NEW ENGLAND APPALACHIAN SYSTEMS 



177 



Metamorphism 



All the sedimentary and metamorphic rocks have been deformed and 

 metamorphosed to various degrees. The metamorphism increases gen- 

 erally to the southeast, and three zones have been recognized by Billings, 

 namely, the low-grade, the middle-grade, and the high-grade. See map 

 of Fig. 11.22. 



The distinction between the zones is based primarily on their mineralogy. 

 The low-grade zone is characterized by chlorite, epidote, albite, sericite, and 

 dolomite; the middle-grade zone, by staurolite, garnet, hornblende, actinolite, 

 diopside, biotite, and intermediate and calcic plagioclase. The mineralogical 

 contrast between these two zones is striking. The high-grade zone differs from 

 the middle-grade zone chiefly in that sillimanite is present and staurolite is 

 absent or is in small crystals. Thus, if aluminous sediments are not present, it 

 is difficult or impossible to distinguish the middle-grade and the high-grade 

 zones on mineralogical criteria alone. In general, the high-grade rocks are 

 coarser than the middle-grade, but this criterion is difficult to apply, and, 

 wherever the rocks might belong to either of the two higher zones, they have 

 been assigned to the middle-grade zone. 



The change in the degree of metamorphism in a southeasterly direction is 

 readily apparent. The cumulative effect of these changes is so great that, for a 

 long time, rocks now known to belong to the same formations were believed to 

 be of very different ages. Whereas, northwest of the Ammonoosuc thrust the 

 rocks are dominandy sandstone, slate, calcareous slate, dolomitic slate, rhyo- 

 lite tuff, and greenstone, composed of such minerals as sericite, chlorite, albite, 

 dolomite, calcite, quartz, and epidote, to the southeast the rocks are mica schist, 

 calcite-biotite schist, actinolite-diopside granulite, biotite gneiss, and amphibo- 

 lite, composed of such minerals as biotite, garnet (almandite), staurolite, silli- 

 manite, actinolite, diopside, hornblende, calcite, quartz, and calcic plagioclase. 

 Moreover, there is a general coarsening in grain. These changes clearly repre- 

 sent progressive metamorphism toward the southeast, for the new rocks are 

 farther and farther removed mineralogically from the original rocks from which 

 they were derived. 



A number of the intrusive rocks are older than the regional metamor- 

 phism and were affected to different degrees. The Highlandcroft grano- 

 diorite was in the zone of low-grade metamorphism, and its original 

 andesine plagioclase has been replaced by albite-oligoclase, epidote, and 

 sericite. Green biotite, which is found in places as a shell around the horn- 

 blende, is of metamorphic origin. The Moulton diorite has been subjected 

 to low-grade metamorphism, and its original condition is much altered. 



7I - 4S" 



Scale of Miles 



LEGEND 



DRIFT AND 

 ALLUVIUM 



WHITE MOUNTAIN 

 MAGMA SERIES? 



NEW HAMPSHIRE 

 MAGMA SERIES 



OLIVERIAN 

 MAGMA SERIES 



HIGHLANDCROFT 

 MAGMA SERIES 



METAMORPHIC 

 ZONES 



HIGH-GRADE ZONE 

 WHERE SILLIMANITE 

 HAS RETROGRESSED 

 TO STAUROLITE 



HIGH-GRADE ZONE 

 (KATAZONE) 



_J 



middle-grace zone 

 (mesozone) 



TRANSITION 

 BETWEEN LOW- 

 GRADE ANO MIDDLE- 

 GRADE ZONES 



i 





LOW-GRADE ZONE 

 (EPIZONE) 



FOSSIL LOCALITIES 

 SOUTHEAST OF 

 AMMONOOSUC THRUS1 



Fig. 11.22. Metamorphic zones in the Littleton-Moosilauke area. Metamorphism is progressive 

 toward the southeast. Reproduced from Billings, 1937. 



