Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Piotr Marian Wojtulek, Jacek Puziewicz, Theodoros Ntaflos

MORB melt metasomatism and deserpentinization in the peridotitic member of Variscan ophiolite: an example of the Braszowice–Brzeźnica serpentinites (SW Poland)

Journal of Geosciences, volume 62 (2017), issue 3, 147 - 164

DOI: http://doi.org/10.3190/jgeosci.240

The Variscan Braszowice-Brzeźnica Massif (SW Poland) consists of gabbros and serpentinized peridotites with gabbro veins. Antigorite serpentinites form the western part of the Massif, whereas tremolite peridotites, tremolite serpentinites and lizardite-chrysotile serpentinites are found at the contact with granite intrusion in the east. Sparse relics of clinopyroxene, olivine and chromite were studied within the antigorite serpentinites. Clinopyroxene I (Mg# 90.9-93.47, 1.92-3.80 wt. % Al2O3) occurs in the neighbourhood of gabbro veins. Its REE patterns are similar to those of clinopyroxene from the mid-ocean ridge gabbros. Clinopyroxene II (Mg# 96.0-97.0) is Al-poor (≥ 0.10 wt. % Al2O3). Olivine I (Fo = 90.1-92.3) contains 0.32-0.50 wt. % NiO, whereas olivine II (Fo = 86.0-91.2) is Ni-poor (0.01-0.25 wt. % NiO) and contains micrometric magnetite intergrowths. Chromite I (Cr# 44.9-54.0, Mg# 45.0-52.1, < 0.17 wt. % TiO2) is associated with olivine I and clinopyroxene I, whereas chromite II (Cr# 43.2-51.4, Mg# = 34.6-47.7, 0.49-0.74 wt. % TiO2) occurs in serpentinites penetrated by gabbro veins.
The serpentinites of the Braszowice-Brzeźnica Massif were formed supposedly immediately below the paleo-Moho in the ocean-spreading setting. Chemistry of clinopyroxene I from antigorite serpentinites resembles, in terms of major elements and REE, clinopyroxenes that originate due to MORB-like melt percolation through abyssal peridotites. The coexisting olivine I with both chromite I and II supposedly shared a similar origin. Composition of chromite suggests the back-arc setting of the Braszowice-Brzeźnica Massif. Clinopyroxene II and olivine II have major-element compositions indicative of metamorphic origin at expense of serpentine ± magnetite (deserpentinization). The deserpentinization assemblage occurring in serpentinites (antigorite-olivine-clinopyroxene) was formed probably under low-grade metamorphic conditions. The tremolite-bearing rocks record the thermal metamorphism by granite intrusion.