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

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1 Journal of the Geological Society of Sri Lanka, Vol. 14, Journal of the Geological Society of Sri Lanka Vol. 14 (2011): C.B. Dissanayake Felicitation Volume Petrology of Metamorphic Rocks from the Highland and Kadugannawa Complexes, Sri Lanka Sanjeewa P.K. Malaviarachchi *1 and Akira Takasu 2 1 Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka. 2 Department of Geoscience, Shimane University, Matsue , Japan. ABSTRACT (*Corresponding author, malavi@pdn.ac.lk). 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 o C. Spinelabsent sillimanite gneiss records peak temperatures of 810 o C. 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 C and a pressure of ~11 k bar. Intermediate rocks show a temperature of C and a pressure of 9 kbar. In KC pelitic rocks, equilibrium pairs of garnet and biotite core compositions recorded a temperature of C whereas mafic rocks yielded a temperature of C 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 metasedimentary 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 The metamorphic basement of Sri Lanka has been considered as a key terrain to understand the evolution of the Gondwana supercontinent. In a palaeogeographic reconstruction, Sri Lanka was located close to India, Madagascar and East Antarctica. The geology of the island is therefore a key to understand the Gondwana evolution. Calc-silicate and Mg-, Al-rich metasedimentary and mafic to felsic meta-igneous high- to ultra-high temperature granulites together with amphibolites, migmatites and minor gabbroic, granitic, pegmatitic and aplitic igneous intrusions characterize the Sri Lankan basement. Due to the tectonic amalgamation of amphibolite to granulite-facies terrains of diverse isotopic signature containing a diversity of rock types in a relatively small area, Sri Lanka is of great interest in the fields of petrology, geochronology and structural geology. Based on the Nd- model age mapping by Milisenda et al (1988) and zircon geochronology (Kröner et al., 1991), the supracrustal rocks of Sri Lanka have been subdivided into four major terrains (e.g. Cooray, 1994): the Highland (HC), 103

2 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks Wanni (WC), Vijayan (VC) and Kadugannawa Complexes (KC) as shown in the (Fig. 1). Although on the basis of similarities in structures Kehelpannala (1997) included the Kadugannawa Complex in the Wanni Complex, we have adhered to Cooray (1994) classification in this paper. Figure 1: Litho-tectonic units of Sri Lanka (after Cooray, 1994) showing the study area. Although petrologic research on Sri Lankan metamorphic rocks has been carried out extensively, this study was undertaken to present an updated dataset particularly on mineral chemistry. Especially, electron microprobe data (EPMA) of constituent minerals of the Kadugannawa Complex gneisses are rarely found in the literature. Therefore, petrological investigation of some pelitic and intermediate to mafic granulites from the central Highland Complex and some pelitic and mafic rocks from the Kadugannawa Complex were carried out in this study. The samples were collected systematically from both Highland and Kadugannawa Complexes (Fig. 2). After careful petrographic observations, selected thin sections were analyzed by an electron probe micro analyser (EPMA) to reveal the constituent mineral chemistry. REVIEW OF PREVIOUS WORK Typical high temperature metamorphic conditions are well established from the Sri Lankan metamorphic basement by various researchers 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 C in the East and South east to 5-6 kbar C in the North West (Faulhaber and Raith, 1991; Schumarcher & Faulhaber, 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 suggest isobaric cooling. Osanai (1989) first reported sapphirine bearing granulites from the HC, and other UHT assemblages have been reported by Kriegsman (1991), Kriegsman and Schumarcher (1999), Osanai et al. (2000, 2003), Sajeev et al (2003), Sajeev and Osanai (2002, 2003, 2004a) suggesting UHT metamorphism above C and kbar. Evidence of isobaric cooling after UHT and a multi stage evolution was presented by Sajeev and Osanai (2002, 2004a). These P/T conditions are in contradiction with the other granulites in the surrounding area, which preserves a maximum of C and 9-10 kbar and determined to be metamorphosed during the Pan African tectonothermal event. Also, Sajeev and Osanai (2004b) reported osumillite from Sri Lanka, and its implications on UHT metamorphism, though they could not distinguish whether it is a product of the Pan African 104

