STRUCTURE AND PETROLOGY OF THE SUNAPEE QUADRANGLE, NEW HAMPSHIRE

Abstract

The rocks of the Sunapee quadrangle, in southwestern New Hampshire, are chiefly metamorphic in origin and range in age from upper Ordovician (?) to lower Devonian. The pre-Devonian rocks, constituted by the Ammonoosuc volcanics, the Clough formation, and the Fitch formation, are confined to the northwest part of the quadrangle. They are nearly completely separated from the rest of the area by the Grantham fault, which is probably Triassic in age. The structure northwest of this normal fault consists of an elongate dome of granodiorite gneiss flanked and overlain by concordant layers of amphibolite, biotite gneiss, quartzite, and schist. Immediately southeast of the fault is the Bethlehem gneiss. This mass occupies the central half of the quadrangle and is composed of well-foliated, “porphyritic” granitic gneiss. East of the gneiss is the Littleton formation. The western (older) portion of this belt of rocks consists of well-bedded mica-sillimanite schist and quartzite. To the east the rocks of this belt become somewhat coarser-grained and more gneissic. The most easterly portions contain little schist but consist predominately of mica and microcline gneiss. East of the Littleton formation is the Kinsman quartz monzonite. This rock resembles the Bethlehem gneiss but is generally coarser-textured and more poorly foliated. Metamorphism, during the late Devonian time, was not uniform throughout the quadrangle. In general zoning is well developed and ranges from the staurolite-kyanite subfacies, in the western part, to the lower portion of the granulite fades in the eastern part of the area. The structural relations and origin of foliation, lineation, major folds, tabular inclusions, large feldspar crystals, and pegmatites are considered in detail. It is concluded that the area southeast of the Grantham fault originally consisted of a fairly uniform sequence of pelitic rocks of the Littleton formation. As a result of regional metamorphism, these rocks were in large part converted to coarse-grained, “porphyritic” feldspathic gneiss (Bethlehem gneiss and Kinsman quartz monzonite). Recrystallization and reconstitution of the original sediments were the predominant processes involved. Certain constituents are considered to have migrated from distances up to many miles, but the source of this nomadic material may have been largely from the more deeply buried sedimentary rocks. In the Bethlehem gneiss, bedding planes controlled the development of foliation, and recrystallization converted much muscovite into large porphyroblasts of potash feldspar. Considerable sodium and some calcium were introduced to form oligoclase-andesine. In the Kinsman quartz monzonite, recrystallization was so severe that foliation was more poorly developed, grain size was greatly increased, and nearly all muscovite was converted to huge microcline porphyroblasts. Also contributing to the formation of micro-dine was the potassium released in the conversion of biotite in the granulite (eastern) facies. Sodium and calcium were introduced quite thoroughly to form oligoclase-andesine. In the eastern part of the area, considerable potassium was introduced to develop the phases very rich in microcline. The history of these metamorphic-metasomatic rocks is extremely complicated but certain processes and sequences are clear. There has been, apparently, a whole sale introduction of sodium, but potassium has been introduced only locally. Recrystallization and reconstitution were long-continued processes which began prior to the introduction of material. Minor bodies of magma were intruded, largely in the form of dikes and sills, in the later stages. These intrusives replaced much of the adjacent country rock and were themselves altered by the introduction of material which was highly mobile up to a very late stage.