asselian (earliest Cisuralian), the northwestern Peninsular Malaysia was still part of the Gondwana continental shelf. During the late Early Permian, northwestern Peninsular Malaysia, as part of the eastern Cimmerian Continent, was located in a low latitude subtropical region of the paleo-equatorial tropical Tethyan Realm. The microfacies analysis of the Kubang Pasu Formation suggests a very high-energy, warm shallow marine environment of the inner ramp (e.g., sand shoals and banks).

Record of the Indosinian Orogeny from conglomerates and detrital zircon U–Pb ages of the western Indochina Block, central Thailand

Middle Triassic to Early Jurassic conglomerates and associated sedimentary rocks that record the Indosinian Orogeny are exposed in the western Indochina Block, central Thailand. To elucidate the tectonic and sedimentary history of the Indosinian Orogeny, we studied the conglomerates and analyzed detrital zircon U–Pb ages of samples from the conglomerates and associated sedimentary rocks. On the basis of provenance analysis and estimation of depositional ages, the three recognized events (I, II, and III) of the Indosinian Orogeny are identified. The Indosinian I Event is represented by Type-1 limestone conglomerates, which were derived from collapse of the Permian carbonate platform. This event took place during the Norian (middle Late Triassic) and was triggered by collision between the Indochina and South China blocks. The Indosinian II Event is represented by Type-2 conglomerates, which are composed of various types of clasts, as well as basaltic sandstones. These sedimentary rocks were derived from the uplift and collapse of the Sukhothai Arc, the Nan Back-arc Basin, the Phetchabun Volcanic Belt, and basement rocks of the Indochina Block. Event Ⅱ occurred as a result of the closure of the Paleo-Tethys and the commencement of collision between the Sibumasu Block, the Sukhothai Arc, and the Indochina Block during the Rhaetian (latest Triassic). The Indosinian III Event is represented by Type-3 conglomerates, which comprise volcaniclastic rocks and volcanic breccias composed of andesite and dacite. Event Ⅲ occurred after the Toarcian (late Early Jurassic), associated with post-collision magmatism that occurred between the Sibumasu and Indochina blocks. Our study clarifies the timing and nature of Indosinian Orogeny I, II, and III events in the western Indochina Block and reveals that the Indosinian Orogeny involved Paleo-Tethyan subduction and micro-continental collision tectonics and associated processes during the Middle Triassic to Early Jurassic.

Carboniferous-Permian sequences of the western margin of Sibumasu Block: Implications for new lithostratigraphic correlation in the eastern Highlands of Myanmar

The Eastern Highlands of Myanmar are characterized by Precambrian to Mesozoic rock formations, with Carboniferous-Permian sequences prominently present in the Shan Scarp and Shan Plateau zones. These sequences are particularly concentrated in the western margin of the Sibumasu Block. Although Permian sequences are abundant in this region, they are complicated by geological faults. Consequently, a lithostratigraphic study was conducted to establish correlations across an extensive area spanning 1200 km from Mandalay City in the north to Kawthaung City in the south and 250 km wide, bordered by the Sagaing Fault to the west and Inle Lake to the east. The Permian sequences can be categorized into lower siliciclastic units and upper carbonate units. These siliciclastic units encompass the Mergui Group, locally referred to as the Lebyin Group, Mawchi Series, Taungnyo Group, and Mergui Group, as well as the Yinyaw Formation. Additionally, there is a carbonate unit known as the Moulmein Formation in the Thaton, Hpa-an, and Mawlamyine areas, and the Tenasserim Formation in the Myeik and Bokepyin areas within the Shan Scarp zone. In the late Lower Permian sequences, the Mwedaw Beds are exclusively found in the Linwe area. However, the Thitsipin Formation, which is an equivalent of the Moulmein Formation, and the Nwabangyi Formation are also exposed in the Shan Plateau zone. Lower Carboniferous sequences, such as the Loikaw Beds, are sporadically visible in the Loikaw area but are overlain by the Thitsipin Formation. This study elucidates the presence of a diamictite unit (Mergui Group) and its associated units (Yinyaw and Thitsipin/Moulmein Formations) in the geo-structurally complex zones of the Shan Plateau and Shan Scarp through lateral lithostratigraphic correlation, shedding light on the Carboniferous-Permian period at the western margin of the Sibumasu Block.

