Sibumasu Research Articles

New insights into the petrogenesis of granitic rocks in southern Thailand, SE Asia tin belt: evidence from petrochemistry and geochronology

ABSTRACT The granitic rocks in the Prachuap Khiri Khan Province, southern Thailand, SE Asia Tin Belt, comprise gneissic biotite monzogranite, porphyritic muscovite–biotite monzogranite, and porphyritic biotite monzogranite. Comprehensive field observations, petrography, mineral chemistry, and geochemistry establish gneissic biotite monzogranites as part of the Central Belt Granites (CBGs), while porphyritic muscovite–biotite monzogranite and porphyritic biotite monzogranite as part of the Western Belt Granites (WBGs). CBG and WBG granitic units exhibit peraluminous, alkali-calcic, and magnesian magmatic series traits, reflecting an S-type granite character resulting from crustal partial melting. Significantly, scatter correlations in variation diagrams, combined with the enrichment of Cs, Rb, Th, U, Pb, Nd, and P, and the depletion of Ba, Nb, and Ti contents, point towards an S-type granite signature formed during collisional events. Crystallization conditions reveal CBG gneissic biotite monzogranite encountered P–T conditions of 2.97–3.85 kbar/680°C–733°C/11.1–14.4 km, while WBG’s porphyritic muscovite–biotite monzogranite and porphyritic biotite monzogranite experienced 2.73–4.15 kbar/628°C–732°C/10.2–15.5 km and 2.47–3.80 kbar/651°C–728°C/9.2–14.2 km, respectively. Geochronological data indicate that CBG’s gneissic biotite monzogranite was emplaced from Early Cretaceous collision (128–119 Ma) of Indochina and Sibumasu terranes, while WBG’s porphyritic muscovite–biotite monzogranite and porphyritic biotite monzogranite were formed during the syn-collision of Sibumasu and West Burma terranes during Early Cretaceous (121–111 Ma) to Paleocene. Mineralogical and geochemical attributes of these granitic rocks exhibit strong relevance to Sn-W mineralization processes, indicative of a profound association.

Late Paleozoic magmatism from northern Sibumasu terrane: Constraints on separation of Cimmeria from Gondwana

The Cimmerian is one of several continental terranes that separated from northern Gondwana during the Carboniferous-Permian period. Magmatic evidence for the rifting of the fragments from Gondwana is preserved in the Baoshan area of eastern Cimmeria. These magmatic rocks outcrop over an area of more than 10,000 km2 and are mainly composed of basalts and dolerites, with limited occurrences of picrites. Our new results show that the basalts are tholeiitic and exhibit enriched trends in light rare earth elements (LREE), negative anomalies in Nb-Ta-Ti, and enriched isotopic signatures in Sr-Nd-Pb. They underwent fractional crystallization of clinopyroxene and olivine, with insignificant crustal contamination. Suppositional primary melt compositions and REE modeling results suggest that they were mainly formed by 2–5 % degrees of melting of spinel lherzolites with minor garnet (<1%) in a shallow enriched EMII lithospheric mantle, with minor components from the depleted mantle. Lack of plume-related ocean island basalts, a narrow south-north distribution, and a relatively low melting temperature may contradict the plume model. The Woniusi basalts could have originated in a back-arc environment, but further research is required to gather a more complete spectrum of volcanic rocks associated with the arc. In contrast, we propose that they were formed on the lower-plate passive Gondwana margin after the northward subduction of the Paleo-Tethys Ocean since the Late Devonian. This subduction may result in a longer-lasting extension of the passive Gondwana margin. The Woniusi basalts are roughly coeval with the subduction of the mid-ocean ridge during the Latest Carboniferous. After the active ridge has subducted, the slab pull is strong enough to separate a terrane, because the ridge spreading no longer absorbs the strain caused by subduction. If this is the case, subduction of the Paleo-Tethys ocean-ridge would enhance slab-pull forces and finally trigger Woniusi magmatism.

