Wollastonite formation during Variscan post-tectonic cooling in the Schwarzwald, Germany

摘要

Mteaquartzdioritic plagioclase-biotite-quartz-gneissat Artenberg, Schwarzwald, Germany, contains up to 10cm widecalcite veins partially replaced and surrounded by a zonedcalcsilicate sequence. The calcsilicate veins consist of two parts: (a)a zoned mineral sequence of reaction products deposited in thevein itself (wollastonite, garnet, clinopyroxene, quartz with relicsof calcite) and (b) a zoned alteration sequence in the wall rockgneiss extending from the vein-gneiss contact several cm into theplagioclase-bearing gneiss (hornblende, chlorite). Based onhornblened-plagioclase thermobarometry and on chlorite stability,the P-T-conditions of calcsilicate formation are estimated to beabout 450℃ at c.2 k bar.The zoned mineral sequence formed by a bi-metasomaticreaction in an open system of the hydrothermal carbonate with theorthogneiss in the presence of an externally derived fluid. Thereaction was started by H2O and SiO2 infiltration along the vein-gneiss contact. The stability of wollastonite at the above P-Tconditions records an XCO2 of the fluid phase below 0.05.Chemical potential gradients of Ca, Al, Fe, Mg and probably Sibetween vein and gneiss resulted in diffusional mass transferperpendicular to the vein-gneiss contact. The sequence of stablecalcsilicate minerals suggests Ca diffusion from vein to gneiss,while Fe, Mg and Al were transported in the opposite direction,from gneiss to vein. It is inferred that the calcsilicates formedduring Variscan post-tectonic cooling.