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M Santosh - One of the best experts on this subject based on the ideXlab platform.
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petrology phase equilibria modelling and in situ zircon and monazite geochronology of ultrahigh temperature granulites from the Khondalite belt of southern india
Lithos, 2019Co-Authors: Bing Yu, M Santosh, Shanshan Li, E ShajiAbstract:Abstract The granulite facies metapelite (Khondalite) belt in the Trivandrum Block of southern India has been central to investigations on extreme crustal metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic-Cambrian. Here we investigate garnet-sillimanite-cordierite-spinel-bearing metapelites from this Khondalite belt using state-of-the-art petrologic, mineral phase equilibria modelling, and coupled zircon and monazite U-Pb geochronology to characterize the nature and timing of metamorphism and their tectonic implications. From textural studies and mineral phase equilibrium modelling, we infer that Sill + Grt + Crd representing prograde metamorphism was stable at 6–9 kbar and 760–790 °C. Equilibrium spinel-quartz assemblage suggests peak metamorphic conditions of 6.5–7 kbar and 1010–1030 °C consistent with ultra-high temperature metamorphism in the Trivandrum Block. Growth of cordierite and biotite at the expense of garnet correlates with retrograde metamorphism at 4.5–6.5 kbar and 770–950 °C. Our results allow an interpretation of the prograde, peak, and retrograde P–T conditions of the Khondalites, where from the peak ultrahigh-temperature stage of >1000 °C, the rocks underwent isothermal decompression as indicated by garnet breakdown to cordierite, followed by isobaric cooling, corresponding to an overall clockwise P–T evolution. We present results from U-Pb geochronology of zircon as well as LA-ICPMS data on monazite grain separates. The zircon grains show two distinct age peaks, with Paleoproterozoic cores surrounded by Late Neoproterozoic-Cambrian rims or recrystallized domains. The younger ages around 550–560 Ma are similar to the lower intercept age, whereas the wide range of Paleoproterozoic ages fall along a discordia. The LA-ICP-MS monazite U-Pb data also show two distinct age populations at Paleoproterozoic and latest Neoproterozoic-Cambrian, although the Paleoproterozoic population is scarce. We correlate the late Neoproterozoic-Cambrian ages to the ultrahigh-temperature metamorphic event, associated with the assembly of the Gondwana supercontinent. The wide range of monazite ages indicate that high temperature metamorphism was long-lived.
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late palaeoproterozoic depositional age for Khondalite protoliths in southern india and tectonic implications
Precambrian Research, 2016Co-Authors: Alfred Kroner, M Santosh, E ShajiAbstract:Abstract SHRIMP dating of detrital zircons from garnet–sillimanite–cordierite paragneisses (Khondalites) of the Trivandrum Block, southern India, revealed nanometer-scale Pb isotopic inhomogeneity in most grains that we ascribe to annealing processes during ultra-high-temperature metamorphism at ca. 570 Ma. Our age data for zircons from six representative Khondalite samples do not document any Neo- or Mesoproterozoic detrital grains, and we conclude from the concordant ages and discordant minimum 207Pb/206Pb ages that the Khondalite precursor sediments were deposited more than 2.1 Ga ago and were subsequently intruded by granitoid rocks at ca. 1765–2100 Ma. Some detrital zircons in the Khondalites contain late Palaeoproterozoic metamorphic domains, suggesting that these grains are derived from an unknown crustal source that most likely experienced late Palaeoproterozoic high-grade metamorphism. Both the metasedimentary assemblage and granitoids were severely ductilely deformed, metamorphosed and migmatized during the pervasive Pan-African event at ca. 550–580 Ma. This caused many detrital zircons in the Khondalites to become variably recrystallized and to develop metamorphic rims. Proper interpretation of cathodoluminescence images and zircon morphology is important in interpreting detrital populations. The southern Indian Khondalites were part of an extensive Palaeoproterozoic metasedimentary assemblage that may have extended from southern Madagascar via the Trivandrum Block to the Highland Complex of Sri Lanka and seems to reflect a continental margin sequence that was deposited on an Archaean to early Palaeoproterozoic continental terrane, possibly the southern margin of the Indian Dharwar craton. The tectonic history of this terrane remains obscure due to the pervasive overprint during the Pan-African event.
