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| Titel |
Formation of a cold ophiolitic sole at the base of the Devonian Balkan Carpathian Ophiolite (Romania, Serbia, Bulgaria) |
| VerfasserIn |
Gaëlle Plissart, Hervé Diot, Christophe Monnier, Marcel Maruntiu, Vinciane Debaille, Franz Neubauer |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072179
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| Zusammenfassung |
| Our study concerns deformed gabbroic rocks from the Balkan Carpathian Ophiolite (BCO -
Romania, Serbia, Bulgaria). The BCO consists of four ophiolitic massifs dismembered during
Alpine tectonic and displaying together a complete classical oceanic lithosphere. Our new
Sm-Nd dating on fresh lower gabbroic rocks give an accretion age for the BCO crust at
409 ± 38 Ma, in agreement with a previous age of 405 ± 3 Ma (Zakariadze et
al. 2012). After removing the Alpine tectonic, the BCO appears as an elongated
E-W body tilted to the south. At the base of the ophiolitic complex occurs a thin
deformed zone (< 800m) of metagabbroic rocks underlined by Cambro-Ordovician
metasediments.
Petrostructural observations on metagabbroic rocks coupled with mineralogical and
geochemical data indicate that their protoliths were mainly upper gabbros statically
metamorphosed in the Greenschist/Amphibolite facies (event 1 = ocean-floor metamorphism
at the ridge axis). These rocks have been affected by a second circulation of fluids (event 2),
contemporaneous to a deformation and inducing local K-enrichment (formation of
Cr-muscovite). Temperature estimates for this event indicate a range of 450°C - 280°C, with
the lower values observed for the more intensively metasomatized rocks. 40Ar - 39Ar dating
on two Cr-muscovites from slightly and highly deformed metagabbros gives plateau ages of
372.6 ± 1.3 Ma and 360.6 ± 1.2 Ma respectively. We interpret the first age as a
mimimum age for the beginning of the event 2, observed into preserved rocks,
and the second one as linked to (neo-/)recrystallisation due to localisation of the
metasomatism/deformation.
The interval of 30 Ma between oceanic crust accretion and initiation of metasomatism/deformation
involves that the upper oceanic crust had cooled down to temperatures close to 100°C before
the beginning of event 2. Consequently, a temperature increase is required to observe the
greenschist facies assemblage. We have tested by tectono-thermal modelling the hypothesis
that these rocks could correspond to a slice of upper crust dragged down during intra-oceanic
subduction: temperatures of 450°C are reached at a depth of 17 km (5 kbar). These PT
conditions are in agreement with the mineralogical assemblage formed during event 2
meanwhile the intense fluids circulation and the K-rich metasomatism (up to 5% K2O for
bulk rock analyse) could be explained by the destabilization of deep oceanic sediments. To
initiate the subduction of a 30 Ma old oceanic lithosphere, we propose a zone of weakness
alongside a transform fault that juxtaposes oceanic lithospheres of different ages and
thicknesses, supported by microstructural criteria that evidence a highly oblique tectonic
obduction.
Our study emphasizes that ophiolitic soles could develop away from the ridge in a context of
intra-oceanic subduction along a transform fault. In that case, the hanging wall will not be hot
enough to produce classical high-grade metamorphic sole and the resulting rocks could be
referred as “cold ophiolitic soles”. |
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