Fogo Island (Fig. 2) comprises salic and mafic igneous rocks of the Fogo Island Batholith and its host, the Botwood Group, which on Fogo Island comprises siltstone, sandstone, and matrix-supported conglomerate of the Fogo Harbour Formation overlain by rhyolite ignimbrite, rhyolitic tuff, and minor tuffaceous sandstone of the Brimstone Head Formation. The lowest unit of the Botwood Group, the Llandovery Lawrenceton Formation (Williams 1972), does not outcrop on Fogo Island, but appears on nearby islands. This formation, consisting of about equal amounts of basalt (flows and breccias) and rhyolite (flows, tuffs, and breccias), reaches thicknesses in excess of 2000 m on Change Islands, 10 km west of Fogo Island (Currie 1997a), but elsewhere does not exceed 300 m in thickness, suggesting presence of a vent area just west of Fogo Island. The presence of several rhyolite ignimbrite sheets within the Lawrenceton Formation indicates subaerial emplacement. The top 100 m of the Lawrenceton Formation contains metre-scale lenses of pale green, thinly laminated siltstone identical to the conformably overlying Fogo Harbour Formation.

The bulk of the Fogo Harbour Formation (Baird 1958) consists of grey-green to brown siltstone and sandstone, laminated on centimetre-scale, and exhibiting grading, ripple marks, and cross-bedding. Most exposures contain no obvious volcanic material, but a few beds contain 2 to 10% by volume of pink feldspar lapilli and crystal fragments up to a centimetre in diameter. Near Rogers Cove one bed contains volcanic bombs. Conglomerate lenses with rounded siltstone cobbles in a homogenised siltstone matrix occupy erosive channels up to 2 m thick. Composite stratigraphic sections for the Fogo Harbour Formation suggest a relatively constant thickness of 1000 to 1300 m. Observations indicate a shallow marine origin for the formation, with the conglomerate intervals attributed to slumping during deposition. No fossils have been found in the Fogo Harbour Formation, but interbedding of its lower part with the Llandovery Lawrenceton Formation fixes a maximum age, and a precise U-Pb zircon age of 422 ± 2 Ma (Ludlow) from a composite dyke cutting the formation on Dog Bay (Elliot et al. 1991) provides a minimum age.

A sharp, conformable contact of the Fogo Harbour Formation with the overlying Brimstone Head Formation (Baird 1958) can be traced across northern Fogo Island at the base of a cliff of rhyolite ignimbrite (Currie 1997b). A single metre-scale lens of tuffaceous sandstone resembling the underlying sedimentary section occurs about 60 m above the base of the Brimstone Head Formation at Brimstone Head. On Fogo Island, the Brimstone Head Formation comprises several sheets of densely welded, brown, rhyolite ignimbrite with a foliation marked by elongate pale streaks of less welded material (fiamme). Offshore islands consist of moderately welded crystal tuff and tuff breccia with a few lenses of red, subaerial sandstone up to 10 m thick. The thickness of the formation is unknown, but locally exceeds 1000 m. The composition of the ignimbrite sheets resembles that of salic parts of the Fogo Island Batholith, and the Brimstone Head Formation may represent an erupted portion of the batholith, as suggested by Sandeman and Malpas (1995).

All of the supracrustal rocks on Fogo Island are openly folded about axial planes trending northeast and dipping about 65° to the south. Limb dips rarely exceed 40°, and fold amplitudes vary from a few metres to a few hundred metres. All of the folds exhibit strong axial planar cleavage, a cleavage which is developed on a regional scale, extending tens of kilometres southwest from Fogo Island. An envelope to the folds dips north-northwest at about 25°, so that stratigraphically higher levels are exposed to the north.

The Fogo Island Batholith, which underlies 80% of Fogo Island (about 250 km²) and also appears on small islands up to 8 km offshore to the east and southeast, intrudes the Fogo Harbour Formation. The batholith comprises five distinct lithologies. 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. In the Tilting Harbour area, Aydin (1995) reported an emplacement age of 422 ± 2 Ma for the amphibole-biotite granite, and 408 ± 2 Ma for agmatitic diorite, consistent with field observations.

