The interpretation of magnetic anomalies North of the dartmoor granite"

Abstract

The large negative magnetic Okehampton anomaly, northwest of the Dartmoor granite, and rock samples from the same locality, were studied to determine the cruse of the magnetic disturbances in this area. Mineralogical examination revealed that the ferromagnetic mineral pyrrhotite (Fe(_7)S(_8) has been developed in rocks within, and probably beyond, the metamorphic aureola. Magnetic measurements of the rock samples suggested that the Carboniferous sediments, notably shales, have a mean Q value of 3.57 and that the direction of magnetisation is near horizontal and reversed. Models made to define the profiles across the Okehampton Anomaly suggested that the casual body was composite, dipped north at 30º and had an undulating surface. This could be interpreted as faulted and folded lower Carboniferous rocks which disappear north from the exposed Meldon inlier under overlying upper Carboniferous sediments. The 30º dip of the slabs of magnetised rock is the same as the dip of the edge of the Permo-Carboniferous granite from which mineralising fluids emanted to deposit pyrrhotite in the lower carboniferous sediments. The directions of magnetisation used in the models correspond to typical early Permian directions suggesting that the magnetic properties of the rock could be attributed to pyrrhotite emplaced at the time of the granite intrusion. Comparison with other magnetic anomalies found around the northern edge of the Dartmoor granite, where it abets against Carboniferous sediments, showed that this explanation is tenable elsewhere. It was also noted that where pyroclastics are interbedded with the Carboniferous sediments the anomaly was the most pronounced. This was attributed to the abundant supply of sulphur and iron in the volcanics which could be a source for pyrrhotite development