Tectonic evolution of the Ailaoshan–Red River shear zone and its relationship with the growth of the southeastern Tibetan Plateau

The Ailaoshan–Red River shear zone (ARSZ), which is ∼ 1000 km long and one of the largest boundary strike-slip fault zones in the southeastern margin of the Tibetan Plateau, provides valuable insights into the growth process of the southeastern Tibetan Plateau since the India-Asia collision. During the Late Miocene to Early Pliocene, the ARSZ underwent a kinematic transition from left-lateral strike-slip to right-lateral strike-slip motion. To reveal the mechanism of this kinematic transition and its relationship with the growth process of the southeastern Tibetan Plateau during the Cenozoic, we conducted paleomagnetic and tectonic deformation analyses on both sides of the ARSZ. The results show that the formation of the S-shaped bending and kinematic transition of the ARSZ were controlled by the changes in clockwise rotation rates of the Chuandian Fragment and the Indochina Block. This suggests that since the Oligocene, the growth of the southeastern Tibetan Plateau progressively extended outward from the southern margin of the Lhasa Terrane to the southeastern margin of the Tibetan Plateau, resulting in the sequential initiation of the clockwise rotation of the Shan-Thai Block, Indochina Block, and Chuandian Fragment since the Late Oligocene, and in variations in the rotational rates of these blocks. The different rotational movement of these blocks controlled the tectonic evolution of strike-slip fault systems on the southeastern margin of the Tibetan Plateau.

Tectonic and metallogenic implications of W-Sn related granitoid rocks in the Kawthaung-Bankachon area, southernmost part of Myanmar: Constraints from petrology, geochemistry, and U-Pb zircon geochronology

Granitoid rocks are widely distributed along the N-S trending tectonic belts encompassing Myanmar, Thailand, Peninsular Malaysia, and Indonesia and these granitoid rocks are host to the world's largest deposits of tin-tungsten, in terms of both production and reserves. The investigated study area belongs to the southernmost part of the Central Cretaceous-Eocene granitoid belt of SE Asia in the border region of Myanmar and Thailand within the Sibumasu or Shan-Thai Terrane. The granitoid units in the area are biotite granite, muscovite-biotite granite, porphyritic biotite granite, and diorite which occur locally as dykes. All granitic rocks in the area fall are recognized in High-K calc-alkaline series and show a high abundance of Cs, Rb, U, Th, K, Pb, Nd, Zr, and Sn. Based on the major oxide composition and tectonic discrimination diagram, all granitoid rocks in the area fall in the peraluminous, S-type granite category and have formed within the Post-Orogenic Granite and Post-Collisional Granite fields. Radiometric dating by LA ICP-MS U-Pb zircon method yielded the ages of 78.97 ± 0.63 Ma (biotite granite), 79.59 ± 0.76 Ma (muscovite-biotite granite), and 79.14 ± 0.91 Ma (porphyritic biotite granite) and 49.48 ± 0.83 Ma for the biotite granite from the Yadanabon W-Sn deposit. According to the zircon U-Pb ages, the timing of the granitoid intrusions of the Kawthaung-Bankachon area is the Campanian (Late Cretaceous) to Eocene. The magmatism and W-Sn mineralization in the area is attributable to Indian Ocean subduction and collision between the west Myanmar Terrane and the Sibumasu Terrane.

Zircon U-Pb geochronology of the Lan Sang gneisses and its tectonic implications for the Mae Ping shear zone, NW Thailand

