||Yasuhito Osanai, Krishnan Sajeev, Nobuhiko Nakano, Ippei Kitano, Wilbert K.V. Kehelpannala, Ryosuke Kato, Tatsuro Adachi, Sanjeewa P.K. Malaviarachchi, UHT granulites of the Highland Complex, Sri Lanka I
Geological and petrological background, Journal of Mineralogical and Petrological Sciences, 10.2465/jmps.151227, 111, 3, 145-156, 2016.07, The ultrahigh-temperature (UHT) regional metamorphism of Sri Lanka has a significant role in understanding the tectonics and formation of the Gondwana supercontinent. Sri Lanka is specifically important because of its central position in Gondwana, located between southern India, Madagascar and eastern Antarctica. In particular, the Highland Complex has been the focus of several previous studies because of the prominence of metasedimentary rocks that experienced UHT metamorphism. The central Highland Complex of Sri Lanka consists of Spr-bearing Mg-Al rich granulites intercalated with other pelitic, mafic granulites and calc-silicates, which preserve several textural evidences for UHT metamorphism. The calculated peak metamorphic conditions for the Mg-Al rich granulite yielded a temperature range from 910 to 1005 °C at 1.0 GPa, and the pressure varies between 0.9 to 1.2 GPa. The estimated metamorphic P-T conditions and evolution path is in good agreement with previous studies and also to that of similar rock-types from southern Madagascar, southern India and East Antarctica..
||Yasuhito Osanai, Krishnan Sajeev, Nobuhiko Nakano, Ippei Kitano, Wilbert K.V. Kehelpannala, Ryosuke Kato, Tatsuro Adachi, Sanjeewa P.K. Malaviarachchi, UHT granulites of the Highland Complex, Sri Lanka II
Geochronological constraints and implications for Gondwana correlation, Journal of Mineralogical and Petrological Sciences, 10.2465/jmps.151230, 111, 3, 157-169, 2016.07, The regional ultrahigh-temperature (UHT) metamorphism of the Highland Complex, Sri Lanka is well established and has an important role in our understanding of the tectonic history of the Gondwana supercontinent. U-Pb zircon dating of sapphirine-bearing Mg-Al granulites yielded two major metamorphic age populations at approximately 620-590 and 563-525 Ma with no evidence of older zircon cores. Pelitic granulite samples with a Grt-Sil-Spl-Crd assemblage have similar metamorphic ages with concordant data clusters at ~ 602, 563, and 526 Ma and inherited zircon cores aged from 2040 to 1600 Ma. The pelitic granulites also underwent two stages of metamorphism (565-520 and 622-580 Ma). Some of these pelitic granulite samples have inherited zircon cores ranging from 3060 to 760 Ma. Zircons in mafic granulite samples have age ranges of 566-533 and 620-578 Ma. A calc-silicate granulite sample also has similar age populations at 591, 541, and 524 Ma. Combining these new results with previously published ages from Sri Lanka and formerly adjacent continental fragments of Gondwana, we propose that the terranes in southern Madagascar (south of Ranotsara Shear Zone), Northern and Southern Madurai and the Trivandrum Blocks of southern India, the Highland Complex of Sri Lanka, and the Skallen Group in the Lützow-Holm Complex of East Antarctica represent a unique metamorphic belt that regionally experienced the Ediacaran-Cambrian UHT event during the amalgamation of the Gondwana supercontinent..
||Yasuhito Osanai, Yoshifumi Nogi, Sotaro Baba, Nobuhiko Nakano, Tatsuro Adachi, Tomokazu Hokada, Tsuyoshi Toyoshima, Masaaki Owada, Madhusoodhan Satish-Kumar, Atsushi Kamei, Ippei Kitano, Geologic evolution of the Sør Rondane Mountains, East Antarctica
Collision tectonics proposed based on metamorphic processes and magnetic anomalies, Precambrian Research, 10.1016/j.precamres.2013.05.017, 234, 8-29, 2013.09, The Sør Rondane Mountains of eastern Dronning Maud Land, East Antarctica can be subdivided into two different crustal terranes: the NE-terrane and the SW-terrane. The former is underlain by basement rocks of amphibolite-facies (unit A) and granulite-facies (unit B), and the latter by granulite-facies (unit C) to greenschist-facies (units D and D') rocks. The metamorphic evolution of the NE-terrane exhibits a clockwise pressure-temperature-time (. P-T-t) path and the SW-terrane exhibits a counter-clockwise P-T-t path, and detrital zircon U-Pb ages are also different between the two terranes. The differences in the metamorphic evolution processes and detrital zircon provenance in these two regions can be explained by the collision of the NE-terrane and the SW-terrane which is constrained to have occurred at 600-650. Ma. The collision is interpreted to reflect convergence between the East Maud-East African orogen terrane and the Maud-Nampula terrane of the Kalahari Craton, which are bounded by the Main Tectonic Boundary and passes through the Sør Rondane Mountains. The RTP magnetic anomalies suggest this suture is cut by the subsequent collision boundary between the Kalahari Craton and the Rayner Belt, which is exposed in the Lützow-Holm region. This collisional belt is considered to have formed at approximately 520-580. Ma..