3 Journal of the Geological Society of Sri Lanka, Vol. 14, 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 (2004b) reported osumillite from Sri Lanka, and its implications on UHT metamorphism, though they could not distinguish whether it is a product of the Pan African metamorphism or a relic of an older metamorphic event due to lack of geochronological data. Sajeev and Osanai (2004a) argued that the UHT granulites of the HC probably evolved along an anticlockwise path. b) Geochronology Milisenda et al (1988) presented Nd model age data, and identified three distinct age provinces. The Highland Complex has model ages of Ga, indicating derivation mainly from late Archean sources, and is bounded to the East and West by late Proterozoic gneisses of the Vijayan Complex and Wanni Complex with model ages of Ga. An ion microprobe (SHRIMP) U-Pb study of zircons (Kröner et al., 1987) documented Ga for detrital grains from the Highland Complex. In addition, this study revealed some indication of Pb loss at about 1.1 Ga, which was attributed to granulite facies metamorphism. Later U-Pb zircon and monazite studies (e.g. Hölzl et al., 1991; Kohler et al., 1991; Baur et al., 1991; Kröner and Williams, 1993) from both orthogneisses and paragneisses assigned an age of Ma for high-grade metamorphism. Osanai et al (1996) reported a ca. 670 Ma metamorphic event from saphirine-bearing granulites, based on Sm-Nd whole rock isochron data. They also identified a retrograde age of ca. 520 Ma, based on the whole rock biotite internal isochron method. Sajeev et al. (2003) reported an internal Sm-Nd isochron age for the UHT metamorphism of ca.1500 Ma based on the analysis of a garnet core, whole rock and felsic fraction of ultra-high temperature (UHT) granulites. They also reported an orthopyroxene reference isochron age of 550 Ma, implying that these UHT granulites were also affected by the Pan-African metamorphism. STUDIED SAMPLES AND THE ANALYTICAL METHODS General geology of the study area and sample localities is shown in the Figs. 1 and 2, respectively. Eight pelitc granulites, three mafic granulites and six intermediate granulites were studied from the Highland Complex. Two pelitic gneisses and six mafic gneisses were studied from the Kadugannawa Complex (Table 1). 105

4 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks Hand specimens and thin sections were studied for petrography, and the mineral textural features were studied using a polarizing microscope. Chemical compositions of constituent minerals of rocks and the back scattered electron images (BSE) were obtained using a JEOL JXA-8800M electron probe micro analyser (EPMA) at Shimane University, Japan. The analytical conditions used were 15 kv accelerating voltage, 25 na probe current and 5µm probe diameter. Representative mineral analyses are given in the Tables 2 to 6. MINERAL TEXTURES HC pelitic granulites Two types of pelitic granulites were identified as, garnet biotite sillimanite gneiss and biotite gneiss (Table 1). They have gneissose foliation defined by preferred orientation of biotite and/or sillimanite, with alternation of layers composed of quartz and feldspar. In garnet biotite sillimanite gneiss, K- feldspar lamellae occur in plagioclase (antiperthite texture), together with fine quartz intergrown in the host (Fig. 3a). Subhedral to anhedral garnet porphyroblasts up to 8 mm in diameter commonly contain biotite, sillimanite, ilmenite, quartz inclusions and rarely hercynitic spinel and kyanite too. Occasionally, garnet porphyroblasts are replaced by sillimanite and/ or by biotite or symplectire of biotte and quartz at the rim. Mainly biotite and quartz inclusions occur in the garnet core, while sillimanite occurs in the mantle. Rare kyanite occurs only as inclusions in garnet (Fig. 3b). Sillimanite makes very fine needles in garnet (Fig. 3c) but is prismatic and medium grained in the matrix with typical transverse fractures, fibrolitic when associated with hercynite symplectites. Aggregates of sillimanite collectively form a shape of a relict porphyroblast, very likely kyanite. Rare sillimanite pseudomorphs after kyanite occur in the rim part (Fig. 3c, d). Hercynitic spinel occurs as rare inclusions in garnet porphyroblasts, in garnet biotite sillimanite gneiss. In addition, it occurs in symplectites associated with fibrolite at garnet rims and along fractures (Fig. 3c). Biotite forms a preferred orientation in the matrix as well as random grain overgrowths replacing garnet rims. Plagioclase grains show well developed polysynthetic twinning in many samples. Quartz commonly occur both as inclusion in garnet and in the matrix with plagioclase and K-feldspar. Ilmenite and rutile occur both as inclusions in garnet as well as in the matrix with other accessory phases like zircon and monazite. HC Mafic granulites These rocks are generally coarse grained and poorly foliated. The garnet amphibole pyroxene mafic granulite of this study consists mainly of garnet, cpx, opx, pargasitic amphibole, plagioclase, quartz, and titanite (Table 1). Garnet occurs as poikiloblasts up to 15 mm in diameter are sometimes idioblastic with plagioclase, quartz, titanite and iron oxide inclusions. Also, some samples have rare cpx plagioclase symplectites (Fig. 4a,c) within outer core of the garnet porphyroblasts. Garnet porphyroblasts are partially replaced by secondary biotite and ilmenite at their margins. Plagioclases in the matrix are porphyroblastic and are mostly untwinned. However, these plagioclase porphyroblasts rarely show lamella twinning and oscillatory zoning (Fig. 4b). These grains contain fine exsolution blebs of K- feldspar. Plagioclase also occurs as inclusions in garnet and as symplectites with opx after garnet (Fig. 4c). Orthopyroxene occurs as porphyroblasts up to 5 mm and also as both fine grained and coarse grained symplectites with plagioclase (Fig. 4c and d, respectively). Also, coarse grained opx symplectites are found to be replaced by pargasitic amphibole. Opx porphyroblasts have biotite, plagioclase and opaque mineral inclusions and these porphyroblasts are later replaced by secondary biotite and ilmenite. Clinopyroxene was found in the symplectite included in the garnet porphyroblasts (Fig. 4a, c), and as well as rare inclusions in garnet. Cpx is totally absent in the matrix. Amphibole grains texturally postdate the garnet porphyroblasts, as evident even in hand specimen scale, by the foliation defined by amphibole wrapping around the garnet. These amphiboles are pargasite. Some of these pargasites replace opx. Due to strong retrogression, chlorite, quartz and hematite assemblages are found between garnet porphyroblasts. Opaque phases include magnetite and ilmenite. 106

