ultrahigh-temperature metamorphism (UHT) A cause study of Achankovil Shear Zone (ACSZ) Omer M.Ahmed University of Kerala India MSc geology 2016

Spinel + quartz assemblage in granulites from the Achankovil Shear Zone, southern India: Implications for ultrahigh- temperature metamorphism By; Hisako Shimizu, Toshiaki Tsunogae and M. Santosh Journal of Asian Earth Sciences, 2009

Introduction The southern Indian granulite terrane is known for granulite - facies rocks which is formed during the ‘Pan-African orogeny. The region is composed of Neoproterozoic to Cambrian crustal blocks, dissected by large-scale shear zones (Palghat-Cauvery and Achankovil). The Palghat-Cauvery Shear Zone System (PCSZ), separates the terrane into two parts, Archean Dharwar Craton in the north and the Neoproterozoic Madurai Block in the south. The southern margin of this block is defined by the Achankovil Shear Zone.

Fig. 1. Simplified geological map of the southern part of Kerala and Tamil Nadu ( after GSI, 1995a,b ) PCSZ: Palghat-Cauvery Shear Zone; DC: Dharwar Craton; MGB: Madurai Block; ACSZ: Achankovil Shear Zone;

Geology of the study area The Achankovil Shear Zone (ACSZ) is major lineament of 8-10 km width and >100 km length. The rocks in the zone display a prominent NW-SE trending foliation with steep dips to southwest. Estimation of pressure and temperature (P-T) of this lithology was first carried out by Santosh (1987) based on conventional geothermometers and mineral equilibrium, that gave ◦ C at kbar. Later study done by Nandakumar and Harley (2000) which is slightly higher 925 ± 20◦ C at kbar.

Available geothermobarometric results from the ACSZ thus demonstrate that the area underwent UHT metamorphism. Geochronological investigation of the high-Al orthopyroxene-bearing rocks suggests that the peak UHT metamorphism took place at Ma. The rocks show NW-SE foliation dipping steeply to southwest, which is with the regional trend of the ACSZ.

Petrography Most of the representative rock samples were collected from the Pakkandom open quarry. The major lithology is comprises of Charnockite Khondalite Leptynite Different types of Gneiss Granulite Quartzite Metacarbonate rocks. The mineral assemblages and approximate modal abundances of minerals are listed in Table 1.

Table 1 Mineral assemblages of studied granulites with approximate abundance. +++, abundant; ++, moderate; +, rare; ‘‘, inclusion in garnet. 1: Grt gneiss; 2: charnockite; 3: Spl-Sil gneiss; 4: Grt-Bt-Spl gneiss

3.1. Grt- Opx- Crd Gneiss The Grt-Opx-Crd gneiss is a coarse-grained, granulite-facies rock with a probable pelitic protolith. The mineralogy of a representative sample (KR19-5G1) is plagioclase (30-40%), ortho-pyroxene (20-30%), garnet (10-20%), K-feldspar (10-20%), quartz (5-10%), and cordierite (2-5%) with accessory of biotite, spinel, and sillimanite (Fig. 2a). Garnet is very coarse-grained (3-6 mm),subidioblastic, and contains numerous fine-grained inclusions of sillimanite ( mm), biotite ( mm), spinel ( mm), and quartz ( mm). The most significant feature of this rock is the direct contact relation of fine grained spinel and quartz (Spl + Qtz), which occur only as inclusions in garnet. Spl + Qtz association has been regarded as one of the indicators for decompression at UHT conditions. This is the first finding of such an assemblage from the ACSZ.

The coexistence of spinel and quartz Grt, Pl-Qtz assemblage in Grt-Opx-Crd gneiss PPL view

Back-scattered electron image photographs of granulites from PKDM showing detailed textures of minerals. (a)The two minerals show direct grain contact with no reaction texture between spinel and quartz

Brownish orthopyroxene is sub-idioblastic and very coarse-grained (up to 3cm) occurs adjacent to coarse garnet. It contains inclusions of quartz, K-feldspar, and Fe-Ti oxide. It is describe the equilibrium. Biotite is also present as an inclusion phase and sometimes occurs in contact with quartz grains coexisting with spinel. Cordierite is present as a matrix phase coexisting with quartz- and sillimanite-bearing garnet (Fig. 2e).

Cordierite around garnet with numerous sillimanite inclusions XPL view.

