Mogok Metamorphic Belt Research Articles

Origin of high-Mg# orthopyroxene-rich cratonic mantle: Insights from the Mogok peridotites (Myanmar)

Cratonic peridotites are typically depleted but have overall higher modal orthopyroxene than young oceanic and continental peridotites. The origin of this enrichment remains debatable. Here we focus on a spinel harzburgite block from the Mogok metamorphic belt, Myanmar, presenting major and trace element data for 27 harzburgite samples. Twelve samples are clinopyroxene-free but have high modal orthopyroxene (mostly 25.3–30.4%); The remaining fifteen are clinopyroxene-bearing (<4%), with only 10.8–22.7% orthopyroxene. The clinopyroxene-free samples display higher Mg# (91.8–92.5) than those with clinopyroxene (91.1–92.1). All samples yield a positive correlation between modal orthopyroxene and bulk Mg#, overlapping with the trend defined by refractory cratonic peridotite xenoliths. This correlation is unlikely explained by post-melting metasomatism, mechanical sorting, or serpentinization. Instead, it is consistent with non-pyrolitic, silica-rich mantle melting. Thermodynamic modeling shows that high-pressure melting (∼15–35 kbar) of the silica-rich mantle proceeds through an orthopyroxene-forming peritectic reaction, leaving residues with higher Mg# compared to those produced at lower pressures. Our harzburgite samples are compatible with this model, with high-Mg# orthopyroxene-rich samples formed at higher pressures (∼20–40 kbar) than the orthopyroxene-poor ones (∼10–20 kbar). We suggest that high-Mg# orthopyroxene-rich cratonic peridotites are likely an important component of the primordial cratonic mantle. Their formation might occur through anhydrous extensive melting of the silica-rich mantle at relatively high pressures, corresponding to the elevated potential temperatures characteristic of the Archean mantle. Progressive mantle cooling from the Archean to the present can account for the rarity of young analogues of high-Mg# orthopyroxene-rich cratonic mantle.

Multiple growth of zirconolite in marble (Mogok metamorphic belt, Myanmar): evidence for episodes of fluid metasomatism and Zr–Ti–U mineralization in metacarbonate systems

Abstract. Fluid infiltration into (meta-)carbonate rocks is an important petrologic process that induces metamorphic decarbonation and potential mineralization of metals or nonmetals. The determination of the infiltration time and the compositional features of reactive fluids is essential to understand the mechanism and process of fluid–rock interactions. Zirconolite (ideal formula: CaZrTi2O7) is an important U-bearing accessory mineral that can develop in metasomatized metacarbonate rocks. In this study, we investigate the occurrence, texture, composition, and chronology of various types of zirconolite from fluid-infiltrated reaction zones in dolomite marbles from the Mogok metamorphic belt, Myanmar. Three types of zirconolite are recognized: (1) the first type (Zrl-I) coexists with metasomatic silicate and oxide minerals (forsterite, spinel, phlogopite) and has a homogeneous composition with high contents of UO2 (21.37 wt %–22.82 wt %) and ThO2 (0.84 wt %–1.99 wt %). (2) The second type (Zrl-II) has textural characteristics similar to those of Zrl-I. However, Zrl-II shows a core–rim zonation with a slightly higher UO2 content in the rims (average of 23.5 ± 0.4 wt % (n=8)) than the cores (average of 22.1 ± 0.3 wt % (n=8)). (3) The third type (Zrl-III) typically occurs as coronas around baddeleyite and coexists with polycrystalline quartz. Zrl-III has obviously lower contents of UO2 (0.88 wt %–5.3 wt %) than those of Zrl-I and Zrl-II. All types of zirconolite have relatively low rare earth element (REE) contents (&lt; 480 µg g−1 for ΣREE). Microtextures and compositions of the three zirconolite types, in combination with in situ zirconolite U–Pb dating using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), reveal episodic fluid infiltration and element mobilization in the dolomite marbles. The first-stage infiltration occurred at ∼ 35 Ma, leading to the formation of Mg-rich silicates and oxides and accessory minerals (Zrl-I, baddeleyite, and geikielite). The reactive fluid was characterized by high contents of Zr, Ti, U, and Th. After that, some Zrl-I grains underwent a local fluid-assisted dissolution–precipitation process, which produced a core–rim zonation (i.e., the Zrl-II type). The final stage of fluid infiltration, recorded by the growth of Zrl-III after baddeleyite, took place at ∼ 19 Ma. The infiltrating fluid of this stage had relatively lower U contents and higher SiO2 activities than the first-stage infiltrating fluid. This study illustrates that zirconolite is a powerful mineral that can record repeated episodes (ranging from 35 to 19 Ma) of fluid influx, metasomatic reactions, and Zr–Ti–U mineralization in (meta-)carbonates. This mineral not only provides key information about the timing of fluid flow but also documents the chemical variation in reactive fluids. Thus, zirconolite is expected to play a more important role in characterizing the fluid–carbonate interaction, orogenic CO2 release, and the transfer and deposition of rare metals.