The Mae Ping shear zone (MPSZ), a major shear zone trending NW-SE in Thailand, is responsible for the left-lateral displacement of the N-S Triassic-Jurassic granitoid and gneiss belt. This displacement is postulated to have contributed to Cenozoic extrusion tectonics. Within the Lan Sang National Park, the MPSZ exposes several intensely deformed lithologies, collectively known as the Lan Sang gneisses. These gneisses have attracted considerable attention for their potential to substantiate the extrusion model. However, the timing of the emplacement of the orthogneiss protolith is still debated. Moreover, the origin and distribution of the Eocene syn-shearing granodiorite within this shear zone are not well understood. To shed light on the magmatic history of the MPSZ, this study utilized zircon U-Pb geochronology to systematically investigate the Lan Sang gneisses. Our findings demonstrate that these gneisses can be categorized into paragneiss and orthogneiss groups. Paragneiss samples feature zircons displaying rounded detrital cores ranging from 3,078 to 450 Ma, with metamorphic rim overgrowth of ca. 200 Ma (most Th/U &amp;lt;0.01). This indicates that their Paleozoic sedimentary protoliths experienced high-grade metamorphism during the Triassic-Jurassic Indosinian orogeny. On the other hand, zircon from orthogneiss samples shows that their magmatic protoliths were predominantly emplaced either around ∼200 Ma or within 45-32 Ma. The Eocene-Oligocene magmatism likely coincided with the proposed Eocene metamorphism. Since these samples were deformed by left-lateral shearing, the left-lateral motion of the MPSZ probably ended after 32 Ma. Eocene-Oligocene magmatic events have also been identified in granite, mylonite, and gneiss samples from other regions along the Sibumasu terrane, including the Three Pagodas, Klaeng, Ranong, Khlong Marui shear zones, and the Doi Inthanon area. The Eocene-Oligocene magmatism was likely linked with the movement of the shear zones and may be responsible for the regional cooling pattern. The spatial and temporal distribution of the Eocene-Oligocene magmatism within the Sibumasu terrane supports the hypothesis that the inward migration of magmatism in the overriding plate resulted from the shallowing of the Neo-Tethyan slab.

Fracture Characterization Explores Potential of Phra Wihan Formation, Thailand

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 22845, “Fracture Characterization and Basement Reservoir Potential of Phra Wihan Formation in Southern Part of Uttaradit Province, Northern Thailand,” by Watcharapong Piewmoh, SPE, PTTEP, and Pitsanupong Kanjanapayont, Chulalongkorn University. The paper has not been peer reviewed. Copyright 2023 International Petroleum Technology Conference. Reproduced by permission. _ The complete paper describes the characterization of fractures in the Mesozoic sandstones of the Phra Wihan Formation in northern Thailand. Fracture characterization of outcrop analogs is important to determine the potential of subsurface reservoirs. The methodology used field study and petrography analysis to determine the relationship of fractures and other structures such as bedding planes and fold geometry. In addition, microstructure analysis can assist in the evaluation of a reservoir’s potential. Introduction Basement reservoir sandstones are one of the main targets in a mature onshore oil field in Thailand. But most basement reservoirs of the onshore Tertiary basin are pre-Tertiary and are associated with low quality. Prediction of reservoir quality in low-porosity and -permeability conditions requires an understanding of fractures in the subsurface. Outcrop analogs are key in characterizing fractured reservoirs. Geological Setting The tectonic evolution of Thailand is detailed in the complete paper. The country’s primary geological provinces are subdivided into the following five components, from west to east: - Sibumasu terrane - Inthanon zone - Sukhothai terrane - Nan and Sra Keao sutures - Indochina terrane The Sibumasu and Indochina terranes are major continental masses that collided during the Devonian-Triassic. Two subparallel suture zones exist along the convergence between the Sibumasu and Indochina terranes; one is an accretionary complex defined by the Inthanon zone. Another is a closed back-arc basin defined by the Nan and Sra Keao sutures. The Sukhothai terrane is an island-arc system formed along the margin of the Indochina terrane.

Coupled sedimentary and δ13C records of western Thailand and South China from Middle to Late Ordovician: Sea-level and climate changes prior to the Hirnantian glaciation

The Middle to Late Ordovician sedimentary records are of great interest because they potentially contain critical signatures about triggers of the Great Ordovician Biodiversification Event and the Late Ordovician Mass Extinction. A long climate cooling has been proposed by some scholars, which may have initiated in Middle Ordovician and eventually culminated in the Late Ordovician (Hirnantian) glaciation; however, the origin, frequency, magnitude and duration of this cooling-glaciation event are still in debate. A sedimentary and multi-stratigraphic study of the Middle to Late Ordovician transition records may help reveal the evolutionary history of this great climate change. Here we document and correlate the sedimentary facies, conodont biostratigraphy and carbon isotope chemostratigraphy of latest Darriwilian through early Katian limestone successions in the Kanchanaburi area of western Thailand and in South China. This correlation reveals two remarkably cross-regional transgression events and their following sea-level highstand periods. The first transgression happened in the latest Darriwilian, leading to the deposition of the Tha Manao Formation of restricted carbonate facies at the Nautiloid Site Geopark in Thailand, Sibumasu Terrane, and the Miaopo and chert-rich Hulo formations of black shale facies in South China. The second transgression was initiated in the earliest Katian, forming the extensively distributed open-marine carbonates of the Pa Kae and Pagoda formations in platform settings, and the debrite-rich, deep-water carbonate of the Yenwashan Formation in slope settings. We also document coupled changes in depositional facies and carbon isotopic compositions, showing that the rising limb of the Guttenberg Isotopic Carbon Excursion (GICE) co-occurred with the second transgression period. The South China and Thailand Sibumasu records are comparable to those of many North American successions, indicating a presumably global sea-level rise in the early Katian, which may have been caused by the waning of the potential Darriwilian–Sandbian icecaps.

