K-Ar geochronology and palaeomagnetism of volcanic rocks in the Lesser Antilles island arc

Abstract

K-Ar age determinations, mainly whole rock, with some corroboration from mineral separates, are presented for lava flows, domes, minor intrusives and blocks in tuffs from 95 localities in the Lesser Antilles. Together with the much smaller number of previously published data, these show a distinction between a range 38-10 million years (Ma) in the outer arc (Limestone Caribbees) and less than 7.7 Ma in the inner arc (Volcanic Caribbees). From southern Martinique southwards, the two arcs are superposed, and the whole range is fragmentarily represented. The observed age ranges in the outer and inner arcs fit between discontinuities in sea floor spreading in the North Atlantic at ca. 38 and ca. 9 Ma and a causal connection between spreading change and relocation of arc volcanicity is suggested. Palaeomagnetic directions at 108 localities in ten of the islands fall into normal ( N = 56, k = 13.8, D = 359°, I = + 22°, pole position 229° E, 89° N with drjr = 3°, d _X = 6°) and reversed groups ( N = 41, k = 14.1, D = 178°, I = -22°, pole position 18° E, 88° S with d xjr = 3°, dx = 6°) plus six sites of intermediate polarity and five sites indeterminate. The mean dipole axis is within 2° of the present rotation axis and is likely to be identical with it with a probability of 99%. The data are generally in accord with the established geomagnetic polarity time scale, but there is some suggestion of a normal polarity event at ca. 1.18 Ma within the Matuyama Reversed Epoch. The palaeomagnetic data relate mainly to be past 10 Ma and suggest that within that time the Lesser Antilles have not changed their latitude or geographic orientation, and that the geomagnetic field has averaged that of a centred axial dipole. The few older palaeomagnetic data are consistent with these same conclusions (though with less certainty) back to ca. 20 Ma ago. There is no evidence for oroclinal bending of the arc since then.