U-Pb Ages of Zircons From the Foot Bay Gneiss and the Donaldson Lake Gneiss, Beaverlodge area, northern Saskatchewan: Reply
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Intensely metamorphosed and deformed basic dykes, the Ameralik dykes, have been used to divide the amphibolite-facies gneisses of the Godthåb district into the Amitsoq gneisses (older) and the Nuk gneisses. Metavolcanic and metasedimentary rocks (the Malene supracrustals), and stratiform meta-anorthosites are also present and are probably younger than the Amitsoq gneisses, but are older than the Nuk gneisses. The Amitsoq gneisses contain abundant fragments of Ameralik dykes. They appear to have been derived from homogeneous granitic (s.l.) parents, but most of them have been intensely reworked before and/or after the intrusion of the Ameralik dykes and are now banded gneisses. They range from dioritic to granitic in composition and potassic varieties are common. Isotopic data indicate that the parent rocks of the gneisses were emplaced or metamorphosed about 3750 Ma ago. Ameralik dykes are absent from the Nuk gneisses, which are the most abundant rocks in the area. These gneisses are derived from intrusive calc-alkaline parents, mainly tonalites and granodiorites, and represent a massive addition of granitic material to this level of the crust about 3080 Ma ago, after the first supracrustal rocks had been laid down.
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We performed petrological, geochemical, and geochronological study for the Pyeonghae granite gneiss and the Hada leuco-granite gneiss intruding the Paleoproterozoic meta-sedimentary rocks (pyeonghae formation and Wonnam formation) of the Pyeonghae area located in northeastem part of the Yeongnam (Sobaeksan) massif. The Pyeonghae granite gneiss generally has higher abundance of mafic minerals (biotite etc.), and posesses higher , MgO, CaO, , contents but lower and contents than the Hada leuco-granite gneiss which tends to have slightly high and contents and slightly high larger negative Eu anomalies. However both gneisses reveal very similar REE concentrations and chondrite-normalized patterns and apparently show differentiation trend affected by crystallization of biotite, plagioclase, apatite and sphene. Their peraluminous and calc-alkaline chemistry suggests tectonic environment of volcanic arc. SHRIMP Zircon U-Pb age determinations yield upper intercept ages of () and (), and weighted mean ages of () and () for the Pyeonghae granite gneiss and the Hada leuco-granite gneiss respectively, showing overlapping ages within the error. Our study suggests that the Precambrian granitoids in this area intruded contemporaneously with the Buncheon granite gneissin volcanic arc environment.
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Extremely 18O-depleted zircons from granitic gneisses, with δ 18O values as low as -7.8‰, were found in the Zaobuzhen area in the Weihai region, at the northeastern end of the Sulu orogen. SHRIMP zircon U-Pb ages and the oxygen isotope compositions were determined for the low δ 18O zircons. CL images reveal that the low δ 18O zircons are primarily of igneous origin, but some grains underwent metamorphic recrystallization. The igneous zircons from a granitic gneiss sample yield a concordant U-Pb age of (760±49)Ma and an upper intercepted age of (751±27)Ma, indicating a protolith of Middle Neoproterozoic age. Metamorphic zircons from the same sample yield a concordant U-Pb age of (232±4)Ma and a lower intercepted age of (241±33)Ma, pointing to Triassic UHP metamorphism. Most of the igneous zircons have unusually variable δ 18O values of -7.76‰ to 5.40‰, indicating that the gneiss protolith was intruded as low δ 18O magma that was generated by the partial melting of altered rocks, which suffered intensive water-rock interaction with a low δ 18O fluid at high temperatures during the Neoproterozoic. The preservation of extreme 18O-depletion in the zircons suggests that there is no remarkable oxygen isotope exchange between the metagranite and the mantle during the processes of Triassic subduction and exhumation. The protolith nature, metamorphic timing and oxygen isotope compositions of the granitic gneisses in the Weihai region are similar to those of granitic gneisses in the Qinglongshan area in the southwestern part of the Sulu orogenic belt, indicating that the gneisses along the Sulu orogenic belt share the same nature of protolith origin, water-rock interaction and UHP metamorphism. The present study provides tight constraints not only on the origin of extremely 18O-depleted zircons, but also on the protolith nature of granitic gneisses in the Sulu orogenic belt.
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The Korean peninsula is part of the Sino-Korean platform and is composed of Precambrian to Holocene rocks. It can be divided into three cratonic blocks, the Nangrim massif of North Korea, and the Gyeonggi and Ryeongnam massifs of South Korea. The Precambrian rocks in the Ryeongnam massif are dominantly plutonic, while in the Gyeonggi massif the metasedimentary rocks are dominant. This Precambrian basement is overlain by Phanerozoic rocks and intruded by Mesozoic plutonic rocks. This study reports U-Pb zircon ages for Precambrian rocks in the southwestern Ryeongnam and southwestern Gyeonggi massifs. The oldest rocks in both massifs belong to the Gneiss complex which is composed of migmatitic and other gneisses, metasedimentary rocks, and mafic plutonic rocks. In the Ryeongnam massif the Gneiss complex is intruded by granite and porphyroblastic orthogneisses. The Kurye granite gneiss has an age of 2120 and 2113 Ma. The Chailbong granite gneiss is younger, having an age of 1923 Ma. The porphyroblastic gneisses were probably emplaced in at least two episodes. The older ones are dated at 1945, 1935, and 1928 Ma, and a younger porphyroblastic gneiss has an age of 1890 Ma. In the Gyeonggi massif the Gneiss complex is intruded by two granite orthogneisses. The Seosan granite gneiss has an age of 1766 Ma and the Hongseong granite gneiss is 687 Ma old. The age for the Hongseong granite gneiss is a lower concordia intercept age, the upper intercept indicates derivation from a ca. 3 Ga protolith.
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