Paleozoic Subduction-Accretion-Closure Histories in the West Mongolian Segment of the Paleo-Asian Ocean: Evidence from Pressure-Temperature-Time-Protolith Evolution of High-Mg and -Al Gneisses in the Altai Mountains

Journal Metamorphic Geology

Owada

et al. 2015

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Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical-zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca-zoning patterns of the garnet are different in each rock type. U-Th-Pb monazite geochronology revealed that all rock units experienced a c. 360 Ma event, and three of them were also affected by a c. 260 Ma event. The variations in the microstructures and garnet-zoning profiles are caused by the differences in the (i) whole-rock chemistry, (ii) pressure conditions during garnet growth at c. 360 Ma and (iii) equilibrium temperatures at c. 260 Ma. The garnet with sillimanite inclusions records an increase in pressure at low-P (~5.2-7.2 kbar) and moderate temperature conditions (~620-660°C) at c. 360 Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (~7.0-8.9 kbar at~600-640°C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360 Ma. The differences in the thermal gradients that resulted in varying prograde P-T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260 Ma event is characterized by a relatively high-T/P gradient (~25°C km À1 ) and may be due to collision-related granitic activity and re-equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.

Section: Discussionmentioning

confidence: 99%

Section: Geological Backgroundmentioning

confidence: 97%

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Multiple growth of garnet, sillimanite/kyanite and monazite during amphibolite facies metamorphism: implications for theP–T–tand tectonic evolution of the western Altai Range, Mongolia

Journal Metamorphic Geology

Owada

et al. 2015

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“…The analytical procedure utilized has been described in detail by Adachi et al (2012) and Nakano et al (2014). Separated zircons were mounted in epoxy resin and polished.…”

Section: U-pb Dating Of Zirconmentioning

confidence: 99%

Late Permian plume–related magmatism and tectonothermal events in the Kontum Massif, central Vietnam

Owada

Journal of Mineralogical and Petrological Sciences

et al. 2016

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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.

“…Highly aluminous metamorphic rocks have been reported from several terranes around the world, and they have been generally regarded as either the restite of metamorphosed pelitic rocks (e.g., Vielzeuf et al, 1990;BernardGriffiths et al, 1996;Ouzegane et al, 2003;Raith et al, 2008), or the metamorphic product of bauxites (e.g., Jansen and Schuiling, 1976;Adusumalli and Schubert, 2001;Franceschelli et al, 2003;Grew et al, 2003) or hydrothermally altered rocks (e.g., Pan and Fleet, 1995;Baba et al, 2008;Nakano et al, 2014). In the first case of restitic origin, the high-Al content is attributed to metamorphic processes, and in the latter two cases, to the movement or exchange of mobile elements prior to metamorphism.…”

Section: Introductionmentioning

confidence: 99%

Low–temperature eclogite–facies bauxite from the Red River shear zone in Vietnam: Its evolution and significance

Journal of Mineralogical and Petrological Sciences

Nam

et al. 2016

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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% Al 2 O 3 ) and oxidized [Fe 3+ /(Fe 3+ + Fe 2+ ) = 0.89] emery-type rock. Its composition is also characterized by very low SiO 2 , CaO, and Na 2 O 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.