Contact metamorphism of the Lucerne pluton, Hancock Co., Maine

Abstract

The Lucerne pluton intrudes rocks of the Penobscot formation Ordovician-Silurian (?)), a quartz-rich sulfidic pelite that contains muscovite, biotite, cordierite, andalusite, plagioclase, pyrrhotite and graphite outside the thermal aureole; and the Bucksport formation (=Vassalboro, Silvian-Devonian (?)), a calcareous, quartzofeldspathic pelite that contains chlorite, biotite, celadonitic muscovite, albite, and ilmenite outside the Lucerne aureole. Within the aureole, the Penobscot formation contains K-feldspar plus andalusite as the result of muscovite reaction with quartz. Corundum occurs at the immediate contact of the granite from the. reaction of the remaining muscovite. The Bucksport formation is recrystallized within the aureole to a purple and green gneiss. The gneissic banding is not present in the low grade calcareous rocks, and represents the segregation of biotite-rich and calc-silicate-rich bands. Vertical or sleepy dipping, the banding parallels both the regional strike and the intrusive contact, and is probably the result of both mechanical and chemical effects. The following sequence of assemblages (+ quartz) is found in the calcareous portions of the Bucksport formation as the Lucerne contact is approached: a) chl + bio + musc + cc + albite; b) bio + cc + plag (An₂₅₋₃₃); c) actinolite + cc + K-feldspar + plag (An₄₀); d) diopside + zoisite + sphene +cc+ plag (An₈₅₋₉₀). Interbedded pelites contain biotite + quartz + plagioclase + pyrite with corundum occurring at the igneous contact in quartz free beds. The mineral assemblages in the Lucerne aureole indicate a lithostatic pressure between 1000 and 3000 bars during metamorphism with temperatures between 700°C and 450°C. Isobaric univariant assemblages in the calc-silicate beds indicate buffering of H₂O/CO₂ fluids produced by prograde reactions. H₂O rich fluids that produced zoisite were probably associated with late stage crystallization of the Lucerne.