U Pb single grain zircon ages for Sanbagawa Metamorphic

U Pb Single Grain Zircon Ages For Sanbagawa Metamorphic-Free PDF

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40 Knittel U Walia M Suzuki S and Lee Y H, For the Besshi unit one of the corner stones for the. assumed peak metamorphism at ca 116 Ma is a Rb Sr, isotope study of Minamishin et al 1979 who analysed 21. whole rock samples collected within a small area in the. upper reaches of the Asemi River in central Shikoku An. isochron age of 116 10 Ma was calculated and has since. been considered to date peak metamorphism of the,Sanbagawa Metamorphic Rocks e g Isozaki and Itaya. 1990 Aoki et al 2007 Otoh et al 2010 Aoki et al 2011. Itaya et al 2011 Itaya and Tsujimori 2015 This view was. supported by single grain ages of 132 112 Ma obtained for. zircon considered to be of metamorphic origin from the. eclogite unit Okamoto et al 2004 In addition eclogite. exhibiting partial melting textures contains zircons with rim. ages of 115 104 Ma Arakawa et al 2013,In contrast Wallis and co workers Endo et al 2009. Wallis et al 2009 Wallis and Endo 2010 based on Lu Hf. dating of garnet pyroxene pairs in eclogite suggest that. Sanbagawa metamorphism actually occurred at ca 88 89. Ma Aoki et al 2009 also reported an U Pb age of 86 3. Ma for metamorphic zircon from the Besshi unit but. ascribed this to retrograde metamorphism Knittel et al. 2014 and Endo et al 2018 report ages for detrital zircon. younger than 90 Ma from the purported Sanbagawa, Metamorphics within the Besshi unit Thus the question.
of the age of the Besshi unit of the Sanbagawa Belt is. contentious in part due to the uncertain interpretation of the. 116 Ma Rb Sr age e g Aoya and Endo 2017, In order to resolve the discussion on the significance of. the Rb Sr age obtained by Minamishin et al 1979 we re. sampled the area studied by these authors and determined. the U Pb ages of detrital zircon in order to determine the. maximum protolith ages of these rocks This is based on the. notion that the youngest detrital zircon is older than the. sediment hosting this zircon,The Sanbagawa Metamorphic Belt extends over a. distance of about 800 km across south western Japan Fig. 1 from Kyushu to the Kanto Mountains in Honshu To the. north it is bounded by the Median Tectonic Line In the. south the Sanbagawa Belt is bounded by the Jurassic. Chichibu composite belt Further south separated from the. former by the Butsuzo Tectonic Line the Shimanto, accretionary belt is exposed The latter is subdivided by the. Aki Tectonic Line into the northern part comprising strata. of late Cretaceous age and the southern part composed of. Paleogene to Miocene strata, In Shikoku three tectonostratigraphic units that differ in. age metamorphic grade and perhaps also chronology are. distinguished within the Sanbagawa Belt the Oboke unit. Fig 2 Profile along the Asemi River showing our sample. locations with ages of the youngest zircons as well as K Ar ages the Besshi unit and the eclogite unit Takasu and Dallmeyer. for selected locations from Itaya and Takasugi 1988 open 1990 Wallis and Aoya 2000. circles and Ar Ar ages obtained by Takasu and Dallmeyer 1990 The Oboke unit composed largely of metasandstone. squares numbers are the ages obtained NAB and NC are Koboke Formation and minor metapelite Kawagushi. youngest zircons of Endo et al 2018 KA5 is the sample dated Formation Kenzan Research Group 1984 records. by Aoki et al 2009 Modified from Itaya and Takasugi 1988. U Pb single grain zircon ages for Sanbagawa Metamorphic Rocks in central Shikoku Japan 41. chlorite zone metamorphism Banno et al 1978 Based on across the Besshi unit along the Asemi River including the. geochemical signatures and petrological evidence area studied by Minamishin et al 1979 in central Shikoku. Kiminami et al 1999 inferred that the Oboke unit is the Fig 2 One sample locality lies within the northern part of. metamorphic equivalent of the Hiwasa Formation of the the albite biotite zone and the other within the southern. Shimanto accretionary complex that was deposited in the chlorite zone Itaya and Takasugi 1988 presented K Ar. Campanian 84 72 Ma Ages between 96 and 102 Ma and ages of white mica for the traverse along the Asemi River. between 89 and 98 Ma for zircon in granite cobbles ranging from ca 65 Ma in the northern chlorite zone to ca. obtained by Manabe et al 1996 and Aoki et al 2007 82 Ma in the albite oligoclase biotite zone Takasu and. respectively are consistent with this suggestion In addition Dallmeyer 1990 presented additional 40Ar 39Ar ages that. Aoki and co workers Aoki et al 2007 Aoki et al 2012 largely confirm the results of Itaya and Takasugi 1988 and. reported ages for detrital zircon from the Oboke unit as also presented detailed samples descriptions that illustrate. young as 82 Ma that likewise support the suggested the lithologies exposed along the traverse Mori and Wallis. correlation 2010 presented a tectonic analysis of the Asemi region and. The Besshi unit consists mainly of metapelite and described kilometer scale folding in the central part of the. metabasite and has been sub divided into chlorite garnet Asemi gawa region that explains the distribution of the. albite biotite and oligoclase biotite zones with increasing metamorphic zones. metamorphic grade e g Higashino 1990 Kiminami, 2010 used geochemical and petrographic arguments to Analytical procedures.
