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| Titel |
Petrogenetic implications from ultramafic rocks and pyroxenites in ophiolitic occurrences of East Othris, Greece |
| VerfasserIn |
P. Koutsovitis, A. Magganas |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070593
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| Zusammenfassung |
| Ultramafic rocks and pyroxenites in east Othris are included within ophiolitic units near the
villages of Vrinena, Karavomilos, Pelasgia, Eretria, Agios Georgios, Aerino and Velestino.
The first five ophiolitic occurrences are estimated to have been emplaced between the
Oxfordian and Tithonian-Berriasian[1,2,3], while the latter two have been emplaced during
the Eocene[4]. Ultramafic rocks include variably serpentinized harzburgites and
lherzolites. Pyroxenites are usually found in the form of crosscutting veins within the
harzburgites.
Ultramafic rocks include depleted lherzolites, with Al2O3 ranging from 1.12 to 1.80 wt%
and Cr from 3250 to 3290 ppm, as well as moderate to highly depleted serpentinized
harzburgites, with Al2O3 ranging from 0.69 to 1.98 wt% and Cr from 2663 to 5582 ppm.
Pyroxenites have generally higher Al2O3 ranging from 1.91 to 3.08 wt% and variable Cr
ranging from 1798 to 3611 ppm.
Lherzolites mostly include olivines (Fo=87.07-89.23) and clinopyroxenes
(Mg#=85.71-90.12). Spinels from Eretria lherzolite (TiO2=0.02-0.08 wt%, Al2O3=36.06-42.45
wt%, Cr#=31.67-36.33) are compositionally similar with those of MORB peridotites[5],
while those from Vrinena lherzolite (TiO2=0.16-0.43 wt%, Al2O3=6.90-22.12 wt%,
Cr#=57.69-76.88) are similar to SSZ peridotites[5]. Serpentinized harzburgites include few
olivines (Fo=90.51-91.15), enstatite porphyroclasts (Mg#=87.42-88.91), as well as fine
grained enstatites of similar composition. Harzburgites from Pelasgia, Eretria and Agios
Georgios include spinels (TiO2=0.03-0.08 wt%, Al2O3=23.21-31.58 wt%, Cr#=45.21-56.85)
which do not clearly show if they are related with MORB or SSZ peridotites[5]. Spinels
from Karavomilos harzburgite (TiO2=0.02-0.05 wt%, Al2O3=45.71-50.85 wt%,
Cr#=16.84-22.32) are compositionally similar with MORB peridotites[5], whereas
spinels from Vrinena harzburgite (TiO2=0.15-0.19 wt%, Al2O3=1.42-1.86 wt%
Cr#=91.64-93.47) with SSZ peridotites[5]. Pyroxenites include clinopyroxenes
(Mg#=84.25-91.78) but also enstatites (Mg#=88.37-91.47). Spinels have been analysed in
pyroxenites from Aerino and Velestino (TiO2=0.79-1.07 wt%, Al2O3=10.88-18.46 wt%
Cr#=60.74-70.78), indicating SSZ settings. Application of the olivine-spinel[6],
olivine-augite[7], Cpx-Opx[8,9] geothermometers, yield equilibration temperatures of
961-1075 oC for lherzolites, 895-1084 oC for harzburgites and 990-1011 oC for
pyroxenites.
Our data indicate that the ophiolitic occurrences of Vrinena, Aerino and Velestino include
ultramafic rocks and pyroxenites related to SSZ processes, while the other ophiolitic
occurrences embrace ultramafic rocks which originated from a MORB-like setting, similar to
west Othris ophiolites. It should be noted that even lherzolites have Cr and Y values
similar to those of a highly depleted mantle source. A supra-subduction zone origin
of the east Othris ophiolites, possibly with a slab rollback in the Pindos oceanic
basin, may explain the different geotectonic environment affinities of the studied
rocks.
References. [1] Rassios 1990: Ofioliti 15, 287-304; [2] Migiros et al. 1997: Ann
Soc Geol du Nord 5, 59-67; [3] Smith and Rassios 2003: Geol Soc Am, Spec pap
373, 337-350; [4] Pe-Piper & Piper 2002: Borntraeger, Stuttgart, pp 1-645.; [5]
Kamenetsky et al. 2001: J Petrol 42, 655-671; [6] Fabries 1979: Contr Min Petrol
69, 329-336; [7] Loucks 1996: Contr Min Petrol 125, 140-150; [8] Wells 1977:
Contr Min Petrol 62, 129-139; [9] Brey & Kohler 1990: J Petrol 31, 1353-1378. |
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