Metamorphic evolution and oxygen isotope geochemistry of rift-precursor amphibolites from Hole 1067A ODP Leg 173 off West Iberian Galicia Bank rifted margin

The paper documents Permian igneous rocks in the vicinity of an ocean–continent transition in the West Iberian Abyssal plain. ODP Leg 173, Hole 1067A was drilled through a sequence of foliated to massive amphibolites of gabbroic protolithic origin. The 270 ± 3 Ma to 246 ± 5 Ma rock sequence experienced several metamorphic events during the interval of time spanning from 138 Ma up to 55 Ma. Metamorphic parageneses record two major thermobarometric events for which the temperature and pressure conditions are 670 ± 40 °C and 0.7 ± 0.1 GPa and 550 ± 50 °C and 0.5 ± 0.1 GPa. According to radiometric dating these metamorphic events might have occurred around 138–136 Ma and 113–110 Ma respectively. The low Cl content and the preserved igneous δ18O of amphibole imply seawater-absent 580 °C isothermal plagioclase–amphibole equilibrium related to the development of the strong foliation of the amphibolite during the second metamorphic event. Late isotopic equilibration involved lower temperature seawater-derived fluid and is younger than 110 Ma. The gabbroic intrusion is believed to be a precursor for initiation of continental rifting. Melting of such metamorphosed rocks in a rift setting would generate magmatic rocks with complex oxygen isotopic signatures.