From the bottom upwards, this Complex consists of a continuum of amphibole-biotite and biotite gneiss with irregular bodies and lenses of granite, migmatitic amphibolitic gneiss (lit-par-lit and agmatite) often showing ptygmaticfolds, and ortho-amphibolites.
The most common facies is one of lit-par-lit amphibolitic gneiss, hosting well developed ptygmatic folds, accompanied by amphibolitic agmatite. These facies are followed, downwards, by amphiboliticbiotitic gneiss that contains irregular bodies of foliated hololeucocratic granite.
In order of decreasing age they are (1) agmatite containing blocks of host rock in a coarse-grained igneous matrix, (2) felsite, rhyolite porphyry, and microgranite, (3) layered and massive gabbro, (4) medium- to coarse-grained amphibolebiotite granite, (5) heterogeneous mafic rocks with a generally dioritic to tonalitic matrix containing schliers and blocks ranging from layered gabbro, to monzonitic, syenitic and granitic varieties.
Although some small-scale contacts of the batholith with its host rocks appear knife-sharp and generally conformable, agmatite (Fig.
The top of the section, northwest of Tilting, consists of coarse-grained, unfoliated plagioclase-hornblende rocks which grade to quartz dioritic agmatite containing metre-scale fragments of layered mafic rocks.
Coarse-grained granite of the Fogo Island Batholith occurs as an east-west belt across the northern part of the island, partially separated into three lobes by narrow belts of hornfelsed host rocks, agmatite, and sheets of felsite.