5 Journal of the Geological Society of Sri Lanka, Vol. 14, 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 Biotite 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 Intermediate granulites include charnockitic gneisses and hornblende and biotite bearing meta-granitoids. Usually, charnockitic gneisses have a characteristic greasy lustre or appearance in hand specimen, exhibiting weak gneissic foliation. In contrast, meta-granitoid shows a preferred orientation of minerals such as hornblende, biotite and ribbon quartz. Also, this rock shows a strong lineation defined by graphite. Many quartz grains are highly stretched and show subgrain boundaries. In some charnockitic gneisses garnet occurs as subhedral to anhedral porphyroblasts up to 5 mm (Table 1) and contain quartz and plagioclase inclusions. Many garnets are replaced by biotite and some show breakdown textures forming fine opx grains and reaction rims of plagioclase. In contrast, garnet porphyroblasts of metagranitoid are free from inclusions and occur in sizes of 3-5mm anhedral grains. Some garnets are completely broken down to form cpxbearing symplectites, associated with amphibole, biotite and opaque. Hypersthene in charnockitic gneiss commonly occurs as anhedral porphyroblasts and is associated with plagioclase rims after garnet. Rare cpx was found occurring in symplectites with plagioclase in metagranitoid where opx is absent. In both rock types, plagioclase occurs as porphyroblasts, inclusions in garnet and coronae on garnet. Generally, plagioclase show albite twining and include fine quartz grains. K- feldspar and quartz occur in excess in both lithologies. Amphiboles occur only in meta-granitoid, as porphyroblasts mainly associated with porphyroblastic titanite. Charnockitic gneisses show retrograde alteration products of greenschist facies such as chlorite and calcite. Symplectite of cpx + rtl + ilm after garnet were also observed in the meta-granitoid. Opaque minerals like ilmenite, magnetite and rutile occur in the charnockitic gneiss and ilmenite is the only opaque phase in the metagranitoid. KC Pelitic gneiss Pelitic gneisses in the Kadugannawa Complex consist of quartz, K-feldspar, plagioclase, and biotite with accessory minerals such as muscovite, rutile, ilmenite, and zircon (Table 1). Garnet is rarely found and occurs as porphyroblasts up to 5 mm with inclusions of biotite 107

