Petrology of the Pawtuckaway Mountains, New Hampshire

Abstract

The Pawtuckaway Mountains are located in Deerfield and Nottingham townships, Rockingham County, in southeastern New Hampshire. The rocks of the area constitute an intrusive complex belonging to the White Mountain magma series which was injected into the Winnipesaukee gneiss (?) of the New Hampshire magma series. The complex consists of a variety of diorites and monzonites with minor amounts of gabbro. Aplitic, monzonitic, and lamprophyric dikes cut all the major rock types. Some aplite dikes are younger than lamprophyre dikes which they intersect, but the age relationships of the monzonite dikes are unknown. The oldest units of the complex are the gabbros followed in order by the diorites, monzonites, and dike rocks. Diorites and monzonites constitute the major units of the complex and are believed to have formed from one parent magma differentiating at depth, although the variations in the diorite body may have resulted from differentiation in place. Cauldron subsidence or ring-dike injection is favored as the method of emplacement on the basis of the circular plan of the complex, the arcuate pattern of the monzonite bodies, and the general structural and petrographic similarity to other ring-dike areas of the White Mountain magma series.