Metamorphic textures in all samples of Iron Creek peraluminous gneiss provide insight
into metamorphic history occurring through the pressure-temperature evolution of the SMC.
Relative timing of these textures and associated reactions is interpreted through inclusion
patterns, crosscutting relationships, and secondary mineral formation. In chronologic order,
these textures are: (1) sillimanite in garnet, (2) embayed muscovite and K-feldspar with
sillimanite, (3) disseminated sillimanite and biotite in cordierite, (4) deformation of cordierite +
sillimanite + biotite + muscovite around garnet grains, (5) plagioclase halos around garnet, (6)
sillimanite mats replacing cordierite, (8) tabular muscovite cross cutting deformational fabric,
and (9) large euhedral andalusite crystal overprinting deformational fabric. These textures are
described and interpreted below.
Aligned needles of sillimanite occur within garnet in Iron Creek peraluminous gneiss
(Fig. 4.11). This suggests that sillimanite was present before and durring garnet growth, and that
garnet grew within the sillimanite field.
The pelitic assemblage of Iron Creek peraluminous gneiss contains embayed muscovite
parallel to the main foliation (Fig. 4.12) suggesting muscovite is breaking down and that
temperature conditions are above muscovite stability. In addition, K-feldspar is present in the
leucosome assemblage and occurs near sillimanite (Fig. 4.13) supporting the interpretation of
muscovite decomposition through the dehydration reaction: muscovite + quartz K-feldspar +
sillimanite + H2O. This reaction produces fluid that could promote partial melting and the
beginning of migmatization.
Fine-grained sillimanite and biotite are disseminated throughout poikiloblastic cordierite
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Figure 4.11: Photomicrographs in PPL (A) and XPL (B) of aligned sillimanite in garnet in Iron Creek peraluminous gneiss (IC08- 01b1) indicating sillimanite was present before and during garnet growth.
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Figure 4.12: Photomicrographs in PPL (A) and XPL (B) of embayed muscovite in Iron Creek peraluminous gneiss (IC08-01b). Texture suggests the rocks attained temperatures above the muscovite stability field.
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Figure 4.13: Photomicrographs in XPL of Iron Creek peraluminous gneiss (IC08-01b) displaying myrmekitic alkali-feldspar and sillimanite suggesting muscovite breakdown.
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Figure 4.14: Photomicrographs in PPL (A) and XPL (B) of disseminated sillimanite and biotite in poikiloblastic colorless cordierite (IC08-01b). This texture suggests that cordierite growing at the expense of biotite and sillimanite through the reaction: biotite + sillimanite garnet + cordierite + H2O. This biotite dehydration reaction produces a large volume of H2O that could be responsible
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describes the dehydration reaction: biotite + sillimanite garnet + cordierite + H2O. This
reaction produces large volumes of supercritical H2O that allow for partial melting of felsic
minerals and migmatization to occur. This is the highest pressure-temperature reaction recorded
in the rocks, therefore, peak metamorphic conditions are marked by migmatization and partial
melting interpreted at approximately T = 800 °C and P = 7 kbar.
After peak metamorphic conditions were reached, several metamorphic textures record
retrograde metamorphic reactions. Fibrous mats of sillimanite occur in poikiloblastic cordierite
(Fig. 4.15) suggesting a second phase of sillimanite growth through the reversal of the previously
described reaction (i.e. garnet + cordierite + H2O biotite + sillimanite). This reaction requires
the presence of H2O to occur. Therefore, it is interpreted that this reaction did not reach
completion because H2O escaped the system and was not present in sufficient quantities,
allowing the preservation of prograde textures.
The main deformation is marked by biotite, sillimanite, muscovite, and cordierite
deforming around elongate garnet (Fig. 4.15). Because these phases are incorporated into the
deformation, this indicates that deformation occurred after growth of the phases described
previously. Plagioclase halos occur around garnet and appear to be replacing deformed garnet
(Fig. 4.16) indicating a post-peak decompression texture (D. Henry, pers. comm..).
Tabular muscovite, perpendicular to the main foliation, cross-cuts the primary rock fabric
(Fig. 4.17). This suggests the muscovite is secondary and that the rocks passed back into
muscovite stability along the retrograde pressure-temperature path.
Large euhedral andalusite crystals (<1 – 6 mm in length) (Fig. 4.18) occur in all samples
of peraluminous gneiss and indicate that the rocks passed through the andalusite field (less than
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Figure 4.15: Thinsection scan of Iron Creek peraluminous gneiss (IC08-01b1) displaying main deformation/foliation, bimodal distribution of minerals, and sillimanite mat replacing cordierite. The main deformation is recorded by the pelitic assemblage deforming around garnet grains. Plagioclase halos surround and appear to be replacing deformed garnet.
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Figure 4.16: Photomicrographs in PPL (A) and XPL (B) of plagioclase halo surrounding garnet in Iron Creek peraluminous gneiss (IC08-01b). Plagioclase appears to be replacing deformed garnet. Tabular muscovite and biotite occur parallel with the main rock fabric within the halo. Cordierite and sillimanite fold around garnet/plagioclase defining the main foliation.
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Figure 4.17: Photomicrographs in PPL (A) and XPL (B) displaying tabular muscovite cross-cutting the main foliation (IC08-01b1). This suggests muscovite grew after peak metamorphism.
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Coupling metamorphic textures, interpreted reactions, and pressure-temperature (P-T)
calculations define a clock-wise P-T path for the Iron Creek peraluminous gneisses (Fig. 4.19).
