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Low-grade Mn-rich metamorphic rocks of the Lienne syncline (westernpart of the VennStavelot Massif, Belgian Ardennes) havebeen re-examined to evaluate the petrological significance ofcarpholite proper, Mn2$ Al2[Si2O6](OH)4. Metamorphic PTconditions of these rocks are estimated to be {small tilde}300C12 kbar, which is in accordance with the exclusive occurrenceof carpholite in low-P rocks such as hydrothermal environmentselsewhere. Carpholite of the Lienne syncline exclusively occursin quartz-rich segregations. Its composition is close to end-member.Thermodynamic calculations confirm that carpholite is a stablephase at low-pressurelow-temperature conditions, in contrastto 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 withspessartine-bearing country rocks, or carpholite is alteredto assemblages with spessartine, sudoite, chlorite, muscoviteand paragonite. Spessartine in these rocks contains minor amountsof hydrogarnet component {(H/4)/[Si$(H/4)] = 0.030.06}.The presence of carpholite-spessartine assemblages in theselow-P rocks is in contrast to high-pressure metamorphic rocksfrom other areas, where parageneses such as fem/magnesiocarpholitechloritoidor magnesiocarpholitechloritekyanite occur. Theappearance of carpholitegarnet assemblages in low-P Mn-richrocks can be explained by contrasting phase relations becauseof a high MnMg partition coefficient between the mineralsunder consideration. In rhodo-chrosite-bearing veins in theLienne syncline, nearly complete replacement of carpholite byspessartine and chlorite is due to the continuous reaction carpholite$ rhodochrosite $ quartz = spessartine $ chlorite $ H2O $ CO2,which defines a very low Xco, in the temperature range underconsideration. It is suggested that spessartine (possibly containingsome hydrogarnet component), during prograde metamorphism atlow pressure, becomes stable at a temperature of {small tilde}300C KEY WORDS: carpholite; spessartine; sudoite; VennStavelot Massif; Ardemes
*Corresponding author. Fax: x49/531/3918131. e-mail: t.theye{at}tu.bs.de 相似文献
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