A major disparity is observed between the late Paleozoic‐early Mesozoic apparent polar wander paths (APWPs) of Laurussia and Gondwana when the landmasses are re‐assembled in a conventional “A‐type” Pangea. This discrepancy has endured from the earliest paleomagnetic reconstructions of the supercontinent, and has prompted discussions of non‐dipole paleomagnetic fields and alternative paleogeographic models. Here we report on a joint paleomagnetic‐geochronologic study of Late Permian and Early to Middle Triassic volcanic and volcaniclastic rocks from Argentina, which demonstrates support for an A‐type model, without requiring modification to the geocentric axial dipole hypothesis. New SHRIMP U‐Pb and 40Ar‐39Ar isotopic dating has reinforced the inferred age of the sequences, which we estimate at ∼264 Ma (Upper Choiyoi Group) and ∼245 Ma (Puesto Viejo Group). Field‐stability tests demonstrate that the volcanic rocks are carrying early/primary magnetizations, which yield paleopoles: 73.7°S, 315.6°E, A95: 4.1°, N: 40 (Upper Choiyoi) and 76.7°S, 312.4°E, A95: 7.3°, N: 14 (Puesto Viejo). A comprehensive magnetic fabric analysis is used to evaluate structural restorations and to correct for magnetization anisotropy. The paleomagnetic results derived from volcaniclastic rocks are interpreted to be affected by inclination shallowing, and corrections are discussed. A comparison of these new results with the existing Permian‐Triassic paleomagnetic data from Gondwana suggests the presence of widespread bias in the latter. We contend that such bias can explain the observed APWP disparity, at least for Late Permian‐Middle Triassic time, and that alternative paleogeographic reconstructions or non‐dipole paleomagnetic fields do not need to be invoked to resolve the discrepancy.

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2016-12-23T18:37:48

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S U M M A R YTo better constrain Baltica's position within Pangea, we conducted a palaeomagnetic study of Permo-Triassic dykes from the Oslo Graben, as a follow-up to an initial, but rather limited, study by Torsvik and colleagues in 1998. The age of these so-called Lunner dykes had previously been determined as ∼240 Ma in that study, but details in their analyses and new 40 Ar/ 39 Ar ages reveal that there may have been some argon loss in the initially dated dyke minerals and that a combined (weighted mean) age of 271 ± 2.7 (2σ ) Myr for the dykes is preferable. We find two major components of magnetization in our samples: one carried by an Fe-sulphide (likely pyrrhotite) and the other carried by low-Ti magnetite; these magnetization components may be found together (superposed) in a given sample or they may occur apart. Micronmetresized crystals of Ti-Fe oxides, observed with a scanning electron microscope (SEM) show exsolution lamellae, formed upon cooling from intrusion temperatures. Assuming that the submicronmetre-sized (Ti)-magnetite grains that carry a stable remanence are of the same generation as the observed larger grains, we interpret the magnetite remanence in the dykes as of primary, thermoremanent origin. The sulphide remanence appears to be slightly younger, as seen by the SEM observations of pyrite framboids and a Fe-sulphide grain invading a Timagnetite grain. Moreover, the sulphide mineralization is likely of region-wide hydrothermal origin. The magnetizations carried by the pyrrhotite and magnetite have nearly identical directions and so, must be nearly of the same age. For this study, we sampled 56 sites including 39 dykes, 10 baked-contact rocks and 7 host rocks removed from the immediate dyke contacts. The dykes and the contact rocks have the same SW and up directions of magnetization, and contain the Fe-sulphide or the magnetite magnetization or both, as diagnosed by their relative unblocking temperatures. However, all the sampled carbonate and igneous host rocks far away from the dykes also have the same directions. Thus, all of the 10 originally planned contact tests are inconclusive. The new palaeopoles of this study are a few degrees apart; the magnetite pole (from dykes only, N = 25) is located at 51 • N, 164 • E, K = 69, A 95 = 3.5 • , whereas the pole calculated from iron sulphide magnetic directions (all rock types, N = 20) is at 54 • N, 166 • E, K = 112, A 95 = 3.1 • . All directions are of reversed polarity, suggesting that the magnetization was acquired during the Kiaman Reversed Superchron. The palaeomagnetic mean result from the magnetite-bearing sites implies a palaeolatitude of Oslo of 23 • N, whereas the palaeolatitude calculated from the pyrrhotite magnetizations is 25-27 • N, depending on choice of host lithologies.As noted in many previous publications, the palaeomagnetic poles for the late Palaeozoic and Early-Middle Triassic are in conflict with classical Pangea reconstructions. The poles with ages of 250 ± 10 Ma, in particular, previously showed a discrepancy of some ...