Hidden Archaean and Palaeoproterozoic crust in NW Ireland? Evidence from zircon Hf isotopic data from granitoid intrusions
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Abstract The presence of major crystalline basement provinces at depth in NW Ireland is inferred from in situ Hf isotope analysis of zircons from granitoid rocks that cut structurally overlying metasedimentary rocks. Granitoids in two of these units, the Slishwood Division and the Tyrone Central Inlier, contain complex zircons with core and rim structures. In both cases, cores have average ϵHf values that differ from the average ϵHf values of the rims at 470 Ma (the time of granitoid intrusion). The Hf data and similarity in U–Pb age between the inherited cores and detrital zircons from the host metasedimentary rocks suggests local contamination during intrusion rather than transport of the grains from the source region at depth. Rims from the Slishwood Division intrusions have average ϵHf 470 values of −7.7, consistent with a derivation from juvenile Palaeoproterozoic crust, such as the Annagh Gneiss Complex or Rhinns Complex of NW Ireland, implying that the deep crust underlying the Slishwood Division is made of similar material. Rims from the Tyrone Central Inlier have extremely negative ϵHf 470 values of approximately −39. This isotopic signature requires an Archaean source, suggesting rocks similar to the Lewisian Complex of Scotland, or sediment derived wholly from it, occurs at depth in NW Ireland.Keywords:
Basement
1. The early Archaean rocks in northern Labrador can be subdivided into the ea. 3.78 Ga Nulliak supracrustal association, the migmatitic Uivak I gneisses, the dominant phase of which was emplaced at ea. 3.73 Ga, and the Uivak II augen gneiss. Inherited low-U rounded inclusions within igneous zircons in the Uivak I gneisses have ages between 3.73 and 3.86 Ga and are more likely to have been derived from a pre-existing high-grade metamorphic gneiss complex than from the Nulliak association. In the early Archaean there were probably several rapid cycles of sedimentary deposition and volcanism followed by emplacement of major plutons. Mid Archaean gneisses are more abundant in northern Labrador than previously realised. The late Archaean metamorphic history of these gneisses is different from the history of the early Archaean gneisses. Whereas an important part of the mid Archaean suite was emplaced in granulite facies and retrogressed at the time of granitoid veining at ea. 2.99 Ga, the major part of the early Archaean rocks were reworked under granulite facies conditions in a sequence of closely spaced events between 2. 7 and 2.8 Ga. The two groups of gneisses had different metamorphic histories until ea. 2.7 Ga, but late and post-tectonic granites of 2.5- 2. 7 Ga age cut across both. It is suggested that the terrane model in southern West Greenland can be extended to Labrador and that tectonic intercalation of early and mid Archaean gneisses took place around 2.7 Ga. Correlation between the Maggo gneisses around Hopedale, mid Archaean gneisses in northernmost Labrador and gneisses from the Akia terrane in West Greenland is suggested. Like the Malene supracrustals in West Greenland the Upernavik supracrustals in Labrador are composite associations, the youngest of which are thought to have been deposited around 2. 7 Ga.
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Baltic Shield
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Migmatite
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The boundary between the Archaean gneiss block and the Nagssugtoqidian mobile belt (Escher et al., 1976) runs through the area south-east of the inner part of Søndre Strømfjord (fig. 42). The Archaean block here is mainly composed ofbrownish hypersthene gneisses cut by numerous basic dykes, the Kangåmiut dyke swarm. The Nagssugtoqidian mobile belt consists mainly of light grey biotite gneisses, formed by shearing and recrystallisation of gneisses from the Archaean block, together with numerous amphibolite layers, derived from the Kangâmiut dykes.
Greenstone belt
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U-Pb ages of zircons from Yanzihe gneiss of Northern Dabie have been determined by single-grain TIMS method. There are two types of zircon: one is the magmatic zircon from protolith which had undergone different degree of Pb loss, and the another is metamorphic zircon. Five points give a good concordia line with lower intercept at 228±13Ma and upper intercept at 880±110Ma ( MSWD = 1.1). The upper intercept point represents the protolith age and the lower intercept point represents the metamorphic age. One metamorphic zircon gives concordant U-Pb age of 238Ma, which is similar to the lower intercept age. The result demonstrates that the Northern Dabie gneiss also underwent Jurassic metamorphic event. The suture of North China Block and Yangtze Block should be north of the North Dabie.
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Gneisses which crop out along the southern coast of Ameralik between Kangimut-sangmissoq and Qasigianguit (K-s-Q) are the subject of long-standing controversy concerning their relationship to the early Archean Amitsoq gneisses of the Godthaab district. On the basis of field observations, it was argued that gneisses at Kangimut-sangmissoq and Qasigianguit are correlatives of the early Archean Amitsoq gneisses. The data were reexamined and it is concluded that the K-s-Q gneisses represent an addition of substantially juvenile mantle-derived material to the Archean craton of West Greenland during the late Archean times. Some of the parent magmas have undergone interaction with older crust, as indicated by Pb isotope evidence for contamination with Amitsoq-derived Pb. However, the positive epsilon Nd(I) value for the K-s-Q gneisses firmly rules out any significant material contribution from the Amitsoq gneisses to the K-s-Q gneisses.
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The gneiss complex of Wildhorse Creek (Wildhorse gneiss) forms the central component of the lowest structural plate in the Pioneer metamorphic core complex of south-central Idaho. The oldest rock in the complex is a felsic orthogneiss, with Neoarchean U-Pb magmatic zircon ages of 2.60–2.67 Ga. The orthogneiss overlaps in age and is interpreted to be part of the Grouse Creek block of the Albion Mountains to the south. This Archean metagranitoid is structurally interleaved with paragneiss containing quartzite and calc-silicate rock. Structurally below the orthogneiss, some quartzites have multiple concordant populations of detrital-zircon grains as young as ca. 1700 Ma, while others have no zircon grains younger than ca. 2500 Ma.
Felsic
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