Determinación del pH

(1976) as compared to the Nashoba Formation of Hansen (1956), from Goldsmith (1991a)

Nashoba Formation (10,890 m)

Beaver Brook Member Sillimanite-muscovite-biotite gneiss and schist;

minor amphibolite, calc-silicate rock, and

Fort Pond Member Upper part – Calc-silicate rock; sillimanite-mica gneiss and amphibolite. Lower part – amphibolite-biotite gneiss and amphibolite;

calc-silicate rock; sillimanite gneiss and schist Nagog Pond Gneiss

Member

Sillimanite-muscovite-biotite gneiss;

subordinate amphibolite and amphibole gneiss Nashoba Brook Member Calc-silicate rock;

sillimanite-muscovite-biotite schist and amphibole-bearing gneiss Tophet Swamp Gneiss

Member

Sillimanite-(muscovite)-biotite gneiss; rare amphibolite

Spencer Brook Member Amphibolite and calc-silicate rock Billerica Schist Member Sillimanite- biotite -muscovite schist Bellows Hill Member Upper part – Amphibolite and subordinate

sillimanite-(muscovite)-biotite schist. Lower part – (sillimanite)-biotite-(muscovite) schist and gneiss; minor amphibolite

Boxford Member Amphibolite and amphibole-bearing gneiss and schist; minor sillimanite gneiss, calc-silicate gneiss; rare marble

Fish Brook Gneiss (1250 m) Mafic-poor biotite-quartz-sodic plagioclase gneiss; very minor biotite-rich gneiss and amphibolite

Shawsheen Gneiss (2600 m) Muscovite-biotite-oligoclase-quartz gneiss, locally sillimanite-muscovite-biotite schist near base

31 State bedrock map (Zen and others, 1983), as it is the only unit recognized in other areas (Goldsmith, 1991a). Bell and Alvord (1976) described the Nashoba Formation as pre-Silurian and determined a thickness of about 10,890 meters.

The Fish Brook Gneiss is fine- to medium-grained, white to light gray, distinctly foliated biotite-quartz-plagioclase rock (Castle, 1965). The rock displays a distinct

“swirly” foliation that is due to uniquely oriented biotite flakes. Bell and Alvord (1976) determined a thickness of about 1,250 meters for the Fish Brook Gneiss (Table 2-2).

The Shawsheen Gneiss is described by Bell and Alvord (1976) as largely sillimanite-bearing muscovite-biotite schist and gneiss. Lithologically, the Shawsheen Gneiss is similar to the Nashoba Formation. It was separated out from the Nashoba Formation because the Fish Brook Gneiss intervenes between the two formations (Goldsmith, 1991a). Bell and Alvord (1976) determined a thickness of about 2,600 meters for the Shawsheen Gneiss (Table 2-2).

Two members of the Nashoba Formation were identified and described in the Marlborough quadrangle to the southwest of the study area. DiNitto (1983) defined the base of the Nashoba as the schist member, which is overlain by the gneiss member. The schist member is described as being micaceous, sillimanitic and garnetiferous. The gneiss member is described as containing abundant muscovite, biotite, hornblende, plagioclase, and quartz interlayered with thin beds of sillimanite-muscovite-biotite schist. The schist member is described as being in fault contact with the upper member of the Marlboro Formation although that contact is not exposed in the Marlborough quadrangle.

32 Exposures

In the Maynard quadrangle the Nashoba Formation is defined as undifferentiated (Plate 1). Several different rock types were identified within the Nashoba Formation and described below. While some of these rock types are certainly similar and likely correlative with formations and members mapped by Bell and Alvord (1976) and Castle (1965), none are shown as individually mapped units on Plate 1. The reason for omitting such units from the map is because exposures were insufficient and hindered tracing detailed lithologies from one outcrop to the next so that they could be included as mappable units on the 1:24,000 scale map. It is important to note that the Nashoba Formation members mapped by Bell and Alvord (1976) are difficult to trace beyond their type area in the Billerica quadrangle (Hepburn personal comm., 2009). Bell and Alvord relied largely on the percentage of rock types within a unit when naming individual members. Possible correlations with Bell and Alvord‟s (1976) members will be discussed within the sections of the various rock types.

