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This study provides SHRIMP-RG data on zircons from garnet gedritites, the products of retrograde metamorphism of eclogite-like rocks constituting belonging to the basement of the Omolon Massif. The earliest episode recorded by oscillatory-zoned cores having high HREE and Ti contents occurred at 3.25–3.22 Ga (Paleoarchean) and is interpreted to represent an upper age limit of a metamorphic or magmatic protolith. One zircon core with a pronounced negative Eu anomaly yielded a concordant age of 2.6 Ga, which is interpreted to mark a Neoarchean episode of granite formation. The studied population of zircons provides the most distinct record of a Paleoproterozoic (1.9 Ga) event, which is marked by formation of garnet gedritites under amphibolite-facies conditions. This event is recorded by transparent recrystallization rims of preexisting large zircon grains and small newly-formed grains, which are characterized, compared with their cores, by lower crystallization temperatures and one order of magnitude lower concentrations of U, Th, and HREE, and the presence of garnet micro-inclusions. 相似文献
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Petrological, geochemical, and isotope geochronological aspects of the evolution of calc-alkaline magmatism were investigated
in the Western Okhotsk flank zone, the Okhotsk segment, and the Eastern Chukchi flank zone of the Okhotsk-Chukotka volcanic
belt (OCVB). The OCVB is a tectonotype of continental margin volcanic belts comprising much greater volumes of felsic ignimbritic
volcanics compared with mature island arcs (MIA, Kuril-Kamchatka and Aleutian) and the Andean continental margin. The volcanic
rocks of continental margin volcanic belts (OCVB and Andean belt) are enriched in K, Ti, and P compared with the rocks of
MIA and show a trend toward the field of high-potassium calc-alkaline series. Primitive andesite varieties (Mg# > 0.6) were
not yet found in the OCVB, but there are relatively calcic varieties unknown in Andean-type structures and a significant fraction
of moderately alkaline rocks, which are not typical of MIA. Variations in trace and major element characteristics in the basalts
and andesites of the OCVB were interpreted as reflecting the competing processes of assimilation/mixing and fractional crystallization
during the evolution of the parental basaltic magma. Significant lateral variations were established in the composition of
the mantle sources of calc-alkaline magmas along the OCVB over more than 2500 km. The initial isotopic ratios of Sr, Nd, and
Pb in the volcanics of the Okhotsk segment are relatively depleted and fall near the mixing line between PREMA and BSE. The
magma source of the Western Okhotsk flank zone is most enriched and approaches EMI, whereas that of the central and eastern
Chukchi zones contains an admixture of the EMII component. The geochronological characteristics of all the main stages of
OCVB magmatism were comprehensively studied by U-Pb SHRIMP and ID-TIMS zircon dating (86 samples) and 40Ar/39Ar analysis (73 samples). In general, a discontinuous character was established for the OCVB magmatism from the middle Albian
to the early Campanian (106–77 Ma). The volcanism is laterally asynchronous. There are several peaks of volcanism with modes
at approximately 105, 100, 96, 92.5, 87, 82, and 77 Ma. The Coniacian-Santonian peaks correspond to the most extensive stages
of the middle and late cycles of felsic volcanism. A decreases and a hiatus in magmatic activity were reconstructed for the
end of the Cenomanian and the beginning of the Turonian. The volcanism was terminated by plateau basalts with ages of 76–78
Ma, which mark a change in the geodynamic setting from frontal subduction to the regime of a transform margin with local extension
in zones normal to the slip direction. A catastrophic character of eruptions with rather narrow ranges of volcanism (<2 Myr)
were established taking into account new reliable age estimates for some individual large calderas. The accumulation rate
of volcanic materials in such structures was up to 0.15–0.36 km3/yr and even higher. 相似文献
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N. V. Makarova V. I. Makarov G. A. Postolenko B. E. Akinin 《Stratigraphy and Geological Correlation》2011,19(4):450-471
Based on a generalized knowledge of conditions and mechanisms for accumulation of the cycle terrace alluvium on plain and
mountain rivers, alluvium is suggested as an instrument for stratigraphic studies, interregional correlation, and correction
of present schemes. Diachronous alluvial formations are uniformly constructed. They include two subunits, a lower “warm” and
an upper “cool” one. In addition the formations are subdivided in dynamic phases that display a dominating regime and stages
of erosion-and-accumulation cycle. Alluvial formations are characterized by a peculiar mechanical and mineralogical composition,
gradation, bedding, thickness, landscape and climatic conditions of accumulation, paleontological content, character of occurrence,
and position in a cycle incision. All these characters of an alluvial formation and its subformations correspond to a complete
climatic rhythm and oscillation stages. Alluvial formations are associated in a certain way with glacial and marine sediments
making up a single formation complex. 相似文献
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Doklady Earth Sciences - First data on oxygen isotopic composition in phenocrysts in volcanic rocks from Okhotsk–Chukotka volcanic belt (106–78 Ma, North-Eastern Russia) together with... 相似文献
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V. V. Akinin A. V. Prokopiev J. Toro E. L. Miller J. Wooden N. A. Goryachev A. V. Alshevsky A. G. Bakharev V. A. Trunilina 《Doklady Earth Sciences》2009,426(1):605-610
U-Pb SHRIMP-dating of zircons from twenty five intrusions representative of the Main granitoid batholith belt and associated
dike swarms (Yano-Kolyma gold bearing province, North East of Asia) are mostly ∼150 ± 3 Ma (Kimmeridgian-Tithonian). Two less
widespread impulses of magmatism dated at 160–155 Ma and 146–143 Ma representing the full range of ages present in the Main
belt. Paleoproterozoic (∼1.8 Ga) inherited zircons were found in three intrusions from the south-western part of the belt
where Precambrian crust of the North-Asia craton is inferred to underlie it.
Published in Russian in Doklady Akademii Nauk, 2009, Vol. 426, No. 2, pp. 216–221.
The article was translated by the authors. 相似文献
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