Petrology of Metamorphic Rocks from the Highland and Kadugannawa Complexes, Sri Lanka

Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Journal of the Geological Society of Sri Lanka Vol. 14 (2011): 103-122. C.B. Dissanayake Felicitation Volume

Sanjeewa P.K. Malaviarachchi*1 and Akira Takasu2

1Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka. 2Department of Geoscience, Shimane University, Matsue 690-8504, Japan.

(*Corresponding author, email: [email protected]).

ABSTRACT

Petrological investigation by electron probe micro analyser (EPMA) was carried out on pelitic, intermediate and mafic granulites from the central Highland Complex (HC) and the Kadugannawa Complex (KC) of Sri Lanka. Among the HC pelitic rocks, spinel-bearing garnet biotite sillimanite gneiss shows the highest temperature conditions. Equilibrium pairs of biotite and garnet cores record peak metamorphic temperatures of 810-830 oC. Spinel- absent sillimanite gneiss records peak temperatures of 810 oC. Peak metamorphic pressure is estimated to be 8 kbar, and that for spinel-absent rocks is 9 kbar. Mafic granulites of the HC yield temperatures of 890-900 0C and a pressure of ~11 k bar. Intermediate rocks show a temperature of 760 0C and a pressure of 9 kbar. In KC pelitic rocks, equilibrium pairs of garnet and biotite core compositions recorded a temperature of 750 0C whereas mafic rocks yielded a temperature of 690 0C from garnet cores and matrix biotite. Suitable equilibrium mineral assemblages for barometry were absent the KC rocks. The P-T trajectory of the Highland Complex pelitic granulites shows a clockwise P-T path. Presence of kyanite as rare inclusions in pre-peak garnet indicates an initial pressure increase before the peak metamorphism. The rocks subsequently experienced continuous temperature increase under slightly decreasing or constant pressure followed by cooling and gradual decompression after peak metamorphism. The P-T paths of mafic and intermediate granulites are consistent with magmatic intrusion or magmatic underplating occurring at depth and subsequent cooling took place during the uplift. Accordingly, the clock-wise P-T path for meta- sedimentary granulites and cooling path for meta-igneous granulites document possible deep crustal processes by which continental crust grows, similar to the phenomena in most granulite terrains of the world.

INTRODUCTION nor gabbroic, granitic, pegmatitic and aplitic igneous intrusions characterize the Sri Lankan The metamorphic basement of Sri Lanka basement. Due to the tectonic amalgamation of has been considered as a key terrain to under- amphibolite to granulite-facies terrains of di- stand the evolution of the Gondwana supercon- verse isotopic signature containing a diversity of tinent. In a palaeogeographic reconstruction, Sri rock types in a relatively small area, Sri Lanka is Lanka was located close to India, Madagascar of great interest in the fields of petrology, geo- and East Antarctica. The geology of the island is chronology and structural geology. therefore a key to understand the Gondwana Based on the Nd- model age mapping by evolution. Calc-silicate and Mg-, Al-rich meta- Milisenda et al (1988) and zircon geochronology sedimentary and mafic to felsic meta-igneous (Kröner et al., 1991), the supracrustal rocks of high- to ultra-high temperature granulites to- Sri Lanka have been subdivided into four major gether with amphibolites, migmatites and mi- terrains (e.g. Cooray, 1994): the Highland (HC),

103 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

Wanni (WC), Vijayan (VC) and Kadugannawa an electron probe micro analyser (EPMA) to Complexes (KC) as shown in the (Fig. 1). Al- reveal the constituent mineral chemistry. though on the basis of similarities in structures Kehelpannala (1997) included the Kadugannawa REVIEW OF PREVIOUS WORK Complex in the Wanni Complex, we have ad- hered to Cooray (1994) classification in this pa- Typical high temperature metamorphic per. conditions are well established from the Sri Lankan metamorphic basement by various re- searchers suggesting a clock-wise P-T path. However, some recent petrological studies have noted ultra-high temperature (UHT) metamorphism at several localities in the HC and these have significant implications on the thermal events and tectonics of Sri Lanka and related Gondwana fragments. Therefore, it is necessary to briefly discuss the previous work.

a) Petrology Previous P-T studies making use of pelitic and felsic to mafic granulites have established a P/T zonation across the Sri Lankan granulite terrain. Pressures and temperatures decrease from 9-10 kbar and 830 0C in the East and South east to 5-6 kbar 700 0C in the North West (Faul- haber and Raith, 1991; Schumarcher & Faul- haber, 1994). The P-T path for pelitic rocks, based on the sequence kyanite and staurolite (inclusions in garnet) followed by sillimanite, and then by andalusite, is clockwise (Hiroi et al., 1994, Raase and Schenk, 1994). By contrast, reaction textures involving garnet formation in metamorphosed mafic rocks (Perera, 1987; Schumacher et al., 1990; Prame, 1991a) and

exsolution of pyroxenes have been used to sug- Figure 1: Litho-tectonic units of Sri Lanka (after Coo- gest isobaric cooling. ray, 1994) showing the study area. Osanai (1989) first reported sapphirine Although petrologic research on Sri Lankan bearing granulites from the HC, and other UHT metamorphic rocks has been carried out exten- assemblages have been reported by Kriegsman sively, this study was undertaken to present an (1991), Kriegsman and Schumarcher (1999), updated dataset particularly on mineral chemis- Osanai et al. (2000, 2003), Sajeev et al (2003), Sajeev and Osanai (2002, 2003, 2004a) suggest- try. Especially, electron microprobe data 0 (EPMA) of constituent minerals of the ing UHT metamorphism above 1050 C and 11- Kadugannawa Complex gneisses are rarely 12 kbar. Evidence of isobaric cooling after UHT found in the literature. Therefore, petrological and a multi stage evolution was presented by investigation of some pelitic and intermediate Sajeev and Osanai (2002, 2004a). These P/T to mafic granulites from the central Highland conditions are in contradiction with the other granulites in the surrounding area, which pre- Complex and some pelitic and mafic rocks from 0 the Kadugannawa Complex were carried out in serves a maximum of 850-900 C and 9-10 kbar this study. and determined to be metamorphosed during The samples were collected systematically the Pan African tectonothermal event. Also, from both Highland and Kadugannawa Com- Sajeev and Osanai (2004b) reported osumillite plexes (Fig. 2). After careful petrographic obser- from Sri Lanka, and its implications on UHT vations, selected thin sections were analyzed by metamorphism, though they could not distinguish whether it is a product of the Pan African

104 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Figure 2: Geology map of the study area showing sample localities plotted on the Kandy-Hanguranketha Sheet published by the Geological Survey and Mines Bureau, (1996). tectonothermal event. Also, Sajeev and Osanai orthogneisses and paragneisses assigned an age (2004b) reported osumillite from Sri Lanka, and of 550-610 Ma for high-grade metamorphism. its implications on UHT metamorphism, though Osanai et al (1996) reported a ca. 670 Ma they could not distinguish whether it is a prod- metamorphic event from saphirine-bearing uct of the Pan African metamorphism or a relic granulites, based on Sm-Nd whole rock isochron of an older metamorphic event due to lack of data. They also identified a retrograde age of geochronological data. Sajeev and Osanai ca. 520 Ma, based on the whole rock biotite (2004a) argued that the UHT granulites of the internal isochron method. Sajeev et al. (2003) HC probably evolved along an anticlockwise reported an internal Sm-Nd isochron age for the path. UHT metamorphism of ca.1500 Ma based on the analysis of a garnet core, whole rock and b) Geochronology felsic fraction of ultra-high temperature (UHT) Milisenda et al (1988) presented Nd granulites. They also reported an orthopyrox- model age data, and identified three distinct ene reference isochron age of 550 Ma, implying age provinces. The Highland Complex has model that these UHT granulites were also affected by ages of 3-2.2 Ga, indicating derivation mainly the Pan-African metamorphism. from late Archean sources, and is bounded to the East and West by late Proterozoic gneisses STUDIED SAMPLES AND THE ANALYTICAL of the Vijayan Complex and Wanni Complex METHODS with model ages of 2-1.1 Ga. An ion microprobe (SHRIMP) U-Pb study of zircons (Kröner et al., General geology of the study area and sam- 1987) documented 3.2–2.0 Ga for detrital grains ple localities is shown in the Figs. 1 and 2, re- from the Highland Complex. In addition, this spectively. Eight pelitc granulites, three mafic study revealed some indication of Pb loss at granulites and six intermediate granulites were about 1.1 Ga, which was attributed to granulite studied from the Highland Complex. Two pelitic facies metamorphism. gneisses and six mafic gneisses were studied Later U-Pb zircon and monazite studies (e.g. from the Kadugannawa Complex (Table 1). Hölzl et al., 1991; Kohler et al., 1991; Baur et al., 1991; Kröner and Williams, 1993) from both

