The large Variscan Karkonosze Granite in the West Sudetes, representative of the vast Variscan granite plutonism in Central Europe and located adjacent to regional tectonic suture and strike-slip-zones, has been difficult to date precisely; a range of published data varies between c. 304 and 328 Ma. However, the granite is cut by locally numerous lamprophyre and other dykes. Dating of the dyke rocks, emplaced shortly after the granite intrusion and cooled more rapidly, provides a promising tool for the verification of published SHRIMP results on the granite itself. SHRIMP zircon geochronology of a studied micromonzodiorite dyke indicates substantial admixture of inherited zircons of c. 2.0, 1.4 Ga ( 207 Pb-206 Pb minimum ages), and c. 570 (and 500?) Ma. The average concordia age of the main magmatic population of the zircons in the dyke is 313 ± 3 Ma (2σ); however, the true magmatic age might be older, around 318 Ma. This would constrain the age of the hypabyssal magmatism in the Karkonosze Massif and the minimum age of the host Karkonosze Granite. Thus, the Karkonosze Granite is confirmed as representative of an early phase of Variscan granite plutonic activity in the central-European Variscides.
Gneisses of the Góry Sowie Block, of probable Late Precambrian–Early Cambrian age, contain numerous small metabasite bodies which may have originated as dykes or sills. Many metabasites have phase assemblages and textures which show that they suVered early granulite-facies and later amphibolite- facies metamorphism, although some exhibit no evidence for the earlier event. Trace element geochemistry enabled four metabasite groups to be distinguished: (a) a dominant meta-tholeiite group (Sowie group) probably incorporating more than one set of intrusive rocks; (b) meta-tholeiites characterized by higher Nb/Y (high Nb/Y group); (c) Ti-poor metabasites (Myslêcin group) that exhibit strongly depleted HFSE; (d) alkali metabasalts (Wlóki group). Sowie group meta-tholeiites were also divided according to their dominant host lithology: granitoid gneiss, migmatitic paragneiss or gneiss containing relict granulite-facies rocks. Those in granitoid gneiss are compositionally restricted (although they fall within the same compositional range as other Sowie meta-tholeiites), and may comprise both early metabasites and a later intrusive group, postdating granite emplacement. Both the latter metabasites and the granitoid gneiss lack phase assemblages indicative of granulite facies metamorphism: this suggests that both were emplaced after the granulite-facies metamorphic event, but before or during amphibolite-facies metamorphism. An Early Ordovician emplacement age for syn-metamorphic granitoid gneiss shows that the Góry Sowie Block underwent an Ordovician (early Caledonian) metamorphic event, following metabasite emplacement, or was at this grade during the Ordovician. Nonetheless, the chemistry of all the metabasite groups and their geological setting is consistent with emplacement either through continental crust undergoing extension at a passive continental margin, or during the formation of an intracratonic basin before subsequent metamorphism recorded a local compressional event.
Mafic, monogenetic vol ca nism is in creas ingly rec og nized as a com mon man i fes ta tion of post-collisional vol ca nism in late Variscan, Permo-Car bon if er ous intramontane bas ins of Cen tral Eu rope. Al though iden ti fi ca tion of in di vid ual erup tive cen tres is not easy in these an cient suc ces sions, the Perm ian Rožmitál andesites in the Intra-Sudetic Ba sin (NE Bo he mian Mas sif) pro vide an ex cep tion ally de tailed re cord of ex plo sive, ef fu sive and high-level in tru sive ac tiv ity. Based on field study and petrographic and geo chem i cal data on pyroclastic and co her ent rocks, the Rožmitál Suc ces sion is in ter preted as the prox imal part of a tuff ring sev eral hun dred metres in di am e ter. Ini tial ac cu mu la tion of pyroclastic fall and surge de pos its oc curred dur ing phreatomagmatic erup tions, with tran si tions to wards Strombolian erup tions. Gul lies filled with re worked tephra doc ument pe ri ods of ero sion and redeposition. Andesitic blocky lavas capped the volcaniclastic suc ces sion. In va sion of lavas into un con sol i dated sed i ments and em place ment of shal low-level in tru sions in near-vent sec tions re sulted in the for ma tion of jigsaw-and ran domly-tex tured peperites. Most geo chem i cal dif fer ences be tween co her ent andesites and pyroclastic rocks can be linked to in cor po ra tion of quartz-rich sed i ments dur ing the ex plo sive erup tive pro cesses and to later ce men ta tion of the volcaniclastic de pos its by do lo mite. The Rožmitál tuff ring could have been one of sev eral phreatomagmatic cen tres in a monogenetic vol ca nic field lo cated on an al lu vial plain.
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