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 meta-pelites and meta-psammites 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 (εNd373 = –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 Early Cambrian (529 ± 6 Ma) and the rocks are isotopically different (εNd530 = –6.7 to –6.4). MELTS modelling (P = 3 kbar; fO2 = 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 million year time period. The Group 2 enclaves are xenoliths derived by melting of meta-sedimentary upper crust rocks of Avalonia.