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Abstract Volume workshop Craton Formation and Destruction with special emphasis on BRICS cratons University of Johannesburg, South Africa 21-22 July 2012 Craton Formation and Destruction 21-22 July 2012 First data on the composition and age of the lower crust of the central part of the Aldan-Stanovoy Shield: results of study of xenoliths from Mesozoic plutons 1 1 2 1 1 Kravchenko A.A. , Smelov A.P. , Popov N.V. , Zaitsev A.I. , Beryozkin V.I. , Dobretsov V.N. 1 1 Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk, Russian Federation – [email protected] 2 Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russian Federation Keywords: Lower crust, Aldan-Stanovoy Shield. To date there has been almost no published data on the composition and age of the lower crust of the North-Asian craton (NAC) (Smelov and Timofeev, 2007). According to Rudnick and Fountain (1995), the lower crust, 20-25 km thick, is composed of metamorphic rocks of granulite facies. Within the limits of the NAC, rocks of granulite facies are widespread in the Aldan-Stanovoy and Anabar Shields and constitute Early Precambrian terranes of different age and composition (Rosen and Turkina, 2007; Smelov et al., 2010). Analysis of PTparameters of granulite metamorphism of some terranes of the Central-Aldan superterrane (CAST) show that they form a PT-path of isothermal decompression and are typical for granulite complexes formed as a result of crustal thickening (Harley, 1989). These terranes are felsic and cannot represent the lower crust. Rocks of high density occur at depths of 20-45 km according to geophysical data. The first data on the composition and age of the pre-Mesozoic lower crust were obtained during study of xenoliths of metamorphic rocks from the Mesozoic syenite plutons which intrude CAST granulite rocks. greywacke. Sm-Nd isotopic data show that the protoliths of high-alumina gneisses formed from rocks of ~ 3.06 to ~2.85 Ga in age, while those of the biotite-hypersthen gneisses were derived from rocks of ~2.4 to ~2.33 Ga in age. The Fedorov Group includes amphibole, diopside-amphibole, and two pyroxene-amphibole plagiogneisses, corresponding to basalt, andesite-basalt and andesite. Rocks of the Fedorov Group formed by metamorphism of protoliths with Nd model ages of ~2.3 to ~2.0 Ga (Smelov and Timofeev, 2007). UPb age of zircons from pyroxene-amphibole plagiogneisses is 2006±3 Ma (Velikoslavinsky et al., 2006). Age of granulite metamorphism (T=700830ºC, P = 5-6 Kb) is estimated to be 2.0-1.90 Ga. Xenoliths of metamorphic rocks in syenite plutons are quite rare. Their number is from 500 to 1000 pieces per1 km2, and size varies from 5 to 20 cm. Their petrographic composition is the same in the different plutons. Xenoliths are represented by amphibolites, amphibole gabbros (metagabbro), garnet-amphibolites, anorthosites, and pyroxenites, this suite having no analogues among rocks exposed at the surface. The CAST consists of the Nimnyr and Sutam terranes. Plutons of Mesozoic syenites intrude metamorphic rocks of the Nimnyr terrane. The Nimnyr granulite-orthogneiss terrane is composed of domes of granite-gneiss. The cores of the domes are granite-, charnockite- and enderbite-orthogneisses, covering more than 50 percent of the terrane. Orthogneisses with Nd model ages of ~2.5 to ~2.3 Ga contain xenoliths of granite-gneisses and tonalite-trondhjemite gneisses, which are older than ~3.35 Ga (Nutman et al., 1992). The external parts of the domes are composed of a paragneiss complex consisting of the Kurumkan and Fedorov Groups. The Kurumkan Group includes quartzite and highalumina gneisses. The high-alumina gneisses are similar in chemical composition to pelite and siltstone and are interlayered with biotitehypersthene plagiogneisses, chemically similar to Amphibolites and garnet-amphibolites have foliation, revealed by amphibole orientation. Amphibole gabbro, anorthosites and