Elsevier

Lithos

Volume 75, Issues 3–4, August 2004, Pages 331-358
Lithos

Mechanism of arrested charnockite formation at Nemmara, Palghat region, southern India

https://doi.org/10.1016/j.lithos.2004.03.005Get rights and content

Abstract

The formation of arrested charnockite is an excellent example of structurally controlled channellised fluid flow along specific sites accompanied by selective elemental mobility and mineralogical changes. The present paper recognises and focuses study on three types of arrested charnockite formation from Palghat region, namely, shear-controlled, foliation parallel and boudin-neck types, and address their spatio-temporal relations to regional-scale charnoenderbite. The shear-controlled and foliation parallel types post-date deformation and migmatisation. The boudin-neck type, on the contrary, is coeval with partial melting and followed the path of cooling and decreasing water activity in the gneiss. K-feldspar veining around plagioclase and quartz, symplectitic intergrowth of biotite+quartz after orthopyroxene and K-feldspar, and fluid inclusion data suggests the presence of alkalic supercritical brine and low-density CO2-rich fluid during charnockite formation. Charnockite domains developed following the breakdown of hornblende, biotite and quartz are characterised by a more or less pronounced depletion of Fe, Ca, Mg and Ti and trace elements Y and Zr, compared to their counterpart gneiss. REE spectra indicate a subtle depletion in the HREE near the centre of the charnockite domain. Although close-pair samples of gneiss–charnockite are isochemical, on a scale of a few millimetres, bi-directional element movement, related to the formation of new mineral was noted. It is postulated that arrested charnockite formation developed in situ on local scale within the granitic domains of the hornblende-biotite gneiss, in the presence of CO2-rich fluids and alkalic supercritical saline brine. This process post-dated the time of regional granulite (charnoenderbite) and large regional scale retrogression and migmatisation.

Introduction

The study of the characteristic conversion of amphibolite-facies gneisses to charnockite along ductile shears at Kabbaldurga, southern India (Pichamuthu, 1960) and the work that followed Janardhan et al., 1979, Newton et al., 1980 opened up a lively discussion on the formation of charnockite and the processes causing the desiccation of the deep continental crust. These workers noted meandering streaks and diffuse patches of greasy green, coarsely crystalline charnockite arrested in its growth. They argued that the destabilisation of amphibole and/or biotite in the gneiss assemblage was due to infiltration of fluids rich in CO2 along localised networks, originated from deep-seated sources. This scenario of a mantle-derived CO2-fluid wave propagating into the deep crust was strongly favoured Newton et al., 1980, Janardhan et al., 1982, Friend, 1981, Friend, 1983 in the absence of an immediate alternative model. The Kabbaldurga outcrop since has been the subject of detailed studies, leading to alternative concepts for the formation of arrested charnockite Friend, 1981, Hansen et al., 1987, Stähle et al., 1987, Buhl, 1987, Friend and Nutman, 1991. Further research far south in the Kerala khondalite belt (KKB) and in the region around Palghat, which supposedly marks a terrain boundary between Archaean and Proterozoic crustal domains, has led to the discovery of widespread and most spectacular examples of arrested charnockite Ravindra Kumar et al., 1985, Srikantappa et al., 1985, Ravindra Kumar and Chacko, 1986, Raith and Srikantappa, 1993, Ravindra Kumar and Venkatesh Raghavan, 1992, Ravindra Kumar and Srikantappa, 1995.

The formation of arrested charnockite by conversion of amphibolite-facies gneiss heralds major changes in mineralogy, notably the development of orthopyroxene at the expense of biotite and/or amphibole. This mineralogical change (occurring under fluid-present conditions along structurally controlled fluid pathways), also implies element mobility and element exchange for the mineral reactions to occur. In the past, comprehensive studies by Stähle et al. (1987) on Kabbaldurga (hornblende-biotite gneiss to charnockite), Raith and Srikantappa (1993) on Kottavattam (garnet biotite gneiss to charnockite), and Burton and O'Nions (1990), Milisenda et al. (1991) on Kurunegala in Sri Lanka (hornblende-biotite gneiss to charnockite) have provided insights into the textural, mineralogical and chemical changes accompanying arrested charnockite formation.

Palghat region provides an ideal situation to study the factors controlling the dehydration processes in the context of local arrested versus regional pervasive regional-charnockite development, as there is close spatial association of arrested charnockite and regional granulite (charnoenderbite). An important feature here is that arrested charnockite has developed in a gneissic terrain that was subjected to large-scale retrogression following regional granulite facies metamorphism. This initiates considerable debate and provides ample scope to study whether arrested charnockite developed in bleached gneisses (retrogressed charnoenderbites) or in compositionally distinct gneissic units which did not undergo granulite facies metamorphism. Further, three different types of arrested charnockite formation do occur which may be related to different processes of dehydration Ravindra Kumar and Venkatesh Raghavan, 1992, Ravindra Kumar and Srikantappa, 1995. Charnockite domains within the hornblende-biotite gneiss developed (1) along ductile shears cross-cutting the gneiss fabric, (2) parallel to the foliation and, (3) in boudin-developed necks in somewhat more melanocratic layers.

In this contribution, the results of a detailed petrological study on sequential samples across gneiss to charnockite transitions, aimed to characterise the processes responsible for the formation of the three types of arrested charnockite, are presented. Similar studies elsewhere have helped in understanding the small-scale compositional changes taking place from the host gneiss through transitional zones into the charnockite domain Stähle et al., 1987, Hansen et al., 1987, Milisenda et al., 1991, Raith and Srikantappa, 1993. Since structurally different types of charnockitization are studied in this work, the extent of elemental variability linked to different controlling factors, such as structurally distinct conduits like shears, foliation boudin necks can be better assessed. Such comparative study, therefore, would add further to the earlier efforts in understanding the process of arrested charnockite formation and in providing further insights into the evolution of the granulite terrain of southern India.

