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 ⁴⁰Ar/ ³⁹Ar ages on hornblende and biotite. Four hornblende samples yielded a mean ⁴⁰Ar/ ³⁹Ar 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°, Inc=-56°, k=106, α₉₅=12°) or NW-downwardly directed magnetizations (mean Dec=270°, Inc=+64°, k=30, α₉₅=11°) that pass a reversal test with a classification of 'C' and an angular difference of 14.4°. 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°S, 001°E (dp=13°, dm=17°). 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.