Genesis and mobility of the H 2 O-CO 2 fluid phase during regional greenschist and epidote amphibolite facies metamorphism: a petrological and stable isotope study in the Scottish Dalradian

Abstract

During high-pressure, low-temperature greenschist and epidote-amphibolite facies metamorphism in Dalradian rocks of the SW Scottish Highlands, mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids; consequently, mineral assemblages capable of buffering the fluid phase composition were rare. Equilibrium prograde H 2 O-CO 2 fluids usually contained less or much less than about 1–2 mol% CO 2 . Three fluid infiltration events are recognized. During prograde greenschist facies metamorphism, metabasic sills were infiltrated by large volumes of CO 2 -bearing hydrous fluid; carbon isotope studies indicate that the CO 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments. This may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f O 2 , initiated by thermal expansion of water during heating, decompression and consequent hydraulic fracturing. In the epidote-amphibole facies (garnet zone), dehydration reactions in metabasites generated large quantities of water, which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular, diopside, K-feldspar, amphibole, clinozoisite and sphene. A late retrograde infiltration of CO 2 ,-bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing K-feldspar + chlorite + rutile ± dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation. Large-scale infiltration of fluid into greenschist-facies metadolerite sills was intimately related to, and possibly controlled by, penetrative deformation and, in the absence of a penetrative deformation, grain-boundary diffusion by itself was an ineffectual mechanism of fluid transport.