3.2. Grt charnockite Garnet-bearing charnockite occurs as layers parallel to the foliation defined by Grt-Opx-Crd gneiss. A representative sample contains K-feldspar ( 20-30% ), quartz ( 20-30% ), plagioclase ( 10-15% ), garnet ( 5-10% ), orthopyroxene ( 5-10% ), and biotite ( 5-7% ). With accessory spinel, Fe-Ti oxide, and zircon (Fig. 2g). Garnet ( mm) is subidioblastic and contains numerous inclusions of quartz ( mm), biotite ( mm), zircon ( mm), and Fe-Ti oxide. Orthopyroxene is in direct contact with garnet (Fig. 2g) and in part slightly aligned along a weak foliation.

Grt-Opx-Pl-Qtz assemblage in Grt charnockite PPL view

3.3. Spl-Sil Gneiss Spl-Sil gneiss is characterized by the occurrence of numerous spinel and sillimanite grains enclosed in plagioclase. The mineralogy of a representative sample is K-feldspar (20- 25%), quartz (20-25%), plagioclase (20-25%), garnet (25%) and spinel (10%) With accessory biotite, sillimanite, and Fe-Ti oxide. Spinel ( mm) is subidioblastic to idioblastic occurs as aggregates mostly in plagioclase and rarely in garnet (Fig. 2h and i). It does not show any contact relation with quartz. The spinel in plagioclase is aligned parallel to the matrix foliations and sillimanites in plagioclase are randomly oriented. Coarse-grained garnet (up to1.5 cm) is subidioblastic and often also elongated along the foliation (Fig. 2i).

Coarse-grained garnet (up to 1.5 cm) is subidioblastic and occasionally elongated along the foliation PPL view sillimanite occurs as aggregates mostly in plagioclase

3.4. Grt-Bt-Spl Gneiss The rock type is composed dominantly of quartz (25%), plagioclase (10%), garnet (10%) and biotite (7-10%) with accessory K-feldspar, spinel, sillimanite, cordierite, Fe-Ti oxide, and zircon (Fig. 2j). Garnet ( mm) is subidioblastic and contains numerous inclusions of sillimanite ( mm), biotite ( mm), spinel ( mm), and quartz ( mm). The inclusion sillimanites are aligned along the matrix foliation defined by biotites. Biotite is mostly present as inclusions in garnet and is rare in the matrix. Fine-grained symplectic aggregates of cordierite and quartz occur around garnet (Fig. 2k). Fe-Ti oxide (complex intergrowth of magnetite and ilmenite) is also present in contact with garnet and/or the symplectite (Fig. 2l).

(j) Grt-Spl-Pl assemblage in Grt-Bt-Spl (k) Fine-grained aggregates of cordierite and quartz occur around garnet (l) The occurrence of Fe-Ti oxide with the symplectite. PPL view XPL view.

Mineral chemistry Chemical analyses of all the minerals were carried out using a WDS electron microprobe analyzer [EMPA] at the University of Tsukuba, Japan. The data were regressed using oxide-ZAF correction method. Below, the description of mineral chemistry of examined gneiss.

4.1. Garnet [ X 3 Y 2 (SiO 4 ) 3 X = Ca, Fe 2+, Mg, Mn 2+ ; Y = Al, Cr, Fe 3+, Mn 3+, Si ] Garnet in Grt-Opx-Crd gneiss and Grt charnockite is essentially a solid solution of pyrope and almandine ( ) with low contents of spessartine (<3 mol.% ) and grossular ( <2 mol.% ) ( Table 2 ). The mineral shows a general rim ward increase of almandine content and slightly pyrope-rich core.

Table 2 Representative electron microprobe analyses of garnet (O = 12).

4.2. Spinel [ XY 2 O 4 ] X may be (Mn,Fe 2+,Mg,Ni,Zn) Y may be (Al,Fe 3+, Cr) 2 The composition of spinel in the studied rocks varies depending on its occurrence and textural association. Matrix spinel coexisting with ilmenite in (Grt-Opx-Crd gneiss) has the highest ZnO content of wt.% and the highest X Mg ratio of In contrast, spinel coexisting with quartz in the same sample shows lower ZnO content of wt.% and negligible Cr 2 O 3 (<0.10 wt.%), and lower X Mg ratio ( ). Low ZnO is indicator to UHT Spinel coexisting with plagioclase in (Spl-Sil gneiss) shows the lowest X Mg (0.34) and ZnO content (1.32 wt.%).

Table 3 Representative electron microprobe analyses of spinel (O = 4).