A clock-wise P-T path suggests metamorphism through collisional processes. Little-to-no
evidence of the prograde path is preserved in samples of the Iron Creek peraluminous gneiss,
therefore, an interpreted prograde path of increasing pressure and temperature represents burial
of the rocks. At some point along the prograde path, the rocks cross into the sillimanite field. In
the sillimanite field, the rocks pass beyond muscovite stability such that dehydration melting
occurs to produce isolated migmatites in the area. Approaching peak conditions, garnet +
cordierite growth occurs through the dehydration of sillimanite and biotite generating fluid with
partial melting and migmitization. Production of migmatites is interpreted as peak
metamorphism along the partial melting curve at ~800 °C and 7 kbar. Subsequently, the rocks
experience a decrease in pressure and temperature such that mats of sillimanite locally replace
cordierite, plagioclase replaces garnet, and tabular muscovite replaces K-feldspar. A kink in the
P-T path results from a pressure decrease. This is interpreted as tectonic de-roofing and is
required for the rocks to drop into the andalusite field.
Comparing P-T paths across the SMC provides insight into the continuity of
metamorphic history across the SMC. A composite P-T path generated by Dutrow et al. (1995)
displays a clock-wise P-T metamorphic history for rocks from the Thompson-Peak area (Fig.
1.8). Overlaying the P-T path calculated from Iron Creek it is clear these parts of the SMC
suggest similar peak and post-peak metamorphic conditions, but different retrograde paths (Fig.
4.20). The Thompson Peak rocks remained at high-pressure conditions (4 – 5 kbar), indicated by
a kyanite pseudomorph after margarite, whereas, the Iron Creek rocks experienced lower
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The garnet-sillimanite-biotite gneiss in the Thompson Peak area (mapped by Dutrow et
al., 1995) is similar in bulk-rock composition to the Iron Creek peraluminous gneiss (Fig. 4.21).
Plagioclase halos surround garnet (ibid.). However, the Thompson Peak samples contain greater
modal amount of biotite (20 – 45 %) that define a moderate-strong foliation and include fine-
grained sillimanite needles accounting for 5 – 20 modal amount. No cordierite is observed
(ibid.). Lack of cordierite and abundance of tabular biotite suggests the Thompson Peak rocks
did not record the reaction: biotite + sillimanite garnet + cordierite + H2O. This reaction
produces H2O and partial melting that allows the rocks to deform more readily. The foliation in
the Thompson Peak rocks is weaker than in the Iron Creek rocks, suggesting Thompson Peak
rocks did not experience the same deformational history. This could be attributed to the earlier
uplift of the Iron Creek rocks, and/or lack of H2O in Thompson Peak garnet-sillimanite-biotite
gneiss.
Calculated pressure-temperature conditions are similar in Thompson Peak and Iron
Creek. Thompson peak garnet-sillimanite-biotite gneisses TWQ calculations record conditions of
765 ± 10°C and 6.54 ± 0.11 kbar (Table 1.1, Dutrow et al., 1995), whereas Iron Creek
peraluminous gneisses record conditions of 670 – 825 ± 40°C and 5.3 – 6.7 ± 0.5 kbar.
Similar bulk rock compositions, peak metamorphic conditions, post-peak metamorphic
conditions, and interpreted prograde-peak P-T paths coupled with different textures and
interpreted retrograde paths between Thompson Peak and Iron Creek suggest the SMC is
composed of two or more tectonic slices of rock from the mid-lower crust that experienced
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Figure 4.18: Photomicrographs in PPL (A) and XPL (B) of Iron Creek peraluminous gneiss (IC08-01b) displaying euhedral andalusite overprinting main foliation indicating post-peak and post-deformation overprinting.
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Figure 4.19: Petrogenetic grid displaying continuous reactions for garnet-bearing peraluminous gneiss (Spear et al., 1999), estimated pressure-temperature (P-T) path and calculated P-T
conditions for five samples of Iron Creek peraluminous gneiss. The prograde path is represented by a dashed line due to a lack of evidence. The rocks cross into the sillimanite field followed by muscovite dehydration (first bold red-line). Here, partial melting occurs. Peak metamorphic conditions are estimated at ~800 °C and 7 kbar where garnet + cordierite occur (second bold line) And fluid isgenerated that contributes to partial melting and migmitization of the rocks. P-T conditions calculated from five samples of Iron Creek peraluminous gneiss record post-peak conditions from ~625-775 and 5-6.75 kbar. The rocks experienced a decrease in pressure and temperature to reside in the andalusite stability field A clockwise P-T path suggests
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Figure 4.20: Comparison of P-T paths from Iron Creek (this study) with Thompson Peak (Dutrow et al., 1995). Similar peak conditions are observed at ~800 °C and 7 kbar, with post-peak conditions range from 625 – 825 °C and 4 – 6.75 kbar for Iron Creek rocks versus 625 – 775 ˚C and 5 – 7 kbar for Thompson Peak rocks. The Thompson Peak rocks remained at higher pressures as indicated by a margarite pseudomorph of kyanite (Dutrow et al., 1995), whereas the Iron Creek rocks passed through conditions of andalusite stability. The similarities suggest the two areas of the SMC experienced similar peak conditions, but the difference between the two retrograde paths suggests the SMC is composed of two or more tectonic slices of rock from the mid-lower crust.
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Figure 4.21: Photomicrographs in PPL (A) and XPL (B) of garnet-sillimanite-biotite gneiss (ST95-08) from the Thompson Peak area. Medium-grained tabular biotite is common throughout the sample and defines a moderate-strong foliation. Plagioclase halos surround deformed garnets. Coarse sillimanite crystals are present throughout the foliation.