Sediment‐hosted disseminated gold mineralization at the Gegalaw deposit in Central Myanmar

AbstractGegalaw is one of the gold deposits situated in the Sagaing Fault zone, which marks the boundary between the Central Myanmar Basin to the west and the Mogok metamorphic belt to the east. The gold mineralization is mainly confined along northeast striking faults between carbonaceous silty limestone of the Kywethe Chaung Formation and carbonaceous and calcareous shale of the Eocene Male Formation. Hydrothermally induced dissolution breccia associated with progressive silica replacement increase near the northeast trending faults. The hydrothermal alteration associated with the mineralization is silicification in addition to illite‐kaolinite argillic alteration, of which illite is dated by the K‐Ar method as 40.26 ± 0.91 Ma. Sulfide minerals contained in the ores are mainly pyrite and arsenopyrite, with minor to trace amounts of marcasite, chalcopyrite, and sphalerite, among which pyrite and arsenopyrite are the hosts of gold. Pyrite and arsenopyrite are divided into diagenetic and hydrothermal stages, based on their mineral textures. Diagenetic stage pyrites are framboidal (Py1A) and coalescent framboidal (Py1B). Those of the hydrothermal stage are zoned pyrite with porous cores (Py2) and compact rims (Py4). In addition, euhedral homogeneous pyrites with or without compact rim are present (Py3). Some pyrites show pseudomorphs of ilmenite (Py3i) and carbonate minerals, possibly ankerite (Py3c). Arsenopyrites are also divided into porous, sooty arsenopyrite (Apy1), and compact arsenopyrite (Apy2) that co‐exists with Py3 and Py4 pyrites. Electron probe microanalysis for sulfides indicates that Py1A and Py1B are deficient in Fe while Py2, Py3 and Py4 are deficient in S compared to stoichiometry of pyrite. Such deficiencies are compensated by metal ions such as Co2+ and Ni2+ for Py1A, As2+ for Py1B, and As1− for Py2, Py3, and Py4. Arsenic equilibrium temperature of Apy2‐Py3 pairs shows a mode range of 250–270°C. Detected gold concentrations (~650 ppm) in arsenian pyrite (Py2, Py3, and Py4) indicate that gold in pyrite are below the solubility, suggesting that gold is present as solid‐solution in pyrite. Sulfur isotopic values of the pyrites in the unaltered host rocks are −18.9‰ and −8.9‰, and those of the pyrites from the hydrothermal altered rocks range from −17.3‰ to −0.5‰. These similar negative δ34S ranges of the pyrites suggest that the source of sulfur is dominated in biogenic reduction origin. Nevertheless, the two relatively heavier sulfur isotopic values (−4.9‰ and −0.5‰) suggest a contribution of an external hydrothermal fluid source. The research results indicate that gold mineralization occurred by infiltration of auriferous reduced hydrothermal fluids into the carbonaceous silty limestone and carbonaceous and calcareous shale through northeast trending secondary faults of the dextrally moving Sagaing Fault system and the fluids reacted with framboidal pyrite as well as iron‐bearing minerals in the host rocks to precipitate gold‐bearing arsenian pyrite. The age of hydrothermal illite suggests a link to the right‐lateral strike‐slip movement of the Sagaing Fault that is related to the collision of the Indian and Asian plates.

