The stratoid granites of central Madagascar: paleomagnetism and further age constraints on neoproterozoic deformation |
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| Institution: | 1. Department of Geological Sciences, University of Florida, 274 Williamson Hall, Gainesville, FL 32611, USA;2. Norwegian Geological Survey, Geodynamics Group, Leiv Elrikssons vei 39, 7491 Trondheim, Norway;3. Université Paul Sabatier, UMR 5563 CNRS, 38 rue des 36-ponts, 31400 Toulouse, France;4. Department of Geological Sciences, University of Michigan, 2563 CC Little Building, Ann Arbor, MI 48109 USA;1. Kosygin Institute of Tectonics and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, 65 Kim Yu Chen Str., Khabarovsk 680000, Russia;2. Faculty of Geology, Lomonosov Moscow State University, 1 Leninskie Gory, Moscow 119234, Russia;3. Geological Institute, Russian Academy of Sciences, 7 Pyzhevsky lane, Moscow 119017, Russia;1. Department of Metallurgical Engineering, Vaal University of Technology, P/Bag X021, Vanderbijlpark, South Africa;2. PRISE, Research School of Earth Sciences, Australian National University, Canberra, Australia;1. Geological Sciences Department, National Research Centre, 12622-Dokki, Cairo, Egypt;2. Geology Department, Faculty of Science, Fayoum University, P.O. Box 63514, Fayoum, Egypt;3. Department of Geoscience, University of Nevada, Las Vegas, USA;1. Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Street, Beijing 100037, China;2. Chinese Academy of Geological Sciences, 26 Baiwanzhuang Street, Beijing 100037, China;3. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Street, Beijing 100037, China;1. Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN, United States;2. Department of Earth Sciences, University of Minnesota, Minneapolis, MN, United States |
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| Abstract: | A paleomagnetic and 40Ar/39Ar study of a 630-Ma alkaline granite suite in Madagascar, the so-called ‘stratoid’ granites, reveals a complex history of remagnetization during the formation of the Antananarivo Zone de Virgation at ～560 Ma (D2) and the Angavo shear zone at ～550 Ma (D3). 40Ar/39Ar dating of hornblende, biotite and potassium feldspar from rocks affected by D2/D3 show initial cooling rates of 8 °C/Ma during the 550–520 Ma interval followed by slower cooling of 2.5 °C/Ma. The thermal effects of the D2 and D3 events appear to be restricted to regions surrounding the shear zones as evidenced by a 40Ar/39Ar biotite age of 611.9±1.7 Ma north of the virgation zone. The paleomagnetic data from the stratoid granites are complex and some sites, particularly in areas to the north of the virgation zone, may have been rotated about non-vertical axes following their emplacement and cooling. Because of these possible rotations, our best estimate for the paleomagnetic pole for Madagascar is derived from sites within the virgation zone. This pole falls at 6.7°S, 352.6°E (a95=14.2°). A post-metamorphic cooling history for the virgation zone indicates a magnetization age of 521.4±11.9 Ma. Our work in central Madagascar, coupled with previous studies, suggests that emplacement of the 630 Ma stratoid granites followed a collisional (?) tectonic event beginning around 650 Ma, recently recognized in southern Madagascar and in Tanzania. Subsequently, the stratoid granites in the Antananarivo virgation zone were reheated (～750–800 °C) at pressures between 3.5 and 3.6 kbars resulting in a pervasive remagnetization. We suggest that the younger shear events are genetically related to collisional tectonics elsewhere during the final stages of Gondwana assembly and are a consequence of the Kuunga Orogeny further south. |
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