The Early palaeozoic intracontinental rifting and incipient oceanic lithosphere development in the central West Sudetes (NE Bohemian Massif): the geochemistry of metabasites of the East Krknoše (Karkonosze) Complex.

Abstract

Low- to medium-grade metabasites are the most abundant metaigneous rocks in the Early Palaeozoic metavolcanic (ąmetasedimentary) East Krkonoše (Karkonosze) Complex located at the Czech/Polish border in the central West Sudetes (NE Bohemian Massif). These mafic rocks are interpreted as metamorphosed equivalents of basic magmatites – both volcanics (lavas and pyroclastics) and subvolcanic intrusives. The correlation of lithostratigraphic units defined in the Czech (southern) and Polish (central and northern) parts of the East Krkonoše Complex is based on a comparison of the geochemical characteristics and petrography of the metabasites: (1) The greenschists to greenstones (associated with abundant felsic metavolcanics) of the Czech East Krkonoše Complex, which are finely interfingered with low-grade metasediments, are correlated with the amphibolites forming small- to medium-sized bodies in medium-grade metasediments of the Polish East Karkonosze Complex. Both the low- and medium-grade metabasites are interpreted as comprising a range of metamorphosed tholeiitic, transitional and alkaline WPBs. (2) The largest mafic rock suite, which dominates the Polish part of the East Krkonoše Complex, has a dismembered promontory along the eastern margin of the East Krkonoše Complex Czech component. Most of these mafic rocks (blueschists, greenschists, greenstones and amphibolites) correspond to N- and E-MORBs. The above groups of rocks are broadly coeval and geochronologically overlap the Cambrian/Ordovician boundary. The similarity in magmatic ages and the diversity in geochemical features suggest that the East Krkonoše Complex metabasites are evidence for intracontinental rift development and the subsequent generation of incipient oceanic basin lithosphere in the NE Bohemian Massif during the Early Palaeozoic. Provided that the East Krkonoše Complex metabasites can be matched with similar rock suites in the West and Central European Variscides, their magmatic origin may be related to the rifting of northern Gondwana and large-scale break-up at the beginning of the Palaeozoic.