Bani Hamid Quartzites

These granulite-facies quartzites, formed at depth beneath the Oman Ophiolite, show a remarkable sequence of quartz microstructures representing overprinting of the original very coarse high-T fabrics by progressively lower-temperature microstructures as the Bani Hamid rock unit was emplaced to high crustal levels by thrusting within the ophiolite. Since the microstructures reflect the deformation mechanisms and therefore also the temperature regime under which deformation took place, while the mineral assemblages also help to constrain pressure and temperature, a P-T-deformation path can be reconstructed.
Overview Granulite-facies microstructure  JC107, crossed-polars slide scan 1.5 x 1 cm. Granulite-facies microstructure of very coarse-grained quartz. Other minerals are enstatite (high relief, birefringent) and opaque oxide. The oxide grains do not pin Qtz grain boundaries; most are now enclosed within quartz crystals. GBM microstructure  JC68, crossed-polars slide scan 1.5 x 1 cm. This quartzite has wholly recrystallized to a grain size of about 1 mm. These grains now have wavy, sutured grain boundaries and a shape preferred orientation (SPO) parallel to the E-W tectonic fabric defined by other minerals (just visible as narrow trains of sheet silicates and opaques) SGR microstructure  JC73, crossed-polars slide scan 1.5 x 1 cm. Much of the matrix is made up of recrystallized grains about 100 µm in diameter, and deformed larger relics have subgrains of similar size, suggesting subgrain rotation as the dominant mechanism. The silicate minerals are diopside and hornblende/actinolite. Mylonite, BLG microstructure  JC84, crossed-polars slide scan 1.5 x 1 cm. A quartz mylonite with scattered diopside porphyroclasts from near the thrust contact against peridotite. Most of the matrix consists of tiny recrystallized grains about 10 µm in diameter, a typical size for the bulging mechanism. The fabric is brought out by patterns of extinction formed by a few elongate relict grains and by the moderate misorientation of neoblasts formed from the same original grain.
Microstructure detail Granulite, chessboard Qtz  JC66, crossed-polars slide scan 1.5 x 1 cm. Granulite-facies microstructure: very coarse-grained quartz strained into roughly square subgrains (chessboard texture). Locally some very fine low-T recrystallization at grain boundaries. Granulite microstructure  JC61, crossed-polars slide scan 1.5 x 1 cm. Banded quartz-diopside calc-silicate rock with fresh granulite-facies assemblage, coarse-grained quartz, and very little later deformation, apart from slight undulose extinction in quartz. Granulite, chessboard Qtz  JC66, crossed polars, field width 2 mm. High-tempeature chessboard subgrain microstructure in quartz. Horizontal subgrain boundaries are aligned roughly parallel to the c axis, vertical subgrain boundaries along the basal plane. GBM microstructure  JC68, crossed polars, field width 4 mm. Medium-grained quartz with wavy boundaries is attributable to a high-temperature recrystallization of the granulite-facies microstructure.
GBM microstructure  JC68, sensitive tint plate, field width 4 mm. This view highlights the differences in orientation between grains. Ongoing deformation is indicated by large-scale subgrain boundaries in some grains, and fine-grained suturing and recrystallization at some grain boundaries. SGR-GBM microstructures  JC75, plane polarized light, field width 4 mm. Quartz, opaques, and scruffy-looking pseudomorphs after cordierite and (probably) Opx. There is a compositional banding running NE-SW marked by the abundance of opaques, etc., but the elongated opaque trains define a WNW-ESE tectonic fabric at a high angle to this, apparently axial-planar to cm-scale folds. SGR-GBM microstructures  JC75, crossed polars, field width 4 mm. Same view showing grain size varying with the compositional banding. The tectonic fabric is not obvious. The matrix has clearly recrystallized from high-grade conditions: the grain size is 50 µm in the impure bands, up to 200 µm in the quartz-dominated bands. SGR-GBM microstructures  JC75, sensitive tint plate, field width 4 mm. This view confirms the thoroughly recrystallized nature of the matrix: there is little obvious sign of lattice preferred orientation. The grain size in impure bands is consistent with a subgrain rotation mechanism, but little subsequent growth has occurred, probably because grain boundaries are pinned by other phases. In quartz-rich bands the grain size been able to increase by GBM. This is consistent with T a little above 500°C during recrystallization. SGR microstructure  JC73, sensitive tint plate, field width 4 mm. Diopside quartzite with recrystallized matrix. Microstructure apparently dominated by subgrain rotation, and showing a whole spectrum of different amounts of misorientation between adjacent grains.
