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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mi mathvariant="bold-italic">N</mml:mi> <mml:mi mathvariant="bold-italic">d</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>373</mml:mn> </mml:mrow> </mml:mover> </mml:math> = −1.2 to +1.5) similar to the host Port Mouton Pluton (PMP). The Group 2 enclaves are Si-rich (SiO 2 = 74.3−79.7 wt%) and Al-poor (Al 2 O 3 = 10.0−12.2 wt%). The youngest zircons are Early Cambrian (529 ± 6 Ma) and the rocks are isotopically different ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mi mathvariant="bold-italic">N</mml:mi> <mml:mi mathvariant="bold-italic">d</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>530</mml:mn> </mml:mrow> </mml:mover> </mml:math> = −6.7 to -6.4). MELTS modeling (P = 3 kbar; ƒO 2 = NNO = +1; H 2 O = 2.00 wt%) shows that fractional crystallization can generate the range of PMP rock compositions. The parental magma of the PMP was likely 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.