105 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

Hand specimens and thin sections were samples. Quartz commonly occur both as inclu- studied for petrography, and the mineral tex- sion in garnet and in the matrix with plagioclase tural features were studied using a polarizing and K-feldspar. Ilmenite and rutile occur both as microscope. Chemical compositions of con- inclusions in garnet as well as in the matrix with stituent minerals of rocks and the back scat- other accessory phases like zircon and mona- tered electron images (BSE) were obtained us- zite. ing a JEOL JXA-8800M electron probe micro analyser (EPMA) at Shimane University, Japan. HC Mafic granulites The analytical conditions used were 15 kV ac- These rocks are generally coarse grained celerating voltage, 25 nA probe current and and poorly foliated. The garnet amphibole py- 5µm probe diameter. Representative mineral roxene mafic granulite of this study consists analyses are given in the Tables 2 to 6. mainly of garnet, cpx, opx, pargasitic amphibole, plagioclase, quartz, and titanite (Table 1). MINERAL TEXTURES Garnet occurs as poikiloblasts up to 15 mm in diameter are sometimes idioblastic with plagio- HC pelitic granulites clase, quartz, titanite and iron oxide inclusions. Two types of pelitic granulites were identi- Also, some samples have rare cpx – plagioclase fied as, garnet biotite sillimanite gneiss and bio- symplectites (Fig. 4a,c) within outer core of the tite gneiss (Table 1). They have gneissose folia- garnet porphyroblasts. Garnet porphyroblasts tion defined by preferred orientation of biotite are partially replaced by secondary biotite and and/or sillimanite, with alternation of layers ilmenite at their margins. composed of quartz and feldspar. Plagioclases in the matrix are porphyroblas- In garnet biotite sillimanite gneiss, K- tic and are mostly untwinned. However, these feldspar lamellae occur in plagioclase (anti- plagioclase porphyroblasts rarely show lamella perthite texture), together with fine quartz in- twinning and oscillatory zoning (Fig. 4b). These tergrown in the host (Fig. 3a). Subhedral to an- grains contain fine exsolution blebs of K- hedral garnet porphyroblasts up to 8 mm in di- feldspar. Plagioclase also occurs as inclusions in ameter commonly contain biotite, sillimanite, garnet and as symplectites with opx after gar- ilmenite, quartz inclusions and rarely hercynitic net (Fig. 4c). spinel and kyanite too. Occasionally, garnet Orthopyroxene occurs as porphyroblasts up porphyroblasts are replaced by sillimanite and/ to 5 mm and also as both fine grained and or by biotite or symplectire of biotte and quartz coarse grained symplectites with plagioclase at the rim. Mainly biotite and quartz inclusions (Fig. 4c and d, respectively). Also, coarse occur in the garnet core, while sillimanite oc- grained opx symplectites are found to be re- curs in the mantle. placed by pargasitic amphibole. Opx porphyro- Rare kyanite occurs only as inclusions in blasts have biotite, plagioclase and opaque garnet (Fig. 3b). Sillimanite makes very fine mineral inclusions and these porphyroblasts are needles in garnet (Fig. 3c) but is prismatic and later replaced by secondary biotite and ilmen- medium grained in the matrix with typical ite. Clinopyroxene was found in the symplectite transverse fractures, fibrolitic when associated included in the garnet porphyroblasts (Fig. 4a, with hercynite symplectites. Aggregates of silli- c), and as well as rare inclusions in garnet. Cpx manite collectively form a shape of a relict por- is totally absent in the matrix. phyroblast, very likely kyanite. Rare sillimanite Amphibole grains texturally postdate the pseudomorphs after kyanite occur in the rim garnet porphyroblasts, as evident even in hand part (Fig. 3c, d). specimen scale, by the foliation defined by am- Hercynitic spinel occurs as rare inclusions in phibole wrapping around the garnet. These garnet porphyroblasts, in garnet biotite silli- amphiboles are pargasite. Some of these par- cross nicol manite gneiss. In addition, it occurs in symplec- gasites replace opx. Due to strong retrogres- tites associated with fibrolite at garnet rims and sion, chlorite, quartz and hematite assemblages along fractures (Fig. 3c). are found between garnet porphyroblasts. Biotite forms a preferred orientation in the Opaque phases include magnetite and ilmenite. matrix as well as random grain overgrowths replacing garnet rims. Plagioclase grains show well developed polysynthetic twinning in many

106 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Figure 3: a) Back scattered electron image of anti-perthite in the sample 8-1of the HC. b) Occurrence of rare kyanite inclusions in garnet in the sample 14A3 of the HC (PPL image). c) PPL image showing inclusions of Bio- tite and sillimanite in garnet of the same sample in b). Same garnet also contains sillimanite pseudomorph after kyanite and hercynite symplectites. d) CPL image showing the close view of the sillimanite pseudomorph after kyanite shown in c).

HC Intermediate granulites curring in symplectites with plagioclase in meta- Intermediate granulites include charnockitic granitoid where opx is absent. gneisses and hornblende and biotite bearing In both rock types, plagioclase occurs as meta-granitoids. Usually, charnockitic gneisses porphyroblasts, inclusions in garnet and coro- have a characteristic ‘greasy’ lustre or appear- nae on garnet. Generally, plagioclase show al- ance in hand specimen, exhibiting weak gneissic bite twining and include fine quartz grains. K- foliation. In contrast, meta-granitoid shows a feldspar and quartz occur in excess in both preferred orientation of minerals such as horn- lithologies. blende, biotite and ribbon quartz. Also, this rock Amphiboles occur only in meta-granitoid, as shows a strong lineation defined by graphite. porphyroblasts mainly associated with porphy- Many quartz grains are highly stretched and roblastic titanite. show subgrain boundaries. Charnockitic gneisses show retrograde al- In some charnockitic gneisses garnet occurs teration products of greenschist facies such as as subhedral to anhedral porphyroblasts up to 5 chlorite and calcite. Symplectite of cpx + rtl + mm (Table 1) and contain quartz and plagio- ilm after garnet were also observed in the clase inclusions. Many garnets are replaced by meta-granitoid. biotite and some show breakdown textures Opaque minerals like ilmenite, magnetite forming fine opx grains and reaction rims of and rutile occur in the charnockitic gneiss and plagioclase. ilmenite is the only opaque phase in the meta- In contrast, garnet porphyroblasts of meta- granitoid. granitoid are free from inclusions and occur in sizes of 3-5mm anhedral grains. Some garnets KC Pelitic gneiss are completely broken down to form cpx- Pelitic gneisses in the Kadugannawa Com- bearing symplectites, associated with amphi- plex consist of quartz, K-feldspar, plagioclase, bole, biotite and opaque. Hypersthene in char- and biotite with accessory minerals such as nockitic gneiss commonly occurs as anhedral muscovite, rutile, ilmenite, and zircon (Table 1). porphyroblasts and is associated with plagio- Garnet is rarely found and occurs as porphyro- clase rims after garnet. Rare cpx was found oc- blasts up to 5 mm with inclusions of biotite

107

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108 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Table 2: Representative electron microprobe analysis for pelitic granulites, HC

Table 2. Representative electron microprobe analysis for pelitic ganulites, HC Mineral Garnet Biotite Hercynite Sillimanite Kyanite Plagioclase Ilmenite Rutile Sample 8 8-1 14A 8 14A 10 14A 14A3 8 14A 14A3 8 14A 8 14A core rim core rim core rim inc sec inc sec sec inc sym inc sym

SiO2 37.76 37.05 37.85 37.41 37.71 37.63 36.83 35.08 36.07 34.51 36.64 0.01 0.02 0.02 0.02 64.47 35.90 36.65 60.86 59.29 0.03 0.12

TiO2 0.01 0.05 0.00 0.00 0.00 0.02 3.47 4.60 4.43 5.59 6.07 0.01 0.02 0.00 0.01 0.01 0.00 0.00 0.00 0.00 42.53 88.74

Al2O3 21.40 21.20 21.64 20.95 21.56 20.91 17.70 17.12 17.33 17.03 13.96 57.04 56.72 56.50 57.32 18.69 61.34 62.70 24.39 25.21 0.04 0.11 FeO* 31.77 35.46 30.91 32.55 30.60 33.99 15.31 17.82 14.99 16.49 17.92 27.55 25.70 28.68 27.10 0.04 0.70 0.74 0.02 0.09 54.75 8.87 MnO 0.72 1.12 0.68 0.91 0.60 0.66 0.00 0.01 0.02 0.00 0.04 0.05 0.00 0.05 0.02 0.00 0.03 0.00 0.02 0.00 0.01 0.02 MgO 7.23 4.57 7.49 6.09 7.20 5.80 13.34 11.67 13.86 11.87 12.47 8.20 8.86 7.90 8.51 0.00 0.00 0.01 0.00 0.01 0.08 0.00 CaO 1.36 1.32 1.41 1.22 1.37 0.89 0.00 0.00 0.00 0.08 0.01 0.03 0.01 0.03 0.00 0.09 0.01 0.00 6.29 6.22 0.01 0.05

Na2O 0.02 0.04 0.00 0.01 0.01 0.01 0.12 0.12 0.13 0.10 0.04 0.17 0.12 0.20 0.14 1.11 0.02 0.00 8.00 7.73 0.02 0.03

K2O 0.03 0.06 0.00 0.03 0.06 0.04 9.55 9.91 9.63 9.96 9.90 0.06 0.04 0.03 0.03 13.95 0.00 0.01 0.29 0.67 0.02 0.06

Cr2O3 0.03 0.00 0.00 0.01 0.08 0.05 0.04 0.09 0.26 0.14 0.04 0.02 0.19 0.58 0.39 0.00 0.01 0.08 0.00 0.00 0.13 0.10 ZnO 5.94 7.22 4.99 6.72 Total 100.33 100.87 99.99 99.19 99.18 100.00 96.36 96.41 96.71 95.76 97.10 99.09 98.90 98.98 100.26 98.36 98.02 100.18 99.86 99.22 97.63 98.07

