Silicic microgranular enclaves of the Late Devonian Port Mouton Pluton: petrological implications for a Peripheral Pluton of the Meguma Terrane, Northern Appalachians

The Meguma Terrane of the Northern Appalachians is primarily composed of Cambro-Ordovician metapelites and metapsammites that were intruded by Late Devonian (380–360 Ma) granitic plutons. The Port Mouton Pluton (373 ± 1 Ma) is composed of tonalite and monzogranite, contains silicic microgranular enclaves and is cross-cut by lamprophyric dykes. The silicic enclaves are peraluminous, magnesian and calc-alkalic to calcic. The enclaves can be divided into two groups. The Group 1 enclaves are temporally, compositionally and isotopically ((Formula presented.) = −1.2 to +1.5) similar to the host Port Mouton Pluton (PMP). The Group 2 enclaves are Si-rich (SiO₂ = 74.3–79.7 wt%) and Al-poor (Al₂O₃ = 10.0–12.2 wt%). The youngest zircons are of Early Cambrian age (529 ± 6 Ma) and the rocks are isotopically different ((Formula presented.) = −6.7 to −6.4). MELTS modelling (P = 3 kbar; ƒO₂ = NNO = +1; H₂O = 2.00 wt%) shows that fractional crystallization can generate the range of PMP rock compositions. The parental magma of the PMP was probably derived by partial melting of sub-Meguma Terrane rocks in a lower crustal hot zone over a 20–30 myr time period. The Group 2 enclaves are xenoliths derived by melting of metasedimentary upper crustal rocks of Avalonia.