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| Titel |
Proterozoic Eastern Sayan ophiolites (Central Asian Orogenic Belt) record subduction initiation in vicinity of continental block |
| VerfasserIn |
Vasilii Belyaev, Marina Gornova, Alexander Medvedev, Sergey Dril, Anas Karimov |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250151490
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| Publikation (Nr.) |
 EGU/EGU2017-16079.pdf |
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| Zusammenfassung |
| Geochemical study of cumulate and volcanic rocks from ∼ 1020 Ma Eastern Sayan
ophiolites1 (Siberia, Russia) is used to provide a correlation between two ophiolitic belts and
link them to subduction initiation setting. Studied areas include Ospin and Ilchir massifs to
the East and Dunzhugur to the West of Early Precambrian Gargan block. Ophiolitic
cumulates represent peridotite-pyroxenite-gabbro-norite suite with crystallization orders of
Cr-Sp – Ol – Cpx – Opx – Plag, and Cr-Sp – Ol – Opx – Amph – (Cpx) – Plag.
Clinopyroxene is augite-diopside with Mg# 85-95, low Al2O3 (1-2.5%) and TiO2
(0.05-0.2%). Amph is Mg-hornblende to edenite (Mg# 84-86, 5-8% Al2O3, 0.3-0.6% TiO2).
Cr-Sp has Cr# 65-83 and 0.05-0.3% TiO2 in cumulates with high Opx proportion,
while in Cpx-dominating pyroxenites chemistry of Cr-Sp is variable (Cr# 40-75,
0.05-0.5% TiO2). Due to alteration, Ol and Opx chemistry is available only for some
samples (Ol: Mg# 88, 0.2-0.3% NiO; Opx: Mg# 89, 1.6% Al2O3). Whole-rock
MgO ranges 9 to 38%. Amph-free pyroxenites and gabbro-norites show flat to
slightly depleted REE pattern with negative HFSE anomalies. Amph-pyroxenites
have fractionated trace-element pattern with LREE enrichment, Nb-Ti minima at
slightly higher HFSE abundances. In-situ LA-ICP-MS analysis of Cpx in Amph-free
pyroxenites and gabbro-norites revealed moderately depleted to flat REE and Nb-Zr-Hf-Ti
depletion, with low trace element abundances (La/SmPM = 0.14-0.9, Zr 0.6-2.3, Nd =
0.2-1.1, Yb = 0.2-0.7 ppm). Melts calculated to be in equilibrium with Cpx using
distribution coefficients2 are REE-flat to slightly LREE-enriched (La/SmPM = 1-4) at low
HREE abundances (0.5-1.5 ppm Yb). Overall, crystallization orders, mineral and
whole-rock chemistry suggest origin of ophiolitic cumulates from low-Ca boninites or
primitive andesites (higher Opx or Amph proportion) to high-Ca boninites or primitive
island arc tholeiites (Cpx-dominating, Amph-free associations with subordinate
Opx).
Ophiolitic volcanics and dikes are low-Ca and intermediate-Ca boninites, andesite-basalts,
andesites, dacites of calc-alkaline (CA) affinity with rare evolved island-arc tholeiitic
(IAT) andesite-basalts. They resemble appropriate rocks of intraoceanic island arcs,
forearcs, and ophiolites. Boninites and CA-andesites are LREE-enriched (La/SmPM
1.2-3.8) at low HREE (0.5-1.6 ppm Yb) contents while evolved IAT show flat REE
(La/SmPM = 1.1) and higher abundances (2.4-2.8 ppm Yb), and both have negative Nb
anomalies.
Nd-isotopic data expressed as epsilon Nd(1020Ma) values are -2.3 to +4.1 in cumulates,
-2.8 to +0.4 in boninites and andesites, and +2.3 to +2.7 in IAT (compared to epsilon
Nd(1020Ma) +7.8 in depleted mantle). The ophiolites obducted on the Gargan continental
block, which contains Archean gneisses with epsilon Nd(1020) = -20 to -281. Subduction and
recycling of sediments derived from these gneisses could explain enriched Nd isotopic
characteristics of the studied ophiolitic rocks.
The boninite-andesite-IAT association is usually found in subduction initiation settings
recorded by modern forearc regions and forearc ophiolites. The difference of the Eastern
Sayan ophiolites is their supposed formation close to ancient continental block which
supplied recycled material into newly formed subduction zone.
1. Sklyarov et al (2016) Russ Geol Geophys 57, 127-140
2. Sobolev et al (1996) Petrology 3, 326-336. |
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