||K. Sajeev, I. S. Williams, Y. Osanai, Sensitive high-resolution ion microprobe U-Pb dating of prograde and retrograde ultrahigh-temperature metamorphism as exemplified by Sri Lankan granulites, Geology, 10.1130/G31251.1, 38, 11, 971-974, 2010.11, Ultrahigh-temperature (UHT) granulites of the central Highland Complex, Sri Lanka, underwent some of the highest known peak temperatures of crustal metamorphism. Zircon and monazite U-Pb systems in granulites near Kandy, the highest grade region (~1050 °C; 0.9 GPa), preserve both a record of the timing of prograde and retrograde phases of UHT metamorphism and evidence for the ages of older protolith components. Zircon grains from a quartz-saturated granulite containing relics of the peak UHT assemblage have remnant detrital cores with dates of ca. 2.5-0.83 Ga. Date clusters of ca. 1.7 and 1.04-0.83 Ga record episodes of zircon growth in the source region of the protolith sediment. Two generations of overgrowths with contrasting Th/U record metamorphic zircon growth at 569 ± 5 and 551 ± 7 Ma, probably in the absence and presence of monazite, respectively. The age of coexisting metamorphic monazite (547 ± 7 Ma) is indistinguishable from that of the younger, low-Th/U zircon overgrowths. Zircon from a quartz-undersaturated monaziteabsent UHT granulite with a mainly retrograde assemblage is mostly metamorphic (551 ± 5 Ma). The ca. 570 Ma zircon overgrowths in the quartz-saturated granulite probably record partial melting just before or at the metamorphic peak. The ca. 550 Ma zircon in both rocks, and the ca. 550 Ma monazite in the quartz-saturated sample, record post-peak isothermal decompression. A possible model for this pressure-temperature-time evolution is ultrahot collisional orogeny during the assembly of Gondwana, locally superheated by basaltic underplating, followed by fast extensional exhumation..
||Nobuhiko Nakano, Yasuhito Osanai, K. Sajeev, Y. Hayasaka, Tomoharu Miyamoto, N. T. Minh, M. Owada, B. Windley, Triassic eclogite from northern Vietnam
Inferences and geological significance, Journal of Metamorphic Geology, 10.1111/j.1525-1314.2009.00853.x, 28, 1, 59-76, 2010.01, The first finding of low-temperature eclogites from the Indochina region is reported. The eclogites occur along the Song Ma Suture zone in northern Vietnam, which is widely regarded as the boundary between the South China and Indochina cratons. The major lithology of the area is pelitic schist that contains garnet and phengite with or without biotite, chloritoid, staurolite and kyanite, and which encloses blocks and lenses of eclogite and amphibolite. The eclogites commonly consist of garnet, omphacite, phengite, rutile, quartz and/or epidote with secondary barroisite. Omphacite is commonly surrounded by a symplectite of Na-poor omphacite and Na-rich plagioclase. In highly retrograded domains, diopside + tremolite + plagioclase symplectites replace the primary phases. Estimated peak-pressure metamorphic conditions based on isochemical phase diagrams for the eclogites are 2.1-2.2 GPa and 600-620 °C, even though thermobarometric results yield higher pressure and temperature conditions (2.6-2.8 GPa and 620-680 °C). The eclogites underwent a clockwise P-T trajectory with a post-peak-pressure increase of temperature to a maximum of >750 °C at 1.7 GPa and a subsequent cooling during decompression to 650 °C and 1.3 GPa, which was followed by additional cooling before close-to-isothermal decompression to ∼530 °C at 0.5 GPa. The surrounding pelitic schist (garnet-chloritoid-phengite) records similar metamorphic conditions (580-600 °C at 1.9-2.3 GPa) and a monazite chemical age of 243 ± 4 Ma. A few monazite inclusions within garnet and the cores of some zoned monazite in garnet-phengite schist record an older thermal event (424 ± 15 Ma). The present results indicate that the Indochina craton was deeply (>70 km) subducted beneath the South China craton in the Triassic. The Silurian cores of monazite grains may relate to an older non-collisional event in the Indochina craton..
