Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
11024649 | Lithos | 2018 | 56 Pages |
Abstract
The Monviso meta-ophiolite complex (Northern Italy, Western Alps) represents an almost intact fragment of Tethyan oceanic lithosphere metamorphosed at ~80â¯km depth (~2.6GPa-550â¯Â°C) during the Alpine subduction. We focus our study on a major shear zone cutting across this slab fragment at low angle (the Lower Shear Zone; LSZ). Here, in its talc and tremolite-rich serpentinite matrix, are embedded (together with metasedimentary lenses) variously brecciated Fe-Ti and Mg-Al metagabbro blocks. The latter were either interpreted as eclogitic breccias resulting from intermediate-depth rupture or as inherited, overprinted oceanic core complex features. Our new field, structural and petrographic data testify the genesis of this metagabbro breccia blocks at eclogite-facies conditions. Three types of eclogitic blocks can be distinguished, with non-random distribution (and decreasing size from top to base) throughout the ~200-m-thick and ~15â¯km-long LSZ: (1) Fe-Ti-metagabbros, brecciated and scattered in the upper to intermediate levels of the LSZ; (2) meter-size blocks and decameter-scale slivers of intact Mg-Al metagabbros, locally brecciated; (3) dm- to m-scale blocks of intact Fe-Ti metagabbros without breccia fabrics. Brecciation at eclogite facies conditions (at ~80â¯km depth) is documented by: i) the eclogitic foliation of intact Mg-Al-rich metagabbros (composed of omphacite + clinozoisite ± rutile and locally garnet) cut by breccia planes (cemented by omphacite + garnet ± lawsonite) and ii) the occurrence in breccia clasts of minerals that are fractured and offset along peak P-T omphacite-bearing planes. Rupture preferentially affected the Fe-Ti metagabbros, suggesting that rheological contrasts controlled the locus of brecciation. The occurrence of a first omphacite-rich matrix (M1, ~2.7GPa - 580â¯Â°C) crosscut by omphacite + garnet-bearing matrix M2 (~2.4GPa - 560â¯Â°C), witnesses multiple brittle rupture events, prior to a stage of eclogite facies fluid ingression marked by massive lawsonite recrystallization (matrix M3).
Keywords
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
Michele Locatelli, A. Verlaguet, P. Agard, L. Federico, S. Angiboust,