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Ewolucja tektonometamorficzna marmurów i otaczających łupków łyszczykowych formacji strońskiej (Sudety)

Jastrzębski Mirosław

Geologos

2008

Vol. 14, No. 1

51--71

Połączone badania strukturalne, petrograficzne oraz geochronologiczne nad relacjami P-T-d (ciśnienie-temperatura-deformacja) w marmurach i otaczających łupkach łyszczykowych kopuły orlicko-śnieżnickiej (KOŚ) przyczyniły się do rekonstrukcji ewolucji tektonicznej wschodniej krawędzi Sudetów Zachodnich. Wyniki przeprowadzonych badań wykazały, iż na wskutek skrócenia tektonicznego w kierunku E-W (etap D1), związanego z waryscyjską kolizją domen kontynentalnych Sudetów Zachodnich oraz Brunowistulii, doszło do powstania stromej foliacji SI, pogrubienia skorupy i pogrążenia skał formacji strońskiej na głębokości odpowiadające ciśnieniom 0.9-1.0 GPa. Początkowe fazy wynoszenia skał, stowarzyszone ze pionowym skracaniem tektonicznym (etap D2), były powiązane z progresją metamorfizmu regionalnego od warunków facji zieleńcowej do facji amfibolitowej. Pod koniec etapu D2, podczas ustalenia się piku temperaturowego metamorfizmu, ukształtowała się strefowość metamorfizmu zapisana w skałach formacji strońskiej. Etap D2 doprowadził do wykształcenia leżących wąskopromiennych fałdów F2, deformujących foliację S1, oraz do spłaszczania więźby wewnętrznej zakonserwowanej w granatach. Progresywna deformacja etapu D2 ostatecznie doprowadziła do ukształtowania się subhoryzontalnej foliacji osiowej S2, którą w łupkach łyszczykowych stanowi powszechnie obserwowane złupkowanie. Podczas dalszego wynoszenia skał doszło do lokalnej reaktywacji foliacjj S2 w reżimie góra-ku-N, odpowiedzialnej za wytworzenie się stref ścinania. Późniejsze skracanie tektoniczne w kierunku NE-SW (etap D4) mogło wiązać się z regionalnym fałdowaniem i reorientacją powierzchni strukturalnych S2 || S3 oraz powierzchni izoterm/izograd metamorficznych, zaś na wskutek skracania tektonicznego w kierunku NW-SE (etap D5), doszło do nachylenia tych powierzchni w kierunku W/NW. Oba końcowe etapy mogły być odpowiedzialne za obserwowany kształt izograd metamorficznych, a także zmniejszanie się stopnia metamorfizmu w kierunku W/NW.

Integrated structural, petrographic and geochronological studies on P-T-d record in marbles and adjacent mica schists of the Orlica-Śnieżnik Dome (OSD) contribute to a model of the tectonic development of the eastern borderland of the West Sudetes. The results of these studies show that marbles of the OSD experienced E-W directed subhorizontal shortening (D1 stage), which resulted from the Variscan collision between the West Sudetes domain and the Brunovistulian terrane. These processes led to crustal thickening, development of a steep metamorphic fabric, and the burial of the Stronie Formation to depths corresponding to 0.9-1.0 GPa. Subsequent vertical shortening (D2 stage) was related to the onset of uplift and progression of metamorphism from greenschist to amphibolite facies conditions. Under peak conditions in the temperature, metamorphic zonation was established in the Stronie Formation. The D2 shortening led to folding of the 51 planes to form recumbent tight folds (F2), and to flattening of the inclusion trails in syn-D2 garnet porphyroblasts. Ongoing D2 deformation led to the development of subhorizontally disposed S2 axial-plane foliation. In mica schists, it is preserved as a penetrative schistosity. During progressive uplift, S2 planes were reactivated as a result of subsequent top-to-N directed transport (D3 stage). The S2 || S3 planes and S2-related metamorphic isograds were regionally folded during the D4 tectonic stage, which was characterised by NE-SW directed tectonic shortening. During stage D5, due to NW-SE directed tectonic shortening, the structural and metamorphic sur- faces became inclined toward west (or north-west). The D4 and DS stages were presumably together responsi- ble for the observed metamorphic isograds and their overall tilting toward W (or NW).

The acid metavolcanic rocks of the Orlica-Śnieżnik Dome (Sudetes): their origin and tectono-metamorphic evolution

Murtezi M.

Geologia Sudetica

2006

Vol. 38

1-38

The Orlica-Śnieżnik Dome (OSD) belongs to the mosaic of geological units forming the Sudetic part (NE margin of the Bohemian Massif) of the Variscan belt. Acid metavolcanic rocks referred to as leptites appear in the OSD as intercalations in the mica schists of the Stronie Formation - the supracrustal cover of the gneiss core of the unit. Two types of acid meta-volcanogenic rocks have been distinguished: massive leptites (ML) and biotite-bearing leptites (BBL); together they are referred the leptite series. The ML originate from acid volcanic rocks that could have contained only small amounts of sedimentary material. The BBL are metamorphosed tuffites with a variable share of sedimentary material. All the analysed leptites from the entire OSD area reveal similar geochemical characteristics. The high concentration of strongly incompatible elements such as REE, as well as the enrichment in large-ion-lithophile elements indicates the continental crust affinities of the leptites' protolith. On the other hand, ascertained the high Th/Ta ratios, negative Nb and Ta anomalies, and low Ti, Zr and Hf content can be interpreted as indicators of magmatism connected with a suprasubduction environment. Such ambiguous characteristics may indicate that an ensialic rift environment, considered as the place of origin for the magmatic rocks dated at c. 500 Ma and occurring in the OSD, developed due to extension behind a collisional zone of Lower Palaeozoic age. An extensional environment created during this event could be the location for the postulated early deformation and metamorphism of the Stronie Formation. The Śnieżnik metagranites would have been emplaced into such, already deformed, supracrustal cover. The signature of an active margin found in these continental crust rocks that were stretched and melted due to the increased heat transfer resulted from an earlier (Neoproterozoic?) tectono-thermal episode. The reconstructed path of the tectono-metamorphic evolution of the leptites entirely results from a Variscan collisional episode occurring in the borderland of the West and the East Sudetes. The sequence of the deformations established for the studied rocks is interpreted as an effect of the continuous convergence of crustal units in an E-W direction. Structural and phase-equilibria lines of evidence show that the tectono-metamorphic evolution of the Stronie Formation was that of early burial (D1) to a depth not exceeding upper amphibolite facies conditions, and subsequent uplift connected with subvertical shortening and flattening strain (D2). As a result of the continuous convergence, thrusting took place along the edges of the colliding units. The final adaptation of the colliding crustal slices led to zonal lateral oblique displacements (D3). This latter event is treated as the cause for disturbance of the isograd and isotherm pattern observed within the rocks of the Stronie Formation. It is also responsible for zonal shearing, generally taking place in a N-S direction. During the collision that occurred along the eastern margin of the OSD, the rocks of the Stronie Formation reached the highest P-T conditions of metamorphism at c. 9 kbar and 650°C. The presented data shows a considerable decrease in the peak-metamorphic conditions from the east to the west of the OSD.