petrology of metamorphic rocks from the highland and ... 14/seismicity.pdf · igneous intrusions...

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

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

Petrology of Metamorphic Rocks from the Highland and

Kadugannawa Complexes, Sri Lanka

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 equi-librium 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 decom-pression after peak metamorphism. The P-T paths of mafic and intermediate granulites are consistent with magmatic intrusion or magmatic underplating occurring at depth and sub-sequent 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

The metamorphic basement of Sri Lanka has been considered as a key terrain to under-stand the evolution of the Gondwana supercon-tinent. 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 meta-sedimentary and mafic to felsic meta-igneous high- to ultra-high temperature granulites to-gether with amphibolites, migmatites and mi-

nor gabbroic, granitic, pegmatitic and aplitic igneous intrusions characterize the Sri Lankan basement. Due to the tectonic amalgamation of amphibolite to granulite-facies terrains of di-verse isotopic signature containing a diversity of rock types in a relatively small area, Sri Lanka is of great interest in the fields of petrology, geo-chronology 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),

Malaviarachchi and Takasu, Petrology of Metamorphic Rocks

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Wanni (WC), Vijayan (VC) and Kadugannawa Complexes (KC) as shown in the (Fig. 1). Al-though on the basis of similarities in structures Kehelpannala (1997) included the Kadugannawa Complex in the Wanni Complex, we have ad-hered to Cooray (1994) classification in this pa-per.

Figure 1: Litho-tectonic units of Sri Lanka (after Coo-ray, 1994) showing the study area.

Although petrologic research on Sri Lankan

metamorphic rocks has been carried out exten-sively, this study was undertaken to present an updated dataset particularly on mineral chemis-try. 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 Com-plexes (Fig. 2). After careful petrographic obser-vations, 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 re-searchers suggesting a clock-wise P-T path. However, some recent petrological studies have noted ultra-high temperature (UHT) metamor-phism 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-gest 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) suggest-ing UHT metamorphism above 1050 0C and 11-12 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 pre-serves a maximum of 850-900 0C 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 distin-guish whether it is a product of the Pan African


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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 prod-uct 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 3-2.2 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 2-1.1 Ga. An ion microprobe (SHRIMP) U-Pb study of zircons (Kröner et al., 1987) documented 3.2–2.0 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 550-610 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 orthopyrox-ene 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 sam-ple localities is shown in the Figs. 1 and 2, re-spectively. 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).


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Hand specimens and thin sections were studied for petrography, and the mineral tex-tural features were studied using a polarizing microscope. Chemical compositions of con-stituent minerals of rocks and the back scat-tered electron images (BSE) were obtained us-ing a JEOL JXA-8800M electron probe micro analyser (EPMA) at Shimane University, Japan. The analytical conditions used were 15 kV ac-celerating 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 identi-fied as, garnet biotite sillimanite gneiss and bio-tite gneiss (Table 1). They have gneissose folia-tion 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 (anti-perthite texture), together with fine quartz in-tergrown in the host (Fig. 3a). Subhedral to an-hedral garnet porphyroblasts up to 8 mm in di-ameter 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 oc-curs 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 silli-manite collectively form a shape of a relict por-phyroblast, 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 silli-manite gneiss. In addition, it occurs in symplec-tites 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 inclu-sion 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 mona-zite. HC Mafic granulites

These rocks are generally coarse grained and poorly foliated. The garnet amphibole py-roxene mafic granulite of this study consists mainly of garnet, cpx, opx, pargasitic amphi-bole, plagioclase, quartz, and titanite (Table 1). Garnet occurs as poikiloblasts up to 15 mm in diameter are sometimes idioblastic with plagio-clase, 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 porphyroblas-tic 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 gar-net (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 re-placed by pargasitic amphibole. Opx porphyro-blasts have biotite, plagioclase and opaque mineral inclusions and these porphyroblasts are later replaced by secondary biotite and ilmen-ite. 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 am-phibole wrapping around the garnet. These amphiboles are pargasite. Some of these par-gasites replace opx. Due to strong retrogres-sion, chlorite, quartz and hematite assemblages are found between garnet porphyroblasts. Opaque phases include magnetite and ilmenite.