4.3. Orthopyroxene All orthopyroxene grains in Grt-Opx-Crd gneiss are compositionally nearly homogeneous in terms of Al 2 O 3 and X Mg content.

Table 4 Representative electron microprobe analyses of orthopyroxene.

4.4. Biotite Biotite is Mg-rich and characterized by high-TiO 2 content (up to 6.4 wt.%) (Table 5). The inclusion of biotite within garnet is relatively Mg- rich (X Mg = ) compared to the matrix phase ( ) Greenish biotite with higher X Mg (0.80) and lower TiO 2 values ( %) occurs together with Spl + Qtz association in garnet (Fig. 2c).

Table 5 Representative electron microprobe analyses of biotite (O = 22).

4.5. Other minerals Plagioclase in the examined samples is albite-rich Ab K-feldspar has a composition of Or 90. Cordierite in all the samples shows a uniform magnesian composition with X Mg = Ilmenite, Magnetite and Sillimanite are close to their ideal composition.

5. Metamorphic P-T conditions If we know the pressure (P) and temperature (T) at which metamorphic rock equilibrated, we can determine where and how the rock has been formed. Methods of determine the P-T Index minerals: characteristic minerals that provide an indication of the temperature and pressure conditions at which a rock formed (e.g kyanite). Not all rocks have a suitable bulk composition to produce index minerals. Metamorphic facies: assemblages of minerals, each characteristic for a particular bulk composition and indicating the range of pressure-temperature conditions at which the rock equilibrated For example, blue schist facies, indicator to high-pressure - low-temperature conditions. Qualitative methods don't necessarily provide information about both pressure and temperature.

Thermobarometry Is the quantitative determination of the temperature and pressure at which a metamorphic or igneous rock reached chemical equilibrium. The study in the area done by using different geothermobarometers calibrated on the basis of experimental, empirical, thermodynamic parameters and using different composition-activity models for the various minerals. Several geothermobarometers are applicable for the Grt-Opx-Crd Gneiss and Grt Charnockite from the studied area.

5.1. Grt-Opx Geothermobarometers The Grt-Opx geothermometer was applied to poikiloblastic garnet and matrix orthopyroxene in Grt-Opx-Crd Gneiss Grt Charnockite. Application of the method of Lee and Ganguly (1988), which is based on experimental calibration of Fe-Mg fractionation between garnet and orthopyroxene Gave temperature ranges of o C (Grt-Opx-Crd gneiss) and o C (Grt charnockite) at 9 kbar. P-T diagram (Fig. 5).

Fig. 5. P-T diagram Showing P-T path (gray array) for Grt-Opx-Crd gneiss and Grt charnockite

5.2. Grt-Crd geothermobarometers The Grt-Crd geothermometer of Bhattacharya et al. (1988) was applied to estimate the conditions of retrograde metamorphism. Application of this method to cordierite and adjacent garnet yielded a temperature range of o C at 4 kbar. for (Grt–Bt–Splgneis) Retrograde pressure was estimated by using an experimental geobarometer of Nichols et al. (1992) yielded a pressure range of kbar at 650 o C. Fig. 5.

Fig. 6. Composition of core of orthopyroxene in Grt-Opx-Crd gneiss y(opx) (= Al + Si2) isotherms of Kelsey et al. (2003b) at 9 kbar are also plotted in the diagram.

Conclusion Based on detailed petrographic and mineralogical data in the ACSZ of Spl + Qtz bearing Grt - Opx - Crd granulites, it has been estimated P - T of C at 8-10 kbar as the peak metamorphic condition for the granulite - facies Such high-temperature condition is provided evidence for UHT metamorphic Generally, UHT metamorphic rocks are characterized by diagnostic minerals assemblages such as : Spr + Qtz, Spl + Qtz, Opx + Sil + Qtz

The Spl + Qtz association reported in this study is a robust evidence for UHT metamorphism within the ACSZ. The assemblage should not be regarded as a diagnostic evidence of UHT metamorphism when (1) the two minerals are not observed as stable major phases or (2) they are separated, and shows no contact relation with each other. The rocks within the accretionary belt in the ACSZ have undergone a rapid decompression history and retrograde mineral growth

Reference Hisako Shimizu,Toshiaki Tsunogae and M.Santosh. Spinel + quartz assemblage in granulites from the Achan kovil Shear Zone, southern India: Implications for ultrahigh- temperature metamorphism. Journal of Asian Earth Sciences 36, 209–222 (2009).

THANKSTHANKS THANKSTHANKS Omer M.Ahmed,