Cr-rich spinels from Moldanubian ultramafic rocksArticle PDF
Cr-rich spinels from Moldanubian ultramafic rocks
Cr-rich spinel is a common minor to accessory mineral in the Moldanubian spinel and garnet peridotites. Recently, Cr-rich spinels were found in the ultramafic tremolite rock near Nová Pec. Cr-rich minerals of spinel group (MSG – minerals of spinel group) from Moldanubian ultramafic rocks form grains disseminated in the rock, banded segregations or massive layers. Two principal compositional types of MSG from spinel peridotites have been identified: i) primary and ii) secondary. In general, the primary MSG exhibit a wide range of Cr# (0.15-0.87) and Mg# (0.35-0.79) with negative correlation between these parameters. They have characteristically low Fe203, Ti02, ZnO and MnO contents. MSG from dunites have higher Cr# than those from harzburgites and lherzolithes. The chemistry of primary MSG is controlled by the bulk rock composition and modal ratio of MSG and silicates in the rock. Primary Cr-spinels remain unaltered, occasionally with magnetite coatings formed during the serpentinization. Al-rich spinels are partly replaced by Cr-rich chlorite and mica. Secondary MSG were formed from primary ones by substitution Fe3+ (Cr3+, Al3+)-1 and (Fe2+, Zn2+, Ni2+, Mn2+)Mg2+-1 in peridotites during variscan metamorphism. Secondary MSG correspond to Fe3+-rich chromite and Cr-rich magnetite, usually with Cr-low magnetite rim. MSG from gamet peridotites have similar chemistry to primary MSG from spinel peridotites. Locally, Cr-poor spinels (Cr# < 0.1) are found in kelyphites after garnet. MSG from tremolite rock exhibit the features of chromites metamorphosed under relatively low fO2. They are Cr-rich, Mg-low, with low Fe2O3, and elevated ZnO and MnO contents.
Renata Čopjaková, Joint Centre for Electron Microscopy and Microanalysis, Masaryk University and Czech Geological Survey, Kotlářská 2,611 37 Brno, Czech Republic, e-mail: firstname.lastname@example.org
Radek Škoda, Joint Centre for Electron Microscopy and Microanalysis, Masaryk University and Czech Geological Survey, Kotlářská 2,611 37 Brno, Czech Republic, e-mail: email@example.com
Jaroslava Pertoldová, Czech Geological Survey, Klárov 3, 11821 Prague I, Czech Republic, e-mail: firstname.lastname@example.org