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Graphite mineralization in Paleoproterozoic Khondalites of the North China Craton: A carbon isotope study
Precambrian Research, 2014Co-Authors: Qiong-yan Yang, M Santosh, Hideki WadaAbstract:Abstract Graphite mineralization in Precambrian supracrustal rocks provide important information on the source characteristics of carbon and has been the focus of studies tracking primitive life in the early Earth as well as a geochemical tracer in fluid-rock interaction processes. Here we present results from a high precision carbon stable isotope study of the different associations of graphites from a graphite deposit in the Paleoproterozoic Khondalite Belt of the North China Craton. Our results show that disseminated graphite occurring as ‘stratabound’ deposits along the compositional planes of the Khondalites possess the lowest δ13C values of ca. −25.5‰. Graphite in the massive ores also show identical composition with δ13C values in the narrow range of ca. −25.3 to −25.7‰. Coarse graphite flakes in quartz pods and veins show slightly heavier values with δ13C in the range of −19.1 to −20.9‰; such flakes with δ13C composition of ca. −20.6‰ were also deposited in some domains of the host Khondalites. The graphite in melt pockets within felsic leucosomes show the heaviest δ13C values in the range of −15.8 to −16.8‰. Micro-sampling perpendicular to the c-axis of the graphite crystals indicates a remarkable homogeneity of carbon isotopic composition within the individual graphite types. The highly negative δ13C values of the dominant graphite type in this deposit, as well as similar values reported for those from other deposits in the region, compare well with the isotopic signature of biogenic carbon occurring in other Precambrian terranes and suggest that microbial life was flourishing in the Paleoproterozoic oceans that closed during the final assembly of the North China Craton. The 13C enriched graphite in veins and melt pockets might have been deposited from mixed fluids derived through degassing of organic material during high grade metamorphism and externally-derived CO2-rich fluids
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retrograde metamorphism of ultrahigh temperature granulites from the Khondalite belt in inner mongolia north china craton evidence from aluminous orthopyroxenes
Geological Journal, 2011Co-Authors: Jianghai Li, M SantoshAbstract:Orthopyroxene-sillimanite-sapphirine-spinel-quartz granulites have been reported from the Khondalite belt in Inner Mongolia, North China Craton (NCC), which record ultrahigh-temperature (UHT) metamorphism. These rocks record metamorphic temperatures up to 1050°C and pressures up to 10 kbar during the peak metamorphism, as indicated by diagnostic UHT mineral assemblages such as equilibrium orthopyroxene + sillimanite + quartz, sapphirine + quartz and low-Zn-spinel + quartz. To well constrain the retrograde P–T conditions, we investigate orthopyroxenes in different textural settings in these UHT rocks and the results reveal marked variations in alumina contents, indicating a multistage formation. The core of coarse orthopyroxene porphyroblasts and some of the grains associated with sillimanite and quartz have the highest alumina content of up to 7.61–8.05 wt%. Rims of coarse orthopyroxene porphyroblasts and those occurring as coronas surrounding garnet grains have intermediate alumina value ranging from 6.21 to 7.61 wt%. Fine grained orthopyroxenes which form symplectite with cordierite have the lowest alumina content between 5.52 and 6.15 wt%. Based on the XAl–XMg isopleths, orthopyroxenes in different textural associations and textures indicate formation under varying temperature and pressure conditions, ranging from 950°C, 10.5 kbar to 840°C, 8.5 kbar. These results are in accordance with the P–T estimates from previous studies on these rocks using Ti-in-zircon geothermometry and pseudosection analysis. The reaction textures and orthopyroxene chemistry suggest an initial stage of isobaric cooling followed by isothermal decompression, and a final stage of cooling. Combined with the available age data which suggest the timing of the UHT event as 1.92 Ga, and the previous models on P–T evolution of the Khondalites, our results suggest a complex exhumation of the UHT granulites within the Inner Mongolia Suture Zone associated with the final assembly of the NCC within the Columbia supercontinent during the Palaeoproterozoic. Copyright © 2010 John Wiley & Sons, Ltd.