Lower Permian (Late Kungurian) conodonts from the Sibumasu Terrane, Malaysia: paleoecological, paleobiogeographical and tectonic implications

AbstractLate Kungurian (Lower Permian) conodonts are described from the Kanthan Limestone, Perak, Peninsular Malaysia and for the first time from the Sibumasu Terrane of the Malay Peninsula. The co-occurrence of Gullodus duani, Gullodus hemicircularis, Gullodus sicilianus, Mesogondolella lamberti and Mesogondolella siciliensis represent the Mesogondolella lamberti International Conodont Zone and the broadly equivalent Mesogondolella siciliensis Regional Conodont Zone. A small fault-bounded basal Pennsylvanian (basal Bashkirian) conodont fauna including Gnathodus girtyi simplex and Declinognathodus inaequalis is also reported. The late Kungurian conodonts from the Kanthan Limestone were deposited in a relatively deep-water environment on the northern passive margin of the Sibumasu Terrane of the eastern Cimmerian Continent located at c. 35oS latitude. Biogeographically, the fauna represents the southern peri-Gondwana Cool Water Province which is consistent with its palaeogeographic location. A new scheme, utilizing characteristics of P1 elements, including position of the 1st denticle, location of 2nd and 3rd denticles, platform shape, platform cross-section, denticle shape in cross-section, and lateral denticle development is proposed for distinguishing between species of the hindeodid genera Gullodus, Hindeodus and Isarcicella. The late Kungurian fauna from the Kanthan Limestone represents the southern peri-Gondwana Cool Water Province supporting palaeogeographic reconstructions placing the Sibumasu Terrane in moderate southern palaeolatitudes in the Kungurian.

Variation in thermal structure with crustal thickness for the crust beneath the Peninsular Malaysia

We present a regional thermal structure as well as a new crustal thickness model beneath the Peninsular Malaysia and the surrounding regions. Curie point depth estimates for the entire area range between ∼ 17 – 46 km with a mean of 29 km. The estimated crustal thickness for the study area varies from ∼ 28 – 35 km with an average of ∼ 31 km. Uplifted Curie depths (20 – 25 km) and deeper Moho depths (30 – 34 km) are observed over most parts of Peninsular Malaysia continent. In contrast, the NW Peninsular Malaysia continent and the southern Thailand are characterized by deeper Curie depths between 30 and 40 km. These regions with deeper Curie depths are coincident with the oldest dated rocks on the Sibumasu terrane. The observation of larger Curie depths in NW Peninsular Malaysia implies that the region is presently thermally stable than the remaining parts of the Peninsular. Consistent with deeper crustal thickness (30 – 35 km), the west Sumatra block yields the deepest Curie point depth that ranges between 30 and 46 km. The west Sumatra block and the NW Peninsular Malaysia have low Bouguer anomalies and comparable crustal thickness indicating similarity in regional features. This suggests that these regions are linked by a common ancient continental core. A comparison between the Curie depths and crustal thickness shows that the upper mantle beneath the Island of Sumatra, Singapore, Malay basin, NW Peninsular Malaysia continent, and southern Thailand are significantly magnetized.