Continental‐scale shearing triggered by Oligocene subduction in Myanmar‐Indochina, SE Asia

Continental‐scale shear zones in Indochina record Cenozoic tectonic processes in SE Asia. Previous extrusion models link these shear zones to northward indentation of the Indian continent and conflict with distributed Oligocene conjugate strike‐slip pairs, formed by E‐W‐directed compression (present coordinates). This paper presents evidence of Oligocene shearing along the Mogok Metamorphic Belt in Myanmar, the western boundary of the Indochina block. The Kyanigan quarry at northern Mogok Metamorphic Belt exposes paragneiss, marble and quartzite schist with E‐W striking, which would be dragged from N‐S striking foliations of the main Mogok Metamorphic Belt by right‐lateral shearing of the brittle Sagaing Fault. After restoration, right‐lateral shearing ‘σ’ structures, cored with garnet, in paragneiss in the Kyanigan quarry are consistent with ‘σ’ and ‘δ’ structures in the Moulmein granitic mylonite to the south. U–Pb ages of metamorphic zircon of a sheared paragneiss and a leucogranite, cross‐cutting shear foliation in the Kyanigan quarry, and a biotite 39Ar‐40Ar age of mylonite at Moulmein constrain dextral shearing to 33–25.4 Ma, coeval with other 31–24.5 Ma shear zones in Indochina. The eleven NW‐striking left‐lateral shear zones and N‐S to NE‐striking right‐lateral shear zones define Oligocene conjugate fault patterns in Indochina. After restoration of ~80–45° clockwise rotation, these Oligocene conjugate shear zones, coupled with the anticlinal configuration of the Xuelongshan and Doi Inthanon‐Doi Suthep domes, reflect approximate N–S‐directed shortening. These structures to the north with coeval Myanmar‐Sumatra magmatic arc to the south demonstrate that they correspond to back‐arc contraction during northward subduction of the Indian Ocean. Oligocene continental‐scale intra‐continental shearing may be triggered by syn‐subduction compression in SE Asia.

Different Cooling Histories of Ultrahigh-Temperature Granulites Revealed by Ti-in-Quartz: An Electron Microprobe Approach

The cooling history of granulite is crucial to understanding tectonic scenarios of the continental crust. Ti-in-quartz, a useful indicator of temperature, can decipher the thermal evolution of crustal rocks. Here we apply the Ti-in-quartz (TitaniQ) thermometer to ancient ultrahigh-temperature (UHT) granulites from the Khondalite Belt (KB) in the North China Craton (NCC) and young UHT granulites from the Mogok Metamorphic Belt (MMB), Myanmar. Ti content in quartz was analyzed using a highly precise method constructed in a CAMECA SXFive electron probe microanalyzer (EPMA). The granulites from the two localities show different quartz Ti contents with a constant deforced beam of 10 μm. Matrix quartz and quartz inclusions from the NCC granulites have 57–241 ppm and 65–229 ppm, respectively, corresponding to the TitaniQ temperatures of 653–810 °C and 666–807 °C. The calculated temperatures are significantly lower than the peak temperatures (850–1096 °C) obtained by other methods, due to the formation of abundant rutile exsolution rods in quartz during cooling. Thus, the low calculated temperatures for the NCC granulites reflect a cooling state near or after the exsolution of rutile from quartz, most likely caused by a slow cooling process. However, the matrix quartz from the MMB granulites is exsolution-free and records higher Ti contents of 207–260 ppm and higher metamorphic temperatures of 894–926 °C, close to the peak UHT conditions. This feature indicates that the MMB granulites underwent rapid cooling to overcome Ti loss from quartz. Therefore, determining the amount of Ti loss from quartz by diffusion can provide new insight into the cooling behavior of UHT granulites. When a large deforced beam of 50 μm was used to cover the rutile rods, the matrix quartz in the KB granulites could also yield the TitaniQ temperatures above 900 °C. Thus, our new data suggest that the TitaniQ thermometer could be useful for revealing UHT conditions.

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.

