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 i sotope 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 stron g cataclastic fabric with orthoclase megacryst 'augen', whereas the younger granitic pebble is less deformed. Chemical discrimination of the initial e mplacement environment of the granites is equivocal, although a syn-collisi onal setting during closure of the Ordovician Rheic ocean is preferred. The granites were finally exposed and released as clasts after rapid uplift an d erosion into the remainder of Lizard-Hart ocean during the Frasnian. The zircons of both granitic pebbles show different degrees of discordance. The ir lower intercept ages with the concordia curves of 373 +/- 6 Ma and 422 /- 4 Ma, respectively, ate interpreted as generation ages of the granites. The upper intercept ages with the concordia curves of 2606 +/- 40 Ma and 24 45 +/- 19 Ma, respectively, indicate the presence of a late Archaean/Early Proterozoic source terrane. The Normannian High, the northern part of the A rmorican plate, is proposed as the initial source region of the granites an d the pebbles derived from them. (C) 1999 Elsevier Science B.V. All rights reserved.