Proe. Indian Acad. Sci. (Earth Planet. Sci.), Vol. 96, No. 1, March 1987, pp. 1-10 {~) Printed in India Gneiss-charnockite relation around Ponmudi, southern Kerala: evidences and implications of prograde charnockite formation in southern India C SRIKANTAPPA and G R RAVINDRA KUMAR* Department of Geology, University of Mysore, Mysore 570 006, India * Centre for Earth Science Studies, Akkulam, Trivandrum 69S 031, India MS received 28 September 1984; revised 7 November 1986 Abstract. Isochemical conversion of garnet-biotite bearing paragneiss to charnockite in the Precambrian Khondalite belt of southern Kerala is described from Ponmudi area. Petrographic evidences indicate the formation of hypersthene by the breakdown of biotite in the presence of quartz following the reaction: Biotite + quartz --) hypersthene + K- feldspar + vapour. The estimated pressure - temperature conditions of metamorphism are around 5-7 kbars and 750~ + 40~ Presence of COz--rich, mixed CO2-H20 and HzO-rich inclusions were noticed in gneiss as well as in charnoekites. Charnockites contain abundant CO2-rich inclusions. Keywords. Charnockite; khondalite; whole rock chemistry; mineral chemistry; P-T conditions; Kerala. 1. Introduction The incipient charnockite formation at Kabbal reported by Pichamuthu (1960) in Karnataka, which grades on to a massive charnockite upland belt further south, opened new vistas and kindled new interest in the study of charnockites in southern Peninsular India. Several workers have suggested that the amphibole-bearing gneisses with basic enclaves are transformed to charnockite by the breakdown of amphibole to orthopyroxene at relatively low H20 fugacities (Janardhan et a11979; Friend 1981; Hansen et al 1984). Now there is a general consensus among petrologists that the widespread tonalitic to trondhjemitic Peninsular Gneiss in South India, which carries abundant enclaves of metasediments and basic rocks, have been metamorphosed under granulite facies conditions around 2.6 b.y. ago (Crawford 1969). So far description of arrested charnockite formation in South India is known only from southern Karnataka and in parts of Tamil Nadu (Janardhan et al 1979; Holt and Wightman 1983; Condie et al 1982). In these localities, the presence of K-rich granitic veins has been commonly noticed which may be genetically linked to charnockite-forming process (Friend 1981). Recent field investigations in southern Kerala have brought out interesting field relations between charnockite and gneiss (Ravindra Kumar et al 1985; Ravindra Kumar and Chacko 1986; Srikantappa et al 1985). The present paper reports new data on the field relations, petrography and fluid inclusions of charnockite formation from garnet-biotite bearing para-gneiss around Ponmudi in the Precambrian 'khondalite belt' in South India. This has a bearing on the understanding of the processes of charnockitization and evolution of this part of South Indian Precambrian terrane. 2 C Srikantappa and G R Ravindra Kumar 2. Regional geology South Kerala is predominantly composed of high-grade metapelites (khondalites) with minor amounts of charnockites (figure 1). The NW-SE trending foliation dips steeply (55-80 ~ towards SW in southern and south-western part of the Kerala khondalite belt. Khondalites with minor amounts of carbonate rocks, quartzites and graphitic gneisses are interlayered with basic and ultrabasic rocks. It is generally believed that the charnockites and khondalites represent original intercalations of igneous and sedimentary suite of rocks developed in a geosynclinal environment which were later metamorphosed to granulite facies. Subsequent retrogression of these charnockites and khondalites on a regional scale lead to the development of amphibolite facies gneiss (Mahadevan 1974; Narayanaswamy 1976; Sinha Roy 1979). The prograde formation of charnockites has not been reported from this tract till recently (Ravindrakumar et al 1985). Crawford (1969) in his reconnaissance dating of South Indian rocks reports poorly defined Rb-Sr ages for charnockites as 2155 m.y. and 2780 m,y. and for khondalites an older age of 3070 m.y. All these dates are from a single rock collected in the vicinity of Po,mudi. These data do not seem to be reliable because of too low initial 87Sr/86Sr ratio of 0.700. Evidence for the late proterozoic age for the regional granulite facies N 13 ~ 12 11 10 8 76 77 78 79*E Figure 1. Generalised geological map of southern India modified after Rao (1978). Gneiss-charnockite relations in southern Kerala 3 metamorphism is obtained from the U-Pb zircon data for charnockites from Ponmudi Hill which gives an age of c.540 m.y. and c.1930 m.y. (Bhul and Grauert 1985 pers. comm.). 