GNGTS 2014 - Atti del 33° Convegno Nazionale

Fig. 3 – Gamma Ray Spectroscopy: Potassium content (%). of high-resolution magnetic and gamma-ray airborne data. The data were measured in 2009 in the frame of a combined geophysical survey including also EM and ground resistivity data. A comparison of these data with available geological information and rocks composition is being carried out in order to put some constrain on the surface and subsurface geology of specific areas of the island. Apreliminary analysis of magnetic data (Fig. 2) performed through the DEXPmethod shows the presence of a low magnetization volume below the surface of the island, possibly connected to high temperatures in the deeper part of the volcano, a low total remanent magnetization of pyroclastic caldera filling due to non-aligned sedimentation, and/or the influence of hydrothermal alteration within the caldera. The airborne gamma-ray data (Fig. 3) show a good correlation between the maxima of the potassium map and the 15 ka old peralkaline trachytes of the Felipe member (Fig. 1). References Bohrson W.A., Reid M.R., Grunder A.L., Heizler M.T., Harrison T.M. and Lee J.; 1996: Prolonged history of silicic peralkaline volcanism in the eastern Pacific Ocean . Journal of Geophysical Research , 101, 11457–11474. Bohrson W.A. and Reid M.R.; 1997: Genesis of silicic peralkaline volcanic rocks in an ocean island setting by crustal melting and open-system processes: Socorro Island, Mexico . Journal of Petrology, 38, 1137–1166. Bohrson W.A. and Reid M.R.; 1998: Genesis of evolved ocean island magmas by deep- and shallow-level basement recycling, Socorro Island, Mexico: constraints from Th and other isotope signatures . Journal of Petrology, 39 , 995–1008. Bryan W.B.; 1966: History and mechanism of eruption of soda-rhyolite and alkali basalt, Socorro Island, Mexico . Bull. Volc . , 29 , 453–480. Carballido-Sanchez E.A.; 1994: The geology and petrology of Socorro Island, Revillagigedo Archipelago, Mexico , PhD Thesis, Tulane University, 253 pp. Fedi M.; 2007 : DEXP: a fast method to determine the depth and the structural index of potential field sources . Geophysics, 72 , 1–11. 178 GNGTS 2014 S essione 3.2