O = 12 12 12 12 12 12 22 22 22 22 22 4 4 4 4 5 5 5 8 8 3 2

Si 2.964 2.953 2.968 2.988 2.977 2.992 5.434 5.280 5.317 5.212 5.479 0.000 0.001 0.001 0.001 2.997 0.992 0.991 2.712 2.667 0.001 0.002 Ti 0.000 0.003 0.000 0.000 0.000 0.001 0.385 0.521 0.491 0.631 0.683 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.871 0.943 Al 1.980 1.991 1.999 1.971 2.007 1.960 3.078 3.036 3.012 3.032 2.461 1.968 1.977 1.949 1.967 1.024 1.998 1.999 1.281 1.337 0.001 0.002 Fe 2.085 2.364 2.026 2.174 2.020 2.261 1.889 2.243 1.848 2.082 2.241 0.674 0.636 0.702 0.660 0.002 0.016 0.017 0.001 0.004 1.246 0.105 Mn 0.048 0.075 0.045 0.062 0.040 0.044 0.000 0.001 0.003 0.000 0.005 0.001 0.000 0.001 0.001 0.000 0.001 0.000 0.001 0.000 0.000 0.000 Mg 0.847 0.543 0.875 0.725 0.848 0.687 2.935 2.618 3.046 2.672 2.781 0.358 0.391 0.345 0.370 0.000 0.000 0.000 0.000 0.000 0.003 0.000

Table 3: Representative electron microprobe analysis for mafic granulites, HC

Mineral Garnet Opx Cpx Amphibole Plagioclase biotite chlorite Ilmenite Titanite Sample 14B 14B1 14B2 14B 14B2 14B 14B1 14B2 14B1 14B2 14B 14B1 14B2 14B 14B1 core rim core rim por f-sym c-sym

SiO2 37.50 36.86 38.07 38.29 49.59 50.74 51.13 50.51 42.89 46.41 53.88 47.90 35.01 37.10 0.05 0.03 30.02 30.37

TiO2 0.04 0.10 0.03 0.03 0.10 0.08 0.04 0.25 1.94 0.00 0.00 0.00 5.50 2.25 47.18 45.80 47.37 36.96 38.31 Al2O3 20.80 21.59 20.31 20.94 1.51 2.42 0.70 1.94 11.16 33.66 28.35 33.07 14.41 9.61 0.01 0.04 0.05 1.95 1.63 FeO* 30.42 23.96 31.51 29.15 32.06 28.07 29.48 13.40 16.28 0.30 0.13 0.25 16.27 16.07 51.21 52.85 50.40 1.03 0.99 MnO 1.42 0.07 0.69 0.61 0.38 0.11 0.30 0.28 0.09 0.02 0.00 0.08 0.00 0.12 0.09 0.20 0.19 0.01 0.07 MgO 3.60 4.08 5.07 5.30 15.50 18.79 17.11 11.26 10.96 0.01 0.02 0.01 12.06 8.68 0.74 0.79 0.81 0.04 0.03 CaO 6.09 13.75 4.30 4.80 0.61 0.23 0.54 21.94 11.64 18.69 11.48 16.47 0.28 11.18 0.07 0.01 0.05 29.69 27.27

Na2O 0.02 0.00 0.01 0.00 0.05 0.00 0.02 0.26 1.05 1.38 5.00 1.94 0.25 0.86 0.11 0.00 0.00 0.01 0.00

K2O 0.03 0.04 0.03 0.04 0.05 0.05 0.02 0.04 1.67 0.10 0.25 0.11 9.45 1.70 0.06 0.04 0.02 0.04 0.03

Cr2O3 0.07 0.00 0.07 0.08 0.04 0.10 0.00 0.07 0.07 0.00 0.01 0.00 0.12 0.00 0.11 0.17 0.03 0.00 0.00 Total 100.00 100.47 100.08 99.24 99.89 100.59 99.34 99.94 97.74 ##### 99.11 99.82 93.34 87.57 99.64 99.93 98.94 99.75 98.68

O = 12 12 12 12 6 6 6 6 23 8 8 8 22 10 3 3 3 4 4

Si 2.990 2.891 3.020 3.029 1.945 1.929 1.985 1.931 6.488 2.133 2.458 2.201 5.422 2.749 0.001 0.001 0.000 0.791 0.802 Ti 0.002 0.006 0.002 0.002 0.070 0.108 0.031 0.088 0.221 0.000 0.000 0.000 0.641 0.125 0.922 0.900 0.930 0.732 0.761 Al 1.955 1.996 1.898 1.952 0.003 0.002 0.001 0.007 1.989 1.823 1.525 1.790 2.629 0.840 0.000 0.001 0.002 0.061 0.051 Fe 2.028 1.571 2.090 1.928 0.000 0.003 0.000 0.002 2.060 0.012 0.005 0.010 2.107 0.996 1.113 1.155 1.100 0.023 0.022 Mn 0.096 0.005 0.046 0.041 0.906 1.065 0.990 0.642 0.011 0.001 0.000 0.003 0.000 0.008 0.002 0.004 0.004 0.000 0.001 Mg 0.428 0.478 0.599 0.625 0.990 0.849 0.957 0.305 2.472 0.001 0.001 0.001 2.784 0.958 0.029 0.031 0.032 0.002 0.001 Ca 0.520 1.156 0.365 0.407 0.013 0.003 0.010 0.009 1.886 0.920 0.561 0.811 0.046 0.888 0.002 0.000 0.001 0.838 0.772 Na 0.004 0.000 0.002 0.000 0.026 0.009 0.022 0.899 0.308 0.123 0.443 0.173 0.075 0.123 0.006 0.000 0.000 0.000 0.000 K 0.003 0.004 0.003 0.004 0.004 0.000 0.002 0.019 0.322 0.006 0.014 0.007 1.866 0.161 0.002 0.001 0.001 0.001 0.001 Cr 0.005 0.000 0.005 0.005 0.002 0.002 0.001 0.002 0.008 0.000 0.000 0.000 0.015 0.000 0.002 0.003 0.001 0.000 0.000 Total 8.031 8.107 8.030 7.992 4.020 4.015 4.000 4.027 ##### 5.019 5.007 4.994 15.586 6.848 2.079 2.097 2.070 2.448 2.411 * Total Fe as FeO; por - porphyroblasts; f-sym - fine grained symplectite; c-sym - coarse grained symplectite and quartz. Sometimes garnet porphyroblasts Garnet-bearing rocks consist of plagioclase, are replaced by biotite along the rim (Fig. 5a). quartz, biotite, rutile and ilmenite. Garnet oc- Quartz in the matrix with plagioclase, K-feldspar curs as porphyroblasts up to 3 mm and occa- and biotite forms a preferred orientation. Pla- sionally contains quartz and biotite as inclusions gioclase rarely shows polysynthetic twinning in (Fig. 5b). Garnet porphyroblasts are replaced by these rocks. Rare muscovite was found in the biotite overprints at their margins. Garnet- KC rocks and ilmenite occurs in the matrix. absent mafic rocks contain hornblende, quartz, Quartz and K-feldspar are also found in excess. plagioclase, biotite and ilmenite and are repre- sented by hornblende gneisses and migmatitic KC Mafic gneiss gneisses where the dominant mineral being Mafic gneises in Kadugannawa Complex in- hornblende and plagioclase. Rare cpx is also clude garnet-bearing and garnet-absent rocks found as porphyroblasts in some garnet and (Table 1). These rocks are generally coarse hornblende absent rocks. Rare hornblende in- grained and poorly foliated and exhibit a grano- clusions are present in plagioclase. blastic polygonal texture.

109 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

Figure 4: a) Relict cpx inclusions with plg preserved in the outer core of a garnet poikiloblast of the sample 14B of the HC. b) Plagioclase in the matrix showing oscillatory zoning with exsolusion blebs of K-feldspar in the same sample. c) Opx+ Plg symplectites at garnet rims in the same sample. d) Occurrence of plg corona in association with Opx and Qtz around garnet in the sample 14A of the HC.

Figure 5: a) Replacement of garnet rims by retrograde biotite in the sample 50 of the KC. b) Occasional inclusions of quartz and biotite in garnet porphyroblasts of the sample 51 of the KC.

MINERAL CHEMISTRY decreases slightly from core to rim. The grossular component also has a similar trend, with Pelitic granulites – Highland Complex maximum ratio of XGrs=0.03 preserved in the porphyroblastic cores. Garnets which are Garnet rimmed by ilmenite and hematite have the Different generations of garnet are present highest almandine contents. Pyrope content in the Highland Complex pelitic gneiss, based on varies from 0.4 to 0.2, whereas the grossular inclusion patterns. These are garnets which content varies from 0.05 to 0.001. contain biotite, sillimanite and quartz; those In addition, some garnets which contain with rare kyanite inclusions; and those with rare hercynite + ilmenite inclusions show com- hercynite and ilmenite inclusions. Garnets in positional heterogeneity. these rocks represent almandine-rich Fe-Mg solid solutions (up to XAlm = 0.8), where XPrp ratio

110 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Plagioclase gneiss and inclusion-free fine grained garnets of Plagioclase is oligoclase to andesine in charnockitic gneiss, but increases in porphyro- composition (XAn = 0.23 to 0.35). There are no blastic garnets of charnockitic gneiss. Meta- significant differences between the An contents granitoid garnets have almost constant compo- of plagioclase inclusions in garnet and matrix sition. Pyrope contents of garnets from the gar- plagioclases; however, garnet-absent biotite net amphibole pyroxene gneiss vary from 0.27 gneiss shows the minimum An content. to 0.19. In the case of charnockitic gneiss, pyrope varies from 0.14 to 0.08. In meta- Spinel granitoid, the pyrope content is almost con- Spinel is rich in hercynite component with a stant.

