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

The Niquelandia intrusion is among the largest layered intrusions in t he world. It is part of a 350 km long belt of large mafic-ultramafic c omplexes in central Brazil. Both intrusion and country rocks were subj ected 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 me ta-anorthosite from the upper gabbro-anorthositic unit are variably di scordant (0-28% with regard to the lower intercept) along a mixing lin e from 777 +/- 10 Ma lower intercept and 1583 +/- 25 Ma upper intercep t. Zircon fractions from a later quartz diorite within the Central Maf ic Unit are variably discordant (0-14% with regard to the lower interc ept) along a mixing line from 794(+4/-6) lower intercept and 1565 +/- 22 upper intercept. The upper intercepts for both rocks, interpreted t o be the magmatic age, overlap and suggest a Mesoproterozoic age for t he intrusion. The lower intercept is interpreted to be the high-grade metamorphic event. Zircons from two quartz-rich blastomylonites repres enting different metamorphic domains and cutting through the layered i ntrusions were analyzed. A single zircon from the amphibolite facies d omain blastomylonite gives a concordant (-0.3%) Pb-207/Pb-206 age of 7 77 +/- 3 Ma, and a single zircon from the granulite facies blastomylon ite gives a Pb-207/Pb-206 age of 786 +/- 5 Ma (- 0.8% discordance). Bo th results are consistent with the lower intercept ages and set the 77 0-795 Ma interval for the high-grade metamorphic event. Rutile ages (a pproximately 650 Ma) are considerably younger than zircon metamorphic ages and suggest a milder regional tectonomagmatic episode or a comple x and long-lasting cooling history for the region. The U-Pb data provi de evidence for an important Mesoproterozoic extensional tectonic even t in central Brazil. It also sets the 770-795 Ma interval for the high -grade amphibolite to granulite facies prograde metamorphism, interpre ted to represent the timing of a continental collision event.