cross nicol


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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 af-ter 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 appear-ance in hand specimen, exhibiting weak gneissic foliation. In contrast, meta-granitoid shows a preferred orientation of minerals such as horn-blende, 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 plagio-clase 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 meta-granitoid are free from inclusions and occur in sizes of 3-5mm anhedral grains. Some garnets are completely broken down to form cpx-bearing symplectites, associated with amphi-bole, biotite and opaque. Hypersthene in char-nockitic gneiss commonly occurs as anhedral porphyroblasts and is associated with plagio-clase rims after garnet. Rare cpx was found oc-

curring in symplectites with plagioclase in meta-granitoid where opx is absent.

In both rock types, plagioclase occurs as porphyroblasts, inclusions in garnet and coro-nae on garnet. Generally, plagioclase show al-bite 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 porphy-roblastic titanite.

Charnockitic gneisses show retrograde al-teration 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 meta-granitoid. KC Pelitic gneiss

Pelitic gneisses in the Kadugannawa Com-plex 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 porphyro-blasts up to 5 mm with inclusions of biotite


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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 are replaced by biotite along the rim (Fig. 5a). Quartz in the matrix with plagioclase, K-feldspar and biotite forms a preferred orientation. Pla-gioclase 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 in-clude garnet-bearing and garnet-absent rocks (Table 1). These rocks are generally coarse grained and poorly foliated and exhibit a grano-blastic polygonal texture.

Garnet-bearing rocks consist of plagioclase, quartz, biotite, rutile and ilmenite. Garnet oc-curs as porphyroblasts up to 3 mm and occa-sionally contains quartz and biotite as inclusions (Fig. 5b). Garnet porphyroblasts are replaced by biotite overprints at their margins. Garnet-absent mafic rocks contain hornblende, quartz, plagioclase, biotite and ilmenite and are repre-sented 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 in-clusions are present in plagioclase.


110

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 asso-ciation 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 inclu-sions 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 XAlm = 0.8), where XPrp ratio

decreases slightly from core to rim. The grossu-lar component also has a similar trend, with maximum ratio of XGrs=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 0.001.

In addition, some garnets which contain rare hercynite + ilmenite inclusions show com-positional heterogeneity.


111

Plagioclase Plagioclase is oligoclase to andesine in

composition (XAn = 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 0.67 – 0.65. Inclusion spinel has higher Zn content (max. ZnO = 7.15 wt %) while the retrograde spinel has a maximum ZnO con-tent 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 4.5 – 6.8 wt % of TiO2, and their Fe/(Fe + Mg) varies from 0.52 to 0.64. Mg ratio vs. Ti (per formula unit, p.f.u) varies depending on the textural setting. Thus, biotite inclusions in garnet and secondary bio-tite overprints on garnet have contrasting com-positions. 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 inclu-sion phases, for a single lithology. Spinel-bearing lithologies have higher Mg contents in biotites, whereas garnet-absent lithologies have higher Ti contents.

Opaque minerals Rutile contains 6.2-8.87 wt % FeO and up to

0.1 wt % Cr2O3. Ilmenite contains up to 1.5 wt % MgO, 0.15 wt % MnO, and 0.35 wt % Cr2O3, while magnetite contains up to 0.43 wt % Cr2O3.

Mafic to intermediate granulites – Highland Complex

Garnet Garnets in these rocks are almandine-rich

and highly variable in composition (Fig. 6), and the highest XAlm 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 porphyro-blastic garnets of charnockitic gneiss. Meta-granitoid garnets have almost constant compo-sition. Pyrope contents of garnets from the gar-net amphibole pyroxene gneiss vary from 0.27 to 0.19. In the case of charnockitic gneiss, py-rope varies from 0.14 to 0.08. In meta-granitoid, the pyrope content is almost con-stant.

