N. G. Berezhnaya

Age of mafic granulites from the early precambrian metamorphic complex of the Angara-Kan terrain (Southwestern Siberian Craton): U-Pb and Lu-Hf isotope and REE composition of zircon

986 The Angara–Kan terrain on the southwestern mar gin of the Siberian Craton is the southern part of the Yenisei Ridge. Early Precambrian metamorphic rocks of the Kan granulite–gneiss and Yenisei amphibolite– gneiss complexes dominate in its structure. Granulites of the Kan Complex were correlated with high grade metamorphic rock associations of the Sharyzhalgai Block of the southwestern part of the Siberian Craton; multi age Archean and Paleoproterozoic formations were established in the composition of the latter [1–3]. The age boundaries of the granulite formation in the Kan Complex are still debatable. The upper age boundary is determined by the intrusion of postcolli sional granites of the Taraka massif (1837 ± 3 Ma) intruding gneisses of the Kan Complex [4]. The U–Pb age of the protolith of biotite–hypersthene gneisses was estimated as <2.6–2.7 Ga, whereas the age of granulite metamorphism was ~1.9 Ga [5]. The maxi mal model Nd age of paragneisses (TNd(DM) = 2.5– 2.8 Ga) [6] provides evidence for the fact that the age of their protoliths does not exceed 2.5 Ga.

U-Pb isotope and REE composition of zircon from the pyroxene crystalline schists of the Irkut terrane, Sharyzhalgai Uplift: Evidence for the Neoarchean magmatic and metamorphic events

The Early Precambrian granulitegneissic com� plexes are typically characterized by a long evolution� ary history, which involves formation of their pro� toliths and repeated metamorphism related mainly with collisional events. Zircons from highgrade meta� morphic rocks usually consist of cores inherited from the protolith, and one or several mantles and rims that were formed during later metamorphism or partial melting. Highresolution methods of U-Pb dating make it possible to determine the age of cores and rims. However, knowledge of the traceelement com� position of zircon is required in order to interpret cor� rectly the obtained dates, because its zones that were grown or recrystallized at hightemperature metamor� phism significantly differ in terms of REE distribution from their magmatic analogues (1-3). This paper reports the results of study of zircons from highgrade basic rocks, i.e., pyroxene crystalline schists, which allowed us to determine the formation time of their magmatic protoliths and metamorphism, as well as to estimate the conditions of zircon formation. The available data indicate that hightemperature metamorphism and granite formation in the Irkut block occurred in several stages. It was reliably estab� lished that Paleoproterozoic metamorphism and related granite and charnockite formation spanned a time range of 1.88-1.85 Ga. These events were recorded in the formation of metamorphogenic zir� cons, their rims, and baddeleyite in the metamorphic rocks, as well as magmatic zircons in the granitoids (4-7). The lower age boundary of the Neoarchean stage of granulite metamorphism is constrained by the crystallization age of magmatic zircon from gabbro (2649 ± 6 Ma) metamorphosed under the granulite� facies metamorphism (7). Vein granites cutting across the crystallization schistosity in metagabbro have an age of 2562 ± 20 Ma. The Neoarchean granulite meta� morphism and collisional granite formation of close age (~2.6 Ga and 2.53 Ga) were also established in the adjacent Kitoi granulite gneiss block (6).

U-Pb age of autochthonous paleoproterozoic charnockite in the Aldan Shield

Autochthonous and parautochthonous charnockites in granulite facies of the Aldan Shield (the Aldan River upper flow) were dated. According to the geological observation data, the autochthonous and parautochthonous granite formation included successive development of nebulite (Lc1), its melting product such as early diatectite (Lc3), later “layer-by-layer” migmatite (Lc4), and diatectite (Lc5). The concordant ages of Lc1 and Lc3 were estimated at 2436 ± 10 and 2453 ± 14 Ma. The age of Lc5 was estimated by the upper concordia crossing at 1960 ± 8 Ma likely corresponding to the diatectic melt crystallization period. The process is accompanied by repeated high-temperature alterations of nebulite, diatectite, and their zircons yielding a concordant age of 1945 ± 13 Ma. This zircon making up the overgrowth rims is characterized by remarkable enrichment in uranium and thorium. The granulite facies metamorphism is confirmed by dating of monazite from migmatite after metapelite (1947.7 ± 8.7 Ma). The two main stages of the autochthonous and parautochthonous charnockite formation initiated the development of the crust magmatic chambers. The first stage (2430–2450 Ma) was synchronous to allochthonous high-K alkali granite in the Olekma granite-greenstone region. The second stage (1900–1960) implied the formation of autochthonous and parautochthonous charnockites under the granulite facies conditions and development of allochthonous charnockite and granite in the central part of the granulite areal.

