Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine
Metamorphism and Migmatization
Metasedimentary and metavolcanic rocks of the Casco Bay Group in southwestern Maine have been metamorphosed in a low pressure Buchan-type metamorphic facies series ranging from middle greenschist to upper amphibolite facies. Within the map sheet the metamorphic grade ranges from staurolite grade to sillimanite + K-feldspar grade (Figure 51). Most of the area is at sillimanite or sillimanite + K-feldspar grade. Lower grades are seen in the Portland area to the west.
Within the staurolite + andalusite zone of metamorphism at the western edge of the map sheet (Figure 51) the typical mineral assemblage in pelitic rocks is muscovite-biotite-quartz- oligoclase-staurolite-andalusite. Chloritoid is occasionally present. Almanditic garnet is present but sparse. Andalusite occurs in two habits, as large poikiloblasts, and as idioblastic to hypidioblastic graphite-choked porphyroblasts commonly with retrograde muscovite rims. In this zone, short milky quartz lenses 1 to 4 meters in length commonly have pink andalusite masses with abundant muscovite. Occasionally within the andalusite there are idiobasts of blue corundum (usually less than 2 mm in length) and staurolite. Sillimanite is very sparse and is usually isolated from contact with andalusite. Where sillimanite occurs in the quartz veins, it can usually be found in minute amounts within biotite grains in the host schist, indicating that the grade of metamorphism is actually slightly higher than is suggested by megascopic occurrences of the critical indicator minerals in hand specimen.
Within metapelites in the staurolite + sillimanite zone, andalusite is absent except for occasional remnants in the quartz veins. Sillimanite is recognizable in hand specimen. In the mafic rocks of the Spring Point Formation, hornblende, andesine, and garnet are typical minerals present. In rocks at Small Point at the southern tip of Phippsburg (Figure 16), andalusite coexists, probably metastably, with sillimanite. Large twinned staurolite porphyroblasts are locally abundant. Other minerals characteristic of staurolite + sillimanite grade metamorphism in metapelites are biotite, plagioclase, cordierite, garnet, and muscovite. Muscovite occurs both as matrix mica, forming the schistosity, and as large (1 to 3 cm long) pseudomorphs after andalusite or possibly untwinned staurolite (Figure 12; Lang and Dunn, 1990; Grover and Lang, 1995). In places, fresh staurolite forms large (1 to 2 cm long) twinned porphyroblasts, and andalusite occurs as large poikiloblasts up to 4 cm in diameter, engulfing all other phases.
In the sillimanite zone, staurolite is not present. The narrow zone marking the transition from staurolite + sillimanite zone to sillimanite zone can be seen along the northwest part of the Harpswell Neck shoreline at Barnes Point (Figure 5). Within the sillimanite zone the typical assemblage in the metapelites is muscovite + quartz + sillimanite + biotite + plagioclase ± garnet ± cordierite. In mafic rocks pale hornblende (pale green in thin section), plagioclase, cummingtonite, and gedrite are common phases. Calc-silicate assemblages include pale bluish green hornblende, diopside, sphene, grossularite, clinozoisite, microcline, quartz, and rarely, scapolite.
The sillimanite + K-feldspar zone coincides closely with the development of migmatites. Migmatization is most extensive in the metapelites, where original schistosity is obliterated and the rock takes on the typical gneissic structure of migmatites. K-feldspar commonly occurs as small grains in the shape of bow ties with rounded ends. In the migmatite areas, amphibolites, quartzo-feldspathic and calc-silicate assemblages of the Sebascodegan and Bucksport Formations are relatively free of migmatite, except for the extensive introduction of even-walled pegmatite sills, stringers, and dikes. This contrast in migmatization is particularly well shown in the two members of the Cross River Formation exposed in the Boothbay anticline (Figure 33). The upper member (O
Ccrg) consisting of fine-grained quartz-plagioclase-biotite-garnet granofels is essentially unmigmatized, whereas the underlying rusty sillimanite-graphite gneiss is so thoroughly migmatized as to mask such structural features as relict bedding, schistosity, and lineations. It should be pointed out that the migmatite front within the Casco Bay Group, shown in Figure 51, is crudely concordant with the geometry of major F2 upright folds.
Rocks of the Falmouth-Brunswick sequence have been metamorphosed to sillimanite and possibly sillimanite + K-feldspar grade within the Bath map sheet. All rocks are significantly migmatized and injected by pegmatites. Evidence for the geobarometric environment during metamorphism of this sequence is lacking within the Bath map sheet, but along strike in the Freedom area, Pankiwskyj (1976, 1996) reports the presence of kyanite in the paragenesis of aluminous schist lenses in the Falmouth-Brunswick sequence, suggesting higher pressure Barrovian metamorphism for that area. The extent of the higher pressure metamorphism is not known. Pankiwskyj (personal communication, 1997) suggests that it may be related to premetamorphic thrust faulting with attendant thickening of the sedimentary pile, and probably is synchronous with minor and map scale F1 recumbent folding.
Introduction Central Maine sequence Falmouth-Brunswick sequence Casco Bay Group East Harpswell Group Fredericton sequence Megunticook sequence Benner Hill sequence Sequence uncertain Correlations Intrusives Structure Metamorphism Timing Minerals Acknowledgements References
Last updated on February 1, 2008