The highest grossular content (XGrs= 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 meta-granitoids, garnet rims are richer in grossular than the cores (max. X Grs= 0.23).

Orthopyroxene Opx occurs as porphyroblasts, in fine-

grained symplectites and coarse-grained sym-plectites after garnets, with X Mg = 0.85 – 0.96. Alumina contents differ markedly in the garnet amphibole pyroxene gneiss. Opx in the sym-plectites after garnet has Al contents from 2.11 to 3.12 wt %, whereas the opx in the coarse grained symplectites has Al2O3 contents ranging from 0.70 to 1.19 wt%. However, porphyroblas-tic opx in the matrix contains 1.35 to 2.09 wt% Al2O3. Symplectitic opx in all lithologies has greater XAl 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 gar-

net, and as internal symplectite with plagioclase in garnets of the garnet amphibole pyroxene gneiss. Rare cpx was found occurring in sym-plectites 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 ma-trix, as inclusions in garnet, in symplectites with opx, and as coronas on garnet. The anorthite content is highly variable, with maximum of XAn = 0.90 in garnet amphibole pyroxene gneiss, and minimum of XAn = 0.20 in charnockitic gneiss. Anorthite contents show marked varia-tion in garnet amphibole pyroxene gneiss where XAn of matrix < symplectite < inclusions in garnet


112

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.


113

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

Mineral Garnet Biotite Plagioclase Amphibole K-feldspar Ilmenite

Sample 14B 14B1 14B 14B-1 14B-2 51 1 2B 3 51 1 2B 3 51 1 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.

Sample Texture Garnet Biotite Plagioclase Nominal P Nominal T K

Calculated

P

Calculated

T

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

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

0.363 0.637 0.25gt-bt-sill gneiss (spl bearing) garnet (I) mantle - inclusion of biotite 0.25 0.72


114

However, anorthite content of plagioclase in meta-granitoids is almost constant. In char-nockitic gneisses, anorthite content is variable in the range from 0.20 to 0.63. Amphibole

Amphibole occurs in garnet amphibole py-roxene 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 6.37-6.47 and Mg/ (Mg + Fe2+) from 0.51 and 0.59 with Al VI > Fe3+, falling into pargasite. In contrast, meta-granitoid amphi-boles have Si from 6.02 and 6.23 and Mg/(Mg + Fe2+) of 0.10-0.18, with AlVI < Fe3+, and are classi-fied as hastingsite.

Titanite

Titanite occurs as numerous inclusions in garnet porphyroblasts, along with plagioclase, and contains about 1.9 wt% Al2O3 and about 1 wt% FeO. In meta-granitoids, titanite occurs mainly in the matrix, associated with amphi-boles. Pelitic gneiss – Kadugannawa Complex Garnet

One generation of garnet was recognized, and it contains no significant chemical zoning. Garnet porphyroblasts are almandine-rich, with XAlm varying from 0.79 to 0.88. Spessartine con-tent is slightly greater than in the HC pelitic gneisses. Grossular content ranges up to XCa = 0.032. 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 an-desine (XAn = 0.11 to 0.43) in composition. Biotite

Biotites contain about 3.35–3.49 wt% TiO2, and Fe/(Fe + Mg) varies from 0.58 to 0.67. There are no significant differences in composi-tion between inclusion phases and retro-grade/later overprinted biotite.

Sam

ple

Text

ure

XF

e-G

tX

Mg-

Gt

XC

a-G

tG

t Fe/

Mg

XF

e-O

pxX

Mg-

Opx

XFe

-Opx

Opx

Fe/

Mg

garn

et a

mph

ibol

e py

roxe

ne g

neis

sga

rnet

-opx

sym

plec

tite

0.62

50.

188

0.12

3.32

0.99

60.

921.

08

0.54

90.

196

0.08

2.8

0.99

60.

921.

08

0.51

40.

156

0.1

3.29

0.97

90.

927

1.06

garn

et a

mph

ibol

e py

roxe

ne g

neis

scp

x in

clus

ions

in g

arne

t0.