Early Permian passive-margin magmatism in the Sibumasu Block, northern Thailand: A product of the slab-pull force of the Paleo-Tethys Ocean?

Continental rift-related magmatism provides important clues to continental rifting and breakup, but the petrogenesis and geodynamic processes remain debatable. Here we report a detailed study on zircon and apatite UPb dating, whole-rock geochemistry, and Sr-Nd-Hf isotopes for the hornblende gabbros in northern Thailand. Zircon and apatite UPb age data reveal emplacement of the hornblende gabbros at ca. 290 Ma. The hornblende gabbros are alkaline in composition and characterized by slight negative Nb-Ta-Zr-Hf and marked negative Ti anomalies. They have enriched Sr-Nd-Hf isotopic values [whole-rock (87Sr/86Sr)i = 0.705970–0.706756; εNd(t) = +0.1 to +0.7; zircon εHf(t) = +0.2 to +9.2]. Their geochemical affinities to within-plate basalts suggest a continental rifting setting. These combined features reveal that the hornblende gabbros in northern Thailand originated from a subcontinental lithospheric mantle previously modified by subducted sediments. Our results combined with published data indicate that the continental rift-related magmatism in the Sibumasu Block took place over a long period of ca. 309–282 Ma. We suggest that the slab pull force of the Paleo-Tethys oceanic lithosphere is a likely mechanism for the Late Carboniferous-Early Permian magmatic event in the passive margin of the Sibumasu Block.

Ordovician geology of the Sibumasu Block, SE Asia

Abstract 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.

Paleogeographic Evolution of Southeast Asia: Geochemistry and Geochronology of the Katha-Gangaw Range, Northern Myanmar

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 &gt; 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 Timing of Collision Between Asia and the West Burma Terrane, and the Development of the Indo‐Burman Ranges

AbstractThe West Burma Terrane (WBT) is a small terrane bounded to the east by the Asian Sibumasu Block and to the west by the Indo‐Burman Ranges (IBR), the latter being an exhumed accretionary prism that formed during subduction of Indian oceanic lithosphere beneath Asia. Understanding the geological history of the WBT is important for reconstruction of the closure history of the Tethys Ocean and India‐Asia collision. Currently there are major discrepancies in the proposed timings of collision between the WBT with both India and Asia; whether the WBT collided with India or Asia first is debated, and proposed timings of collisions stretch from the Mesozoic to the Cenozoic. We undertook a multi‐technique provenance study involving petrography, detrital zircon U‐Pb and Hf analyses, rutile U‐Pb analyses and Sr‐Nd bulk rock analyses on sediments of the Central Myanmar Basins of the WBT. We determined that the first arrival of Asian material into the basin occurred after the earliest late Eocene and by the early Oligocene, thus placing a minimum constraint on the timing of WBT‐Asia collision. Our low temperature thermochronological study of the IBR records two periods of exhumation, in the early‐middle Eocene, and at the Oligo‐Miocene boundary. The Eocene event may be associated with the collision of the WBT with India. The later event at the Oligo‐Miocene boundary may be associated with changes in wedge dynamics resulting from increased sediment supply to the system; however a number of other possible causes provide equally plausible explanations for both events.

Newly discovered Early Carboniferous and Late Permian magmatic rocks in eastern Myanmar: Implications for the tectonic evolution of the eastern Paleo-Tethys

Eastern Myanmar is located at the junction of the Changning-Menglian and Chiang Rai-Chiang Mai zone and is a crucial region for constraining the evolution of the eastern Paleo-Tethys. This study presents new zircon U-Pb geochronological, mineral and whole-rock geochemical, and Sr-Nd-Hf-O isotopic data for magmatic rocks from the Sukhothai arc in eastern Myanmar. The rock suites analyzed include 360–355 Ma basaltic rocks and trondhjemitic dikes, and 257–254 Ma volcanic rocks and gabbroic cumulates. The basaltic rocks were derived from partial melting of mélange pair with peridotite and experienced assimilation and fractional crystallization (AFC). The trondhjemitic dikes were formed by partial melting of the basaltic rocks and experienced fractional crystallization at shallow depth. We suggest that andesite and dacite were derived from partial melting of depleted mantle wedge and underwent AFC process. The gabbroic cumulates are a crystallizing phase associated with the melts that produced the coeval volcanic rocks. We propose that eastern Myanmar, Central Tibet, SW Yunnan and Southeast Asia share a similar three-staged magmatic history, forming a ∼ 4000 km long magmatic belt. Stage I records the magmatic events related to subduction of the eastern Paleo-Tethys Ocean during the Early Carboniferous. A back-arc basin was opened during the Late Carboniferous, and extensive subduction-related magmatism was followed since the Permian. Stage II records the igneous rocks formed during the final amalgamation between the Indochina and Sibumasu Blocks during the Late Permian to Middle Triassic. Stage III is defined by the post-collisional magmatism distributed across the suture zone during the Late Triassic.

