Shelburne Falls, Massachusetts: Arc Domain Rocks (Field Trip 4)

Figure 4.1: The outcrops of this field trip are related to the collision of the Shelburne Falls arc with Laurentia. 

Stop 1: Moretown Formation

Figure 4.2: Darker and lighter layers are the more mafic and
felsic materials, respectively.  A precipitated quartz vein can be
seen running from top-left to bottom-right of photo, with an
arrowpointing to it.  Hammer is photographed for scale. 

The Moretown Formation was originally part of Gondwana.  Detrital zircons have given the age of the earliest sediments in this formation as 514 Ma.  The oldest intrusion is dated at 504 Ma.  Older models had suggested that pieces of Gondwana had collided with Laurentia during the Devonian; the ages of Moretown zircons suggest that this fragment of Gondwana would have actually arrived during the Ordovician.  
     In general, this outcrop consists of highly foliated gneiss with alternating quartz- and mica-rich bands intruded by mafic dike and quartz veins.  Mineralogy also includes reddish garnets ranging from <.5 mm to about 7 mm in size, green epidote, and actinolite in the mafic intrusion.  The thinly laminated micaceous layers are composed mostly of muscovite.  Along the top of the outcrop, actinolite and weathered mafic textures were particularly noticeable.  Boudinage, where more ductile layers are deformed preferentially around more robust layers, suggests at least a local extension of fold limbs.
     Strike and dip at this outcrop are approximately 045, 49. The trend and plunge of lineation is about 100, 42.

Stop 2: Hallockville Pond Gneiss

Figure 4.3: Crenulated gneissic texture serves as evidence
for two deformations.

From zircon dating, the age of this outcrop, composed of intrusive rock forming part of the Shelburne Falls arc, has been determined to be 475 Ma.  The mica-rich gneiss is intruded by numerous quartz veins.  Evidence in the fabric suggests that it has been subjected to at least two deformations, possibly as part of the Taconic and Acadian orogenies.  Evidence consists of aplite dikes that have been folded and redeformed, as well as crenulated gneissic texture.







Stop 3: Hawley Volcanics

Figure 4.4: Possible deformed pillow lavas.  Field
notebook is photographed for scale.

Dating of detrital zircons show a clearly Laurentian origin for this outcrop, with a date of approximately 475 Ma for the mafic rock, intruded by a more felsic sill. The mafic rock consists of quartz, feldspar, and amphibole, with small amounts of garnet and epidote that likely formed through hydrothermal alteration of basalts in a marine environment by percolating seawater.
     In pillow lava; the cusp of the pillow should be pointing down with the bulbous part pointing up.  Some structures at this outcrop appear similar to pillows.  However, if pillow lavas, they must be highly deformed as the cusp and top are not readily distinguishable.



Stop 4: Shelburne Falls Dome: Collinsville Formation

Figure 4.5: Mafic dike intruded into tonalite

and intruded by quartz veins. 

This outcrop is composed primarily of foliated tonalite, rich in plagioclase and quartz and deficient in K-feldspar, a composition typical of volcanic arc rocks.  The tonalite is intruded by mafic dikes, which would have required a magma temperature of 1200 degrres C, compared with roughly 700 degrees C for the felsic material.  Formation required partial cooling and the beginning of crystallization of the tonalite followed by mafic magma intrusion.  Strike and dip are about 100, 10.  Trend and plunge of lineation on the approximately isoclinal folds are 275, 6.   Mica is strongly aligned with the fold axis.


"Glacial" potholes are present, and contain quartz veins where other minerals have been eroded away preferentially.  Pegmatites indicate areas where unusual elements that did not mix well with the rest of the magma crystallized into larger grained rock.

Figure 4.6: Pegmatite vein in tonalite

Additional Tectonic Context

• According to Karabinos et. al. (2013), the Taconian orogeny occurred when the Shelburne Falls arc, at 485 to 470 Ma, formed over an east-dipping subduction zone and collided with Laurentia soon after.  The polarity of the subduction zone then switched, ending the period of convergence.  The Bronson hill arc then formed above the new, west-dipping subduction zone.
• The Collinsville Formation represents the core of the volcanic arc, while the Hallockville Pond gneiss is a pluton that originated from subduction.  The Hawley volcanics are erupted extrusives from the volcanic arc.