Fe/(Fe + Mg) of 0.67 – 0.65. Inclusion spinel has The highest grossular content (XGrs= 0.38) is higher Zn content (max. ZnO = 7.15 wt %) while found in a garnet from amphibole pyroxene the retrograde spinel has a maximum ZnO con- gneiss. Grossular content decreases from core tent of 5.9 wt %. to rim in both garnet amphibole- pyroxene gneiss and charnockitic gneiss garnets. In meta- Kyanite granitoids, garnet rims are richer in grossular Kyanite occurs as rare inclusions in garnet than the cores (max. X = 0.23). porphyroblasts and contains ~1 wt % of FeO. Grs

Orthopyroxene Sillimanite Opx occurs as porphyroblasts, in fine- Sillimanite contains 1 wt% FeO and 0.1 wt % grained symplectites and coarse-grained sym- Cr; however, the oxide total was around 97.5 %. plectites after garnets, with X = 0.85 – 0.96. Mg Alumina contents differ markedly in the garnet Biotite amphibole pyroxene gneiss. Opx in the sym- Biotites contain about 4.5 – 6.8 wt % of plectites after garnet has Al contents from 2.11 TiO , and their Fe/(Fe + Mg) varies from 0.52 to 2 to 3.12 wt %, whereas the opx in the coarse 0.64. Mg ratio vs. Ti (per formula unit, p.f.u) grained symplectites has Al O contents ranging varies depending on the textural setting. Thus, 2 3 from 0.70 to 1.19 wt%. However, porphyroblas- biotite inclusions in garnet and secondary bio- tic opx in the matrix contains 1.35 to 2.09 wt% tite overprints on garnet have contrasting com- Al O . Symplectitic opx in all lithologies has positions. In garnet biotite sillimanite gneiss, 2 3 greater X content than opx porphyroblasts biotite occurs as symplectites with quartz, after Al (Fig. 6). Opx in lithologies lacking garnet also garnet. These biotites have lower Mg ratio. The have relatively higher X Mg ratios. secondary biotites have lower Mg ratios and higher Ti (p.f.u) contents compared to the inclu- Clinopyroxene sion phases, for a single lithology. Spinel- Cpx occurs as rare inclusion phases in gar- bearing lithologies have higher Mg contents in net, and as internal symplectite with plagioclase biotites, whereas garnet-absent lithologies have in garnets of the garnet amphibole pyroxene higher Ti contents. gneiss. Rare cpx was found occurring in symplectites with plagioclase in meta-granitoids. No Opaque minerals great compositional variations were observed Rutile contains 6.2-8.87 wt % FeO and up to among these occurrences except for variable 0.1 wt % Cr O . Ilmenite contains up to 1.5 wt % 2 3 aegerine content in cpx in meta-granitoids. MgO, 0.15 wt % MnO, and 0.35 wt % Cr2O3, while magnetite contains up to 0.43 wt % Cr2O3. Plagioclase Plagioclase occurs as porphyroblasts in the ma- Mafic to intermediate granulites – Highland trix, as inclusions in garnet, in symplectites with Complex opx, and as coronas on garnet. The anorthite

Garnet content is highly variable, with maximum of XAn Garnets in these rocks are almandine-rich = 0.90 in garnet amphibole pyroxene gneiss, and highly variable in composition (Fig. 6), and and minimum of XAn = 0.20 in charnockitic gneiss. Anorthite contents show marked varia- the highest XAlm of 0.95 was recorded from meta-granitoids. Almandine content decreases tion in garnet amphibole pyroxene gneiss from core to rim in garnet amphibole pyroxene where XAn of matrix < symplectite < inclusions in garnet

111 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

Table 4: Representative electron microprobe analysis for intermediate ganulites, HC.

Mineral Garnet Opx Biotite Chlorite Plagioclase Ilmenite Rutile Sample 12A 11 9 12 9 11A 17 11A 9 11A 12A 12A 17 17 core rim core rim por por sym

SiO2 37.14 36.84 36.83 36.71 50.25 49.07 49.00 35.61 34.83 36.65 28.82 57.85 60.30 61.117 0.019 0.01 0.19

TiO2 0.04 0.01 0.02 0.01 0.09 0.11 0.07 5.55 4.64 5.47 0.05 0.00 0.00 0 44.757 44.00 96.72

Al2O3 20.77 20.52 19.99 20.12 1.04 1.55 1.72 14.28 12.39 13.17 12.83 26.12 23.67 23.85 0.029 0.03 0.04 FeO* 33.11 34.25 32.87 32.70 28.57 32.73 32.59 19.60 24.61 20.64 39.78 0.18 0.34 0.084 51.235 50.46 0.53 MnO 1.32 1.31 1.42 1.37 1.33 0.42 0.47 0.19 0.03 0.03 0.08 0.05 0.00 0.01 0.342 0.42 MgO 5.07 4.37 2.11 1.98 17.75 15.55 15.77 11.51 8.47 10.84 6.54 0.491 0.14 0.00 CaO 2.79 2.35 5.48 5.72 0.76 0.23 0.24 0.00 0.00 0.03 0.28 8.07 5.58 5.311 0.124 0.20

Na2O 0.02 0.03 0.02 0.04 0.00 0.00 0.00 0.03 0.05 0.32 0.11 6.68 8.32 8.366 0.040

K2O 0.04 0.05 0.05 0.00 0.06 0.05 0.03 9.52 9.50 9.32 0.12 0.58 0.35 0.338 0.037 0.02 0.04

Cr2O3 0.04 0.12 0.05 0.04 0.03 0.03 0.03 0.05 0.02 0.00 0.06 0.00 0.02 0 0.111 0.00 0.00 Total 100.33 99.83 98.84 98.68 99.88 99.74 99.92 96.33 94.53 96.47 88.67 99.53 98.58 99.086 97.185 95.08 97.72

O = 12 12 12 12 6 6 6 22 22 22 10 8 8 8 3 3 2

Si 2.962 2.968 3.010 3.004 1.947 1.934 1.926 5.416 5.557 5.578 2.324 2.606 2.725 2.739 0.001 0.000 0.003 Ti 0.002 0.001 0.001 0.000 0.003 0.003 0.002 0.634 0.556 0.626 0.003 0.000 0.000 0.000 0.905 0.911 0.992 Al 1.952 1.949 1.925 1.940 0.048 0.072 0.080 2.560 2.330 2.362 1.219 1.387 1.261 1.260 0.001 0.001 0.001 Fe 2.208 2.307 2.246 2.237 0.926 1.078 1.071 2.492 3.283 2.627 2.682 0.007 0.013 0.003 1.152 1.161 0.006 Mn 0.089 0.089 0.098 0.095 0.001 0.001 0.001 0.024 0.004 0.004 0.005 0.002 0.008 0.010 Mg 0.603 0.525 0.257 0.242 1.025 0.914 0.925 2.609 2.015 2.460 0.786 0.020 0.006 Ca 0.238 0.202 0.480 0.502 0.044 0.014 0.016 0.001 0.000 0.004 0.024 0.390 0.270 0.255 0.004 0.003 Na 0.002 0.005 0.003 0.006 0.032 0.010 0.010 0.008 0.016 0.095 0.017 0.584 0.729 0.727 0.002 K 0.004 0.005 0.005 0.000 0.000 0.000 0.000 1.846 1.933 1.809 0.013 0.033 0.020 0.019 0.001 0.001 0.001 Cr 0.001 0.007 0.003 0.002 0.003 0.002 0.001 0.005 0.002 0.004 0.001 0.002 Total 8.062 8.058 8.028 8.028 4.027 4.028 4.032 15.595 15.696 15.567 7.077 5.009 5.019 5.004 2.095 2.089 1.005 * Total Fe as FeO; por - porphyroblasts; sym - symplectite

Table 5: Representative electron microprobe analysis for pelitic gneisses, KC.