pyroxenites show weakly manifested metamorphic textures. There are gradual transitions from amphibole gabbros to garnet-amphibolites, then to amphibolites. In some cases, garnet-amphibolites have massive structure and zonal plagioclase, connecting them with amphibole gabbros. The anorthosites are light-grey coarse-grained rocks with a distinct lineation caused by alignment of darkcolored minerals. Garnet-amphibolites consist of Grt+Cpx±Opx+ Hbl+Pl±Qtz. Garnet and clinopyroxene are often surrounded by amphibole rims, which separate them from plagioclase. Garnets show a narrow interval of composition variation: Alm – 51.0-59.0%, Pyr – Craton Formation and Destruction 21-22 July 2012 20.0-30.0%, Sps – 1.5-2.5, Grs – 11.0-20.0%, Adr – 0-7.0%. In clinopyroxenes Na2O content = 0.7-1.0 wt. %, Al2O3 = 3.1-5.5 wt. %. In orthopyroxenes Al2O3 content = 0.4-2.3 wt. %. Amphibole compositions are the most variable. Their TiO2 content varies from 0.1 to 2.3 wt. %, Na2O+K2O content – from 1.9 to 4.5 wt. %. Anorthite content in plagioclases varies from 40 to 55 %. The PT-path of metamorphism shows isobaric character and indicates that temperature varies from 900º to 700ºC at almost constant pressure of 11.0 kb. Regarding chemical composition, xenoliths correspond to basic and intermediate rocks and belong to a layered gabbro-diorite-anorthosite complex of normal and sub-alkali series. The position of the majority of composition points on the AFM diagram shows a single tholeiitic trend. The several points of analyses in the calc-alkalic field are explained by plagioclase fractionation. The LaN/YbN mean value = 8.7, with different concentrations of REE in rocks. Heavy REE concentration exceeds chondrite in 10-20 times, and light REE – in 30-120 times. All basic and intermediate xenoliths have characteristic negative Eu anomalies. In anorthosites, the level of relations of heavy REE to chondrite < 1, light REE = 116, LaN/YbN = 197. Eu positive anomalies are recorded by anorthosites. Trace element and lead isotope compositions were determined in three zircon grains from one sample of amphibolite (Cpx+Hbl+Pl+Ilm) by the LAM-ICP-MS method. According to the classification and regression trees of Belousova et al. (2002), such zircons are typical for mafic rocks. The Pb-Pb model age of zircons varies within the interval 1900-1963 Ma, and probably reflects time of emplacement and metamorphism, which is confirmed by estimations of zircon crystallization temperature by Watson et al. (2006) at 815-7300C. Thus, we can draw the following conclusions: 1. Chemical and petrographic composition of xenoliths, specifically the presence of early Hbl with high TiO2 content, in association with Grt, Cpx, Opx and Pl, indicate that these rocks are derivatives of basalt melt crystallization. 2. PT-parameters of metamorphism show that, magma crystallization occurred at a depth of ~30 km. Trends of isobaric rock cooling support the fact that, the lower crust of the central part of the AldanStanovoy Shield was formed as a result of magmatic mafic underplating. 3. Estimation of the average composition of the Paleoproterozoic lower crust, taking into account percentage ratio of different petrographic types of rocks and their size, indicates the following values: SiO2 – 53.61, TiO2 – 0.98, Al2O3 – 16.35, Fe2O3 – 9.56, MnO – 0.21, MgO – 4.5, CaO – 7.86, Na2O – 3.21, K2O – 1.58, P2O5 – 0.21, LOI – 1.57 (wt.%) and Ba – 897.59, Rb – 27.79, Sr – 642.57, Y – 22.28, Zr – 198.1, Nb – 7.69, Th – 1.72, Ni – 132.12, V – 167.44, Cr – 419.96, Hf – 4.84, Ta – 0.41, Co – 27.19, U – 0.73, La – 24.98, Ce – 48.27, Pr – 5.96, Nd – 24.69, Sm – 5.02, Eu – 1.18, Gd – 4.79, Tb – 0.66, Dy – 4.17, Ho – 0.86, Er – 2.48, Tm – 0.33, Yb – 2.35, Lu – 0.35 (ppm). 4. Formation of the lower crust was at the final stage of collision processes between terranes of the North-Asian craton, as the part of the Nuna (Columbia) supercontinent. 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