550.

140.

053.

93

0.54

20.

139

0.05

3.9

0.54

80.

140.

053.

91

garn

et a

mph

ibol

e py

roxe

ne g

neis

sga

rnet

-opx

por

phyr

obla

sts0

.648

0.11

70.

47

0.66

0.08

30.

49

0.66

60.

077

0.87

6

garn

et a

mph

ibol

e py

roxe

ne g

neis

sga

rnet

-opx

sym

plec

tite

0.56

30.

029

0.67

9

0.58

0.03

50.

687

Opx

(Al-

M1)

XF

e-C

pxX

Mg-

Cpx

Cpx

Fe/

Mg

XA

n-P

lgK

Nom

inal

PN

omin

al T

(%)

(H 8

4a)

(E&

G,7

9)(P

&C

,85)

(H&

G,8

2)

365

05

2.6

700

5

3.1

650

5

0.44

0.65

60.

675.

989

510

0.42

50.

628

0.68

5.8

900

10

0.42

30.

653

0.65

689

110

0.81

11.

111

800

0.77

61.

610

.580

0

70.

211

800

0.79

117

660

0

0.75

412

660

0

Cal

cula

ted

PC

alcu

late

d T

Tabl

e 8:

Tem

pera

ture

and

pres

sure

calcu

latio

ns fo

r maf

ic g

ranu

lites

, Hig

hlan

d Co

mpl

ex (%

Al M

1= m

olar

ratio

of A

l in

the

M1

site

of O

px. H

84: H

arle

y, 1

984;

E&

G, 7

9: E

llis a

nd G

reen

, 197

9; P

&C,

85: P

erki

ns an

d Ch

iper

a, 19

85; H

&G,

82:

Har

ley a

nd G

reen

, 198

2).


115

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+ in-dicating it is magnesio-hastingsite (Leake et al., 1997). Opaque minerals

Magnetite is the dominant opaque phase, and contains up to 6.57 wt % TiO2.

Mafic gneiss – Kadugannawa Complex; Garnet

Garnets are almandine rich (XAlm=0.74 - 0.77). These garnets are richer in the spessar-tine component than HC garnets. Grossular content (XGrs= 0.1) shows no significant varia-tion. No zoning was observed in garnet. Plagioclase

All the plagioclase in these rocks is ande-sine. No significant variation of anorthite con-tent was observed between plagioclase inclu-sions in garnet and matrix plagioclase. Biotite

Biotites contain about 4.1 -5.0 wt% TiO2, and Fe/(Fe + Mg) vary from 0.48 to 0.63. There is no significant difference in composition be-tween 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 am-phiboles (values of (Ca+Na)B > 1.00 and (Na)B < 0.50). Si varies from 6.19 to 6.47 and Mg / (Mg + 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 and Newton (1988) were used (Table 7).

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 condi-tions, of which peak metamorphic assemblage is; garnet + sillimanite + K-feldspar + quartz + hercynite. Equilibrium pairs of biotite and gar-net cores which have hercynitic spinel and il-menite inclusions records peak metamorphic temperatures about 810-830 oC at a nominal pressure of 7 kbar. Reaching such high tem-peratures 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 oC, 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 630 -700 oC, at 8 kbar (Fig. 8). Presence of kyanite as rare inclu-sions 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 us-ing core compositions of peak garnet in spinel-bearing gneisses, and that for spinel-absent rocks is 9 kbar at a nominal temperature of 800

oC .

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 pre-exsolution temperature of 630-700 °C.


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HC Intermediate and mafic granulites Temperatures were determined by garnet–

opx thermometer of Harley (1984) and garnet – cpx thermometer of Ellis and Green (1979). Ba-rometry 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 tempera-tures of 890-900 oC at a nominal pressure of 10 k bar (Table 8). Symplectite opx yields tempera-ture of formation between 650 – 700 oC at a nominal pressure of 5 kbar. Assemblage of opx porphyroblasts, plagioclase, garnet and quartz yielded a pressure range of 10.5 – 11 k bar using the calibrations of Perkins and Chipera (1985) and Harley and Green (1982) at a nominal tem-perature of 800 oC. Symplectite opx gave a pres-sure of 6 kbar at a nominal temperature of 600 oC, using the calibration of Perkins and Chipera (1985).