Mineral Garnet Biotite Plagioclase K-feldspar Ilmenite 50 50 15 50 15 15 15

SiO2 37.77 35.77 36.55 60.69 61.20 64.58 0.07 TiO2 0.01 5.05 3.57 0.00 0.00 0.00 5.06 Al2O3 20.85 16.51 15.10 24.75 24.06 18.47 0.05 FeO* 32.31 16.96 20.62 0.10 0.13 0.01 82.80 MnO 1.59 0.02 0.27 0.02 0.00 0.00 0.05 MgO 5.84 12.36 10.51 0.01 0.00 0.00 0.03 CaO 1.60 0.02 0.00 6.33 6.14 0.01 1.10 Na2O 0.01 0.11 0.10 8.14 8.18 0.90 0.00 K2O 0.05 9.10 9.32 0.27 0.29 15.93 0.05 Cr2O3 0.03 0.13 0.04 0.00 0.00 0.02 0.03 Total 100.06 96.03 96.08 100.31 100.02 99.91 89.24 O = 12 22 22 8 8 3 Si 2.997 5.352 5.568 2.696 2.724 2.988 0.003 Ti 0.001 0.568 0.409 0.146 Al 1.949 2.911 2.711 1.296 1.262 1.007 0.002 Fe 2.144 2.123 2.627 0.004 0.005 2.649 Mn 0.107 0.002 0.035 0.001 0.002 Mg 0.691 2.757 2.388 0.002 Ca 0.136 0.003 0.000 0.301 0.293 0.001 0.045 Na 0.001 0.032 0.029 0.701 0.706 0.080 K 0.005 1.738 1.812 0.154 0.017 0.941 0.002 Cr 0.001 0.015 0.005 0.001 0.001 Total 8.031 15.501 15.584 5.015 5.006 5.018 2.851 * Total Fe as FeO.

112 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Table 6: Representative electron microprobe analysis for mafic gneisses, KC.

Mineral Garnet Biotite Plagioclase Amphibole K-feldspar Ilmenite Sample 51 14B 1 2B 14B1 3 51 1 2B 14B3 14B-151 14B-21 2B 3 1 2B

SiO2 36.93 65.19 36.44 35.59 35.70 59.37 57.38 57.64 57.43 42.80 43.72 43.55 63.65 0.02

TiO2 0.00 0.04 4.83 5.14 4.08 0.00 0.00 0.00 0.00 1.77 1.69 2.08 0.03 13.19

Al2O3 20.35 19.41 15.29 14.09 15.32 25.28 25.18 25.46 26.56 10.55 10.25 10.57 18.56 0.14 FeO* 27.53 0.11 11.71 13.22 19.92 0.11 1.29 0.14 0.07 17.30 14.71 14.95 0.36 78.97 MnO 3.73 0.02 0.16 0.14 0.10 0.01 0.01 0.43 0.49 0.22 0.01 0.10 MgO 5.12 0.00 15.69 15.03 10.83 1.28 11.21 13.32 12.81 0.00 0.18 CaO 4.95 0.14 0.04 0.05 7.29 7.14 7.42 9.77 11.72 11.24 10.94 0.08 0.09

Na2O 0.01 2.58 0.06 0.04 0.03 7.37 7.11 6.99 6.79 1.73 1.63 1.68 1.18 0.00

K2O 0.01 13.12 9.60 9.14 9.79 0.55 0.45 0.45 0.36 1.45 0.83 0.83 15.72 0.04

Cr2O3 0.02 0.00 0.03 0.03 0.05 0.02 0.05 0.01 0.02 0.03 0.06 0.02 0.14 Total 99.66 95.59 93.83 92.46 95.81 ##### 99.84 98.15 ##### 98.98 97.90 97.67 99.62 92.86

O = 12 22 22 22 22 8 8 8 8 23 23 23 8 3

Si 2.974 5.469 5.462 5.462 5.461 2.656 2.592 2.628 2.564 6.462 6.538 6.523 2.965 0.002 Ti 0.501 0.545 0.593 0.469 0.201 0.190 0.234 0.001 1.143 Al 1.932 2.540 2.702 2.548 2.761 1.332 1.341 1.368 1.398 1.878 1.806 1.866 1.019 0.019 Fe 1.854 2.405 1.468 1.696 2.547 0.004 0.049 0.005 0.003 2.184 1.839 1.873 0.014 7.609 Mn 0.255 0.033 0.020 0.018 0.013 0.055 0.062 0.028 0.001 0.009 Mg 0.615 2.827 3.506 3.439 2.469 0.086 2.523 2.971 2.862 0.030 Ca 0.427 0.007 0.006 0.008 0.000 0.349 0.346 0.362 0.467 1.896 1.801 1.755 0.004 0.011 Na 0.001 0.017 0.016 0.013 0.010 0.639 0.622 0.617 0.588 0.505 0.472 0.487 0.106 K 0.001 1.937 1.837 1.790 1.909 0.032 0.026 0.026 0.020 0.279 0.159 0.158 0.934 0.006 Cr 0.001 0.001 0.003 0.003 0.006 0.001 0.000 0.002 0.000 0.002 0.003 0.007 0.001 0.013 Total 8.060 15.737 15.566 ##### 15.646 5.013 5.062 5.009 5.041 ##### 15.840 15.791 5.045 8.842 * Total Fe as FeO.

Figure 6: Variation of the grossular component of garnets in mafic and intermediate granulites.

Table 7: Temperature and pressure calculations for pelitic granulites, Highland Complex.

Calculated Calculated Sample Texture Garnet Biotite Plagioclase Nominal P Nominal T K P T

XMg XFe XCa XFe XMg Xpl-an K&N,88 F&S,78

gt-bt-sill gneiss (spl bearing) garnet (I) mantle - inclusion of biotite 0.25 0.72 0.363 0.637 5 0.2 621 0.29 0.669 0.308 0.692 5 0.19 602

garnet (III) core - biotite 0.281 0.704 0.438 0.562 7 0.31 831 0.291 0.695 0.417 0.583 7 0.3 813

garnet (III) rim - biotite 0.254 0.73 0.41 0.59 5 0.24 694 0.281 0.693 0.389 0.611 5 0.26 731 0.27 0.691 0.378 0.622 5 0.24 694 0.288 0.673 0.363 0.637 5 0.24 694 0.265 0.69 0.348 0.652 5 0.2 621

garnet (III) rim - plagioclase 0.021 0.289 800 0.0004 8

gt-bt-sill gneiss (spl absent) garnet (II) core – biotite 0.276 0.666 0.407 0.592 10 0.28 790 garnet (II) mantle – biotite 0.297 0.651 0.392 0.608 10 0.29 809

garnet (II) rim - biotite 0.24 0.705 0.392 0.608 5 0.22 658 0.223 0.724 0.408 0.592 5 0.21 639

garnet (II) rim – symp. of biotite 0.18 0.789 0.429 0.571 3 0.17 556 0.18 0.789 0.436 0.563 3 0.18 575

garnet (II) core - plagioclase 0.017 0.241 800 0.0004 6 0.03 0.3 800 0.001 9 garnet (II) rim - plagioclase 0.019 0.3 700 0.0003 5 0.015 0.283 575 0.0001 2 F&S, 78 - Ferry and Spear, 1978; K&N, 88 - Koziol and Newton, 1988. garnet (I): garnets with biotite + sillimanite + quartz inclusions; garnet (II): garnets with kyanite inclusions; garnet (III): garnets with hercynite + ilmenite inclusions

113

Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

However, anorthite content of plagioclase ,

n 0 0 0 0 0 P

in meta-granitoids is almost constant. In char- i

0 0 0 0 0

;

m 8 8 8 6 6

9 o

nockitic gneisses, anorthite content is variable 7

)

1 2

in the range from 0.20 to 0.63. 8

n 1

e (

Amphibole l

)

c 5

a 5

Amphibole occurs in garnet amphibole py- ,

C C 6 6

s 1

i 1

l l

roxene gneiss and meta-granitoid rocks, and is P

(

P 9

classified using the method of Leake et al

7 a

, n

i 0 0 0

5 5 5

1 1

(1997). Accordingly, all the amphiboles have 1

N E

(Ca+Na)B > 1.00 and (Na)B < 0.50, thus belong to

;

)

9 8

the calcic amphibole group (Fig. 7). In garnet 7

5 0 1

9 0 9

8 9 8

& ,

amphibole pyroxene gneiss, all amphiboles d

y E

( e

2+ a

l r

have Si from 6.37-6.47 and Mg/ (Mg + Fe ) u

a l

VI 3+ )

H a

from 0.51 and 0.59 with Al > Fe , falling into C

4 0 0 0

5 0 5

6 7 6

8 H

pargasite. In contrast, meta-granitoid amphi- (

boles have Si from 6.02 and 6.23 and Mg/(Mg + x

6 1 9 8 1 6 2

7 2

p ......

3 6

1 1

3 5 5 1 1 0

2+ VI 3+ 2

Fe ) of 0.10-0.18, with Al < Fe , and are classi- f

1 6 1 4

1 7 9 5 -

8 7 7 7

. . . .

fied as hastingsite. A

i 0 0 0 0

7 8 5 e

6 6 6

. . .

0 0

Titanite 0

e p

C t

Titanite occurs as numerous inclusions in

n 6 8 3

5 2 5

6 6 6

. .

garnet porphyroblasts, along with plagioclase, .

0 0 0 M

and contains about 1.9 wt% Al2O3 and about 1 x

5 3 p

o 4

i 2 2

t . 4 4 e

. .

F 0

wt% FeO. In meta-granitoids, titanite occurs a

0 0

r X

r ) 1

mainly in the matrix, associated with amphi- a

l M

)

(

% (

boles. x

O 1

8 8 6

l F

0 0 0

. .

Pelitic gneiss – Kadugannawa Complex .

1 1 1

p O

Garnet %

(

9 7 p

7 9

7 8 O

4 4 -

. . 6 6

One generation of garnet was recognized, e

. .