||Nobuhiko Nakano, Yasuhito Osanai, Masaaki Owada, Yasutaka Hayasaka, Tran Ngoc Nam, Permo-Triassic Barrovian-type metamorphism in the ultrahigh-temperature Kontum Massif, central Vietnam
Constraints on continental collision tectonics in Southeast Asia, Island Arc, 10.1111/j.1440-1738.2008.00646.x, 18, 1, 126-143, 2009.05, The Kontum Massif in central Vietnam is composed of various metamorphic complexes including a high-temperature southern part (Kannak and Ngoc Linh complexes) and a low- to medium-temperature northern part (Kham Duc complex). The Kham Duc complex exhibits Barrovian-type medium-pressure metamorphism evidenced by kyanite- and/or staurolite-bearing metapelites. The garnet-gedrite-kyanite gneiss, which is the focus of the present study, preserves several mineral parageneses formed during a prograde and retrograde metamorphic history: staurolite + quartz in gedrite, garnet + gedrite + kyanite in the matrix, and spinel + cordierite symplectite between gedrite and sillimanite. The calculated semiquantitative petrogenetic grid reveals peak pressure conditions of 620-650°C at 1.1-1.2GPa and peak temperature conditions of 730-750°C at 0.7-0.8GPa. The monazite U-Th-Pb electron microprobe ages of the garnet-gedrite-kyanite gneiss and associated gneisses yield 246 ± 3 Ma for the Kham Duc complex, which is similar to the age of the high- to ultrahigh-temperature metamorphism in the adjacent Kannak and Ngoc Linh complexes of the southern Kontum Massif. The present results indicate that both the Barrovian-type and ultrahigh-temperature metamorphism occurred simultaneously in the Kontum Massif during an event strongly related to Permo-Triassic microcontinental collision tectonics in Asia..
||Yasuhito Osanai, Nobuhiko Nakano, Masaaki Owada, Tran Ngoc Nam, Tomoharu Miyamoto, Nguyen Thi Mihn, Nguyen van Nam, Tran van Tri, Collision zone metamorphism in Vietnam and adjacent South-eastern Asia
Proposition for Trans Vietnam Orogenic Belt, Journal of Mineralogical and Petrological Sciences, 10.2465/jmps.080620e, 103, 4, 226-241, 2008.12, Recent geological and petrological investigations of the metamorphic rocks from Vietnam revealed the following new evidences to understand the tectonic evolution of Southeast Asia; 1) findings of ultrahigh-T(∼ 1000 °C) pelitic granulites, high- to ultrahigh-P (∼ 40 kbar) mafic metamorphic rocks and high-P/medium-T gneisses from the Kontum Massif, 2) eclogite and high-P granulite from the Song Ma suture zone, and 3) ultrahigh-T aluminous granulite rrom the Red River zone. These lines or evidence are strongly indicative of the highest-metamorphic conditions in each metamorphic terrane. Estimated P-T conditions and reaction textures from these rocks delineate a characteristic clockwise pressure-temperature-time (P-T-t) path for each other. Based on the combination of P-T paths from these complexes, two-stages of metamorphic field gradient are identified. An earlier MO-stage of high-P/T gradient is recognized, based on the peak-P conditions from the Kontum Massif and Song Ma suture zone. A later M1-stage of low-P/T gradient is also identified by linking the peak metamorphic conditions from the Kontum Massif, the Song Ma suture zone and the Red River zone. The former metamorphic field gradient could represent an early continental collision event and the latter would indicate a peak metamorphic stage caused by very high-T magmatic intrusion (asthenosphere upwelling) as a heat source of ultrahigh-T metamorphism. A simultaneous collision metamorphism throughout Vietnam should have taken place during the continental collision between Indochina and South China cratons, which led to the formation of Trans Vietnam Orogertic Belt..
