E. S. Bogomolov

New U-Pb and Sm-Nd ages and P-T estimates for eclogitization in the Fe-rich gabbro dyke in Gridino area (Belomorian Mobile Belt)

New data from isotope geochronology (U-Pb, Sm-Nd) petrological study provide evidence of the Svecofennian age (∼1.9 Ga) for eclogitization in the Fe-gabbro dyke inductile shear zones of Gridinrea. P-T estimates of eclogitization were computed using the THERIAK/DOMINO software. A close timing relationship between dyke magmatism and eclogitization is inferred.

The eastern boundary of the Baikal collisional belt: Geological, geochronological, and Nd isotopic evidence

New geological. geochronological, and Nd isotopic data are reported for the rocks occurring at the interfluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodiorite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol’khon, Southwest Baikal, and Sayan regions (480–470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60–0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.

Complex Isotopic-Geochemical (Sm-Nd, U-Pb) Study of Salma Eclogites

Salma eclogites distinguished in the northwestern part of the Belomorian mobile belt (BMB) is one of the key objects supporting the application of the plate tectonics mechanism to the Early Precambrian [1]. In this connection, age dating of eclogite metamorphism acquires principal importance and requires a complex approach. As a result of local U–Pb dating and study of the geochemistry of zircons from Salma eclogites of two types (after basic and ultrabasic rocks), grains and rims of zircon crystals of Svekofennian age (~1.9 Ga) with a complete set of geochemical characteristics of standard eclogitic zircons were found [2].

New data on the age (U-Pb, Sm-Nd) of garnetites from Salma eclogites of the Belomorian mobile belt

The origin of garnetites, which are quite abundant in highpressure metamorphic complexes, is still debatable. The idea about primary magmatic differen� tiation of basites to Fe-Ti (garnetite protolith) and Mg (protolith of metabasite complimentary to garnetite) parts is the most popular (1 and others). There are assumptions about the formation of garnetite as a result of metamorphic differentiation from active infiltration of fluid (2) and metasomatism with the for� mation of a metasomatic column (3). Extensive garnetization of eclogitic bodies as linear bands up to the appearance of garnetite containing up to 50% garnet and more was registered in Salma eclog� ites within the northwestern part of the Belomorian mobile belt (BMB). The authors studied in detail the body of massive eclogites (Sample 46) with a size of up to 10 m in diameter in the key area of Salma eclogites, in the KuruVaara deposit mine. This body occurs in migmatizes tonolite-trondhjemite gneiss intruded by numerous veins of ceramic pegmatites (4). Eclogites are strongly amphibolized at the contact with host gneiss with the formation of a garnet amphibolite rim (Sample 50) with a thickness of 1-2 m. The garnetite layer with a thickness of up to 60 cm (Sample 48) occurs between the amphibolite rim and the eclogite. Garnetite (Sample 48) contains garnet porphyro� blasts with a size of ~1 mm (up to 50%), intergranular brownishgreen amphibole (20%), andesine (14%), rutile, and ore mineral (5%). In contrast to eclogitic garnet (Sample 46), garnet from garnetite contains numerous poikilitic inclusions of prevailing quartz (10% of the whole rock volume), abundant horn� blende and rutile, and single grains of monoclinic pyroxene and biotite. Garnetite (Sample 48) and eclogite (Sample 46) located within the same body differ significantly in the chemical composition. Garnetite differs from eclogite by the high concentration of FeO* (18.0 and 12.1 wt %, respectively) and TiO2 (1.38 and 0.43 wt %) and the low concentrations of MgO (6.1 and 12.1 wt %) and CaO (11.1 and 13.4 wt %). Garnetite is significantly enriched in V (by a factor of 6) and depleted in Ni, Cr, and Ba by one order of magnitude in comparison with eclogite. The concentrations of Y, Zr, Hf, Th, and REE in garnetite are almost two times higher. A difference in major and minor elements is regu� larly observed in characteristic minerals of garnetite and eclogite as well. Garnet from garnetite differs from eclogitic garnet by the high concentrations of Fe, Ca, HREE, Y, and V and by low contents of Mg and Cr (4); amphibole and monoclinic pyroxene, by the high Fe#, Ti, and V contents; and rutile, by the high con� centrations of V, Zr, and Hf and the low contents of Cr and Nb. The contrasting chemical compositions of garnetite and eclogite do not result in qualitative change of the mineral association upon transforma� tion of eclogite to garnetite, but have an impact on the compositions of rockforming, as well as accessory, minerals.

