Abstract
A complex system of mono- and polymineralic centimeter-thick veins occurs within the ultrabasic amphibolites of Montigiu Nieddu hill in northeastern Sardinia, and they are filled with garnet, amphibole, chlorite, and epidote. Some garnet-rich veins are margined by an amphibole layer at the interface with the host rock and/or show replacement of epidote concentrated in the vein core. Together with homogeneous matrix garnet (Grt1), millimetric, euhedral, and strongly zoned garnet porphyroblasts occur within these veins. The estimated pressure–temperature conditions (P = 1.0–1.7 GPa, T = 650–750 °C) for the formation of Grt1 match the metamorphic peak and early exhumation derived previously for the host rocks and confirm that the garnet veins also formed under high-pressure (HP) conditions. The igneous protolith of the host rocks experienced HP metamorphism in a subduction zone and underwent exhumation in an exhumation channel. The vein system in the ultrabasic amphibolites formed by cyclic hydrofracturing as rapid and transient events such as crack-seal veining. The growth of multiple vein-filling mineral assemblages indicates the formation of separate vein-producing cycles.
Highlights
Progressive changes in deformation style and pressure–temperature conditions during prograde and retrograde metamorphism influence the style, scale, and distribution of fractures and veins in metamorphic terranes
The vein-system evolution accompanied the metamorphic history of the host rocks, which is related to continent–continent collision in one of the best-preserved branches of the southern Variscan belt
We demonstrate that evolution of the host rock with reference to exhumation processes in subduction zones related to the vein system developed through different steps during the exhumation of the ultrabasic amphibolites in collisional orogens
Summary
Progressive changes in deformation style and pressure–temperature conditions during prograde and retrograde metamorphism influence the style, scale, and distribution of fractures and veins in metamorphic terranes. Throughout each metamorphic event, such fractures are filled with a sequence of mineral phases that testifies to the presence of fluids and their flow and reactions with the host rock [1]. These rocks are crosscut by a complex system of mono- and polymineralic veins developed in response to ductile to brittle deformation. This system comprises garnet veins, as well as clinozoisite and amphibole veins, recording several events of fracturing and crystallization likely associated with fluid–rock interaction and/or metasomatism. The vein-system evolution accompanied the metamorphic history of the host rocks, which is related to continent–continent collision in one of the best-preserved branches of the southern Variscan belt
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