Uplift-related retrogression history of aragonite marbles in Western Crete (Greece) |
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| Authors: | Thomas Theye Eberhard Seidel |
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| Institution: | (1) Institut für Mineralogie, Ruhr-Universität, Universitätsstrasse 150, W-4630 Bochum, Germany;(2) Mineralogisch-Petrographisches Institut, Universität zu Köln, Zülpicher Strasse 49b, W-5000 Köln, Germany;(3) Present address: Abteilung Mineralogie, Institut für Geowissenschaften, Gaussstrasse 29, W-3300 Braunschweig, Germany |
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| Abstract: | In the low-grade, high-pressure ( 400°C, 10 kbar) metamorphic Phyllite-Quartzite Unit of Western Crete, widespread occurrences of aragonite marbles have been discovered recently. A sedimentary precursor is proved by relic structures (bedding, fossils). Partial or complete transformation of aragonite into calcite is ubiquitous. Compositional and microstructural features reflect the metamorphic history: (1) The high-pressure stage is documented by aragonite that is chemically characterized by incorporation of variable amounts of Sr and the lack of Mg. The most Sr-rich aragonite has about 9 wt% SrO (X
Sr
arag
=0.09). A compositional zoning observed in some aragonite crystals may be due to the prograde divariant calcite aragonite transformation in the system CaCO3-SrCO3. Because the parent rocks probably were Sr-poor calcite limestones, one can speculate that strontium has been supplied from an external source under high-pressure conditions. (2) During uplift, calcite replacing aragonite did not equilibrate with unreplaced aragonite. Disequilibrium is indicated by highly variable compositions of calcite crystals that show topotactic relations to the host aragonite. The calcite compositions range from that of the host aragonite (Sr-rich and Mg-free) to Mg-bearing and Sr-poor. (3) Calcite that recrystallized during retrogression is generally Sr-poor (mean value ofX
Sr<0.005), Mg-bearing (X
Mg0.010), and chemically homogeneous. Because practically no Sr remains in the calcite, an interaction with a fluid phase is indicated. In fine-grained calcite marbles rich in solid organic matter, microstructural features indicative of former aragonite may be present. (4) The last stage of retrogression is documented by the appearance of radiating aragonite in veins and nodules. This aragonite, which shows neither deformation nor retrogression, was probably formed metastably in a near-surface environment. |
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