Petrology of the tephrite-phonolite suite and cognate xenoliths of the Fortaleza district (Cearä, Brazil)

Abstract

The alkaline district of Fortaleza (Cearä, Brazil) consists of a number of phonotephritic plugs, domes, and dykes cropping out in the Precambrian basement, aged around 30 Ma.Chemically and petrographically the rocks range in composition from tephrites and phonolitic tephrites to peralkaline phonolites. At one locality (Caruru), olivine clinopyroxenite, essexite, and nepheline syenite xenoliths of cognate origin were also found included in phonolites. Whole rock and mineral chemistry variations coherently indicate that the various magmatic bodies of the suite essentially evolved by fractional crystallization. Mass balance calculations between bulk rocks and constituent minerals show that the evolution from tephrites to phonolitic tephrites and phonolites can be generally accounted for by subtraction of cumulates, which partly correspond to essexites and nepheline syenite xenoliths found in phonolites. Tephrites which are the least fractionated alkaline magmas in the area could, in turn, be derived from parental basanitic melts as suggested by the presence of olivine clinopyroxenite xenoliths that represent the proper fractionated solid complementary to tephritic liquids. 87<5r /86sΓ initial ratios (Ro) range from 0.7024-0.7057 suggesting significant crustal contamination processes only for the samples with high isotopic values. The inferred initial Sr ratios of the parental magmas (Ro = 0.7029-0.7036) are remarkably lower than those of southern Brazilian alkaline complexes (Ro = 0.704-0.706) supporting the existence of the Dupal Anomaly in the southern hemisphere mantle.