Field Trip 5: Basement

Stop 5.1 - Beckett Quarry - Sill intruding basement

When the magma intruded, it dissolved bits of the wall rock. Therefore, the zircons in the sill contain xenocrystic cores that indicate Laurentian basement, which is the wall rock that was intruded.

Figure 5.1. Upper contact with the darker biotite gneiss (basement). Above that there is a smaller, coarser-grained sill of the same age as the large sill.

The sill contains many pegmatites, which formed when the sill cracked during cooling.

Stop 5.2 - Beckett Quarry - Felsic sill intruding Hoosac Schist

The granite sill is bifurcated and has a foliation fabric parallel to the contact. This intrusion may be associated with forearc basin rifting, because it is difficult for small bodies like the sill to intrude without extension.

Figure 5.2. Notice the foliation (outlined by dashed lines) within the sill.

There is no contact metamorphism in the schist, indicating that either the granite was too low in temperature when it was intruded, or the schist was still pretty hot during intrusion. Large pegmatites can be found in between the sill and schist (Fig. 5.3).

Figure 5.3. Large pegmatite, which is lighter in color than the schist it has intruded.

We don't see any evidence of the Taconic orogeny. Perhaps the Acadian reset everything?

 Stop 5.3 - Felsic sill intruding Tyringham Gneiss

The coarse-grained augen-granodiorite Tyringham gneiss is intruded at many points by finer grained tonalite sills (Fig. 5.4). The augen are stretched (Fig. 5.5), a deformation which goes back to Grenvillian activity.

Figure 5.4. Contact (highlighted in red) between the tonalite on the bottom and the gneiss on the top. Notice the lack of contact metamorphism.

Figure 5.5. Stretched augen.

There is no contact metamorphism (Fig. 5.4), partly because the sill is so small, and partly because the Tyringham is already so metamorphosed that there aren't many additional reactions that could occur. The zircons in the gneiss have osillatory zoning layers: one from the initial crystallization at 1170 Ma, and one at the sill intrusion at 1000 Ma.

Stop 5.4 - Biotite gneiss located close to fault

Here, a biotite gneiss has been thrust ofer the Stockbridge Formation. The fault itself is located just east of the outcrop. The proximity of the gneiss to the fault has created a mylonitic fabric of alternating dark and light laminations (Fig. 5.6). Mylonitic fabric is formed when movement occurs at high enough temperature that minerals recrystlaize, often to as smaller grain size. It is possible to see fold noses being truncated by laminations. The mylonitic fabric is wiping out the folds, and as you look farther up the outcrop (moving away from the fault) there are more folds than by the fault.

Figure 5.6. Mylonite fabric (outlined by dashed line). The rusty color is from the weathering of biotite and magnetite.

Stop 5.5 - The top of the Cobble

The Tyringham gneiss is much less deformed here because it is farther away from the fault.  It is still foliated, but the separation into laminations is less extreme (Fig. 5.8). We don't see any folds here, so the folds seen at Stop 4 must be associated with the fault.
Parts of an amphibolite layer, which is also part of the basement, is also visible. This amphibolite layer may have acted as a plane of weakness that was exploited by the alaskite sill that is found near the top of the Cobble.

Figure 5.7. The gneiss at the top of the Cobble

Figure 5.8. Still foliated (highlighted by dashed black lines), but not to the same extent as at Stop 5.4

Summary

 This field trip takes a closer look at deformation of the basement underlying all of western Mass. The trip focuses on intrusions into the basement, specifically small felsic sills that intruded 430 and 1000 Ma. It is thought that most of the sills are associated with forearc basin rifting, because it is difficult to intrude such small bodies without extension. The Berkshire Massif may have been emplaced by thrusting, which may have begun at the start of the Acadian Orogeny.