Megan Diehl
10/18/07
Sample TR07DH2
Mineral Assemblage and Textures
Garnet
Biotite
Plagioclase
Quartz
Sillmanite
Titanite
Iron Oxide
Alteration Product (maybe sericite)
Sample TR07DH2 is a fine to medium grained phaneritic rock with a schistose
structure defined by the layers of biotite (Figure 1).
The garnet crystals are subhedral and range in size from 1.5mm to 0.25mm, with
an average size of 0.75mm. Garnet makes up 5% of the rock. The garnet either tends to
be poikiloblastic with inclusions or it tends to be broken up with an alteration product
filling the cracks (Figures 2,3,4). The garnet tends to be located in areas high in biotite,
rather than near the quartz or plagioclase. The biotite crystals are euhedral to subhedral
and range in size from 1mm to 0.05mm, with an average size of 0.25mm. The rock is
comprised of 45-50% biotite. Some biotite grains are internally bent or slightly folded,
and because in some cases the biotite is as large as the other crystals, in a blunt way, it
slightly wraps around the garnet, quartz and feldspar crystals (Figures 3,4,5). The
plagioclase crystals are subhedral and range in size from 3mm to 0.2mm, with an average
size of 0.1mm. They are the largest crystals in the rock and make up about 15-20% of the
rock (Figures 5,6,7). The quartz crystals are subhedral and range in size from 2.5mm to
0.2mm, with an average size of 0.25mm. Quartz makes up about 15-20% of the rock
(Figures 5,8,9). The sillimanite crystals are euhedral to subhedral and are all typically
0.05mm in size. It makes up 5 % of the rock. The crystals are prismatic and tend to be
found in clusters (Figures 10,11,12). The titanite crystals are subhedral to anhedral and
range in size from 0.05mm to 0.25mm, with an average size of 0.05mm. The crystals are
too small to see in hand sample and make up less than 1% of the rock (Figure 13,14). The
iron oxide crystals are anhedral, but a few are euhedral. They range in size from 0.05mm
to 0.25mm, with an average size of 0.1mm. The crystals are too small to see in hand
sample and they make up less than 1% of the rock (Figures 15,16). The alteration product
is found throughout the rock and makes up less than 1% of the bulk composition (Figures
8,9,13-16).
Relationship of Mineral Growth to Fabric
The garnet is prekinematic because it does not include any of the exterior fabric
within its boundaries and the exterior fabric slightly wraps around the crystals. The fact
that most of the crystals are broken also suggests that they had grown before the
deformation. The biotite and sillmanite are the two minerals that define most of the fabric
in the rock. The biotite is pre-to-synkinematic and the sillmanite is synkinematic. The
plagioclase and quartz are pre-kinematic due to the fact that the biotite also tends to
slightly wrap around both. The titanite and iron oxides may have been pre-topostkinematic, but because they typically do not show any strain it is difficult to place
them in any order. The alteration product is likely a syn-to-postkinematic mineral. As
other minerals begin to break down it forms in the cracks or within the grain boundaries.
Bulk Compositional Group
Silica rich metapelite.
Protolith
Shale with higher amounts of quartz.
P/T Conditions
Barrovian, medium pressure and temperature, 0.4GPa/600ºc
Figure 1. Whole rock showing schistose structure at 6.3x.
(A)
Figure 2. Garnet (A) crystals in hand sample at 16x.
(B)
(A)
(B)
(A)
Figure 3. Garnet(A) and biotite(B) in pp
Figure 4. Garnet(A) and biotite(B) in xp
light at 16x. Biotite slightly wraps around light at 16x
the garnet, and on the bottom of the picture
it is bordering a plagioclase crystal.
.
(C)
(B)
(D)
Figure 5. Biotite(B), plagioclase(C), and
quartz(D) in hand sample at 16x
(C)
Figure 6. Plagioclase(C) in pp light at 16x.
(C)
Figure 7. Plagioclase(C) in xp light at 16x.
