Petrogenesis of a porphyritic granite east of Oporto (Portugal)

... One of the fundamental branches of plutonic geology, ... is concerned with textures as indicating the time-relations of the rock components. H. H. Read (1949)

Abstract

A coarsely porphyritic granite of great areal extent in northern Portugal is described from the locality of Cete, cast of Oporto. Geological and petrographical evidence go to show that it originated by consolidation from a magma of much the same composition.

The relative ages and modes of formation of the constituents are deduced from textural relationships. A paragenetical table is given. Initial magmatic conditions are demonstrated by the preferential distribution of the primary accessories in biotite, the earliest of the main constituents. Rare andalusite seems to be an early constituent. Intrusion is correlated with early crystallization. The magma had been emplaced before oligoclase started to separate. The microclineperthite megacrysts are shown to have largely crystallized from a magmatic fluid, ending on a replacement stage. Foliation and lineation, defined by planar and linear parallelism of the megacrysts, have formed in situ, not as flow structures due to intrusion. Orientation of older biotite and plagioclase inclusions in the megacrysts is described. Perthite is inferred to have formed by exsolution.

The orthomagmatic stage passed into a final metasomatic period, with replacement textures. The main generation of quartz is partly magmatic, partly metasomatic. Primary plagioclase in contact with microcline suffered decalcification of its borders. There also occur albite fringes around primary plagioclase which were formed by albitization of surrounding microcline. Six different habits of quartz, representing two or three generations, and two types of myrmekite are distinguished. A new type of myrmekite quartz is described. The features of the metasomatic or endoblastic stage are discussed.