AR-40 AR-39 AGES AND PALEOMAGNETISM OF SAO-MIGUEL LAVAS, AZORES/

We present new Ar-40/Ar-39 ages and paleomagnetic data for Sao Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and o ne dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. Ar-40/Ar-39 age determinations on 12 flows from th e Nordeste complex were attempted successfully: ages obtained are betw een 0.78 Ma and 0.88 Ma, in contrast to published K-Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity p aleomagnetic field directions, and indicate that the entire exposed pa rt of the Nordeste complex is of a late Matuyama age. The duration of volcanism across Sao Miguel is significantly less than previously beli eved, which has important implications for regional melt generation pr ocesses, and temporal sampling of the geomagnetic field. Observed stab le isotope and trace element trends across the island can be explained , at least in part, by communication between different magma source re gions at depth. The Ar-40/Ar-39 ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0-0.1 Ma) and up to 0.78 Myr (0-0.78 Ma) of paleosecular variation and our reverse polarity dat a sample approximately 0.1 Myr (0.78-0.88 Ma) of paleosecular variatio n. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomag netic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the d ata uncertainties, normal and reverse polarity field directions are an tipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction. (C) 1998 Elsevier Science B. V. All rights reserved.