Week 5: All Day Field Trip

All Day Field Trip
Monterey, MA

October 10, 2013

This all-day field trip looked at formations related to the Berkshire Massif. A reconstruction of the Berkshire Massif follows in this blog post, drawing mostly on the work done by Karabinos and others in the 2008 field trip guide "Age and style of thrusting in the Berkshire Massif, Massachusetts."

5.i: Lithotectonic map showing the general location of Field Trip 5 in Monterey, MA

5.ii: Zoomed in lithotectonic map showing the location of stops within field trip 5

Stop 1: Quarry in Becket, MA

Spotted muscovite-biotite granite and granodiorite
Mid-Ordovician

At Becket Quarry, we saw two different outcrops. One had a sill intruding the Laurentian Basement Rocks (5A), while the second has a sill that intrudes the Hoosac Schist (5B).

From the geologist's notebook- field notes
5A:
-3 main components to this outcrop: The basement rocks (biotite gneiss), the felsic sill, and the large pegmatites
-The felsic sill intrudes the basement rocks, the biotite gneiss
-The biotite gneiss is darkest in color, while the felsic sill is light gray and the pegmatites are white (Figure 5.1)
-The biotite gneiss is coarse-grained and has layers
-Pegmatite-like white lines in the basement rocks
-Sill is fractured, filled with pegmatic material


5B:
-There is a fine-grained, granite sill that intrudes the Hoosac Schist
-The sill is a lighter gray than the Hoosac Schist
-The Hoosac Schist has a more even texture and color than the felsic sill
-Hoosac Schist is highly foliated, fine-grained, micaceous
-Graphite is present in the Hoosac Schist
-Foliations seem to follow a pre-existing layering, parallel to schistocity of Hoosac Schist
-Thin and thick coarse-grained pegmatites

-Strike: 340 (NW)*
-Dip: 25 (NE)*

5.1: Becket Quarry, image showing biotite gneiss, a pegmatite, and a felsic sill outlined in red, shows the 3 main components found in 5A

Interpretation:
The sill observed in stop 5A is dated to roughly the same period as the sill observed in stop 5B. It is hard to determine an exact age because the Acadian Orogeny created enough heat to reset the zircons, making it difficult to find ages as old as the Taconian Orogeny. We can infer from the ages that we do have that the two sills were likely formed during the same tectonic event, such as rifting, which could allow the asthenosphere to upwell to the crust.

Stop 2: Tyringham Gneiss with a sill of Felsic Gneiss of Harmon Brook
Proterozoic

From the geologist's notebook- field notes
-Tyringham Gneiss= Coarse-grained Augen gneiss
-Contains biotite, muscovite, and silliminite, or kyanite and garnet
-Augen grains plunge down and to the left --> lineations are west plunging
-Highly foliated gneiss
-A felsic sill of tonalite intrudes the Tyringham gneiss
-The sill is fine-grained, lighter colored
-The sills bifurcate
-There is no contact metamorphism around large inclusions
-Some grains are zoned

-Strike: 282
-Dip: 70
-Trend: 285
-Plunge: 27

5.2: Outcrop in the quarry (Stop 1) showing Washington Augen gneiss with a felsic sill, the type of rock dated in the 2008 paper by Karabinos et al. ; "Augen" is named after the eye-shaped grains

Stop 3: Stockbridge Formation, Tyringham Cobble
Lower Cambrian

From the geologist's notebook- field notes
-Alternates between light and dark layers
-The dark layers are biotite gneiss, the light layers are rich in quartz, feldspar
-Gneiss contains biotite, k-feldspar, and plagioclase
-Mylenite fabric- highly recrystalized, generally found near faults and forms at a high temperature
-Folds, some isolated fold noses
-The mylenite fabric has obscured or obliterated the folds in some parts of the rock


-Strike: 122
-Dip: 20

3.3: Rock from Stop 3, the Stockbridge Formation, shows strongly foliated biotite gneiss (foliations outlined in blue), product of distance from summit

