STRUCTURE AND PETROLOGY OF THE LOVEWELL MOUNTAIN QUADRANGLE, NEW HAMPSHIRE

Abstract

The Lovewell Mountain quadrangle in southwestern New Hampshire is underlain by metasedimentary rocks of the lower Devonian Littleton formation and plutonic rocks of the New Hampshire magma series, which is probably of late Devonian age, The lower portion of the Littleton formation in the area consists mainly of well-bedded sillimanite schist, pseudo-sillimanite schist, and mica-quartz schist, whereas the upper portion consists of poorly banded gneisses. The thickness of the Littleton formation exposed in the area is probably of the order of 20,000 feet. The plutonic rocks range from quartz monzonite to quartz diorite but granodiorite predominates. The area is on the eastern flank of a large anticline. The foliation in the metasedimentary rocks is mainly of the axial plane type and is parallel to bedding except at the noses of folds. The plutonic bodies are generally concordant although some parts of the bodies cross-cut several thousand feet of the adjacent metasedimentary rocks. Small scale discordant relationships are common along the contacts of the Kinsman quartz monzonite. The metasedimentary rocks have been subjected to high-grade metamorphism. Orthoclase formed at the expense of muscovite in a large part of the area in response to the high temperatures which prevailed during the peak of metamorphism, but the orthoclase altered to muscovite and quartz under declining intensity conditions during the final stages of metamorphism in most of the eastern two-thirds of the quadrangle. Changes in chemical composition during metamorphism were apparently not important, although potash was probably added to the pseudo-sillimanite schists and soda and lime were added to a few of the paragneisses. Most of the bodies of plutonic rocks are intrusive, but the gradational nature of the contacts in a few places suggests that some of the plutonic rocks may have undergone little movement. Definite conclusions regarding the source of the granitic material could not be drawn. Magma rising from great depth may have entered certain favorable horizons to form large sill-like masses. The mineralogical and textural similarity between the plutonic rocks and some of the metamorphic rocks suggests that the granitic material may have originated at a comparatively shallow depth by the solution or partial solution of favorable beds. The mobilized material may have moved along approximately the same stratigraphic horizon at which it originated to form the intrusive sheets.