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| Titel |
Isotope-geochemical features of tholeiites from the Spiess Ridge (South Atlantic) |
| VerfasserIn |
Boris Belyatsky, Anton Antonov, Robert Krymsky, Elena Lepekhina, Natalia Migdisova, Nadezhda Sushchevskaya |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056428
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| Zusammenfassung |
| Oceanic crust formation is a complicated nonlinear process enhanced by tectonic activity
which course origination of numerous geochemical anomalies in the composition
of MOR basalts and involves the ancient material like non-spreading blocks or
subducted slabs into the crust formation. The regions of triple-junction of spreading
ridges where the tectonic processes are often of the leading role and determine
the character and evolution of magmatism, are marked by especially complicated
construction and evolution. One of such regions is Bouvet triple-junction in the South
Atlantic where three ridges meet: Mid-Atlantic, American-Antarctic and South-West
Indian (SWIR). At the most western point of the SWIR there is situated the rift
segment with massive volcanic structure formed not more than 2 m.y. ago with
which the modern position of plume – Spiess Ridge is correlated (Peive, Skolotnev,
2001).
One of the samples represented by olivine-plagioclase porphyritic basalt dredged within
central part of the small separate volcanic edifice within Spiess Ridge has been studied for
trace element and isotope geochemistry. The studied sample is a typical tholeiitic
basalt: SiO2 – 50.75÷51.19, Al2O3 – 16.4÷16.7, FeO – 8.7÷9.1, MgO – 7.9÷8.01,
CaO – 12.4÷12.6, K2O – 0.2÷0.3, Na2O – 1.92÷1.99 %; with phenocrysts of
olivine (up to 1-2 mm) and plagioclase (up to 1 mm) and aphanitic groundmass
composed mainly by three mineral phases – plagioclase microlites (60 vol.%),
pyroxene microlites (35 vol.%) with more abundant FeO and CaO in the grain
cores and accessories (Ti-magnetite up to 3 vol.%). Phenocrysts constitute not more
than 20% of the sample volume. Olivine phenocrysts are characterized by clear
composition zoning: from center to grain margin FeO content is growing and MgO
decreasing and mainly correspond to Fo 81.2-89.5 (aver. 85.4). At the same time, this
sample has evidently lower titanium content – TiO2: 0.63÷0.65 and negative Ta
and Nb anomalies, and positive – Pb and Sr – well-known characteristics for arc
volcanics and typical for subduction tectonic settings (Niu, O’Hara, 2009). Isotope
composition for studied sample and mineral separations varies within: 143Nd/144Nd:
0.513010-0.513024; 87Sr/86Sr: 0.703632-0.704345; 206Pb/204Pb: 17.863-18.638;
207Pb/204Pb: 15.522-15.631; 208Pb/204Pb: 37.750-38.683 that means more radiogenic Sr
and less radiogenic Nd and Pb content in comparison with published data for Spiess
Ridge basalts: 143Nd/144Nd: 0.512910-0.513074; 87Sr/86Sr: 0.703260-0.703634;
206Pb/204Pb: 19.005-19.28; 207Pb/204Pb: 15.53-15.65; 208Pb/204Pb: 38.69-38.99.
Moreover, Re-Os isotope systematics of the studied constituent phases of the basalt
allows to construct some line which corresponds to the age 13.44±0.77 Ma and
initial 187Os/188Os = 0.1714, but the whole-rock sample after acid leaching has
clearly less radiogenic composition 187Os/188Os = 0.14758 corresponding only
to the 1.6 Ma model age comparable with the supposed age of magmatic activity
within Spiess Ridge (Peive, Skolotnev, 2001). Moreover, in the course of mineral
separation we found two zircon grains in this sample of tholeiite with morphology
and geochemistry typical for the zircons of primary magmatic origin and studied
their U-Pb system on SIMS SHRIMP II. The age of these grains is Archean, as
old as 2418 and 2720 Ma, and is comparable in time with the processes within
Kalahari Craton – Limpopo orogeny, metamorphism and volcanism accompanying
accretion Barberton Craton with Limpopo mobile belt and Zimbabwe Craton which
resulted in formation of Kalahari Craton 2.7 Ga ago (Jacobs et al., 2008; Zeh et al.,
2009).
Thus the whole data-set of isotope-geochemical characteristics of the studied sample of
abnormally enriched porphyry basalt unequivocally evidence to the presence of old crust
(oceanic or low continental) material within magma source. The share of this ancient matter
can be estimated as 45% on the basis of Os and Sr isotope data (Krymsky et al., 2009) and
not less than 50% by Pb and Nd isotope data. One of the plausible explanations of such
considerable amount of the ancient substance is the formation of Spiess Ridge over the
ancient crustal block – Lafonia microplate (Ben-Avraham et al., 1993) which was separated
from Kalahari Craton in the course of complicated tectonic movements during or after
Gondwana break-up. |
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