Abstract

The Pliny region, in the northern portion of the Mt. Washington quadrangle, New Hampshire, contains some of the finest examples of ring dikes described in North America. These arcuate bodies intrude older gneiss, quartzite, and quartz diorite, and are, in turn, cut by later granitic stocks. The dikes and stocks, which are grouped about two distinct centers, are composed of differentiates of the White Mountain magma series, which in this area range from quartz monzodiorite to granite. Two of the ring dikes are strikingly expressed in the topography as arcuate mountain ridges.

All the ring dikes and some of the stocks are thought to have originated by cauldron subsidence or ring-fracture stoping. However, this mechanism operated differently for different dikes. In a composite ring dike in the southwestern part of the area, an arcuate zone of intense fracturing developed, resulting in the subsidence of a large cylindrical or domical block of country rock Numerous small dikes of hastingsite-quartz syenite penetrated the northern part of this zone to form a more or less solid dike of hastingsite-quartz syenite. Later, pink biotite granite intruded the southeastern part of the fractured zone, forming a network of irregular dikes.

The Crescent Range ring dike of granite porphyry, however, is a symmetrical crescent with regular, broadly sweeping boundaries. This suggests that it was formed by intrusion en masse along a clean, sharp ring fracture. The dense groundmass of the rock implies rapid cooling.

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