A pilot rock magnetic and ore microscopy study of xenolith-bearing young basaltic rocks from the Camargo cinder cone field, Chihuahua, Northern Mexico

Initial results of a rock magnetic, paleomagnetic, microscopic study of the Camargo volcanic field from Chihuahua (northern Mexico) are reported. The field is formed by more than 300 cinder cones and extends over an area of 200,000 km(2) Mantle and lower crust xenoliths in the alkaline basaltic rocks of La Olivina, in the northern sector of the field, suggest rapid magma ascent rates. This study concentrates on the rock magnetic properties of the cinder cone field. Remanences are carried in most cases by Ti-poor titanomagnetites and magnetites as a result of the oxy-exsolution of original titanomagnetite during initial flow cooling. For a few units, remanence seems to be carried by (titano)hematites, as shown by unblocking temperature spectra and secondary titano(maghemite). Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain state region. Most units (13) show reverse polarity magnetizations, and only 2 show normal polarity. Radiometric dating gives the field's age range as between 2 and 1.7 Ma, which, together with the magnetostratigraphic results, suggests a relatively short emplacement time for the volcanic field. The mean preliminary paleomagnetic direction obtained for the Camargo volcanic field is I = 30.6degrees, D = 341.3degrees, k = 19, and alpha(95) = 9.8degrees, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. In turn, this suggests a vertical axis tectonic counterclockwise rotation of approximately 20degrees relative to stable North America. (C) 2002 Elsevier Science Ltd. All rights reserved.