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Patchy Charnockites from Jenapore, Eastern Ghats Granulite Belt, India: Structural and Petrochemical Evidences Attesting to Their Relict Nature

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Patchy Charnockites from Jenapore, Eastern Ghats Granulite Belt, India: Structural and Petrochemical Evidences Attesting to Their Relict Nature Patchy charnockites from Jenapore, Eastern Ghats granulite belt, India: Structural and petrochemical evidences attesting to their relict nature Rajib Kar∗ Geological Studies Unit, Indian Statistical Institute, 203 B. T. Road, Calcutta 700 035, India. e-mail: rajib−[email protected] ∗Present address: Department of Geology, J. K. College, Purulia 723 101, West Bengal, India. The charnockite patches that occur within leptynite host, in and around Jenapore, northern sec- tor of the Eastern Ghats granulite belt, are disposed in a linear fashion and generally have sharp lithological contact with the host leptynite. Sometimes the patches and foliations of the host are cofolded. Also, the patches sometimes have the internal S1 foliation, while the host leptynite records only S2 foliation. Mineralogically and chemically patchy charnockites and host leptynites are distinct entities, and cannot be related by any prograde and retrograde reactions. Particu- larly important is the peraluminous granitic composition and high Rb/Sr ratios of the leptynites, presumably resulting from biotite-dehydration melting; as against metaluminous granodioritic to tonalitic composition and low Rb/Sr ratios of the patchy charnockites, presumably resulting from hornblende-dehydration melting. The charnockite patches here can be interpreted as caught up patches or xenolith within granitic melt (leptynite). Mg-rich rims of garnet in the charnockite patch were probably caused by heat from the crystallising melt or decompression during ascent of melt. 1. Introduction mation or ‘charnockitisation’ (Janardhan et al 1979; Hansen et al 1987a, b; Stahle 1987; New- Charnockitic rocks are important constituents of ton 1992b). Some workers even interpreted the high grade terrains, and could have originated by larger charnockite bodies as products of ‘pro- both igneous and metamorphic processes (New- gressive charnockitisation’ (Raith et al 1990; ton 1992a). From its type area, near Pallavaram, Srikantappa et al 1992; Janardhan et al 1994; Madras (presently Chennai), Holland (1893, 1900) Harley and Santosh 1995; Janardhan and Francis first suggested a plutonic igneous origin of the 1996). charnockite. Later on, Howie (1955) elaborated this These diverse petrogenetic interpretations or idea and proposed the concept of plutonic meta- models for the origin of charnockitic rocks morphism, according to which charnockitic magma are based primarily on petrological observa- was emplaced at lower crustal depths. Pichamuthu tions; the structural setting of these rocks, (1960) first described ‘patchy charnockites’ within are, in many cases, not well documented. In host amphibolite facies gneisses from Kabbal- this context it is important to note that from durga, Karnataka, south India, and interpreted some south Indian localities Naha (1988) and them as arrested growth from the host. Subse- Naha et al (1993) described charnockitic rocks quently, many workers suggested that the ‘patchy that occur in a variety of different struc- charnockites’ from several places in south India tural setting and differ in style and time rela- are products of in situ metamorphic transfor- tion. Keywords. Patchy charnockite; leptynite; distinct entities; relict. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 110, No. 4, December 2001, pp. 337–350 © Printed in India. 337 338 Rajib Kar Also, in the Eastern Ghats granulite belt, where also use the term charnockite in a similar India, the charnockitic rocks occur in different manner (De Waard 1969; Ridley 1992; Kilpatrick modes, namely, as larger massif-type bodies and and Ellis 1992). It is also important to note that the as patches in the host leptynitic gneisses (Bhat- natural occurrence of charnockitic rocks includes tacharya et al 1993a, 1994; Rao et al 1995; Sub- intimately associated varieties that may be termed baRao et al 1995; Kar 1995). Several workers charnockite, charno-enderbite and enderbite. have assumed an in situ transformation origin for the patchy charnockites (Aftalion et al 1988; Paul et al 1990), but Bhattacharya et al (1993a, 2. Geological setting 1994) described the relict nature of the patchy charnockites from the Chilka Lake area. Recently, Besides the presence of the massif-type charnock- Dobmeier and Raith (2000) presented an alter- ite suite of rocks in the study area, metapelitic native interpretation—“arrested growth” for the granulites and orthopyroxene-free granites are also patchy charnockites of the Chilka Lake area. This important lithologies. Metapelitic granulites are is one of the leading controversies, and for a proper primarily of two