Re-Os isotope constraints on subcontinental lithospheric mantle evolution of Southern South America

SCHILLING MANUEL; CARLSON R.W.; KOESTER E.; CONCEIÇÃO R.V.; BERTOTTO G.W.; DANTAS C.

Punta del Este (Uruguay)

Simposio; V South American Symposium on Isotope Geology; 2006

We present Re-Os isotopic data for a widely dispersed mantle xenoliths carried to the surface of southern South America (36º-52ºS) by Eocene to recent alkaline magmatism. Our hypothesis is that the lithospheric mantle sections formed as the roots of southern South America can reflect the history of crust formation and amalgamation at different periods of time and under different geological settings, and so, present different petrogenetic and geochronological characteristics The Re-Os isotopic system gives unique chronological information about the time of mantle depletion process which is associated to the lithospheric formation. Our data shows coherent model ages for the lithospheric mantle formation that can be correlated with some hypothesis for the crust evolution of this region. Most samples show Os isotopic values similar to the convective mantle with relatively young model ages from 0.03 to 0.91 Ga, suggesting a relatively recent lithospheric mantle formation. Xenoliths from Agua Poca and probably Prahuaniyeu represent the roots of the Grenvillian terrane of Cuyania, inferred to be a fragment derived from Laurentia, were samples with old model ages could reflect relicts of the original lithopheric mantle (~1.1 Ga), that could be partially replaced for younger convecting mantle during the accretion of Chilenia terrane as suggested by some younger model ages. Samples erupted in the southwest corner of the Deseado Massif show only Proterozoic depletion ages suggesting that this microplate had been formed during this period of time, and thus, could be correlated to the Folkland Islands which shows similar ages for their basement rocks.