Paleomagnetism, geochronology and tectonic implications of the Cambrian-age Carion granite, Central Madagascar

The Carion granitic pluton in central Madagascar was intruded into warm. continental crust following orogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP dating of the pluton yields an emplacement age of 532.1 +/- 5.2 Ma followed by relatively slow cooling as constrained by Ar-40/Ar-39 ages on hornblende and biotite. Four hornblende samples yielded a mean Ar-40/Ar-39 age of 512.7 +/- 1.3 Ma. A biotite sample yielded an age of 478.9 +/- 1.0 Ma. Paleomagnetic samples from the pluton and surrounding country rocks exhibit either SE-upwardly directed magnetizations (mean Dec = 113 degrees, Inc = -56 degrees, k = 106, alpha (95) = 12 degrees) or NW-downwardly directed magnetizations (mean Dec = 270 degrees, Inc = +64 degrees, k = 30, alpha (95) = 11 degrees) that pass a reversal test with a classification of 'C' and an angular difference of 14.4 degrees. The 'normal' (negative inclinations) and 'reverse' (positive inclinations) directions also show a spatial bias within the pluton, suggesting a field transition from reverse to normal during cooling. The paleomagnetic pole calculated from the mean direction falls at 6.8 degreesS, 001 degreesE (dp = 13 degrees, dm = 17 degrees). Estimates Of the blocking temperature for the magnetization are compared to the cooling history of the pluton and an age of 508.5 +/- 11.5 Ma is assigned to the pole. The Carion pole falls near similar-age poles from elsewhere in Gondwana, supporting the idea that the major orogenic events during Gondwana assembly were complete. A slight revision of the Gondwana apparent polar wander path (APWP) is proposed with rapid APW from 540 to 520 Ma; however, the proposed mechanisms to explain this rapid APW (including intertial-interchange true polar wander (TPW) or enhanced mantle driving forces) cannot fully explain all the data. (C) 2001 Elsevier Science B.V. All rights reserved.