Paleomagnetic constraint on the formation of the Eastern Himalayan Syntaxis: A new late Eocene result from the Mangkang area of the eastern Tibetan Plateau

The Eastern Himalayan Syntaxis (EHS) is a major structural and geomorphological unit in Eastern Tibet; however, its mechanism of formation and the extent of its northward effect of compression remain unclear. To address these questions, we conducted a paleomagnetic and geochronologic study of the late Eocene trachyte intrusive mass in the Mangkang area of Tibet. The U-Pb SHRIMP II age estimate of the trachyte intrusive mass is ∼40 Ma, and the estimated mean paleomagnetic direction we obtained is D=27.0°, I=36.8°, k=42.9, α95=1.3°, and n=282 specimens. These results indicate that the Mangkang area was located at a paleolatitude of 20.5 ± 0.9° N during the late Eocene. A comparison of paleolatitudes of the Qiangtang block with those inferred from the apparent polar wander paths (APWP) of India and Eurasia suggests that the Eastern Himalayan Syntaxis was formed after 35 Ma when Eurasia was penetrated by ∼600 km by northward-moving India. This is consistent with geologic evidence of intra-continental shortening in the foreland of the Eastern Himalayan syntaxis. The northward counterclockwise penetration of the corner of India also resulted in the large right-lateral strike-slip system of the Sagaing fault (SGF), with ∼400-600 km of displacement between the EHS and the Shan-Thai block. Thus, the northward wedging of the EHS indicates minimal southward displacement of the Shan-Thai block. Our results link the tectonic activity of the EHS to the lateral extrusion of the southeastern margin of the Tibetan Plateau, providing a new perspective on the evolution of this region since the late Eocene.

Two-layer model of anisotropy beneath Myanmar and Thailand revealed by shear-wave splitting

The first model of two layers is presented to study the anisotropy pattern beneath Myanmar and Thailand using shear-wave splitting. Teleseismic activity recorded by 15 permanent broadband stations was analysed to investigate the anisotropy and to understand the flow direction in the mantle. The flow direction and speed were observed in the forms of fast polarisation direction (𝜙) and delay time (𝛿𝑡) between fast and slow components. The measurements showed that a two-layer model beneath stations better explicates the splitting observations than a single-layer model. The upper and lower layers were interpreted as lithosphere and asthenosphere in similar patterns and compared with GPS (Global Positioning System) velocity fields and strain rate fields. Two groups of 𝜙 can be classified and matched with West-Burma Terrane (WBT) and Shan-Thai Terrane (STT). The 𝜙 represents that West-Burma Terrane moves in a northward direction, Shan-Thai Terrane and Indo-China Terrane (ICT) move in a south-eastern direction, and West-Burma Terrane has less anisotropy of 𝜙 than Shan-Thai Terrane.

Petrology, geothermobarometry and geochemistry of granulite facies wall rocks and hosting gneiss of gemstone deposits from the Mogok area (Myanmar)

The Mogok Metamorphic Belt (MMB) of Myanmar formed during the Paleogene collision between the West Burma block and the Shan-Thai block. The MMB is mainly composed of medium to high-grade metamorphic marble, calc-silicate rocks, gneiss, quartzite, peridotite and igneous rocks such as granite, syenite and gabbro. The Mogok area in the central part of the MMB is well-known for magnificent quality ruby, spinel, sapphire, and peridot. To unravel the metamorphic PT-conditions prevailing during the formation of spinel and ruby from primary marble deposits in the Mogok area, three different types of high-grade quartz-garnet gneiss from the neighbourhood of gemstone mines were investigated by electron microprobe. Geothermobarometry reveals granulite facies PT-conditions of 756–792 °C at 7.4–7.6 kbar, which is reproduced by Theriak-Domino modelling within the error of both methods at water activities of 0.34–0.4. Shoshonitic and high-K calc-alkaline mafic dykes occur within marble forming conspicuous garnet-nepheline and clinopyroxene-clinoamphibole gneiss. Petrologic and geochemical investigations of these metadykes verify their granulite facies metamorphism and classify them as subduction-related magmatic rocks, which intruded the marble sequences. These investigations as well as previous studies show that spinel and ruby in marble of the Mogok area may have formed not only by metasomatism around alkaline intrusions, but also by granulite facies regional metamorphism.