SGR microstructure  JC49, crossed polars, field width 2 mm. Recrystallization by subgrain rotation, size of recrystallized grains 20 to 80 µm. SGR microstructure  JC49, sensitive tint plate, field width 2 mm. Recrystallization by subgrain rotation, size of recrystallized grains 20 to 80 µm. This view highlights the differing amounts of misorientation between areas of new grains. SGR microstructure  JC63, sensitive tint plate, field width 0.8 mm. Detail of grain size (50 to 100 µm) and shape in a thoroughly recrystallized diopside quartzite. SGR microstructure  JC85, crossed polars, field width 0.8 mm. A sigma-porphyroclast of amphibole (indicating dextral shear) in finely-recrystallized quartz, which shows neoblasts of similar size to the fine subgrain structure of strained grains. SGR microstructure  JC85, sensitive tint plate, field width 0.8 mm. Another view of the same specimen showing strong tectonic fabric defined by granule trains of silicate minerals, and recrystallized quartz apparently formed by subgrain rotation. The grain size is 30 µm, smaller than typical for SGR but probably controlled by high strain rate.
BLG recrystallization  JC57, crossed polars, field width 0.8 mm. The recrystallized grain size (~10 µm) is indicative of the bulging mechanism and is smaller than the indistinct subgrains in porphyroclasts. Compare with the previous photograph. BLG microstructure  JC65, crossed polars, field width 2 mm. Mylonite with strong fabric defined by opaques and elongate strained quartz porphyroclasts, with bulging recrystallization along their margins. The new grains are distinctly smaller than the subgrains seen in the porphyroclasts. BLG microstructure  JC65, sensitive tint plate, field width 2 mm. As the last image, but also revealing some detail of grain orientations. Subgrain diameters ~25 µm, neoblasts ~10 µm. BLG microstructure  JC67, crossed polars, field width 2 mm. Mylonite with strong fabric defined by extremely elongated opaque trains and strained quartz porphyroclasts, which are extensively recrystallized to 10–20 µm neoblasts. Note that the larger neoblasts in the middle of the field of view have acquired an oblique shape fabric, suggesting further deformation by dislocation creep, and a dextral shear sense. BLG microstructure  JC67, sensitive tint plate, field width 4 mm. View of a larger area of this sample to show the overall fabric.
Ultramylonite (BLG)  JC84, crossed polars, field width 10 mm. Ultramylonite with a few porphyroclasts of rigid diopside and highly-strained elongate quartz (hardly visible at this scale). The mean grain size of neoblasts is around 8 µm. Inferred to be the end product of high-strain deformation and BLG recrystallization as seen in the preceding images. Overprinting Granulite with BLG  JC107, crossed polars, field width 0.8 mm. Granulite-facies rock with a largely fresh mineral assemblage, but BLG recrystallization just beginning at some grain boundaries. Granulite to GBM  JC46, crossed-polars slide scan 1.5 x 1 cm. Calc-silicate quartzite (or silicified volcanic rock). Quartz vein-stringer near the top of the image preserves coarse granulite-facies microstructure, but much of the rest appears to have recrystallized to grains 0.5 to 1 mm across. Amphibole appears to have replaced much of the original diopside. Granulite and GBM with BLG  JC46, crossed polars, field width 2 mm. From the upper part of the field of view in the previous image, a granulite-facies chessboard quartz relic is modified by BLG recrystallization at its margins.
Granulite with subgrains and BLG  JC62, crossed-polars slide scan 1.5 x 1 cm. Granulite-facies microstructure modified by intracrystalline deformation marked by strongly undulose extinction and narrow subgrains. There is fine BLG recrystallization at grain margins. Granulite overprinted by BLG  JC97, crossed-polars slide scan 1.5 x 1 cm. A mostly granulite-facies microstructure modified by intracrystalline deformation in the larger quartz grains, and by BLG recrystallization along foliation traces and at grain margins. Granulite/GBM overprinted by BLG  JC106, crossed-polars slide scan 1.5 x 1 cm. Coarse granulite relics and smaller 1-2 mm grains (the GBM generation?), all showing internal strain, mantled by relatively coarse neoblasts of 30-50 µm, in the upper part of the range considered typical for the BLG mechanism. There are no subgrains of this scale in the older grains. Granulite overprinted by BLG  JC57, crossed-polars slide scan 1.5 x 1 cm. Coarse granulite-facies microstructure affected by intense intracrystalline deformation at relatively low temperature and/or high strain rate, traversed by zones of BLG recrystallization. SGR-GBM microstructures  JC68, crossed polars, field width 2 mm. A rock dominated by recrystallization to mm-sized grains (GBM) locally shows areas where recrystallization to finer neoblasts can be attributed to subgrain rotation. This could be inferred to be the natural consequence of continued or repeated low-strain deformation with falling temperature in the range 650 - 450°C
GBM overprinted by BLG  JC72, crossed polars, field width 2 mm. A granoblastic texture of around 0.3 mm grains, presumably formed in the GBM regime, with intracrystalline strain, sutured boundaries and the beginnings of BLG recrystallization. Note the crumpled train of sheet silicates near the bottom of the photograph, reflecting this later deformation. Mylonite (BLG on granulite/GBM)  JC65, crossed-polars slide scan 1.5 x 1 cm. Highly-elongated porphyroclasts derived from coarse-grained quartz (relics of high-T microstructure), extensively overprinted by BLG recrystallization. Mylonitic Mn-quartzite  JC53, plane polarized light, field width 2 mm. A manganiferous quartzite with Mn-silicates, red piemontite (Mn-epidote) and a pale brown mica. The mineral assemblage implies some recrystallization at medium grade to form the sheared, recrystallized microstructure. Mylonite (BLG on granulite/GBM)  JC53, crossed polars, field width 2 mm. The quartz microstructure consists of highly elongated, strongly strained porphyroclasts, with irregular internal subgrain patterns, overprinted by BLG recrystallization with neoblasts that for the most part appear unrelated to the internal subgrains. Mylonite (BLG on granulite/GBM)  JC53, sensitive tint plate, field width 2 mm. This view, same area as the previous photograph, highlights the large size of the strained quartz porphyroclasts, and some of the trains of neoblasts with contrasting lattice orientation.