In intermediate rocks, equilibrium pairs of garnet and opx porphyroblasts recorded a tem-perature of 760 oC at a nominal pressure of 9 k bar (Table 9). Symplectite opx compositions yielded a temperature range of 650 – 690 oC at 6 k bar of nominal pressure. Pressure calcula-tions are 9 k bar at 800 oC nominal temperature and 6 k bar at 700 oC of nominal temperature, for porphyroblastic and symplectite opx com-positions, respectively.

Kadugannawa Complex rocks

For thermometry calculations, the garnet–biotite thermometer (Ferry and Spear, 1978) was used (Table 10). However, mineral assem-blages suitable for barometry were not avail-able. In pelitic rocks, equilibrium pairs of garnet and biotite core compositions recorded a tempera-ture of 750 oC at a nominal pressure of 5 k bar. Inclusion biotites of garnet cores and the garnet mantle compositions gave temperatures 583 oC and 639 oC, respectively. Garnet rims indicated temperatures from 620 to 730 oC. In mafic rocks, garnet cores and matrix biotite gave a temperature of 690 oC and biotite inclusion in garnet core recorded 560 oC. Rims of garnet and retrograde biotite gave a temperature of 620 oC.

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 ini-tial P-T increase due to heating and loading fol-lowed by a stage of rapid increase of pressure. Subsequently, the rocks experienced continu-ous temperature increase under slightly de-creasing or constant pressure. Then the pelitic granulites underwent peak metamorphism fol-lowed by cooling and gradual decompression. Stage A

Calculated lines of equilibrium constant (K) (Table 7) are plotted on the P-T space to elabo-rate the P-T evolution. Equilibrium K values for biotite inclusions in garnet and coexisting gar-net+ sillimanite + plagioclase + quartz assem-blage are in the range of 0.19 – 0.20 and 0.0004–0.001 respectively, suggesting equilibra-tion under Stage A. While passing from stage A to B, the pelites crossed the melting curve pro-ducing leucosomes which are observed in the outcrop scale. Stage B

Stage B is constrained by antiperthite exso-lution suggesting a minimum temperature for the process at a nominal pressure of 8 kbar. There is no way to determine the upper pres-sure constrain, due to non-availability of a suit-able coexisting assemblage for GASP barome-ter. 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 pseu-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


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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 kyanite formation has taken place between two sillimanite forming stages of A and C. For the stage C, calculated K values are 0.290 and 0.001 for the thermometer and barometer respec-tively. 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 sug-gests a higher temperature and pressure origin (Dasguptha et al., 1995). Absence of opx or cor-dierite 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 co-existing at peak metamorphism and reversal of

the hercynite forming reaction may have given rise to stage E, during cooling after peak meta-morphism. New garnet of stage E contains her-cynite inclusions (with ilmenite) in the core and is free from other inclusions. The spinel inclu-sions produce a nearly linear pattern, suggest-ing deformation at the metamorphic peak. Cal-culated K values for the stage E is 0.30 – 0.31 and 0.0004 for the thermometer and barome-ter, respectively. The minimum pressure for this stage is constrained by the absence of cordier-ite. Stage F

Temperature constraints for the stage F is from rim compositions of garnet and coexisting biotite, and P constraints also from rim compo-sition of garnet and matrix plagioclase. These values confine the range of K values of 0.22 - 0.26 and 0.0003-0.0004 for thermometer and barometer, respectively. Lower values of K re-flect 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 ag-gregates 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.