Layering in the batholith, commonly present in intermediate to mafic units and locally in granitic units, parallels bedding in nearby sedimentary rocks, and exhibits open folds congruent to those in the host rocks. However, folds in the igneous rocks do not exhibit axial planar cleavage, suggesting that folding in the host rocks pre-dated emplacement of the batholith, at least in part. Both top and bottom contacts of the batholith with the Fogo Harbour Formation can be observed in the western part of the island, dipping moderately north. These observations indicate a sill-like form for the exposed part of the batholith. Geological mapping shows the batholith to be about 6.5 km thick at its western end increasing to 8 km or more toward the east. The batholith comprises a homogeneous granitic upper part, 3– 4 km thick, and a texturally complex intermediate to mafic lower part about 3 km thick.

Minor intrusive bodies related to the batholith occur both above and below the main mass. Dykes related to the batholith tend to strike parallel to bedding in the host rocks, but dip at high angles to bedding, lying in the axial planar cleavage of open folds. Below the body, on the southwest corner of the island, composite dykes up to 60 m thick (Fig.3) have a core of gabbro up to 20 m thick, commonly plagioclase-porphyritic, with thick rims of porphyry or microgranite chemically indistinguishable from granite of the main body. The contacts between mafic and salic phases are sharp, but lobate, with occasional rounded blobs or streaks of one lithology in the other, indicating some degree of mingling. These distinctive dykes occur up to 20 km from Fogo Island. One gave a precise U-Pb zircon date of 422 ± 2 Ma (Elliot et al. 1991), identical to that of the main batholith. In some cases these dykes connect to small microgranite sills or laccoliths. Above the main body near Fogo Harbour, porphyry and microgranite dykes abound, but they lack mafic cores and rarely exceed 15 m in width. Many of these dykes connect to sills up to 10 m thick, with the dyke-sill transition forming prominent rock ridges, possibly due to thickening of the erosionally resistant igneous bodies at this point. Sandeman and Malpas (1995) considered these sills to be ignimbrite sheets, but their intrusive nature is demonstrated by (1) equal and intense hornfelsing of the host on both sides of the sills, and (2) presence of laminated siltstone on both sides with no increase in tuffaceous material.

Although some small-scale contacts of the batholith with its host rocks appear knife-sharp and generally conformable, agmatite (Fig. 4) commonly appears along internal and external contacts of the Fogo Island Batholith and may be up to 100 m thick. Agmatite in contact with the Fogo Harbour Formation consists of 50 to 80% of centimetre to metre-scale hornfelsed blocks of Fogo Harbour Formation in a medium- to coarse-grained matrix which is granitoid along the northern and southwestern margins of the complex, but becomes more tonalitic to the south. Similar agmatite forms a semi-continuous fringe, 10 to 100 m thick, separating granite bodies between Joe Batts Arm and Barr'd Islands. Within the agmatite, the blocks become larger and more coherent with distance from more massive igneous rocks, passing outward into irregularly folded sedimentary rocks with a few granite dykes. Throughout the agmatite, the blocks exhibit a strong preferred orientation, in many examples preserving only slightly disrupted stratigraphy. Within 50 m of the main body, some centimetre-scale beds appear to have melted to thin layers of brownish aphanitic material which locally forms a selvedge around blocks. Despite intense hornfelsing and possible local partial melting, new mineral growth was confined to sparse growth of fine biotite, suggesting that cooling took place rapidly enough that thermal equilibrium was never attained.

The upper margin of the granite consists of an almost continuous layer of fine-grained, pink, felsic rock varying from featureless felsite to microgranite with abundant granophyric intergrowth and drusy miarolitic cavities, to rhyolite porphyry that is commonly spherulitic. Similar to identical lithologies occur as sills along the base of the granite where it contacts the mafic phase, and as sills and dykes above and below the batholith. In general felsite occurs close to the coarse-grained granite, whereas microgranite tends to form sills below the batholith, and rhyolite porphyry forms dykes above the batholith. However, there is no sharp division and all these rocks have similar or identical compositions (Appendix 1). A typical feature of all these rocks is presence of thin seams or clots of amphibole and magnetite. These segregations occur erratically in the felsic rocks (which otherwise contain virtually no mafic minerals) and also in hornfelsed country rocks adjacent to them.

