Field Trip Four
Shelburne Falls, MA
October 3, 2013
In Field Trip 4, we saw formations that are predominantly related to the Shelburne Falls Arc Complex. The Shelburne Falls Arc was formed after subduction began on the East side of the Proto-Atlantic, closing the ocean. It was this active margin that allowed for the formation of the Shelburne Falls Arc Complex, a volcanic arc on Moretown, a microcontinent rifted from Gondwana. During the Taconic Orogeny, the Shelburne Falls Arc collided with Laurentia.
Cambrian-Ordovician:
Opening of Proto-Atlantic (Iapetus Ocean)
475 Ma: Formation of the Shelburne Falls Arc
Rifting of Gondwana
Ordovician:
460 Ma: Taconic Orogeny
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| 4.i: Lithotectonic map showing the general location of the Week 4 field trip in Shelburne Falls, MA |
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| 4.ii: Zoomed in lithotectonic map showing the stops within the Peri-Laurentian Arc System |
Stop 1: Moretown Formation
Mid-Ordovician
Description from USGS MA geological map:
-Green-gray or buff in color
-Fine-grained, pinstriped granofels (granoblastic, weak foliation) and schist
From the geologist's notebook- field notes
-3 layers within outcrop: schist, mafic, and epidote-rich
-Visible bedding, sedimentary protolith
-The schist layer is light-colored
-Schist layer has a pinstripe pattern that is also folded, deformed
-Schist contains quartz and feldspar
-The mafic layer appears to be a dike
-Dike is nearly horizontal, appears squished
-There is an alignment of grains within the mafic layers, shows flow structure
-Epidote-rich layer likely caused by hydrothermal alteration
-Multiple large, boudinaged folds
-Garnet and actinolite needles
-Strike: 45
-Dip: 49
-Trend: 100
-Plunge: 42
What is hydrothermal alteration?
Hydrothermal metamorphism occurs when extremely hot water that is rich in ions flows through rock fractures. This causes chemical alterations and is often associated with igneous/volcanic activity, since that generates enough heat and pressure to circulate the ion-rich fluids.
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| Figure 4.1: Mafic rock from Moretown Fm, Stop 1. Note the dark color and the fact that the grains are weakly aligned. |
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Interpretation:
The Moretown, as previously stated, was a microcontinent of Gondwana that rifted off of the supercontinent. Originally, Moretown was thought to be associated with the Shelburne Falls Arc Complex, specifically a fore-arc wedge of the Shelburne Falls Volcanic Arc. However, the zircons of Moretown date back farther than the formation of the Shelburne Falls Arc; Moretown is too old to be formed as part of the arc complex. The zircons show affinities for Gondwana, which is why it is now suspected that Moretown rifted off of Gondwana and that the volcanic arc formed on top of it. The ultramafic rocks found in the Moretown Formation were probably dikes that intruded later in the Moretown's history.
Stop 2: Hallockville Pond Gneiss
Ordovician
Description from MA USGS geologic map:
-Light gray in color, foliated
-Microcline-plagioclase-quartz biotite gneiss
-Microcline found in large crystals (megacrysts)
From the geologist's notebook- field notes
-Granite diorite (felsic, plutonic gneiss)
-Alternates between very light and very dark layers
-Dark layers are biotite-rich, shiny
-Captures two different deformations: one caused the layering, the second folded the layers
-Appelite dikes present in gneiss
-Some dikes, apear to be folded
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| Figure 4.3: Hallockville Pond Gneiss (granite diorite) from Stop 2. The annotated orange lines to the left of number 1 show the first stage of deformation, the pinstripe pattern. The line annotated to the right of number 2 shows the second deformation, folding of the pinstripes. |
Interpretation:
An east-dipping subduction zone underneath Moretown occurred before the formation of the volcanic arc. It was this subduction zone that produced the Hallockville Pond Gneiss, a pluton that intruded Moretown as granodiorite. There are two evident stages of deformation seen in this outcrop: the first caused layering, the second folded the layers. These two deformations likely occurred first when Shelburne Falls collided with Laurentia (the Taconic Orogeny), and second during the Acadian Orogeny, when Avalonia collided with Laurentia.
Stop 3: Hawley Volcanics
Mid-Ordovician
Description from MA USGS geologic map:
-Interbedded amphibolite, greenstone, feldspathic schist, and granofels
-Underlies the Goshen Formation
-Coarse hornblende occurs in certain locations, coarse plagioclase in some amphibolites near the top of the formation
From the geologist's notebook- field notes
-Mafic rocks
-Black in color
-Contains hornblende, actinolite, plagioclase, quartz feldspar
-Actinolite caused by lower-grade metamorphism than previous sites
-"Green pods" of epidote, caused by either percolating sea water, a MOR, or is a product of metamorphosis
-Basalt is likely pillowed, hard to tell for sure
-Pillows are deformed, not perfect circles
-Visible vesicles in basalt
What is an MOR?
"MOR" stands for mid-ocean ridge, which is an oceanic ridge within underwater mountain chains that serves as spreading centers. "Spreading centers" are locations of underwater rifting where the upwelling of magma occurs, cooling into new oceanic crust that spreads outward from the spreading center.
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| 4.4: Hawley Volcanics, Mid-Ordovician, from stop 3. Outlined in orange is what could be interpreted as a single "pillow," caused by rapid cooling of magma when ejected into water. |
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| Figure 4.5, Hawley Volcanics, Mid-Ordovician, close-up of single pillow. Note the vesicles, highlighted with an orange box, a common feature of mafic rocks that cooled quickly. Also note the deformed circularity of the pillow, indicating deformation of this meta-basalt. |
The Hawley Volcanics were deposited by underwater volcanoes. We know this because they are mafic rocks, and the pillowed texture indicates that they cooled very rapidly underwater. They were a result of eruptions of the Shelburne Falls Volcanic Arc, probably when the volcanic arc and the underlying Moretown were close to Laurentia, since the zircons from Hawley have Laurentian signatures.
