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.
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.
Chemically precipitated carbonate sediments directly record seawater composition that helps to decode the Earth’s paleo–environment, the existence of paleo–oceans, and provide valuable clues on the paleo–tectonic position of continents through Earth’s history. In addition, the geochemical and isotopic composition of carbonate rocks have a strong dependence on the depositional tectonic setting and surrounding source rock composition. This was particularly important in the Precambrian, during which biological activity was less prominent and vegetation was virtually absent. Here we present evidence for the existence of an extinct East Antarctic Ocean and its peripheral oceanic island arc system that preceded the formation of the East Antarctic continent in the Neoproterozoic before the final assembly of Gondwana. Applying a multi–element isotope geochemical approach on chemostratigraphically well–constrained metacarbonate rocks collected from the remote Sør Rondane Mountains in East Antarctica, we present a model on carbonate deposition surrounding an island arc system, mid–ocean volcanic islands and a shallow marine continental shelf of a yet unidentified interior Antarctic continent, all of which accreted in the late Neoproterozoic to early Paleozoic to form the present day East Antarctic continent prior to the final amalgamation of Gondwana supercontinent. Our results support the presence of an oceanic island arc system that might have separated the Mozambique ocean and East Antarctic ocean.
The Kontum Massif is situated in the southern part of Trans Vietnam Orogenic Belt (TVOB), central Vietnam, and contains various types of magmatic and metamorphic rocks, the latter including both ultrahigh–pressure and ultrahigh–temperature units. While geochronological data indicate the existence of two main tectonothermal events at 480–420 Ma and 270–240 Ma, the most intense metamorphic and magmatic activity occurred between the Late Permian and Early Triassic due to continental collision between the South China and Indochina cratons. In this study, U–Pb LA–ICP–MS geochronological analyses of zircon obtained from two samples of metagabbro and one sample of charnockite from the massif yielded a magmatic age range of 260–250 Ma for all three samples and an inherited age of ~ 1400 Ma for the charnockite. These magmatic ages overlap with those documented for peak metamorphism in the Kontum Massif. When combined with Nd isotopic data for granitic rocks and pelitic gneisses from the region, these data suggest that the massif may have been derived from reworked continental crust. Geochemical characteristics of metagabbros from the massif reveal that the parental basaltic magma can be correlated with the Song Da igneous suite situated in the northern part of the TVOB, and was assimilated by crustal materials. The Song Da igneous suite is a member of the Emeishan large igneous province and resulted from Late Permian mantle plume activity. We conclude that the plume–related magma intruded into the deeper part of Kontum Massif and induced ultrahigh–temperature metamorphism of the lower crust by acting as a heat source.
We report first finding of low–temperature eclogite–facies metamorphism from the Red River shear zone in northern Vietnam that has been known as the Paleogene high–temperature metamorphic belt. The rock is extremely aluminous (50.13 wt% Al2O3) and oxidized [Fe3+/(Fe3+ + Fe2+) = 0.89] emery–type rock. Its composition is also characterized by very low SiO2, CaO, and Na2O with high concentrations in high field strength elements, suggesting that the protolith is lateritic bauxite. The analyzed rock is composed mainly of porphyroblastic titanomaghemite, kyanite, allanite, and chloritoid with abundant fine–grained corundum in the matrix. The mineral paragenesis and phase equilibria modeling suggest that the aluminous rock underwent a clockwise pressure–temperature trajectory with the peak metamorphic condition corresponding to low–temperature eclogite–facies approximately 520–550 °C at 1.2–1.5 GPa. Monazite U–Th–Pb dating constrains the timing of the metamorphism to be older than 214 Ma. Our results require two geological events; 1) tropical weathering and bauxitization on the erosional surface, and 2) pre–Late Triassic subduction of supracrustal materials most likely during the Permo–Triassic collisional events in East and Southeast Asia. Our study provides the first evidence for the subduction of supra–continental crustal materials up to 40–50 km depth during the collision between the Indochina and South China blocks.
This contribution reports the metamorphic evolution of the high–pressure metamorphic rocks from the Bantimala Complex, South Sulawesi, Indonesia. Barroisite–bearing and barroisite–free eclogites were examined to assess their metamorphic evolutions, which have implications regarding the tectonic conditions in this region. The eclogites mainly consist of garnet, omphacite, phengite, rutile, and epidote, with or without barroisite. The variations in mineral assemblages are interpreted to depend upon local changes in the bulk chemical composition. The barroisite–bearing eclogites contain two types of euhedral garnet: coarse– (1–1.5 mm) and fine–grained (<0.5 mm). Mineral inclusions in the coarse–grained garnet core and mantle show epidote + titanite and glaucophane + epidote assemblages, that stabilized at 0.9–1.5 GPa and 350–550 °C within epidote blueschist–facies conditions. Mineral chemistry and chemical–mapping analyses indicate that both fine–grained garnet and the rim of coarse–grained garnet formed at peak P–T conditions, which were estimated as 2.3–2.7 GPa at 615–680 °C based on the garnet–omphacite–phengite–quartz equilibrium. Peak P–T conditions for barroisite–free eclogite were similar (2.5–2.7 GPa at 650–690 °C) to those for barroisite–bearing eclogite. Actinolite rims overgrowing matrix sodic–calcic amphiboles attest to retrogression at P < 0.5 GPa and T < 350 °C in a clockwise P–T path. The very low geothermal gradient experienced during the prograde path (~ 5 °C/km) likely suggests the subduction of an old and cold oceanic crust. The low geothermal gradient on the retrograde path suggests decompressional cooling during exhumation, possibly favored by a serpentinite–dominated matrix within a subduction channel environment.
We propose a Ti–in–garnet thermometer for ultrahigh–temperature granulites calibrated from experimentally reversed data of the TiO2 solubility in garnet coexisting with orthopyroxene, rutile and quartz at pressures 7–23 kbar and temperatures 850–1300 °C. We confirm that the combined substitution TiVIAlIV ⇌ AlVISiIV, quasi–chemically equivalent to Ti ⇌ Si, is predominant rather than the coupled substitution MVITiVI ⇌ AlVIAlVI (M: Ca, Mg, Fe). The chemical formula of Ca– and Ti–poor garnet under ultrahigh–temperature metamorphic conditions can be expressed as M3Al2Si3−xTixO12, which indicates that the relation NSi + NTi = 3 is not an evidence of TiIV ⇌ SiIV.
The TiO2 content of garnet increases with temperature and pressure, though the pressure dependence is small, as is given by the following equation:
where N is the number of cation per formula unit based on a 12–oxygen atom normalisation. Temperature T and pressure P are given in Kelvin and kbar, respectively.
The present thermometer is useful to estimate metamorphic conditions. This new thermometer is applied to natural garnet in geologically and petrologically well–characterised Antarctic ultrahigh–temperature granulites. The resultant pressure and temperature estimates are consistent with those reported from granulites of these areas.
Vol.31 (1944) No. 5 and No.6 in the predecessor journal ″The Journal of the Japanese Association of
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April 03, 2015
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