The origin of quartz-carbonate veins hosted by ductile shear zones of the Astridal Supracrustal Belt, West-Troms Basement Complex.

Abstract

An approximately 1 meter wide NE-SW-oriented mylonitic shear zone hosts hydrothermally precipitated quartz and carbonate veins. The shear zone mainly consists of a secondary mylonitic fabric enriched in chlorite. A similar mylonitic shear zone striking NE-SW, oriented parallel to the axial plane of a F3-fold, consists of the same secondary mylonitic chloritized fabric and are therefore assumingly related to each other. The rocks have undergone multiple folding and are cut by several generations of pegmatite bearing anastomosing ductile shear zones that are both parallel to lithological contacts, parallel to the mylonitic foliation and parallel to axial-surfaces and/or other truncation fabrics. Enrichment of hydrothermal fluids and the occurrence of prominent quartz-carbonate veins in the syn/post D3 shear zone at Småsandnesset are likely linked to reactivation of shear zones arranged parallel to the axial plane of small-scale F3-folds. Complex stress and strain pattern at fault tips may have contributed to opening, and fluids under high pressure (CO2) in the crust were free to move upwards and triggered chloritization of these shear zones. Fluid inclusions studies of quartz-carbonate veins, reveals that these fluids circulating the chloritized syn/post D3 mylonitic shear zones where low to moderate saline (6-7 wt% NaCl), high temperature fluids (290-320 ºC) with enriched in CO2 and N2. Combined fluid inclusions data and chlorite geothermometry revealed that the hydrothermally quartz and calcite veins hosted in the syn/post D3 chloritized shear zones have been deposited at temperatures of 500ºC and pressure of 600MPa. Isotopic compositions of carbonates from hydrothermal quartz-carbonate veins and carbonate deposited in a chloritized shear zone containing carbonate, suggest influence of both magmatic CO2 and marine carbonates.