The Nashoba Formation occupies the northwest corner of the Maynard quadrangle, encompassing over half of the study area. The formation extends into the Hudson quadrangle to the west, the Billerica quadrangle to the north, and the Concord quadrangle to the east. As noted above, outcrop exposures are mostly small, discontinuous, and weathered knobs found in yards in residential areas, moss covered in wooded areas and soot covered railroad cuts (Figure 2-7). The largest exposures are found along the road cut at the interchange between Routes 2 and 27 in Acton.

33 FIGURE 2-7 Typical exposure of weathered biotite gneiss of the Nashoba Formation.

MY-01-01, railroad cut, Boxborough, northeast corner of study area (see Appendix A for outcrop locations).

34 Lithological Description

The dominant lithology throughout the formation is a light to medium gray, medium- to coarse-grained, biotite gneiss that displays classic gneissic banding of alternating darker biotite layers and near white quartz and sodic plagioclase layers (Figure 2-7). Foliation is prominent and generally reflects the regional trend, striking northeast, and dipping steeply to the northwest. Other lithologies recognized within the formation include quartzo-feldspathic gneiss and interbedded layers of sulfidic schist, amphibole gneiss and schist, and granulite. The thickness of these layers range from a half a centimeter to several meters. Most outcrops also contain pegmatites, veins of quartz, and pods of granite resembling the Andover Granite (SOag). The biotite gneiss grades to a more quartzo-feldspathic gneiss to the east and northeast. This rock is lighter gray and less foliated to massive.

Various rock types within the Nashoba Formation:

Biotite Gneiss – The lithology referred to as biotite gneiss is made up of varying

amounts of muscovite, quartz, plagioclase, biotite, plus or minus sillimanite. It is a northeasterly-trending unit that can be found throughout the north and northwest area of the quadrangle. It is especially observable in the far northwest corner of the quadrangle where the Boston and Maine railroad cuts through several outcrops (Figure 2-7), and in the towns of Acton and South Acton in railways and roads cuts. The biotite gneiss may be in part correlative with the following Bell and Alvord (1976) Nashoba Formation members mapped in adjoining areas, the Beaver Brook Member, the Long Pond Gneiss Member, and the Nagog Pond Gneiss Member (Table 2-2).

35 The biotite gneiss is light to medium gray to white in color. Classic gneissic banding of alternating light and dark layers of 1-6 mm in width are prominent in most samples (Figure 2-8). Lighter colored bands are made up of plagioclase and smoky gray quartz, darker bands consist of biotite and muscovite mica. The rock is medium- to coarse-grained with porphyroblasts of plagioclase up to 7 mm. Weathered surfaces are gray to rusty. Foliation is prominent and is generally gently warped or folded (Figure 2-9), and in some cases highly crenulate. Late “fish-scale” muscovite defines foliation at some exposures.

The dominant minerals throughout the biotite gneiss are sodic plagioclase which composes 34 to 64 percent of the rock, and quartz which composes 15 to 31 percent (Table 2-3, Modes 1-4). For the samples studied petrographically, the composition of the plagioclase is oligoclase, An₂₃ to An₂₆. The plagioclase is rarely zoned, but displays albite (Figure 2-10) and minor pericline twinning. The texture is characteristically medium- to coarse-grained and granoblastic. Plagioclase tends towards idioblastic, as does biotite and muscovite on occasion. Quartz displays undulatory extinction a sutured texture due to strain (Figure 2-11). A few samples exhibit small (0.02mm) prismatic crystals of sillimanite within larger muscovite grains (Figure 2-12). Most samples contain trace amounts of apatite, magnetite/ilmenite, epidote, and zircon. Zircons enclosed in biotite grains exhibit “radiation haloes” (Figure 2-11).

The biotite mica abundance, as seen in Modes 1-4 in Table 2-3, ranges from as low as 10 percent to as high as 35 percent, with approximately 19 percent being the average. The biotite is pleochoric, and includes muscovite that is present in all of the

36 FIGURE 2-8 Photomicrograph of biotite gneiss of the Nashoba Formation (On).

Sample consists of classic gneissic banding of layers. MY-01-01 under plain (top) and cross (bottom) polarization at 4X (see Appendix B for thin section locations).

37 FIGURE 2-9 Typical minor outcrop scale folding of biotite gneiss, MY-01-03, recent

exposure behind the Acton Police Station (see Appendix A for location).

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