Rock Type: Metamorphosed
ultramafic rocks
Geologic terrane or major geologic element: Falls Lake terrane
Age: Late Proterozoic
to Cambrian – approximately 620-520 million years old
USGS 7.5-minute Quadrangle:
Bayleaf
Site Access: Park in the lot for fishing access on the
west side of Six Forks Road, just south of the Upper Barton Creek arm of Falls
Lake, and south of the boat ramp (Figure 1).
Follow the trail west across a tiny creek and then head to the left onto
a low ridge.
Figure 1. Parking for fishing access on west side of
Six Forks Road.
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Technical Information: Stoddard, E. F., and Blake, D. E., 1994, Carolina
Geological Society Field Trip Guide, 1994).
See the description for Stop 6 on pages 96-97 and the section on the
Falls Lake mélange on page 7.
Horton, J. W., Blake, D. E.,
Wylie, A. S., Jr., and Stoddard, E. F., 1986, Metamorphosed
mélange terrane in the eastern Piedmont of North Carolina: Geology, vol. 14, no. 7, p. 551-553.
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Introduction
Although it is not an extensive rock unit, the Falls Lake
terrane (formerly known as the Falls Lake mélange) is very distinctive. Bounded on the west by the Carolina terrane
and on the east by the Crabtree terrane, it pinches out south of Falls Lake and
continues to the north into Granville County.
The terrane consists mainly of mica schist within which there are
scattered hundreds of blocks and pods of other rock types. These rocks include soapstone, serpentinite, chlorite
schist, and amphibolite, and a variety of characteristic minerals including
talc, chlorite, actinolite, hornblende, serpentine, magnetite, chromite, and
Cr-rich corundum (ruby). Before
metamorphism and deformation, the blocks were ultramafic igneous rocks, mainly
peridotite, with lesser amounts of the mafic igneous rocks gabbro and
basalt. Blocks and pods that have been
studied in the Falls Lake terrane range from fist-sized to more than a
kilometer in length (Figure 2).
Figure 2. Portion of
geological map of the Bayleaf Quadrangle, showing location of this site (red
diamond). Falls Lake schist is in gray;
ultramafic pods in blue.
On the ridge, you will encounter a large outcrop of massive,
light green to dark green ultramafic rock (Figure 3). This very hard rock consists of a dense
aggregate of very fine-grained actinolite and/or serpentine. If you look closely, you will see small
metallic grains of magnetite, which will attract a sensitive magnet. You will also see veinlets of tiny quartz
crystals. These were introduced very
recently into the rock (geologically speaking); they have nothing to do with
the rock’s origin and early history.
Figure 3. Outcrop of
metamorphosed ultramafic rocks on low ridge.
If you explore this ridge, walking uphill toward the south,
and downhill toward the west, you should be able to find examples of talc
(slippery to the touch and softer than a fingernail), chlorite (green and
platy, will peel into flakes), hornblende (jet-black and lustrous), and perhaps
crystals of actinolite (dark green, lustrous, and breaks into long fragments
with smooth crystal faces). In this part
of Falls Lake, the lakeshore has numerous exposures of similar ultramafic
rocks. To see more, you can follow one
of these on-line
Falls Lake geology guides. One guide
is for a trip by boat; the other is along a hiking trail.
Significance of the Falls Lake terrane
There have been several different interpretations for the
rocks of the Falls Lake terrane; each proposes a different mechanism to achieve
the “block-in-matrix” nature of the unit.
The earliest studies interpreted the ultramafic rocks as intrusions into
the schist. In this scenario, extremely
hot ultramafic magma was injected into the host “country rock.” The magma cooled, and much later both rock
types were metamorphosed. In this case,
we would expect to see cross-cutting contacts between the ultramafic rocks and
the schist, as well as evidence of heating of the schist where it was in
contact with the hot magma. (We don’t
see either.)
In the early 1980s, these rocks were inferred to represent a
characteristic assemblage of rocks that forms at the site of an ocean trench
above a subduction zone, where an oceanic plate descends beneath another plate. Such assemblages are called accretionary
wedges, or mélanges. They consist mostly
of sediment that may be scraped off the down-going plate or derived from
erosion of the upper plate. In the
subduction process, pieces of lower crustal and upper mantle material
(ultramafic and mafic rock) may be broken off and incorporated in the sediment,
producing a chaotic mixture (mélange in French). In this scenario, the precursor of the schist
would be sedimentary rocks such as siltstone, mudstone, and sandstone. We might expect to be able to trace out
specific layers of the original sedimentary rock within the schist. (We can’t).
Because feldspar weathers quickly in the sedimentary environment, we
would not expect to find much in the schist.
(In fact, the schist is unusually rich in feldspar.) We would also expect that individual grains
of highly resistant minerals like zircon would be rounded from sedimentary
transport. (They aren’t. Instead they are well-formed crystals like
those that crystallize in igneous rocks, directly from a magma).
Tracing the Falls Lake terrane north into Granville County,
the metamorphic intensity decreases.
Whereas the Wake County area was subjected to a medium to high grade of
metamorphism, the Granville rocks only experienced low-grade metamorphism. We therefore gain a more confident
understanding of the original rocks because they have not been modified so
much. In fact, it seems the precursor of
the Falls Lake schist was itself an igneous rock related to granite – the Gibbs
Creek metagranodiorite. The Falls Lake
schist represents a large igneous pluton that is related to the volcanic rocks
of the Carolina terrane. It may
represent the roots of the ancient volcanic arc. The ultramafic and mafic blocks and pods are
pieces of wall rock that were incorporated into the magma as it rose
(xenoliths). In this scenario, the
evidence that counters the first two theories makes perfect sense. Further testing of the third theory is
underway.
The evolving theory of the Falls Lake mélange/Falls Lake
terrane is a very good case study of how science works, by suggesting
hypotheses and then testing them.
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