A Study on the Mineralogy and Volatile Fraction of Scapolite from Mogok, Myanmar

The Mogok metamorphic belt (MMB) of Myanmar is a famous geographical origin acknowledged by the whole world for its large mineral resources. In this study, basic gemological tests (density, UV fluorescence, refractive index, etc.), spectroscopic tests (infrared spectrum, Raman spectrum, and ultra-violet visible spectrum) and chemical composition analysis (electron microprobe and laser ablation inductively coupled plasma mass spectrometer) were carried out on scapolites from Myanmar. This paper explores the mineralogical characteristics of Burmese scapolites and provides additional information on its origin. Burmese scapolites are colorless and transparent with a yellow tone and belong to Cl-rich dipyre. The infrared spectra of the samples show the vibrations of Si-O and Al-O (400–1300 cm−1), (CO3)2− (1400–300 cm−1), and OH (3048 cm−1 and 3568 cm−1). The Raman spectra are mainly the vibrational spectra of Si (Al)-O-Si (Al) and the absorption peaks at 992 cm−1 and 1110 cm−1 caused by the vibrations of sulfate and carbonate ions, respectively. Black inclusions were found inside the scapolites, and the dark inclusions were identified as graphite by Raman spectroscopy. Moreover, the composition of the scapolite could be influenced by the αCl−/α(CO3)2− of the fluid. The Cl activity degree could control the scapolite content as the ion exchange between scapolite and plagioclase was gradually balanced. The enrichment of rare earth elements and the apparent positive Eu anomaly indicate that Burmese scapolites have a high degree of light and heavy rare earth element differentiation and an oxidizing environment of mineralization.

Telluride-bearing Au-Ag mineralization in the Singu-Tabeikkyin gold District, Mogok metamorphic Belt, Myanmar: New constraints on an intermediate-sulfidation epizonal orogenic ore system

The Singu-Tabeikkyin gold district is located in the Mogok Metamorphic Belt in Myanmar. Gold mineralization in this district occurs in a set of mainly N − S trending quartz veins, which are associated with minor replacement zones and skarns. This study investigates the field geology, ore mineralogy, mineral chemistry, and fluid inclusion characteristics of three gold prospects (CG-1, CG-2, and CG-3) containing Au-Ag along with telluride assemblages. Gold has formed as two generations, and only the second gold mineralization stage is associated with major telluride assemblages. The tellurides are associated with chalcopyrite, a late generation of sphalerite, and electrum. The telluride phases identified in the study are hessite (Ag2Te), petzite (Ag2AuTe3), altaite (PbTe), tellurobismuthite (Bi2Te3), and tetradymite (Bi2Te2S), and unnamed Au-Ag telluride phases. Liquid-rich two-phase fluid inclusions have homogenization temperatures in the range of 174–310 °C, and salinities of 1–12 wt% NaCl equiv. The chlorite geothermometry in this study yielded a variable temperature range from as low as 250 °C to 380 °C, in general agreement with the fluid inclusion temperature range. The thermometric data, in conjunction with the analysis of telluride-sulfide phase stability, constrain the conditions of ore formation to logƒTe2 and logƒS2 values of − 17 to − 9.5, and of − 13.4 to − 12.5. The geological and mineralogical data and the reconstructed conditions of ore formation as well as high-grade metamorphic host rocks of gneiss, marble and calc-silicates suggest that the telluride bearing gold-silver mineralization in the Singu-Tabeikkyin gold district forms part of an intermediate-sulfidation epizonal orogenic Au system.

MK-1 Orthopyroxene—A New Potential Reference Material for In-Situ Microanalysis

Orthopyroxene, an important phase in mantle-derived rocks, has become a powerful tool to unravel mantle nature and magma processes. However, the applications have been hindered by the lag in the development of analytical techniques, such as shortage of reference materials. Orthopyroxene grains derived from an ultramafic intrusion at the Mogok metamorphic belt (Myanmar) were evaluated for the potential use of orthopyroxene as a reference material for in-situ microanalysis. Approximately 20 g of 0.5–3 mm pure orthopyroxene grains were separated under binocular microscope and analyzed using EPMA, LA-ICPMS, and bulk analytical methods (XRD, XRF, and solution-ICPMS) for major and trace elements at four institutions. Eleven core-to-rim profiles carried out using EPMA and twelve core-to-rim profiles determined using LA-ICPMS suggest that MK-1 orthopyroxene grains are sufficiently homogeneous, with RSD &lt; ±2% (1σ) for major elements (Mg, Si, and Fe) and RSD &lt; ±10% (1σ) for trace elements (Na, Al, Ca, Ti, Cr, Co, Zn, Ni, Mn, Sc, and V). In addition, the composition of MK-1 orthopyroxene was also measured by XRF and solution-ICPMS measurements in two different laboratories, to compare with the results measured using EPMA and LA-ICPMS. The results indicated a good agreement with RSE &lt; ±2% (1σ) for major elements and RSE &lt; ±5% (1σ) for most trace elements, except for Na (±9.73%) and Ti (±6.80%). In an overall assessment of these data, MK-1 orthopyroxene can be considered as a reference material for in-situ microanalysis, which would provide solid trace elements data for a better understanding of mantle source and magmatic evolution.

