Petrographic Problem Solving Problem #3
Riccilee Keller
The Rock
Sample #300 is a metamorphic rock called Hornfels, more specifically a Quartz-Epidote Hornfels. Hornfels “is a
fine-grained rock that lacks any foliation and generally retains relict features (minerals/textures) of the original
protolith. A hornfels is produced via contact metamorphism”
(Earth's Materials: Minerals and Rocks).
Mineralology
The two major rock forming minerals in this sample are quartz and
epidote. Muscovite, tourmaline, and zircon are also present in the
sample. Quartz makes up about 80% of the sample. Zircon appears
to be relict because muscovite has grown around it while
tourmaline is a product of
metamorphosis (Figure 1).
Figure 1 – Showing a relict zircon with
muscovite growing around it.
Textures
In hand sample, the rock looks like a quartzite. It has inter-grown
crystals and a slightly green tint. Individual crystal grains are not
visiable, there is no “platey” jointing, and it lacks a “schisty”
sheen. Mineralology, as stated above, does not allow this rock to
be considered a quartzite. Thin section analysis reveals more in
depth textural features that help to identify the rock as a Hornfel
and suggests metamorphosis. Sample 300 has a granoblastic
texture, more specifically heterogranular (Figure 2).
Heterogranular texture implies different grain sizes. Quartz grains
have serrated edges and subgrain boundaries. In general, there is
no foliation in the rock with the exception of the epidote and
muscovite grains. Relict minerals are present and characteristic of
Hornfels.
Figure 2 – Shows Heterogranular texture,
serrated edges, and subgrain boundaries
Problem/Hypothesis
Based on the textural and mineralogical features, what are the metamorphic conditions under which this rock was
formed? Sample 300 is a Quartz-Epidote Hornfels formed by contact metamorphism of a sedimentary rock like a
greywacke or a clastic sandstone.
Evidence of a Hornfel
A hornfels “is a fine-grained rock that lacks any foliation and
generally retains relict features (minerals/textures) of the
original protolith.
A hornfels is produced via contact
metamorphism” Sen, Gautam. The protolith of this sample is a
type of sandstone, like greywacke or a clastic sandstone. Both
types of sandstone contain mostly quartz and a variety of other
minerals like feldspars, muscovite, and epidote. Sample 300
contains one large porhyroblast (Figure 3) with a long axis of
Figure 3 – Showing a Feldspar porphyroblast
with muscovite growing around it
approximately 1300 microns. The porhyroblast is considered idio-subidioblastic, meaning well-moderately formed,
synonymous with euhedral-subhedral. The porhyroblast is not in equilibrium; it shows undutlatory extinction. The
extinction of the crystal is suggestive of a quartz, however it appears to have cleavage and the interference figure
looks biaxial. Based on these observations, the porphyroblast is a feldspar that originates from the protolith.
Tourmaline and zircon also appear to be relict minerals of the protolith; both have minor amounts of muscovite
growing around them.
Evidence of Contact Metamorphosis
Figure 4 – Showing preferred orientation to
muscovite grains
Contact metamorphism occurs when a magma body comes into contact
with pre-existing rocks. Contact metamorphism produces non-foliated
rocks
such
as
marble,
quartzite,
and
hornfels
(http://volcano.oregonstate.edu). When a magma body intrudes a solid
rock complex, it causes deformation and metamorphism to occur. As
heat moves out from a magma body, it causes the protolith to
recrystallize and deform. Dynamic recrystallization supports many of the
textural features found in the rock. Serrated grain boundaries are
evidence of disequilibrium between crystal grains. “Grain sizes
decrease, with increased differential stress, in an attempt to reduce
strain energy during deformation” (1). Grain sizes in the sample appear
to be trending toward isogrannular as opposed to heterogrannular; this
supports the idea that as the magma body intrudes the protolith, strain
and stress occur and continue as intrusion continues. Preferred
orientations of muscovite (Figure 4) grains are also suggestive of contact
metamorphism. Pressure during regional metamorphism increases with
temperature and would cause the rock to show more foliation. Intrusion
of a magma body will cause slight pressure on the rock as it intrudes;
this is the cause of the preferred orientation observed with the
muscovite.
Conclusion
Sample 300 is a Quartz-Epidote Hornfel. Supporting evidence includes
the presence of a porphyroblast, lack of foliation, and mineralology.
Because there is less than 90% quartz, the idea that this sample is a
quartzite has been refuted. Contact metamorphism is supported by the
idea of dynamic recrystallization causing serrated edges and changes in
subgrain boundaries. By using the metamorphic facies diagram as a
general template for the rock type, it can be generalized that this rock,
during contact metamorphism, formed at moderate to high
temperatures under low pressure.
References
1. Shelley, David. Igneous and Metamorphic Rocks under the Microscope: Classification, Textures, Microstructures,
and Mineral Preferred-orientations. London: Chapman & Hall, 1983. Print.
2. http://web.arc.losrios.edu/~borougt/TempPressureEnv.jpg