C3: A TRAVERSE THROUGH THE SUTURE ZONE BETWEEN LAURENTIA AND THE
MORETOWN TERRANE IN NORTHWESTERN MASSACHUSETTS
Paul Karabinos
Department of Geosciences
Williams College
Williamstown
MA 01267,
pkarabin@williams.edu
Francis A. Macdonald
Department of Earth Science
Webb Hall
UC Santa Barbara
Santa Barbara, CA 93106,
francism@geol.ucsb.edu
James L. Crowley
Boise State University
Boise, ID 83725,
jimcrowley@boisestate.edu
ABSTRACT
U-Pb dates on magmatic and detrital zircon from samples in the hinterland of the Taconic orogen place new constraints on the timing and plate tectonic geometry of terrane accretion and magmatic arc activity. The Moretown terrane, a Gondwanan-derived exotic block, extends from the Rowe Schist-Moretown Formation contact in the west to the Bronson Hill arc in the east. Arc-related plutonic and volcanic rocks formed above an east-dipping subduction zone under the western leading edge of the Moretown terrane from approximately 500 to 470 Ma, until it collided with hyperextended distal fragments of Laurentia, represented by the Rowe Schist. Magmatic arc rocks formed during this interval are primarily located in the Shelburne Falls arc, although some rocks that formed above the early east-dipping subduction zone are located in the northern part of the Bronson Hill arc to the east. Metasedimentary rocks in the Shelburne Falls arc contain detrital zircon derived from mixing of Gondwanan, Laurentian, and arc sources, suggesting that the Moretown terrane was proximal to Laurentia by 475 Ma. Explosive eruptions at 466 to 464 Ma preserved in the Barnard Volcanic Member of the Missisquoi Formation in Vermont and as ash beds in the Indian River Formation in the Taconic allochthons may record slab-break-off of subducted lithosphere following collision of the Moretown terrane with distal Laurentian crustal fragments. Between 466 and 455 Ma a reversal in subduction polarity lead to a west-dipping subduction zone under Laurentia and the newly accreted Moretown terrane. Magmatic arc rocks in the Bronson Hill arc formed above this west-dipping subduction zone along the eastern trailing edge of the Moretown terrane at approximately 455 to 440 Ma. The western boundary of Ganderia in New England is east of the Bronson Hill arc, buried beneath Silurian and Devonian rocks deformed during the Acadian orogeny.
INTRODUCTION
Before plate tectonic theory was applied to the Appalachians (e.g. Bird and Dewey, 1970; St. Julien and Hubert, 1975), and even before absolute radiometric ages were available, three major orogenies were recognized based on paleontological constraints on deformed rocks below angular unconformities: the Ordovician Taconic, the Devonian Acadian, and the Pennsylvanian to Permian Alleghenian orogenies. During the 1970s and 1980s, it was common for geologists working in western New England to ascribe deformation to either the Taconic or Acadian orogenies. This simplified view of early Paleozoic tectonism held that the Taconic orogeny resulted from the collision of Laurentia with a ‘Taconic arc’ (for example, Rowley and Kidd, 1981; Stanley and Ratcliffe, 1985) and that the Acadian orogeny marked the collision of a Gondwanan-derived microcontinent called Avalonia (for example, Rast and Skehan, 1993). The Alleghenian orogeny occurred when the main Gondwanan continent arrived in the Late Paleozoic, and it was believed to have only affected rocks in southeastern New England (for example, Quinn and Moore Jr, 1968).
Detailed work in New England and in the Canadian Appalachians, where oceanic tracts, arcs, and Gondwanan-derived microcontinents are better preserved, has resulted in a much more intricate and complex history of arc and microcontinent accretion, reversals in subduction polarity, and intermittent back-arc rifting (for example, Van Staal et al., 1998; Zagorevski et al., 2008). More recent tectonic syntheses have expanded the number of orogenies to the Taconic, Salinic, Acadian, Neoacadian, and Alleghenian, and some of these involve multiple phases (for example, Van Staal and Barr, 2012). Gondwanan-derived microcontinents now include the Moretown terrane (Macdonald et al., 2014), Ganderia (Van Staal et al., 1998), Avalonia (Rast and Skehan, 1993), and Meguma (Schenk, 1997, White and Barr, 2010). Furthermore, we now know that Alleghenian deformation affected rocks as far west as the Bronson Hill arc and Connecticut Valley Trough in western Massachusetts (for example, Robinson et al., 1992). The expanding geochronological database in the northern Appalachians suggests that the Laurentian margin was continuously active from approximately 475 to 270 Ma.
Hibbard et al. (2006) mapped the suture between peri-Laurentian and peri-Gondwanan terranes in the New England Appalachians along the western margin of the Bronson Hill arc (Fig. 1). This interpretation required that the Shelburne Falls arc (Fig. 1) formed on a Laurentian-derived microcontinent as proposed for the Dashwoods block in Newfoundland (Waldron and van Staal, 2001), and that the Bronson Hill arc formed on the western leading edge of Ganderia (Hibbard et al., 2006). Based on detrital zircon data from the Moretown Formation in Massachusetts and Vermont, Macdonald et al. (2014) and Karabinos et al. (2017) argued that the suture between Laurentia and Gondwanan-derived terranes is located approximately 50 km further west along the contact between the Rowe Schist and the Moretown Formation, which is characterized by a high concentration of mafic and ultramafic lenses, interpreted as remnants of subducted oceanic lithosphere. Because the evidence for a Gondwanan source for the metasedimentary rocks came from the Moretown Formation, we called this newly identified Gondwanan-derived block the Moretown terrane.
The relocation of the suture has some important tectonic implications for the deep crustal structure of the New England Appalachians. 1) It requires that the Shelburne Falls arc formed on Gondwanan-derived crust rather than Laurentian crust. 2) Instead of the Bronson Hill arc having formed above an east-dipping subduction zone on the western leading edge of Ganderia, we suggested that it formed above a west-dipping subduction zone along the eastern trailing edge of the Moretown terrane after a reversal in subduction polarity. 3) If the Moretown terrane is distinct from Ganderia, the suture between these two Gondwanan-derived terranes must be located somewhere under the Silurian-Devonian rocks of the Central Maine basin (Fig. 1).
The focus of this trip is the evidence for a plate suture between Laurentia and a Gondwanan-derived microcontinent, the Moretown terrane, proposed by Macdonald et al. (2014) and Karabinos et al. (2017). In Massachusetts, this suture coincides with the boundary between the Rowe Schist and the Moretown Formation. There is nothing cryptic about this suture; it is characterized by a dramatic difference in sediment source as revealed by detrital zircon populations, and is characterized by a high concentration of mafic and ultramafic lenses, which we interpret as remnants of oceanic crust and mantle from an east-dipping subduction zone. It also coincides with a sharp 10 to 15 km decrease in depth to MOHO from the Laurentian margin to the Moretown terrane (Li et al., 2018). We will also discuss evidence suggesting that much of the 475 Ma arc magmatism in the Shelburne Falls arc occurred in close proximity to Laurentia even though the arc was built on the Gondwanan-derived Moretown terrane.

