Geochemistry of amphibolites from the Strzelin crystalline massif, Fore-Sudetic Block, SW Poland
Szczepanski, Jacek; Oberc-Dziedzic, Teresa
published: Mar 24, 1998
ArtNo. ESP154017301002, Price: 29.00 €
Amphibolites in the Strzelin crystalline massif form conformable intercalations of several centimetres to several metres thick within the fine to medium-grained Strzelin orthogneisses, the so-called older schist series (?Proterozoic, ?lower Paleozoic) composed of mica schists, amphibolites, marbles and calc-silicate rocks and within paragneisses. Unfortunately, the relationships between the paragneisses, and the orthogneisses and between the paragneisses and the older schists series are unknown. Because of poor exposure, amphibolite samples were collected from boreholes in the northern part of the massif. The geochemical data indicate that the amphibolites from the Strzelin massif represent a homogeneous group of tholeiitic composition. A very good linear correlation between incompatible elements suggests a common magmatic source of the Strzelin amphibolites and the lack or very low degree of contamination during ascent and metamorphic processes. Different concentrations of Ni, Cr and Mg in particular samples, controlled by the early crystallizing mineral phases (pyroxene and olivine), suggest the presence of two batches of magma from the same or similar source: (a) Ni, Cr and Mg-rich primitive melts, and (b) Ni, Cr, and Mg-poor representing more evolved magmas. A probable geotectonic enviromnent of the emplacement of the Strzelin amphibolite protoliths was a within-plate setting, e. g. that of early stages of continental splitting (IRT - initial rift tholeiites). The amphibolites found in various geological positions (within rocks of different provenance and age) are geochemically similar. It means that the protoliths of all amphibolites were most probably emplaced in the same geotectonic environment during lower Paleozoic times. Geochemically, the amphibolites correspond to mafic members found in the lower Paleozoic of several other tectonic units of the Sudetes interpreted to represent the early stages of intracontinental rifting locally followed by the formation of the oceanic crust.