Pink to red monzogranite gradational to syenogranite forms the northwestern part of Porcupine Mountain, as well as smaller area in the south and southeast (Fig. 2). Especially in the northwestern areas, where it is well exposed in the main quarry, it is mixed with abundant enclaves of mafic plutonic rocks and metavolcanic rocks of andesitic to rhyolitic composition, and cut by abundant mafic dykes. Texture varies from coarse-grained equigranular to medium-grained porphyritic, the latter with phenocrysts of quartz and plagioclase up to 1 cm in length (Fig. 3a). Interstitial granophyre generally is present in syenogranite samples. Mafic minerals may have originally been hornblende and biotite, but they have been entirely altered to chlorite and iron oxides. Accessory minerals include zircon, titanite, apatite, and magnetite. White et al. (2001) published a U-Pb (zircon) age of 610 ± 3 Ma from a quartz-porphyritic syenogranite sample typical of the unit.

Leucodiorite forms small areas associated with monzogranite/syenogranite in the southwestern part of the mountain (Fig. 2). The leucodiorite is generally white to light grey, medium- to coarse-grained and inequigranular, and typically consist of a framework of subhedral plagioclase grains and interstitial mafic minerals (chlorite and amphibole), locally forming up to 30% of the rock (Fig. 3b). The plagioclase is completely altered to saussurite. The rock is named leucodiorite because it contains amphibole rather than pyroxene. The relationship of these highly altered rocks to the ca. 610 Ma monzogranite/syenogranite is uncertain, but their close spatial association suggests that they may be of similar age.

Quartz alkali-feldspar syenite gradational to alkali-feldspar syenite occurs in the southwestern part of the mountain (Fig. 2). These rocks are typically red, medium-to coarse-grained, and highly magnetic (measured susceptibility values up to 63 × 10 ^-3 SI units). They are composed of euhedral to subhedral, coarsely perthitic alkali feldspar and varying amounts of interstitial quartz (Fig. 3c). Dark green euhedral amphibole, altered fayalite(?), and chlorite (aſter biotite?) locally constitute up to 10% of the rock. Magnetite is abundant (up to 10%), much of it replacing what may have been fayalite crystals, by comparison with similar alteration in syenite in the West Barneys River plutonic suite of the Antigonish Highlands in which elict fayalite has survived (Escarraga 2010). Titanite and apatite are additional accessory phases.

Quartz alkali-feldspar leucosyenite occurs in the central part of Porcupine Mountain, west of the metasedimentary unit (Fig. 2). It consists mainly of perthitic alkali feldspar and quartz, and mafic minerals are less abundant than in the quartz alkali-feldspar syenite to alkali-feldspar syenite (Fig. 3d). It also tends to be finer grained, and interstitial granophyric texture is typically present. Magnetite is an abundant accessory mineral, and this unit also is magnetic (measured susceptibility values up to 40 × 10^-3 SI units). The leucosyenite intruded the coarse-grained alkali-feldspar syenite, based on what appears to be a chilled margin in the leucosyenite at a sharp contact observed in the southwestern part of the area (Fig. 3e).

Alkali-feldspar granite gradational to syenogranite forms much of the southeastern part of Porcupine Mountain, and is in faulted contact with the metasedimentary and metavolcanic units, and locally with older granitic rocks (Fig. 2). These rocks are typically more deformed than those in the western belt and in places discrete mylonite zones are present, which appear to correspond to the “gneissoid” beds of Fletcher (1881). In these zones, few of the primary igneous features are preserved, but the rocks appear to have been coarse grained, with abundant quartz and alkali feldspar (Fig. 3f). The mylonite contains abundant quartz ribbons and K-feldspar augen in a fine-grained cataclased quartz-feldspar groundmass. Local areas of quartz alkali-feldspar syenite similar to that in the western area are also present but could not be mapped out as separate units. Overall, these rocks are much less magnetic than the syenitic units in the western part of the complex; the highest measured susceptibility is 9 × 10 ^-3 SI units, and most samples have susceptibilities of less than 1. Gabbroic rocks occur in the northwestern part of the unit, intruded by small cross-cutting dykes of alkali-feldspar granite. Although the rocks are poorly exposed in that area, the relationships and rock types are similar to those described in the Poor Farm Brook composite pluton in the Antigonish Highlands (White and Archibald 2011; White et al. 2011; Archibald 2012).

Mafic dykes occur in all units of the Cape Porcupine Complex, as well as in the surrounding Carboniferous rocks (e.g., White and Barr 1998a; Giles et al . 2010). They are typically 1 to 2 m wide, although the largest observed dyke is about 5 m wide, and are fine to medium grained with well developed chilled margins. Dykes vary from undeformed with relatively unaltered plagioclase and pyroxene, to intensely altered and dominated by actinolitic amphibole and/or chlorite instead of clinopyroxene. Some dykes cut mylonitic fabrics in the metasedimentary and metavolcanic units.

The monzogranite/syenogranite samples (referred to collectively as the granitic suite) range in SiO₂ from about 70 to 80%, with one monzogranitic sample containing 67% (Fig. 5). As a group, they are characterized by negative correlations between SiO₂ and TiO₂, Al₂O₃, Fe₂O₃^t, MgO, CaO, and P₂O₅, whereas Na₂O and K₂O are more scattered.