6 Table 1: Constituent minerals in the studied samples (+ present as major minerals, ± present as minor minerals; grt: garnet; qtz: quartz; plg: plaioclase; Kfs: K- feldspar; sil: sillimanite; ky: kyanite; bt: biotite; amp: amphibole; opx: orthopyroxene; cpx: clinopyroxene; mus: muscovite: spl: spinel; tit:titanite; grp: graphite; ilm: ilmenite; rtl: rutile). Highland Complex pelitic gran ulites garnet-biotitesillimanite gneiss S am ple n o. grt qtz p lg Kfs sil ky bt am p opx cpx 8, A,14A biotite g neiss 10, , m afic granu lites garnet am phibo lepyroxene gneiss 14B, 14B B intermediate granulites charnockitic gn eiss A, 11, m etagranito id 6,6C ± Kadugannawa C om plex pelitic gneiss garnet bio tite gneiss biotite g neiss mafic gneiss amphibole gn eiss 2,2B, biotite g neiss ± m ig matitic gneiss 1, m us spl tit g rp ilm rtl m gt zir mo n ± ± ± ± ± ± ± ± + + ± ± + ± ± ± ± Malaviarachchi and Takasu, Petrology of Metamorphic Rocks 108

7 Journal of the Geological Society of Sri Lanka, Vol. 14, Table 2: Representative electron microprobe analysis for pelitic granulites, HC Table 2. Representative electron microprobe analysis for pelitic ganulites, HC MineralGarnet Biotite Hercynite Sillimanite Kyanite Plagioclase Ilmenite Rutile Sample A 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 SiO TiO Al 2 O FeO* MnO MgO CaO Na 2 O K 2 O Cr 2 O ZnO Total O = Si Ti Al Fe Mn Mg 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 SiO TiO Al2O FeO* MnO MgO CaO Na 2 O K 2 O Cr 2 O Total ##### O = Si Ti Al Fe Mn Mg Ca Na K Cr Total ##### * Total Fe as FeO; por - porphyroblasts; f-sym - fine grained symplectite; c-sym - coarse grained symplectite and quartz. Sometimes garnet porphyroblasts are replaced by biotite along the rim (Fig. 5a). Quartz in the matrix with plagioclase, K-feldspar and biotite forms a preferred orientation. Plagioclase rarely shows polysynthetic twinning in these rocks. Rare muscovite was found in the KC rocks and ilmenite occurs in the matrix. Quartz and K-feldspar are also found in excess. KC Mafic gneiss Mafic gneises in Kadugannawa Complex include garnet-bearing and garnet-absent rocks (Table 1). These rocks are generally coarse grained and poorly foliated and exhibit a granoblastic polygonal texture. Garnet-bearing rocks consist of plagioclase, quartz, biotite, rutile and ilmenite. Garnet occurs as porphyroblasts up to 3 mm and occasionally contains quartz and biotite as inclusions (Fig. 5b). Garnet porphyroblasts are replaced by biotite overprints at their margins. Garnetabsent mafic rocks contain hornblende, quartz, plagioclase, biotite and ilmenite and are represented by hornblende gneisses and migmatitic gneisses where the dominant mineral being hornblende and plagioclase. Rare cpx is also found as porphyroblasts in some garnet and hornblende absent rocks. Rare hornblende inclusions are present in plagioclase. 109

8 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 Pelitic granulites Highland Complex Garnet Different generations of garnet are present in the Highland Complex pelitic gneiss, based on inclusion patterns. These are garnets which contain biotite, sillimanite and quartz; those with rare kyanite inclusions; and those with hercynite and ilmenite inclusions. Garnets in these rocks represent almandine-rich Fe-Mg solid solutions (up to X Alm = 0.8), where X Prp ratio decreases slightly from core to rim. The grossular component also has a similar trend, with maximum ratio of X Grs =0.03 preserved in the porphyroblastic cores. Garnets which are rimmed by ilmenite and hematite have the highest almandine contents. Pyrope content varies from 0.4 to 0.2, whereas the grossular content varies from 0.05 to In addition, some garnets which contain rare hercynite + ilmenite inclusions show compositional heterogeneity. 110