0 0

X x

and it contains no significant chemical zoning. m

2 2

o O

9 9

C . . 9

0 0

Garnet porphyroblasts are almandine-rich, with M

6 6 9 6 p

XAlm varying from 0.79 to 0.88. Spessartine con- a

l O

9 9 7 7

9 9 9 8 e

. . . .

g 0 0 0 0 i

tent is slightly greater than in the HC pelitic X

2 9 3 1 /

8 9

3 . 2 9 . 9

gneisses. Grossular content ranges up to XCa = s

. . . . F

2 3

e 3 3 3 3

. G

) l

0.032. However, there is no significant variation t

3 7 9 5

2 G

u 2 8 5 5 5

1 1 8 7 2 3

1 0 . 0 0 0

n . . . . . 1 0 0 0 0

0 . . . . .

9 0 0 0 0 0

in grossular content. a

0 0 0 0 0

8 6 6 9

4 4 -

n 8 9 5 3 c

1 1

i 1 1 1 . 1 .

e . . . . M

0 0

0 0 0 0

a X

Plagioclase r

5 9 4 2 8 8 6 3

5 6 8 G

2 4 1 4 4 4 6 6

5 6 5

6 5 5 . 5 5 6 . 6 5 .

d ......

0 0

Plagioclase occurs as inclusions in garnet 0

0 0 0 0 0 0 0 0

f n

t a

and as a matrix mineral, and is oligoclase to an- y

n e

e l e

t t

e n b

i i

l t t

r c r c

g e desine (X = 0.11 to 0.43) in composition. e

An y

a l l

p n p

H i

m m

: s

c o y

l n

s p s

a i

x x x

p p p

Biotite u

e o o o

e G

- - -

r c

r t t t

u e e e

& i

n n

Biotites contain about 3.35–3.49 wt% TiO2, n

s r x r r

e a a a

;

T g c g g

e 5

and Fe/(Fe + Mg) varies from 0.58 to 0.67. r

d 1

There are no significant differences in composi-

s s s s

n s s s s

i i i i

e e e e

n n n

tion between inclusion phases and retro- n

g g g g

e e e e

n n n n

grade/later overprinted biotite. t

h e e e e

x x x x

o o o o

r r r r

y y y

n p p p p

a e e e e

l l l l m

o o o o

b b b b

i i i i

h h h h

p p p

m m m m

e a a a a

l P

t t t

p e e e e

n n n n

a m

r r r r

a a a a

T 8

S g g g g 114 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Amphibole Amphibole occurs only in biotite gneiss.

(Ca+Na)B > 1.00 and (Na)B < 0.50 indicates it be- longs to the calcic amphibole group (Fig. 7). Si varies between 6.02 and 6.23 and Mg / (Mg + Fe2+) ranges from 0.66-0.72, with Al VI < Fe3+ indicating it is magnesio-hastingsite (Leake et al., 1997). Opaque minerals Magnetite is the dominant opaque phase, Figure 7: Composition of amphiboles (after Leake et and contains up to 6.57 wt % TiO . al. 1997) in mafic and intermediate rocks of the HC 2 and KC.

Mafic gneiss – Kadugannawa Complex; Spinel-bearing garnet biotite sillimanite Garnet gneiss shows highest grade metamorphic condi- Garnets are almandine rich (X =0.74 - Alm tions, of which peak metamorphic assemblage 0.77). These garnets are richer in the spessar- is; garnet + sillimanite + K-feldspar + quartz + tine component than HC garnets. Grossular hercynite. Equilibrium pairs of biotite and gar- content (X = 0.1) shows no significant varia- Grs net cores which have hercynitic spinel and il- tion. No zoning was observed in garnet. menite inclusions records peak metamorphic o temperatures about 810-830 C at a nominal Plagioclase pressure of 7 kbar. Reaching such high tem- All the plagioclase in these rocks is ande- peratures is consistent with the occurrence of sine. No significant variation of anorthite con- Zn-rich hercynitic spinel (Dasguptha et al., tent was observed between plagioclase inclu- 1995), at a high oxygen fugacity indicated by sions in garnet and matrix plagioclase. associated ilmenite. Spinel-absent sillimanite o gneiss records peak temperatures of 810 C, at Biotite a nominal pressure of 10 kbar. Two-feldspar Biotites contain about 4.1 -5.0 wt% TiO , 2 thermometry (Furhman and Lindsey, 1988) was and Fe/(Fe + Mg) vary from 0.48 to 0.63. There applied to antiperthites in the spinel-absent is no significant difference in composition be- garnet sillimanite gneiss. This yields minimum tween inclusion phases and retrograde/later o pre-exsolution temperatures of 630 -700 C, at 8 overprinted biotite. However, garnet-bearing kbar (Fig. 8). Presence of kyanite as rare inclu- mafic gneisses have slightly lower Mg/(Fe+Mg) sions in pre-peak garnet indicates the pressure ratios. conditions were increased before the peak

metamorphism. Peak metamorphic pressure as Amphiboles estimated by GASP barometer yields 8 kbar us- Amphiboles occur in hornblende gneiss and ing core compositions of peak garnet in spinel- migmatitic gneiss. All belong to the calcic am- bearing gneisses, and that for spinel-absent phiboles (values of (Ca+Na) > 1.00 and (Na) < B B rocks is 9 kbar at a nominal temperature of 800 0.50). Si varies from 6.19 to 6.47 and Mg / (Mg o C . + Fe2+) values range from 0.59-0.74, with Al VI < Fe3+ (Fig. 7) falling into magnesiohastingsite (Leake et al., 1997).

THERMOBAROMETRY

HC Pelitic granulites For temperature calculations, the garnet– biotite thermometer of Ferry and Spear (1978) and for pressure the garnet-aluminosilicate- quartz-plagioclase barometer (GASP) of Koziol Figure 8: Application of Two-Feldspar Thermometer and Newton (1988) were used (Table 7). (Furhman and Lindsey, 1988) to antiperthite in the sample 8-1 of the HC. This yields a minimum pre- exsolution temperature of 630-700 °C.

115 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

HC Intermediate and mafic granulites P-T PATH AND TECTONIC Temperatures were determined by garnet– INTERPRETATION opx thermometer of Harley (1984) and garnet – cpx thermometer of Ellis and Green (1979). Ba- a) P-T path: rometry based on alumina solubility in opx Using mineral textures and P-T estimations, the coexisting with garnet, plagioclase and quartz of P-T and tectonic evolution of each rock unit are Perkins and Chipera (1985) and Harley and elaborated in the following sections. Green (1982) were applied. In mafic granulites, rare cpx inclusions in HC pelitic granulites garnet cores yielded crystallization tempera- Pelitic granulites o tures of 890-900 C at a nominal pressure of 10 Fig. 9 shows the P-T trajectory for the Highland k bar (Table 8). Symplectite opx yields tempera- Complex pelitic granulites of the present study. o ture of formation between 650 – 700 C at a This shows a clockwise P-T path, consisting ini- nominal pressure of 5 kbar. Assemblage of opx tial P-T increase due to heating and loading fol- porphyroblasts, plagioclase, garnet and quartz lowed by a stage of rapid increase of pressure. yielded a pressure range of 10.5 – 11 k bar using Subsequently, the rocks experienced continu- the calibrations of Perkins and Chipera (1985) ous temperature increase under slightly de- and Harley and Green (1982) at a nominal tem- creasing or constant pressure. Then the pelitic o perature of 800 C. Symplectite opx gave a pres- granulites underwent peak metamorphism fol- sure of 6 kbar at a nominal temperature of 600 lowed by cooling and gradual decompression. o C, using the calibration of Perkins and Chipera (1985). Stage A In intermediate rocks, equilibrium pairs of Calculated lines of equilibrium constant (K) garnet and opx porphyroblasts recorded a tem- (Table 7) are plotted on the P-T space to elabo- o perature of 760 C at a nominal pressure of 9 k rate the P-T evolution. Equilibrium K values for bar (Table 9). Symplectite opx compositions biotite inclusions in garnet and coexisting gar- o yielded a temperature range of 650 – 690 C at net+ sillimanite + plagioclase + quartz assem- 6 k bar of nominal pressure. Pressure calcula- blage are in the range of 0.19 – 0.20 and o tions are 9 k bar at 800 C nominal temperature 0.0004–0.001 respectively, suggesting equilibra- o and 6 k bar at 700 C of nominal temperature, tion under Stage A. While passing from stage A for porphyroblastic and symplectite opx com- to B, the pelites crossed the melting curve pro- positions, respectively. ducing leucosomes which are observed in the outcrop scale. Kadugannawa Complex rocks For thermometry calculations, the garnet– Stage B biotite thermometer (Ferry and Spear, 1978) Stage B is constrained by antiperthite exso- was used (Table 10). However, mineral assem- lution suggesting a minimum temperature for blages suitable for barometry were not avail- the process at a nominal pressure of 8 kbar. able. There is no way to determine the upper pres- In pelitic rocks, equilibrium pairs of garnet and sure constrain, due to non-availability of a suit- biotite core compositions recorded a tempera- able coexisting assemblage for GASP barome- o ture of 750 C at a nominal pressure of 5 k bar. ter. However, the associated garnets contain Inclusion biotites of garnet cores and the garnet rare kyanite inclusions, suggesting a minimum o mantle compositions gave temperatures 583 C pressure for this stage using the kyanite– o and 639 C, respectively. Garnet rims indicated sillimanite line of Holdaway (1971). o temperatures from 620 to 730 C. In mafic rocks, garnet cores and matrix biotite gave a Stage C o temperature of 690 C and biotite inclusion in P-T path between B and C is uncertain, due to o garnet core recorded 560 C. Rims of garnet and lack of petrographical evidence. Probable pseu- o retrograde biotite gave a temperature of 620 C. domorphs of nearly sub-parallel sillimanite, and coarse sillimanite after kyanite at the marginal zone of garnet (e.g. Fig. 3c, d) suggests that the rock re-entered sillimanite field from kyanite stability field. Early fine needle shaped

