Geochemistry of amphibolites and related graphitic gneisses from the Suchý and Malá Magura Mountains (central Western Carpathians) – evidence for relics of the Variscan ophiolite complex

Three small bodies of amphibolites and associated graphitic gneisses from the Suchý and Malá Magura Mountains (Tatric Megaunit, central Western Carpathians) have been studied by petrographic and geochemical methods. Isolated, fault-bounded bodies first hundreds of meters in size are located in the complex of the Early Paleozoic paragneisses and migmatites intruded by the Lower Carboniferous granitoid rocks. Amphibolites (locally actinolite schists) were formed from effusive basalts, dolerites or isotropic gabbros hydrothermally altered and veined before the regional metamorphic transformation. Distribution of the trace elements relatively immobile during the metamorphic alteration (HFSE, REE, Cr, V, Sc) is similar to E-MORB type in the Malá Magura Mountain or to N-MORB/E-MORB types in the Suchý Mountain. Graphitic gneisses to metacherts are rich in silica (up to 88 wt. %) and Ctot, poor in other major element contents and display negative Ce-anomaly, enrichment in HREE, V, Cr and U. They were probably originally deposited as non-carbonate and silica-rich deep-sea sediments in anoxic conditions. The oceanic provenance of amphibolites and related graphitic gneisses clearly indicates their oceanic crust affinity and identity with the uppermost part of the ophiolite sequence. Ophiolite bodies from the Suchý and Malá Magura Mountains are supposed to be relic fault blocks identical with the Upper Devonian Pernek Group which represents a Variscan ophiolite nappe preserved to large extent in the Malé Karpaty Mountains located in the Tatric Megaunit further to the southwest. All these ophiolite relics are vestiges of the original ophiolite suture created by oceanic closure in the Lower Carboniferous.

Clay minerals from Permo-Carboniferous very low-grade metamorphic rocks from the central part of the Fore Sudetic Monocline (western Poland)

The mafic rocks from boreholes studied here contain the assemblage typical for upper-subgreenschist to lower-greenschist facies (albite, chlorite, illite, titanite, quartz, Ti-oxide ± actinolite) as well as relict clinopyroxene.Metapelites underlying a metabasic rock sequence also record metamorphic alteration as demonstrated by the chlorite and illite crystallinities. Chlorite crystallinity suggests formation in conditions close to the subgreenschist/greenschist facies boundary, but this mineral is partly decomposed during metamorphic retrogression.

Geoquímica de Rochas Metabasálticas da Mina da Palma, Bloco São Gabriel, Escudo Sul-rio-grandense: Um Possível Platô Oceânico

The Mina da Palma oceanic plateau is described for the first time in the São Gabriel Block, southern Brazilian Shield. It is composed mostly of metabasalts, marbles and metachert. Two deformational events are recognized in the Palma region, thrust faulting followed by dextral subvertical shearing. The rocks were metamorphosed in the greenschist facies, as shown by the dominant mineral assemblage albite-actinolite-chlorite-epidote-opaques. Metamorphic alteration was intense, because only a few relicts of the original magmatic textures are preserved, such as flow structure of amphibole and plagioclase. In spite of intense alteration during metamorphism, geochemical studies show that the chemical composition of many basalts is still preserved. The metabasalts are transitional tholeiite/alkaline and reflect intraplate conditions of formation. The tectonic environment was identified by field relations, such as predominance of basalts and small volume of marble and chert and minor clastic sedimentary rocks. The tectonic environment of formation is reflected in the geochemistry, particularly in multi-element spidergrams, because no significant Nb anomaly is observed in the least altered samples.Previous investigations showed the presence of an island-arc basaltic-andesitic association in the Palma region, which indicated that the geological unit contains one oceanic association and one active plate margin association. This is possibly the first description of remnants of an oceanic plateau in the Brazilian Shield.

Textures of diffusion-controlled reaction in contact-metamorphosed Mg-rich granulite, Kokchetav area, Kazakhstan

AbstrsctIn a granulite from the Kokchetav massif, a complex mineral assemblage and intricate textures have resulted from a combination of unusual rock composition and two–stage metamorphic history. The second, contact metamorphism produced mainly cordierite and anthophyllite, reflecting a bulk composition attributed to pre–metamorphic alteration of basic igneous rock. From the first, highpressure metamorphism, garnet relics persist while another mineral has been completely pseudomorphed. The garnet is partly replaced by a symplectite of three minerals: orthopyroxene vermicules in a coarser intergrowth of cordierite and calcic plagioclase. Despite variable proportions of cordierite and plagioclase, the Al:Si ratio of the symplectite is almost constant, because the proportion of orthopyroxene is smaller where the dominant aluminous mineral is cordierite (Al:Si ≈ 0.8) than where the even more aluminous plagioclase (Al:Si ≈ 0.89) is prominent. The bulk Al:Si ratio of this symplectite, approximately 0.69, is very close to that of reactant garnet (0.66), indicating that Al and Si have been retained almost completely during the local reaction, while other elements were more mobile. In the pseudomorphs, aluminous cores (with Al:Si ratios 1.61–1.93) indicate that the mineral which has been completely replaced was probably kyanite. These cores comprise plagioclase, zoisite, corundum and spinel, and are surrounded by layers of plagioclase and cordierite. Fe, Mg, and Ca have diffused to the core, through layers with low bulk concentrations of these elements, probably by grainboundary diffusion in the solid state.