PETROGENESIS OF COEXISTING SIO2-UNDERSATURATED TO SIO2-OVERSATURATED FELSIC IGNEOUS ROCKS - THE ALKALINE COMPLEX OF ITATIAIA, SOUTHEASTERN BRAZIL

The Itatiaia alkaline complex is a Late Cretaceous intrusion (72 Myr) made up of felsic differentiates, with syenitic rocks dominant through out and with presence of both nepheline-and quartz-rich varieties. Dyk es with phonolitic or trachytic composition cross-cut the coarse-grain ed facies. The rocks are arranged concentrically, with the core of the complex being formed by SiO2-oversaturated syenites (with a small out crop of granites), and are radially displaced by faults related to reg ional tectonic lineaments. The minerals show gradual but significant c hanges in composition (salitic and augitic to aegirine-rich pyroxenes, hastingsite and actinolite to richterite and arfvedsonite amphiboles, sodic plagioclase to orthoclase feldspars and so on) and the whole-ro ck trends are broadly consistent with fractional crystallization proce sses dominated by alkali feldspar removal. Sr-isotopic data indicate m ore radiogenic ratios for the SiO2-oversaturated rocks (0.7062-0.7067 against 0.7048-0.7054 for the SiO2-undersaturated syenites), consisten t with small amounts of crustal input. The favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differe ntiation starting from differently SiO2-undersaturated mafic parental magmas (potassic alkali basalts to ankaratrites, present in the Late C retaceous dyke swarms of the area), accompanied by variable crustal co ntamination prior to the final emplacement. The lack of carbonatite as a significant lithotype, the potassic affinity of the Itatiaia comple x, and the relatively high Sr-isotopic ratios match the characteristic s of the other complexes of the Rio de Janeiro-Sao Paulo states coastl ine and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source. (C) 1997 Elsevier Science B.V.