"Rock of the Month # 43, posted January 2005" --- Sample GCW 1329 (photos by Karyn Gorra). With special thanks to Dr. G.R. Ravindra Kumar for a tour of granulite localities in Kerala in 1991. Collected in situ, 10 January 1991. Khondalites are quartz- feldspar- sillimanite gneisses, with graphite, garnet and biotite, ± cordierite.

Khondalite is a regional rock name rooted in the history of petrological research in the Indian subcontinent. The use of the term remains overwhelmingly linked to this region, although it has been transported beyond its original use in India to Burma (Iyer, 1949), Sri Lanka (e.g., Vitanage, 1959), Vietnam, China, Antarctica and even the Grenville-age metamorphic terrane of the Adirondack Mountains in New York state, in North America. More than 95% (176 out of 182) of the records on khondalites in the MINLIB database are from India and Sri Lanka. Within India, most references to khondalite follow an arc of southern and eastern states: Kerala, Tamil Nadu, Andhra Pradesh and Orissa. The usage of the name khondalite dates to Walker (1902). Walker originated the term to refer to the suite of rocks including quartz- garnet- sillimanite- graphite gneisses, together with garnet gneiss, quartzofeldspathic gneiss and quartzite, which host manganese deposits in the state of Orissa.

The khondalite suite are a distinctive group of rocks, but the minimal "export" of the term beyond India and Sri Lanka led P.G. Cooray (J.Geol.Soc.India 51 no.5, p.710, May 1998) to suggest that it be dropped in favour of garnet-sillimanite granulite/gneiss. In comparison, the charnockite suite, first described in Tamil Nadu, and often found in geographic proximity to the khondalites, were readily adopted by petrologists. Varietal names were added, principally in Scandinavia, the term is in use worldwide, and MINLIB now lists 770 records on charnockites, 1900 onwards.

The figured sample (1329, above) is a coarse khondalite, sensu strictu from the Kunnanpara quarry, just north of Trivandrum (Thiruvananthapuram) in Kerala, south India. The locality is described in Ashwal (1988, "site 2" of pp.361-368). For an introduction to the regional geology, see also the fine field guide of Ravindra Kumar et al. (1990). This is a good sample of true khondalite with dark cordierite-rich bands, sillimanite and graphite (1-5% graphite is typical). Greenish feldspathic bands occur, plus thinner bands rich in dark grey cordierite, or quartz, or fibrous white sillimanite. All are overgrown by lilac (Mn-rich?) garnet crystals up to 4 mm in dia.

The piece weighs 1473.75 grams, and the average bulk magnetic susceptibility is estimated at 0.760x10^-3 SI units, with slightly higher values in garnet-rich bands.

A similar sample from the same region, 1332, is displayed at the foot of the page. Sample 1332 was collected in situ at the Elavattayam quarry, Trivandrum district. Most of the quarry is in garnetiferous gneiss, with minor acid charnockite. This is a felsic gneiss dominated by perthitic feldspar plus quartz, with scattered lilac garnet, plagioclase feldspar, dark biotite mica and graphite flakes. The garnet contains inclusions of biotite and rutile, and most probably has a spessartine-rich (Mn-rich) composition. Traces of muscovite, zircon, magnetite, pyrite, chalcopyrite and pyrrhotite are also visible in polished thin section. The oxides and sulphides may both be found on garnet crystal margins. The sample is a metasediment of the khondalite suite, lacking the sillimanite found in `definitive' khondalite.

The Kerala khondalite belt contains three sets of interlayered rocks: (1) garnet- biotite ± orthopyroxene ± graphite gneisses; (2) khondalites (graphite- garnet- biotite- sillimanite ± cordierite gneisses); and (3) cordierite gneisses (garnet, biotite, cordierite ± orthopyroxene). The pre-metamorphic protoliths of these rocks were probably a spectrum of quartzitic, arkosic and argillaceous sediments, perhaps with interbedded felsic volcanic units (see, e.g., Chacko et al., 1992).

The name persists to this day - it appears useful within the subcontinent, a well-understood term, the physical conditions of metamorphism which generated these striking rocks now well-constrained by pressure-temperature estimates from numerous areas of India and beyond (Lal, 2003). Metamorphic terranes with khondalites may contain a range of metallic and industrial mineral deposits. Indian examples include iron and manganese oxides, bauxite, graphite and gemstones.

References

Ashwal,LD (editor) (1988) Workshop on the Deep Continental Crust of South India. Lunar and Planetary Institute Tech.Rep. 88-06, in conjunction with Geol.Soc.India, 388pp. [with notes by G.R. Ravindra Kumar and M. Santosh on eight sites in the Kerala khondalite belt, pp.359-377].

Chacko,T, Ravindra Kumar,GR, Meen,JK and Rogers,JJW (1992) Geochemistry of high-grade supracrustal rocks from the Kerala khondalite belt and adjacent massif charnockites, South India. Precambrian Research 55, 469-489.

Iyer,LAN (1949) The Geology and Gem-Stones of the Mogok Stone Tract, Burma. Geol.Surv.India Memoir 82, 100pp. plus 6pp. of plates and 2 maps.

Lal,RK (2003) Metamorphic evolution of granulites from southern Indian shield. In `Milestones in Petrology and Future Perspectives' (Mohan,A editor), Geol.Soc.India Memoir 52, 471pp., 61-108.

Ravindra Kumar,GR, Rajendran,CP and Prakash,TN (1990) Charnockite- Khondalite Belt and Tertiary- Quaternary Sequences of Southern Kerala. Geol.Soc.India, Bangalore, 116pp.

Vitanage,PW (1959) Geology of the country around Polonnaruwa. Department of Mineralogy, Geological Survey of Ceylon, Memoir 1, 75pp. plus 2 maps.

Walker,TL (1902) The geology of the Kalhandi state. Geol.Surv.India Memoir 33 part 3.

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