Ordovician palaeogeography with new palaeomagnetic data from the Montagne Noire (Southern France)

A joint palaeomagnetic and 40Ar/39Ar study has been performed on two olistolithic blocks from the Cabrieres Wildflysch in the Montagne Noire region of the Massif Central in France. There, andesitic volcanic and volcaniclastic rocks of Llanvirn-Early Caradoc age (ca 470-458 Ma) occur. Despite extensive secondary alteration, destruction of the dominant magnetic mineral phase and 40Ar/39Ar whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss, a positive fold test and the presence of dual polarities suggest that a primary, Ordovician magnetisation has mostly survived. This is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small, relatively unaltered magnetite grains, probably hosted in silicates, still carry the original, in this case Ordovician, remanence. The new data show that the Montagne Noire region was located at high southerly latitudes (68degrees (+19)/(-15)) during the Mid-Ordovician. This latitude represents the location for NW Gondwana of which the Massif Central was a part. Palaeomagnetic data from all the Central European massifs and terranes demonstrate a close link to the Gondwana Margin during the Lower and Middle Ordovician. (C) 2002 Elsevier Science B.V. All rights reserved. A joint palaeomagnetic and 40Ar/39Ar study has been performed on two olistolithic blocks from the Cabrieres Wildflysch in the Montagne Noire region of the Massif Central in France. There, andesitic volcanic and volcaniclastic rocks of Llanvirn-Early Caradoc age (ca 470-458 Ma) occur. Despite extensive secondary alteration, destruction of the dominant magnetic mineral phase and 40Ar/39Ar whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss, a positive fold test and the presence of dual polarities suggest that a primary, Ordovician magnetisation has mostly survived. This is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small, relatively unaltered magnetite grains, probably hosted in silicates, still carry the original, in this case Ordovician, remanence. The new data show that the Montagne Noire region was located at high southerly latitudes (68degrees (+19)/(-15)) during the Mid-Ordovician. This latitude represents the location for NW Gondwana of which the Massif Central was a part. Palaeomagnetic data from all the Central European massifs and terranes demonstrate a close link to the Gondwana Margin during the Lower and Middle Ordovician. (C) 2002 Elsevier Science B.V. All rights reserved.