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Metamorphic disturbances of magnetite chemistry and the sm-nd isotopic system of reworked Archean iron formations from NE Brazil

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Geoscience Frontiers 2022年第5期13卷 227-241页

作者： Joice Dias de Moraes Pedro Cordeiro Eduardo Abrahão Filho Juliana Rezende Oliveira Carlos Victor Rios da Silva Filho Instituto de Geociências Universidade de Brasília(UnB)70910-900 BrasíliaDFBrazil Department of Mining Engineering Pontifical Catholic University of Chile7820436 SantiagoChile

Iron formations are valuable archives of sedimentary conditions and post-depositional events.However,geochemical proxies commonly used to determine genetic characteristics can be variably modified during metamorphism and deformation,hampering their use as records of regional geological events.This work focuses on strongly reworked magnetite-quartz-rich rocks from the São Josédo Campestre Massif,one of the oldest fragments of preserved crust in South America.The genetic classification of these magnetite-quartz-rich rocks is not straightforward because primary assemblages and textures were variably modified by granulite facies metamorphism during a regional Paleoproterozoic migmatization event.To address genetic ambiguities,we analyzed their magnetite and pyroxene chemistry,wholerock geochemistry,and sm-nd isotopes.Magnetite chemistry indicates that pyroxene-poor iron formations(Type B)are low in trace elements such as Ti,Al,V,and Mn,suggesting a chemical similarity to iron formations elsewhere.In contrast,magnetites from pyroxene-enriched Type A iron formations are rich in trace elements and more akin to magnetite crystallized from higher temperature systems,such as skarn and IOCG.The^(147)sm/^(144)nd of these rocks show substantial variation even at the outcrop scale,indicating a locally-controlled,highly heterogeneous mixture of Archean,Paleoproterozoic,and Neoproterozoic sources.Therefore,our geochemical tools point out to heterogenous signatures of these magnetitequartz rocks and proxies compatible with both low and high-temperature conditions and age of deposition spanning sources from the Archean to the Neoproterozoic.We interpret that the studied São Josédo Campestre magnetite-quartz rocks represent Archean iron formations with original magnetite chemistry and isotopic signatures variably modified by metamorphism and by at least one deformation-related hydrothermal event.These results contrast with similar examples from China and Greenland where iron formations either preserved the magnetite chemistry or th

关键词： Metamorphic disturbances of magnetite chemistry and the sm-nd isotopic system of reworked Archean iron formations from NE Brazil

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Effects of retrograde-zoning of garnet on sm-nd isotopic dating of eclogite and oxygen isotopic disequilibrium between eclogitic minerals

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Science China Chemistry 2002年第Z1期45卷 28-37页

作者：李曙光刘贻灿徐树桐郑永飞 School of Earth and Space Sciences University of Science and Technology of China Hefei 230026 China Anhui Institute of Geology Hefei 230001 China

The compositional zoning of the garnet in a strongly deformed eclogite from Raobazhai foliated peridotite has been recognized. The CaO concentrations of the garnet are decreased from the core to the rim, while its MnO concentrations are increased, suggesting the retrograde origin of such CaO-MnO zoning. The tie line of garnet + omphacite from this eclogite gives a sm-nd age of (187±5) Ma, which is less significant than the sm-nd ages of (221±5)-(228±3) Ma and (210±6)-(214±6) Ma for ultrahigh-pressure eclogites in the southern Dabie zone and in the northern Dabie zone, respectively. This younger sm-nd age could result from the 143nd/144nd ratio decrease of the retrograde zone in the garnet. The δ18O values of the garnet and omphacite show that their fractionation values are less than the equilibrium fractionation value between the garnet and omphacite at 500-900℃, which suggests an oxygen isotopic disequilibrium between them.

关键词： eclogite, sm-nd isotopic dating, oxygen isotope, zonation of garnet, Dabie Mountains.

