A LOWER CRETACEOUS, SYN-EXTENSIONAL MAGMATIC SOURCE FOR A LINEAR BELTOF POSITIVE MAGNETIC-ANOMALIES - THE PACIFIC MARGIN ANOMALY (PMA), WESTERN PALMER-LAND, ANTARCTICA

Ar-Ar laserprobe dating suggests that in western Palmer Land, plutons associated with a curvilinear belt of positive magnetic anomalies alon g the Pacific margin of the Antarctic Peninsula, the Pacific Margin An omaly (PMA), are Early Cretaceous in age. The new ages, combined with published structural and geochemical studies, suggest that highly magn etically susceptible gabbroic to tonalitic-granodioritic rocks, the pr obable source of the Palmer Land segment of the PMA, were generated du ring Early Cretaceous extension when mantle-derived basaltic magma int ruded mafic lower to middle crust. Continued extension uplifted newly generated, lower to middle crust through the Curie Isotherm (ca. 600 d egrees C) forming the magnetic anomaly. The PMA broadly tracks an arc- parallel band in western Palmer Land where crustal extension and uplif t of lower crust were greatest. The close spatial relationship between the PMA and Early Cretaceous, syn-extensional plutons suggests that a nomaly area can be used as a crude proxy for the volume of a related p lutonic complex; the areal extent of the PMA indicates that a signific ant proportion of the are crust was newly generated during the Early C retaceous in western Palmer Land. (C) 1998 Elsevier Science B.V. All r ights reserved.