9 Journal of the Geological Society of Sri Lanka, Vol. 14, Plagioclase Plagioclase is oligoclase to andesine in composition (X An = 0.23 to 0.35). There are no significant differences between the An contents of plagioclase inclusions in garnet and matrix plagioclases; however, garnet-absent biotite gneiss shows the minimum An content. Spinel Spinel is rich in hercynite component with a Fe/(Fe + Mg) of Inclusion spinel has higher Zn content (max. ZnO = 7.15 wt %) while the retrograde spinel has a maximum ZnO content of 5.9 wt %. Kyanite Kyanite occurs as rare inclusions in garnet porphyroblasts and contains ~1 wt % of FeO. Sillimanite Sillimanite contains 1 wt% FeO and 0.1 wt % Cr; however, the oxide total was around 97.5 %. Biotite Biotites contain about wt % of TiO 2, and their Fe/(Fe + Mg) varies from 0.52 to Mg ratio vs. Ti (per formula unit, p.f.u) varies depending on the textural setting. Thus, biotite inclusions in garnet and secondary biotite overprints on garnet have contrasting compositions. In garnet biotite sillimanite gneiss, biotite occurs as symplectites with quartz, after garnet. These biotites have lower Mg ratio. The secondary biotites have lower Mg ratios and higher Ti (p.f.u) contents compared to the inclusion phases, for a single lithology. Spinelbearing lithologies have higher Mg contents in biotites, whereas garnet-absent lithologies have higher Ti contents. Opaque minerals Rutile contains wt % FeO and up to 0.1 wt % Cr 2 O 3. Ilmenite contains up to 1.5 wt % MgO, 0.15 wt % MnO, and 0.35 wt % Cr 2 O 3, while magnetite contains up to 0.43 wt % Cr 2 O 3. Mafic to intermediate granulites Highland Complex Garnet Garnets in these rocks are almandine-rich and highly variable in composition (Fig. 6), and the highest X Alm of 0.95 was recorded from meta-granitoids. Almandine content decreases from core to rim in garnet amphibole pyroxene gneiss and inclusion-free fine grained garnets of charnockitic gneiss, but increases in porphyroblastic garnets of charnockitic gneiss. Metagranitoid garnets have almost constant composition. Pyrope contents of garnets from the garnet amphibole pyroxene gneiss vary from 0.27 to In the case of charnockitic gneiss, pyrope varies from 0.14 to In metagranitoid, the pyrope content is almost constant. The highest grossular content (X Grs = 0.38) is found in a garnet from amphibole pyroxene gneiss. Grossular content decreases from core to rim in both garnet amphibole- pyroxene gneiss and charnockitic gneiss garnets. In metagranitoids, garnet rims are richer in grossular than the cores (max. X Grs = 0.23). Orthopyroxene Opx occurs as porphyroblasts, in finegrained symplectites and coarse-grained symplectites after garnets, with X Mg = Alumina contents differ markedly in the garnet amphibole pyroxene gneiss. Opx in the symplectites after garnet has Al contents from 2.11 to 3.12 wt %, whereas the opx in the coarse grained symplectites has Al 2 O 3 contents ranging from 0.70 to 1.19 wt%. However, porphyroblastic opx in the matrix contains 1.35 to 2.09 wt% Al 2 O 3. Symplectitic opx in all lithologies has greater X Al content than opx porphyroblasts (Fig. 6). Opx in lithologies lacking garnet also have relatively higher X Mg ratios. Clinopyroxene Cpx occurs as rare inclusion phases in garnet, and as internal symplectite with plagioclase in garnets of the garnet amphibole pyroxene gneiss. Rare cpx was found occurring in symplectites with plagioclase in meta-granitoids. No great compositional variations were observed among these occurrences except for variable aegerine content in cpx in meta-granitoids. Plagioclase Plagioclase occurs as porphyroblasts in the matrix, as inclusions in garnet, in symplectites with opx, and as coronas on garnet. The anorthite content is highly variable, with maximum of X An = 0.90 in garnet amphibole pyroxene gneiss, and minimum of X An = 0.20 in charnockitic gneiss. Anorthite contents show marked variation in garnet amphibole pyroxene gneiss where X An of matrix < symplectite < inclusions in garnet 111

10 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks Table 4: Representative electron microprobe analysis for intermediate ganulites, HC. MineralGarnet Opx Biotite Chlorite Plagioclase Ilmenite Rutile Sample 12A A 17 11A 9 11A 12A 12A core rim core rim por por sym SiO TiO Al 2 O FeO* MnO MgO CaO Na 2 O K 2 O Cr 2 O Total O = Si Ti Al Fe Mn Mg Ca Na K Cr Total * 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 SiO TiO Al 2 O FeO* MnO MgO CaO Na 2 O K 2 O Cr 2 O Total O = Si Ti Al Fe Mn Mg Ca Na K Cr Total * Total Fe as FeO. 112