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sm-nd isochron age evidence for Precambrian metamorphic rocks from the eastern Junggar in northern Xinjiang, China

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Chinese Science Bulletin 1996年第22期41卷 1896-1899页

作者：张前锋胡霭琴张国新陈义兵聂勇郭德英 Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China No. 704 Geological Team BNGE Hami 839000 China

The characterstic of the basement of Junggar block has been a topic of long-standingdebate.Based on the data of gravity and magnetic anomaly,some researchers deduced thatthere were Precambrian basic-ultrabasic complex on the basement of Junggar block.However,others maintained that the basement of Junggar block was probably the Paleozoicoceanic crust because of the lack of isotopic evidence and older outcrops aroundthe Junggar Basin. In recent years,molecules of fossil community were discovered from Aleandao Groupat the Qingshuiquan and Kamuster of eastern Junggar.Most of the fossils,such asPylocorpus,Retincnlafopora,Asteropyla,were found at the boundary between Cambrianand Sinian in Qinling and South China.An isotopic age of 1908 Ma for leuco-pink graniticgneiss from the lower Huangcaopo Group was gained by evaporated Pb-isotope method forsingle zircon in Xiaoluigou of eastern Junggar by Zhang et al.But all of these results onlyindicate a possibility that the basement of Junggar block consists of

关键词： eastern Junggar Precambrian metamorphic rocks sm-nd age.

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sm-nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry

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Science China Earth Sciences 2005年第12期48卷 2081-2091页

作者： HOU Zhenhui1, LI Shuguang1, CHEN Nengsong2, LI Qiuli1 & LIU Xiaoming3 1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China 2. The Key Laboratory of Continental Dynamics of the Ministry of Education, Northwest University, Xi’an 710069, China 3. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China School of Earth and Space Sciences University of Science and Technology of China Hefei China The Key Laboratory of Continental Dynamics of the Ministry of Education Northwest University Xi’an China Faculty of Earth Sciences China University of Geosciences Wuhan China

The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The sm-nd isochron defined by granulite-facies meta-morphic minerals (garnet + clinopyroxene + hypersthene) yields an age of 136 ± 18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the granulite show clearly core-mantle-rim structures. The zircon cores are charac-terized by typical oscillatory zoning and highly HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb ages rang-ing from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neopro-terozoic mafic rocks. The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3―10 times lower REE, Th, U, Y, Nb and Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb age (716―780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granu-lite-facies metamorphism underneath the Dabie Mountains. The similarity between the granulite metamorphic age (136 ± 18 Ma) defined by sm-nd isochron and K-Ar age of 123―127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome, which may result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.

关键词： Dabie orogen, granulite, zircon U-Pb ages, sm-nd age.

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sm-nd isotopic age and genesis of gabbro xenoliths in Daoxian County, Hunan Province

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Chinese Science Bulletin 1997年第21期42卷 1814-1817页

作者： Guo, F Fan, WM Lin, G Lin, YX Chinese Acad Sci Changsha Inst Geotecton Changsha 410013 Peoples R China

MANY geologists pay attention to Mesozoic tectonomagmatic activity and the associating multi-metal ore deposits in South China. However, the age of the transformatiori of tectonic regimein the area is little studied. Occurrence of gabbro xenoliths in Mesozoic alkaline basalts indaoxian region, Hunan Province, shows that the lithosphere in the region was intensely ex-tended and thinned during the Mesozoic, followed by underplating of mantle-derived mag-

关键词： gabbro xenoliths sm-nd dating petrogenesis.

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Fossil sm-nd Isotopic Dating of the Precambrian-Cambrian Boundary in the Meishucun Section, Jinning, Yunnan, China

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Science China Chemistry 1993年第4期36卷 463-471页

作者：杨杰东王银喜孙卫国王宗哲 Center of Modern Analysis Naniing University Nanjing 210008 PRC Nanjing Institute of Geology and Palaeontology Academia Sinica Nanjing 210008 PRC

It was for the first time that the fossil sm-nd isochron method was used to date the Precambrian-Cambrian boundary. The samples were collected from the Zhongyicun Member in the Meishucun section of Jinning County,Yunnan Province, southern China. The date of 562.8±7.9Ma B.P. was obtained from four hyolithid fossil samples; another date of 562.1±5.7Ma B.P. was provided by four hyolithid fossil samples and two collophanite samples. These figures suggest that the Precambrian-Cambrian boundary reference point A, which is slightly below the Zhongyicun Member, might be about 570Ma old, and the reference point B, which is near the top of the Zhongyicun Member, could be approximately 560Ma in age. This result contributed valuable reference to precise calibration of the base of the Cambrian System in the global geological time scale.

