Sam Bruno
Geo. 525
Progress Report 1
10/19/07
Two samples were analyzed to complete this report, samples TB07JW16 and
TB07AH1. The samples were collected from the Tobacco Root mountains area in
western Montana. Sample TB07JW16 was determined to be a glaucophane garnet
quartzo-feldspathic gneiss. Sample TB07AH1 was determined to be an actinolite
granofelsic amphibolite.
Hand-sample analysis of sample TB07JW16 shows a distinct banding or
gneissose texture, with large garnet porphyroblasts (.1-1cm in diameter) in matrices of
iron-rich bt/act and qtz/plag (See figure 1). In thin section, the banding is not observed
via mineral orientation, however segregation of qtz and plag, and bt, act, and grt are
indicative of the macroscopic texture. Sample TB07AH1 shows no distinct fabric and has
a micro- to cryptocrystalline groundmass, thus identifying the texture as hornfelsic. Some
garnet porphyroblasts can be seen, as well as large porphyroblasts of bt, hbl, and act (See
figure 2). The hornfelsic texture is observed in thin section as well as hand sample, with
random orientation and dominant grain sizes of garnet and hbl clusters.
Thin-section analysis of both samples yielded expected mineral assemblages with
only a few anomalous cases. In sample TB07JW16, qtz was the dominant mineral. It
appeared as broken subidioblastic grains and masses that dominated the northern half of
the thin section and often surrounded grains of actinolite and biotite (See figure 3). Other
minerals found included grt, gla, ep plag, bt, and actinolite (figures 3-6). These minerals
appear to show no real relationship to the banding seen in hand sample other than the
above-mentioned weak segregation of qtz and amphiboles. Only sieve/poikiloblastic
textures were observed in grt grains with qtz and bt inclusions, and occasionally epidote
inclusions as well (see figure 7). Dominant minerals in the actinolite granofelsic
amphibolite included act, hbl, and bt (figure 8). Other constituents were plagioclase, qtz,
and some minor grains of epidote (figure 9). The presence of epidote is somewhat
questionable as the thin section is slightly too thick, thus making some grains falsely high
in birefringence. Chlorite was also noted in some cases spaced throughout the
groundmass (figure 10). A few garnets were recognized; their structure, however, was
nearly destroyed, and the grains were only identifiable based on their isotropic nature
(figure 11).
Based on the above mineral assemblages, it is most likely that sample TB07JW16
consists of a quartzo-feldspathic bulk composition (correlating it to an AKF diagram) and
sample TB07AH1 has a mafic bulk composition (also correlating it to an AKF diagram).
Due to the high Na, Al, Si, and K values it is most likely that the protolith of sample
TB07JW16 is a granitoid. The Fe rich (as observed by dark ferro-actinolite and dark
biotite) and calcium-rich nature of sample TB07AH1, a plausible protolith would be a
gabbro (due to the pyroxene-rich nature of gabbros which would supply the Fe, Ca, and
Mg needed for bt, act, and hbl).
Possible reactions for sample TB07JW16 occur in the lower blueschist zone, with
act + chl + ab producing gln + ep + qtz + fluids. This reaction occurs around 0.8 GPa, and
ranges from 240-400oC, though the reaction temperature most likely occurred around
250-300oC as the qtz is still roughly subidioblastic. The reaction that best describes the
assemblage observed in sample TB07AH1 occurs at the low-mid end of the amphibolite
facies. The reaction is described as the chl out reaction, where act + chl + E + ab + qtz
produces Hbl + plg + qtz + f. However, due to the minor presence of grt and chl, it is
most likely that the reaction began at around .5 GPa and 600oC with the E + Qtz reaction
producing an + grt + Mt + F, followed by a retrograde path that intersected the chl-out
reaction which occurs at ~525oC and ranges in pressure from .1-.5 GPa. This would
account for the given mineral assemblage and the minor amount of unhappy grt and chl.
Writing: Needs better proofreading (14/20)
Met.Pet.: Mineral ID is okay, but could be better, and you need to consider some
reasonability when identifying minerals (e.g., Glaucophane in a q-f rock, or act & hbl
coexisting. Both are unlikely). Thin section quality could be better. Determination of
conditions/history is good. (23/30)
Figure 1) actinolite bearing garnet quartzo-feldspathic gneiss in handsample. Note the
two distinct groundmasses, qtz and plag rich on the left side of the photograph and dark
Fe-rich bt and act on the right side of the photo. For scale, the garnet in the center of the
photograph is approximately .2 cm in diameter.
Figure 2) Epidote
bearing actinolite
amphibolite in
hand sample.
Note the lack of
distinct fabric as
well as masses of
interstitial plag
and large grains
of hbl. For scale
the hbl grain in
the center
oriented E-W is
approximately .5
cm long. A garnet
cluster can be
seen in the upper
right corner.
Figure 3) Quartz can be seen around all of the edges of the photograph. The east and west
margins contain thick quartz that was identified by its undulose extinction and uniaxial
interference figure. Near the center of the photo is a grain of epidote (slightly NE with
compositional zoning). In the top center of the photo (just under the orange qtz grain)
there is a grain of plagioclase which is somewhat unexpected as the presence of
glaucophane would suggest that minerals such as plagioclase would have been destroyed.
Figure 4) Garnet as
seen in thin section with
grains of quartz and
actinolite. Actinolite
can be seen to the left
of the garnet in the
upper corner and quartz
is in the right half of the
photograph. Bt, and qtz
inclusions can be seen
in the grt as well as a
possible retrograde
chlorite rim around the
east rim of the grt.
Figure 5) Close up of a chlorite reaction rim around grt. Qtz is in the north section of the
photograph with biotite grains and a small grain of Glc in the northwestern section of the
photograph.
Figure 6)
Glaucophane
in plain polar.
The grain (in
the north
western
corner)
exhibits
excellent bluegreen
pleochroism.
Another
smaller cluster
can be seen
just to the
right of the
grain as well.
Figure 7) Close up of poikiloblastic texture of grts in the QFP gneiss. Note the biotite
inclusions and the green yellow epidote grain in the center of the photo. Also a small
grain of glaucophane can be seen in the bottom left corner.
Figure 8)
Amphibole
rich matrix
of the
actinolite
granofelsic
amphibolite.
Note the
dominance
of dark fe
rich
actinolite
and hbl
grains. Also
some
interstitial
qtz in the
NW portion
of the photo.
Figure 9) Poor polysynthetic twinning in a slightly thick grain of plagioclase in the
actinolite granofelsic amphibolite. The plag grain can be seen in the SW portion of the
potograph. In the NE portion there is a slightly thick grain of hbl, and a grain of Epidote
in the center of the photo.
Figure 10) Chlorite
peppering the
groundmass in the
center of the
photograph, lighting
causes the chlorite
to appear slightly
more blue than in
reality.
Figure 11) Severely unhappy garnet grain in the center of the photograph, with a large
vein of qtz through the center and many hbl and actinolite grains around the edges.