Monterey, Massachusetts: Berkshire Massif (Field Trip 5)

Figure 5.1: In this field trip, outcrops are of gneiss from the Grenville basement, though they contain felsic/granitic sills.  Some of the sills intruded in a rifting environment; others are dated as being almost as old as the basement rock.

Stop 1A: Becket Quarry Granite

Figure 5.2: Biotite gneiss overlies granite; contact between
the two is shown.  A band of pegmatite precipitated within
the gneiss.

A granitic sill, dated at 432 Ma, intrudes basement 1150 Ma.  On the north end of the quarry are basement xenoliths of biotite-rich gneiss.  On that same side of the quarry is a contact between granite overlain by biotite gneiss.  The gneiss is crosscut by a wide band of pegmatite, which is the same age as the granite.

Stop 1B: Becket Quarry sills

Figure 5.3: A pegmatite vein lies at the contact between
granite sill and Hoosac schist.

At this outcrop, two granite sills intrude through Proterozoic Hoosac schist containing plagioclase, quartz, and micas; pegmatite can also be seen.  The schist is too high-grade to show contact metamorphism, and its contact with the granite shows no indication of faulting.  The granite is interpreted to be about 434 Ma.  Grain size and composition for the two rocks are similar; the difference is mainly in the degree of foliation.  The strike and dip of the granite sill are about 340, 25.
        These sills may have been intruded during basement rifting; it is difficult for such bodies to intrude in a compressional zone.




Stop 2: Felsic sill in Tyringham gneiss
The two rock types here are a fine-grained felsic sill of tonalite and a coarser grained gneiss of granodioritic composition.  This Tyringham gneiss contains eye-shaped feldspars called augen, shaped in response to deformation.  Dark material, most likely magnetite, is contained within the augen.

Figure 5.4: Augen texture in the Tyringham gneiss. Pencil
is photographed for scale.  

     Felsic sills intruded the Tyringham gneiss. Zircon dating indicates that the Tyringham gneiss was crystallized at about 1179 Ma, and the felsic sill intruded at 1004 Ma.  Zircon dating, and the fact that grain structure is roughly east-west aligned compared with a typical post- Precambrian north-south alignment, suggests that the Taconic orogeny did not affect this area.  Oscillatory zoning of the zircons might be a result of pulses of magma, each with a slightly different composition and temperature.
     Contact metamorphism can most likely not be seen here because small intrusions like these cool relatively fast.  Strike and dip are about 275, 27.  Trend and plunge of lineation are 285, 28.

Stop 3: Tyringham Cobble

Figure 5.5: The mylonitic texture of this rock indicates that
metamorphism took place at high temperatures.  Pen is
photographed for scale.

Walking up the east slope of Tyringham Cobble, marble and then mylonitic gneiss is encountered. Argon dating in this locality and nearby monazite dating of around 400 Ma suggests that recrystallization occurred as a result of emplacement of the Berkshire Massif.

Figure 5.5: Biotite gneiss at the top of Tyringham Cobble.

At the top of Tyringham Cobble, biotite-rich gneiss looks similar to the basement rock.  It is intruded by a sill of granodioritic composition.  The sill hasn't been dated yet, but is probably similar in age to the sills from the previous stops, and so probably intruded at around 1000 Ma, meaning the thrust is not related to the Taconic orogeny.