||Toshisuke Kawasaki, Yasuhito Osanai, Empirical thermometer of TiO2 in quartz for ultrahigh-temperature granulites of East Antarctica, Geological Society Special Publication, 10.1144/SP308.21, 308, 419-430, 2008.12, Two preliminary experiments, heating of rutilated quartz grains with 0.082 wt% TiO2 on average from Bunt Island, Napier Complex, East Antarctica and a synthetic TiO2-SiO2 (rutile-cristobalite) system in air at 1300 °C for 39 days, showed increasing solubility of TiO2 in silica minerals with temperature. Bunt quartz was converted to cristobalite and traversed by many transparent seams with the disappearence of needles and spots of rutile. Unreacted host grains retained many fine needles of rutile. The seams are homogeneous and slightly enriched in TiO2 up to 0.149 wt% on average, which is about one-fifth lower than that of the synthesized TiO2-SiO2 cristobalite (0.767 wt% on average). Area analyses with an electron beam in the raster mode at a magnification of × 5000 gave 0.308 wt% TiO2 for the bulk composition of the Bunt quartz. This indicates that needles of rutile exsolved from the TiO2-saturated quartz at the cooling stage, or during retrograde metamorphism. Natural examples of quartz in geologically and petrologically well-characterized metamorphic rocks were chemically analysed to examine the temperature controls on the Ti saturation level in quartz. The TiO2 content of quartz in equilibrium with rutile increases sensitively with the metamorphic temperature, which can be expressed as where XQtz TiO2 is the mole fraction of TiO2, or the number of Ti atoms per formula unit based on a two-oxygen atom normalization. This empirical equation is very useful to evaluate the metamorphic temperatures for ultrahigh-temperature granulites. The temperatures calculated by the existing Ti-in-quartz thermometer are about 200 °C higher than those estimated by the present thermometer, potentially because of underestimates of Ti solubility in quartz in the previous calibration..
||T. Toyoshima, Yasuhito Osanai, Y. Nogi, Macroscopic geological structures of the Napier and Rayner Complexes, East Antarctica, Geological Society Special Publication, 10.1144/SP308.6, 308, 139-146, 2008.12, This paper presents a form-line map of the Napier and Rayner Complexes, East Antarctica, constructed from attitude data for foliations shown on published geological maps, and discusses the macroscopic geological structures. The form-line map shows that the two complexes consist of several, structurally distinct, units or blocks bounded by east-west-,NE-SW- and NW-SE-striking faults. The major boundary between the two complexes, as indicated on the published geological maps, is a structural discontinuity shown as a large fault on the form-line map. On the form-line map, east-west- and NE-SW-trending folds are abundant and NW-SE-trending ones occur locally in both complexes. North-south-trending folds are also abundant in the Napier Complex. Dome-and-basin fold patterns on a regional scale occur in some regions. The regional strikes, macroscopic structures, and the major boundary between the two complexes are considered to have resulted from the same later deformation episode. The form-line map and distribution map of key mineral assemblages show that the Napier Complex is not uniform and includes at least two types of metamorphic units or fragments of the Archaean crust that were formed through distinct P-T-t evolutionary processes and divided by several faults..
||Nobuhiko Nakano, Yasuhito Osanai, Nguyen Thi Minh, Tomoharu Miyamoto, Yasutaka Hayasaka, Masaaki Owada, Discovery of high-pressure granulite-facies metamorphism in northern Vietnam
Constraints on the Permo-Triassic Indochinese continental collision tectonics, Comptes Rendus - Geoscience, 10.1016/j.crte.2007.10.013, 340, 2-3, 127-138, 2008.02, High-pressure mafic granulites containing granoblastic garnet, quartz, and minor hornblende have been found from the Song Ma Suture zone in northern Vietnam, regarded as a microcontinental boundary between the South China and Indochina blocks. Fine-grained symplectite formed during the decompression stage is developed in the granulite and is divided into orthopyroxene + plagioclase and orthopyroxene + clinopyroxene + plagioclase ± hornblende. The former replaces garnet and the latter is regarded as a breakdown of sodic clinopyroxene. Detailed observation and careful data selection revealed that the high-Mg and low-Ca garnet should be in equilibrium with the precursor sodic clinopyroxene, and the pair indicates high-temperature and -pressure conditions (910-930 °C at 1.9-2.0 GPa). Although we could not obtain quantitative age data from the high-pressure granulite, the U-Th-Pb age (233 ± 5 Ma) of pelitic gneiss strongly suggests a Middle to Early Triassic metamorphic event. If the age indicates the timing of the high-pressure granulite-facies metamorphism, it might be related to a continental collision setting by following crustal subduction. According to the metamorphic signatures, north to central Vietnam may be regarded as an orogenic belt formed by the micro-continental collision between the South China and Indochina cratons..