The quartz alkali-feldspar syenite/alkali-feldspar syenite, quartz alkali feldspar leucosyenite, and alkali-feldspar granite/syenogranite samples (referred to collectively as the syenitic suite) also have SiO₂ between 70–80%, except for two alkali-feldspar syenite samples (CP00-26 from the western body and CP09-79A from the eastern body) with 62% and 66.7% SiO₂, respectively). Both of these samples are dominated by K-feldspar and contain very little modal quartz, consistent with their relatively low SiO₂ content compared to other samples. The syenite sample from the western body also contains some separate plagioclase grains and a higher abundance of mafic minerals (including magnetite) than other syenitic samples, consistent with its higher Al₂O₃, Fe₂O₃^t, and CaO (Fig. 5b, c, e). With the exception of these two samples, the other analyzed samples show overall similarity in major element compositions. Compared to the granitic suite, they tend to have lower Al₂O₃, CaO, and Na₂O, and higher Fe₂O₃^t and especially K₂O (Fig. 5).

The metarhyolite samples also have high SiO₂ contents overlapping with those in both the granitic and syenitic suites. Their overall chemical features are more similar to those of the syenitic suite than to the granitic suite, including low Al₂O₃, CaO, and Na₂O and high Fe₂O₃^t and K₂O, although one sample has low K₂O.

Although designed for volcanic rocks, a plot of Zr/TiO₂ against Nb/Y is useful for further illustrating the differences between the syenitic and granitic suites (Fig. 6a). Even with similar SiO₂ contents, the syenitic suite has higher Nb/Y than the granitic suite, and the samples plot in the alkalic fields as the intrusive equivalents of volcanic rocks such as trachyte and alkali rhyolite. The metarhyolite samples also plot in those fields. In contrast, the granitic samples are subalkalic and plot mainly in the rhyolite +dacite fields. On a plot of FeO^t/FeO^t + MgO ratio against SiO₂, the granitic, syenitic, and metarhyolite samples all plot in the ferroan and A-type granite fields, although the granitic samples tend to have lower FeO^t/FeO^t + MgO ratios and some cross into the magnesian granite field of Frost et al. (2001). On diagrams designed to characterize A-type granitoid rocks compared to I- and S-type, the syenitic and metarhyolite samples have high Ga/Al ratios and elevated Zr and Nb contents typical of A-type granitoid rocks (Fig. 7a, b). In contrast, the granitic samples plot in the field for I- and S-type granitoid rocks. The differences are also apparent on the tectonic setting discrimination diagrams (Fig. 7c, d), on which most of the syenitic and all the metarhyolite samples plot in the within-plate granite fields, whereas the granitic samples lie in the volcanic-arc granite field or slightly into the within-plate granite field, as is typical of evolved I-type granitoid suites.

On all of these diagrams, the granitic samples are similar to the ca. 610–605 Ma plutonic rocks with similar SiO₂ contents in the Antigonish Highlands, shown as fields on the diagrams. The Antigonish Highlands plutons are part of an expanded I-type granitoid suite that ranges in composition from dioritic to granitic and likely formed in a continental margin subduction zone (Escarraga 2010; Murray 2011). In contrast, the syenitic samples from the Cape Porcupine Complex, as well as the metarhyolite samples, show similarity to the Ordovician West Barneys River plutonic suite of the Antigonish Highlands. The tectonic setting for the West Barneys River suite is uncertain, but they are A-type granitoid rocks formed in an extensional setting (Escarraga et al. 2012). Hence the chemical data suggest that the igneous units in the Cape Porcupine Complex are correlative with units of similar ages in the Antigonish Highlands.

Rare-earth element data are not available for all of the samples from the Cape Porcupine Complex, but existing data are consistent with this proposed correlation (Fig. 8). Two granitic samples show moderate light REE enrichment and flat-heavy REE patterns, similar to the field defined by samples from ca. 610–605 Ma plutons in the Antigonish Highlands (Fig. 8a; Table 3). Four syenitic samples and one metarhyolite sample show similar chondrite-normalized patterns, with light REE enrichment (200 to 400 times chondritic values) and gently sloping heavy REE at 20 to 40 times chondritic values, and lie in the field defined by felsic samples from the West Barneys River suite (Fig. 8b). The strong negative Eu anomalies in all of these samples are consistent with feldspar fractionation, documented to be a major factor in magma evolution in both the ca. 610–605 Ma and Ordovician suites in the Antigonish Highlands (Escarraga 2010; Archibald 2012; Escarraga et al. 2012).

The four analyzed leucodiorite samples are low in SiO₂ (40–45%), suggesting that they may originally have been leucogabbro. Their high Al₂O₃ and CaO are consistent with the abundance of plagioclase, although its original composition cannot be determined due to pervasive alteration to epidote, zoisite/clinozoisite, and high MgO and Fe₂O₃^t with the abundance of chlorite and amphibole. These highly altered rocks are notably low in TiO₂ and P₂O₅ . Their distinctly different compositions suggest that they are not directly related to the other igneous units in the Cape Porcupine Complex.

Four mafic dykes, one cutting each of the metasedimentary and metarhyolite units and two in the ca. 610 Ma monzogranite/syenogranite unit, were also analyzed. All four dykes contain about 48% SiO₂ and show chemical similarities to one another and to gabbroic samples with similar SiO₂ contents in the West Barneys River plutonic suite (Fig. 5). Especially notable are the high TiO₂ and Fe₂O₃^t (Fig. 5a, c). Like the West Barneys River gabbroic samples, the dykes are tholeiitic transitional to alkalic based on Nb/Y ratio (Fig. 6a). On tectonic setting discrimination diagrams for mafic rocks, they plot in the within-plate basalt fields, and also in the field defined by mafic samples from the West Barneys River suite (Fig. 9a, b). These chemical similarities to gabbroic rocks in the West Barneys River plutonic suite suggest that the analyzed mafic dykes may also be Ordovician. If so, they indicate a minimum age for the metasedimentary and metarhyolite units, and for the mylonitic deformation as the mafic dykes in those units are not mylonitic.