Evolution of a Cambro-Ordovician active margin in northern Gondwana: Geochemical and zircon geochronological evidence from the Gory Sowie metasedimentary rocks, Poland

摘要

The Gory Sowie Massif (GSM) in Poland represents a Devonian high-pressure-ultrahigh-pressure (HP-UHP) terrane dominated by paragneiss, with subordinate orthogneiss, metabasite and felsic granulite. Whole-rock geochemistry of four migmatitic paragneiss and five granulite samples from the northern part of the GSM was combined with U-Pb-Lu-Hf isotopic data on zircon in order to constrain the sedimentary provenance, tectonic setting and paleogeographic location of the sedimentary paleo-basins now forming the GSM. Granulite and paragneiss protoliths are geochemically analogous to flysch-like graywacke and point either to a continental arc or an active continental margin setting dominated by Cambrian felsic arc detritus. Detrital zircon age spectra record a dominant Ediacaran to Cambrian population (493-600 Ma) with rare clusters of Palaeoproterozoic to Neoarchean age (1900-2100 Ma and 2400-2700 Ma). These reflect a peri-Gondwana provenance in the vicinity of the Trans-Saharan belt and place firm constraints on the age of sedimentary protolith of paragneisses at the middle to late Cambrian. Except one sample, the felsic granulites show only late Cambrian depositional ages. The zircon overgrowth rims give two age clusters, one at c. 397-402 Ma that possibly records a HP metamorphic event, and the other at c. 379-393 Ma that probably records the timing of a high-temperature event including migmatization during exhumation. This tectono-thermal event was coeval with emplacement of Devonian ophiolitic rocks that surround the GSM. The GSM rock association, degree and timing of metamorphism and coeval ophiolite emplacement resemble closely the Galicia-Tras-os-Montes middle and upper allochthonous units in NW Iberia and in the Munchberg Massif in Germany. We suggest that all these massifs were in close spatial proximity before the Devonian and may form part of a single, now tectonically dismembered, terrane or archipelago, which was located along the northern periphery of the Gon