ELSEVIER Sedimentary Geology 124 (1999) 131–147 U–Pb ages and geochemistry of granite pebbles from the Devonian Menaver Conglomerate, Lizard peninsula: provenance of Rhenohercynian flysch of SW England

The geochemical composition of some garnet-bearing biotite granite pebbles within the Menaver Conglomerate were investigated, together with the U–Pb isotope systematics of zircons extracted from two samples. The granites have suffered low-grade metamorphism as shown by the development of a secondary mineral assemblage of chlorite, Fe-oxides, muscovite and stilpnomelane. In terms of their determined ages, the older granitic pebble exhibits a strong cataclastic fabric with orthoclase megacryst ‘augen’, whereas the younger granitic pebble is less deformed. Chemical discrimination of the initial emplacement environment of the granites is equivocal, although a syn-collisional setting during closure of the Ordovician Rheic ocean is preferred. The granites were finally exposed and released as clasts after rapid uplift and erosion into the remainder of Lizard–Harz ocean during the Frasnian. The zircons of both granitic pebbles show different degrees of discordance. Their lower intercept ages with the concordia curves of 373 6 Ma and 422 4 Ma, respectively, are interpreted as generation ages of the granites. The upper intercept ages with the concordia curves of 2606 40 Ma and 2445 19 Ma, respectively, indicate the presence of a late Archaean=Early Proterozoic source terrane. The Normannian High, the northern part of the Armorican plate, is proposed as the initial source region of the granites and the pebble