Fluid Infiltration and Transport of Major, Minor, and Trace Elements During Regional Metamorphism of Carbonate Rocks, Wepawaug Schist, Connecticut, USA

Abstract

Geochemical alteration of metacarbonate layers due to fluid infiltration during greenschist and amphibolite facies metamorphism was investigated based on 149 whole-rock analyses of the Wepawaug Schist, USA. Ankerite-Albite (Ank-Ab), rare Ankerite-Oligoclase (Ank-Ol), Biotite (Bt), Amphibole (Amp), and Diopside (Di) index mineral zones developed during progressive metamorphism (Acadian Orogeny) and devolatilization (CO2, H2O, S loss). Ank-Ab, Ank-Ol, and Bt zone rocks were altered along some lithologic contacts and in reaction selvages around veins. Mass changes included gains of Na, Y and, in many cases, P and Fe. K, Rb, and Ba were typically lost, and Sr was lost except where calcite precipitated in and around calcite-bearing veins. Na gain and K loss reflect muscovite destruction and albite growth in response to infiltration of fluids from surrounding metapelitic and metapsammitic (metaclastic) rocks. At higher metamorphic grades, K, Rb, and Ba were lost when biotite broke down to form amphibole or diopside; these elements were transported out of metacarbonate layers down chemical potential gradients at lithologic contacts and in selvages. Na was lost as plagioclase reacted to form clinozoisite/zoisite (Czo/Zo). Calc-silicates (cm to dm scale thickness) rich in amphibole and Czo/Zo (Amp zones) or diopside and Zo (Di zones) formed at layer margins and in selvages via extreme metasomatism including major: 1) addition of Al, Si, and, in many rocks, Fe, 2) destruction of calcite, and 3) loss of Ca and volatiles. Volatile losses near -90 percent relative to low-grade precursors demonstrate that calc-silicate formation resulting from Al-Si metasomatism is an important source of CO2 to be considered when assessing regional devolatilization. Al mass gains for calc-silicates indicate that large quartz veins and some lithologic contacts were loci for massive time-integrated fluid fluxes of ∼105 m3 m-2. Heavy rare earth elements (HREE) were gained in the Ank-Ab through the Amp zones, and a Di zone example gained light- and mid-REE including Sm and Nd. The REE gains coincide with gains of Y and, in most cases, P, suggesting that P and Y complexes transport REE. Volume was lost progressively from the unveined interiors of metacarbonate beds, attaining -20 to -30 percent loss in the Di-II zone. Mass addition due to veining reduced volume losses and, in some cases, produced volume gains. Reaction-transport modeling indicates that fluid flow was mostly layer-parallel, was focused along lithologic contacts, fractures, and permeable horizons, and was accompanied by diffusion and/or mechanical dispersion at high angles to the flow directions. The metasomatic fluids infiltrated from surrounding metaclastic and/or syn-metamorphic igneous rocks. Thus, interaction of fluids derived from different rock types must be considered when assessing mass transfer in lithologically diverse mountain belts.