C3: A TRAVERSE THROUGH THE SUTURE ZONE BETWEEN LAURENTIA AND THE
MORETOWN TERRANE IN NORTHWESTERN MASSACHUSETTS

STOP 1. View to west and Hoosac Schist.
The gift shop has been closed for a while, but the platform next to it still provides a nice view of Mount Greylock (south), the Taconic Crest (distant ridge), and the southern termination of the Green Mountain massif (north). Across the road from the gift shop is an outcrop of Neoproterozoic to Cambrian Hoosac Schist. It is very similar lithologically to the Greylock Schist on Greylock and the Nassau Formation on the Taconic Crest. They were all deposited as silty mudstones on the continental slope and rise of the Laurentian margin following rifting of Rodinia. Metamorphic grade decreases from our location on Hoosac Summit to the west. Here the Hoosac Schist is a quartz, albite-porphyroblast, muscovite, biotite, garnet schist with lots of quartz veins. The Nassau Formation on the Taconic crest in the distance is a fine-grained phyllite; the Greylock Schist is at an intermediate grade of metamorphism. Small relict garnet crystals can be found in albite porphyroblasts on the eastern most ridges of Mount Greylock. The southern end of the Green Mountain massif is a south-plunging anticline cored by Mesoproterozoic basement unconformably overlain by Neoproterozoic to Cambrian Dalton Formation and Cambrian Cheshire Quartzite. The east-west valley that connects Williamstown and North Adams between the Green Mountains and Mount Greylock must contain a lateral thrust ramp with many kilometers of structural relief because the Taconic sequence rocks of Mount Greylock must have formerly covered the Green Mountains before erosion stripped them away.

Outcrop of Rowe Schist on right.