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timing of paleoproterozoic ultrahigh temperature metamorphism in the north china craton evidence from shrimp u pb zircon geochronology
Precambrian Research, 2007Co-Authors: M Santosh, Simon A Wilde, Jianghai LiAbstract:Abstract We report SHRIMP U–Pb zircon ages from two samples of Mg–Al granulite and one of normal Khondalite from the Khondalite belt of Inner Mongolia, North China Craton. These data constrain the timing of extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions in the region. The UHT indicator assemblages in these rocks include equilibrium sapphirine + quartz, high alumina orthopyroxene + sillimanite + quartz and high temperature perthites. The surrounding Khondalites also carry spinel + quartz equilibrium assemblage. Based on the stability of sapphirine + quartz and orthopyroxene + sillimanite + quartz in the matrix assemblages, T > 1000 °C and P > 10 kbar for peak metamorphism was estimated in a previous study, with initial isobaric cooling and subsequent isothermal decompression along a broad anticlockwise trajectory. Metamorphic zircons from the UHT rocks are characterized by ovoid shapes with numerous high-order crystal faces and low luminescence in cathodoluminescence (CL) images. As metamorphic temperatures increase, based on the presence of UHT indicator minerals, relict oscillatory zones are progressively destroyed and the grains become uniformly dark in CL. The oldest detrital cores reveal 207 Pb/ 206 Pb ages up to 2090 ± 22 Ma, but most have ages of ∼1970 Ma. Importantly, the highest grade sample with sapphirine-bearing UHT assemblage, contains only a single zircon population with a weighted mean 207 Pb/ 206 Pb age of 1919 ± 10 Ma. This is the same age, within error, as the youngest populations in the other two samples and is interpreted to record the time of UHT metamorphism in the North China Craton. The zircons in the highest-grade sample have unusually high Th/U ratios (average 1.44) for metamorphic zircon, which may prove to be a feature of UHT rocks. The Khondalite belt defines a collisional orogen between the Yinshan block to the north and the Ordos block to the south, that together form the Western Block of the North China Craton. The ∼1.92 Ga UHT metamorphic event in the North China Craton preceded the collision that amalgamated the North China Craton along the Trans-North China Orogen at ∼1.85 Ga, but both coincided with the assembly of the Paleoproterozoic supercontinent Columbia.
Guochun Zhao - One of the best experts on this subject based on the ideXlab platform.
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U–Pb and Hf isotopic study of detrital zircons from the Wulashan Khondalites: Constraints on the evolution of the Ordos Terrane, Western Block of the North China Craton
Earth and Planetary Science Letters, 2020Co-Authors: Guochun Zhao, Fuyuan Wu, Ping Xu, Ji-heng ZhangAbstract:Detrital zircons from the Wulashan Khondalites in the Ordos Terrane, western block of the North China Craton, give U-Pb ages between 1.84 and 2.32 Ga, showing that their provenance was dominated by Paleoproterozoic rocks, much younger than those exposed in the Eastern Block. These zircons have oscillatory growth zoning and possess ε Hf values between -8 and +9, suggesting derivation of their precursor magmas from old crust and juvenile materials from the mantle. The lowest ε Hf values of zircons with different ages give a line that intersects the depleted mantle line at about 2.6 Ga in a ε Hf vs. time diagram. This may imply that the precursor magma source in the deep level of the crust was largely separated from the mantle ∼2.6 Ga ago and therefore 2.6 Ga may be an important crustal formation period for the terrane. One Khondalite sample has detrital zircons with a single age population at ∼2.0 Ga and positive ε Hf values from +1 to +9, clearly recording the significant addition of juvenile materials at ∼2.0 Ga in the source region. This may inspire us to seek whether there was a connection between a mantle superevent and the formation of Columbia supercontinent. Data of this study exclusively demonstrate that the Ordos Terrane was developed independently from the Eastern Block until assembly of the Columbia supercontinent. Therefore, tectonic models involving that the whole North China Craton was cratonized 2.5 Ga ago should be dismissed. © 2005 Elsevier B.V. All rights reserved.postprin
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U–Pb and Hf isotopic study of zircons of the Helanshan Complex: Constrains on the evolution of the Khondalite Belt in the Western Block of the North China Craton