Petrogenesis of the Early Jurassic Longtang and Menglong Peraluminous Granites in Tengchong Terrane, and their Tectonic Implication

AbstractA comprehensive study of zircon U‐Pb geochronology, in situ Hf isotopes, whole‐rock major and trace element geochemistry, and Nd isotopes was carried out for two Early Jurassic two‐mica granites (Longtang and Menglong) in the southern part of the Tengchong terrane, which is in the northern part of the larger Sibumasu terrane. We assess the origin of the granites and explore their possible genetic relationship to the Paleo‐Tethyan regime. LA‐ICP‐MS zircon U‐Pb dating shows that they were simultaneously emplaced in the Early Jurassic (ca. 199 Ma). They have SiO2 contents of 69.7–75.1 wt% and are mainly strongly peraluminous with alumina saturation index (ASI) values ranging from 1.06 to 1.46. They show similar Mg# (0.29–0.42) to experimental partial melts of metasedimentary rocks under continental pressure‐temperature (P‐T) conditions. They are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with moderately negative Eu anomalies and flat HREEs patterns. They show negative εNd(t) values (–9.0 to –12.4) and εHf(t) values (–8.0 to –9.1). Elemental and isotopic data suggest that they most likely to formed by muscovite‐dehydration melting of a metapelitic source at lower temperatures in the range of 700°C to 750°C. The granites might represent a post‐collisional tectonic setting response to Paleo‐Tethyan regime.

Magnetostratigraphic Evidence From the Gengma Basin for the Fast Response of the Crustal Rotation of the Northern Part of the Shan‐Thai Block to the India‐Eurasia Convergence

AbstractThe Cenozoic tectonic evolution of the southeastern edge of the Tibetan Plateau is interpreted in terms of two alterative dynamic mechanisms: lateral extrusion of coherent lithospheric blocks and viscous lower crustal flow. To contribute to this debate, we conducted a magnetostratigraphic study of the Early Miocene sedimentary strata of the Gengma Basin, in the northern part of the Shan‐Thai Block (STB). The results show that variations in the deposition rate were synchronous with the rate of clockwise rotation during ∼18.05–15.93 Ma, indicating that the crustal clockwise rotation of the northern STB was the driver of the tectonic transformation of the NE‐SW trending strike‐slip system within the STB by at least ∼18.05 Ma. The rapid decrease in the clockwise rotational rate of the Gengma Basin was also synchronous with the sharp decrease in the convergence rate between the Indian Plate (IDP) and Eurasia at ∼17.23 Ma, which is similar to that occurred in the interior of the Tibetan Plateau during the Paleocene and Eocene, demonstrating the fast response of the southeastern edge of the Tibetan Plateau to the convergence of the IDP and Eurasia, in the form of the continuous upper crustal ductile deformation of the Tibetan Plateau and its periphery. The lateral flow of the viscous lower crust beneath the southeastern edge of the Tibetan Plateau may be the predominant driver of the tectonic evolution of the southeastern edge of the Tibetan Plateau since the Early Miocene.

Mid-Cretaceous drainage reorganization and exorheic to endorheic transition in Southeast Tibet

To understand the early topographic growth of the Tibetan Plateau and the causes and effects of climate change in this region, it is important to reconstruct the drainage evolution across Southeast Tibet. Based on age constraints provided by detrital zircons in the fluvial sandstones in the Khorat Plateau Basin, we identified a major Early Cretaceous drainage disruption and reorganization event. The ages and Hf isotope compositions of the detrital zircons in the Upper Jurassic-Lower Cretaceous Phu Kradung to Phu Phan formations (166–122 Ma) suggest the existence of an exorheic continental-scale paleo-Mekong River with headwaters in the Songpan-Garze and Qiangtang terranes. This paleo-river flowed southwards across the Khorat Plateau to what is presently the offshore Phuquoc Basin, which was then part of the Neo-Tethys Ocean. In contrast, the U-Pb-Hf compositions of the detrital zircons in the Lower Cretaceous Khok Kruat Formation (122–113 Ma) indicate that a prominent detrital supply from the Sibumasu magmatic rocks was transported by the paleo-Salween River. This major change reflects the occurrence of ancestral transcontinental river disruption at around 122 Ma, which was driven by the final collision between the Lhasa and Qiangtang and Woyla Arc and Sibumasu terranes. The tectonic uplift and ensuing arid climate corresponded to the reorganization of the ancestral Mekong drainage system during the Mid-Cretaceous (113–94 Ma).

