Géochronologie par méthode conventionnelle [1] Bruguier O, Dada SS & Lancelot JR (1994) Early Archean crust (>3.56 Ga) within a 3.05 Ga orthogneiss from Northern Nigeria. U-Pb zircon evidence. Earth and Planetary Science Letters 125: 89-103. [2] Bruguier O (1996) U-Pb ages on single detrital zircon grains from the Tasmiyele Group: Implications for the evolution of the Olekma Block (Aldan Shield, Siberia). Precambrian Research 78: 197-210. [3] Bruguier O, Lancelot JR & Malavieille J (1997). U-Pb dating on single detrital zircon grains from the Triassic Songpan-Ganze flysch (Central China): evidence for a Sino-Korean provenance and tectonic correlations. Earth and Planetary Science Letters 152: 217-231. [4] Bosch D & Bruguier O (1998) An Early Miocene age for a high temperature event in gneisses from Zabargad Island (Red Sea, Egypt): mantle diapirism. Terra Nova 10: 274-279. [5] Bruguier O, Becq-Giraudon JF, Bosch D & Lancelot JR (1998) Late Visean hidden basins in the Internal Zones of the Variscan belt: U-Pb zircon evidence from the French Massif Central. Geology 26: 627-630. [6] Bruguier O, Bosch D, Pidgeon RT, Byrne D & Harris LB (1999) U-Pb chronology of the Northampton Complex, Western Australia - evidence for Grenvillian sedimentation, metamorphism and deformation and geodynamic implications. Contributions to Mineralogy and Petrology 136: 258-272. [7] Dhuime B, Bosch D, Bruguier O, Caby R & Archanjo C (2003) An early Cambrian U-Pb apatite cooling age for the high-temperature regional metamorphism in the Pianco area, Borborema province (NE Brazil). Compte-Rendus Geosciences, 335: 1081-1089. [8] Bruguier O, Becq-Giraudon J.F, Clauer N & Maluski H (2003) From late Visean to Stephanian: Pinpointing a two-stage basinal evolution in the Variscan Belt. A case study from the Bosmoreau basin (French Massif Central) and its geodynamic implications. International Journal of Earth Sciences 92: 338-347. [9] Bosch D, Bruguier O. Kranshobaiev A & Efimov A (2006) A Middle Silurian age for the Uralian Platinum-bearing Belt (Central Urals, Russia): U-Pb zircon evidence and geodynamic implication. In "European Lithosphere Dynamics", Gee, D.G. and Stephenson, R.A. (eds), Memoirs of the Geological Society of London 32: 443-448. Pre[nmbrinn Resenrth ELSEVIER PrecambfianResearch 78 (1996) 197-210 U-Pb ages on single detrital zircon grains from the Tasmiyele Group: implications for the evolution of the Olekma Block (Aldan Shield, Siberia) O. Bruguier Laboratoire de G£ochronologie-Gdochimie-P£trologie, CNRS-URA 1763, Case courrier 066, Universit£ de Montpellier 11, PI. Eugene Bataillon, 34,095 Montpellier Cedex 5, France Received 22 December 1994; revised version accepted 19 September 1995 Abstract The Aldan Shield of Siberia is one of the largest exposures of the Siberian Craton and has been divided into different units according to their geological characteristics. In the central part of the shield, the main divisions are the Olekma and West Aldan Blocks. The former contains supracrustal rocks and typical greenstone belts. We report U-Pb isotopic analyses on 51 single detrital zircon grains from 5 samples of quartzite collected at different stratigraphical levels from clastic metasediments of the Tasmiyele Group situated in the Olekma Block. The youngest sub-concordant grain (2963 ___ 5 Ma) provides an older limit to the deposition. Combined with other information on the geological evolution of this part of the Aldan Shield, the results show that sediments were deposited between 2500 and ~ 2960 Ma, and that detritus was derived from the neighbouring basement of the Olekma Block. The age spectrum presented by detrital zircons implies the creation of large amounts of differentiated material during the period 2900-3000 Ma which represents an important crustal event for this part of the Aldan Shield. Moreover, it appears from these results and previous works that the Tasmiyele Group and the Tungurcha Group, initially grouped together to form the Tungurcha Greenstone Belt, are two distinct and unrelated units. The Tasmiyele Group, whose affiliation with greenstone belts is uncertain, was deposited at the same time or after the formation of the Olondo Greenstone Belt. 1. Introduction carry important information on the composition, tec- tonic setting and evolution of the source(s) region(s) Despite covering up to 80% of the Earth surface, from which they come from (e.g. Maas and McCul- sediments have not been intensively studied using loch, 1991). Furthermore, they may represent the conventional geochemical techniques. This is mainly only remnants of source rocks which have since due to mixing between components of various origin disappeared (Froude et al., 1983; Compston and and also to the complexity of the processes occurring Pidgeon, 1986). In sedimentary environments, detri- during diagenesis and/or alteration. However, clas- tal zircons constitute a mixture of grains from source tic sedimentary rocks and their detrital minerals, can rocks whose