Petrogenesis of Pre-Tertiary A-Type Granitoid in Jambi Area and its Implications of Rare Earth Element Potential on Main Range Sumatra Belt

Granitoid rocks are one of the main sources of rare earth elements (REE). This makes granitoid characterization become important in the early stages of REE exploration. Almost all granitoids in Indonesia have been mapped. However, more detailed granitoid studies in Indonesia are still focused on Bangka and Belitung granites (tin belt granite). In contrast to Bangka and Belitung granites, studies related to petrogenesis and granite characteristics on the mainland of Sumatra Island (Sumatra Main Range) are rarely done, such as granitoid in Jambi area. The aim of this study is to determine the characteristics of the Pre-Tertiary granitoids located in the Tigapuluh and Duabelas Mountains, Jambi. The Tanjungjabung Barat granitoid represent the Tigapuluh Mountains area while the Sarolangun granitoid represent the Duabelas Mountains area. These two granitoids interpreted to be Triassic to Jurassic in age. Granitoid characteristics include petrological and geochemical characters. This study also focuses on the petrogenesis of Pre-Tertiary granitoid and its implications for the abundance of REEs. Megascopic observation, petrographic, and geochemical analysis are done in this study. Geochemical analysis was done at the Center of Geological Survey Laboratory, Bandung using the ICP-MS Thermo Icap-Q and XRF ADVANT XP Thermo ARL9900 instruments. Based on megascopic and petrographic observations, both of the granitoids are classified as granite. Geochemically, these two granitoids show the character of A-type granite which is formed in the post-collision environment, and derived from the crustal melting with ferrous alkalic to alkali-calcic peraluminous affinities. This crustal melting happened due to the collision of the Sibumasu Block with Indochina resulting in crustal thickening and crustal melting. The magma then contaminated effectively in the rift environment due to the subduction roll-back of Meso-Tethys in the Late Triassic. Subduction in the West Sumatra also play roles in the genesis and it is shown by the geochemical character of the Sarolangun granitoid. Effective contamination derives the characteristics of A-type granite so that the REE content in both granites are abundant. The abundance of REE is indicated by the presence of the allanite, monazite, apatite, zircon, and titanite. The REE concentration of the Sarolangun granitoid reaches 330 ppm, while the Tanjungjabung Barat granitoid reaches 261 ppm. The REE concentrations of A-type granitoid in Jambi then compared with A-type granitoids from the world and showed relatively the same REE concentrations. The REE concentrations of these granitoids are also higher than the other type granitoids in Indonesia. However, the REE concentrations of Jambi granitoids are similar to the fractionated S-type granite in Bangka. With a recent study showing the presence of A-type granitoid in Sarudik (North Sumatra) and Bukit Batu (South Sumatra), the A-type granitoid in this study indicates the existence of A-type granitoid belt in the Sumatra Main Range. This belt will have a high abundance of REE concentrations and potentially become the source for REE deposits. The author hopes that this study could improve the understanding of tectonic in Sumatra and suggestion for REE exploration in the area.

Petrogenesis of Late Carboniferous–Early Permian mafic igneous series in the Baoshan block: Implications to birth of Neo-Tethys and generation of magmatic sulfide deposit

