Palaeomagnetism of Permian multiple intrusion dykes in Bohuslän, SW Sweden

Abstract

Summary. A detailed rock and palaeomagnetic investigation of the three major dyke systems of late Palaeozoic age in Bohuslän SW Sweden is reported. The three dyke systems are dolerite dykes, porphyritic dolerite dykes and rhomb porphyry (RP) dykes, the latter ones being of special interest as examples of multiple intrusion dykes. The rhomb porphyry dykes are closely related to the rhomb porphyry lavas and dykes of the Permian igneous province of the Oslo Rift zone some 100–200 km NNW. Geometrical mean values of natural remanence and susceptibility are of the order of 10−3 emu cm−3, and the Q‐ratio is usually < 1. Magnetic fabric indicates vertical flow of the magma, and discrepancies in the Curie‐temperatures when measured and calculated by microprobe analysis, respectively, suggest the very stable primary component of the TCRM to reside in single domain titanomagnetites exsolved in larger grains where a soft VRM dominates. Detailed sampled profiles across two well developed multiple intrusion RP dykes show a marked consistency in the symmetrical build‐up of the dykes and the magnetic parameters (bulk susceptibility, Q‐ratio, anisotropy and coercivity spectrum), which strongly supports the theory of the multiple intrusion mechanism of these composite RP dykes. During af cleaning, the remanent direction of the RP dykes migrates along great circles and stabilizes around (D, I) = (199°–32°) α95= 6.6°, N= 8 sites, with the apparent pole position 46°S, 345°E, which is very close to other poles of lower Permian age from stable Europe. In the dolerites and porphyritic dolerites VRMs of very low coercivities dominate, and a stable direction was difficult to isolate by af demagnetization. Because of the higher scatter the cleaned mean directions of the dolerite dykes and porphyritic dolerite dykes however are not significantly different from that of the rhomb porphyry dykes, and all dyke types are considered to be of approximately the same age. Copyright © 1980, Wiley Blackwell. All rights reserved