116 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Figure 9: Inferred P-T path of the studied pelitic granulites of the Highland Complex. Letters A-G show evolu- tionary stages of the rocks, as discussed in the text (K- equilibrium constant). sillimanite grains in garnet suggest that the the hercynite forming reaction may have given kyanite formation has taken place between two rise to stage E, during cooling after peak meta- sillimanite forming stages of A and C. For the morphism. New garnet of stage E contains her- stage C, calculated K values are 0.290 and 0.001 cynite inclusions (with ilmenite) in the core and for the thermometer and barometer respec- is free from other inclusions. The spinel inclu- tively. The maximum pressure is constrained by sions produce a nearly linear pattern, suggest- kyanite – sillimanite line of Holdaway (1971), ing deformation at the metamorphic peak. Cal- and the minimum temperature is constrained culated K values for the stage E is 0.30 – 0.31 by the biotite dehydration at the expense of and 0.0004 for the thermometer and barome- sillimanite that produced garnet which contains ter, respectively. The minimum pressure for this prismatic sillimanite (different from previous stage is constrained by the absence of cordier- sillimanites) together with biotite and quartz ite. inclusions. Stage F Stage D Temperature constraints for the stage F is Stage D is implied by the formation of hercynite from rim compositions of garnet and coexisting at the outer margins of garnet which contain biotite, and P constraints also from rim compo- kyanite inclusions (Fig. 3c). The kyanite-bearing sition of garnet and matrix plagioclase. These garnet may have crystallized at high P at stage B values confine the range of K values of 0.22 - and near isobaric heating from C to D made 0.26 and 0.0003-0.0004 for thermometer and garnet + sillimanite assemblage unstable. Also barometer, respectively. Lower values of K re- the high Zn content (~5 wt %) of the spinel sug- flect the extensive retrograde Fe – Mg exchange gests a higher temperature and pressure origin between garnet and biotite. Further, when (Dasguptha et al., 1995). Absence of opx or cor- reaching from stage E to F, prograde sillimanite dierite at Stage D suggests opx or cordierite consuming reaction is reversed, as evidenced by forming reaction lines have not been crossed. the formation of late biotite and sillimanite ag- Also, K line calculated (= 0.31) is also well agree gregates over garnet porphyroblasts and in the with the lower boundary of stage D. Stage D matrix. Also, the P-T path from stage F to G represent the peak metamorphic stage of these cannot cross the staurolite forming reaction in granulites. the sillimanite stability field, since there is no evidence for staurolite in any of the studied Stage E rocks. Hercynitic spinel + quartz was probably coexisting at peak metamorphism and reversal of

117 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

Table 9: Temperature and pressure calculations for intermediate granulites, Highland Complex.

Calculated T Calculated P Sample Texture XFe-Gt XMg-Gt XCa-Gt Gt Fe/Mg XFe-Opx XMg-Opx Opx Fe/Mg XAn-Plg K Nominal P Nominal T (H 84) (P&C,85)

Charnckitic gneiss garnet-opx symplectite 0.701 0.193 0.036 3.630 0.573 0.427 1.340 2.7 690 6 0.7 0.186 0.03 3.76 0.56 0.44 1.27 3 650 6 garnet-opx porphyroblasts 0.680 0.196 0.038 3.460 0.610 0.408 1.500 2.3 760 9

garnet-opx porphyroblasts 0.573 0.036 0.700 3.700 0.706 1.3 9 800 0.560 0.056 0.700 2.000 0.677 0.7 9 800 garnet-opx symplectite 0.688 0.029 0.551 18.000 0.784 2.9 6 700

H84: Harley, 1984; P & C 85: Perkins and Chipera, 1985.

HC mafic and intermediate granulites Stage A2 P-T evolution of mafic and intermediate In mafic rocks, Stage A2 is characterized by granulites is shown in the Fig. 10. wide spread occurrence of symplectite of opx and plagioclase after garnet. This is one of the Pre A1 stage typical petrographic evidence for granulite de- This stage is inferred by the presence of cpx compressional stages (Thost et al., 1991). K val- in mafic rocks. However, cpx does not occur in ues for symplectite opx and garnet rim compo- the matrix and found only as inclusions and as sitions range from 2.6 – 3.1. Calculated values an internal symplectite with plagioclase within of ln K from Perkins and Chipera (1985) ranges garnet porphyroblasts. These garnets have nu- from 2.5-2.8 and has largely increased than that merous inclusions of titanite + plag ± quartz, of stage A1 implying the pressure has markedly and occur in a matrix with opx porphyroblasts + decreased. Also in intermediate granulites, Opx- plagioclase + pargasitic amphibole. Therefore, symplectite and garnet rim give K values of 2.7 the P-T path crosses the amphibole forming to 3, and ln K = 2.9, higher than those of Stage reaction at the expense of cpx + opx + plagio- 1. Also when cooling under decomp pression clase, at high temperature and pressure condi- from stage A1 to A2, the P-T path crosses two tions inferred from garnet-cpx pairs and core different Ca-amphibole (pargasite) forming re- compositions of garnet and opx for the stage actions at the expense of pyroxenes, garnet and A1. Intermediate granulites show no evidence plagioclase. of this stage. These rocks have relict evidence for the occur- Stage A1 rence of these reactions as some of opx grains In mafic rocks, garnet-cpx inclusion pairs show are replaced by pargasitic amphibole. Secon- K = 6 and ln K = 0.1 – 0.5 from geothermometry dary or late biotite partially replacing opx por- (Ellis and Green, 1979) and barometry (Perkins phyroblasts as well as some of the symplectitic and Chipera, 1985), respectively. However, opx and overprinting on amphiboles and the there is no opx porphyroblasts which are in di- occurrences of chlorite implies lower amphibo- rect contact with garnet to estimate tempera- lite to greenschist facies conditions after stage ture. For Harley and Green (1982) barometric A2. Similar retrograde assemblages also present equilibrium, Al content in the M1 site of opx has in intermediate granulites. strong dependence on pressure, and for this considered rock the isopleth line of Al (in M1) Kadugannawa Complex rocks and the ln K line of Perkins and Chipera (1985) It was difficult to construct P-T path for closely intersect the K line of Ellis and Green Kadugannawa Complex rocks due to lack of (1979), constraining the stage A1. In intermedi- mineral assemblages for suitable and sufficient ate granulites K= 2.3 and ln K ranging from 0.7 geobarometry. The geothermometry data are to 1.3 for garnet and opx core compositions, presented in the Table 10. reflect the equilibration of the rock at a higher pressure and temperature conditions.

118 Journal of the Geological Society of Sri Lanka, Vol. 14, 103-122

Table 10: Temperature calculations for pelitic and mafic rocks, Kadugannawa Complex.

Sample Texture XFe-Gt XMg-Gt XFe-Bt XMg-Bt K Calculated T Nominal (F & S, 78) P Pelitic gneiss Bioite gneiss garnet core-biotite 0.434 0.176 0.750 0.566 0.18 583 5 inclusion garnet mantle-biotite 0.370 0.239 0.682 0.632 0.21 639 5 inclusion garnet mantle-biotite 0.350 0.253 0.664 0.650 0.21 639 5 inclusion garnet core-biotite 0.454 0.253 0.664 0.546 0.32 750 5 garnet rim-biotite 0.439 0.206 0.718 0.561 0.22 658 5 0.442 0.213 0.712 0.556 0.24 694 5 0.365 0.240 0.679 0.635 0.2 621 5 0.436 0.224 0.698 0.564 0.25 712 5 0.435 0.231 695 0.565 0.26 731 5 Mafic gneiss Garnet biotite gneiss garnet core-biotite 0.338 0.202 0.6 0.662 0.17 564 5 inclusion garnet core-biotite 0.427 0.193 0.61 0.573 0.24 694 5 garnet rim-biotite 0.375 0.195 0.6 0.625 0.2 621 5 F & S, 78: Ferry and Spear, 1978

Figure 10: Inferred P-T trajectory of the studied mafic and intermediate granulites of the Highland Complex. K= equilibrium constant. (see text for details).

Tectonic interpretation could indicate the over-thrusting of the Wanni A possible tectonic interpretation of the P-T Complex onto the Highland Complex. Heating paths obtained in this study can be explained as accompanying magmatic intrusions (e.g. Holzl follows. et al., 1991) to the thickened crustal unit repre- senting the HC and the WC at depth probably a) P-T path for pelitc granulites led to the peak metamorphism during the Pan The P-T path shown in the Fig. 9 indicates African times (~550 Ma; Milisenda et al. 1988; an initial P-T increase in HC granulites, followed Baur et al.1991; Kröner et al., 1991), followed by rapid increase of pressure. This type of P-T by cooling and decompression. [Early tempera- path suggests tectonic crustal thickening (Eng- ture increase before rapid pressure increase land and Thompson, 1984; Spear, 1993) in a event can be attributed to rifting and extension collision zone. From a Sri Lankan context this (Kriegsman, 1996)].

