SHRIMP U-Pb age for the emplacement of the Santana Granite and reactivation of the Porto Alegre Suture, southern Brazil

SHRIMP U-Pb zircon ages are reported from a sample of the Santana Granite t hat represents the beginning of alkaline magmatic activity in the Neoproter ozoic of the Sul-Rio-Grandense Shield, Brazil. This granite outcrops in the city of Porto Alegre and has a NE-SW elongated shape (20 km long and 5 km wide). It is a medium- to coarse-grained syenogranitic rock with subvertica l magmatic foliation and subhorizontal lineation with NE-SW trends, overpri nted by a weak solid-state fabric. Zircons of this rock with (206)Pp/U-238 ages between ca. 575 and ca. 617 Ma are interpreted as having formed during magmatic crystallisation; the best fit for this age using five analyses pr ovided a mean (206)pb/U-238 age of 600 +/- 10 Ma. This age is also interpre ted as the age of the reactivation of the Porto Alegre Suture, a crustal di scontinuity along which the Santana Granite was emplaced. Younger isotopic data (< 575 Ma) for zircons from this sample are interpreted as due to Pb l oss from metamietization. The 600-Ma age is consistent with previous result s and marks a magmatic episode, probably related to an extensional tectonic episode that affected the NE of the Sul-Rio-Grandense Shield at the end of the Brasiliano Cycle. (C) 2001 Elsevier Science Ltd. All rights reserved.