The Vinalhaven intrusive complex consists mainly of coarse-grained granite, inward-dipping gabbro-diorite sheets, and a fine-grained granite core. Small bodies of porphyry occur throughout the coarsegrained granite. The largest porphyry body (roughly 0Á5 km by 2Á5 km) occurs with coeval gabbro, hybrid rocks, and minor finegrained granite in the Vinal Cove complex, which formed during the waning stages of solidification of the coarse-grained Vinalhaven granite. Porphyry contacts with surrounding coarse-grained granite are irregular and gradational. Compositions of whole rocks and minerals in the porphyry and the coarse-grained granite are nearly identical. Neighboring phenocrysts in the porphyry vary greatly in degree of corrosion and reaction, indicating that the porphyry was well stirred. Thermal rejuvenation of a silicic crystal mush by a basaltic influx can explain the composition and texture of the porphyry. Comparable rejuvenation events have been recognized in recent studies of erupted rocks. Weakly corroded biotite phenocrysts in the porphyry require that hydrous interstitial melt existed in the granite during remelting. Field relations, along with thermal calculations, suggest that cooling and crystallization of coeval mafic magma could have generated the porphyry by thermal rejuvenation of granite crystal-mush containing about 20% melt. Field relations also suggest that some of the porphyry matrix may represent new felsic magma that was emplaced during remelting.

In the field, pumice clasts were taken from four unconsolidated ignimbrite deposits and a welded tuff in the Roseau Valley (Fig. DR1). The sample location distribution was controlled by exposures, often limited to road cuts, but spans the length of the Roseau Valley. The most distal sample was from a 20 m thick deposit in the city's Botanical Gardens (King's Hill), not previously documented, ~8 km from the summit of Micotrin. The basal St. Aromont sample is from a 50 m thick pyroclastic sequence previously described by Howe et al. (2014) as part of the Link Flow sequence, ~8 km from Micotrin. The Casso sample was located on the southern ridge flank of the Roseau River, 6 km from Micotrin at an elevation of 250 m. The Casso deposit is a ~15 m thick pyroclastic deposit, dominated by ash and has not previously been studied. The Trafalgar sample was from a 2 m boulder of welded tuff at the base of Trafalgar Falls, a ~200 m thick sequence, which is the most proximal deposit to the proposed vent, ~2 km from Micotrin.

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