Primary magnetic fabric in an ultramafic body (Moldanubian Zone, European Variscides) survives exhumation-related granulite-amphibolite facies metamorphism

Abstract

Serpentinized peridotite–clinopyroxenite, approximately 11 × 5 m in size, enclosed in high-pressure felsic granulite, occurs in a quarry near the village of Bory (Gföhl Unit of the Moldanubian Zone of Western Moravia, Czech Republic). The anisotropy of magnetic susceptibility (AMS) was used to investigate the fabrics of both the ultramafic body and the host granulite. The granulite shows a PT path from above 2 GPa/850–900 °C following nearly isothermal decompression to sillimanite stability field with subsequent cooling. Two foliations (S 1-preserved only in isoclinal folds and the dominant S 2 foliation parallel to leucocratic bands) are visible in granulite. Through investigating low-field variation of the AMS, one could resolve the AMS into the field-independent and field-dependent components. In the ultramafic body, the former component is due to mafic silicates and pure magnetite, while in the host granulite it is due to mafic silicates. The latter component is due to pyrrhotite in both the ultramafic body and granulite. The field-independent and field-dependent components are coaxial within ultramafic body and within granulite, but show different orientations between the ultramafic body and granulite. Consequently, the magnetic fabric in the ultramafic body is different from that in the host granulite even though they experienced common structural history during granulite facies metamorphism. The componental movements forming the granulite fabric were evidently not strong enough to strongly overprint the magnetic fabric of ultramafic rocks. The ultramafic rocks have therefore sufficient mechanical strength to maintain their pre-granulite facies metamorphic fabric even at such high temperatures and high pressure that are characteristic of granulite facies metamorphism.