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Polyphase evolution and reaction sequence of compositional domains in metabasalt: a model based on local chemical equilibrium and metamorphic differentiation
Author(s) -
Toóth T. M.,
Grandjean V.,
Engi M.
Publication year - 2000
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.857
Subject(s) - metamorphic rock , geology , petrography , eclogite , plagioclase , mineral , mineralogy , metamorphic facies , geochemistry , facies , quartz , tectonics , chemistry , paleontology , structural basin , organic chemistry , subduction
Eclogitic garnet amphibolite samples from the Southern Steep Belt of the Central Alps show evidence of several stages of metamorphic evolution and exhumation. A method for unravelling this evolution is presented and applied to these samples. It involves a combination of detailed petrographic analysis and microchemical characterization with quantitative models of the thermodynamically stable phase relations for specific compositional domains of each sample. Preserved mineral relics and textural evidence are compared to model predictions to identify the important irreversible reactions. The interpretation of the exhumation history is thus based on the consistency of a wide spectrum of observations with predicted phase diagrams, leading to robust reconstruction of a pressure–temperature (P–T) path even where the mineralogical relics in samples are insufficient, due to retrogression, to warrant application of multi‐equilibrium thermobarometric techniques. The formation of compositionally different domains in the metabasalt samples studied is attributed to prograde growth of porphyroblasts (e.g. garnet, plagioclase, zoisite) in the matrix, implying substantial metamorphic differentiation at the scale of a few millimetres. Chemical interaction among different domains during the subsequent P–T evolution is shown to have been very limited. This led to different reaction sequences during exhumation, in which relics preserved in different domains reflect a range of continually changing P–T conditions. For samples from a single outcrop, we deduce a Barrovian prograde path to eclogite facies (23 ± 3 kbar, 750 ± 50°C), followed by (rapid) decompression to 8 ± 1 kbar and 675 ± 25°C, and a final heating phase at similar pressures reaching 750 ± 40°C. This evolution is attributed to the Alpine cycle involving subduction→collision and slab breakoff→extrusion of tectonic fragments that make up the Southern Steep Belt of the Central Alps. Copyright © 2000 John Wiley & Sons, Ltd.