(D)
Figure 8. Quartz(D) in pp light at 16x.
(D)
Figure 9. Quartz(D) in xp light at 16x.
(E)
Figure 10. Sillmanite(E) in hand sample at 16x.
(E)
(E)
Figure 11. Sillmanite(E) in pp light at 40x. Figure 12. Sillmanite(E) in xp light at 40x.
The ends of the crystals are shown here.
The ends of the crystals are shown here.
(F)
Figure 13. Titanite(F) in pp light at 40x.
(G)
(F)
Figure 14. Titanite(F) in xp light at 40x.
(G)
Figure 15. Iron oxide(G) in pp light at 40x. Figure 16. Iron oxide(G) in xp light at 40x.
-------------------------------------------
Sample TB97K
Mineral Assemblage and Textures
Enstatite
Orthoamphibole
Serpentine
Iron Oxide
Sample TB97K is a fine to slightly coarse grained phaneritic rock with a
granofelsic texture (Figure 17).
The enstatite crystals are subhedral and range in size from 0.25mm to 1cm, with
an average of 0.5mm. Enstatite makes up 40-45% of the rock. The larger crystals are
surrounded by serpentine and in some cases serpentine also shows up inside some of the
larger crystals (Figures 18,19,20). The orthoamphibole is euhedral to subhedral and
ranges in size from 0.25mm to 2.5mm, with an average of 0.5mm. Orthoamphibole
makes up 30% of the rock (Figures 21,22,23). The serpentine is anhedral and ranges in
size from less than 0.02 to .5mm. The serpentine formed tiny masses of crystals as well
as larger ones. It usually surrounded the pyroxene and is likely an alteration product of
the pyroxene as well. It is too small to be seen in hand sample and makes up 5-10% of the
rock (Figures 19, 20). The iron oxide is anhedral to slightly subhedral and ranges in size
from 0.05mm to 0.5mm, with an average of 0.3mm. It makes up 5-10% of the rock and is
too small to be seen in hand sample (Figures 19,20).
Relationship of Mineral Growth to Fabric
This sample has a granofelsic texture and because of this, determining exactly
when each of the minerals grew is not the easiest task. The orthoamphibole seems to have
grown pre-to-synkinematic because it is included in other minerals in some areas and not
in other areas. The iron oxide mineral is randomly oriented throughout the rock an could
have grown at any time. Sometimes it is an inclusion within another mineral and
sometimes it is not. The enstatite possibly formed from a previous clinopyroxene that
used to be in the rock. As the clinopyroxene broke down enstatite may have formed along
with the serpentine, which is likely an alteration product of either the clinopyroxene or
the now present enstatite. If this is true that would make the enstatite and serpentine
possibly syn-to-postkinematic. There is also the possibility that the enstatite formed
because the rock is in the granulite facies, which would make the enstitite a synkinematic
mineral. However, if that were the case you would not expect to see an amphibole present
in the rock.
Bulk Compositional Group
Magnesium rich metabasite.
Protolith
Mafic volcanic rock.
P/T Conditions
Amphibolite facies, 0.5-0.7GPa/600ºc---if it is in the granulite facies both the temperature
and pressure will be higher.
Figure 17. Whole rock showing granofelsic texture at 6.3x.
(H)
Figure 18. Enstatite(H) in hand sample at 16x.
(H)
(J)
(H)
(G1)
Figure 19. Enstatite(H), serpentine(J),
iron oxide(G1) in pp light at 16x.
(J)
(G1)
Figure 20. Enstatite(H), serpentine(J),
iron oxide(G1) in xp light at 16x.
(I)
Figure 21. Orthoamphibole(I) in hand sample at 16x.
(I)
(I)
Figure 22. Orthoamphibole(I) in pp light Figure 23. Orthoamphibole(I) in xp light
at 40x.
at 40x.
Nice job. Very good descriptions. A little more proofreading would help, and the figures
need scales.
Writing: 18/20
Met Pet: 25/30