||K. Sajeev, Y. Osanai, J. A.D. Connolly, S. Suzuki, J. Ishioka, H. Kagami, S. Rino, Extreme crustal metamorphism during a neoproterozoic event in Sri Lanka
A study of dry mafic granulites, Journal of Geology, 10.1086/519778, 115, 5, 563-582, 2007.09, Garnet-clinopyroxene-quartz granulites of the central Highland Complex of Sri Lanka preserve textural and compositional features indicative of high-pressure, ultrahigh-temperature (HP-UHT) crustal metamorphism and multistage retrogression. Grains of the peak metamorphic assemblage, garnet-clinopyroxene-quartz, are commonly separated and embayed by late orthopyroxene-plagioclase symplectitesj however, in some domains, rare grain-to-grain associations of the peak assemblages are still preserved. Thermodynamic modeling in the CaO-Na2O-K2O-FeO-MgO-Al 2O3-SiO2 system indicates peak metamorphic conditions of 12.5 kbar at 925°C. The temperature estimates using garnet and clinopyroxene core compositions are in the range 844°-982°C, in agreement with the thermodynamic modeling. In conclusion, the textural, geochemical, and thermodynamic modeling and thermobarometric data indicate a multistage decompression after HP-UHT metamorphism. U-Pb zircon (laser ablation-inductively coupled plasma mass spectrometry) ages represent the timing of the peak metamorphism at ca. 580 Ma. A Sm-Nd internal isochron from mineral phases (garnet, clinopyroxene, orthopyroxene, and felsic fraction) and from a whole rock yields an age of 534 ±12 Ma interpreted as the time of isothermal decompression (retrogression). Our results from the central Highland Complex of Sri Lanka provide important constraints on the Neoproterozoic orogeny associated with the assembly of Gondwana..
||Yasuhito Osanai, Krishnan Sajeev, Masaaki Owada, K. V.Wilbert Kehelpannala, W. K.Bernard Prame, Nobuhiko Nakano, Sarath Jayatileke, Metamorphic evolution of high-pressure and ultrahigh-temperature granulites from the Highland Complex, Sri Lanka, Journal of Asian Earth Sciences, 10.1016/j.jseaes.2004.09.013, 28, 1, 20-37, 2006.10, Sapphirine+quartz-bearing pelitic granulites, garnet-clinopyroxene-quartz-bearing mafic granulites and quartzo-feldspathic granulites with corundum-garnet-quartz constitute rare but important members of the Highland Complex in Sri Lanka. Peak metamorphic conditions from the Highland Complex generally have been considered to be up to ∼850-900 °C and ∼8 10 kbar. However, this study on the above mentioned rocks indicates that ultrahigh-temperature and high-pressure conditions (>1100 °C and ∼12 kbar) were attained during peak conditions. A metamorphic evolution of the ultrahigh-temperature metamorphic rocks was determined from careful analyses of shifts in divariant assemblages and reaction textures. This shows a clockwise P-T path from more high-pressure conditions (∼1000 °C and ∼17 kbar; stage 0 as part of the prograde metamorphic path) to lower-pressure and -temperature conditions (∼950 °C and ∼9 kbar; stage 4 as part of the retrograde path) through the peak metamorphic conditions (stage 1). Widespread lower pressure and temperature granulite-facies metamorphic rocks surround the ultrahigh-temperature granulites and are interpreted to have formed by the strong effect of the retrograde metamorphism and deformation. The widely reported Pan-African metamorphic ages derived from similar granulite-facies metamorphic rocks in the Gondwana fragments (Highland Complex in Sri Lanka, Lützow-Holm Complex in east Antarctica, etc.) may also be the result of retrograde metamorphism of ultrahigh-temperature metamorphic rocks. There remains a possibility that this early ultrahigh-temperature/high-pressure granulite-facies metamorphism in the Highland Complex, as well as that in the Lützow-Holm Complex, might pre-date Pan-African metamorphism..
||Sajeev, K. and Osanai, Y., Thermal gradients in the Sri Lankan granulite terrane: a garnet-biotite thermometric approach., J. Metamorphic Geol., 10.1111/j.1525-1314.2005.00582.x, 23, 5, 383-397, 23, 383-397., 2005.07.
||Sajeev, K., Osanai, Y. and Santosh, M., Ultrahigh-temperature metamorphism followed by two-stage decompression of Grt-Opx-Sil granulites from Ganguvarpatti, Madurai block, southern India, Contrib. Mineral. Petrol., 10.1007/s00410-004-0592-0, 148, 1, 29-46, 148, 29-46, 2004.10.
||Osanai, Y., Nakano, N., Owada, M., Tran, N. Nam, Toyoshima, T., Tsunogae, T. and Binh, P., Permo-Triassic ultrahigh-temperature metamorphism in the Kontum massif, central Vietnam, Jour. Mineral. Petrol. Sci., 10.2465/jmps.99.225, 99, 4, 225-241, 99, 230-247., 2004.07.