Stop 4: Collinsville Formation, Core of Shelburne Falls Dome
Mid-Ordovician
Description from the MA USGS map:
-Garnetiferous biotite gneiss
-Contains "clots" of chlorite
-Depletion halos
-Homogenous
From the geologist's notebook- field notes
-Contains dark (mafic) plutonic rocks and light (felsic) plutonic rocks
-Felsic layers are tonalite, somewhat granitic-looking (tonalite is sodium and calcium-rich granite with no potassium feldspar)
-Tonalite is strongly foliated
-The mafic layers are interbedded and also appear to be dikes that chilled as they intruded the cool tonalite
-Potholes evident, caused by steam erosion
-Lots of veins cutting through the tonalite, many cross-cutting each other
-Some pegmatites are still straight, show obvious cross-cutting
-Numerous quartz veins present
-Numerous quartz veins present
-East/West lineations
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| Figure 4.6: View of the Deerfield River at Stop 4 |
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| Figure 4.7: Cross-cutting pegmatites (the white lines) within the tonalite(black rock) of the Collinsville formation, stop 4 |
Interpretation:
The Collinsville Formation is the core of the Shelburne Falls Volcanic Arc Complex. The rocks have differentiated layers of mafic and felsic rocks and strong foliations. The mafic layers intruder the cool tonalite and cooled. This gneissic fabric was caused by deformation, likely during the Acadian Orogeny. Many of the pegmatites cross-cut the mafic and felsic foliated layers, telling us that they intruded the Collinsville Formation at a layer time than the original formation of the tonalite (Figure 4.9). The final phase of deformation caused doming.
To see these sites on a Google map, please look here!
Conclusions
In this section, I will go into more detail about the chronology of the events involving the Shelburne Falls Arc Complex.
The Shelburne Falls Volcanic Arc was built on the Moretown Formation, previously interpreted as a fore-arc basin of the volcanic arc. However, when Moretown was dated, it was discovered that the zircons pre-date the formation of the volcanic arc and actually show affinities with Gondwana (Africa). It is now interpreted as a microcontinent that rifted away from Gondwana.
During the Taconic Orogeny of the Ordovician (460 Ma), the Shelburne Falls Arc on the Moretown collided with Laurentia. The Hallockville Pond Gneiss intruded the Moretown as a pluton that was produced during subduction, which began after the collision transformed Laurentia back into an active margin.
When the volcanos that formed the volcanic arc exploded, they erupted mafic magma into water, causing it to cool quickly into pillow basalts on top of the Moretown, which was sitting at a depth that allowed it to be covered in water. These formed the Hawley Volcanics. There may also be feeder dikes that traverse from the Hawley to the Moretown and allowed for this deposition, but we have no evidence of feeder dikes. If no feeder dikes were present, it is likely that the Moretown was originally to the East of Hawley but that it was thrust over to the West during the collision. We know this because the Hawley contains zircons with Laurentian affinities, implying that it was, at one time, likely adjacent to Laurentia.
Finally, the Collinsville Formation represents the core of the Shelburne Falls Arc Complex.
In the past, it was thought that the Bronson Hill Volcanic Arc Complex collided with Laurentia during the Taconic Orogeny. We now know, from the work done by Karabinos et al. in 1998, that the Bronson Hill Arc was formed after the Taconic Orogeny as a result of the collision and the ensuing active margin. It was the Shelburne Falls Volcanic Arc that collided during the Taconic Orogeny, however. Karabinos et al. used the following information to conclude this:
(2) 40Ar/39Ar cooling ages from Laurentian margin rocks related to Taconian metamorphism in southern Quebec, Vermont, and western Massachusetts range in age from 470 to 460 Ma (Laird et al., 1984; Sutter et al.,1985; Hames and Hodges, 1993; Castonguay et al., 1997) and are 15 to 30 m.y. older than arc related rocks dated by Tucker and Robinson (1990) in the Bronson Hill arc.
(3) The age of emplacement of the Giddings Brook thrust sheet in the Taconic Range in Vermont is constrained by the presence of Orthograptus truncatus in flysch below the thrust (Zen, 1967) to have taken place during late Trenton time. Thus, thrusting in the western part of the Taconian thrust belt occurred ca. 460 to 455 Ma.
(4) The Middlefield Granite in Middlefield, Massachusetts, intruded the Moretown Formation and the Rowe Schist at their contact, which was interpreted as a Taconian thrust by Stanley and Hatch (1988), but the pluton is not offset at the contact. Therefore, the 447 ±3 Ma zircon evaporation age of the Middlefield Granite (Karabinos and Williamson, 1994) indicates that Taconian thrusting in the eastern part of the thrust belt ended before or possibly during formation of the Bronson Hill arc" (Karabinos et al. 1998)
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| 4.8: Figure adapted from Karabinos et al, 1998. Included are the likely locations of the formations from the field trip before any thrusting occurred. The lower image shows the reversal in subduction polarity that occurred after the collision of the Shelburne Falls Arc turned the Laurentian margin from a passive margin to an active one with a west-dipping subduction zone. This subduction allowed for the intrusion of the Hallockville Pond Gneiss.
Karabinos, P.M., Samson, S., Hepburn, J.C.,Stoll, H., 1998, Taconian Orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls Arc
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