The Petrogenesis of the Niquelandia Layered Basic-Ultrabasic Complex, Central Goias, Brazil

The Niquelandia igneous layered complex is made up of a lowersequence (LS), consisting of a basal layered zone (gabbros,peridotites, and pyroxenites), followed by an ultramafic zone(peridotites and pyroxenites), followed in turn by layered gabbros,and an upper sequence (US), composed of coronitic gabbros andanorthosites and upper amphibolites. Its stratigraphy and theproportion between ultramafic and gabbroic units resemble thoseof other layered complexes of the world, e.g. Stillwater andBushveld. The LS and US have different fractionation trendsrelated to the different liquidus phases, viz. ol (?sp), cpxand opx in the LS and ol, pl, and cpx in the US. Differentiatesappear at the top of both the LS and US. The possibility thatthe Niquelandia complex is formed by two unrelated intrusionsis rejected and a model accounting for the fractionation patternis presented. This model implies that, during crystallization,water caused a decrease in the temperature of the liquidus ofthe magma in the upper portion of the chamber. Taking into account the shape of the geotherm and its intersectionswith the liquidus temperature gradient, a point was reachedat which crystallization may have occurred simultaneously inthe lower and upper portions of the magma chamber, leaving differentiatedresidua both at the roof and in the middle of the complex. The inferred composition of the parent magma is that of a low-Tipicritic basalt of possibly transitional character, poor inincompatible elements and rich in Cr and Ni. These characteristicsimply a derivation by extensive partial melting of a relativelydepleted source in a tensional environment. The compositionof the liquid and the occurrence of metamorphic xenoliths ofthe country-rock in the uppermost gabbros suggest that the complexintruded the continental crust during a rifting episode. Theabnormal thickness of the complex (14 500 m) implies a continentalcrust comparable in thickness to the present one.