origin, age and evolution may be totally 0301-9268/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0301-9268(95)00056-9 198 O. Bruguier / Precambrian Research 78 (1996) 197-210 different. Therefore, even two grains which appear which (such as the Kurulta Group) may have been identical in shape and colour, cannot be considered deposited and subsequently metamorphosed in the to have been derived from the same source rock. Age Early to Mid-Archaean (> 3.3 Ga) (Bibikova et al., determinations of single zircon grains are therefore 1989; Glebovitsky and Drugova, 1993). Geochrono- essential to identify different age populations and logical results (Bibikova et al., 1989; Morozova et this paper presents U-Pb ages on 51 detrital zircon al., 1989a; Nutman et al., 1992) indicate that the grains from the Tasmiyele Group (Olekma Block, basement of the West Aldan Block is made up of Aldan Shield). The study aims to resolve the age Archaean and even Early Archaean rocks (> 3.5 Ga) spectrum presented by the zircon populations and to as demonstrated by the 3500 Ma minimum 2°Tpb/ obtain information on the provenance of the detritus 2°6pb age of a discordant zircon fraction from a and on the age of deposition of the Tasmiyele Group. tonalitic gneiss (Morozova et al., 1989b). Furthermore, the age patterns reflect the evolution of The basement of the Olekma Block consists the source regions and allow a direct comparison mainly of granitoids and tonalitic to granodioritic between the Tasmiyele Group and other supracrustal gneisses. Supracrustal belts (metavolcanic and units of the Olekma Block. Another purpose of this metasedimentary rocks) are also preserved. All these study was to search for evidence of very old (Early rocks have undergone amphibolite-facies metamor- Archaean) zircon grains. With the discovery of 3.4 phism. Eastwards, the grade of metamorphism and Ga old Archaean rocks within the Omolon massif deformation increases and relict eclogites are found (Bibikova, 1984), it was hoped to find, in the Olekma in the easternmost part of the Olekma Block (Smelov, Block, the most ancient core of the Aldan Shield. 1989), assigned to be of Mesoproterozoic age (Nut- man et al., 1992). Published geochronological results (Jahn et al., 1991; Nutman et al., 1992; Glebovitsky 2. Geological setting and Drugova, 1993; Neymark et al., 1993; Ve- likoslavinsky et al., 1993), emphasize that the base- The Tasmiyele Group outcrops on the Olekma ment of the Olekma Block is mainly constituted of Block (Fig. 1) in the middle part of the Archaean 2.9-3.0 Ga old rocks. So far, the oldest rocks identi- Aldan Shield. The Aldan Shield, which constitutes fied are 3.25 Ga old orthogneisses (Nutman et al., the largest exposure of the Siberian Craton, has been 1992), although Neymark et al. (1993) proposed for traditionally subdivided into various geological units the 2.98 Ga Amnunnakta granitoid massif a Nd according to structural and metamorphic characteris- model age of 3700 Ma, that reflects the occurrence tics (Dook et al., 1989). In the middle part of the of much older sialic material. Greenstones belts out- shield, the main divisions are constituted by the cropping on the Olekma Block have not been dated Olekma granite-greenstone terrain and the West AI- yet, except for the Olondo Greenstone Belt. Ages dan granulite-gneiss Block (Fig. 1). These two from volcanics of this typical greenstone belt are blocks are separated by the Amga fault which ap- indistinguishable from ages of the basement rocks pears to represent a ductile shear zone, related to ( ~ 3.0 Ga) (Baadsgaard et al., 1990). The Tungurcha thrusting of the West Aldan Block westward over the Greenstone Belt, more than 170 km long and 25-30 edge of the Olekma Block (Smelov, 1989; Smelov km wide, is constituted by the Tungurcha Group and Beryozkin, 1993). The age of this event has been (lower part) and the Tasmiyele Group (upper part). clearly shown to be Proterozoic (1.9-2.0 Ga) (Nut- The Tungurcha Group outcrops as isolated tectonic man et al., 1992). The West Aldan Block consists fragments which may have constituted a single se- mainly of granulites and amphibolite-facies migma- quence (Bogomolova and Smelov, 1989). The slabs titic gneisses and it is generally thought that high- differ from each other in their constituent rocks grade events occurred several times during Archaean which comprise volcanics, mafic plutonic rocks, and Proterozoic time (Bibikova et al., 1989; Gle- schists, carbonates and clastic sediments. A mini- bovitsky and Drugova, 1993). The block also pre- mum age for the deposition and subsequent thrusting sents supracrustal sequences of mature sediments of the Tungurcha Group is given by a tonalitic gneiss (quartzites, mafic schists and calc-silicate), some of (3016_+8 Ma) intruding gneisses and ultramafic O. Bruguier / Precambrian Research 78 (1996) 197-210 199 rocks of the group on the west side of the Olekma mineral assemblages in the diabases indicate that the ri,/er (Nutman et al., 1992).