ELSEVIER Earth and Planetary Science Letters 158 (1998) 143–155 A Lower Cretaceous, syn-extensional magmatic source for a linear belt of positive magnetic anomalies: the Pacific Margin Anomaly (PMA), western Palmer Land, Antarctica A.P.M. Vaughan a,Ł , C.D. Wareham a , A.C. Johnson a , S.P. Kelley b a British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK b Department of Earth Sciences, Walton Hall, Open University, Milton Keynes MK7 6AA, UK Received 19 June 1997; accepted 12 November 1997 Abstract Ar–Ar laserprobe dating suggests that in western Palmer Land, plutons associated with a curvilinear belt of positive magnetic anomalies along the Pacific margin of the Antarctic Peninsula, the Pacific Margin Anomaly (PMA), are Early Cretaceous in age. The new ages, combined with published structural and geochemical studies, suggest that highly magnetically susceptible gabbroic to tonalitic–granodioritic rocks, the probable source of the Palmer Land segment of the PMA, were generated during Early Cretaceous extension when mantle-derived basaltic magma intruded mafic lower to middle crust. Continued extension uplifted newly generated, lower to middle crust through the Curie Isotherm (ca. 600ºC) forming the magnetic anomaly. The PMA broadly tracks an arc-parallel band in western Palmer Land where crustal extension and uplift of lower crust were greatest. The close spatial relationship between the PMA and Early Cretaceous, syn-extensional plutons suggests that anomaly area can be used as a crude proxy for the volume of a related plutonic complex; the areal extent of the PMA indicates that a significant proportion of the arc crust was newly generated during the Early Cretaceous in western Palmer Land.  1998 Elsevier Science B.V. All rights reserved. Keywords: granites; extension; Gondwana; island arcs 1. Introduction Large, linear magnetic anomalies are often associ- ated with anomalously thick (20–25 km) sections of underplated igneous rock at rifted volcanic margins (e.g. [1]) and with magmatic arcs (e.g. [2]). Ren- ner et al. [3] identified a belt of positive magnetic anomalies along the Pacific margin of the Antarc- tic Peninsula: the west coast magnetic anomaly of Renner et al. [4], labelled the WCMA by Garret and Ł Corresponding author. Tel.: C44 (1223) 251442; Fax: C44 (1223) 362616; E-mail: a.vaughan@bas.ac.uk Storey [5]. Maslanyj et al. [6] showed that this belt also extends along the west coast of Ellsworth Land and forms a curvilinear belt ca. 2000 km long and up to 120 km wide (Fig. 1) (cf. [7]). Maslanyj et al. [6] called this the Pacific Margin Anomaly (PMA); this includes the WCMA. In Palmer Land, reconnaissance aeromagnetic surveys [4,6,7] showed that the western edge of the PMA closely follows the eastern edge of George VI Sound (Fig. 2), and is limited to the western half of the Palmer Land plateau. The PMA is ca. 100 km wide in Palmer Land, and has peak amplitudes of 600 nT (Fig. 2). Garrett [8] showed that positive iso- 0012-821X/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S0012-821X(98)00054-5