types, namely, khondalite and resolution of this issue, many more specific exam- quartzite. Quartzite occur as minor bands within ples should be investigated. The present commu- khondalite. On the other hand, orthopyroxene-free nication deals with a case of patchy charnockite granites are principally of three types, namely, lep- from the northern sector of the Eastern Ghats tynite, leucogranite and porphyritic granite-gneiss. belt. The definition of dominant lithologies is presented The charnockitic rocks here occur in two differ- in table 1. ent modes. One variety occurs as large massif-type Three phases of folding are recorded from bodies associated with minor bands and lenses of the study area (Kar 1995). Large massif-type enderbite, two-pyroxene granulite and hornblende- charnockite and khondalite are characterised by bearing mafic granulites. The other variety occurs the presence of streaky gneissic foliation and perva- as patches within leptynitic gneisses. It can be sive gneissosity respectively. Minor quartzite bands shown on the basis of field-structural and petro- in the khondalite host and hornblende-bearing logical data that, the massif-type charnockite is a mafic granulite bands in the charnockite host define product of hornblende-dehydration melting in the reclined, intrafolial F1 folds. The gneissic foliations protolith of hornblende-bearing mafic granulite and of the hosts are axial planar to these folds, and the associated two-pyroxene granulite and ender- hence designated as S1 fabric. F2 folds, fairly tight, bite are cumulates of solid peritectic phases, equi- commonly developed on these S1 fabrics and have librating with the charnockitic melt (Kar et al axial planar leptynitic foliation in both the khon- 2000). dalite and massif-type charnockite. Hence the lep- Any scientific discussion of charnockitic rocks tynitic foliation is designated as S2 fabric. F2 folds should take into account its nomenclature. Hol- commonly occur on a mappable scale. Except the land (1900) first defined the charnockite as mesoscopic F2 fold hinges, S1 fabric is parallel to S2 orthopyroxene-bearing granite, found abundantly fabric on a regional scale. The composite S1kS2 fab- in south India, with the conviction that they are ric defines broad F3 warps. This three-phase fold- of plutonic igneous origin. Later on, Streckeisen ing history is correlatable to those described from (1976) also subdivided the orthopyroxene bear- the other areas in the Eastern Ghats (Halden et al ing quartzo-feldspathic rocks into charnockite, far- 1982; Bhattacharya et al 1994; Shaw 1996). sundite, charno-enderbite and enderbite, with the Orthopyroxene-free granites generally occur as assumption that they are plutonic rocks. On the minor stock-like bodies at the core of the mapable other hand, in his review paper Newton (1992a) F2 folds in the charnockite and khondalite host (fig- noted that many of the south Indian charnockitic ure 1, insets A & B). The foliations present in them, rocks are poor in K-feldspar, corresponding to gra- i.e., leptynitic foliation in leptynite and leucogran- nodiorite or quartz monzonite, rather than gran- ite passes undeviated into adjacent charnockite ite, in mineralogy. He also pointed out that these and khondalite, and is axial planar to F2 folds, rocks are complexly foliated, interlayered with var- and hence this foliation is designated as S2. Sim- ious metasediments, and some contain garnet and ilarly, the gneissic foliation in porphyritic gran- even graphite, and are definitely metamorphic in ite is parallel to the axial plane of the meso- origin. For these reasons, he suggested that it is scopic F2 fold in the host charnockite and/or better to adopt Pichamuthu’s (1969) most gen- khondalite, and hence it is also designated as eral and non-genetic definition of charnockite as S2. any quartzo-feldspathic rocks with orthopyroxene. On the other hand, dismembered charnockite Some Indian workers (Bhattacharya et al 1993a; patches are generally disposed in a linear fash- Sen and Bhattacharya 1993) and workers from else- ion within the leptynitic host (figure 2). Notably, Relict patchy charnockites from Eastern Ghats 339 Table 1. Definition of different rock types used in the text. Charnockite Quartzo-feldspathic rock with orthopyroxene as essential mineral phase; minor clinopyroxene and garnet also present. Khondalite Quartz-rich rock with alkali feldspar, sillimanite and garnet as essential mineral phases. Leptynite Quartzo-feldspathic rock with plattung structure; biotite and garnet are common ferromagnesian phase. Leucogranite Quartzo-feldspathic rock with plattung structure, with minor or no ferro- magnesian phases. Porphyritic granite Granite with common porphyritic texture; alkali feldspar megacrysts and biotite rich foliation swerves around it. Note: Plattung structure – Platy granoblastic texture, as defined by Mehr (1962) and Sen (1987). boundaries between them are very sharp. On a feldspar (table
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