An investigation of the timing of orogenic events in southern Spain and southern California using the Lutetium/Hafnium and Samarium/Neodymium isotopic systems

This study examines the timing of metamorphism in the Betic Cordillera of southern Spain and the Catalina Schist in southern California, using Lu/Hf and Sm/Nd isotopic geochronology on garnet-bearing rocks. Garnets are very common metamorphic minerals; they sequester 176Lu as they grow, and they also incorporate a measurable amount of 147Sm. Both isotopes can be used in conjunction with their respective daughter isotopes to determine the growth or cooling age of the garnets, giving an insight into the timing of the metamorphism.;Five samples are studied from the Internal Zone of the Betic Cordillera. A graphitic-mica schist sample from the structurally lowest tectonic unit (the Nevado-Filabride Complex) produced a Lu/Hf age of 13.3 +/- 7.3 Ma and a Sm/Nd age of 15.6 +/- 7.4 Ma. The other four Betic samples come from the structurally central tectonic unit (the Alpujarride Complex), and include a garnet-mica schist (Sm/Nd age: 23.8 +/- 5.7 Ma) from Almunecar, a granulite facies metapelite from the Jubrique unit above the Ronda peridotite massif (Lu/Hf age: 192 +/-47 Ma, Sm/Nd age: 44 +/- 12 Ma), an amphibolitized eclogite from the Blanca unit from beneath the peridotite (Sm/Nd age: 25.1 +/- 4.9 Ma), and a garnet peridotite from the Ronda massif in the Sierra Bermeja (Sm/Nd age: 28.3 +/- 6.4 Ma).;One sample comes from the Franciscan Complex on Santa Catalina Island in southern California. It is from the Blueschist Unit of the Catalina Schist. This is a garnet-glaucophane-lawsonite schist, from which two separate Lu/Hf ages were obtained. Glaucophane and lawsonite separates defined an isochron with an age of 112.3 +/-7.3 Ma, while the garnet separates produced an age of 152 +/-23 Ma.;Several of these ages provide useful insights into the metamorphic histories of their respective rock units, but some of the Lu-Hf ages may have been affected by zircon inclusions with isotopically old Hf. The significance of each age is discussed in the light of current knowledge about the rock history, and the strengths and weaknesses of the Lu/Hf and Sm/Nd isotopic systems are compared.