Occurrence of Early Carboniferous Radiolarians and Middle Triassic Conodonts from Ban Rai, Southwestern Uthai Thani, Central Thailand and Its Geological Significance

A bedded chert succession, intercalated with layers of coarse-grained sandstone and chert conglomerate, and a limestone block with a long axis of about 10 m are distributed in the Ban Rai area, southwestern Uthai Thani, central Thailand. The chert yielded early Carboniferous (Tournaisian to Visean) radiolarians composed of 10 species of five genera; the limestone yielded Middle Triassic (early Anisian) conodonts comprising five species (P1 element). The lower Carboniferous chert was presumably deposited on the upper continental rise of the Sibumasu Terrane in the Paleotethys Ocean, based on radiolarian faunal characteristics. The Middle Triassic limestone was probably deposited on the continental shelf of the eastern margin of the Sibumasu Terrane, judging from the inclusion of silt-sized quartz grains. Because the blocks of the lower Carboniferous chert and Middle Triassic limestone are currently located in a narrow area, these blocks are inferred to have been mixed with each other by submarine sliding after at least the Middle Triassic at the eastern margin of the Sibumasu Terrane.

The boundary between the Inthanon Zone (Palaeotropics) and the Gondwana-derived Sibumasu Terrane, northwest Thailand—evidence from Permo-Triassic limestones and cherts

The boundary between the Inthanon Zone (Palaeotropics) and the Gondwana-derived Sibumasu Terrane, northwest Thailand—evidence from Permo-Triassic limestones and cherts

Zircon Hf‐isotope constraints on the formation of metallic mineral deposits in Thailand

AbstractThe effects of the Sibumasu–Indochina Terranes collision created several kinds of mineral deposits in Thailand, which include porphyry–skarn copper–gold, epithermal gold and antimony, orogenic gold–antimony–tungsten and tin‐tungsten mineralization among others. The deposits show a distinct spatial zonal distribution and occur in specific tectonic terranes. Combining regional geological data and ore deposit distribution data with Hf‐isotopic data of zircons in igneous rocks can be used to investigate the relationship between crustal construction processes and metallogeny. In this study, we investigated the Sukhothai Fold Belt, which is composed of quartz monzodiorite, granodiorite, syenogranite, and monzogranite of I‐ and S‐type affinities. All granitoids were analyzed for zircon U–Pb geochronology and Lu–Hf isotopic analysis. The granitoids of the Sukhothai Fold Belt yielded U–Pb zircon ages ranging from ~243 to 202 Ma, which mark the timing of subduction to the syn‐collisional stage between the Sibumasu–Indochina terranes at ~243–237 Ma and the timing of post‐collision between the Sibumasu–Indochina terranes during 230–202 Ma. In addition, an age of ~43 Ma in the south of the Sukhothai Fold Belt may indicate intrusion during the sinistral movement of the Klaeng and Mae Ping fault zones resulted from the Indian–Eurasian plate collision. The Doi Tung quartz monzodiorite provided an age of ~350 Ma as a timing of formation of the Sukhothai Fold Belt. The negative and positive initial εHf values (−8.0 to +9.2) with two‐stage depleted mantle model ages (TDMC of 2.2–0.6 Ga) of zircons from the Sukhothai Fold Belt granitoids indicate that the sources of their magma derived from partial melting of old continental crust and young oceanic crust, which probably mixed with a mantle‐derived magma. A zircon Hf‐isotope compilation including the data obtained in this study and previously reported values was used to prepare a map that allows a comparison between magmatic source and mineral deposit distribution in Thailand. The spatial distribution of Hf isotopic data reveals a distinct zonation, with initial εHf values decreasing from the east to the west, that is, from the western margin of the Indochina Terrane or the Loei Fold Belt to the Sukhothai Fold Belt, the Inthanon Zone and the Sibumasu Terrane. The magmatic source for the granitoids in the Loei Fold Belt is dominated by mantle‐derived components, as shown by positive average initial εHf values (+1.0 to +12.7), and contributed to porphyry‐related skarn copper–gold and iron and epithermal gold mineralization. In contrast, magmas in the Sibumasu Terrane and the Inthanon Zone originated from melting of old crustal materials, as indicated by mostly negative average initial εHf values (−15.1 to +0.8), and are responsible for S‐type granite‐related tin‐tungsten mineralization. The average initial εHf values (−5.0 to +11.0) from the intrusions in the Sukhothai Fold Belt suggest mixed sources, including evolved and juvenile magmatic materials, which generated the orogenic gold deposits and other vein‐type antimony, tungsten, fluorite, and base metal deposits. These results imply a close spatial correlation between the source of magmatism in each tectonic terrane of Thailand and different metal ore deposits. These isotope maps can be used as a powerful tool in the exploration for various commodities at the regional scale.