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

Sample Texture XFe-Gt XMg-Gt XCa-Gt Gt Fe/Mg XFe-Opx XMg-Opx Opx Fe/Mg XAn-Plg K

Calculated T

Nominal P

Calculated 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

P-T evolution of mafic and intermediate granulites is shown in the Fig. 10. Pre A1 stage

This stage is inferred by the presence of cpx in mafic rocks. However, cpx does not occur in the matrix and found only as inclusions and as an internal symplectite with plagioclase within garnet porphyroblasts. These garnets have nu-merous inclusions of titanite + plag ± quartz, and occur in a matrix with opx porphyroblasts + plagioclase + pargasitic amphibole. Therefore, the P-T path crosses the amphibole forming reaction at the expense of cpx + opx + plagio-clase, at high temperature and pressure condi-tions inferred from garnet-cpx pairs and core compositions of garnet and opx for the stage A1. Intermediate granulites show no evidence of this stage. Stage A1 In mafic rocks, garnet-cpx inclusion pairs show K = 6 and ln K = 0.1 – 0.5 from geothermometry (Ellis and Green, 1979) and barometry (Perkins and Chipera, 1985), respectively. However, there is no opx porphyroblasts which are in di-rect contact with garnet to estimate tempera-ture. For Harley and Green (1982) barometric equilibrium, Al content in the M1 site of opx has strong dependence on pressure, and for this considered rock the isopleth line of Al (in M1) and the ln K line of Perkins and Chipera (1985) closely intersect the K line of Ellis and Green (1979), constraining the stage A1. In intermedi-ate granulites K= 2.3 and ln K ranging from 0.7 to 1.3 for garnet and opx core compositions, reflect the equilibration of the rock at a higher pressure and temperature conditions.

Stage A2 In mafic rocks, Stage A2 is characterized by wide spread occurrence of symplectite of opx and plagioclase after garnet. This is one of the typical petrographic evidence for granulite de-compressional stages (Thost et al., 1991). K val-ues for symplectite opx and garnet rim compo-sitions range from 2.6 – 3.1. Calculated values of ln K from Perkins and Chipera (1985) ranges from 2.5-2.8 and has largely increased than that of stage A1 implying the pressure has markedly decreased. Also in intermediate granulites, Opx-symplectite and garnet rim give K values of 2.7 to 3, and ln K = 2.9, higher than those of Stage 1. Also when cooling under decomp pression from stage A1 to A2, the P-T path crosses two different Ca-amphibole (pargasite) forming re-actions at the expense of pyroxenes, garnet and plagioclase. These rocks have relict evidence for the occur-rence of these reactions as some of opx grains are replaced by pargasitic amphibole. Secon-dary or late biotite partially replacing opx por-phyroblasts as well as some of the symplectitic opx and overprinting on amphiboles and the occurrences of chlorite implies lower amphibo-lite to greenschist facies conditions after stage A2. Similar retrograde assemblages also present in intermediate granulites.

Kadugannawa Complex rocks

It was difficult to construct P-T path for Kadugannawa Complex rocks due to lack of mineral assemblages for suitable and sufficient geobarometry. The geothermometry data are presented in the Table 10.


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

Sample Texture XFe-Gt XMg-Gt XFe-Bt XMg-Bt K Calculated T (F & S, 78)

Nominal P

Pelitic gneiss Bioite gneiss garnet core-biotite

inclusion 0.434 0.176 0.750 0.566 0.18 583 5

garnet mantle-biotite inclusion

0.370 0.239 0.682 0.632 0.21 639 5

garnet mantle-biotite inclusion

0.350 0.253 0.664 0.650 0.21 639 5

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 inclusion

0.338 0.202 0.6 0.662 0.17 564 5

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

A possible tectonic interpretation of the P-T paths obtained in this study can be explained as follows. a) P-T path for pelitc granulites

The P-T path shown in the Fig. 9 indicates an initial P-T increase in HC granulites, followed by rapid increase of pressure. This type of P-T path suggests tectonic crustal thickening (Eng-land and Thompson, 1984; Spear, 1993) in a collision zone. From a Sri Lankan context this

could indicate the over-thrusting of the Wanni Complex onto the Highland Complex. Heating accompanying magmatic intrusions (e.g. Holzl et al., 1991) to the thickened crustal unit repre-senting the HC and the WC at depth probably led to the peak metamorphism during the Pan African times (~550 Ma; Milisenda et al. 1988; Baur et al.1991; Kröner et al., 1991), followed by cooling and decompression. [Early tempera-ture increase before rapid pressure increase event can be attributed to rifting and extension (Kriegsman, 1996)].


