Petrogenesis of the Kinsman Intrusive Suite: Peraluminous Granitoids of Western New Hampshire

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Abstract
The Kinsman Intrusive Suite occurs in six major plutons of western New Hampshire, covering a total area of 2240 km2. It is an Acadian-age syntectonic gneissic S-type peraluminous granitoid, ranging in composition from quartz diorite to granite. Much of the Kinsman is characterized by very large (up to 120 mm in maximum dimension) megacrysts of alkali feldspar, but the bulk chemistry of the rocks indicates that these cannot be phenocrysts crystallized from initially homogeneous melts. Locally, there is abundant (20 per cent) almandine-rich garnet, and graphite is a common accessory. In contrast to the unannealed orthoclase in surrounding metapelites, the alkali feldspar of the Kinsman has, for the most part, inverted to maximum microcline. The garnets have core temperatures in the range of 800 to 900 °C, and are pseudomorphed by, or show reaction rims to, biotite. Plagioclase commonly shows zoning, some of it oscillatory. These features are magmatic in nature, and argue against the conclusions of previous investigators that the mineralogy and textures of the rock are due to regional metamorphism of a previously-crystallized two-mica granitoid which has undergone prograde reactions such as: muse + bio + 3 qtz ↔ 2 Kfs + gar + 2H2O. The intrusives have also produced recognizable contact-metamorphic features in the wallrocks and are probably coeval with the dominant M2 Acadian metamorphism. Major element analytical data for the Kinsman suite has been examined by least squares mixing-model and extended Q-mode factor analysis. These calculations, supported by consideration of REE data, suggest that the most likely origin for the Kinsman magmas is by deep-crustal anatexis of slightly calcareous metapelites, and involves a reaction such as: bio + Al2SiO5 + qtz + feldspars ↔ gar + cord + Kfs + plag + melt. In non-calcareous pelites this reaction produces a water-undersaturated peraluminous melt at temperatures above 700 °C, and allows for the early crystallization or recrystallization of K-feldspar, plagioclase, and garnet in a crystal-liquid mush or migma. Geochemically, garnet + plagioclase are treated as restite, and a minimum-melt granite as the magma in the Q-mode and mixing-model calculations. The variability in chemistry of the Kinsman Intrusive Suite is best explained on the basis of mixing of leucogranitic anatectic melts with garnet-plagioclase restitic material and a quartz-feldspar-sillimanite-biotite rock, but only very slightly affected by crystal settling.

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