Polymetamorphic events in the Jiao-Liao-Ji Belt, North China Craton：Evidence from integrated zircon, xenotime, and monazite LA–ICP–MS U–Pb dating
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Abstract:
Poly-metamorphism has been observed in many Precambrian metamorphic terranes. The metamorphic overprinting in the poly-metamorphic rocks makes it difficult to determine the timing and P − T path of each metamorphic event. In this study, in situ U–Pb dating of zircon, xenotime, and monazite, with simultaneously determination of trace element compositions, is utilized to provide new constraints on the timing of discrete metamorphic events involving reactivation of the Jiao-Liao-Ji Belt, the eastern edge of the North China Craton. Detrital zircons from metasandstones in the Gaixian Formation record a mixed provenance that includes late Neoarchean (ca. 2532 Ma) and middle Paleoproterozoic (ca. 2181 Ma). Xenotimes from the Gaixian pelitic schists yield an upper intercept age of 1906 ± 25 Ma, and this age is interpreted as the timing of greenschist-facies regional metamorphism. However, monazites from the same pelitic schists yield a distinctly younger age of 242 Ma, with late Paleoproterozoic inherited ages of 1894 and 1809 Ma, which are broadly consistent with the xenotime ages. In addition, magmatic zircons from a granitic mylonite vein yield a similar age of 245 Ma, making it coeval with the monazite. The interpretation of these ages is also reinforced by the mineral assemblage of inclusions in the dated accessory mineral grains. These new geochronological data, combined with published data from the Jiao-Liao-Ji Belt and the related regions, suggest that the Gaixian Formation in the Yanghe area was likely some time deposited at 2181–1900 Ma, and experienced both late Paleoproterozoic and Triassic low-grade metamorphic events related to two distinct orogenic phases in the eastern North China Craton.Keywords:
Geochronology
Radiometric dating
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Pelitic metamorphic rocks of the Ryoke Belt are distributed in the Rokuroshi area, southern part of Iwakuni district. Dark-brown biotite schist was locally silicified and decolorized to form milky-white “silicified domain”. Quartz veins were developed in both of the biotite schist and silicified domain. The veins in biotite schist are generally parallel to the schistosity and form boudinage due to ductile flow of the host rock. The veins in silicified domain are oblique to the schistosity with medium to high angles and have not undergone ductile deformation except for some schistosity-parallel veins. The mode of occurrence of these veins indicates that the silicified domain is much more competent than the biotite schist. Ductile deformation after the silicification was accommodated by viscous flow of biotite schist. The silicification probably results from dissolution-precipitation processes which may have raised pore pressure to cause hydraulic fracturing.
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Abstract Cretaceous (possibly older) metamorphic rock occurs mainly in the Blue Mountain inlier in eastern Jamaica. Fault‐bounded blocks reveal two styles of metamorphism, Westphalia Schist (upper amphibolite facies) and Mt. Hibernia Schist (blueschist (BS)–greenschist (GS) facies). Both Westphalia Schist and Mt. Hibernia Schist preserve detailed records of retrograde P–T paths. The paths are independent, but consistent with different parts of the type‐Sanbagawa metamorphic facies series in Japan. For each path, phase relationships and estimated P–T conditions support a two‐stage P–T history involving residence at depth, followed by rapid uplift and cooling. Conditions of residence vary depending on the level in a tectonic block. For the critical mineral reaction (isograd) in Westphalia Schist, conditions were P ∼7.5 kbars, T ∼600°C (upper amphibolite facies). Retrograde conditions in Hibernia Schist were P = 2.6–3.0 kbars, T = 219–237°C for a(H 2 O) = 0.8–1.0 (GS facies). Mt. Hibernia Schist may represent a volume of rock that was separated and uplifted at an early time from an otherwise protracted P–T path of the sort that produced the Westphalia Schist. Reset K–Ar ages for hornblende and biotite indicate only that retrograde metamorphism of Westphalia Schist took place prior to 76.5 Ma (pre‐Campanian). Uplift may have commenced with an Albian–Aptian (∼112 Ma) orogenic event. Copyright © 2008 John Wiley & Sons, Ltd.
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ズ 〈1〉SlidIng surfaces observed by
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Abstract MnO zoning profiles for various sizes of garnets from two pelitic schists were examined. The data from garnets of specimen A are consistent with a constant growth law, while the compositional-crystal size relationships of garnets from specimen B are best explained by heterogeneous growth rates.
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Journal Article Calc—Silicate Diffusion Zones between Marble and Pelitic Schist Get access ALAN BRUCE THOMPSON ALAN BRUCE THOMPSON Department of Geological Sciences, Harvard University20 Oxford Street, Cambridge, Massachusetts 02138, U.S.A. Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 16, Issue 2, 1975, Pages 314–346, https://doi.org/10.1093/petrology/16.2.314 Published: 01 January 1975 Article history Received: 05 February 1974 Revision received: 17 June 1974 Published: 01 January 1975
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The Taku Schist, which is located in the north-east Peninsular Malaysia, is characterized by its North-South oriented elongated body. It forms part of the Indonesian orogenic build-up that was generated via the convergence of the Sibumasu continental unit and Sukhothai Arc. Subsequent petrography analyses of the metasedimentary rocks sourced from the Taku Schist revealed that their formation was attributable to the metamorphism of greenschist into amphibolite facies, which could be observed near the Triassic and Cretaceous intrusions of the Kemahang Granite. The evolutionary process of the rocks could be linked with the interactions occurring between contact and regional metamorphisms. The resulting chemical classification upon their assessment disclosed that the metasedimentary rocks of Taku Schist were made up of greywacke and shale, grouped into the quartzose sedimentary provenance, and belonged to the Continental Island Arc (CIA). This information is required for the tectonic setting discrimination purpose. It is a reflection of the episodic contractions underwent by the Taku Schist, wherein they would lead to the Sibumasu sedimentary cover along with both an accretionary wedge and the genetically-correlated Bentong-Raub melange to different greenschist. Otherwise associated with amphibolite facies, the conditions and depths of the facies were determined according to their position in relation to the upper plate of the Sukhothai Arc.
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