Publication details
CR-BOHATÉ SPINELY Z ULTRAMAFICKÝCH HORNIN MOLDANUBIKA
| Title in English | CR-RICH SPINELS FROM MOLDANUBIAN ULTRAMAFIC ROCKS |
|---|---|
| Authors | |
| Year of publication | 2005 |
| Type | Article in Periodical |
| Magazine / Source | Acta Mus. Moraviae, Sci. geol. |
| MU Faculty or unit | |
| Citation | |
| Field | Geology and mineralogy |
| Keywords | Cr-spinels; peridotites; Moldanubian Zone; Bohemian Massif; Czech Republic |
| Description | 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. Primary 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 Fe2O3, TiO2, 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 garnet 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. |