3. Field relations and petrography The area around Ponmudi is a hill resort situated about 60 km north of Trivandrum, at an elevation of 1074 metres above mean sea level. It is located in the northern portion of the Kerala khondalite belt (figure 1). The data presented here come from the quarries examined around Ponmudi and adjacent areas in southern Kerala. The proportion of gneiss-to-charnockite observed in these quarries can be approximated as 60:40. The gneiss is a medium-to-coarse grained leucocratic rock composed of garnet, biotite, feldspar, quartz and graphite. Mineral foliation trending N" 70 ~ W is defined by flakes of biotite and subhedral garnets. Graphite flakes are commonly noticed along the foliation planes. Thin leucocratic quartz and plagioclase-bearing acid veins in these rocks may be related to a period of migmatization preceding the formation of charnockite. Garnet, biotite and graphite are absent in these leucocratic acid veins. The garnet-biotite gneisses are interlayered with garnet-sillimanite and garnet-sillimanite-biotite- cordieirte gneisses on various scales. As a result of deformation and migmatization, the metapelitic bands within the gneisses have been thinned and now occur as streaks and schlieren rich in garnet, sillimanite, biotite and cordierite. All these field evidences clearly indicate that the garnet-biotite gneisses undoubtedly represent metamorphosed pelitic sediments. This is also supported by abundance of graphite in the gneiss and their highly aluminous nature (table 1). The garnet biotite gneiss is traversed by numerous coarse-grained, dark brownish charnockitic patches whose contacts with gneisses are diffused. These patches generally vary. in size from 1 to 2 cm. Some reach to a maximum width of one metre (figures 2, 3 and 4). The development of charnockite with clots of orthopyroxene is seen concentrated in the sheared limbs and hinges of small scale folds. With the development of irregular greasy charnockitic patches there is a variable degree of obliteration of earlier foliation in the gneisses (figure 2 and 3). The distinct coarse-grained nature of charnockitic patches suggests thorough recrystallization. Quite often the gneissic foliation is bent or swerved at the borders of the charnockitic patches (figure 4). Close observation reveals the development of orthopyroxene along conjugate shears trendng N 50 ~ E and N 50~ W, cross-cutting the earlier foliation. Orthopyroxene, often measuring" upto 1-2 cm in size, is characteristic of the charnockite patches. Even some of the acid veins within the gneisses show development of charnockite patches suggesting that the granulite facies metamorphism followed migmatization. A drastic reduction of biotites, the most common ferromagnesian mineral in the gneiss, is seen with an increased process of charnockitization. Garnet and graphite continue to persist in the charnockitic patches and appear to have undergone little or no change during metamorphism. Coarsening of graphite flakes in charnockites indicates remobilization and recrystalization of graphite during prograde meta- morphism. C Srikantappa and G R Ravindra Kumar Table 1. Chemical analyses of rocks and minerals from Ponmudi, Southern Kerala. CS 35.1 CS 35.2 CS 41-1 CS 41.2 CS 35.1 CS 35-2 CS 35.1 CS 35.2 CS 35-2 wh. rock wh. rock wh. rock wh. rock garnet garnet biotite biotite opx gneiss charn, gneiss charn, gneiss charn, gneiss charn, charn. SiO, 65-24 65.96 67.92 68.10 37.94 37.89 36.03 36-27 48.51 TiO2 0.86 0-84 0.90 0-77 0-04 0-04 4-82 3.91 0-23 AI203 15.12 15.38 13.94 14.46 22.09 22-25 13-05 13.95 1.94 FeO t 5-98 5.51 5-80 4.94 33-88 33.65 19-10 19.61 37.13 MnO 0.05 0.06 0.08 0.04 0-45 0,67 0.02 0.02 0.21 MgO 1-89 1.36 1.06 0.95 3,95 3-62 11.32 10.85 11.04 CaO 2.95 2.73 2.39 2.62 1.63 2.06 0.02 0.04 0.23 Na20 2-44 2,16 2.45 2.81 ...... 0.46 0.05 0-02 K20 5-36 4-91 4.42 4-94 ...... 9-40 9.90 . P2Os 0-31 0-27 0.38 0-36 ............... Total 100-20 99.18 99.34 99.99 99.98 100.18 94.22 94.60 99.31 FeO t~ Total iron as FeO Trace elements in ppm Rb 251 196 190 153 Sr 101 120 98 136 Anorthite content of plagioclase: Charm (35.2) = An33, Charn. (41.2) = An36 Modal analyses Garnet Feldspar Biotite Ortho- Quartz Accessories (pig + pyroxene K-felds) Gneiss (CS 35.1) 12.20 48.85 16.40 ,-- 22.20 0-35 Charn. (CS 35.2) 10.10 57.50 5.20 10-50 16-28 0-42 Gneiss (CS 41.1) 15.04 45.60 18.85 ... 19-50 1.01 Charn. (CS 41.2) 12.50 55.20 3-10 11.23 17.12 0.85 Petrographic examination of the gneiss shows the presence of colourless, inclusion-free granular garnet, pethites, dark brown biotite and quartz. Thin section study of charnockite shows the presence of large euhedral orthopyroxenes developing after biotite (figure 5).