suggest that the low grade chlorite zone of the Besshi unit The samples were thoroughly cleaned before crushing. are the metamorphic equivalents of late Albian to Zircons were separated by conventional methods and were. Campanian ca 100 80 Ma sediments of the Shimanto then hand picked and mounted using epoxy resins Prior to. accretionary complex In contrast Aoki et al 2007 2001 analysis Cathodoluminescence images CL were taken at. and others suggest that large parts of the Besshi unit are part the Institute of Earth Sciences Academia Sinica Taipei. of the older Sanbagawa metamorphic rocks Subsequently The U Pb analyses were carried out using an Agilent 7500s. detrital zircon younger than 90 Ma has been found at a quadruple inductively coupled plasma mass spectrometer. number of sites considered to be part of the Sanbagawa ICP MS equipped with a Photon Machines Analyte G2. metamorphics Knittel et al 2014 Endo et al 2018 laser ablation system with a beam diameter of 30 m at. The eclogite unit nappe records the highest metamorphic the Department of Geosciences National Taiwan. grade Wallis and Aoya 2000 Miyamoto et al 2007 Endo University Taipei The analytical procedures are the same. and Tsuboi 2013 but eclogite facies metamorphism as described in Chiu et al 2009 and Knittel et al 2010. appears to have not been restricted to the eclogite unit as Zircon GJ Jackson et al 2004 was used for calibration. relicts of such metamorphism were also detected in parts of Data quality was controlled by analyzing zircon standards. the Besshi unit The units traditionally considered as 91500 207Pb 206Pb age 1065 Ma Wiedenbeck et al 1995. eclogite unit consist largely of mafic and ultramafic rocks and Ple ovice 206Pb 238U age 337 1 0 4 Ma Sl ma et al. Isotopic and petrologic evidence suggest that the evolution 2008 at the beginning of each run Values obtained during. of the eclogite unit was not straightforward comprising only the period of our study were 336 1 1 6 Ma n 20 for the. one phase of progressive metamophism and one phase of Ple ovice zircon and 1070 1 4 6 Ma n 22 for 91500 U. retrograde metamorphism e g Endo et al 2009 Endo et Th Pb isotope ratios were calculated using the GLITTER. al 2012 Aoya and Endo 2017 Radiometric ages 4 0 GEMOC software and common lead was corrected. determined for the eclogite unit Okamoto et al 2004 following Andersen 2002 Concordia plots and. Endo et al 2009 Wallis et al 2009 Arakawa et al 2013 probability density plots were produced using Isoplot v 3 0. therefore may have no implications for the Besshi unit Ludwig 2003 Analyses of broad rims in sample SJ 13. In the past few years evidence has emerged that parts of the were calculated on the basis of the first 15 seconds of data. traditional Besshi unit were initially subjected to eclogite acquisition in order to avoid including the core in the. facies metamorphism prior to retrograde metamorphism in analysis. the epidote amphibolite facies in particular in the. geographic Besshi area Kouketsu and Enami 2010 Samples and Results. Kouketsu et al 2010 Kouketsu et al 2014 but also in the Samples JS 11 JS 13 and JS 14 Fig 2 were collected. Asemi River area Taguchi and Enami 2014 Taguchi et al in the area studied by Minamishin et al 1979 244 spots. 2018 Knittel et al 2019 based on their own results and were dated and 61 97 and 50 concordant ages were. those of Endo et al 2018 have shown that the schists in obtained for samples JS 11 JS 13 and JS 14 respectively. the Asemi River area that initially experienced eclogite Fig 3 Table 1 Most grains exhibit oscillatory zoning in. facies metamorphism have protoliths younger than 90 Ma CL images Fig 4 and all but one has Th U 0 21 1 25. consistent with the views of Wallis and coworkers Endo et Both features are typical for magmatic zircon Corfu et al. al 2009 Wallis et al 2009 2003 Hoskin and Schaltegger 2003 The youngest ages. Our study is focused on the well studied N S traverse are 80 4 Ma 2 error Fig 3 for sample JS 11 85 4. 