Palaeomagnetis and rock magnetism of selected intrusive igneous bodies from southern Mexico i reconnaissance study of the Acapulco and Tierra Colorada intrusives

Abstract

Palaeomagnetic data from a total of 60 oriented samples collected from two intrusive bodies, the Acapulco granite (Rb-Sr date on biotite of 48 ± 0.5 m.y.) and the Tierra Colorada monzonite (Rb-Sr date on biotite of 36.6 ± 2 m.y.), are reported. Alternating field and thermal demagnetization reveal that the palaeomagnetic record of the intrusives is complex. The results sugest that relatively rapid apparent geomagnetic variations are recorded by the intrusives, and different explanations are considered: (1) geomagnetic field variations being recorded by the cooling of the bodies, (2) remagnetization effects, (3) relative tectonic movements within the bodies, (4) tectonic movements of the bodies as coherent units, and (5) extreme magnetic instability. No definitive explanation is given. The magnetic response of the instrusives is dominated by the induced magnetizations (Q-coefficients are very low), which are higher in the monzonite than in the granite.