Lithos, 2020Co-Authors: Guochun Zhao, X ZhouAbstract:The Helanshan Complex is located at the westernmost segment of the Khondalite Belt, a continent-continent collisional belt along which the Yinshan Block in the north collided with the Ordos Block in the south to form the Western Block of the North China Craton. The complex consists of S-type granites and high-grade pelitic granulite/gneiss, felsic paragneiss, quartzite, calc-silicate rock and marble, together called the Khondalite Series rocks. LA-Q-ICP-MS U-Pb geochronology and LA-MC-ICP-MS Lu-Hf isotopic data of zircons, combined with cathodoluminescence (CL) imaging, enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the Khondalite Belt in the Western Block of the North China Craton. CL images reveal the coexistence of magmatic-type detrital zircons and metamorphic zircons in most of the Khondalite Series rocks, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic-type detrital zircon cores. LA-Q-ICP-MS U-Pb analyses on magmatic-type detrital zircons reveal two distinct age populations, with one in the Archaean (2.85-2.53Ga) and the other in the Palaeoproterozoic (2.2-2.0Ga), suggesting that the sedimentary protoliths of the Khondalite Series rocks in the Helanshan Complex must have been deposited at some time after 2.0Ga. The Hf analyses show that the Archaean detrital zircons possess negative Ε Hf(t) values from -7.64 to -0.15 with depleted mantle model ages ranging from 3.34 to 3.10Ga. This implies the existence of a Paleo-Mesoarchaean (3.34-3.10Ga) crust in the Western Block, which underwent a recycling event in the period 2.85-2.53Ga. Nearly all Palaeoproterozoic (2.2-2.0Ga) detrital zircons possess positive Ε Hf(t) values (0.86-9.38) with depleted mantle model ages of 2.45-2.15Ga, suggesting a significant crustal growth event in the Western Block in the Palaeoproterozoic. Metamorphic zircons yield two groups of age, with one at ~1.95Ga and the other at ~1.87Ga, of which the former is considered as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the latter is consistent with the age of ~1.86Ga from two S-type granites which are considered to have formed by the partial melting of pelitic granulites/gneisses at the stage of the exhumation of the Helanshan Complex. © 2010 Elsevier B.V.link_to_subscribed_fulltex
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metamorphism and partial melting of high pressure pelitic granulites from the qianlishan complex constraints on the tectonic evolution of the Khondalite belt in the north china craton
Precambrian Research, 2014Co-Authors: Guochun Zhao, X ZhouAbstract:Abstract High-pressure pelitic granulites with a peak assemblage of garnet + kyanite + K-feldspar + plagioclase + biotite + quartz have been found from the Qianlishan Complex in the westernmost part of the Khondalite Belt, an east–west-trending Paleoproterozoic orogen between the Ordos and Yinshan Blocks in the North China Craton. Petrographic evidence indicates that high-pressure pelitic granulites underwent the prograde, peak and post-peak decompression stages. The early prograde metamorphic stage (M1) is represented by an inclusion-type mineral assemblage of quartz + plagioclase + muscovite + biotite + sillimanite, which occurs within the core of garnet porphyroblasts. The peak metamorphic stage (M2) is marked by the mantle growth of garnet porphyroblasts and matrix minerals kyanite, K-feldspar, biotite, plagioclase and quartz. The decompression stage (M3) is indicated by sillimanite replacing kyanite, the formation of the cordierite + sillimanite symplectite in the matrix (M3–1), and the cordierite corona replacing garnet (M3–2). These mineral assemblages and their P–T estimates based on the assemblage stability fields of P–T pseudosection constructed in NCKFMASHTO define a clockwise P–T path involving near-isothermal decompression. Combined with available metamorphic age data, the clockwise P–T paths reconstructed for high-pressure pelitic granulites from the Qianlishan Complex and medium-pressure pelitic granulites from other complexes in the Khondalite Belt are in accord with collision between the Yinshan and Ordos Blocks to form the Western Block at ∼1.95 Ga, followed by the exhumation and associated decompressional partial melting at ∼1.88 Ga.
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geochronology of Khondalite series rocks of the jining complex confirmation of depositional age and tectonometamorphic evolution of the north china craton
International Geology Review, 2011Co-Authors: Xuping Li, Guochun Zhao, Zhenyi Yang, Rodney GrapesAbstract:A belt of Khondalite-series rocks in the Western Block of the North China craton (NCC) are considered to represent products of the collision between the north Yinshan and the south Ordos terranes before final amalgamation of the NCC basement. The Jining Complex of Inner Mongolia occurs in the eastern part of the Khondalite Belt and is crosscut by the Trans-North China Orogen. Khondalite rocks of the Jining Complex mainly comprise sillimanite-garnet gneiss, garnet/sillimanite-bearing granite, massive porphyritic granite, garnet quartzite, calc-silicate, and marble with minor felsic gneiss and mafic granulite. LA-ICP-MS, U–Pb dating and cathodoluminescence (CL) image analysis of zircons from five rocks from the complex, i.e. Sil-Bt-Grt leptynite gneiss, Spl-Sil-Ksp-Grt vein in (Crd)-Sil-Grt gneiss, Sil-Grt-K-Fsp mylonite from a shear zone, Crd-bearing Sil-Grt gneiss, and granite were used to determine protolith and metamorphic ages of the Khondalite-series rocks. Results of 315 detrital zircon grains indica...
