Paleoposition of the northern margin of Armorica in Late Devonian times: Paleomagnetic and rock magnetic results from the Frankenstein Intrusive Complex (Mid-German Crystalline Rise)

Abstract

A detailed rockmagnetic and paleomagnetic study of the Frankenstein Intrusive Complex (FIC, 363 ± 7 Ma in age), southwestern Germany, has been carried out in order to test whether the paleomagnetic declinations within the internal parts of the European Variscides follow a common trend controlled by the structural grain of the orogen or whether the distribution of declinations follows a random pattern. In addition, new paleomagnetic data will contribute to further refine the drift history of the Armorican Terrane Assemblage during late Paleozoic times. The unmetamorphosed Frankenstein Igneous Complex forms part of the Mid-German Crystalline Rise within the Armorican Terrane Assemblage, amalgamated to Avalonia and Baltica during the Variscan orogeny. Stepwise thermal and alternating field (AF) demagnetization experiments identify four components (A, B, Cn, and Cr) of magnetization. Component A is a recent viscous overprint that parallels the present-day geomagnetic field. The high unblocking temperatures of A in some samples can be explained by the presence of multidomain magnetite. Component B is carried by hematite and is considered to be a late Carboniferous remagnetization. Components Cr and Cn, found in 110 samples from 20 sites, are antiparallel and pass a class C reversal test. Their primary origin is further supported by a contact test. Cr and Cn have maximum unblocking temperatures of up to 580°C and occasionally above over 600°C, indicating magnetite and hematite to carry the characteristic remanent magnetization. Ore microscopy revealed that primary hematite lamellae in ilmenite carry Cr and Cn and that a secondary generation of hematite is the carrier of B. The overall site mean direction of Cr and Cn (20 sites) based on endpoints and remagnetization circle analysis of 198°/40° (declination/inclination) with a 95% confidence limit (α95) of 4.9° and a precision parameter (k) of 45.0 defines a paleopole at 15° S, 9° W. When comparing to the apparent polar wander path for stable Europe this result indicates up to 27° counterclockwise rotation of the FIC region. This is significantly less than reported previously for the Odenwald, Spessart, Black Forest, and the Vosges Massifs of the central European Variscides. Furthermore, the resulting paleolatitude indicates significant separation between Baltica and this sector of the Armorican Terrane Assemblage in the latest Devonian. Copyright 2000 by the American Geophysical Union.