STOP 2. Rowe Schist.
There is abundant outcrop of Rowe Schist all along Whitcomb Hill Road. This is the best one and it contains a prominent contact between graphitic and non-graphitic schist near the bend in the road. Both contain the same mineral assemblage except for the greater graphite and sulfide content in the rusty-weathering rocks. The Rowe Schist is characterized by alternating graphitic and non-graphitic layers. The graphitic variety tends to be more concentrated in the structurally upper part of the unit close to the contact with the Moretown Formation and near the abundant lens of mafic and ultramafic rocks. The Rowe Schist is a typical pelitic schist with quartz, plagioclase, muscovite, biotite, and garnet. Locally it is more aluminous and contains paragonite (Na-white mica) instead of albit and chloritoid instead of biotite. It also contains a surprisingly large amount of quartz for a sediment that was presumably deposited on the distal Laurentian margin; the quartz-rich layers are best observed on faces perpendicular to the foliation. The two samples of Rowe Schist used for our detrital zircon study came from along this road (Figs. 1C and 3). Both zircon populations are typical of Laurentian-derived sediment with prominent peaks between 1.0 and 1.2 Ga corresponding to the Grenville of North America, and minor Neoproterozoic peaks at approximately 550 Ma, reflecting rift-related magmatism from the breakup of Rodinia.
Note that the boundary between the Rowe Schist and the Moretown Formation is a first-order geophysical feature- the depth to MOHO, as determined by P to S wave conversions, abruptly decreases by 10 to 15 km from west to east approximately below this contact in southern New England (Li et al., 2018).

Dirt Road on right is Torrey Hill Road.
This road leads to the Reed Brook Preserve managed by the Nature Conservancy. A long and winding trail leads uphill to a large ultramafic lens with rare plants. Some large boulders of ultramafic rock are visible in the road a short distance from the paved road.

STOP 3. Rowe Schist, Mafic and Ultramafic lenses.
After crossing the bridge, continue straight on foot along the dirt road. Go uphill and across the railroad tracks. Turn right and avoid the drive ways to follow Tunnel Road on the far side of the tracks through the woods to get to Stop 3 outcrops. Lots of outcrop of Rowe Schist in woods upslope to left along the road.

STOP 3A. Rowe Schist
The large outcrop of Rowe Schist next to the road contains abundant graphite and large garnet porphyroblasts. Note the steeply-dipping well-developed foliation and prominent down-dip lineation.

STOP 3B. Mafic Schist
The prominent outcrops on the left and right of dirt road are composed of mafic schist and gneiss, likely derived from oceanic crust caught up in the subduction zone between the Moretown terrane and the down-going Laurentian margin rocks.

STOP 3C. Highly altered Ultramafic Rocks of the Hoosac Tunnel Soapstone Quarry.
The large blocks on the right side of the dirt road are waste from a soapstone quarry and talc mine. Soapstone was quarried between 1885 to 1895. In 1909 an adit was drilled in the quarry to mine talc. The operation did not last long. This fine-grained rock contains talc, actinolite, garnet, antigorite, and calcite. Ultramafic lenses are common in the Rowe Schist and Moretown Formation near their contact, and are probably derived from altered mantle peridotite.

Outcrop of Moretown Formation.

STOP 4. Moretown Formation.
This enormous outcrop is a convenient place to see the Moretown Formation in good light. Most of the light-colored rock is quartz, plagioclase, muscovite, biotite granofels with a distinctive pin stripe texture. There are some quartz-rich layers, one of which provided one of our many Moretown Formation detrital zircon samples. Dark layers are mafic dikes and sills with diverse geochemistry (Kim and Jacobi, 1996; Karabinos et al., 2017). The Moretown Formation is Cambrian in age (Macdonald et al., 2014; Karabinos et al., 2017). Based on geochemistry, the undated mafic intrusions most likely include Cambrian to Early Ordovician arc rocks, some of which may have formed during back-arc rifting, and Silurian or possibly Devonian rift-related rocks coeval with the initiation of the Connecticut Valley basin. Detrital zircons from the Moretown Formation from northern Vermont to southern Massachusetts show prominent 550 to 650 Ma peaks typical of Gondwanan-derived sediment (Figs. 1C and 4) Macdonald et al., 2014; Karabinos et al., 2017). There is a remarkable lack of Grenville peaks in the zircon populations.

