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Geotectonics - The dunite–wehrlite–clinopyroxenite–gabbro massif in Eastern Chukotka, a key object for geodynamic reconstructions of the Vel’may terrane, which represents... 相似文献
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G. V. Ledneva B. A. Bazylev A. V. Moiseev S. D. Sokolov A. Ishiwatari D. V. Kuzmin B. V. Belyatsky 《Geotectonics》2018,52(4):447-467
The Matachingai River basin is known among the few ophiolitic complexes on eastern Chukotka as the southern boundary of the Chukotka Fold System (in terms of tectonics, the Chukotka microcontinent or a fragment of the Arctic Alaska–Chukotka microplate). This complex comprises tectonic blocks of residual spinel harzburgite with dunite bodies and pyroxenite, olivine gabbro, and leucogabbro veins; blocks of hornblende gabbro, diorite, and plagiogranite; and Upper Jurassic–Lower Cretaceous basaltic–cherty and cherty–carbonate rocks. The geological relationships of rocks within tectonic blocks, the compositions of primary minerals, the bulk geochemistry of rocks, as well as the strontium, neodymium, and lead isotopic compositions, make it possible to consider individual tectonic blocks of the complex as fragments of a disintegrated oceanic-type lithosphere that formed in a back-arc spreading center. The melts, crystallization products of which are represented by hornblende gabbro of blocks, olivine gabbro of veins, and basalts, separated from geochemically and isotopically heterogeneous mantle. Blocks composed of rocks with various modal composition are likely relicts of an oceanic lithosphere of different segments of a back-arc basin. The studied complex may be a lithosphere of one of the Middle–Late Jurassic back-arc basins. Fragments of these basins are retained in ophiolitic complexes on Great Lyakhovsky Island of the New Siberian Islands Archipelago, western Chukotka, and the Brooks Range in Alaska. 相似文献
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G. V. Ledneva B. A. Bazylev V. V. Lebedev N. N. Kononkova A. Ishiwatari 《Geochemistry International》2012,50(1):44-53
The Ust’-Belaya mafic-ultramafic massif is assigned to the Western Koryak fold belt and largely composed of residual spinel
peridotites, layered spinel and plagioclase peridotites, and gabbros. These rocks are crosscut by occasional plagiogranite
and diorite veins and exhibit locally a close spatial association with basalts and carbonate-sedimentary deposits of Late
Devonian and Early Carboniferous age. Based on this evidence, the massif was ascribed to the pre-Late Devonian ophiolite association.
Our study presents new U-Pb SHPIMP II zircon ages and petrographic and mineralogical data on samples of the layered amphibole
gabbro and vein diorite from the Ust’-Belaya massif. The approximate concordant U-Pb age corresponding to a timing of of amphibole
gabbro crystallization is 799 ± 15 Ma, and the concordant U-Pb age reflecting a timing of of vein diorite crystallization
is 575 ± 10 Ma. These ages coupled with geological studies of the massif, petrological and mineralogical investigations of
the dated samples, as well as literature data on the petrology of peridotites and the age of formed plagiogranites suggest
that the peridotites and layered gabbros of the Ust’-Belaya massif were formed by the Late Riphean, whereas the vein diorite
and plagiogranite were resulted from a later (Vendian-Cambrian) magmatic stage. The peridotites and gabbros of the massif
display no genetic relationship with spatially associated basalts and sedimentary rocks and, thus, they cannot be considered
as members the pre-Late Devonian ophiolitic association. The results of this study will inevitably lead to a significant revision
of geological and geodynamic interpretations of the Ust’-Belaya mafic-ultramafic massif. However, uneven study of the Precambrian
complexes of the Koryak and Chukchi areas, their evolution in different structures of the region cannot yet be described by
a single geodynamic scenario. 相似文献
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The detailed mapping of the blocks of the Pekul’ney complex revealed that cumulate ultramafics occur as separate tabular bodies among metamorphic rocks and are only fragmentarily observed in some of the blocks. Within these bodies, different types of ultramafics are regularly and multiply intercalated, forming banded structures, which supports their assignment to a single cumulate series. The tabular ultramafic bodies investigated in different blocks of the Pekul’ney complex are from 350 to 1100 m thick, and their internal structure is made up of intercalated regular rhythms of dunites-peridotites and olivine pyroxenites-olivine-free ultramafics (garnet, ceylonite, and clinozoisite clinopyroxenites, websterites, and hornblendites) and units of irregularly interlayered dunites, peridotites, and olivine pyroxenites. The thickness of individual regular rhythms ranges from 50 to 410 m. The cumulate ultramafics of the Pekul’ney complex were derived from a water-rich highly magnesian primary melt, which was equilibrated with mantle harzburgites, within a wide temperature range at pressures of 11–13 kbar in the geodynamic setting of the base of an ensialic arc. The Pekul’ney complex can be considered as a reference object for the petrological and geochemical investigation of the evolution of suprasubduction mantle melts during their high-pressure fractionation. 相似文献
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G. V. Ledneva B. A. Bazylev D. V. Kuzmin N. N. Kononkova 《Geochemistry International》2017,55(4):330-340
Pyroxenite veins in mantle peridotites of the Unnavayam sheet of the Kuyul ophiolite terrane (Koryak—Kamchatka folded area) are composed of clinopyroxenite and websterite crystallized from a boninite-like melt. The host clinopyroxene-bearing spinel harzburgites are moderately depleted residues, whose mineral compositions and conditions of formation correspond to those of peridotites from mid-oceanic and back-arc spreading centers. Mantle peridotites of the Unnavayam sheet may have formed a part of the mantle wedge above a subduction zone and have been intruded by boninitic melts at a certain stage of their evolution. 相似文献
