Paleomagnetism of a paleocene pluton on jamaica

A Paleocene granodiorite pluton on Jamaica has been subject to extensive weathering caused by the tropical marine environment of the island. The natural remanence of 29 samples obtained from relatively fresh rock in two localities was found to consist of two components with overlapping coercivity ranges. Alternating field treatment proved ineffective for removing the secondary component without destroying the primary one. Thermal demagnetization of samples from the two localities was more effective and yielded paleomagnetic poles at 14.7°N, 11.6°W and 58.9°N, 15.9°E respectively. These pole positions are different from those available from contemporaneous North American rocks and from poles derived from Jamaican Cretaceous and Upper Miocene rocks. Mineralogical studies showed that the granodiorite has undergone an extensive maghemitization superposed on earlier class 2 deuteric oxidation and related to the weathering process. Some of the titanomaghemite has, however, been converted to titanohematite. Hence although the secondary remanence carried by the former was removable by thermal treatment at 500°C, its part carried by the latter could not be removed without simultaneously destroying the primary remanence carried by the residual titanomagnetite. The observed paleopole positions do not, therefore, represent the true Paleocene geomagnetic field, but suggest that the direction of magnetization of the pluton has been approximately equatorial and was probably acquired in a reversed geomagnetic field. This could be interpreted as having been caused by the behavior of the geomagnetic field during a polarity transition, but a more favorable interpretation appears to be a large anticlockwise tectonic rotation of the islands since the Paleocene.