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Centre for Earth and Environmental Science Research

Petrology and Geodynamics of pre-Alpine complexes in the Caucasian segment of the Mediterranean fold belt

Executive Summary
ResearchersDr Peter J. Treloar
Dr Fritz Finger (Salzburg)
Mr Franz Mayringer (Salzburg)
Dr Axel Gerdes ( Berlin)
Professor David Shengalia (Georgian Academy of Sciences, Tbilisi, Georgia)
Professor Sergey Korikovsky (Moscow)
Professor Vilen Agmalyian (National Academy of Sciences, Yerevan, Armenia)
Funding Body/SourceINTAS, Brussels
Duration2002 - 2005
Project SummaryTo quantify and date the igneous and metamorphic history of the eastern end of the Variscan belt as exposed in the Caucasus and to comment on overall Variscan evolution of Europe.

The project aims to assess the pre-Alpine history of the Caucasian region of the Variscan orogenic belt. Four major questions relate to the metamorphic and magmatic evolution of the Caucasus. 1) what is the provenance and timing of magmatism within the region; 2) what are the pressure and temperature conditions of metamorphism; 3) what is the age of metamorphism and 4) what is the tectonic setting of magmatism and metamorphism and how are they interlinked? Abundant new data collected during the course of this project directly address these questions.

The Caucasus mountain belt has a complex tectono-thermal history Published data suggest a poly-metamorphic history culminating in high-temperature low-pressure (HT/LP) Variscan regional metamorphism. Cambro-Ordovician (> 500 and c. 480Ma) and Variscan (350 - 300 Ma) ages are recorded. The external zones, including rocks exposed in the Caucasian Main Range and the Dzirula, Loki and Khrami inliers to the south in Georgia show meta-sedimentary rocks to contain biotite, cordierite (often pinitised), sillimanite (sometimes pseudomorphosed to andalusite) and rare garnet but no muscovite. These rocks were metamorphosed at temperatures above that (850oC) of the biotite vapour-absent melt reaction. Pressures, estimated on the basis of petrogenetic grids and thermodynamic criteria were < 5 kbar This indicates a regionally distributed, Variscan low pressure/high temperature regional metamorphism which likely developed as a result of extremely high heat flows. Similar conditions are identified elsewhere in the Variscan terranes of western and central Europe where there is no general consensus as to the heat source. Ongoing thermal modelling, supported by geochemical data, is designed to enable us to propose a solution to this enigma.

New monazite and zircon data from the Dzirula massif in the southern Caucasus which permit erection of a model for Variscan HT/LP metamorphism in the Caucasus.

The Dzirula massif comprises four key units.

  1. Deformed granodiorite gneisses with U-Pb zircon ages of c. 540 Ma.
  2. HT/LP crdierite-biotite-sillimanite migmatites and paragneisses with biotite vapour-absent melting at >800oC. Zircon and monazite data date metamorphism at c. 330 Ma. Some paragneisses contain relict 480 Ma monazites implying Variscan metamorphism overprinted a previous thermal event.
  3. Un-foliated calc-alkaline to high-K, I-type granodiorites, diorites and gabbros intrusive into the gneisses and migmatites. Monazite ages of c. 330 Ma suggest the intrusions represent the heat source that drove the metamorphic event.
  4. Medium- to high-K peraluminous granites intrusive into all other units. Ar ages of 321 + 7 Ma document a Variscan intrusion age. They represent a crustal melt related to HT/LP Metamorphism.

Age and petrographic data from elsewhere in the Caucasus are similar to those from Dzirula. The massif can thus be used as a proxy to model evolution of the whole belt. HT/LP metamorphism, anatexis of previously metamorphosed sequences, and generation of peraluminous granites followed emplacement of TTG type magmas into the upper crust. As there is no evidence in the Caucasus of heating due to obduction of a HT/HP mantle slab it is probable that Variscan HT/LP metamorphism is best linked to supra-subduction zone processes related to closure of the Caucasian, eclogite studded, suture or, and more likely, interaction with a mantle plume synchronously with closure of the Caucasian suture.

Publications

Mayringer, F.; Gerdes, A.; Treloar, P.; Dorr, W.; Finger, F.; Shengelia, D. & Starijas, B. 2005: Geological evolution of The Dzirula massif, Georgia. Mitt. Osterr. Miner. Ges 151

Treloar, P.J, Finger, F., Mayringer, F. Gerdes, A & David Shengelia, D.M.. 2007. New age data from the Dzirula massif, Georgia: implications for the evolution of the European Variscides. Frontiers in Mineral Science, Cambridge, UK.

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