CRITERIA FOR THE MODE OF EMPLACEMENT OF THE ALKALINE STOCK AT MOUNT MONADNOCK, VERMONT

Abstract

The alkaline stock at Mount Monadnock, Vermont, described briefly by Wolff (1929), has been restudied in detail. Its petrography and structure are discussed here and conclusions are drawn as to its mode of emplacement. The stock consists of plutonic and hypabyssal rocks which intrude folded Ordovician (?) schist and quartzite. The longer axis, trending north-northwest across the strike of the country rock, is 3 miles long, and the shorter one about 2½ miles. Essentially the stock consists of quartz syenite, but it encloses a long arcuate mass of older essexite and transition rock, and along its eastern edge are later intrusions of granite. Late dikes of various compositions cut the plutonics and surrounding metamorphic rocks. The following facts have been established: (1) The igneous rocks are typical representatives of the White Mountain magma series (Mississippian?), quite lacking in foliation and lineation. (2) The stock is discordant and has an elliptical ground plan. (3) In detail the boundary is irregular and characterized by abundant dikes and xenoliths. (4) The igneous rocks make sharp contacts with the metamorphosed country rock. (5) The arcuate mass of older essexite is undoubtedly a screen. (6) The small bodies of late granite resemble ring dikes. (7) The stock is cut by prominent sets of steeply dipping radial and tangential joints. (8) The late dikes show radial and tangential patterns. (9) Along the northern and southern margins of the stock the country rock shows strikes and dips which differ from the regional ones. From these criteria it is concluded that the plutonic rocks have invaded the crust by cauldron subsidence accompanied by the stoping of large arcuate slabs and smaller blocks from the walls of the magma reservoir