Petrology and geochemistry of ultramafic rocks in the Mogok belt, Myanmar: Cumulates from high‐pressure crystallization of hydrous arc melts

The Mogok metamorphic belt (MMB) in central Myanmar is well known for its complex tectonics, magmatism, and metamorphism in the framework of Tethyan subduction and India–Asia collision. It is also world renowned due to the gemstone‐class rubies and sapphires. Identification and discrimination of those petrological units in this region therefore are important for understanding the gemstone mineralization. Ultramafic massifs also crop out together with the gemstone‐bearing metamorphic rocks in the MMB. It is still poorly constrained about their origin (i.e., mantle peridotites of Jurassic–Cretaceous ophiolites or cumulate rocks) and their relationship with the gemstone generation. Here, we report petrological and geochemical data for the Mogok ultramafic rocks. Petrographic observations show that they have typical cumulate textures, with a crystallization order of olivine/spinel‐orthopyroxene‐clinopyroxene. They have low whole‐rock Al2O3 and CaO, and extremely high spinel Cr# (=Cr/[Cr + Al]; up to 0.86). Clinopyroxenes of these rocks show light rare earth element enriched patterns when normalized to CI chondrite. The results suggest that the Mogok ultramafic rocks have different compositions, both whole‐rock and mineral, from mantle peridotites of Myanmar ophiolites, such as the Kalaymyo and Myitkyina ophiolites, but resemble typical Alaskan‐type ultramafic cumulates. Therefore, the Mogok ultramafic rocks are cumulates generated by high‐pressure crystallization of hydrous arc melts, probably during the subduction of the Tethyan oceanic slab. We argue that the emplacement of these arc melts may have provided additional heat for the earlier magmatic rocks and regional high‐temperature metamorphism, although there is a need to further constrain the ages of these rocks. This, along with magmatism and metamorphism during post‐collisional extension, probably collectively contributed to the generation of world‐class coloured gemstone mineralization.

Cenozoic ultrahigh-temperature metamorphism in pelitic granulites from the Mogok metamorphic belt, Myanmar

Cenozoic ultrahigh-temperature metamorphism in pelitic granulites from the Mogok metamorphic belt, Myanmar

A multi-proxy provenance study of Eocene to Oligocene sandstones in the Salin Sub-basin, Myanmar

The Salin Sub-basin, Myanmar, contains up to 15,000 m of Cenozoic sediments, but their provenance remains ambiguous. Here, a multi-proxy provenance study that employed Raman Spectroscopy-assisted heavy mineral analysis, light mineral petrography, and U-Pb detrital zircon geochronology, is used to identify source areas and sediment pathways of nine samples from three formations of Eocene to Oligocene age. The heavy mineral assemblages are diverse and highly immature and indicate that sediments were principally provided from a range of igneous and metamorphic lithologies, with some recycling of older sediments. The metamorphic basement rocks in northern Myanmar are identified as a source in all three formations, suggesting that the headwaters of the sedimentary pathway were situated in this area. Detrital zircons overwhelmingly yield Cretaceous and Palaeogene ages. A population of Late Cretaceous and Palaeocene zircons could indicate input from the Mogok Metamorphic Belt, or a currently buried section of the Wuntho-Popa Arc; the diminishing of this signal by the end of the Rupelian suggests reduction in transport from the former, or burial of the latter. Late Cretaceous grains exhibit rounded and euhedral morphologies, suggesting input from recycled Wuntho-Popa Arc material via the sediments of the Chin Hills, and direct input from the Wuntho-Popa Arc, respectively. A persistent Palaeoproterozoic and Triassic signal in the Shwezetaw and Padaung Formations suggests provenance in the Pane Chuang Formation of the Indo-Myanmar Ranges, showing that by the early-mid Oligocene, the ranges were topographically prominent enough to be an important source of sediment for the Central Myanmar Basin.

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.