关键词： fossil sm-nd method Precambrian-Camhrian boundary.

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sm-nd age dating of high-pressure granulites and amphi-bolite from Sanggan area,North China craton

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Chinese Science Bulletin 2001年第2期46卷 106-111页

作者： Jinghui Guo Mingguo Zhai Chinese Acad Sci Inst Geol & Geophys Beijing 100029 Peoples R China

The high pressure (HP) metamorphic age has been dated to HP rocks from the Sanggan area, North China craton. We have got garnet+whole rock isochron ages of (1 842±38) Ma for HP granulite, and (1 856± 26) Ma for HP amphibolite. The sm-nd whole rock isochron of HP granulites give out an age of (1870±150) Ma with nd depleted mantle model age of (2402-2 482) Ma. Considering the nd isotope homogenization during the peak metamorphism of the HP granulite, sm-nd closure temperature and the retention of nd isotopic memory in garnets partially broken down during decompression, all these isochron ages are thought to be HP metamorphic age. Furthermore, we proposed that the HP metamorphism took place at the end of Paleoproterozoic during the large-scale collision and assembly of the North China craton.

关键词： sm-nd dating high pressure granulite Paleoproteroz-oic collision North China craton.

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sm-nd isochron age of ophiolite along northern margin of Altun Tagh Mountain and its tectonic significance

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Chinese Science Bulletin 1999年第5期44卷 456-458页

作者： Zhaojie Guo Zhicheng Zhang Jianjun Wang Peking Univ Dept Geol Beijing 100871 Peoples R China

An ophiolite belt is exposed to the northern edge of Altun Tagh Mountain. Geochronological researches were made on gabbro and basalt. sm-nd isochron age of gabbro is (829 ± 60) Ma, while the age of gabbro mixed basalt is (949 ± 62) Ma. The dating of sm-nd isochron proves that ophiolite formed in (829 ± 60) Ma, which implies that the northern half of Tarim (or north Tarim Block) had been separated by an ocean from the southern half of Tarim and Qaidam (or south Tarim Block) until (829 ± 60) Ma. The south Tarim Block could accrete to the north Tarim Block at the beginning of Sinian,thereby forming the north Altun Tagh suture. The Sinian system would be the first cover on the amalgamated Tarim craton.

关键词： Alton Tagh Mountain ophiolite sm-nd isochron age.

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sm-nd and Rb-Sr isotopic geochronology of the Xilin Gol Complex, Inner Mongolia

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Chinese Science Bulletin 1996年第13期41卷 1107-1110页

作者：徐备陈斌邵济安 Department of Geology Peking University Beijing 100871 China

1 Geologic setting The Xilin Gol Complex outcrops in the Bayan Gol area about 100 km to the northeast of Xilinhot City, Inner Mongolia (fig.1). Shao considered that the Xilin Gol Complex was an old tcrrane in the Pale... 详细信息

关键词： sm-nd and Rb-Sr isotopic geochronology Inner Mongolia.

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sm-nd isotope study and dating on Honguleleng ophiolite in Xinjiang, China

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Chinese Science Bulletin 1999年第20期44卷 1914-1917页

作者： Zhangdong Jin Jianhua Huang Fuchun Li Nanjing Univ State Key Lab Res Mineral Deposits Nanjing 210093 Peoples R China

A study of the sm-nd isotope composition of Honguleleng ophiolite in Xinjiang, China, isochron age ((626±25)Ma),εnd(t) = 8.40,and its model age (TndDM) in the range of 1 180-1 208 Ma, indicates that the ophiolite was formed in the late stage of the Upper Sinian. In combination with its facies association, petrochemistry, REE patterns, trace elements

关键词： ophiolite sm-nd isochron age Honguleleng Xinjiang.

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