Petrology and geochemical characteristics of phlogopite pyroxenite related to durbachites, Moldanubian Zone, Bohemian Massif

The phlogopite pyroxenite with a high content of pyrrhotite forms a body of circular outline about 1 km in diameter at Předni Zvonkova in Moldanubian Zone (Sumava/Bohemian Forest/Bohmerwald), Czech Republic. The contours of pyroxenite body are indicated by magnetometry and gravity data. Loose blocks up to 70 cm in size scattered on fields in the flat terrain indicate that the rock is at least partly exposed at the current erosion level. The best preserved sample SV235A is a medium-grained rock containing 27 vol. % phlogopite, 23 % diopside, 25 % enstatite, abundant apatite (7 %), minor plagioclase (3 %, An39–42), and pyrrhotite (3 %). Tendency to sulphide segregation is indicated by several mm pyrrhotite aggregates enclosing individual grains of pyroxenes, phlogopite and apatite. In the mg# vs. MgO diagram the studied pyroxenites plot along the linear trend defined by granitoid durbachites (melanocratic quartz syenites, quartz monzodiorites to melagranites), biotite–amphibole quartz diorites and biotite–amphibole ultramafic rocks (all from the Moldanubian Zone in southern and south–central Bohemia), after the whole-rock compositions of pyroxenites are corrected for the respective content of pyrrhotite. Several pyroxene thermometers yield equilibration temperatures in the range of 970 to 1040 °C for the sample SV235A. The Re–Os and Sr–Nd isotope composition of whole-rock samples indicates important crustal component in the sulphides and silicates. Petrological and geochemical data show a major role of fractional crystallization/crystal accumulation in formation of these rocks but magma mixing process is also recorded. The phlogopite pyroxenite is a rare rock important for understanding genesis of durbachitic magmatism.