Magnetic studies and U-Pb geochronology of the Uruguayan dyke swarm, Rio de la Plata craton, Uruguay: paleomagnetic and economic implications

A U-Pb age on baddeleyite of 1790 +/- 5 Ma has been obtained from a single dyke of the Uruguayan tholeiitic dyke swarm from the Rio de la Plata craton . Palaeomagnetic results from this and 10 other dykes yield two components, one (A) has the following paleomagnetic parameters: declination = 12 degre es, inclination = -16 degrees, alpha (95) = 7 degrees, latitude of paleopol e = 61 degrees, longitude = -31 degrees, dp = 4 degrees, dm = 7 degrees, pa leolatitude = -8 degrees, whereas another (B), about 20 degrees steeper, ha s a direction closer to the present Earth's field direction in Uruguay. Nei ther magnetisation is considered primary, although the A component may repr esent a composite between B and a primary component. A field test for the p rimary nature of the remanences was inconclusive, and no primary pole posit ions of similar age from other South American cratons are available for com parison. The magnetic results demonstrate that darker-coloured andesite dyk es that are sought for facing and monument stone (as 'black granite') can b e easily differentiated from more andesi-basaltic, lighter coloured, less v aluable dykes of the same swarm, using aeromagnetic data. The results sugge st that a high resolution airborne magnetic survey could constitute an impo rtant exploration tool for the Uruguayan 'black granite' industry. (C) 2001 Elsevier Science Ltd. All rights reserved.