Reappraisal of the applicability of MK-1 apatite as a reference standard for (U[sbnd]Th)/He geochronology

The (UTh)/He dating technique is widely used in studying the shallow-crustal geological processes including exhumation of orogenic belts, surface weathering, fault motion, topographic evolution. A reference material with homogeneous composition and highly reproducible (UTh)/He dates is crucial to get accurate and precise date results for the unknown samples. As the most commonly used reference material, the Durango apatite has its limitations such as heterogeneous distribution of parent isotopes and large date dispersion for different fragments. In order to overcome the shortcomings of Durango apatite and meet the increasing demand for reference material, we developed a new reference material, MK-1 apatite, which was collected from the Mogok metamorphic belt (MMB) of Myanmar, for the (UTh)/He community. In this study, we presented dating results of 191 apatite fragments in six different laboratories, which showed highly reproducible dates and Th/U ratios, with an average date of 18.01 ± 0.37 Ma (sd.) and Th/U ratio of 0.72 ± 0.06 (sd.) respectively. Combined with textural and compositional results in the previous study (Wu et al., 2019), we concluded that he MK-1 apatite could serve as a good new reference material for (UTh)/He geochronology. We use the weighted mean dates of 17.99 ± 0.02 Ma that obtained from six different laboratories as the intercalibrated dates for the new reference material.

Late Eocene-Oligocene granulite facies garnet-sillimanite migmatites from the Mogok Metamorphic belt, Myanmar, and implications for timing of slip along the Sagaing Fault

The Mogok Metamorphic Belt (MMB) in Myanmar is a polymetamorphic, mainly Paleogene granulite-uppermost amphibolite facies terrane consisting mainly of marbles and calc-silicates hosting spinel, ruby and sapphire. Jurassic charnockite-syenite intrusions, as well as Eocene-Miocene leucogranite intrusions are also present. Pelitic rocks are uncommon, and where present, have sillimanite, both as primary inclusions in garnet and as secondary Bt + Sil coronas around garnet. Core samples from the Kyi-Tauk-Pauk gold mine at Thabeikkiyin, north of Mandalay, are mostly Grt + Bt + Sill gneisses and migmatites with uncommon interbanded Opx + Grt + Bt gneisses. Pseudosection modelling suggests prograde garnet growth occurred by biotite-dehydration melting that reached peak P–T conditions of 870–970 °C and ~ 0.9 GPa, and was followed by garnet breakdown forming coarse retrograde Bt + Sil coronas. UPb monazite data show an early high-grade granulite event at 43–32 Ma, and a later upper amphibolite sillimanite-grade event peaking at 23–20 Ma, with a change in monazite chemistry after c. 22 Ma that is consistent with fluid/(melt) interaction and garnet breakdown. Elevated Th/U ratios from ~35 to 22 Ma, and at ~18 Ma are compatible with melt influx at that time, timing that is similar to the age of the regional Kabaing leucogranite in the Mogok valley area. Our data show that peak granulite facies metamorphism along the Mogok Metamorphic belt was mainly Middle Eocene-Early Oligocene, with upper amphibolite facies metamorphism extending to earliest Miocene. The MMB is cut abruptly by the Sagaing fault, a large-scale dextral fault that extends from the Andaman Sea north to the East Himalayan syntaxis. Our new UPb monazite data constrain the oldest age of initiation of the eastern branch of the cross-cutting Sagaing dextral strike-slip fault at <20–18 Ma.

Age determination of oriented rutile inclusions in sapphire and of moonstone from the Mogok metamorphic belt, Myanmar

Abstract The Mogok metamorphic belt (MMB), Myanmar, is one of the most well-known gemological belts on Earth. Previously, 40Ar/39Ar, K-Ar, and U-Pb dating have yielded Jurassic-Miocene magmatic and metamorphic ages of the MMB and adjacent areas; however, no reported age data are closely related to the sapphire and moonstone deposits. Secondary ion mass spectrometry (SIMS) U-Pb dating of acicular rutile inclusions in sapphire and furnace step-heating 40Ar/39Ar dating of moonstone (antiperthite) in syenites from the MMB yield ages of 13.43 ± 0.92 and 13.55 ± 0.08 Ma, respectively, indicating both Myanmar sapphire and moonstone formed at the same time, and the ages are the youngest published in the region. The ages provide insight into the complex histories and processes of magmatism and metamorphism of the MMB, the formation of gemstone species in this belt, and the collision between India and Asia. In addition, our high field strength element data for the oriented rutile inclusions suggest an origin by co-precipitation rather than exsolution. In situ age determination of this nature is particularly significant since rutile inclusions in other gemstones, such as rubies, can be used to help constrain the geological history of their host rocks elsewhere.