Summit of the cobble:
-Biotite gneiss with weaker foliations, easier to make out folds, probably a mesoproterozoic fabric
-Alternates between light and dark layers
-Dark layers are biotite-rich
-Quartz veins can be found

5.4: Summit of Tyringham Cobble, shows weaker foliations of biotite gneiss, annotated in blue. The difference in folations is a result of the added distance from the fault, a product of our position on the summit of the cobble

5.5: View from Tyringham Cobble

Interpretation:

As we move away from the thrust fault, the deformation fabric becomes less pronounced. In the lower outcrop, which is closer to the thrust fault, we see extremely strong foliations and more folds. The mylonitic fabric indicates that the rock is highly recrystallized. In comparison, the outcrop at the summit has weak foliations and fewer folds. We see a gradient of decreasing intensity of deformation fabric as we move away from the fault. 

Click here for the corresponding Google map!

Conclusion

According to popular interpretations, there are two adjacent massifs that make up the basement rocks of New England, the Green Mountain and Berkshire Massifs (Figure 5.i). There is a significant structural difference in these two massifs: the Green Mountain has rocks overlying it unconformably, and it behaved as a rigid tectonic unit during the Taconic Orogeny (Karabinos 2008). In contrast, the Eastern and Western sides of the Berkshire Massif were both interpreted as thrust sheets with many thrust faults within them. In other words, the Berkshire massif likely did not act as one rigid plate during the Taconic Orogeny.

5.i:  Shows the location of the Berkshire Massif, denoted with a blue arrow

New data from Karabinos and others:

Karabinos and others dated five felsic sills in order to try to constrain the age of Taconic thrusting. They found that the sills had drastically different ages: some were from the Mesoproterozoic (1000 Ma) and others dated to the Silurian (430 to 435 Ma). The older group of sills were found along the Western margin of the massif, while the younger group was found on the East. In addition, they examined mylonites (recrystalized rocks) from the West of the Berkshire Massif for evidence of the physical conditions during thrusting and found that monazite (a phosphate rock) is common in rocks along a fault-zone. They were able to date these rocks (Karabinos 2008). 

This data was used to propose a few things:

1) Many contacts that were previously interpreted as faults co-appear with exposures of the older felsic sills. These contacts did not contain mylonites. Karabinos et al. conclude that the interior of the massif likely does not contain many thrusts, as previously suspected.

2) If the Silurian sills on the Eastern side of the massif intruded an active fault, the fault must have formed after the Taconic Orogeny.

3) The sills on the western boundary of the massif, previously interpreted as a thrust contact from the Taconic, do not date to the Taconic. Their ages suggest that the fault zone displacement more likely occurred during the Salinic or Acadian orogenies (Karabinos 2008).

What does this conclude?

"In light of the approximately 430 to 435 Ma age of the granite sills, and the fact that younger rocks are juxtaposed over older rocks, it seems worth considering the possibility that the contact reflects Silurian extension. Karabinos and others (1998) suggested that the Connecticut Valley trough formed as a Silurian back-arc rift basin above a west-dipping subduction zone. A back-arc basin model in which deposition was synchronous with rifting (Karabinos, 1998) can explain many of the stratigraphic and structural problems in the Connecticut Valley trough. This model is also consistent with the back-arc basin geochemistry of the Silurian Standing Pond Volcanics in Vermont (Karabinos and others, 1998), and with work by Castonguay and others (1997)... [who] suggested that they record Silurian extension" (Karabinos 2008)."

Karabinos, P.M., Morris, D., Gordon, R., Hamilton, M., Rayner, N., Pyle, J., 2008, Age and style of thrusting in the Berkshire Massif, Massachusetts: Field Trip guide for GSA. 
*Strike/Dip data marked with an asterisk was cross-referenced or taken from the field notes of Eloise Andry