Upward and downward flow in a camptonite dike as recorded by deformed vesicles and the anisotropy of magnetic susceptibility (AMS)

Abstract

A 1.6-m wide Cretaceous camptonite dike on Higby Mountain, Connecticut, contains deformed vesicles that record an initial brief period of steeply upward intrusion followed by a prolonged vertical downward flow of magma prior to solidification. The camptonite is unusual in that it is almost aphyric, but it does contain abundant skeletal microphenocrysts (∼ 0.3 mm) of magnetite, which give the rock a strong magnetic susceptibility (∼ 127 × 10 − 3 SI). These crystals are slightly elongate, and their long axes became preferentially aligned parallel to the dike walls but perpendicular to the magma flow direction, which is indicated by the stretched vesicles. Consequently, the rock has a strong anisotropy of magnetic susceptibility (AMS) with the mean value of P′ being 1.03. The AMS ellipsoid is everywhere prolate ( T = − 0.9 to − 0.02), with the maximum susceptibility axis being almost parallel to the strike of the dike, that is, parallel to the walls of the dike but perpendicular to the magma flow direction. In the outer part of the dike where the deformed imbricate vesicles indicate upward flow at an inclination of 70–75°, the maximum susceptibility direction plunges ∼ 11°, perpendicular to the flow direction. In the interior of the dike where the vesicles indicate almost straight downward flow, the maximum susceptibility direction is almost horizontal and again is perpendicular to the flow. In many, but not all samples, the sense of flow (i.e., up or down) is indicated by imbrication of the intermediate magnetic susceptibility axis. Although the major magnetic susceptibility axis is commonly assumed to parallel the direction of flow of magma in dikes, the present study indicates that, at least in this camptonite dike, the maximum susceptibility axis is perpendicular to the flow of magma.