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

Abstract

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 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. Braszowice-Brzeźnica Massif, Bohemian Massif Received: 4 December 2016; accepted: 21 June 2017; handling editor: E. Jelínek constitute group of gabbroic and serpentinitic outcrops, with subordinate metabasalts (Kryza and Pin 2010). The geochemical affinity of the Central-Sudetic ophiolites has been relatively well studied. The Ślęża Ophiolite, which is the most complete ophiolitic sequence in the region, was considered to having originated in the mid-ocean ridge or back-arc setting (Pin et al. 1988).The peridotite members of the Central-Sudetic ophiolites are heavily serpentinized, nevertheless they contain non-serpentine phases which record information about ocean rift processes and later metamorphism (Wojtulek et al. 2016a, b, and references therein). Keywords: Variscan ophiolite, peridotitic member, MORB melt percolation, clinopyroxene,In this paper we describe the non-serpentine phases occurring in highly serpentinized ultramafic rocks from the Braszowice-Brzeźnica Massif, a small ophiolite slice belonging to the Central-Sudetic ophiolites. We show that some of these minerals originated due to melt percolation in the uppermost part of the mantle, probably close to the paleo-Moho. We also show that part of the

Late Silurian–Early Devonian tuffites from the Małopolska and Łysogóry blocks reflect arc-back-arc magmatic activity on the southern margin of Laurussia

“…Its ac tiv ity al ready in the Si lu rian and Early De vo nian in the Brunovistulian Block is in di cated by the re sults of iso tope dat ing, made in re cent years (Ta ble 1): Velké Vrbno Dome am phi bo lite 407.5 ±2.5 Ma (Jastrzêbski et al, 2021), granophyre diorite from the Sosnowiec IG 1 bore hole 420 ±2 Ma (Nawrocki et al, 2020), Hlina gran ite 430 ±6.4 Ma (Hönig, 2016), ba saltic dike from Zelesice 438 ±16 Ma (Pøichystal, 1999). More over, ac cord ing to re cent data (Kryza and Pin, 2010;Wojtulek et al, 2017;Awdankiewicz et al, 2021), the Early De vo nian Cen tral Sudetic Ophiolite was also formed dur ing seafloor spread ing in the back-arc ba sin. Mazur et al (2015b) im ply that the Cen tral Sudetic Ophiolite was formed on the mar gin of Avalonia/Bunovistulia.…”

Eo‐Variscan metamorphism in the Bohemian Massif: Thermodynamic modelling and monazite geochronology of gneisses and granulites of the Góry Sowie Massif, SW Poland

“…In the Sudetes, dismembered bodies of the Sudetic Ophiolite, which occur to the NE, E, and SW of the GSM, are often thought to represent the Saxothuringian Ocean (Floyd et al., 2002; Franke et al., 2017; Kryza & Pin, 2010). Both plutonic and volcanic components of the ophiolite carry N‐MORB signature, probably generated in a normal ocean ridge (Floyd et al., 2002; Kryza & Pin, 2010) or back‐arc setting (Wojtulek et al., 2017). Nevertheless, little can be said about the original relationships as only secondary tectonic contacts of the GSM and fragments of ophiolite/oceanic crust are accessible.…”