The petrogenesis of the rift-related Late Carboniferous–Early Permian mafic igneous rocks (basaltic lavas, dolerite dykes and mafic–ultramafic intrusive rocks) from the Sibumasu block in the eastern end of Cimmeria was still ambiguous. This petrogenesis is related to understand the drive for the formation of Neo-Tethys which was developed along the margin of Cimmeria, and to the generation of magmatic sulfide deposit via partial melting of previously subduction-modified mantle induced by rifting of continental margin. Most of the mafic rock samples belong to the subalkaline series, with lithologies varying from basalt to basaltic andesite. Magmatic zircons from the doleritic dykes yield weighted mean 206Pb/238U ages of 308.7 ± 2.5 Ma. The εHf(t) values of zircon are ranging from 3.2 to 11.8, which are much lower than those of the coeval depleted mantle (εHf(t) ≈ 16 at 305 Ma). The igneous series share similar features, such as elevated light rare earth elements, negative Nb-Ta anomalies and enriched Sr-Nd-Hf isotopes. Based on geochemical analyses of the mafic igneous series in the northern part of Sibumasu, we suggest that the primary melts of the mafic igneous rocks were derived from a fertile subcontinental lithospheric mantle (SCLM) which was previously modified by oceanic subduction, and these rocks were intruded and erupted in a passive plate margin during plate extension. The subsequent melting-induced softening of SCLM triggered the evolution of continental margin from the rift to the open of Neo-Tethys Ocean. The proposed model emphasizes that the melt generated from previously-modified SCLM in a continental rift setting is beneficial for the generation of magmatic sulfide deposit.

A late Cisuralian (early Permian) brachiopod fauna from the Taungnyo Group in the Zwekabin Range, eastern Myanmar and its biostratigraphic, paleobiogeographic, and tectonic implications

AbstractThe 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.

A newly discovered Late Cretaceous metamorphic belt along the active continental margin of the Neo-Tethys ocean

Abstract High-grade metamorphic rocks and crustal melts provide crucial evidence for growth and differentiation of the continental crust, and are widespread in collisional orogens. However, their importance in the evolution of continental arcs remains poorly understood. Metamorphism and related anatexis in the preserved continental margin of the Neo-Tethys ocean serves as a key natural laboratory to investigate this process. Along the Neo-Tethyan arc margin, the Gaoligong shear zone, Yunnan region of China, is an important locality for linking Lhasa in the north with Sibumasu and Burma in the south. Here, Late Cretaceous granulite-facies metamorphism and crustal anatexis have been identified for the first time in the Gaoligong area. Zircon and monazite U-Pb dating indicates that S-type granites formed at 87–73 Ma, granites and buried pelitic sediments were simultaneously metamorphosed at 75–70 Ma during Neo-Tethyan subduction, and all lithologies were overprinted by a younger 40–30 Ma magmatic and strike-slip event related to India-Asia collision. Phase equilibria modeling of high-grade anatectic gneiss in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system indicates peak pressure-temperature (P–T) conditions of 780–800 °C and 6.5–7.5 kbar and defines a cooling and decompressional P–T path for the metapelites. This demonstrates that sediments within the Neo-Tethyan active continental arc were buried to &amp;gt;20 km depth at 75–70 Ma. In combination with the metamorphic record of the Lhasa, Burma, and Sibumasu blocks, an extensive Late Cretaceous metamorphic belt must have formed along the Neo-Tethyan subduction zone. This spatially correlates with coeval gabbro-diorite suites exposed in the Gangdese, Sibumasu and Burma terranes that were triggered by thinning of the lithospheric mantle. This prolonged Late Cretaceous mantle-derived magmatism and lithospheric thinning may have provided a regional-scale heat source for high-grade metamorphism and crustal anatexis along the active continental margin of the Neo-Tethys ocean.

Dextral transpressional shearing and strike-slip partitioning developments in the Central Myanmar Basin during the collision between the India Plate and West Myanmar Block

