Ion microprobe and ID-TIMS U-Pb dating on zircon grains from leg 173 amphibolites: evidence for Permian magmatism on the West Iberian margin

In this paper, we report evidence for Permian magmatism at the ocean-continent transition zone of the West Iberian margin, by dating magmatic intrusion of amphibolite precursors drilled at site 1067 during ODP leg 173. The P-T conditions recorded by the amphibolites indicate that the latter was emplaced at the bottom of a 25-km-thick crust. Subhedral to rounded and colourless zircon grains were obtained from samples at the base of the cored section. ID-TIMS dating on these zircon grains yielded a lower intercept age of 247 +/- 5 Ma and a poorly defined upper intercept around 2 Ga. In situ dating of single zircons by ion-microprobe (CAMECA IMS 1270) provided an age of 246 +/- 5 Ma. This Permian age is interpreted as a minimum estimate for the emplacement of the gabbroic melts at the base of the Hercynian crust during the early stage of rifting between the Iberia-Newfoundland conjugate margins.