Golpayegan Metamorphic Complex (Sanandaj–Sirjan Zone, Iran) as Evidence for Cadomian Back-Arc Magmatism: Structure, Geochemistry and Isotopic Data
S. Ahmadi-BonakdarS. M. Tabatabaei ManeshAlireza NadimiAkramossadat MirlohiJ. F. SantosO. V. Parfenova
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Quartzofeldspathic gneisses near zinc–lead–gold mineral deposits at Montauban-les-Mines, Quebec, have been examined geochemically to determine whether their protoliths were dominantly sedimentary or igneous. The gneisses are generally similar in average compositions to both rhyolitic rocks and sandstones (greywackes in particular). The most useful methods of protolith discrimination were found to be: (1) log (SiO 2 /Al 2 O 3 ) versus log [(CaO + Na 2 O)/K 2 O]; (2) Niggli si versus mg; (3) normative quartz; (4) Shaw's discriminant function. Application of these criteria indicates that the Montauban quartzo-feldspathic gneisses were derived dominantly from quartzose sandstone or greywacke protoliths (or both).
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To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The206Pb/238U concordant age of zircon is505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P2O5and Al2O3content decreased with SiO2increasing. Th and Y contents have a good positive correlation with Rb content;the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andeantype orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.
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Three gneiss samples were taken from Wanjialaowu at Yingshan and Yinjiadian at Qidaogou, Hubei Province, and Liji of Dingbuxiang, Anhui Province, in the western part of the northern Dabie Mountains. Backscattered electron (BSE) analysis of the microstructure of zircons from the three samples was performed, and on that basis zircon U-Pb ages were determined using the U-Pb ion probe. The results indicate that the protoliths of two types of gneiss in the study area were formed in the Jinningian and Yanshanian respectively. The protolith of the Wanjialaowu gneiss at Yingshan was formed at 794 Ma and its zircon U-Pb age was little affected by the late-stage geological event. The age of the Yinjiadia gneiss at Qidaohe is 730 Ma, and as it was probably affected by the Yanshanian magmatic event, a metamorphic age of about 140 Ma was obtained. The age of the Liji gneiss at Dingbuxiang is 140 Ma. From these ages combined with the available chronological results , it may be inferred that the Jinningian magmatic event occurred extensively in the northern Dabie terrain, which indicates that the area was likely to be the northern margin of the Yangtze plate.
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Tonalitic orthogneiss from the Brookville Gneiss in southern New Brunswick has an igneous protolith age of $$605 \pm 3 Ma$$ based on U-Pb dating of zircon. Metamorphic titanite from the same sample gives a minimum age of $$564 \pm 6 Ma$$ for upper amphibolite facies metamorphism of the unit. Based on U-Pb analyses of single detrital zircons, a maximum sedimentary protolith age of about 641 Ma is suggested for associated paragneiss. These ages indicate that the igneous protolith of the orthogneiss was at most 35 Ma younger than the sedimentary rocks it intruded, and that the Brookville Gneiss is younger than the Green Head Group to which it was previously considered basement. Both orthogneiss and paragneiss in the Brookville Gneiss contain Proterozoic and Archean detrital zircon grains, suggesting an old, heterogeneous continental source. We conclude that the Brookville Gneiss does not represent the basement to the Avalon Terrane.
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The Mohe gneiss is located at the eastern part of the Quanji Massif.It is a strongly metamorphosed and deformed granodioritic-monzograntic pluton.Previously reported zircon ages for intrusion of its protolith show a difference about ~120 Ma.On the basis of detailed geological survey in the field,we re-collected sample and determined 18 U-Pb data on 15 magmatic zircons using LA-ICP-MS technique.The results show an upper intercept age of(2 394±31)Ma(2σ)(MSWD=0.82).Since the zircons have been experienced intensive metamorphism and deformation,we consider that our result is more reasonable because of the intrusion and crystallization age of the gneiss protolith.Our new dating result reveals a granitic intrusion event occurred at ~2.4 Ga in the Quanji Massif.
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