The long (3.7–2.1 Ga) and multistage evolution of the Bug Granulite–Gneiss Complex, Ukrainian Shield, based on the SIMS U–Pb ages and geochemistry of zircons from a single sample

Abstract Multidisciplinary studies of zircons, rock-forming minerals and the whole-rock composition of granulite samples from the Bug Granulite–Gneiss Complex, Ukraine (including ion microprobe REE analysis, secondary ion mass spectrometry (SIMS) U–Pb and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) Lu–Hf analysis of zircons from a single sample) have revealed three major stages in the geological evolution of the complex. (i) At 3.66 Ga, a mafic intrusion contaminated with felsic rocks formed, as evidenced by 3.74 Ga zircon xenocrysts with inclusions of plagioclase, K-feldspar and quartz. (ii) At 3.59–3.55 Ga, high-temperature and high- to moderate-pressure granulite-facies metamorphism accompanied by migmatization and deformation resulted in the formation of mafic granulites. (iii) At 2.1–2.0 Ga, metamorphic overprinting occurred, and metatrachybasaltic dykes intruded at approximately 2.0 Ga. The metamorphic mineral assemblages recorded in the dykes formed at temperatures similar to those of the 3.59–3.55 Ga metamorphism but at pressures 2–3 kbar lower. This metamorphism disturbed the Sm–Nd whole-rock system, altered the Hf isotope system of the older zircons and resulted in Pb loss in small zircon grains. This complex event history recorded in zircons from a single rock corresponds to major stages of the geological evolution of both the Dniester–Bug Province and the entire Ukrainian Shield.

U-Pb age and Lu-Hf isotope systematics of detrital zircons from paragneiss of the Bulun block (Sharyzhalgai uplift of the Siberian Craton Basement)

1265 The Bulun block occurs in the northwest of the Sharyzhalgai uplift of the Siberian Craton (Fig. 1). It is dominated by plagiogneiss and plagiogranite of tonalite–trondhjemite–granodiorite composition (TTG Complex), while variably deformed and meta morphosed up to amphibolite facies sedimentary– volcanogenic rocks of a greenstone belt form tectoni cally isolated sheets and blocks [1]. Most of the dating so far has been done on the TTG Complex rocks. The plagiogranite and the plagiogneiss protoliths were formed during two magmatic pulses about 3.3 and 3.25 Ga; biotite gneisses (some are amphibole bear ing) of diorite–granite composition have close age of 3.3 Ga [2, 3]. The interpretation of the later geological evolution of the Bulun block is hampered by insuffi cient data on the ages of the greenstone belt rocks, which are supposed to have been formed in the Meso– Neoarchaean. Key information on crust formation may be provided by study of metasediments, since their geochemistry serves as an indicator of the com positions of rock associations in a provenance area, while the age and isotope characteristics of detrital zir cons reflect a staging of geological events. This study concerned the compositional characteristics of the Bulun block metasediments, and the first results on the ages and isotope Lu–Hf compositions of detrital zircons are presented, allowing some estimations on Late Archaean geological evolution. Composition and Age of the Greenstone Belt (GSB) Metamorphic Rocks

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.

Paleoarchean orthopyroxenites of the Bug granulite–gneiss domain at the Ukrainian shield

This report presents data on the geological structure and location of the orthopyroxenite inclusion in gneissic enderbites of the Bug granulite–gneiss domain. Three stages of orthopyroxenite formation were identified on the basis of studies of the mineral composition along with the U–Pb and Lu–Hf isotope systems of zircons.

Isotopic timing of the magmatic and metamorphic events at the turn of the Archean and Proterozoic within the Belomorian belt, Fenno-Scandinavian shield

It is proved that dating high-grade metamorphism events through dating of migmatites is quite efficient. Our investigation has made it possible to identify two events of 2500 and 2700 Ma and to estimate the age of an igneous protolith for both tonalite gneiss, the most ancient in the Belomorian belt, and related metagabbroid. Based on the upper crossing of the concordia and the discordia, the zircon core age is estimated at 2796 ± 63 Ma; this age is slightly different from that of a growth rim of rhythmically zoned prismatic zircon (2816 ± 110 Ma). A linear approximation of all measured points yields an upper crossing of 2803 ± 55 Ma. The error of these estimates is high for quite understandable reasons, and yet it should be taken into account when analyzing the geodynamic development regimes of Neo-Archaean endogenic processes.