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u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the Khondalite belt in the western block of the north china craton
Lithos, 2011Co-Authors: Guochun Zhao, X ZhouAbstract:Abstract The Helanshan Complex is located at the westernmost segment of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north collided with the Ordos Block in the south to form the Western Block of the North China Craton. The complex consists of S-type granites and high-grade pelitic granulite/gneiss, felsic paragneiss, quartzite, calc-silicate rock and marble, together called the Khondalite Series rocks. LA-Q-ICP-MS U–Pb geochronology and LA-MC-ICP-MS Lu–Hf isotopic data of zircons, combined with cathodoluminescence (CL) imaging, enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the Khondalite Belt in the Western Block of the North China Craton. CL images reveal the coexistence of magmatic-type detrital zircons and metamorphic zircons in most of the Khondalite Series rocks, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic-type detrital zircon cores. LA-Q-ICP-MS U–Pb analyses on magmatic-type detrital zircons reveal two distinct age populations, with one in the Archaean (2.85–2.53 Ga) and the other in the Palaeoproterozoic (2.2–2.0 Ga), suggesting that the sedimentary protoliths of the Khondalite Series rocks in the Helanshan Complex must have been deposited at some time after 2.0 Ga. The Hf analyses show that the Archaean detrital zircons possess negative eHf(t) values from − 7.64 to − 0.15 with depleted mantle model ages ranging from 3.34 to 3.10 Ga. This implies the existence of a Paleo-Mesoarchaean (3.34–3.10 Ga) crust in the Western Block, which underwent a recycling event in the period 2.85–2.53 Ga. Nearly all Palaeoproterozoic (2.2–2.0 Ga) detrital zircons possess positive eHf(t) values (0.86–9.38) with depleted mantle model ages of 2.45–2.15 Ga, suggesting a significant crustal growth event in the Western Block in the Palaeoproterozoic. Metamorphic zircons yield two groups of age, with one at ~ 1.95 Ga and the other at ~ 1.87 Ga, of which the former is considered as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the latter is consistent with the age of ~ 1.86 Ga from two S-type granites which are considered to have formed by the partial melting of pelitic granulites/gneisses at the stage of the exhumation of the Helanshan Complex.
Shujuan Jiao - One of the best experts on this subject based on the ideXlab platform.
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the timing and duration of high temperature to ultrahigh temperature metamorphism constrained by zircon u pb hf and trace element signatures in the Khondalite belt north china craton
Contributions to Mineralogy and Petrology, 2020Co-Authors: Shujuan Jiao, Noreen J Evans, Brad J Mcdonald, Dongjian Ouyang, I C W FitzsimonsAbstract:Understanding the timing and duration of metamorphism is fundamental to correctly interpreting the geodynamic evolution of orogenic events. Metamorphic constraints are still ambiguous in the Khondalite Belt, North China Craton, so Paleoproterozoic orogenic processes remain unclear. Here, we investigate a suite of high-grade rocks from the Dongpo locality in the Daqingshan Terrane by integrating zircon trace elements and Hf isotopic signatures with U–Pb ages to track zircon formation events and confidently constrain the timing and duration of metamorphism. The application of the Ti-in-zircon thermometer confirms the occurrence of regional HT-UHT metamorphism. Linking zircon HREE patterns and U–Pb ages, in combination with HREE partitioning between zircon and garnet, reveals zircon formation during partial melting to thermal peak stage, and during the post-peak cooling stage. Local reaction control by garnet breakdown has affected some zircons, which show a more radiogenic 176Hf/177Hf ratio (0.28164–0.28174) relative to unaffected grains (0.28128–0.28156). Metamorphic zircon U–Pb ages, combined with results from previous studies, constrain partial melting to UHT conditions with decompression-heating at 1.94–1.90 Ga, post-peak near isobaric cooling at 1.90–1.86 Ga, and the thermal peak very close to ca. 1.90 Ga. Local garnet breakdown occurred during another decompression-heating stage to HT-UHT conditions at 1.86–1.84 Ga, and further cooling occurred until ca. 1.80 Ga. Our work reveals relatively short-lived partial melting (less than 40 million years) in comparison to long-lived cooling stages (nearly 100 million years) in the eastern part of the Khondalite Belt. Two pulses of decompression-heating to HT-UHT conditions in the deep crust could result from repeated asthenospheric upwelling driven by lithospheric extension after subduction-collision. Our study sheds new light on zircon behaviour during HT-UHT metamorphism and elucidates the underlying driver of UHT metamorphism in this part of the Khondalite Belt.
