Fluid transfers at the basement/cover interface: Part I. Subsurface recycling of trace carbonate from granitoid basement rocks (France)

Basement rocks usually contain trace amounts of disseminated carbonate minerals and sometimes also fractures/veins filled with carbonates which may have a variety of origins. Extensive drilling has been performed of a granitoid body (Vienne, western part of the French Massif Central) overlain by a carbonate sedimentary sequence, a situation which is representative of most of the Variscan basement in western Europe. These unique samples provide the opportunity to investigate the behaviour of basement trace carbonate across the basin/basement interface and the mass transfers throughout a series of fluid migration ranging from high (ca. 450 °C) to low (down to 50 °C) temperature conditions. Study of quartz and carbonate found in fracture infillings reveals three major stages of fluid circulation, namely: (1) high to medium temperature late Hercynian fluids which circulated in fractures developed during basement uplift; (2) medium temperature Mesozoic basinal brines migration laterally along the basement/cover contact; (3) late infiltration of diagenetic/marine waters in the upper part of the granitoid body. rity of analyzed vein carbonates from the three stages displayed a rather restricted range of C isotopic compositions (δ13C between −14‰ and −9‰/PDB) whatever the stage in which they were deposited and despite large variations of O isotopic ratios (δ18O in the range +3‰ to +30‰/SMOW). Carbon in pervasively altered rocks displays the same features and is distributed among the two broad groups defined using O isotopic ratios in veins and fluid inclusions data, namely the Hercynian and the Mesozoic carbonates. ic data argue that carbon was introduced as carbonates in the early stages of the retrograde Hercynian metamorphism and was systematically reworked by subsequent fluids without any significant carbon introduction from external sources. The only exception corresponds to the stage III calcite veins which are located in the upper sections of the cores, at depths lower than 350 m beneath the basement/cover boundary and are the only witnesses of element transfer from the sediments towards the basement. esent data show that trapping of carbon by Ca-rich plutonic rocks during their cooling history confers on these rocks an efficient self-sealing capacity during later fracturing and fluid flow because carbonates possess high solubility and rapid dissolution/precipitation kinetics. Fluid circulation occurred not only in the macroscopic system of fractures but also within some volumes of the granitic “matrix” in order to scavenge and redistribute the early granitic trace carbonate. In the event of poly-cyclic carbonate redistribution, the use of carbon isotopic composition to unravel the fluid origins must be exercised with caution, as early C could be easily redistributed during later fluid migration episodes.