Geological and geochemical studies of the sintra alkaline igneous complex, portugal

Abstract

The Sintra igneous complex, Portugal was an important centre of activity in late Cretaceous times. The great proportion of thealkaline rocks are felsic and include five large quartz syenite intrusions and trachyandesite, trachyte and alkali rhyolite lavas and dykes, most of which are oversaturated. Mafic rocks are sparse, but vary widely from alkaline and highly undersaturated types containing high K₂O, TiO₂ and Ba, similar to the contemporaneous Lisbon lavas, to hypersthene normative trachybasalts and one hypersthene normative basalt. The various magma types are intimately associated and a well-developed netveined complex of alkali gabbro, monzonite and syenite is recognised at Cabo da Roca. A study of the dyke distributions, intersections and orientations suggest a close propinquity of both oversaturated and undersaturated and of both felsic and matic magmas.

The basic magmas of Sintra and Lisbon show a continuous range in undersaturation (0 to 16% normative nepheline) and rare hypersthene normative basalts. Derivation of the hypersthene normative and mildly undersaturated basalts from the more undersaturated melts by low pressure fractionation or contamination by siliceous crust is shown to be unlikely. High pressure eclogite fractionation of a hypersthene normative basalt or variations in the percentage partial melting of a mantle under conditions where titanphlogopite is a low melting fraction are both processes compatible with the variations in undersaturation and proportions of TiO₂, K₂O and Ba. The quartz syenites and over satured felsic lavas of Sintra are thought to be derived from hypersthene nor mative parents.