HR: 0830h
AN: GP41A-02    [Abstracts]
TI: Incidence of Metamorphism on Magnetic Mineralogy in the Sarmiento Ophiolitic Complex, Southern Chile
AU: * Singer, S
EM: singer@gl.fcen.uba.ar
AF: INGEODAV, Dept. Cs. Geol., Univ. Bs.As., Pab.2, C. Universitaria, Buenos Aires, Argentina
AU: Rapalini, A
AF: INGEODAV, Dept. Cs. Geol., Univ. Bs.As., Pab.2, C. Universitaria, Buenos Aires, Argentina
AU: Calderon, M
AF: Dept. Geologia, Univ. de Chile, Santiago, Chile
AU: Herve, F
AF: Dept. Geologia, Univ. de Chile, Santiago, Chile
AB: The Sarmiento Ophiolitic Complex (SOC), located in the southern Andes of Chile, represents the mafic portion of the Late Jurassic to Early Cretaceous oceanic floor of a back-arc basin closed and uplifted in the Mid Cretaceous. Its igneous pseudostratigraphy consists of mainly mafic pillow lavas, dikes, and gabbros. These rocks underwent a non - deformative "ocean floor metamorphism" which developed secondary mineral assemblages in a vertical steep metamorphic gradient passing from zeolite to actinolite facies, followed by a transition to fresh gabbros Syntectonic greenschist facies assemblages in mylonitic rocks bordering the thrust slices that expose the SOC, represent a different metamorphic event, that probably occured before the latest Cretaceous. A paleomagnetic study of these rocks has shown that a post-tectonic widespread remagnetization affected the whole ophiolite. Incidence of the mentioned metamophic processes on the natural remanent magnetization of the ophiolite was evaluated thru a susceptibility survey and a study of ferromagnetic (s.l ) minerals. These were identified by microscope observations under reflected and transmitted light. A very good agreement between magnetic susceptibility values and optical observations was found. Metamorphic processes have produced strong effects on ferromagnetic minerals. These metabasites are disctintly poor in titanomagnetites, except the metagabbros and dikes crosscutting gabbros, which show high susceptibilities due to magnetite from serpentinisation of olivines and uralitization of pyroxenes. Sheeted dikes and pillow lavas show low susceptibilities controlled by paramagnetic minerals with titanite as witness of a primary titanium-bearing mineral. Pure microcrystalline magnetite, main carrier of natural remanent magnetization, likely formed during the greenschist post-tectonic overprint. Sulfide mineralization in the Complex requires further studies. Finally, equilibria between Fe3+ in oxides and Fe3+ in silicates seem to have played an important role in determining the ocurrence and abundance of magnetite during the metamorphism.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1519 Magnetic mineralogy and petrology
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly