Podiform chromitites from the Variscan ophiolite serpentinites of Lower Silesia (SW Poland) - petrologic and tectonic setting implications

摘要

The GogolOw-JordanOw Serpentinite Massif (GJSM) and the Braszowice-Brzenica Massif (BBM) are the largest serpentinite outcrops in the Fore-Sudetic Block (NE part of the Bohemian Massif, Central Europe). The GJSM is a peridotitic member of the Variscan Sleza Ophiolite (SW Poland). Podiform bodies (veins and pockets) of chromitite are found on the Czernica Hill (GJSM) and on the Grochowiec Hill (BBM) within strongly serpentinised harzburgites which occur several hundred metres below Palaeo-Moho. Chromitites consist of rounded chromite grains up to 3 cm across, and of chlorite filling the interstices. The veins are embedded in serpentine olivine chlorite aggregates. Relics of Mg-rich olivine (Fo(95-96)) occur in massive chromitite in the BBM. The bulk-rock total PGEs content is very low (42-166 ppm) and the PGE pattern is negatively sloped towards Pt and Pd and depleted relative to chondrite. The primary chromite I is aluminous (Cr# 0.50-0.52, Mg# 0.60-0.70). The highly aluminous and magnesian (Cr# 0.38, Mg# 0.80) chromite la occurs locally in the BBM. The secondary chromite II is enriched in Cr and impoverished in Al (Cr# 0.57-0.69), it replaces chromite I. Both chromite I and II contain small amounts of Ti (<0.14 wt.% Ti0(2)). Silicate inclusions in chromite are scarce. The composition and mode of occurrence of both the GJSM and the BBM chromitites are similar, thus they were formed probably under the same conditions. Textures of the chromitites suggest their magmatic origin. Their current geological position indicates their emplacement and crystallisation in the uppermost mantle harzburgites occurring below the Moho Transition Zone. The chromitites and hosting harzburgites were subjected to the greenschist-facies metamorphic overprint. The moderate Cr# and low PGEs contents suggest that the chromitites originated in the arc setting, thus their host ophiolite is of supra-subduction type.