Multiple Episodes of Fluid Infiltration Along a Single Metasomatic Channel in Metacarbonates (Mogok Metamorphic Belt, Myanmar) and Implications for CO2 Release in Orogenic Belts

AbstractFluid infiltration into metacarbonates is a key mechanism to induce orogenic decarbonation, which influences the global carbon cycle and long‐term climate evolution. Little is known regarding the fluid pathways during episodic infiltration events and how flow patterns control time‐integrated CO2 outflux. We investigate the “vein‐like” polycrystalline mineral reaction zones (PMRZs) in dolomite marbles (Mogok metamorphic belt, Myanmar), which are formed by metasomatism via the infiltration of Si–Al–K–Ti–Zr‐bearing fluids. The petrographic textures and mineral U–Pb chronology reveal three episodes of fluid influx in a single PMRZ: (1) the initial episode (Stage‐I) transformed most dolomite into Mg‐rich silicates/oxides and calcite at ∼35–36 Ma indicated by baddeleyite cores; (2) baddeleyite rims gave ages of ∼23–24 Ma, representing a subsequent infiltration episode (Stage‐II) that modified Stage‐I minerals via a dissolution–precipitation mechanism; (3) the final episode (Stage‐III) is recorded by zircon replacing baddeleyite, which yielded ages of ∼17 Ma. Stage‐III fluid has a higher SiO2 activity and [CO2/(CO2 + H2O)] than Stage‐I/Stage‐II fluids. Thermodynamic and mass‐balance analyses indicate that Stage‐I infiltration causes &gt;62–67% loss of CO2 by both dolomite‐consuming reactions and calcite dissolution, whereas the latter two infiltration episodes induce &lt;12–18% loss of CO2 via calcite dissolution. Our results provide compelling evidence that repeated episodes of infiltration (each separated in time by 7–13 Ma) occurred along a single channel in marbles. The initial infiltration episode may create high‐permeability regions, offering favorable channels for later‐stage fluids that transfer obviously less CO2 than the initial metasomatism. This considerably complicates a quantitative assessment of CO2 liberation from metacarbonates during orogenesis.

Timing of Syenite‐Charnockite Magmatism and Ruby and Sapphire Metamorphism in the Mogok Valley Region, Myanmar

AbstractThe Mogok metamorphic belt (MMB) extends for over 1,000 km along central Burma from the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle crustal metamorphic rocks of the Sibumasu plate. In the Mogok valley region, the MMB consists of regional high‐grade marbles containing calcite + phlogopite + spinel + apatite ± diopside ± olivine and hosts world class ruby and sapphire gemstones. The coarse‐grained marbles have been intruded by orthopyroxene‐ and clinopyroxene‐bearing charnockite‐syenite sheet‐like intrusions that have skarns around the margins. Syenites range from hornblende‐ to quartz‐bearing and frequently show layering that could be a primary igneous texture or a later metamorphic overprint. Calc‐silicate skarns contain both rubies and blue sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, occasional diopside, and blue apatite. Both marbles and syenites have been intruded by the Miocene Kabaing garnet‐muscovite‐biotite peraluminous leucogranite. New mapping and structural observations combined with U‐Th‐Pb zircon, monazite, and titanite geochronology from syenites, charnockites, leucogranites, meta‐rhyolite‐tuffs, and skarns have revealed a complex multiphase igneous and metamorphic history for the MMB. U‐Pb zircon ages of the charnockite‐syenites fall into three categories, Jurassic (170–168 Ma), latest Cretaceous to early Paleocene (~68‐63 Ma), and late Eocene–Oligocene (44–21 Ma). New ages from five samples suggest that metamorphism in the presence of garnet and melt occurred between ~45 and 24 Ma. U‐Pb titanite ages from the ruby marbles and meta‐skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma). U‐Th‐Pb dating shows that all the metamorphic ages are Late Cretaceous–early Miocene and related to the India‐Sibumasu collision.