11 Journal of the Geological Society of Sri Lanka, Vol. 14, Table 6: Representative electron microprobe analysis for mafic gneisses, KC. Mineral Garnet Biotite Plagioclase Amphibole K-feldspar Ilmenite Sample B B B 3 1 2B SiO TiO Al 2 O FeO* MnO MgO CaO Na 2 O K 2 O Cr 2 O Total ##### ##### O = Si Ti Al Fe Mn Mg Ca Na K Cr Total ##### ##### * 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 X Mg X Fe X Ca X Fe X Mg Xpl-an K&N,88 F&S,78 gt-bt-sill gneiss (spl bearing) garnet (I) mantle - inclusion of biotite garnet (III) core - biotite garnet (III) rim - biotite garnet (III) rim - plagioclase gt-bt-sill gneiss (spl absent) garnet (II) core biotite garnet (II) mantle biotite garnet (II) rim - biotite garnet (II) rim symp. of biotite garnet (II) core - plagioclase garnet (II) rim - plagioclase F&S, 78 - Ferry and Spear, 1978; K&N, 88 - Koziol and Newton, garnet (I): garnets with biotite + sillimanite + quartz inclusions; garnet (II): garnets with kyanite inclusions; garnet (III): garnets with hercynite + ilmenite inclusions 113

12 Table 8: Temperature and pressure calculations for mafic granulites, Highland Complex (% Al M1= molar ratio of Al in the M1 site of Opx. H84: Harley, 1984; E&G, 79: Ellis and Green, 1979; P&C, 85: Perkins and Chipera, 1985; H&G, 82: Harley and Green, 1982). garnet amphibole pyroxene gneiss garnet-opx symplectite garnet amphibole pyroxene gneiss cpx inclusions in garnet garnet amphibole pyroxene gneiss garnet-opx porphyroblasts garnet amphibole pyroxene gneiss garnet-opx symplectite Calculated T Calculated P Opx (Al-M1) X Fe-Cpx X Mg-C px Cpx Fe/ Mg X An-Plg K Nominal P Nominal T (%) (H 84a) (E&G,79) (P&C,85) (H&G,82) Malaviarachchi and Takasu, Petrology of Metamorphic Rocks However, anorthite content of plagioclase in meta-granitoids is almost constant. In charnockitic gneisses, anorthite content is variable in the range from 0.20 to Amphibole Amphibole occurs in garnet amphibole pyroxene gneiss and meta-granitoid rocks, and is classified using the method of Leake et al (1997). Accordingly, all the amphiboles have (Ca+Na) B > 1.00 and (Na) B < 0.50, thus belong to the calcic amphibole group (Fig. 7). In garnet amphibole pyroxene gneiss, all amphiboles have Si from and Mg/ (Mg + Fe 2+ ) from 0.51 and 0.59 with Al VI > Fe 3+, falling into pargasite. In contrast, meta-granitoid amphiboles have Si from 6.02 and 6.23 and Mg/(Mg + Fe 2+ ) of , with Al VI < Fe 3+, and are classified as hastingsite. Titanite Titanite occurs as numerous inclusions in garnet porphyroblasts, along with plagioclase, and contains about 1.9 wt% Al 2 O 3 and about 1 wt% FeO. In meta-granitoids, titanite occurs mainly in the matrix, associated with amphiboles. Pelitic gneiss Kadugannawa Complex Garnet One generation of garnet was recognized, and it contains no significant chemical zoning. Garnet porphyroblasts are almandine-rich, with X Alm varying from 0.79 to Spessartine content is slightly greater than in the HC pelitic gneisses. Grossular content ranges up to X Ca = However, there is no significant variation in grossular content. Plagioclase Plagioclase occurs as inclusions in garnet and as a matrix mineral, and is oligoclase to andesine (X An = 0.11 to 0.43) in composition. Biotite Biotites contain about wt% TiO 2, and Fe/(Fe + Mg) varies from 0.58 to There are no significant differences in composition between inclusion phases and retrograde/later overprinted biotite. 114 Sample Texture X Fe-Gt X Mg-Gt X Ca-Gt Gt Fe/Mg X Fe-Opx X Mg-Opx X Fe-Opx Opx Fe/Mg