Picrite-basalt complex in the Baoshan-Gongshan Block of northern Sibumasu: Onset of a mantle plume before breakup of Gondwana and opening of the Neo-Tethys Ocean

Abstract Mantle plumes are thought to play key roles in Earth's geodynamics, including mantle convection, continental formation, and plate tectonics. The connection between plume activity and continental dispersion, as exemplified by the breakup of Gondwana and the generation of the Neo-Tethys Ocean, is a key question for the geosciences. Here, we present detailed investigations for the picrite-basalt sequence in the Baoshan-Gongshan Block of the northern Sibumasu terrane, southwest China. Field relations and petrological and geochemical data reveal that these volcanic rocks are continental flood picrites and basalts, consistent with a mantle plume origin. The estimated mantle potential temperatures range from 1527 ± 86 °C to 1546 ± 98 °C, and melting depths vary from the spinel to garnet stability fields (1.1–5.3 GPa), similar to Cenozoic Hawaiian picrites. Zircon geochronological data show that the mantle plume activity started at ca. 335 Ma and lasted to 280 Ma; this range is earlier than the breakup of the Gondwana continent and opening of the Neo-Tethys Ocean (270–260 Ma). We conclude that the long-lived mantle plume impacted the continental lithosphere but it did not drive continental breakup and the opening of Neo-Tethys Ocean, which took place because of the subduction-induced stress generated by initial subduction of the Paleo-Tethys Ocean.

Erosion and sedimentation in SE Tibet and Myanmar during the evolution of the Burmese continental margin from the Late Cretaceous to Early Neogene

The evolution of large fluvial drainages in SE Tibet and adjacent areas reflects the degree of coupling between tectonics, erosion, and climate. Reconstruction of their spatio-temporal development provides significant constraints for our understanding of the palaeogeographic evolution of numerous blocks in this domain. The Burmese continental margin drained by the Irrawaddy River and its tributaries includes blocks of the Burma Terrane and the Mount Victoria Land (Eastern Belt of the Indo-Burman Ranges). To investigate how was this margin developed from the amalgamation of the two blocks since the mid-Cretaceous, we present seismic profiles, sedimentary budget, sandstone petrography, bulk-rock geochemistry, and detrital apatite U-Pb ages from the margin. Our results show that provenance of the Burmese Forearc and Backarc depressions shifted from the proximal Western Myanmar Arc and equivalents within northern Burma Terrane to the northern Mogok Metamorphic Belt and Dianxi-Burman batholiths within SE Tibet during the late Paleogene. The provenance transition is synchronous with tectonic evolution of the Burma Terrane from an(a) oceanic/continental island arc setting to an active continental margin. We propose an evolved Western Pacific-type margin for the Burma Terrane, which is dominated by typical accretionary wedge, calc-alkaline island arc, and extensional forearc/backarc basins. An increase in sediment accumulation rates (SARs) in the Burmese Forearc indicates that the input from basement rocks within northern Burma Terrane (Tagaung–Myitkyina Belt) became more prominent and the Eastern Belt of the northern Indo-Burman Ranges were totally exposed in the Middle-Late Eocene. This result corroborates the Burma Terrane assembled with SE Asia (Sibumasu Terrane) before the Middle Eocene. The first presence of SE Tibet-like detritus and the diversification of SARs in the Burmese Forearc and Backarc in the Oligocene are compatible with dextral motion on the Sagaing Fault, which may have triggered the birth of the palaeo-Irrawaddy river and fluvial drainage reorganization in its upper reaches. Higher sedimentary fluxes recorded in the Burmese Forearc and Backarc depressions and the Andaman Sea during the Neogene are compatible with enhanced rock erosion within SE Tibet and thus, can be explained by rapid tectonic uplift and an intensified Asia monsoon in this region.