Abstract

Study of the Croydon and Unity domes sheds considerable light on the mode of intrusion of the Oliverian magma series in western New Hampshire. Each dome consists of an igneous core (late Devonian) surrounded by concentric belts of older (Ordovician?, Silurian, and early Devonian) metamorphosed sedimentary and volcanic rocks. The foliation of the plutonic cores is chiefly a primary flow structure as indicated by the parallel arrangement of inclusions, the parallelism between foliation and contacts, and the marked development of foliation near contacts. The bedding-plane schistosity of the surrounding metamorphic rocks is to be attributed to effects of the magma because (1) it is confined to the rocks immediately surrounding the igneous cores; (2) it parallels the contacts of the igneous cores; (3) it is relatively flat-dipping over large areas and thus rules out regional compression as a possible explanation. Several lines of evidence show that the numerous Oliverian domes are laccoliths: (1) The structure of each dome resembles that of a laccolith; (2) all the domes have roofs at the same horizon; (3) the intense folding and crumpling of the metamorphic rocks between domes indicate crowding of adjacent and isolated igneous cores; (4) laccolithic intrusion best explains the bedding-plane schistosity of the surrounding metamorphic rocks; (5) laccolithic intrusion best explains the foliation of the igneous cores. The eastern contact of the Mt. Clough pluton (New Hampshire magma series) is briefly considered, and the mode of intrusion of the body is explained.

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