PETROLOGIC AND PALEOMAGNETIC STRUCTURE OF THE UPPER MESOZOIC TORTUGA OPHIOLITE, FUEGIAN ANDES

A mineralogical and paleomagnetic study of the Tortuga Ophiolite, on samples covering different lithologies from extrusive basalts to sheeted dikes and cumulate gabbro, provided insights about the magnetic remanence of oceanic crust generated in back-arc basins that subsequently were obducted onto continental crust. Although the igneous textures within the ophiolitic pseudostratigraphy are well preserved, basalts, diabases and dikes in upper levels are vastly hydrothermally altered. Prevailing Prehnite to Greenschist facies secondary mineral assemblages grade downwards through the ophiolitic section and consist of prehnite, albite, chlorite, epidote, titanite, actinolite, andesine, hornblende, with rare white mica and biotite. Subordinate opaque minerals vary downward, being characteristic pyrrhotite, chalcopyrite, pyrite, limonite and magnetite in basalts; hematite pseudomorphs -after magnetite (maghemite)- pyrrhotite, chalcopyrite, pyrite and limonite in diabases and dikes (6-8%;); and pyrrhotite, chalcopyrite, cubanite, bornite, magnetite, hematite pseudomorphs and limonite in gabbros (2-4%). Paleomagnetic data reveal variations of the low natural remanent magnetization of the thick pillow basalts (c. 0.01 A/m) and diabase-dike layers (c. 0.05 A/m) with higher values in the gabbro layer (c. 2 A/m). Mean magnetic suceptibility is higher in the lower gabbro layer (66.1*10 -4 SI) compared to that measured in diabase-dikes and basalts (6.4*10 -4 and 6.7*10 -4 SI, respectively). Remanence demagnetization curves for samples subjected to alternating magnetic fields indicate that “invisible” single and/or pseudo-single domain magnetite and/or low-Ti titanomagnetite is the likely magnetic carrier of the magnetic remanence in the ophiolitic pillow lavas as well as in the underlying dikes and gabbros. The magnetic structure of the Tortuga Ophiolite is thougt to be inherited from the seafloor hydrothermal alteration processes that occurred in the vicinities of a spreading center, in which high thermal gradients and possibly the low permeability prevented the fluid penetration and mineral modification by water–rock interactions in the deep gabbro layer.