Accretionary history and magma sources in the Southern Andes.

RAPELA, C.W..

Rio de Janeiro

Simposio; 31st t International Geological Congress, Rio 2000, Special Simposia F-2 "Andean Tectonics and Magmatism" -; 2000

IUGS

The crustal framework of the southern Andes was formed during Early Paleozoic continental collisions that completed the amalgamation of southwest Gondwana. At least two ¡major episodes of subduction were associated with these events, during the Early Cambrian (ca. 530 Ma) and the Early to Middle Ordovician (465—490 Ma), preceding the accretion of micro-continents that caused the lateral –westward- growth of the supercontinent. In order (o assess the role of mantle and crust during the Phanerozoic construction of the Andean orogen, new initial Sr and Nd isotopic ratios and geochemical data from plutonic rocks of different ages are presented, along an integrated 700 km cross-section of the Andes at 30°—34°S. The combined isotopic and geochemical evidence indicates that while Early Paleozoic events reworked an Paleo to Meso-proterozoic Iithosphere, the Late Paleozoic Pacific subduction regime established at ca. 290—320 Ma on accreted Grenvillian crust, involved reworking of a Meso- to Neo-proterozoic lithosphere. Mesozoic—Tertiary Andean subduction represents a radical change in this trend, as it is characterized by melting of relatively young depleted sources, probably associated with the different plate dynamics that began to prevail during and after the break-up of Gondwana in Jurassic—Early Cretaceous times.