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The purpose of our work was to obtain the most possible detailed information about the composition, concentration, and structural
features of the magnetic minerals contained in the rock to reveal the differences in the magnetic properties of the peridotites
under various circumstances of the mantle magmatism and different conditions of metamorphism. To do this, we examined and
analyzed the magnetic and petrographic characteristics of four collections of oceanic and alpinotype spinel peridotites. The
main object for comparing the magnetic characteristics was the Gorringe ridge, which lies in the eastern part of the Atlantic
Ocean. The peridotite samples from the Gorringe ridge differ from the other collections in many magnetic parameters: I
n
, χ, Q, I
rs
/I
s
, H
c
, H
cr
, and H
m
. The principal question of our work was to clarify the nature of the Earth’s crust where the Gorringe ridge formed. This
subject was studied many times in the literature, but the researchers did not reach a common opinion. In accordance with our
data, the spinel peridotites from the Gorringe ridge represent a subcontinental lithosphere mantle of the Iberian continental
margin. During the metamorphism, the formation of magnetite occurred in the peridotites of the Gorringe ridge in several stages
and had a regressive character. Our investigations explain the results of the analysis of the anomalous magnetic field over
the Gorringe ridge, which is characterized by sharp roughness and high intensity of the local signchanging anomalies. 相似文献
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S. A. Silantyev L. V. Danyushevsky A. A. Plechova L. Dosso B. A. Bazylev V. E. Bel’tenev 《Petrology》2008,16(1):36-62
Data presented in the paper suggest significant differences between the thermodynamic conditions under which magmatic complexes were formed in MAR at 29°–34° N and 12°–18° N. The melts occurring at 29°–34° N were derived by the melting of a mantle source with a homogeneous distribution of volatile components and arrived at the surface without significant fractionation, likely, due to their rapid ascent. The MAR segments between 12° and 18° N combine contrasting geodynamic environments of magmatism, which predetermined the development of a large plume region with the widespread mixing of the melting products of geochemically distinct mantle sources. At the same time, this region is characterized by conditions favorable for the origin of localized zones of anomalous plume magmatism. These sporadic magmatic sources were spatially restricted to MAR fragments with the Hess crust, whose compositional and mechanical properties were, perhaps, favorable for the focusing and localization of plume magmatism. The plume source between 12° and 18°N beneath MAR may be geochemically heterogeneous. 相似文献
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Guram S. Zakariadze Yildirim Dilek S.A. Adamia R.E. Oberhnsli S.F. Karpenko B.A. Bazylev N. Solov'eva 《Gondwana Research》2007,11(1-2):92
The Transcaucasian Massif (TCM) in the Republic of Georgia includes Neoproterozoic–Early Cambrian ophiolites and magmatic arc assemblages that are reminiscent of the coeval island arc terranes in the Arabian–Nubian Shield (ANS) and provides essential evidence for Pan-African crustal evolution in Western Gondwana. The metabasite–plagiogneiss–migmatite association in the Oldest Basement Unit (OBU) of TCM represents a Neoproterozoic oceanic lithosphere intruded by gabbro–diorite–quartz diorite plutons of the Gray Granite Basement Complex (GGBC) that constitute the plutonic foundation of an island arc terrane. The Tectonic Mélange Zone (TMZ) within the Middle-Late Carboniferous Microcline Granite Basement Complex includes thrust sheets composed of various lithologies derived from this arc-ophiolite assemblage. The serpentinized peridotites in the OBU and the TMZ have geochemical features and primary spinel composition (0.35) typical of mid-ocean ridge (MOR)-type, cpx-bearing spinel harzburgites. The metabasic rocks from these two tectonic units are characterized by low-K, moderate-to high-Ti, olivine-hypersthene-normative, tholeiitic basalts representing N-MORB to transitional to E-MORB series. The analyzed peridotites and volcanic rocks display a typical melt-residua genetic relationship of MOR-type oceanic lithosphere. The whole-rock Sm–Nd isotopic data from these metabasic rocks define a regression line corresponding to a maximum age limit of 804 ± 100 Ma and εNdint = 7.37 ± 0.55. Mafic to intermediate plutonic rocks of GGBC show tholeiitic to calc-alkaline evolutionary trends with LILE and LREE enrichment patterns, Y and HREE depletion, and moderately negative anomalies of Ta, Nb, and Ti, characteristic of suprasubduction zone originated magmas. U–Pb zircon dates, Rb–Sr whole-rock isochron, and Sm–Nd mineral isochron ages of these plutonic rocks range between 750 Ma and 540 Ma, constraining the timing of island arc construction as the Neoproterozoic–Early Cambrian. The Nd and Sr isotopic ratios and the model and emplacement ages of massive quartz diorites in GGBC suggest that pre-Pan African continental crust was involved in the evolution of the island arc terrane. This in turn indicates that the ANS may not be made entirely of juvenile continental crust of Neoproterozoic age. Following its separation from ANS in the Early Paleozoic, TCM underwent a period of extensive crustal growth during 330–280 Ma through the emplacement of microcline granite plutons as part of a magmatic arc system above a Paleo-Tethyan subduction zone dipping beneath the southern margin of Eurasia. TCM and other peri-Gondwanan terranes exposed in a series of basement culminations within the Alpine orogenic belt provide essential information on the Pan-African history of Gondwana and the rift-drift stages of the tectonic evolution of Paleo-Tethys as a back-arc basin between Gondwana and Eurasia. 相似文献
10.
Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al2O3. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them. 相似文献