Mylonite (BLG on granulite/GBM)  JC53, crossed polars, field width 2 mm. Area of strained quartz relics with well-defined BLG recrystallization at grain boundaries. Intense BLG on granulite  JC57, crossed polars, field width 2 mm. Well-developed BLG recrystallization among and between coarse early quartz with strong intracrystalline deformation. Intense BLG on granulite  JC57, sensitive tint plate, field width 2 mm. Well-developed BLG recrystallization among and between coarse early quartz with strong intracrystalline deformation. This view emphasises the lattice orientation contrasts of old grains and neoblasts. Fracture on BLG  JC84, crossed-polars slide scan 1.5 x 1 cm. Intense mylonitic fabric (NE-SW on photo) cut by quartz veins showing a variety of widths and grain sizes. Formed by hydraulic fracture after almost all the ductile deformation. Fracture on BLG  JC84, crossed polars, field width 2 mm. Quartz vein cutting ultramylonite. Vein has much coarser grain size, but indistinct margins and some intracrystalline deformation (subgrains, undulose extinction).
Fracture on BLG  JC84, sensitive tint plate, field width 2 mm. Millimetre-scale quartz vein cutting quartz ultramylonite. Note evidence for post-vein low-T ductile deformation in the form of minor BLG recrystallization near vein margins and at some interior grain contacts. P-T-deformation path Fresh granulite  JC61, crossed polars, field width 2 mm. Diopside quartzite with high-temperature microstructure and fresh granulite-facies assemblage, T ~850°C. Fresh granulite  JC107, crossed polars, field width 2 mm. Granulite-facies assemblage in Mg-Al quartzite with well-preserved orthopyroxene and chessboard microstructure in quartz. Amphibolite-facies Ca-Mg quartzite  JC63, plane-polarized light, field width 4 mm. Calc-silicate quartzite with pale green Ca-amphibole, ragged diopside and sphene. Amphibolite-facies assemblage.
Amphibolite-facies Ca-Mg quartzite  JC63, sensitive tint plate, field width 4 mm. Calc-silicate quartzite with amphibolite-facies assemblage has fully-recrystallized quartz microstructure (SGR with some GBM). Upper greenschist alteration  JC75, crossed polars, field width 0.8 mm. Grain of cordierite showing slight marginal alteration, in matrix of quartz showing evidence for SGR recrystallization. Matrix in quartz-rich domains shows evidence for coarsening by GBM. SGR/GBM regime boundary is estimated to be ~500°C. Upper greenschist alteration  JC75, plane-polarized light, field width 0.6 mm. Cordierite grain pseudomorphed by Mg-chlorite and Al-silicate (kyanite and andalusite). Kyanite granules lie at outer edge of pseudomorph, andalusite extends towards interior, consistent with crossing from Ky to And stability fields during alteration and recrystallization. These cordierite breakdown reactions can occur at T   ~450°C SGR and upper greenschist alteration  JC85, plane polarized light, field width 0.8 mm. Sigmoidal porphyroclast of pale amphibole, with narrow rim and tails formed, during deformation, of slightly darker green amphibole, associated with granules of epidote. This suggests deformation at lowermost amphibolite or upper greenschist facies conditions. This is the specimen shown earlier with well-developed SGR recrystallization. BLG recrystallization and alteration  JC65, crossed polars, field width 2 mm. Blocky pseudomorph of fine-grained Mg-chlorite and sericite/muscovite after cordierite, in a mylonitic matrix of quartz undergoing BLG recrystallization. If the cordierite had broken down to this fine-grained aggregate before the observed deformation in the quartz, it too would have been deformed. So, this style of alteration most probably occurs at temperatures no greater than that of the BLG recrystallization.
Low-T alteration  JC62, sensitive tint plate, field width 4 mm. Granulite-facies quartz microstructure with minor BLG at grain boundaries. However, cordierite grains are completely altered. Evidently, little deformation is needed to allow low-temperature fluid into the rock. P-T-D path  P-T path reconstructed from metamorphic evidence combined with estimated temperature ranges for the different regimes of quartz recrystallization, maily from Stipp et al., 2002, J. Structural Geol.. The experimentally determined reaction boundary for breakdown of Mg-cordierite to chlorite + Al-silicate is also shown, along with the Al-silicate phase boundaries. Tectonic interpretation