Field geology of high-grade gneiss terrains
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Journal Article Zoned Ultrabasic and Basic Gneiss Masses in the Early Lewisian Metamorphic Complex at Scourie, Sutherland Get access M. J. O'HARA M. J. O'HARA Grant Institute of Geology, University of EdinburghScotland Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 2, Issue 2, 1961, Pages 248–276, https://doi.org/10.1093/petrology/2.2.248 Published: 01 June 1961
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U-Pb dating of zircon was undertaken with the Beijing SHRIMP II (sensitive high resolution ion microprobe) on anamphibolite facies granodiorite and an almandine-albite granulite from the Santa Maria Chico Granulitic Complex, southern Brazilian Shield. This work was also done to unravel protolith ages which are often hidden in the array of partly reset data. The obtained metamorphic ages of the granodiorite gneiss and the granulite are 2035 ± 9 Ma and 2006 ± 3 Ma, respectively. These data are within the range of metamorphic ages determined in previous studies (2022 ± 18 Ma and 2031 ± 40 Ma). However, protolith ages for the granodiorite (2366 ± 8 Ma) and the granulite (2489 ± 6 Ma) were obtained which are outside the previously recognized range (> 2510-2555 Ma). The magmatic protolith age of the granodiorite refers to a previously little known magmatic event in the shield. Further investigations may demonstrate that amphibolite facies zircon crystals are useful as a window into geological events in associated granulites, because zircon ages are blurred in the studied granulites.
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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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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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Abstract Ultrahigh‐temperature ( UHT ) granulites in the S outh A ltay orogenic belt of N orthwestern C hina provide important clues for the lower crustal components and tectonic evolution of the C entral A sian O rogenic B elt during the P aleozoic. In this paper, we studied whole‐rock geochemistry and mineral characteristics to understand the protolith and metamorphic evolution of the A ltay UHT granulite. The A ltay granulite shows negative discriminant function values ( DF ) of −9.27 to −3.95, indicating a sedimentary origin, probably an argillaceous rock. The peak metamorphic temperature–pressure conditions of 920–1010 °C and > 9 kbar were estimated from the geothermobarometry, together with the stability of spinel (low ZnO ) + quartz and orthopyroxene ( Al 2 O 3 up to 9.2 wt.%) + sillimanite + quartz in the Altay UHT rock, indicate a UHT metamorphic condition has been achieved. Two stages of retrograde conditions are recognized in these rocks; the first is an isothermal decompression to approx. 750 °C at 5.2–5.8 kbar at the early stage, and the second is the cooling down to 520–550 °C at 4.8–5.2 kbar. Combined with previous study, the formation of the A ltay UHT pelitic granulite with a clockwise retrograde P – T path is inferred to be related with collisional and accretional orogenic process between the S iberian and K azakhstan– J unggar plates.
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