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RISS 인기검색어
- 검색키워드
- 저자 : Nutman, A.P. (검색결과 4 건)
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Nutman, A.P.,Bennett, V.C.,Friend, C.R.L.,Yi, K.,Lee, S.R. Elsevier Scientific Pub. Co., etc.] 2015 Precambrian research Vol.258 No.-
<P>The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of similar to 2690 Ma metamorphic zircon and no 2980-2950 Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two similar to 2690 Ma shear zones, and has locally preserved high pressure granulite fades assemblages and widespread growth of 2980-2950 Ma metamorphic zircon and also sporadic growth of similar to 2690 Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying >3600 and similar to 3070 Ma detrital zircons), mafic and ultramafic rocks, with similar to 2970 Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing similar to 3070 Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970-2960 Ma granites. Zircons of an Ivisartoq supracrustal belt similar to 3075 Ma intermediate volcanic rock have initial elif values of +2 to +5 thus are juvenile crustal additions. similar to 3070 Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial epsilon(Nd) values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960 Ma granites intruded into juvenile similar to 3075 Ma supracrustal rocks of the Kapisilik terrane have initial epsilon(Hf) values of -5 to -10, and must have involved the partial melting of >3600 Ma Isukasia terrane rocks. The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980-2950 Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisartoq supracrustal belt and melted at 2970-2960 Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they intruded the overlying allochthonous juvenile similar to 3075 Ma Ivisartoq supracrustal belt arc assemblages. The southern edge of the Isukasia terrane is interpreted as an interior nappe of Eoarchaean basement rocks interfolded with a klippe of Mesoarchaean metasedimentary and mafic/ultramafic rocks, both of which are affected by 2980-2950 Ma metamorphism. The mixed Eoarchaean-Mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust. The northern part of the Isukasia terrane is interpreted as foreland, free of 2980-2950 Ma high-grade metamorphic overprint. This shows that the Isukasia terrane is not a coherent block, but contains ancient rocks that are parautochthonous or allochthonous to each other, with contrasting later metamorphic history. At similar to 2690 Ma the crustal architecture arisen from Mesoarchaean collision between an older continental block and an island arc was reworked along intra-crustal shear zones, coeval with amphibolite facies metamorphism. This reworking followed on from major terrane assembly at 2710-2700 Ma in the southern part of the Nuuk region, when the Eoarchaean Faeringehavn terrane was juxtaposed with 2840-2825 Ma arc rocks. Thus the 2980-2950 Ma assembly of the Isukasia and Kapisilik terranes is distinct from the later 2710-2700 Ma terrane assembly further south in the Nuuk region. (C) 2015 Elsevier B.V. All rights reserved.</P>
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Yi, K.,Bennett, V.C.,Nutman, A.P.,Lee, S.R. Elsevier Scientific Pub. Co., etc.] 2014 Precambrian research Vol.255 No.3
The Archaean gneisses of the Nuuk area (southern West Greenland) are partitioned into tectonostratigraphic terranes - blocks of arc-like crust that evolved independently until they coalesced by collisional orogeny. To 'map' terranes to the east under the Inland Ice, sand samples were taken from rivers issuing from the edge of the Icecap; three from the Isua area and one from ~20km to the south. Bedrock along this part of the ice front consists of ~3820-3600Ma amphibolite facies rocks. 40km south of Isua (Kapisilik terrane in the Ivisaartoq area) and also from ~10km to the north there are Mesoarchaean amphibolite facies gneisses (3070-2950Ma) exposed at the ice front. In the moraine fields in the Isua area there are erratic blocks of granulite facies gneisses. These were sourced from a hidden terrane to the east under the ice because no such rocks are exposed in the Isua area. The majority of the zircons from the sand samples yielded close to concordant U-Pb ages. Apart from one 2414Ma grain, all are Archaean. The Isua sands show ~2695, 2710 and 2730Ma; then 2790, 2805 and 2840Ma clusters with a few grains back to 2950Ma and then a complex 3440-3960Ma spectrum. Less than 1% of the grains have ages between 3000 and 3400Ma. In the Isua sands, Neoarchaean and late Mesoarchaean zircons form >50% of the population. The sand 20km to the south shows a similar span of zircon ages, but there is a 2960Ma peak not seen in the Isua sands. As the Eoarchaean and late Mesoarchaean-Neoarchaean cycles progress, the spread of Th/U and @?<SUB>Hf</SUB> values seen in each zircon age population increases. This is interpreted as repeated addition of juvenile material to the crust, but also with increasing amounts of high temperature tectonothermal recycling of older materials within a package. However, there is no isotopic evidence for any contribution of Eoarchaean crust to the later ≥2750Ma Archaean cycle(s), showing evolution in disparate terranes. Furthermore, no evidence for Hadean material beneath the ice is seen in either the U-Pb ages or Hf isotopic compositions of the detrital zircons. On the other hand, 2750-2500Ma metamorphic zircons show a spread of initial @?<SUB>Hf</SUB> values of zero to -25, with the most negative indicating zircon growth in Eoarchaean rocks. Overall these results closely mirror those obtained from direct sampling of rocks of the Archaean terranes exposed beyond the Icecap. The zircon data indicate that Mesoarchaean rocks equivalent to the Kapisilik terrane are absent under the ice. Instead, zircons were sourced from a late Mesoarchaean to Neoarchaean terrane affected by high grade metamorphism, whose age spectra perfectly matches the Tasiusarsuaq terrane to the southeast of Nuuk. Guided by the zircon results and aeromagnetic signatures, a terrane map is presented that extrapolates the Tasiusarsuaq terrane northwards under the Inland Ice towards the Tuno terrane in the north of the craton. Implications for different syntheses on the evolution of Archaean crust in southern West Greenland are discussed.
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