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Texturally Controlled U–Th–Pb Monazite Geochronology Reveals Paleoproterozoic UHT Metamorphic Evolution in the Khondalite Belt, North China Craton
Journal of Petrology, 2020Co-Authors: Shujuan Jiao, Noreen J Evans, Jianwei Zi, Ian C.w. Fitzsimons, Brad J McdonaldAbstract:Abstract Sapphirine-bearing UHT granulites from the Dongpo locality in the Khondalite Belt of the North China Craton have been comprehensively characterized in terms of petrology, mineral chemistry, metamorphic evolution and zircon geochronology. However, the precise timing of the peak-UHT metamorphism and other key stages in the P–T–t evolution remain controversial due to the complexity of multiple metamorphic overprints and the lack of petrographic context for zircon age data. In this study, monazite from four samples of the Dongpo granulite are divided into six groups based on chemical composition and textural context, and dated (in-situ SHRIMP and LA–ICP–MS U–Pb). An age population of 1·91–1·88 Ga was obtained from high-Y cores of monazite inclusions in garnet (Group 1) and on grains in the rock matrix (Group 2). The maximum age of c.1·91 Ga is interpreted as the minimum timing for prograde metamorphism before UHT metamorphism (M1). An age population of 1·90–1·85 Ga was obtained from low-Y domains of monazite inclusions (Group 3) and of matrix grains (Group 4). Combined with previous zircon dating results, the age population from low-Y Mnz constrains the timing and duration of the UHT metamorphism to 1·90–1·85 Ga and 50 (±15) million years, respectively. The large (50 m.y.) age spread is interpreted to reflect continuous monazite formation, and it is consistent with the slow post-peak near-isobaric cooling stage (M2). An age of c.1·86 Ga was obtained from monazite in textural contact with sapphirine/spinel + plagioclase intergrowths (Group 5), which is interpreted as the timing of the subsequent decompression–heating stage (M3). The younger age clusters at c.1·80 and 1·77 Ga, obtained from Th-rich monazite rims (Group 6) and one single Th-depleted monazite in textural contact with matrix biotite, respectively, indicate dissolution–reprecipitation and new monazite growth from fluid released by crystallizing anatectic melt during retrogression. These results, along with the previous 1·93–1·91 Ga data for UHT metamorphism, suggest that there was a very long-lived Paleoproterozoic UHT metamorphism (1·93–1·85 Ga) in the Khondalite Belt of the North China Craton. This supports the large hot orogeny model for the generation of Paleoproterozoic UHT metamorphism in the Khondalite Belt during the amalgamation of the Nuna supercontinent.
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geochronology and trace element geochemistry of zircon monazite and garnet from the garnetite and or associated other high grade rocks implications for palaeoproterozoic tectonothermal evolution of the Khondalite belt north china craton
Precambrian Research, 2013Co-Authors: Shujuan Jiao, Simon L Harley, Peng PengAbstract:Abstract It is well documented that the Khondalite Belt of the North China Craton formed by the collision of the Yinshan and Ordos Blocks during ca. 1.95 Ga, but the onset of the post-collisional exhumation stage has not been well dated. The garnetite and garnet-bearing quartz-rich lens that are regarded as residue-melt phases occur occasionally in the foliated metasedimentary rocks in the Jining terrane of the Khondalite Belt. They are free of regional gneissosity, and may be generated during the post-collisional exhumation stage. Zircon U–Pb dating, monazite chemical dating and trace element analyses of zircon, monazite and garnet have been carried out on the garnetites and associated other high-grade metamorphic rocks. The geochronological results reveal a major age group of ca. 1890 Ma from the zircon grains/overgrowths and inclusion-type monazites within the garnet poikiloblasts in the garnetites. The trace elements analyses show that zircon hosted in the mantle of the garnet poikiloblasts in the garnetite contains the highest HREE and Y contents, whereas zircon hosted in the garnet rims shows higher HREE and Y contents than matrix zircon grains. Inclusion-type monazite in the garnetite shows higher LaN/SmN ratio than matrix monazite grains. Large garnet poikiloblasts in the garnetite exhibit bell-shaped HREE and Y zoning profiles attributed to Rayleigh fractionation during their growth. Compared to garnet poikiloblasts, coronal garnet shows higher Gro, Cr and Li contents, but lower XMg, Sm, HREE, Zn and Zr contents and weaker Eu negative anomalies, reflecting their different formation environments. Based on evidences from microstructural analyses, REE concentrations and patterns, HREE partition coefficients and the Ti-in-zircon thermometer, it is proposed that most of the zircon grains/overgrowths in the garnetite formed coevally with garnet poikiloblasts. Consequently, ca. 1890 Ma represents the timing of garnet poikiloblast growth during the partial melting of the protolith. Coronal garnet or Zn-rich spinel plus cordierite coupled with the matrix-type monazite formed after garnet poikiloblasts growth during near-isothermal decompression. Given the high metamorphic temperature and pressure revealed by the garnetite (820–850 °C and up to 950 °C; 8.5–9.5 kbar), it is suggested that the ca. 1890 Ma age group represents the beginning of extension/exhumation event in the Khondalite Belt, which is also recorded in the associated other high-grade rocks such as the garnet-bearing quartz-rich lenses, sillimanite–garnet-bearing quartzo-feldspathic gneisses, garnet-bearing quartzo-feldspathic gneisses and a pure quartz vein.
