Pegmatyt z kordierytem w serpentynitach okolic Lubachowa (Góry Sowie)

Ryszard Kryza


Cordierite-bearing pegmatite in serpentinites of the environs of Lubachów (Sowie Góry Mts, Sudetes)

Generally, pegmatites of gneissic block of the Sowie Góry Mts. display simple mineral composition (quartz, plagioclase, potassium feldspar, biotite, muscovite, tourmaline). However, some of them also contain typical metamorphic minerals, as: dysthene, andalusite, sililimanite and cordierite. Cordierite was found to occur in pegmatitic dense cutting thin '(3 m thick) intercalation of serpentinites embedded in layered gneisses in the environs of Luibachów (R. Kryza 1973). Serpentinite in question consists essentially of minerals of serpentine group, tremolite, talc and olivine (table 1), being, most probably a product of serpentinization of lerzolite. Considerable amounts of unaltered enstatite and olivine were preserved close to the contact of senpentinite with pegmatite under consideration. Pegmatite occurs in lenticular form 2.3 m long and up to 0.5 m thick. Two zones can be distinguished in this lense. The external one, showing pegmatoid structure, consists of cordierite, biotite, quartz and oligoclase (18% An), while the internal one, displaying graphic texture, is composed of quartz-potassium feldspar intergrowths. Minor constituents of pegmatite are represented by muscovite, tourmaline, apatite, riebeckite, sillimanite (fibrolite), titanite, chalcopyrite (?), chlorite and fine scaly pinite aggregate. Cordierite occurs in prismatic crystals showing pseudorhombic shape and as irregular grains, 5—8 cm in size. It is dark blue — gray in colour and exhibits vitreous lustre. In thin section it is colourless and shows no pleochroism. Measured birefringence amounts to 0.011 and the optic axial angle 2Va = 74°. In thin section imperfect (010) cleavage and simple (110) twinnings are observed. Cordierite-ibearing. pegmatite represents a peculiar mineral paragenesis which was not yet reported for pegmatites of the Sowie Góry block. In order to get some indications on the origin of this pegmatite and to explain its specific mineral composition, chemical and X-ray examinations of cordierite, biotite and feldspars from cordierite-'bearing pegmatite, as well as of feldspars from a thick pegmatite body in Lutomia (5 km SE from Lubachów) were carried out. As follows from the obtained data (fig. 2, 3 and 4), crystallization temperature of the main stage of formation of both pegmatites was similar, amouting to 770—820°C. It is thus concluded that the formation of cordierite is due to direct contact of this pegmatite with ultrabasic rock. Moreover, attempts were made to determine thermal and chronological interrelation between migmatitization of surrounding rocks and formation of pegmatites. It is supposed that these phenomena were separated in time and that during this interval the temperature of surrounding rocks was lowered to at least 500° C.

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