Fluid pathways and high‐P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

Abstract

The Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed at granulite facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite facies remnants of the Tethyan seafloor (Zermatt‐Saas zone). Near the base of the Mt. Emilius massif, there is a shear zone with eclogite facies hydrofracture systems associated with deformation‐induced re‐equilibration of granulites during high‐P metamorphism. In the basal part of the massif, a pluri‐hectometre domain of sheared mafic boudins is hosted in the granulitic paragneiss. In these mafic boudins, there are garnetites, garnet veins and clinopyroxenites, as well as clinozoisite and calcite veins. These features record multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid–rock interaction, metasomatism and volume change. This integrated petrological, micro‐textural and geochemical investigation illustrates the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite–eclogite‐facies field (at ~2.15–2.4 GPa, 500–550 °C) during subduction of the Mt. Emilius slice. The calcite veins crosscutting the garnetites have relatively low δ18OVSMOW values (∼+6.5‰) near those for marble layers (and nearby calcsilicates) embedded within the metasomatized granulites (+8 to +10‰). It is proposed that infiltration of externally‐derived H2O‐rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short‐distance transport and re‐precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate and creating long‐lasting mechanical instabilities during subduction zone deformation.

Fluid Pulses During Stepwise Brecciation at Intermediate Subduction Depths (Monviso Eclogites, W. Alps): First Internally Then Externally Sourced

“…The systematic presence of metasomatic ultramafic rinds surrounding the dismembered breccia blocks (i.e., metasomatic fluids; Step E; Figures e and ) indicates that the LSZ concentrated large‐scale circulation of ultramafic fluids along the plate interface during the first stages of exhumation of the Monviso slice, as proposed by Angiboust et al (). Similar shear zones collecting slab‐derived fluids have been reported in Syros melange (Breeding et al, ), Sesia zone (Konrad‐Schmolke et al, ), Mont Emilius Arbolla shear zone (Angiboust et al, ), and Cima di Gagnone (e.g., Scambelluri et al, ).…”

“…Fluid circulation also triggers strain localization through mineralogical and associated rheological changes (e.g., strain weakening; e.g., Mancktelow & Pennacchioni, ; Wassmann & Stöckhert, ) and/or high fluid pressure‐induced brittle deformation (e.g., Lund & Austrheim, ; Philippot & Selverstone, ). Several studies evidenced the key role of brittle precursors on strain localization and shear zone development (Angiboust et al, ; Hertgen et al, ; Locatelli et al, ; Mancktelow & Pennacchioni, ; Pennacchioni & Mancktelow, ), but the exact role of fluids on such transient switches between ductile and brittle deformation has remained unclear. For example, it is the presence of fluids triggering or resulting from brittle deformation (e.g., Hertgen et al, ; Locatelli et al, ; Pennacchioni & Cesare, ).…”

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

“…Most probably the formation of fractures and growth of garnet inside these fractures were closely related, and mineral compositions indicate that this occurred under eclogite facies conditions. Moreover, comparable garnet textures in a similar geological context (Monte Emilius klippe) have been interpreted to reflect fracturing and sealing at Alpine eclogite facies conditions (Pennacchioni, 1996;Angiboust et al, 2017;Hertgen et al, 2017).…”

“…The quantitative micro-mapping strategy employed in this study has wellestablished advantages (e.g. Marmo et al, 2002;Lanari et al, 2013;Ortolano et al, 2014b;Angiboust et al, 2017) over traditional spot analyses: (1) it allows key relationships, such as the successive growth zones, to be identified and relevant compositions to be constrained; (2) it permits testing if chemical zoning patterns are consistent over several grains, which helps support (or refute) the assumption of grain boundary equilibrium (e.g. ; (3) it can be used to approximate local reactive bulk composition by accounting for (non-reactive) mineral relics.…”

Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

“…Thermal breakdown of hydrous minerals will liberate fluids, and these can trigger processes as diverse as seismic failure (Abers et al, 2013;Barcheck et al, 2012), mantle infiltration (Soret et al, 2016), or hydrous melting (Labrousse et al, 2015). But continental slabs comprise mostly pretty infertile lithotypes, with sparse hydrates, and little is known about how fluids in the subduction channel interact, especially with very dry rocks (Angiboust et al, 2017) before these reach their solidus.…”