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b) P-T path for mafic and intermediate granu-lites

The P-T path of mafic and intermediate granulites (Fig. 10) can be interpreted by mag-matic underplating models adopted for granu-lites (e.g. Wells, 1980, Bohlen, 1987; Spear, 1993). Accordingly, the meta-igneous rocks were produced from magmatic intrusion or magmatic underplating occurring at depth and subsequent cooling took place during the uplift. The magmatism envisaged occurred in over-thickened crust composed of both the HC and the WC since the metamorphic age of both pe-litic and mafic granulites of the HC and the WC is synchronous (Milisenda et al. 1988, Baur et al.1991, Holzl et al, 1991). This interpretation is supported by the field evidence that mafic granulites are generally intercalated and/or closely associated with pelitic granulites in both the HC and the WC of Sri Lanka.

However, the intrusion or underplating of mafic magmas to produce mafic granulites and felsic to intermediate magmas to produce in-termediate granulites has taken place at differ-ent crustal levels. As shown by the Fig. 10, the intermediate granulites have originated at shal-lower crustal levels than the mafic granulites. However, there is no evidence found in this study for isobaric cooling segment after peak metamorphism, as revealed by previous works (e.g. Perera, 1987; Schumarcher et al., 1990; Schenk et al, 1991; Prame, 1991b).

Accordingly, the clock-wise P-T path for pe-litic granulites and cooling path for meta-igneous rocks document the possible deep crustal processes by which continental crust grows similar to the phenomena in most granu-lite terrains of the world.

CONCLUSIONS

This study reconfirms that the pelitic granu-lites of the Highland Complex of Sri Lanka evolved through a clock-wise P-T path. The Peak metamorphism took place at sillimanite stability field; however an early pressure increase in ex-cess of 9 kbar was inferred by relic kyanite in-clusions in garnet. Due to that, it can be con-cluded that these granulites were equilibrated twice in the sillimanite stability field at distinctly different P-T conditions, in their evolution.

In contrast, metabasic and intermediate granulites evolved through a cooling path from

the peak metamorphism, after emplacement of their protolith to the lower crustal units. How-ever, the studied rocks have not preserved evi-dence for initial isobaric cooling after the em-placement of their igneous protoliths, evi-denced by the growth of garnet, cpx and quartz from orthopyroxene and plagioclase and subse-quent breakdown to the same assemblage, which they interpreted in terms of isobaric cool-ing.

The KC mineral assemblages represent lower metamorphic grade than those of the HC. Prograde metamorphic evidence could be sup-ported by a few inclusion phases in garnet por-phyroblasts and retrograde evidence by over-printing textures of secondary or late stage as-semblages. There was no characteristic decom-pressional/retrograde textures like symplec-tites, reaction rims or coronae formation ob-served in the studied samples of the KC. Fur-ther, this study presents new mineral chemistry dataset for the KC that lacks published mineral chemistry data. Acknowledgements

We thank Dr. B. Roser of Shimane Univer-sity for his comments on the manuscript and to Profs. M. Akasaka, H. Komuro and H. Ohira, Dr. A. Kamei and the members of the ‘Metamor-phic Seminar’ of the Shimane University (2003-2005) for helpful comments and discussions. Constructive comments by Mr. L.R.K. Perera significantly improved the manuscript. This study was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (Monbukagakusho) Scholarship to S.P.K.M for the M.Sc degree.

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petrology of metamorphic rocks from the highland and ... 14/seismicity.pdf · igneous intrusions...

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