Central Myanmar Basin (CMB) lies between the two major strike-slip faults: Sagaing Fault to the east and Kabaw Fault to the west. Central Volcanic Line of Myanmar emplaced en-echelon array of P-shears along the fault zone, which was active dextral as surrounding major faults at a low angle about 15° to the shear zone boundary. CMB was developed as strike-slip partitioned transpression model as proposed in this study. The transprsession tectonic is common with oblique convergence subduction as well as collision of India Plate to West Myanmar Block, especially effects on the CMB. Structural evidence are occurred as en-echelon folding and fracturing by satellite photos, field observation and 3D strain ellipsoid construction showing plane strain by pure shear upon ideal strike-slip with simple shear. In this study, two types of transpression distinguished as homogeneous and strike-slip partitioning which are developed within the frame of the strike-slip fault with progressive deformation. Under the condition of the strike-slip partitioned transpression, strain partitioning is distinctly occurred together with progressive deformation and reactivation of pre-existing structures such as cylindrical fold to double plunging fold, normal fault to oblique slip fault and positive flower structure. Deformation process related to the India Plate movement deduced the period of Late Miocene and Pliocene-Pleistocene based on the structural analysis in Rakhine Coastal region, western Myanmar. During the period between the Late Miocene to Pliocene-Pleistocene, pre-existing ophiolite zone in the Indo-Myanmar Ranges (IMR) was split up different zones by the Sagaing Fault displacement, accompanied to the West Myanmar Block movement with India Plate subduction. Accordance with the Sagaing Fault displacement and group of mud volcanoes shifting, the CMB moved northward about 450km with displacement rate 5–7cm per annum by the 45–50°convergent angle of India Plate collision to Eurasia Plate. Transpression deformation of northward compression was developed in the CMB, simultaneously on the other side transtension as pull-apart basin was formed by Andaman Sea opening at the south on the Sagaing Fault. During the Miocene time, collision of India Plate to Eurasia Plate was principal and caused the deformation at northern border the India, in which India Plate moved both in northwards and rotated anti-clockwise, whereas West Myanmar Block together with Sibumasu Block rotated clockwise becoming final closure of the northeasternmost Assam and Myanmar regions. During the Oligocene time, deposition of the shallow marine Barail formation in the Bengal Basin and shallow marine to continental shelf sequences in the CMB started as twin trough. This study provides the transpression deformation in the CMB and related regions during syn- and post-collision of India Plate to West Myanmar Block with position and kinematic structural interpretation.

Chau Thoi and Nui Gio volcanic rocks: A remnant of the Loei Phetchabun volcanic belt in Viet Nam

Introduction: Extrusive volcanic rocks, such as dacite, andesite, basalto-andesite, basalt… of Chau Thoi and Nui Gio hills in Bien Hoa and Binh Phuoc provinces, southern Viet Nam, characterize volcanic island arc rocks. These rock suites formed as the convergent tectonic between the Indochina and Sibumasu geological blocks.&#x0D; Methods: Geochemical data of rock samples collected on the field were examined and analyzed by the Academia Sinica I E S (Institute of Earth Science, Taiwan and processed with a GCD kit (Geochemical Data Toolkit) to ascertain their characteristics and geotectonic setting. Result: Geochemical data both in major elements and trace elements of the Chau Thoi – Nui Gioshow a specific characteristic of a volcanic island arc environment.&#x0D; Discussion: Chau Thoi and Nui Gio rocks are suitable to correlate to the Permian Thailand Loei Phetchabun volcanic belt. However, at the current time, Chau Thoi and Nui Gio rocks have been classified as Deo Bao Loc formation - late Jurassic to early Cretaceous in ages - belong Truong Son magmatic belt. This magmatic belt resulted from the Yanshanian orogeny by the subduction of the Paleo-Pacific oceanic plate beneath the Eurasia (Indochina) continental plate. More studies needed to be performed, specially geochronological data to support the study.&#x0D; Conclusion: Chau Thoi and Nui Gio rock suites characterize volcanic island arc rocks, products of a convergence tectonic between Indochina and Sibumasu blocks. They are remnants of the Thailand Loei Phetchabun volcanic belt, the first time reported in Vietnam.

First occurrence of brachyopid temnospondyls in Southeast Asia and review of the Mesozoic amphibians from Thailand

Abstract. The non-marine Mesozoic sedimentary rocks of Thailand, which consist of the Indochina block and the Sibumasu block, have yielded several terrestrial and aquatic vertebrate fossils, but only few amphibian remains have been reported. Here, we present an overview on the Thai amphibian palaeo-diversity based on the literature, re-examination of published material, new findings, and unpublished material. Thai amphibian fossil remains are assigned to Stereospondyli (Cyclotosauridae, Plagiosauridae, and Brachyopoidea) and Anura and were discovered from four formations, ranging from the Upper Triassic to Lower Cretaceous of Thailand. The occurrence of Brachyopidae in Thailand, which are related to Chinese forms, supports the previous hypothesis of physical connections between the Indochina blocks and the Sibumasu block during the Mesozoic era.

