Metamorphic reprocessing of a serpentinized carbonate-bearing peridotite after detachment from the mantle wedge: A P-T path constrained from textures and phase diagrams in the system CaO-MgO-Al2O3-SiO2-CO2-H2O

Journal article

Authors/Editors

Proyer, Alexander

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Publication Details

Author list: Mposkos E, Baziotis I, Proyer A

Publisher: Elsevier

Place: AMSTERDAM

Publication year: 2010

Journal: Lithos: Journal of Petrology, Mineralogy and Geochemistry (0024-4937)

Journal acronym: LITHOS

Volume number: 118

Issue number: 3-4

Start page: 349

End page: 364

Number of pages: 16

ISSN: 0024-4937

Languages: English-Great Britain (EN-GB)

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Abstract

In the central Rhodope mountains of Greece a carbonate-bearing metaperidotite lens similar to 200 x 500 m in size crops out as part of the high- to ultrahigh-pressure metamorphic Upper Sidironero Complex similar to 500 m SE of the Gorgona Village, north of Xanthi town. It is composed primarily of coarse grained (3-20 mm in size) olivine and orthopyroxene, medium grained clinohumite and medium to fine grained tremolite, chlorite, dolomite, magnesite, talc, antigorite and various spinel phases.Whole-rock chemistry, mineral textures and compositions, and phase diagram calculations show that the metaperidotite was subjected to a prograde HP metamorphism, isofacial with the surrounding migmatitic gneisses, metapelites and amphibolites. The prograde character of metamorphism is demonstrated by inclusions of talc, antigorite, chlorite, dolomite, magnesite and Ti-clinohumite in ferrit-chromite, olivine, and orthopyroxene, as well as of olivine in orthopyroxene, and by the typical change in composition of zoned spinel minerals from ferrit-chromite in the core to chromian spinel at the rim.The prograde path is characterized by successive growth of amphibole, Ti-clinohumite, olivine and orthopyroxene, followed by the breakdown of Ti-clinohumite to olivine + Mg-ilmenite and of chlorite to olivine + spinel, probably during exhumation. The construction of a partial petrogenetic P-T grid in the system CaO-MgO-Al2O3-SiO2-CO2-H2O (CMASCH) for Ca-poor ultramafic bulk compositions has proven highly useful for the reconstruction of the metamorphic evolution and a P-T path, indicating that the use of univariant reactions in mixed volatile systems is highly warranted. The P-T path is clearly constrained to pressures below 1.5-1.7 GPa by the absence of clinopyroxene. These pressures are slightly lower than those recorded in the closely associated Jurassic eclogites and much lower than those recorded in the diamond-bearing gneisses 5 km to the south in the same tectonic unit.The carbonate-bearing metaperidotite from Gorgona probably represents a fragment of the hydrated mantle wedge. This is indicated by the REE compositions which differ from those of ophiolitic peridotites and resemble those of spinal or garnet peridotites of sub-continental origin. The ultramafic slice was incorporated tectonically into the subduction channel, most likely by tectonic erosion in the Early Jurassic, but did not experience ultrahigh-pressure metamorphism like the nearby metapelites that exhumed along the same subduction channel. (C) 2010 Elsevier B.V. All rights reserved.

Keywords

Carbonate-bearing metaperidotite, High-pressure metamorphism, Mantle wedge, Rhodope

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