119 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

the peak metamorphism, after emplacement of b) P-T path for mafic and intermediate granu- their protolith to the lower crustal units. How- lites ever, the studied rocks have not preserved evi- The P-T path of mafic and intermediate dence for initial isobaric cooling after the em- granulites (Fig. 10) can be interpreted by mag- placement of their igneous protoliths, evi- matic underplating models adopted for granu- denced by the growth of garnet, cpx and quartz lites (e.g. Wells, 1980, Bohlen, 1987; Spear, from orthopyroxene and plagioclase and subse- 1993). Accordingly, the meta-igneous rocks quent breakdown to the same assemblage, were produced from magmatic intrusion or which they interpreted in terms of isobaric cool- magmatic underplating occurring at depth and ing. subsequent cooling took place during the uplift. The KC mineral assemblages represent The magmatism envisaged occurred in over- lower metamorphic grade than those of the HC. thickened crust composed of both the HC and Prograde metamorphic evidence could be sup- the WC since the metamorphic age of both pe- ported by a few inclusion phases in garnet por- litic and mafic granulites of the HC and the WC phyroblasts and retrograde evidence by over- is synchronous (Milisenda et al. 1988, Baur et printing textures of secondary or late stage as- al.1991, Holzl et al, 1991). This interpretation is semblages. There was no characteristic decom- supported by the field evidence that mafic pressional/retrograde textures like symplec- granulites are generally intercalated and/or tites, reaction rims or coronae formation ob- closely associated with pelitic granulites in both served in the studied samples of the KC. Fur- the HC and the WC of Sri Lanka. ther, this study presents new mineral chemistry However, the intrusion or underplating of dataset for the KC that lacks published mineral mafic magmas to produce mafic granulites and chemistry data. felsic to intermediate magmas to produce intermediate granulites has taken place at differ- Acknowledgements ent crustal levels. As shown by the Fig. 10, the We thank Dr. B. Roser of Shimane Univer- intermediate granulites have originated at shal- sity for his comments on the manuscript and to lower crustal levels than the mafic granulites. Profs. M. Akasaka, H. Komuro and H. Ohira, Dr. However, there is no evidence found in this A. Kamei and the members of the ‘Metamor- study for isobaric cooling segment after peak phic Seminar’ of the Shimane University (2003- metamorphism, as revealed by previous works 2005) for helpful comments and discussions. (e.g. Perera, 1987; Schumarcher et al., 1990; Constructive comments by Mr. L.R.K. Perera Schenk et al, 1991; Prame, 1991b). significantly improved the manuscript. This Accordingly, the clock-wise P-T path for pe- study was supported by the Japanese Ministry litic granulites and cooling path for meta- of Education, Culture, Sports, Science and igneous rocks document the possible deep Technology (Monbukagakusho) Scholarship to crustal processes by which continental crust S.P.K.M for the M.Sc degree. grows similar to the phenomena in most granulite terrains of the world. REFERENCES CONCLUSIONS Baur, N., Kröner, A., Todt, W., Liew, T.C. and This study reconfirms that the pelitic granu- Hofmann, A.W. (1991) U-Pb isotopic syste- lites of the Highland Complex of Sri Lanka matics of zircons from prograde and retro- evolved through a clock-wise P-T path. The Peak grade transition zones in high grade orthog- metamorphism took place at sillimanite stability neisses, Sri Lanka. J. Geol., 99: 527-545. field; however an early pressure increase in ex- Bohlen, S.R. (1987) Pressure-temperature-time cess of 9 kbar was inferred by relic kyanite in- paths and a tectonic model for the evolution clusions in garnet. Due to that, it can be con- of granulites. J. Geol., 95: 617-632. cluded that these granulites were equilibrated Cooray, P.G. (1994) The Precambrian of Sri Lan- twice in the sillimanite stability field at distinctly ka: a historic review. Precamb. Res., 66: 3-18. different P-T conditions, in their evolution. Dasguptha, S., Senguptha, P., Ehl, J., Raith, M., In contrast, metabasic and intermediate and Bardhan, S. (1995) Reaction textures in a granulites evolved through a cooling path from suit of spinel granulites, India: evidences for

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ZnO and Fe3O4. J. Petrol., 36: 435-461. granulites from central Sri Lanka. J. Petrol., Ellis, D.J and Green, D.H. (1979) An experimen- 40: 1211-1239. tal study of the effect of Ca upon garnet- Kriegsman, L.M. (1991) Structural geology of clinopyroxene Fe-Mg exchange equilibria. the Sri Lankan basement – a preliminary re- Contrib. Mineral. Petrol., 71: 13-22. view. In: Kröner, A (ed), The Crystalline crust England, P.C. and Thompson, A.B. (1984) Pres- of Sri Lanka, Part I. Geol. Surv. Dept. of Sri sure-temperature-time paths of regional me- Lanka, Professional Paper, 5: 52-68. tamorphism of the continental crust, I, Heat Kriegsman, L.M. (1996). Divariant and trivariant transfer during the evolution of regions of reaction line slopes in FMAS and CFMAS: thickened continental crust. J. Petrol., 25: theory and applications. Contrib. Mineral. Pe- 894-928. trol., 126: 38-50. Faulhaber, S. and Raith, M. (1991) Geothermo- Kröner, A and Williams, I.S. (1993). Age of me- metry and geobarometry of high grade rocks: tamorphism in the high-grade rocks of Sri a case study on garnet pyroxene granulites in Lanka. J. Geol., 101: 513-521. southern Sri Lanka. Mineral. Mag., 55: 33-56. Kröner, A., Williams, I.S., Compston, W., Baur, Ferry, J.M. and Spear, F.S. (1978) Experimental N., Vithanage, P.W., and Perera, L.R.K. (1987). calibration of the partitioning of Fe and Mg Zircon ion microprobe dating of high grade between biotite and garnet. Contrib. Mineral. rocks in Sri Lanka. J. of Geol., 95: 775-791. Petrol., 66: 113, 117. Kröner, A, Cooray, P.G and Vithanage, P.G. Furhman M.L. and Lindsey, D.H. (1988) Ternary (1991) Lithotectonic sub division of the Pre- feldspar modelling and thermometry. Am. cambrian basement in Sri Lanka. In: Kröner, A Mineral., 73: 201-215. (ed), The Crystalline crust of Sri Lanka, Part I. Harley, S.L. (1984) The solubility of alumina in Geol. Surv. Dept. of Sri Lanka, Professional opx coexisting with garnet in FeO-MgO-Al2O3- paper 5: 5-21. SiO2 and CaO-FeO-MgO- Al2O3-SiO2. J. Petrol., Leake, B.E., Wooley, A.R., Arps, C.E.S., Birch, 25: 665-696. W.D., Gilbert, M.C., Grice, J.D., Hawthorne, Harley, S.L. and Green, D.H. (1982) Garnet – F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., orthopyroxene barometry for granulites and Linthout, K., Laird, J., Mandarino, J., Maresch, peridotites. Nature, 300: 697-700. W.V., Nickel, E.H., Rock, N.M.S., Schumacher, Hiroi, Y., Ogo, Y. and Namba, L. (1994) Evidence J.C., Stephenson, N.C.N., Ungaretti, L., Whit- for prograde metamorphic evolution of Sri taker, E.J.W., and Youzhi, G. (1997) Nomen- Lankan politic granulites and implications for clature of amphiboles. Report of the sub- the development of continental crust. Pre- committee on amphiboles of the Internation- camb. Res., 66; 245-263. al Mineralogical Association Commission on Holdaway, M.J. (1971) Stability of andalusite New Minerals and Mineral Names. Mineral. and the aluminium silicate phase diagram. Mag., 61: 295-321. Am. J. Sci., 271: 97-131. Milisenda, C.C, Leiw, T.C, Hofmann, A.W, Hölzl, S., Kohler, H. and Tordt, W. (1991) Geoch- Kröner, A. (1988) Isotopic mapping of age ronology of Sri Lanka basement, Part I: U-Pb provinces in Precambrian high grade terrains: mineral systematics, Terra. Abs., 3: 504. Sri Lanka. J. Geol., 96: 608-615. Kehelpannala, K.V.W. (1997) Deformation of a Osanai, Y. (1989) A preliminary report on saphi- high-grade Gondwana fragment, Sri Lanka. rine/kornerupine granulite from Highland se- Gondwana Res., 4: 174-178. ries, Sri Lanka, (Extended abstract). Seminar Kohler, H., Dratch, V, Hölzl, S and Fehr, T. (1991) on recent advantages in Precambrian Geology Geochronology of Sri Lanka basement. Part II: of Sri Lanka, IFS Kandy, Sri Lanka. Rb-Sr and Sm-Nd dating on mineral and whole Osanai, Y., Ando, K.T., Miyashita, Y., Kusachi, I., rock samples. Terra. Abs., 3: 504. Yamasaki, T., Doyama, D., Prame, W.K.B.N., Koziol A.M and Newton, R.C. (1988) Redetermi- Jayatileke, S., and Mathavan, V. (2000) Geo- nation of the anorthite break down reaction logical fieldwork in the southwestern and and improvement of the plagioclase-garnet- central parts of the Highland Complex, Sri

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