U–Pb Dating of Zircon and Zirconolite Inclusions in Marble-Hosted Gem-Quality Ruby and Spinel from Mogok, Myanmar

The Mogok area in Myanmar (Burma) is known since historic times as a source for some of the finest rubies and spinels in the world. In this study, we focus on in-situ U–Pb geochronological analyses of zircon and zirconolite, either present as inclusions in gem-quality ruby and spinel or as accessory minerals in ruby- and spinel-bearing marble and adjacent granulite facies gneisses. The age determination was carried out using both laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS) and sector-field mass spectrometry (LA-ICP-SF-MS). In addition, we present multi-element data (REE) of zircon and zirconolite collected with LA-ICP-TOF-MS to further characterize these inclusions. Most of the studied zircon grains display growth zoning (core/rim) regardless if as inclusion in gemstones, or as accessory mineral in host rock samples. U–Pb dating was conducted on both core and rim of zircon grains and revealed most ages ranging from ~200 Ma in the core to ~17 Ma in the rim. The youngest U–Pb ages determined from the rim of zircon inclusions in gem-quality ruby and spinel are 22.26 ± 0.36 Ma and 22.88 ± 0.72 Ma, respectively. This agreement in U–Pb ages is interpreted to indicate a simultaneous formation of ruby and spinel in the Mogok area. In ruby- and spinel-bearing marble from Bawlongyi, the youngest zircon age was determined as 17.11 ± 0.22 Ma. Furthermore, U–Pb age measured on the rim of zircon grains in a biotite-garnet gneiss reveals a Late Oligocene age (26.13 ± 1.24 Ma), however older ages up to Precambrian age were also recorded in the cores of zircon as accessory minerals from this gneiss. These old ages point to a detrital origin of the analysed zircon cores. Although non-matrix matched standard was applied, zirconolite U–Pb age results are narrower in distribution from ~35 Ma to ~17 Ma, falling within the range of zircon ages. Based on results which are well in accordance with previous geochronological data from the Mogok Metamorphic Belt (MMB), we deduce that gem-quality ruby and spinel from Mogok probably formed during a granulite-facies regional metamorphic event in Oligocene to Early Miocene, related to post collision tectonics of the Eurasian and Indian plates. Our data not only provide key information to understand the formation of gem-quality ruby and spinel in the so-called Mogok Stone Tract, but also provide assisting evidence when determining the country of origin of gemstones in gemmological laboratories.

Vein Mineralogy and Hydrothermal Gold Mineralization at the Kyaw Soe Thu Prospect in the Wabo Deposit, Mandalay Region, Central Myanmar

The Kyaw Soe Thu Prospect is a part of the Wabo deposit which is situated along Mogok Metamorphic Belt (MMB) in Central Myanmar. Gold bearing veins are hosted in the banded garnet-biotite gneiss, calc-silicate, marble and granitoid of the Wabo area. The granitic rocks intruded into banded garnet-biotite gneiss. The MMB was intensely deformed and it is a highly fractured metamorphic terrain in which metamorphic facies ranges from amphibolite to granulite facies. Gold mineralization at the Kyaw Soe Thu Prospect in the Wabo deposit occurred in the forms of gold-quartz veins. The quartz veins are a few centimeters in width. The color of the quartz vein at the Kyaw Soe Thu Prospect is commonly smoky and milky. The quartz vein exhibits chalcedonic, saccharoidal and granular textures. Three mineralizations were indicated. Stage I is the thin layered crustiform chalcedonic quartz. Stage II is chalcedonic quartz vein and stage III is Ankerite-quartz-sericite vein. Stage II and stage III are gold rich hydrothermal stages, and Ankerite-quartz-sericite veins were formed with a small amount of sericites in the Kyaw Soe Thu Prospect. The Ankerite-quartz-sericite veinlets range from a centimeter up to three centimeter in width within the stage II chalcedonic quartz vein. Textures of mineralized quartz vein samples were petrographically examined. The textures include a feathery texture that is most closely associated with electrum in both stage II chalcedonic quartz veins and stage III Ankerite-quartz-sericite veins. Ankerite varies in color from pink, white to buff. Among them, pink color is more common in the Kyaw Soe Thu Prospect. Ankerite in the Kyaw Soe Thu Prospect exhibits rhombohedral idiomorphic, which exhibits growth patterns. Ore minerals of the veins are electrum, pyrite, galena, and trace amount of very fine-grained chalcopyrite. On the basis of textures of the chalcedonic quartz veins, the Kyaw Soe Thu Prospect is ascribed to be a low-sulfidation epithermal deposit.