Moretown Formation on left

STOP 5. Hawley Formation.
Park on right before bridge. Proceed on foot along Middle Road across bridge and turn left on pleasant dirt road. Make your way down to the Chickley River to the left. Note that this is private property and that permission is needed to access this outcrop.
The mafic gneiss exposed here is one of several common rock types in the Hawley Formation. Jon Kim studied these rocks for his doctoral dissertation and Kim and Jacobi (1996) is an excellent account of the geochemistry of this diverse unit. Structures widely interpreted as pillow basalts are present is some parts of this long exposure. The Hawley Formation is part of the Shelburne Falls arc and its Ordovician age is tightly constrained by a TIMS date of 475 Ma on the intrusive Dell Metatrondhjemite and essentially coeval detrital zircons in metasediments (Fig. 6; Macdonald et al., 2014). At least some of the mafic rocks (Kim and Jacobi, 1996) and perhaps most, if not all, of the metasediments formed during an episode of back-arc rifting. An anoxic environment, consistent with the restricted circulation of a rift basin, is required to explain the highly graphitic schist present in the western and eastern exposures of the Hawley Formation. Notable sulfide and manganese deposits in the Hawley Formation are also consistent with intra-arc rifting/splitting.
Our tectonic model described above is strongly driven by the fact that the detrital zircon populations in the Hawley Formation samples indicate both Laurentian and Gondwanan sources. The Gondwanan source is easy to explain because the Shelburne Falls arc formed on the Moretown terrane. The Laurentian detrital zircons component indicates that by about 475 Ma, the Moretown terrane and the Shelburne Falls arc were close enough to Laurentia to receive detritus from the Grenville rocks so prominent along the Laurentian margin.

Hallockville Pond. Lunch stop

STOP 6. Hallockville Pond Gneiss.
The Hallockville Pond Gneiss is a 475 Ma tonalite (Karabinos and Williamson, 1994; Macdonald et al., 2014) that intruded the Moretown Formation. The intrusive contact is not exposed at this stop, but it is visible nearby on the small hill 0.5 km northwest of the intersection of Hallockville and King Corner Roads in Dubuque State Forest. The arc geochemistry of this pluton (Karabinos et al., 1998) together with the observation that it intruded the Moretown Formation, provides compelling evidence that the Shelburne Falls arc was constructed on the Moretown terrane. The Hallockville Pond Gneiss contains a well-developed foliation that was overprinted by a younger crenulation cleavage. The older fabric, and possibly both, formed during the Taconic orogeny. 20 km to the south the Middlefield Granite at Glendale Falls in Middlefield, MA, is a 447 Ma weakly-foliated granodiorite (Karabinos and Williamson, 1994; Macdonald et al., 2014) that intruded both the Rowe Schist and the Moretown Formation at their contact, but the pluton is not offset at this major terrane boundary. Thus, the suture zone formed before this 447 Ma intrusion. These observations suggest that the strong deformation fabric in the Hallockville Pond Gneiss formed during the Taconic orogeny, and that the Rowe Schist-Moretown Formation fault contact was not greatly modified during the Acadian orogeny.

Outcrop of Collinsville Formation on right in arc of road.

Outcrop of Collinsville Formation on both sides of road.
A sample from this outcrop gave a 475 + 1.4 Ma TIMS date (Karabinos et al., 1998).

Trailhead for Mahican-Mohawk Trail
Alternate way to Stop 7 for those with very low clearance vehicles.

Turn right on Wilcox Hollow Road.
Dirt road can be rough after severe storms. Use alternate route and hike down Mahican-Mohawk trail if desired.

STOP 7. Collinsville Formation in the Shelburne Falls dome.
Park without blocking dirt roads. Continue down to power line. Look for small path on right that leads down to the Deerfield River. Long outcrop along the river continues upstream. Pay attention to rising water warnings; this outcrop is just below a hydroelectric dam.
This accessible exposure of the Collinsville Formation includes both tonalite and mafic gneiss. It is great fun to examine and argue about the intrusive relationship between the felsic and mafic rocks. The rocks are strongly foliated, lineated, and folded. The Collinsville Formation makes up the cores of the Shelburne Falls, Goshen, Granville domes. Core rocks from both the Shelburne Falls and Goshen domes are approximately 475 Ma (Karabinos et al., 1998) and surrounded by much younger Devonian metasediments. The contact is not an unconformity, but rather a major extensional shear zone, tops to the south, similar to that described for the Chester dome (Karabinos et al., 2010). Another fascinating feature about this part of the western New England dome belt is the occurrence of potentially ultrahigh-pressure garnet bearing rocks from the Goshen dome just south of the Shelburne Falls dome (Peterman et al., 2016).