Sandeman and Malpas (1995) reported that microgranite sheets cut the batholith, and that coarse-grained granite grades to microgranite. However, contacts between coarse and fine-grained to aphanitic felsic rocks are always sharp where exposed, and where intrusive relations can be determined, the coarse-grained phase invariably cuts the fine-grained one. Chemically no significant differences are present in major element composition between coarse-grained and fine-grained felsic rocks (Appendix 1). Sandeman and Malpas (1995) drew attention to enrichment of the fine-grained rocks in high-field-strength elements and Ga/Al ratio, suggesting an A-type affinity. These tendencies are evident in all the felsic compositions (Fig. 5), and there appear to be no systematic differences in composition between fine-grained rocks and coarse-grained rocks, with the possible exception of slightly lower FeO*/MgO in the latter.

Baird (1958) noted the presence of layered mafic rocks around Tilting Harbour and Seldom Come By. The Tilting Layered Suite, which formed the subject of studies by Cawthorn (1978) and Aydin et al. (1994), underlies an oval area about 2 by 1.5 km in size, surrounded by hybrid dioritic rocks into which it grades, and by which it is intruded. Centimetre to metre-scale layering faces and dips 35– 85° to the north, and consistently pinches out to the southwest. Layers exhibit modal and textural variation, including repeated cycles from websterite bases (cumulate ortho- and clinopyroxene ± olivine) to gabbro or leucogabbro tops (cumulate clinopyroxene and plagioclase). Large poikilitic hornblende and in some cases plagioclase overgrow the cumulates. Local convolute layering and prominent size and density sorting in some layers suggest deposition from magmatic currents, although much of the fine-scale layering may be due to in situ recrystallisation (Aydin et al. 1994). 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.

Gabbroic rocks around Seldom Come By form two ovoid masses. At Burnt Point, massive, pegmatitic gabbro contains cumulus ortho- and clinopyroxene, and large poikilitic post-cumulus amphibole and plagioclase. The body contains about 10% of sinuous, boudinaged basalt dykes up to 50 cm wide. At the head of Seldom Harbour irregularly layered gabbro and diorite grade to heterogeneous mafic rocks which contain enclaves of layered gabbro, some sharply bounded in metre-scale blocks, others apparently gradational to larger areas of massive gabbro.

The layered mafic rocks of the Fogo Island Batholith resemble large layered mafic intrusions, and imply that such a body was formed or disrupted during emplacement of the Fogo Island Batholith. Within the layered mafic rocks, abrupt reversals to a more primitive crystallisation order (clino pyroxene+plagioclase to orthopyroxene+olivine) imply repeated influx of primitive magma into a relatively stable magma chamber (Brown 1956; Wager and Brown 1968).

Gabbroic compositions commonly occur in dykes and sheets below the main batholith in the southwest corner of the island. Distinctive plagioclase-porphyritic gabbro, resembling the layered gabbro around Seldom Come By, occurs as discrete dykes, in the cores of composite dykes, as synplutonic dykes within the granite, and as synplutonic dykes within gabbro at Burnt Point.

Chemical analyses of non-cumulate gabbroic rocks from Tilting and Seldom show a considerable range of composition, ranging from subalkaline basalt across the andesite/basalt and andesite fields (Fig. 6d), a range approaching that of the apparently much more heterogeneous diorite complex (Appendix 1). On chemical discrimination diagrams (Fig. 6), compositions fall into both arc (Fig. 6a) and non-arc fields (Fig. 6b), with a large amount of scatter in some plots (Fig. 6c).

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. All three lobes exhibit hastingsitic amphibole with minor biotite, and a colour index consistently <15. The central feldspar-porphyritic lobe contains 1 to 2% of digested mafic inclusions up to 5 cm across. The more equigranular eastern and western lobes lack mafic inclusions. All three lobes tend to be locally homogeneous, but become slightly more potassium feldspar-rich and leucocratic toward the north. All three lobes contain sinuous synplutonic basalt dyke segments exhibiting chilled margins, commonly 1 to 2 m wide and 5 to 10 m long (Fig. 7). Dyke lithologies include plagioclase-porphyritic varieties similar to the cores of composite gabbro-rhyolite dykes. Felsic dykes and pegmatite are absent and quartz veins extremely rare (one observed).

