Geology of the Andover Granite and surrounding rocks, Massachusetts

Abstract

Field and petrographic studies of the Andover Granite and surrounding rocks have afforded an opportunity for an explanation of its emplacement and crystallization. The investigation has contributed secondarily to an understanding of the geologic history of southeastern New England, particularly as it is revealed in the Lawrence, Wilmington, South Groveland, and Reading quadrangles of Massachusetts. The Andover Granite and Sharpners Pond Tonalite together comprise up to 90 percent of the Acadian(?) subalkaline intrusive series cropping out within the area of study. The subalkaline series locally invades a sequence of early to middle Paleozoic and possibly Precambrian metasedimentary and metavolcanic rocks. Much of the subalkaline series and most of the Andover Granite is confined between two prominent east-northeast trending faults or fault systems. The northern fault separates the mildly metamorphosed Middle Silurian(?) Merrimack Group on the north from a highly metamorphosed and thoroughly intruded Ordovician(?) sequence on the south. The southern 'boundary '' fault is a major structural discontinuity characterized by penetrative, diffuse shearing over a zone one-half mile or more in width. The magmatic nature of the Andover Granite is demonstrated by: (1) sharply crosscutting relationships with surrounding rocks; (2) the occurrence of tabular-shaped xenoliths whose long directions parallel the foliation within the granite and whose internal foliation trends at a high angle to that of the granite; (3) continuity with the clearly intrusive Sharpners Pond Tonalite; (4) the compositional uniformity of the granite as contrasted with the compositional diversity of the rocks it invades; (5) its modal and normative correspondence with (a) calculated norms of salic extrusives and (b) that of the ternary (granite) minimum for the system NaAlSi3O8-KAlSi3O8-SiO2. Orogenic granites, as represented by the Andover, contrast with post-orogenic granites, represented locally by the Peabody Granite, in their phase composition and texture. Unlike the Peabody, the Andover Granite is thought to have been thoroughly recrystallized through the unmixing of initially homogeneous phases with the concomitant development of extremely intricate, allotriomorphic textures. Textural relationships between potassium and plagioclase feldspars and among quartz and the two feldspars, suggest that the Andover Granite has evolved through exsolution of a single hypersolvus feldspar (or two coexisting subsolvus feldspars of only slightly disparate compositions) into discrete grains of plagioclase and potassium feldspar, much along the lines proposed by Tuttle (1952). A hypothesis is proposed for the origin of myrmekite whereby it is evolved indirectly through exsolution of a homogeneous, hypersolvus, calcalkali feldspar in the presence of a silica reservoir. Where the An 'molecule' is contained in the primary mix crystal, exsolution into potassium and plagioclase feldspar phases normally requires a paired exchange between Ca-Al and K-Si. Should the silicon requirements of the developing potassium feldspar be met by the matrix silica reservoir, the concomitantly evolving plagioclase may become stoichiometrically enriched in silicon and ultimately develop into myrmekite. Discrete unmixing of pure alkali feldspar proceeds through simple alkali ion exchange; ternary compostions high in An are more apt to fall initially in the two-feldspar field, thereby reducing the unmixing potential. General restriction of myrmekite to plagioclase of calcic albite to oligoclase composition is explained accordingly.