13 Journal of the Geological Society of Sri Lanka, Vol. 14, Amphibole Amphibole occurs only in biotite gneiss. (Ca+Na) B > 1.00 and (Na) B < 0.50 indicates it belongs to the calcic amphibole group (Fig. 7). Si varies between 6.02 and 6.23 and Mg / (Mg + Fe 2+ ) ranges from , with Al VI < Fe 3+ indicating it is magnesio-hastingsite (Leake et al., 1997). Opaque minerals Magnetite is the dominant opaque phase, and contains up to 6.57 wt % TiO 2. Mafic gneiss Kadugannawa Complex; Garnet Garnets are almandine rich (X Alm = ). These garnets are richer in the spessartine component than HC garnets. Grossular content (X Grs = 0.1) shows no significant variation. No zoning was observed in garnet. Plagioclase All the plagioclase in these rocks is andesine. No significant variation of anorthite content was observed between plagioclase inclusions in garnet and matrix plagioclase. Biotite Biotites contain about wt% TiO 2, and Fe/(Fe + Mg) vary from 0.48 to There is no significant difference in composition between inclusion phases and retrograde/later overprinted biotite. However, garnet-bearing mafic gneisses have slightly lower Mg/(Fe+Mg) ratios. Amphiboles Amphiboles occur in hornblende gneiss and migmatitic gneiss. All belong to the calcic amphiboles (values of (Ca+Na) B > 1.00 and (Na) B < 0.50). Si varies from 6.19 to 6.47 and Mg / (Mg + Fe 2+ ) values range from , with Al VI < Fe 3+ (Fig. 7) falling into magnesiohastingsite (Leake et al., 1997). Figure 7: Composition of amphiboles (after Leake et al. 1997) in mafic and intermediate rocks of the HC and KC. Spinel-bearing garnet biotite sillimanite gneiss shows highest grade metamorphic conditions, of which peak metamorphic assemblage is; garnet + sillimanite + K-feldspar + quartz + hercynite. Equilibrium pairs of biotite and garnet cores which have hercynitic spinel and ilmenite inclusions records peak metamorphic temperatures about o C at a nominal pressure of 7 kbar. Reaching such high temperatures is consistent with the occurrence of Zn-rich hercynitic spinel (Dasguptha et al., 1995), at a high oxygen fugacity indicated by associated ilmenite. Spinel-absent sillimanite gneiss records peak temperatures of 810 o C, at a nominal pressure of 10 kbar. Two-feldspar thermometry (Furhman and Lindsey, 1988) was applied to antiperthites in the spinel-absent garnet sillimanite gneiss. This yields minimum pre-exsolution temperatures of o C, at 8 kbar (Fig. 8). Presence of kyanite as rare inclusions in pre-peak garnet indicates the pressure conditions were increased before the peak metamorphism. Peak metamorphic pressure as estimated by GASP barometer yields 8 kbar using core compositions of peak garnet in spinelbearing gneisses, and that for spinel-absent rocks is 9 kbar at a nominal temperature of 800 o C. THERMOBAROMETRY HC Pelitic granulites For temperature calculations, the garnet biotite thermometer of Ferry and Spear (1978) and for pressure the garnet-aluminosilicatequartz-plagioclase barometer (GASP) of Koziol and Newton (1988) were used (Table 7). Figure 8: Application of Two-Feldspar Thermometer (Furhman and Lindsey, 1988) to antiperthite in the sample 8-1 of the HC. This yields a minimum preexsolution temperature of C. 115