South of Fogo Harbour, granite cuts across a carapace of felsite and rhyolite sills, and is in essentially conformable contact with strongly hornfelsed sedimentary rocks. This region exhibits crude sheeting parallel to the contact, and prominent joints perpendicular to the contact.

Chemically, all analyses of the granite fall in a restricted field (Fig. 5). On the diagram of Batchelor and Bowden (1985) this region straddles the boundary between late orogenic and post-orogenic granites (Fig. 5a), whereas in the diagrams of Maniar and Piccoli (1989), it clearly falls in the post-orogenic granite field (Fig. 5b, c). The trend to more differentiated compositions from south to north in all lobes can be seen in Fig. 5c (increasing SiO₂ and FeO*/(FeO*+MgO)) The tendency toward A-type compositions noted by Sandeman and Malpas (1995), seen in Fig. 5d, results from relatively high contents of (Zr+Nb+Ce+Y) combined with extreme depletion in MgO (producing extreme FeO*/MgO ratios). Alkali contents are not unusually high.

The diorite complex forms the largest and most complex unit of the Fogo Island Batholith. Some dioritic rocks exhibit coarse, massive textures comparable to those of the gabbro and granite. However, large exposures invariably include patchy, inequigranular textures, streaks of one lithology in another, and schlieren and blocks of more mafic lithologies in a dioritic to granitic matrix. Characteristic gradational, centimetre-scale pegmatitic patches with large amphibole and feldspar crystals have widely varying alkali feldspar/plagioclase ratios, and may contain a significant amount of quartz. Igneous breccias of diverse mafic blocks in a more salic matrix form a major part of the diorite complex (Fig. 8). The mafic component varies from rare examples of layered gabbro, to hornblende-rich amphibolite to hornblende-plagioclase rocks with minor quartz. In coastal exposures at Wild Cove, Cape Fogo, and Kippen Cove some mafic blocks exhibit intimate, amoeboid interfingering with the matrix, and form pillow-like masses, phenomena suggestive of coexisting magmas. The composition of the more homogeneous parts of the diorite complex ranges from gabbroic through hornblende diorite and quartz diorite to tonalite, monzodiorite, and rare syenite. Mafic varieties contain mafic inclusions, whereas more silicic varieties contain more silicic inclusions. Much of the interior part of the diorite complex comprises locally homogeneous, fine to medium-grained, granoblastic hornblende-plagioclase rocks with variable but minor amounts of clinopyroxene, quartz, and potash feldspar. Many of these rocks exhibit nebulous pegmatitic patches of hornblende+plagioclase, commonly associated with healed, epidote-filled fractures.

Near the contact between the diorite complex and granite, diorite and granite are typically observed in alternate outcrops, or in small adjacent areas with few exposed contact relations. In a large highway quarry, felsite and diorite form conformable, non-intrusive, metre-scale sheets. Coastal exposures at Cape Fogo and Cape Cove exhibit a variety of complex relations ranging from complexes of sills with contrasting compositions, through diorite cross-cut by granite, or pillows of mafic in salic phases, to completely gradational contacts over a few tens of centimetres. In the absence of definitive contact relations, Baird (1958) and Sandeman and Malpas (1995) interpreted the salic phases of the batholith to intrude the mafic phases. However, field observations during the present study generally suggested originally non-intrusive contacts within a zone of mingling.

Although the textural range of the diorite complex is bewilderingly large, chemical analyses (Appendix 1) show a limited range of composition, with SiO₂ content ranging fairly continuously from 52 to 61%. Two samples yielded SiO₂ contents of 64 and 75%. The former is the only one of 65 analyses of the batholith with an SiO₂ content between 61 and 70%. In general, analyses of the diorite complex plot in the same fields as the layered mafic rocks (Fig. 8) but contain much higher concentrations of K, Zr, and U, and lower contents of Mg. An obvious possible cause for these anomalies could be mixing with the granitic component of the batholith, as suggested by field observations. The strongly bimodal nature of the analyses shows that if this model is correct, mixing involved incorporation of the salic component into the mafic phase but little incorporation of the mafic component into the salic phase.