Flow duration of a dike constrained by palaeomagnetic data

Abstract

SUMMARY We have conducted a palaeomagnetic study of a Permian red sandstone layer intruded by a 70cm wide Plio-Pleistocene basaltic dike, with the ultimate goal of estimating the flow duration of the magma before its solidification. 31 samples were collected at various distances from the dike/sandstone interface and were thermally demagnetized in order to separate the overprinted PlioPleistocene magnetization component from the primary Permian one and to determine for each sample the unblocking temperature, Tub, necessary to erase totally the overprint. This temperature can be used to determine the maximum palaeotemperature reached at any distance from the contact, provided the secondary magnetization is a (P)TRM and the thermal fluctuation effects and actual cooling rates are taken into account. Examination of various magnetic parameters (NRM intensity, 15 days VRM intensity, initial magnetic susceptibility and hysteresis curves) allowed us to establish the nature of the overprinted magnetization and consequently to exclude samples carrying a CRM overprint. An apparent palaeotemperature profile and two corrected ones were thus deduced from the samples in which the dike emplacement produced no crystallization of new magnetic minerals. These profiles were compared with the calculations of a new conductive thermal model which includes the basalt's latent heat of crystallization, the flow duration of the dike, the width of the circulating magma, and the temperature dependence of the thermal conductivity, k, in the basalt and in the sandstone, or alternatively convection simulated by an increase of the k values. Good fits with the experimental data are obtained for a flowing width consistent with field observations and when the latent heat is taken into account, in the two following cases. First, when k is temperature dependent in the basalt and in the sandstone, which implies a magma flow duration of 5 days. Second, when a limited convection process is assumed in the sandstone only; here, a flowing time of only 1 day is required. The corrected palaeotemperature profiles argue in favour of the second hypothesis, suggesting that when a dike is emplaced, heat transfer by fluids may play an important role in the surrounding rock.