14 Malaviarachchi and Takasu, Petrology of Metamorphic Rocks HC Intermediate and mafic granulites Temperatures were determined by garnet opx thermometer of Harley (1984) and garnet cpx thermometer of Ellis and Green (1979). Barometry based on alumina solubility in opx coexisting with garnet, plagioclase and quartz of Perkins and Chipera (1985) and Harley and Green (1982) were applied. In mafic granulites, rare cpx inclusions in garnet cores yielded crystallization temperatures of o C at a nominal pressure of 10 k bar (Table 8). Symplectite opx yields temperature of formation between o C at a nominal pressure of 5 kbar. Assemblage of opx porphyroblasts, plagioclase, garnet and quartz yielded a pressure range of k bar using the calibrations of Perkins and Chipera (1985) and Harley and Green (1982) at a nominal temperature of 800 o C. Symplectite opx gave a pressure of 6 kbar at a nominal temperature of 600 o C, using the calibration of Perkins and Chipera (1985). In intermediate rocks, equilibrium pairs of garnet and opx porphyroblasts recorded a temperature of 760 o C at a nominal pressure of 9 k bar (Table 9). Symplectite opx compositions yielded a temperature range of o C at 6 k bar of nominal pressure. Pressure calculations are 9 k bar at 800 o C nominal temperature and 6 k bar at 700 o C of nominal temperature, for porphyroblastic and symplectite opx compositions, respectively. Kadugannawa Complex rocks For thermometry calculations, the garnet biotite thermometer (Ferry and Spear, 1978) was used (Table 10). However, mineral assemblages suitable for barometry were not available. In pelitic rocks, equilibrium pairs of garnet and biotite core compositions recorded a temperature of 750 o C at a nominal pressure of 5 k bar. Inclusion biotites of garnet cores and the garnet mantle compositions gave temperatures 583 o C and 639 o C, respectively. Garnet rims indicated temperatures from 620 to 730 o C. In mafic rocks, garnet cores and matrix biotite gave a temperature of 690 o C and biotite inclusion in garnet core recorded 560 o C. Rims of garnet and retrograde biotite gave a temperature of 620 o C. P-T PATH AND TECTONIC INTERPRETATION a) P-T path: Using mineral textures and P-T estimations, the P-T and tectonic evolution of each rock unit are elaborated in the following sections. HC pelitic granulites Pelitic granulites Fig. 9 shows the P-T trajectory for the Highland Complex pelitic granulites of the present study. This shows a clockwise P-T path, consisting initial P-T increase due to heating and loading followed by a stage of rapid increase of pressure. Subsequently, the rocks experienced continuous temperature increase under slightly decreasing or constant pressure. Then the pelitic granulites underwent peak metamorphism followed by cooling and gradual decompression. Stage A Calculated lines of equilibrium constant (K) (Table 7) are plotted on the P-T space to elaborate the P-T evolution. Equilibrium K values for biotite inclusions in garnet and coexisting garnet+ sillimanite + plagioclase + quartz assemblage are in the range of and respectively, suggesting equilibration under Stage A. While passing from stage A to B, the pelites crossed the melting curve producing leucosomes which are observed in the outcrop scale. Stage B Stage B is constrained by antiperthite exsolution suggesting a minimum temperature for the process at a nominal pressure of 8 kbar. There is no way to determine the upper pressure constrain, due to non-availability of a suitable coexisting assemblage for GASP barometer. However, the associated garnets contain rare kyanite inclusions, suggesting a minimum pressure for this stage using the kyanite sillimanite line of Holdaway (1971). Stage C P-T path between B and C is uncertain, due to lack of petrographical evidence. Probable pseudomorphs 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

15 Journal of the Geological Society of Sri Lanka, Vol. 14, Figure 9: Inferred P-T path of the studied pelitic granulites of the Highland Complex. Letters A-G show evolutionary stages of the rocks, as discussed in the text (K- equilibrium constant). sillimanite grains in garnet suggest that the kyanite formation has taken place between two sillimanite forming stages of A and C. For the stage C, calculated K values are and for the thermometer and barometer respectively. The maximum pressure is constrained by kyanite sillimanite line of Holdaway (1971), and the minimum temperature is constrained by the biotite dehydration at the expense of sillimanite that produced garnet which contains prismatic sillimanite (different from previous sillimanites) together with biotite and quartz inclusions. Stage D Stage D is implied by the formation of hercynite at the outer margins of garnet which contain kyanite inclusions (Fig. 3c). The kyanite-bearing garnet may have crystallized at high P at stage B and near isobaric heating from C to D made garnet + sillimanite assemblage unstable. Also the high Zn content (~5 wt %) of the spinel suggests a higher temperature and pressure origin (Dasguptha et al., 1995). Absence of opx or cordierite at Stage D suggests opx or cordierite forming reaction lines have not been crossed. Also, K line calculated (= 0.31) is also well agree with the lower boundary of stage D. Stage D represent the peak metamorphic stage of these granulites. Stage E Hercynitic spinel + quartz was probably coexisting at peak metamorphism and reversal of the hercynite forming reaction may have given rise to stage E, during cooling after peak metamorphism. New garnet of stage E contains hercynite inclusions (with ilmenite) in the core and is free from other inclusions. The spinel inclusions produce a nearly linear pattern, suggesting deformation at the metamorphic peak. Calculated K values for the stage E is and for the thermometer and barometer, respectively. The minimum pressure for this stage is constrained by the absence of cordierite. Stage F Temperature constraints for the stage F is from rim compositions of garnet and coexisting biotite, and P constraints also from rim composition of garnet and matrix plagioclase. These values confine the range of K values of and for thermometer and barometer, respectively. Lower values of K reflect the extensive retrograde Fe Mg exchange between garnet and biotite. Further, when reaching from stage E to F, prograde sillimanite consuming reaction is reversed, as evidenced by the formation of late biotite and sillimanite aggregates over garnet porphyroblasts and in the matrix. Also, the P-T path from stage F to G cannot cross the staurolite forming reaction in the sillimanite stability field, since there is no evidence for staurolite in any of the studied rocks. 117