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application of the two feldspar geothermometer to ultrahigh temperature uht rocks in the Khondalite belt north china craton and its implications
American Mineralogist, 2011Co-Authors: Shujuan JiaoAbstract:The Paleoproterozoic Khondalite belt in the North China craton preserves evidence for ultrahigh-temperature (UHT) crustal metamorphism associated with the collision of the Yinshan and Ordos Blocks. Here we apply two-feldspar geothermometry to UHT granulites from two localities newly reported in this study (Tuguishan and Xuwujia) and another two localities from previous studies (Dajing/Tuguiwula and Dongpo) in the Khondalite belt. The presence of abundant perthite/mesoperthite in these rocks reflects post-peak slow cooling. The minimum estimated peak metamorphic temperatures are 832–998, 819–952, 844–1037, and 966–1019 °C computed at 8 kbar for the Dajing/Tuguiwula, Dongpo, Tuguishan, and Xuwujia areas, respectively. These results confirm the previous report of extreme metamorphism at Dajing/Tuguiwula and Dongpo, and reveal similar conditions in the new localities reported here, suggesting that UHT metamorphism is widespread in the Khondalite belt of the North China craton. Our study demonstrates that UHT metamorphism can be recognized using the two-feldspar geothermometry in rocks that do not possess other key UHT assemblages.
T B Nimalsiri - One of the best experts on this subject based on the ideXlab platform.
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p t evolution of a spinel quartz bearing Khondalite from the highland complex sri lanka implications for non uht metamorphism
Journal of Asian Earth Sciences, 2014Co-Authors: P L Dharmapriya, Sanjeewa P K Malaviarachchi, Andrea Galli, Benxun Su, N D Subasinghe, C B Dissanayake, T B NimalsiriAbstract:Abstract Here, we report a natural field example for the coexistence of spinel + quartz as a non-UHT assemblage in spinel- and cordierite-bearing garnet-sillimanite-biotite-graphite gneiss (Khondalite) interbedded with orthopyroxene-garnet-biotite bearing intermediate granulites from the Highland Complex (HC) in Sri Lanka. The Khondalite contains Zn-rich spinel mainly in four textural assemblages namely: (a) spinel co-existing with tiny quartz (ZnO = 12.67–12.85 wt%), (b) spinel surrounded by sillimanite moates and in intergrowth with skeletal sillimanites (ZnO = 9.03–9.17 wt%), (c) symplectitic spinels at the margin of sillimanite (ZnO = 4.09–4.28 wt%) and (d) spinel co-existing with ilmenite or as isolated grains (ZnO = 7.61–7.97 wt% and Cr 2 O 3 = 5.99–6.27 wt%). Assemblage (a) and (b) occur within garnet while assemblages of (c) and (d) are present within cordierite moates after garnet in the matrix. Pseudosections calculated in the NCKFMASHTMnO system and conventional geothermobarometry suggest that the metamorphic peak conditions attained by the spinel + quartz bearing Khondalites and associated intermediate granulites did not exceed T of 900 °C and P of 7.5–8.5 kbar. Post-peak evolution was characterized by a stage of nearly-isobaric cooling down to T of 770 °C and P of 7.5 kbar, followed by a late stage of isothermal decompression down to P T of 770 °C. We propose that the incorporation of large amount of Zn into spinel from exotic, metasomatic fluids and possibly incorporation of Fe 3+ into spinel under high oxidizing conditions may have shifted the stabilization of co-existing spinel + quartz to T
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P–T evolution of a spinel + quartz bearing Khondalite from the Highland Complex, Sri Lanka: Implications for non-UHT metamorphism
Journal of Asian Earth Sciences, 2014Co-Authors: P L Dharmapriya, Sanjeewa P K Malaviarachchi, Andrea Galli, Benxun Su, N D Subasinghe, C B Dissanayake, T B NimalsiriAbstract:Abstract Here, we report a natural field example for the coexistence of spinel + quartz as a non-UHT assemblage in spinel- and cordierite-bearing garnet-sillimanite-biotite-graphite gneiss (Khondalite) interbedded with orthopyroxene-garnet-biotite bearing intermediate granulites from the Highland Complex (HC) in Sri Lanka. The Khondalite contains Zn-rich spinel mainly in four textural assemblages namely: (a) spinel co-existing with tiny quartz (ZnO = 12.67–12.85 wt%), (b) spinel surrounded by sillimanite moates and in intergrowth with skeletal sillimanites (ZnO = 9.03–9.17 wt%), (c) symplectitic spinels at the margin of sillimanite (ZnO = 4.09–4.28 wt%) and (d) spinel co-existing with ilmenite or as isolated grains (ZnO = 7.61–7.97 wt% and Cr 2 O 3 = 5.99–6.27 wt%). Assemblage (a) and (b) occur within garnet while assemblages of (c) and (d) are present within cordierite moates after garnet in the matrix. Pseudosections calculated in the NCKFMASHTMnO system and conventional geothermobarometry suggest that the metamorphic peak conditions attained by the spinel + quartz bearing Khondalites and associated intermediate granulites did not exceed T of 900 °C and P of 7.5–8.5 kbar. Post-peak evolution was characterized by a stage of nearly-isobaric cooling down to T of 770 °C and P of 7.5 kbar, followed by a late stage of isothermal decompression down to P T of 770 °C. We propose that the incorporation of large amount of Zn into spinel from exotic, metasomatic fluids and possibly incorporation of Fe 3+ into spinel under high oxidizing conditions may have shifted the stabilization of co-existing spinel + quartz to T
