178 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Rasic dikes and sills have attained equilibrium under the new metamor- 

 phic conditions. 



Rillings regards the main alteration to have occurred after the Northey 

 Hill thrust and during the intrusions of the New Hampshire magma 

 series. Then the Ammonoosuc thrust brought different metamorphic 

 zones into sharp contact with each other. Also in certain places retro- 

 grade metamorphism set in with the formation of much chlorite. 



The cause of the metamorphism is apparently the intrusions of the vari- 

 ous plutons of the New Hampshire magma series. Northwest of the 

 Ammonoosuc thrust where metamorphism is least, the intrusions of the 

 New Hampshire magma series are absent except that a few small bodies 

 of the Bethlehem gneiss and Kinsman quartz monzonite appear. Billings 

 points out that, as intrusions are common eastward to the Maine border, 

 and as the sedimentary rocks almost invariably are recrystallized to high- 

 grade metamorphic rocks, there must be a causal connection between the 

 increase in metamorphism and these intrusions. Not only is there a gen- 

 eral increase in the intensity of metamorphism toward the area where 

 igneous intrusions are most abundant, but there is an increase locally 

 toward individual bodies. Such high-grade zones surrounding intrusive 

 masses are not well defined in the map of Fig. 11.22, but it is suggested 

 that the contact metamorphic zones vary in width greatly, and that cer- 

 tain zones betray the presence of unexposed plutons. 



Mechanics of Instrusion 



Introduction. The post-tectonic White Mountain magma series is 

 characterized by ring-dikes, stocks, and a batholith (Billings, 1945). The 

 ring-dikes, most of which range in composition from monzonite to quartz 

 syenite, intruded arcurate and circular vertical fracture zones by piece- 

 meal stoping and related mechanisms. Cauldron subsidence, although 

 associated with some ring-dikes, is not essential for their intrusion. The 

 stocks of the White Mountain magma series were emplaced by under- 

 ground cauldron subsidence. 



The New Hampshire magma series, emplaced during the Acadian 

 ^orogeny, occurs chiefly as great sheets, lenses, and stocks, forcefully 

 injected into the older formations. 



Ring-Dikes. Altogether, 36 ring-dikes associated with the White 

 Mountain magma series have been discovered in New Hampshire. A ring- 

 dike complex is a structural unit containing one or more ring-dikes. Ac- 

 cording to Billings ( 1945 ) : 



There are five ring-dikes at Mt. Tripyramid, four each in the Pliny region and 

 the Franconia quadrangle, and six in the Belknap Mountains, although the six 

 separate intrusions could be considered to belong to two composite ring-dikes. 

 Ring-dikes have also been described from adjacent areas in Quebec and Maine. 



Complete ring-dikes that encompass 360 degrees are rare, but the ring-dike 

 of the Ossipee Mountains and some of those on Mt. Tripyramid are of this type. 

 Most ring-dikes are arcuate in plan and those in New Hampshire encompass, 

 on the average, 170 degrees of the total possible 360 degrees. The average 

 radius of ring-dikes in New Hampshire, measured from the outer margin of the 

 ring-dike to its center of curvature, is three miles. A ring-dike composed of 

 Albany quartz syenite in the Franconia quadrangle has a radius of 9.2 miles 

 and is one of the largest known anywhere in the world. The smallest ring-dike 

 in New Hampshire, with a radius of only 0.8 mile, is on Mt. Tripyramid. The 

 average width of ring-dikes in New Hampshire is 1900 feet. The arcuate body 

 of amphibole granite in the southern part of the Franconia quadrangle is 14,000 

 feet wide, but this may not be a true ring-dike. 



Inside some of the ring-dikes are accumulations of extrusive rocks, 

 known as the Moat volcanics. They are never found outside the ring-dike. 

 The volcanics also have the same composition as the ring-dike within 

 which they have subsided. 



The Moat volcanics are at least 10,000 feet thick and rest with pronounced 

 angular unconformity on the older metamorphic rocks of the Litdeton forma- 

 tion and the plutonic rocks of the New Hampshire magma series. It is almost 

 always impossible to determine the attitude of the Moat volcanics, because 

 many of the pyroclastic rocks and lavas are devoid of bedding and flow struc- 

 ture. Available data indicate, however, that near the ring-dikes the volcanics 

 are essentially vertical, but toward the center of the complex the dips become 

 progressively less [Fig. 11.23]. 



Unfortunately, precise data concerning the amount of subsidence are diffi- 

 cult to obtain in New Hampshire. The key horizon used for such studies is the 

 base of the Moat volcanics. It is apparent from Fig. 11.23 that the center of the 

 subsided block has settled 10,000 feet relative to the margins of the block near 

 the ring-dike. Moreover, the edge of the subsiding block just inside the ring-dike 

 has apparendy settled at least 5,000 feet relative to the rocks some distance out- 

 side of the ring-dike. Therefore, the center of the subsided block has dropped 

 at least 15,000 feet relative to the rocks some distance outside of the ring-dike. 



