Paleomagnetic dating of the alteration overprint on Jurassic lavas in Central Patagonia: Relationship with mineralizing events

GEUNA, S.E.; VIZÁN, H.

Perugia, Italia

Congreso; XXIV General Assembly of the International Union of Geodesy and Geophysics (IUGG); 2007

The extrusive flows of a Middle to Late Jurassic volcanic event occur throughout a wide area of southern South America, representing the culminating event of an extensional period that preceded Gondwana break-up. The volcanism in Central Patagonia is characterized by basic to intermediate composition lavas overprinted by a very low-grade metamorphic event. Occasionally the Jurassic andesites are spatially related with polymetallic, Au-bearing, mineralised veins. Some authors have related both the metamorphism, and mineralization, to the activity of low-temperature geothermal systems. A palaeomagnetic study of 24 basaltic-andesitic flows from the Late Jurassic Cañadón Asfalto Formation revealed the presence of two magnetic components, both of them carried by magnetite. The harder component (?A?), with coercive force > 70 mT and unblocking temperature 590oC was interpreted as primary, acquired on cooling of the lavas. In addition, heating up to 350-400oC allowed isolating a softer component (?B?) in 22 sites, in a consistent direction: Dec. 15o, Inc. -53o, α95 4.8. The acquisition of the component B postdates the tilting of the lavas (negative fold test); however, it differs from the present magnetic field, and therefore a recent viscous origin can be discarded. The uncorrected component B is coincident with the primary remanence carried by the overlying Upper Jurassic-Lower Cretaceous sedimentary sequence (Upper Cañadón Asfalto/Cañadón Calcáreo Formation), and both are rotated respect to the expected Late Jurassic to Recent geomagnetic field for South America. Previous studies have shown that rotation occurred in the earliest Cretaceous, coeval with, and perhaps closely related to the opening of the South Atlantic Ocean. According to the magnetic relaxation time versus blocking temperature curves established for magnetite, to remagnetise the magnetic fraction with blocking temperatures below 350oC would require a metamorphic peak of about 150oC that lasted for a period of about 1 m.y., compatible with the low-grade (zeolite) metamorphic assemblage observed in these lavas. The component B predates the tectonic event that lead to the rotation of widespread areas of Cañadón Asfalto basin in the Early Cretaceous; on the other side, the negative fold test indicates that it postdates the tilting of the Late Jurassic flows. That puts the remagnetisation event in a time span restricted to the Jurassic-Cretaceous transition. Because of the potential economic importance of the mineralization it is important to establish the relationship between the remagnetization event and the major tectonic rotations in the area.  This relationship would be a valuable exploration guide for the low-sulfidation epithermal systems formed following the volcanism.