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petrology phase equilibria modelling and in situ zircon and monazite geochronology of ultrahigh temperature granulites from the Khondalite belt of southern india
Lithos, 2019Co-Authors: Bing Yu, M Santosh, Shanshan Li, E ShajiAbstract:Abstract The granulite facies metapelite (Khondalite) belt in the Trivandrum Block of southern India has been central to investigations on extreme crustal metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic-Cambrian. Here we investigate garnet-sillimanite-cordierite-spinel-bearing metapelites from this Khondalite belt using state-of-the-art petrologic, mineral phase equilibria modelling, and coupled zircon and monazite U-Pb geochronology to characterize the nature and timing of metamorphism and their tectonic implications. From textural studies and mineral phase equilibrium modelling, we infer that Sill + Grt + Crd representing prograde metamorphism was stable at 6–9 kbar and 760–790 °C. Equilibrium spinel-quartz assemblage suggests peak metamorphic conditions of 6.5–7 kbar and 1010–1030 °C consistent with ultra-high temperature metamorphism in the Trivandrum Block. Growth of cordierite and biotite at the expense of garnet correlates with retrograde metamorphism at 4.5–6.5 kbar and 770–950 °C. Our results allow an interpretation of the prograde, peak, and retrograde P–T conditions of the Khondalites, where from the peak ultrahigh-temperature stage of >1000 °C, the rocks underwent isothermal decompression as indicated by garnet breakdown to cordierite, followed by isobaric cooling, corresponding to an overall clockwise P–T evolution. We present results from U-Pb geochronology of zircon as well as LA-ICPMS data on monazite grain separates. The zircon grains show two distinct age peaks, with Paleoproterozoic cores surrounded by Late Neoproterozoic-Cambrian rims or recrystallized domains. The younger ages around 550–560 Ma are similar to the lower intercept age, whereas the wide range of Paleoproterozoic ages fall along a discordia. The LA-ICP-MS monazite U-Pb data also show two distinct age populations at Paleoproterozoic and latest Neoproterozoic-Cambrian, although the Paleoproterozoic population is scarce. We correlate the late Neoproterozoic-Cambrian ages to the ultrahigh-temperature metamorphic event, associated with the assembly of the Gondwana supercontinent. The wide range of monazite ages indicate that high temperature metamorphism was long-lived.
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late palaeoproterozoic depositional age for Khondalite protoliths in southern india and tectonic implications
Precambrian Research, 2016Co-Authors: Alfred Kroner, M Santosh, E ShajiAbstract:Abstract SHRIMP dating of detrital zircons from garnet–sillimanite–cordierite paragneisses (Khondalites) of the Trivandrum Block, southern India, revealed nanometer-scale Pb isotopic inhomogeneity in most grains that we ascribe to annealing processes during ultra-high-temperature metamorphism at ca. 570 Ma. Our age data for zircons from six representative Khondalite samples do not document any Neo- or Mesoproterozoic detrital grains, and we conclude from the concordant ages and discordant minimum 207Pb/206Pb ages that the Khondalite precursor sediments were deposited more than 2.1 Ga ago and were subsequently intruded by granitoid rocks at ca. 1765–2100 Ma. Some detrital zircons in the Khondalites contain late Palaeoproterozoic metamorphic domains, suggesting that these grains are derived from an unknown crustal source that most likely experienced late Palaeoproterozoic high-grade metamorphism. Both the metasedimentary assemblage and granitoids were severely ductilely deformed, metamorphosed and migmatized during the pervasive Pan-African event at ca. 550–580 Ma. This caused many detrital zircons in the Khondalites to become variably recrystallized and to develop metamorphic rims. Proper interpretation of cathodoluminescence images and zircon morphology is important in interpreting detrital populations. The southern Indian Khondalites were part of an extensive Palaeoproterozoic metasedimentary assemblage that may have extended from southern Madagascar via the Trivandrum Block to the Highland Complex of Sri Lanka and seems to reflect a continental margin sequence that was deposited on an Archaean to early Palaeoproterozoic continental terrane, possibly the southern margin of the Indian Dharwar craton. The tectonic history of this terrane remains obscure due to the pervasive overprint during the Pan-African event.
