Trip 4: Shelburne Falls Arc

Stop 4.1 - Moretown

Contains pinstripe unit (metamorphosed sediments) and mafic layers (dikes?). Both the dikes and the metamorphosed sediments are deformed so that they look like original bedding.

Figure 4.1. What looks like original bedding is outlined by dashed lines. But these "beds" are actually just the result of severe deformation.

The Mafic units contains needle-like grains with a strong orientation. On foliation surfaces it is possible to see the amphibole needles that have grown parallel to lineation. Certain layers are more foliated and folded than others, and less ductile layers have been pulled apart into boudinage.

Stop 4.2 - Hallockville Pond Gneiss

This gneiss started as a grano-diorite pluton that was a part of the Shelburne Falls arc. It intruded the the Moretown during the Ordovician and later experienced two deformation sequences. One produced the foliation texture and the other produced the fold texture. Both textures are younger than 475 Ma, but it is unclear when exactly they occurred.

Figure 4.2. Two textures in the Hallockville Pond gneiss: foliation and folding. The foliation is outlined by dashed lines, while a couple of fold axes have been highlighted in red.

Stop 4.3 - Hawley Formation

The Hawley is made up of sediments interlayered with mafic volcanics. The zircons from the Hawley sediments are Laurentian and the volcanics are about the same age as the Shelburne Falls arc. If the volcanics are interpreted as a part of the arc, then the piece of Gondwana would have to be very close to Laurentia in order for the Laurentian sediments to be transported into the forearc basin.

We also see epidote pods (Figures 4.3 and 4.4) that could have formed from seawater percolation, or could be the remnants of lava pillows.

Figure 4.3. Good example of the epidote pods (outlined by dashed black lines) in the mafic volcanics.

Figure 4.4. Close up view of lighter pods in the mafic volcanics.

Stop 4.4 - Core of the Shelburne Falls Dome

The dome is made of tonalite (a felsic rock with no K-spar) that was co-magmatic with mafic intrusions that lie parallel to the foliation of the tonalite. Felsic magam usually have a temperature of 600 to 700 degrees, while mafic magmas are usually at about 1200 degrees. Therefore, the intrusion history probably went something like this: the tonalite intruded and partially crystallized, then the mafic magma intruded and was chilled because it was so much hotter than the tonalite magma. Since intrusion, both the tonalite and the mafic layers have been extremly deformed. The dome also contains some late pegmatites that cross-cut the foliation (Figure 4.5).

Figure 4.5. Lighter colored, undeformed pegmatites cutting through the tonalite

Additionally, quartz veins run through much of the rock. Some cross-cut both the felsic and mafic layers, while others cross-cut only one.

Figure 4.6. Quartz veins cross-cutting both the felsic and mafic layers.

In order to get a gneissic fabric (Fig. 4.7) as strong as we're seeing in a rock of this felsic composition, very strong deformation is required. A high level of deformation was possible because the we are located deep in the pluton, where the rock is still hot and fluid during the deformation period. The severity of deformation is indicated by number of isoclinal folds in the quartz veins (Fig. 4.7). This folding is also evidence for the later formation of the pegmatites, as they are undeformed.

Figure 4.7. Intense gnessic deformation fabric with a quartz vein folded into an isoclinal fold.

Summary

The area covered in this field trip has been interpreted as arc related sediments, volcanics and intrusions. The zircons of the Moretown appear to be Gondwana zircons, not Laurentian, so it is thought that the Shelburne Falls arc intruded a fragment of Gondwana located east of the Laurentian margin (Fig. 4.8). This fragment must have been close to Laurentia because the Hawley volcanics and sediments (which are found in the forearc basin) contains Laurentian zircons. Therefore, the forearc basin was receiving sediment from both the Shelburne Falls arc/Gondwana complex and Laurentia.

Figure 4.8. Schematic diagram of the Shelburne Falls Arc intruding a fragment of Gondwana close enough to Laurentia that Laurentian sediments can be transported into the fore-arc basin. From Karabinos et al. 1998.