Revisiting the tectonic evolution of the Triassic Palaeo-Tethys convergence zone in northern Thailand inferred from detrital zircon U–Pb ages

AbstractDetrital zircon U–Pb ages for sediments in and around the Palaeo-Tethyan convergence zone in northern Thailand provide constraints for tectonic interpretations of the Indochina Block, the Sibumasu Block, the Inthanon Zone accretionary complex and the Nan Back-arc Basin during the Triassic. In sedimentary rocks of the Indochina Block, almost all of the Palaeozoic and Triassic zircons were sourced from the collision zone between the Indochina and South China blocks, and an active continental margin in the western Indochina Block. Sediments of the Sibumasu Block were supplied by erosion of Archaean basement and from the Grenville and the Pan African orogenies, but show no record of Permian to Triassic igneous activity. Accretionary complex sediments have provenances of both the Sukhothai Arc and the Indochina and South China blocks, with detrital zircons of various ages being supplied from crustal uplift and erosion related to the Indosinian I orogeny. Sedimentary rocks of the Nan Back-arc Basin are widely distributed not only in the Nan–Uttaradit but also in northern Sukhothai areas. The origin of the Pha Som Metamorphic Complex and associated formations can be traced to basin-filling sediments in the Nan Back-arc Basin. These detrital zircon U–Pb ages have also allowed identification of the changing tectonic setting in the Palaeo-Tethys convergence zone from the ‘erosion of Proterozoic continental basement’ to ‘Palaeozoic active continental margin in the western Indochina Block’ and ‘Palaeozoic, Permian to Triassic collision zone between the South China and Indochina blocks’ through to ‘Triassic active Sukhothai Arc’.

An Overview of the Tectonic Evolution of the Indochina block and Granitoid Emplacement, particularly in the central and south Vietnam

Introduction: Vietnam is mainly located within the Indochina block in Southeast Asia. Asmall northern part of Vietnam belongs to the South China block, the southwest part liesadjacent to the Sibumasu block and opens to the East Sea on the east side. Tectonicactivities in Vietnam were very complicated they relate to intense interactions betweenmany geological blocks at different times. Magmatic emplacement is the final and instantproduct of tectonic activities.&#x0D; Methods: Geochemical data analysis from rock samples withinVietnam collected by other researchers has been reused in the scope of this study to verifythe relation between tectonic evolutions and their granitic magmatism. GCD (GeochemicalData Toolkit), an R language program for handling and recalculation of geochemical data.&#x0D; Results: Geochronology and geotectonic model derived from rock analysis have beenascertained main tectonic evolutions of the Indochina. The current granitoidclassification in Vietnam mostly based on petrographical studies. The Nui Cam granitoid isbeing classified as Deo Ca, Dinh Quan granitoid. However, based on trace elements, they aredifferent. They may belong to different granitoid system.&#x0D; Conclusion: Major tectonic eventswithin the Indochina block are well supported by the nature of granitoid emplacements. Petrological studies of these magmatic rocks would bring out valuable information toconfirm and clearly understand the tectonic evolutions of the region. Igneous rocksclassification must based on tectonic fundamental instead of petrographical studies.

Middle Permian fusulines from the Thitsipin Formation of Shan State, Myanmar and their palaeobiogeographical and palaeogeographical implications

AbstractFusuline faunas including 29 species belonging to 19 genera/subgenera are described from the Thitsipin Formation in eight sections/localities on the Shan Plateau in eastern Myanmar. These fusulines broadly indicate a Midian (middle Permian, Guadalupian) age. The lower diversity and the presence of some genera, such as Monodiexodina and Eopolydiexodina, suggest that the Sibumasu Block belonged to the palaeobiogeographic Cimmerian Province during the Midian. Furthermore, quantitative cluster analysis of middle Permian fusulines from the Shan Plateau and adjacent Cimmerian blocks suggests close faunal affinities between the Sibumasu Block and the Baoshan Block in western Yunnan. More importantly, the widespread occurrence of fusulines Eopolydiexodina afghanensis and Jinzhangia shengi on the Shan Plateau is similar to that of contemporaneous fusuline faunas from the Baoshan Block and the South Qiangtang Block but different to that of the Tengchong and Lhasa Blocks, which are dominated by the characteristic Nankinella–Chusenella assemblage. This faunal discrepancy provides strong evidence that the Bangong–Nujiang suture passes through the Gaoligong Orogen in western Yunnan rather than through the Myitkyina ophiolites in northern Myanmar. Additionally, palaeobiogeographical analysis of these fusuline faunas from the Lhasa, Tengchong, South Qiangtang, Baoshan and Sibumasu Blocks implies that the Bangong–Nujiang Ocean might have been present before the Midian.