New occurrence of eclogite in the Belomorian mobile belt: Geology, metamorphic conditions, and isotope age

Finds of eclogitelike associations within the Belomorian mobile belt (BMB) in the areas of Gridino [1],Shirokaya and Uzkaya Salma bays, KuruVaara mine[2], and Krasnaya Guba [3] have different geologicalinterpretations in relation to their age and geologicalsetting, as well as geological models of the formation.The eclogitic mineral associations in basic and ultrabasic rocks of the Belomorian area have been knownfor more than 70 years. In the first part of the 20th century, they were registered in the BMB by the authors of[4–6]. Eclogites were repeatedly mentioned byK.A. Shurkin and colleagues (Institute of Precambrian Geology and Geochronology) in the 1950s. Wediscovered a number of new eclogite bodies on theislands of the Keretskii archipelago in the central partof the BMB; previously described [5–7] analogousobjects on Sidorov and Ileika islands have been studiedby the authors in detail as well.Two nappes with different rock compositions maybe distinguished in the geological structure of SidorovIsland. The upper nappe represented by strongly granitized biotite (rarely epidote) gneiss practically doesnot contain basic bodies. The lower nappe with athickness of <30 m is represented by gray granite–gneiss with numerous boudined bodies of basic rocks.Eclogitized bodies of basic rocks were registered in themost outcropped northern and southern parts of theisland. All of them have the character of boudins (up to30–40 m in diameter) surrounded by a granite–gneissmatrix (Fig. 1). Eclogitization is reflected in the formation of linear zones and veins composed of garnet,monoclinic pyroxene with a high Na content, andamphibole (Sample 202) in metabasites. Basic bodiesare usually altered with the formation of rims of intenseamphibolization along the perimeter (Sample 216) witha thickness up to 0.5 m and higher, and intersected bylate pegmatoid and carbonate–quartz veins(Sample 205). Sample 223 was investigated from thethin linear zone of eclogitization of the basic bodyfrom SW Ileika Island, which differs from metabasitesof Sidorov Island in the composition and form ofeclogitization. Metabasites of Sidorov Island correspond to the complex of gabbroanorthosites, whereasrocks of Ileika Island correspond to the complex ofmetaporphyrites–garnet gabbro [7].Eclogites have porphyroblastic and granoblastic,sometimes symplectitic texture. Garnet porphyroblasts are distributed in rock matrix represented bymonoclinic pyroxene with a dependent portion ofamphibole and plagioclase quite regularly. In additionto these minerals, biotite, quartz, magnetite, ilmenite,titanite, rutile, apatite, and pyrite (a total of <2–5% ofrock volume) were registered in eclogites. Garnet ischaracterized by poor zoning reflected in a decrease inthe grossular and pyrope contents from the center tothe margin of porphyroblast and an increase of almandine and spessartine contents. Inclusions of quartz,monoclinic pyroxene, rutile, amphibole, and chloriteare irregularly distributed in garnet. Monoclinicpyroxene of the matrix is represented by prismaticgrains and rarely symplectitic aggregates. According tothe composition, it corresponds to sodic (

Ophiolites of the Ust-Belskii terrane (Chukotka): A consequence of late precambrian breakup of the Rodinia supercontinent in structures of the NE framing of the Siberian craton (structural, petrological-mineralogical, and isotope data)

351 Ophiolites of the Ust Belskii terrane (Chukotka) [1–8]. are part of the ophiolite belt related to the con vergence boundary between the Siberian paleoconti nent and the Alazeya–Oloy and Koryak segments of the Koryak paleo oceanic domain (Fig. 1). However, existing opinions about the nature of this paleo oce anic domain are controversial. At present, almost all researchers traditionally refer it to the Pacific [3, 5–11]. Based on the idea of the Katasian type of Okhotsk– Kamchatka continental framing and the superimposi tion of the domain by structures of the Mongol– Okhotsk and Paleo Asian Ocean paleo oceanic domains along the strike (Fig. 1), we suggest that for mation of the Koryak paleo oceanic domain occurred in the Mesozoic stage of evolution of the lithosphere of the Paleo Asian Ocean [12, 13].

