Low-Temperature, Low-Pressure Metamorphism of Mn-rich Rocks in the Lienne Syncline, Venn—Stavelot Massif (Belgian Ardennes), and the Role of Carpholite

Low-grade Mn-rich metamorphic rocks of the Lienne syncline (western part of the Venn–Stavelot Massif, Belgian Ardennes) have been re-examined to evaluate the petrological significance of carpholite proper, Mn2$ Al2[Si2O6](OH)4. Metamorphic P–T conditions of these rocks are estimated to be ˜300°C 1–2 kbar, which is in accordance with the exclusive occurrence of carpholite in low-P rocks such as hydrothermal environments elsewhere. Carpholite of the Lienne syncline exclusively occurs in quartz-rich segregations. Its composition is close to end-member. Thermodynamic calculations confirm that carpholite is a stable phase at low-pressure–low-temperature conditions, in contrast to ferro- and magnesiocarpholite, which are high-pressure minerals. No information is available on the high-P behaviour of carpholite. The occurrence of carpholite is partly closely associated with spessartine-bearing country rocks, or carpholite is altered to assemblages with spessartine, sudoite, chlorite, muscovite and paragonite. Spessartine in these rocks contains minor amounts of hydrogarnet component {(H/4)/[Si$(H/4)] = 0.03–0.06}. The presence of carpholite-spessartine assemblages in these low-P rocks is in contrast to high-pressure metamorphic rocks from other areas, where parageneses such as fem/magnesiocarpholite–chloritoid or magnesiocarpholite–chlorite–kyanite occur. The appearance of carpholite–garnet assemblages in low-P Mn-rich rocks can be explained by contrasting phase relations because of a high Mn–Mg partition coefficient between the minerals under consideration. In rhodo-chrosite-bearing veins in the Lienne syncline, nearly complete replacement of carpholite by spessartine and chlorite is due to the continuous reaction carpholite $ rhodochrosite $ quartz = spessartine $ chlorite $ H2O $ CO2, which defines a very low Xco, in the temperature range under consideration. It is suggested that spessartine (possibly containing some hydrogarnet component), during prograde metamorphism at low pressure, becomes stable at a temperature of ˜300°C