Abstract
The Niquelândia Complex is a major Proterozoic mafic and ultramafic layered intrusion in central Brazil. Ductile deformation and associated metamorphic recrystallization are widespread along ductile shear zones. Bands of alumina and silica-rich rocks (less than few meters thick) occur in the central parts of these ductile shear zones. Metamorphic grade, ranging from amphibolite to granulite facies, increases progressively downward in the layered intrusion stratigraphy. The three mapped metamorphic zones: amphibolite zone, amphibolite-granulite transition zone and granulite zone, are parallel to the intrusion stratigraphy. Metabasites show progressive changes in mineral assemblages, texture, and Ca-amphibole composition in a traverse covering the three metamorphic zones. With increasing metamorphic grade, amphibolites (hbl+pl ± cpx ± grt ± ep) give way to hornblende granulites (hbl+pl+cpx+opx) and anhydrous mafic granulites (pl+cpx+opx). The Ti, A1 IV and Na+K content of amphiboles increase progressively with metamorphic grade. Quartz-rich rocks have kyanite as the Al 2SiO 5 polymorph in the amphibolite zone, whereas sillimanite occurs in the granulite zone. Geothermobarometry and mineral stability data indicate P-T conditions of peak metamorphism at about 700 °C and 6–8 kbars in the amphibolite zone and temperatures higher than 800 °C in the granulite zone. In quartz-rich rocks of the granulite zone, retrogressive processes are indicated by reaction coronas of sil+grt between peak metamorphic assemblages of hc+qtz and replacement of sillimanite by kyanite. These reactions have an appreciable temperature dependence and together they indicate a retrogressive path characterized by an initial period of nearly isobaric cooling. Previously reported U-Pb zircon dating demonstrates the coeval nature of the amphibolite and granulite facies metamorphism and supports the notion that the entire terrain represents a single continuous crustal section. The metamorphic age is well constrained in the 770–795 Ma interval, indicating the importance of the Neoproterozoic (Brasiliano tectono-magmatic geodynamic Cycle) continent-continent collision event in the region. The large scale gradational metamorphic zonation is interpreted as an oblique cross-section through the continental crust. This interpretation is also supported by a regional linear paired gravity anomaly and a large mylonitic zone at the base of the section, and by the juxtaposition of high-grade against low-grade rocks at the basal fault zone.
Published Version
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