Charlemont, Massachusetts: Oceanic Realm Crust and Deposits (Field Trip 3)

Figure 3.1:During this field trip, we moved from the slope-rise deposits of the Hoosac schist to the deep
oceanic realm Rowe schist.

Stop 1: Hoosac Schist

Figure 3.2: Different colors in the schist indicate graphite

(dark grey), muscovite and albite (lighter grey), and quartz

veins (rusty color).  Hammer photographed for scale.

The green color of the schist at this outcrop is from the associated chlorite and epidote.  The pelitic schist is of Ordovician age, and also contains muscovite, garnet, albite crystals and quartz in veins and as part of the matrix.  Graphitic inclusions include graphite cores derived from biogenic material with  with white rims where feldspar forms after the graphite.
     Foliation is fairly strong; some quartz veins are isoclinally folded with parallel limbs, indicating that they were intruded before deformation. Folding of foliation indicates that the rock underwent two deformations; the original pelitic sediment was metamorphosed in the Taconic orgeny, and folding occurred during the Acadian orogeny.  Strike and dip are 345, 25.

Stop 2: Rowe Schist

Figure 3.3: This schist is lustrous and silver due to the presence

of chloritoid.  The most notable deformation features are the

lineations.  Pencil photographed for scale. 

Compared to the Hoosac schist at stop 1, the Rowe schist is finer-grained, more lustrous, and more finely layered although schistocity is similar.  Composition is more uniform, though there are still many quartz veins.  There is an abundance of chloritoid, a mark of aluminum rich rocks.  There is no albite in this rock, which is lower in both magnesium and iron than the Hoosac schist, but higher in sodium and Aluminum.  Instead of muscovite, there is paragonite, which has a similar crystal structure, but is the sodium end-member.  Like at the first stop, pelitic sediment was deposited here and underwent metamorphism during the Taconic orogeny.  At this outcrop, there is no evidence for a later deformation.
     Strike and dip are 001, 82, and the trend and plunge of crenulation lineation is 140, 50.  




Stop 3: Rowe Schist continued on Whitcomb Hill Road

Figure 3.4: Color banding in this schist is resultant of
quartzite-rich and chlorite-rich layers.  Pen photographed
for scale. 

This schist has a greenish tint similar to at the previous stop, but is less shiny, probably because of a smaller percentage of paragonite and other mica. Quartzite is interbedded with the chlorite, which calls into question whether the Rowe schist is really part of the oceanic realm because there shouldn't be much quartzite far away from land.  The presence of quartz "pebbles" can be explained if they are not actually pebbles, but boudinaged quartz veins.
     Strike and dip of schistocity are 345, 66, and trend and plunge of lineation is 130, 48.






Stop 4: Carbonaceous Rowe Schist

Figure 3.5: The contact between the two forms of Rowe
Schist is distinct in color and consistency differences. The
typical Rowe schist is similar to that in Figure 3.4, while
the carbonaceous schist falls apart, is darker, and is not
lustrous.

The rusty, platy "cruddy black schist" of the Rowe Formation forms a contact here with the paragonite-dominated Rowe schist.  The cruddy black schist is porous, weak, and covered in vegetation.  Although it is not mica-rich, it breaks parallel to schistocity.  The sedimentation rate must have been high, because the fact that quartz was weathered out is indicative of reducing conditions, which means the material must have been buried before it got oxidized.

Stop 5: Ultramafics at the Reed Brook Preserve

Figure 3.6: This rock is harder than the others we visited,
because it has a mantle source.  Note the hint of dark green
ultramafic in the right-hand foreground.  Hammer is photo-
graphed for scale. 

To the southeast of the Rowe formation, small blocks of ultramafic material indicate mantle rock.  The rock is too fine-grained for olivine to be present.  The general belief is that it is part of a suture zone in the Iapetus Ocean, but it might actually be from near the Laurentian continental margin.

Figure 3.7: Mafic schist of the Moretown formation is 

intruded by large veins of quartz.

Stop 6: Moretown Formation
The mafic schist with slaty cleavage was originally a basalt or diabase mafic intrusive, containing a lot of alternating biotite and quartz-rich layers, feldspars, amphibole, quartz veins, and some pyrite, chalcopyrite, epidote, and actinolite.   The Moretown is supposedly from the Shelburne Falls forearc region, but zircon dating data has not confirmed this.





Additional Tectonic Context

• The presence of the Moretown Formation, broken off from Gondwana accreted onto the postrift Laurentian Margin, is explained by Cawood et. al. (2001).  They suggest that rifting at the Iapetus margin has multiple stages, where microcontinents broke off separately after the main rift.  They also disagree with Williams and Hiscott (1987), who claim that the rift-drift transition is marked by a change from siliciclastic to carbonate deposits.  Instead, they point to siliciclastic and volcanic rocks bounded by faults as the rift transition zone.  They consider the oxidation of these rocks to be an indicator that time passed between their accumulation and further siliciclastic deposition.