ZIRCON AND RUTILE U-PB GEOCHRONOLOGY OF THE NIQUELANDIA LAYERED MAFIC AND ULTRAMAFIC INTRUSION, BRAZIL - CONSTRAINTS FOR THE TIMING OF MAGMATISM AND HIGH-GRADE METAMORPHISM

The Niquelandia intrusion is among the largest layered intrusions in the world. It is part of a 350 km long belt of large mafic-ultramafic complexes in central Brazil. Both intrusion and country rocks were subjected to high-grade metamorphism ranging from amphibolite to granulite facies, and associated ductile deformation. The first U-Pb zircon and rutile ages for this region are presented. Zircon fractions from a meta-anorthosite from the upper gabbro-anorthositic unit are variably discordant (0-28% with regard to the lower intercept) along a mixing line from 777 +/- 10 Ma lower intercept and 1583 +/- 25 Ma upper intercept. Zircon fractions from a later quartz diorite within the Central Mafic Unit are variably discordant (0-14% with regard to the lower intercept) along a mixing line from 794(+4/-6) lower intercept and 1565 +/- 22 upper intercept. The upper intercepts for both rocks, interpreted to be the magmatic age, overlap and suggest a Mesoproterozoic age for the intrusion. The lower intercept is interpreted to be the high-grade metamorphic event. Zircons from two quartz-rich blastomylonites representing different metamorphic domains and cutting through the layered intrusions were analyzed. A single zircon from the amphibolite facies domain blastomylonite gives a concordant (-0.3%) Pb-207/Pb-206 age of 777 +/- 3 Ma, and a single zircon from the granulite facies blastomylonite gives a Pb-207/Pb-206 age of 786 +/- 5 Ma (- 0.8% discordance). Both results are consistent with the lower intercept ages and set the 770-795 Ma interval for the high-grade metamorphic event. Rutile ages (approximately 650 Ma) are considerably younger than zircon metamorphic ages and suggest a milder regional tectonomagmatic episode or a complex and long-lasting cooling history for the region. The U-Pb data provide evidence for an important Mesoproterozoic extensional tectonic event in central Brazil. It also sets the 770-795 Ma interval for the high-grade amphibolite to granulite facies prograde metamorphism, interpreted to represent the timing of a continental collision event.