Section snippets

Arrested charnockite in southern India: a review

Previous studies of arrested charnockite phenomena have documented transformation of gneiss to charnockite along structurally controlled zones such as fractures, shears or foliation with passage of dehydrating carbonic fluids. These examples are from the southern parts of the Dharwar craton (Kabbaldurga: Janardhan et al., 1979, Janardhan et al., 1982, Friend, 1981, Friend, 1983, Stähle et al., 1987), the Kerala Khondalite Belt Srikantappa et al., 1985, Ravindra Kumar and Chacko, 1986, Raith and

Study area and regional geology

The study area is located in the Palghat region which is considered as the western part of a major shear zone, referred to as Palghat–Cauvery shear zone Drury et al., 1984, D'Cruz et al., 2000, Meißner et al., 2002. It forms the northern part of the extended Southern Granulite Terrain (SGT) which is considered to represent late-Archaean crust that has experienced a metamorphic overprint of Pan-African age (Harris et al., 1994). The study area is bounded on the northern side by the hill ranges

Phenomena of arrested charnockite formation in the Palghat area

Arrested-type charnockite occurs over the entire study area of the Palghat gap Ravindra Kumar and Venkatesh Raghavan, 1992, Ravindra Kumar and Srikantappa, 1995 exclusively in migmatitic hornblende-biotite gneisses of granitic composition. Bodies of enderbite to charnoenderbite, mafic granulite and amphibolite may be present in the same outcrops. The enderbite/charnoenderbite exhibits a pronounced migmatitic structure which pre-dates the structures of the second deformation. The general

Sample preparation

Three large rock slabs representing the three distinct types of arrested charnockite development and showing the gneiss to charnockite transition were taken from active faces of the Nemmara quarry.

Sample 37 represents type A. The charnockite domain develops discordant to the distinct gneissic fabric with a gradual transition from gneiss to charnockite (Fig. 6). The gneissic fabric dies out in the marginal zone of the charnockite. The sample has been cut into six slices, # 37-1 representing the

Petrography of host gneiss and arrested charnockite

The hornblende-biotite gneiss and arrested charnockite are characterised by granoblastic equigranular to inequigranular interlobate textures. The distinct planar fabric in the gneiss, defined by the alignment of biotite and hornblende, gradually disappears into the charnockite. This is noted in two ways, firstly, by increasing replacement of biotite and hornblende by orthopyroxene and secondly, by grain size coarsening through recrystallization accompanying the dehydration reaction. This change

Mineral chemistry

Orthopyroxene in arrested charnockite domains is strongly altered and commonly replaced by hydrous phases like biotite and chlorite. Only two grains of orthopyroxene in sample 37-5 could be analysed (Table 1). XFe is 0.46 with a composition of Wo1En51Fs48. Orthopyroxene in 183 II do not provide stoichiometrically good analysis.

Amphiboles show a narrow compositional variation within the transition profiles. Fe/(Fe+Mg) ratios vary from 0.47 to 0.49. TiO2 varies moderately from 2.05 to 3.15 (wt.%)

Chemical characteristics of gneiss, charnoenderbite and arrested charnockite at Nemmara

The majority of gneiss/arrested charnockite pairs are granitic to granodioritic while the charnoenderbite and their retrogressive counterparts (grey banded gneisses) have tonalitic to granodioritic compositions (Fig. 12). Gneiss/arrested charnockite pairs show SiO2 ranging from 65 to 72 wt.% and Na2O/K2O ratios <1, whereas charnoenderbites have SiO2< 65 wt.%, and Na2O/K2O ratios >1, typical of I-type granitoids. Al2O3, Fe2O3 CaO, MgO, Na2O are higher than in gneiss-arrested charnockite pairs.

Fluid inclusions

A recent fluid inclusion study has documented several generations of fluid inclusions from pure CO2 to CO2–H2O and CO2–H2O–NaCl in gneiss, charnoenderbite and arrested charnockite, trapped in quartz (Sukumaran et al., submitted for publication). The authors opine that these fluid inclusions were trapped and evolved at different stages of metamorphism and uplift. Important observations are that the hornblende biotite gneiss contains abundant biphase CO2–H2O inclusions, whereas monophase CO2

PT conditions

The evaluation of PT conditions for formation of charnockite is limited by the absence of garnet in the assemblages. However, some of the mafic granulite occurring in the north and north western part are garnetiferous and provided PT estimate of regional metamorphism (Ravindra Kumar and Chacko, 1994). The thermobarometric data suggest near-peak conditions of 9–10 kbar and 780–880 °C. The equilibration conditions of the gneiss–charnoenderbite assemblages in the Nemmara quarry is estimated based

Arrested charnockite formation in relation to the tectono-metamorphic history

The south Indian granulite terrain documents excellent examples of gneiss to charnockite transformation triggered by changing PT–fluid conditions along structurally controlled zones of fluid infiltration. In all the studied occurrences the dehydration process took place after the completion of regional metamorphism and anatexis when, with the near-isothermal uplift of terrain (Ravindra Kumar and Chacko, 1994), the rheology of the gneisses changed from ductile to semi-brittle.

Among the many

Acknowledgements

A large part of this work was carried out at the University of Bonn while the author was a Visiting Fellow of DAAD. The guidance of Prof. M. Raith and stimulating hour-long discussions with him and his detailed comments on the manuscript have vastly benefited the author.

The fieldwork and laboratory work at CESS were supported by the Department of Science and Technology project to the author (ES/CA/A9-35/94 and ES/16/110/98). The manuscript was finalised and completed during DST-DAAD PPP

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