42 Knittel U Walia M Suzuki S and Lee Y H, Fig 5 Probability density plots for Mesozoic zircons based on. concordant grains number refers to the number of concordant. Mesozoic grains, Fig 3 Concordia diagrams for Mesozoic zircons insets show the. youngest zircons, Fig 6 Concordia diagram for depth profiles and rim analyses of. zircons in sample JS 13 Insets show CL pictures of analyzed. Fig 4 CL pictures of the youngest zircons scale bar is 100 m grains. U Pb single grain zircon ages for Sanbagawa Metamorphic Rocks in central Shikoku Japan 43. Ma for sample JS 13 and 92 4 for sample JS 14 In sample will be addressed in the following sections. JS 11 most concordant grains fall into the range 80 96Ma. Protolith age of the schists of the Besshi unit, n 39 whereas in sample JS 13 most grains fall into the Large parts of the Besshi unit were considered to be part. range 91 110 Ma n 77 and in JS 14 into the range 161 of the Sanbagawa Metamorphic Rocks metamorphosed at. 197 Ma n 27 Fig 5 All three samples contain a ca 110 120 Ma This was based on the Rb Sr age of 116. number of grains of Precambrian age but no grains were 10 Ma obtained by Minamishin et al 1979 and the lack. found in the range 300 Ma to 1 8 Ga of zircon younger than 150 Ma in two samples from the. Some zircons of sample JS 13 in CL exhibit very bright Besshi unit Aoki et al 2007 2012 Subsequent work has. rims that may be of metamorphic origin We have attempted shown however that zircon younger than 90 Ma exists in. to date these where they are relatively broad Fig 6 In the chlorite zone in the northwest of Shikoku Knittel et al. order to restrict the analysis as much as possible to the rim 2014. we used in general only the first 20 seconds of measuring The samples studied here come from the Asemi River. time for the age calculation and obtained ages ranging from area previously considered to be underlain by Sanbagawa. 80 4 Ma to 91 4 Ma In all cases the rim ages are Metamorphic Rocks Samples JS 11 13 and 14 were. younger than the core ages but only the youngest rim has collected in the area studied by Minamishin 1979 and. Th U 0 01 typical for metamorphic zircon whereas the contain numerous detrital zircon grains in the range 80 90. others have Th U 0 11 0 23 Nevertheless all rims have Ma This shows that the results of the Rb Sr study of. lower Th U than the cores Thus most rim ages may Minamishin et al 1979 do not date the metamorphism as. represent mixtures of core and rim obviously metamorphism cannot have occurred prior to. In addition we attempted depth profiling i e we mounted deposition The age of the youngest zircon of sample JS 11. zircons on a sticky tape without embedding them and. 80 4 Ma 2 error would suggest that the maximum age. analyzed them As we did not have CL pictures before. of the protoliths is 84 Ma This is also compatible with the. analysis we could not be sure whether or not the analyzed. average age of the five youngest zircon grains of this sample. zircons had broad bright rims Since the laser in all cases. of 82 2 Ma Zircon in samples JS 13 and 14 are slightly. penetrated through the rim we again calculated the ages on. older but it must be kept in mind that the ages of detrital. the basis of the first 20 seconds of measuring time The. zircon are maximum ages that the zircon spectrum, youngest ages obtained this way are 82 4 and 87 4 Ma.
apparently may be quite variable even for samples taken in. respectively, close proximity and that younger zircons might have. 58 grains separated from sample JS 17 Figs 2 3 4,escaped detection in some samples. were analyzed and 43 grains yielded concordant ages The. Recently age data for detrital zircon has been presented. two youngest grains have ages of 92 4 Ma and 99 6 Ma. for a number of samples taken along the Asemi River. In the probability density plot a broad peak is formed by. between our sample locations JS 11 13 14 and location. zircon dated at 173 197 Ma Fig 5 similar to JS 14 A. JS 17 including samples JS 16 Knittel et al 2019 NAB. number of grains have Precambrian ages, and NC Endo et al 2018 and samples studied by Aoki et. al 2019 as shown on the map Fig 2b These samples,Discussion. Ulrich KNITTEL1 2 Monika WALIA 2 4 Shigeyuki SUZUKI 3 Yuan Hsi LEE 4 The high P low T Sanbagawa Metamorphic Belt that traverses SW Japan has been subdivided into two belts thought to have been metamorphosed at ca 120 Ma and at ca 65 Ma Sanbagawa Metamorphic Rocks and Shimanto Metamorphic Rocks The subdivision was based on the assumption that metamorphism occurred at

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