Compositional Variation in the Tunk Lake Granite Pluton, Southeastern Maine
The Tunk Lake granite pluton is exposed over a circular area of 70 square miles in southeastern Maine. It intrudes the Ordovician (?) Ellsworth Schist, the middle Paleozoic Bays-of-Maine gabbroic-granitic complex, and Middle Devonian biotite granite and quartz monzonite. From the margin of the pluton to the core, six concentric gradational zones are recognized: 1 magnetite-aegirine augite granite chill zone; II hornblende-aegirine augite granite; III hornblende granite; IV hornblende-biotite granite; V biotite granite; VI biotite quartz monzonite.

Modal and chemical data for the pluton show regular compositional variation from margin to core. Quartz and oligoclase increase inward, whereas total feldspar, perthite, percentage of albite in perthite, and total mafics and accessories decrease. The dominant mafic mineral varies inward according to the series magnetite-aegirine augite-hornblende-biotite. SiO2 increases toward the core of the pluton paralleling the quartz variation, whereas Al2O3, Na2O + K2O, FeO + Fe2O3, and CaO decrease corresponding to the variation in total feldspar and total mafics and accessories.

Emplacement of the pluton at an assumed depth from one to three miles occurred by piecemeal stoping along marginal zones of ring fracturing and by cauldron subsidence or stoping of large blocks in the central zones. The magma at the time of intrusion is assumed to have had a composition, indicated by a calculated average for the pluton, approximately equal to the biotite granite of zone V, a volatile content from 2 to 3 percent, and a temperature of 800° C.

Crystallization began in the outer portions of the pluton with precipitation of magnetite and aegirine augite followed by sodium-rich alkali feldspar and quartz. Exsolution of feldspar was followed by the partial replacement of primary mafic minerals by hornblende and biotite. During crystallization, silica-rich aqueous fluids migrated from the margins to the center of the pluton. The marginal rocks were left quartz-poor, and feldspar- and mafic-rich. The core rocks became quartz-rich. In the central zones additional water lowered crystallization temperatures allowing oligoclase, hornblende, and biotite to form as primary minerals and also facilitated late-stage recrystallization. Recrystallization fluxed by hydrous fluids accounts for fine-grained intergranular material in the central granites and also for sheets, lenses, and irregular bodies of fine-grained rock throughout the pluton.

The pluton is related to several circum-Atlantic groups of intrusions which are similar in size, shape, petrography, and emplacement. Mineralogical and compositional variations in the granitic rocks of the White Mountain plutonic-volcanic series of New Hampshire and the “Younger Granites” of Northern Nigeria are similar to trends observed in the zones of the Tunk Lake pluton. In the Tunk Lake body, the variations occur within a single, gradational intrusion with a relatively simple crystallization history which may closely parallel those of the White Mountain and Nigerian intrusive complexes.