Find in Library

<p>The study of rock chemistry is a milestone in understanding fluid–rock interactions and fluid migration in subduction zones. When combined with thermodynamic models, it can provide direct insight into fluid composition, metasomatic reactions, and pressure–temperature (<span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span>) conditions, as well as their role in rock deformation. Here, a shear zone – located in the Mont Avic area of the Zermatt-Saas zone (Western Alps) – is analyzed. This shear zone consists of several blocks of different lithotypes, including a Ca-rich metasomatite block embedded in a serpentinite mylonitic matrix, and structurally underlies a coherent eclogitic mafic unit. This work aims to estimate the pressure–temperature conditions of the Ca-rich metasomatism and the amount of fluid involved. The brecciation exhibits mosaic breccia textures with clasts comprising <span class="inline-formula">∼80</span> vol % of garnet, together with omphacite, epidote, titanite, rutile, and apatite hosted in an omphacite matrix. Quantitative chemical mapping of the garnet reveals primary garnet cores with embayment and lobate edges with a chemical composition similar to unaltered reference eclogite garnet. These primary garnet cores are overlain by Ca-rich metasomatic garnet rims with oscillatory chemical zoning. The oscillatory chemical zoning, together with the morphology of the primary garnet cores, suggests repeated influxes of external Ca-rich fluid that destabilized the primary garnet cores and promoted the growth of Ca-rich rims. Mass balance calculations between precursor metabasite and Ca-metasomatite indicate multiple fluid sources involving dehydrated serpentinite, calcic metasediments, and metabasites with time-integrated fluid fluxes calculated between <span class="inline-formula">11.5×10³</span> and <span class="inline-formula">5.5×10⁴</span> m<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mi mathvariant="normal">fluid</mi><mn mathvariant="normal">3</mn></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="17pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="a49491509ecbc5dbe32b9d7b01ac6799"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-767-2024-ie00001.svg" width="17pt" height="17pt" src="ejm-36-767-2024-ie00001.png"/></svg:svg></span></span> m<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mi mathvariant="normal">rock</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="17pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="23baa97ec7f2d777447399cfaa47ceba"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-767-2024-ie00002.svg" width="17pt" height="17pt" src="ejm-36-767-2024-ie00002.png"/></svg:svg></span></span>, consistent with channelized fluid flow in an open system. Thermodynamic modeling of garnet from unbrecciated and non-metasomatized metabasites – from the Savoney eclogitic mafic unit – indicates peak metamorphic conditions of <span class="inline-formula">2.5±0.1</span> GPa and <span class="inline-formula">535±40</span> °C, consistent with regional estimates. Pressure–temperature conditions of metasomatism were constrained using <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>X</i></span> and <span class="inline-formula"><i>T</i></span>–<span class="inline-formula"><i>X</i></span> phase modeling (where <span class="inline-formula"><i>X</i></span> represents changes in bulk CaO and Na<span class="inline-formula">₂</span>O composition) between 2.6–2.2 GPa and 570–500 °C, showing that Ca-rich fluid percolation occurred close to the metamorphic peak (i.e., prograde to the peak or early exhumation path).</p>