Jennifer Wright
Stage 1 Progress Report: Thin Sections
Orthoamphibole Gneiss
The coarse grained orthoamphibole gneiss is composed of anthophyllite (60%)
and gedrite (40%). In hand sample, the anthophyllite occurs as euhedral elongate pale
brown crystals. They are up to 20mm in length, and most range between 5-10mm.
Gedrite occurs as stubby or rounded black crystals, ranging from 2-5mm in size, with no
discernable regular crystal shape. The anthophyllite crystals define both the foliation and
the lineation of the gneiss.
In thin section, the anthophyllite occurs as large bladed euhedral – subhedral
crystals, ranging from 4-10mm in length. The gedrite is distinguished by occurring in
subhedral, rounded crystals with more evident cleavage. Gedrite crystals sometimes
occur as inclusions in the anthophyllite crystals. Both minerals contribute to the fabric
insofar as they are oriented in a specific direction, with the anthophyllite, due to its
elongate nature, being lepidoblastic.
The protolith of this rock was either an Mg-rich ultramafic igneous rock, such as a
dunite or lherzolite, or an impure dolomitic shale. Due to the uniformity and lack of
variation in the mineral assemblage, it is more likely that the protolith was an ultramafic
igneous rock. The presence of anthophyllite indicates that this is a medium to high grade
metamorphic rock. Anthophyllite and gedrite exist in a solid solution, and the textures
and sizes of the minerals indicate that anthophyllite was the more stable mineral.
Figure 1: Orthoamphibole gneiss, in plane light. Field of view is 10mm
Figure 2: Orthoamphibole gneiss, in crossed polarized light. Field of view is 10mm
Kyanite Schist
This medium – coarse grained kyanite-garnet-silliminite schist exhibits kyanite
and sillimanite as porphyroblasts, with matrix minerals consisting of biotite, quartz,
plagioclase, and garnet. The kyanite occurs as subhedral elongate porphyroblasts
approximately 5-10mm in length. The sillimanite occurs as fibrous masses, which appear
to be pseudomorphed after kyanite porphyroblasts. The matrix minerals, especially the
biotite, plagioclase and garnets, are relatively small and equally sized, around 1mm. The
quartz appears in slightly larger bands throughout the sample, with crystals up to 5mm.
There is a weak foliation defined by the biotite, and no lineation.
In thin section, this rock presents an interesting texture and mineral assemblage.
The biotite occurs as euhedral crystals, ranging from 0.8 – 1.2mm, and is 20% of the
sample. Because it is the only elongate mineral, it defines the weak foliation. There are
abundant small subhedral rounded garnet crystals. These range from 0.4 – 1.6mm, and
compose 20% of the sample. The garnets are essentially inclusion free, except for minor
retrograde white mica growing in cracks in the crystals. The garnet is not anastamosed
by the biotite, and overprints the weak foliation. The quartz primarily exhibits undulose
extinction and irregular sutured grain boundaries, and composes 25% of the sample.
However, the presence of some poorly polygonal grain boundaries indicate that some
recovery took place in the absence of strain, or faster than the rates of strain. The
plagioclase also displays irregular anhedral grain boundaries, and composes 15% of the
sample. Most of the kyanite in the sample occurs as anhedral rounded crystals and lacks
the typical elongate bladed crystal shape typical of kyanite. The crystals range from 0.8 –
2.4mm, and compose 15% of the sample. Kyanite is often included with quartz and
plagioclase. The sillimanite occurs in fibrolite mats, composing 5% of the sample. In
this thin section, the sillimanite occurs mostly in a band parallel to foliation, and
surrounded by retrograde white mica.
This rock is of pelitic bulk composition, and the protolith was probably a shale or
mudstone. The aluminosilicate relationship in this rock indicates that it experienced the
high pressure metamorphic conditions necessary to form kyanite, after which conditions
increased in temperature, causing sillimanite to replace kyanite as the stable
aluminosilicate mineral. The presence of kyanite and sillimanite indicate high grade
metamorphic conditions, both high pressures and temperatures. The weak foliation and
partially annealed quartz indicates that the rock experienced recovery in the absence of
strain, or at least at faster rates than the strain rates at the peak metamorphic conditions.
The garnet textures indicate that the garnets formed post tectonically, perhaps in the
period of recovery when the quartz was able to anneal.
Figure 3: Kyanite schist in plane polarized light. Field of view is 8mm
Figure 4: Kyanite schist in crossed polarized light. Field of view is 8mm.
Nice work, Jen. Well written and thorough.
Writing: 19/20
Met Pet: 27/30