Composition and age of metaorthopyroxenite xenoliths and host enderbite gneisses in the Pobuzhie granulite complex, Ukrainian Shield

The SIMS U-Pb isotopic age of zircons from enderbite gneisses and their metaorthopyroxenite xenoliths in the Pobuzhie granulite complex, Ukrainian Shield (48°13′57.3″ N and 29°59′21.5″ E, WGS84 system), was determined. The chemical compositions of these rocks and composing minerals were studied. Enderbite gneisses contain quartz, antiperthite plagioclase, K-feldspar, clinoenstatite, diopside, amphibole, and a small amount of biotite; accessory minerals are ilmenite and apatite. The age of zircon from enderbite gneiss is estimated at about 3.15 Ga. Metaorthopyroxenites are composed of orthopyroxene, clinopyroxene, phlogopite (up to 6% TiO2), and plagioclase. The age of magmatic zircons from metaorthopyroxenite determined by the upper intercept of the discordia with the concordia is 3485 ± 33 Ma (MSVD = 1.6), and the age of metamorphic zircons is 2742 ± 22 Ma (MSVD = 0.22). Hence, the enderbite gneisses studied pertain to a young group of enderbites in the Pobuzhie granulite complex, while the age of metaorthopyroxenites from xenoliths in these rocks is similar to that of ancient Pobuzhie enderbites and pyroxenites of the Novopavlovsk complex in the Azov Region.

The age of high-pressure metasomatism in shear zones during collision-related metamorphism in the Lapland granulite belt: The Sm-Nd method of dating the paragenesises from sillimanite-orthopyroxene rocks of Por’ya Guba nappe

The Sm-Nd isotope-chronological method is used for the first time to investigate the whole rock and rock-forming minerals from metasomatic sillimanite orthopyroxene rocks in the Por’ya Guba nappe of the Lapland granulite belt. As a result isochrone values of the age are obtained, which allows estimate the minimum time intervals that have passed from the moment of formation of the investigated rocks. They indicate that peak conditions of collision metamorphism in fluid-permeable shear zones of the LGB occurred in the Paleoproterozoic Svekofennian age when high-temperature high-pressure sillimanite-orthopyroxene metasomatic rocks were formed. The obtained data make it possible to assume that, with regard to the measurement errors, high-pressure metasomatism did not last over 10 Ma, and this process did not coincide in time with older medium pressure metamorphism.

Age stages of metamorphism at the Kitoi Sillimanite schist deposit, southeastern Prisayan’e

The Irkut Block with dominant rocks of the Sharyzhalgai Series and Kitoi Block with prevalence of the Kitoi Series rocks are the main structures of the Presayanian basement elevation of the Siberian Craton. Two stages of metamorphism, Neoarchean (2.6–2.7 Ga) and Paleoproterozoic (1.85–1.87 Ga), were established for the granulitic complexes of these blocks. The rocks of the Kitoi sillimanite schist deposit composed of sillimanite, andalusite-sillimanite, and garnet-sillimanite schists and gneisses underwent by intense ultrametamorphic transformations which led to the formation of sillimanite- and garnet-bearing plagioclase and feldspar migmatites, and also granites, sienites, and granite-pegmatites. The geochronological study of melanocratic schists and leucocratic plagiogneisses-the typical rocks of the Kitoi deposit-showed the manifestation of metamorphism only at the Archean and Proterozoic boundary (2450–2550 Ma).

The first Sm–Nd isotope–geochemical data on the Paleoproterozoic age of metamorphic rocks from the crystalline basement of the Yurovsk rise (Okhotsk massif)

Rocks with Paleoproterozoic Sm–Nd model ages (TNd(DM) = 2096–2350 Ma) are the sources of protoliths with gneiss, amphibolite, marble, and calciphyre of Yurovsk rise. This fact makes possible estimation of the lower age boundary of the formation of the crystalline basement of Yurovsk rise as Paleoproterozoic. According to the results of Sm–Nd, U–Pb (SHRIMP), and Pb–Pb geochronological studies, two isotope provinces are distinguished within the crystalline basement of the Okhotsk massif. The Paleoarchean province occupies the territory of Kukhtui rise, in which crust-forming processes played a key role (3250–3650 Ma). The Paleoproterozoic province includes Yurovsk rise, in which the major stages of crust formation have an age of 1900–2350 Ma.