Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

Abstract

The Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al2O3–SiO2–H2O–CO2 and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (>780–810°C) and XnormalCO2 [=CO2/(CO2+H2O) >0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (<620°C) and XnormalCO2 (<0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The XnormalCO2 values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower XnormalCO2 conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher XnormalCO2 conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low XnormalCO2 conditions were probably due to far‐ranging infiltration of H2O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.

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

“…In contacts between the magmatic rocks and marbles, metasomatic rocks develop by the influx of fluids or melts, leading to the formation of gemstone-bearing reaction zones (e.g., Themelis, 2008;Guo et al, 2016Guo et al, , 2021Searle et al, 2020). The peak metamorphic pressures and temperatures of the MMB rocks range from 5 to 12 kbar and from 625 to > 950 • C (Searle et al, 2007;Yonemura et al, 2013;Win et al, 2016;Thu and Enami, 2018;Chen et al, 2021;Lamont et al, 2021). Ultrahigh-temperature metamorphism (> 900 • C) has been documented in the Thabeikkiyin and Bernardmyo areas (Chen et al, 2021;Lamont et al, 2021).…”

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

“…In addition to ruby and spinel, marble also contains diopside, phlogopite, olivine, scapolite, titanite, garnet, and graphite. Granite intrudes into marble, and the influence of contact metamorphism is manifested in the presence of feldspar and diopside in the coarse-grained portion contacting the granites [7][8][9][10][11]. Modified from [9] and the references from it.…”

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

“…The main lithologies of the MMB include ortho-gneisses and para-gneisses, migmatites, schists, marbles, and calc-silicate rocks (Mitchell et al, 2007;Searle et al, 2017). The peak metamorphic P-T conditions are ∼5 kbar and 625-680°C for the gneisses in the Kyanigan area (Searle et al, 2007), 6-10 kbar and 780-850°C for the gneisses in the Sagaing town (Win et al, 2016), and ∼8 kbar and >780-810°C for the marbles in the Thabeikgyin area (Thu & Enami, 2018). Previous age determinations indicate that the MMB metamorphism occurred during the Tertiary, with ages ranging from 60 to 31 Ma (Searle et al, 2017).…”