Mafic rocks containing eclogite relics are fairly widespread in the crystalline nappe stack of the Swiss Central Alps. This study addresses the spatial distribution of eclogite relics in the Central Alps, their field relations, structural and petrological characteristics, and their PTt-history. Implications for the assembly of the nappe-stack are explored. The majority of eclogite facies relics is confined to a single super-unit of tectonic mélange, interpreted as a tectonic accretion channel (TAC). Numerous mafic high-pressure (HP) lenses have been discovered through systematic fieldwork in the TAC units of the Central Alps, an up-to-date inventory of which is presented. Systematic documentation of select samples with HP-imprint yields clockwise P-T paths. Prograde phase relations are seldom preserved, except in the chemical zoning of garnet porphyroblasts. However, when present, relic assemblages indicate HP-LT conditions indicative of a subduction setting. Maximum recorded pressures are substantially different from one location to the next (1.9 to 3.3 GPa). Depending on the degree of rehydration, reaction sequences are derived from observed relics, local replacement relations and assemblages. Quantitative constraints on the detailed PT-path are extracted by combining isochemical phase diagrams and TWQ-thermobarometry with petrographic information. HP-lenses from different locations display substantially different paths, both within and between different mélange zones of the TAC. PT-conditions reflecting the late-Alpine Barrovian overprint of mafic HP-lenses are in agreement with the coherent regional pattern derived from metasediments, i.e., maximum temperatures (~600 °C in the central Lepontine belt, 700-750 °C in the southern parts) were reached at pressures between 0.75 and 0.55 GPa Four samples have been dated by Lu-Hf isotopic analysis of garnet, clinopyroxene, matrix phases and whole-rock powders. The age span covers a range from >70 to ~36 Ma, much larger than previously documented for Alpine HP-rocks from the Central Alps. Petrological data of the samples and their Lu-Hf isotopic system indicate a protracted HP-history for at least some of the sub-units of the TAC, with garnet growth under eclogite-facies conditions starting before 70 Ma in some parts of the TAC, and continuing as late as 36 Ma in others. These data have implications for the dynamics of mélange formation within the TAC, with internal fragmentation and mixing, and pronounced mobility of the tectonic zones, probably during the early, subductional stages and again during the post-collisional extrusion along the plate boundary. After 32 Ma, the TAC appears to have been exhumed as part of the then-coherent crystalline nappe stack, when the Barrovian overprint reached its maximum temperature.

This research was supported by the Swiss National Science Foundation (SNF).

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Last modified 12th October, 2007

Eclogite relics in the Central Alps: PT-evolution, Lu-Hf ages, and implications for formation of tectonic mélange zones