The Kondapalli Layered Complex, Andhra Pradesh, India : A Synoptic Overview

Gondwanci Research, V l , No. I , pp. 95-114. 0 I997 International Association for Gondwana Researclz. Japan ISSN: 1342-937X

The Kondapalli Layered Complex, Andhra Pradesh, India : A Synoptic Overview C. Leelanandam Department of Geology, Osmania University, Hyderabad-500 007, India (Manuscript accepted June 25, 1997)

Abstract The Kondapalli Layered Complex (KLC) consists dominantly of gabbroic and anorthositic rocks, with subordinate ultramafic rocks (orthopyroxenites, websterites, clinopyroxenites, dunites and harzburgites) which contain chromitites (with orthopyroxene, clinopyroxene or amphibole). The KLC is a stratiform intrusion possibly similar to Bushveld, and its various components occur as sheets, bands or lenses in the enclosing sea of charnockites; small scale folded structures are not uncommon. Chilled margins, contact metamorphic zones, xenoliths and late differentiates have not been found for the KLC, which is cut by rare 85 If28 Ma (whole rock K-Ar age) metadolerite dykes containing intensely clouded plagioclase and a trace of garnet. Most rocks of the KLC display layered characteristics, and they essentially comprise plagioclase, orthopyroxene and clinopyroxene in different combinations, with variable proportions and diverse textural relationships. The rocks exhibit textures that formed both during the magmatic phase of crystallization and during high-temperature post-cumulus to subsolidus deformation and re-equilibration. ~ )orthopyroxenes ), (Enw5,), 13clinopyroxenes (Ca,,,,Mg,,,Fe,,J, I1 amphiboles (C~,,,Mg,,,,Fe,,). 13plagioclases Mineral analyses for 4 olivines ( F O ~ ~ . I9 (An,w,6; An,,) and I3 chromites [ lOOMg/(Mg+Fe?+)= 66-26; 100Cr/CR’+= 73-35], together with 45 whole rock analyses from the KLC are utilized in the present study. The remarkably wide variations in the host rock chemistries reflect the significant changes in modal mineralogies. Two parental magmas (a magnesian liquid and an alumina-rich tholeiitic liquid) are tentatively proposed for the KLC. Some quartz-bearing (anorthositic, enderbitic and other felsic) rocks with conspicuous deformational textures are interpreted as contaminated or mixed rocks occurring at the tectonized junction zones between the KLC and charnockites. The KLC exhibits several distinctive mineralogical features: pure anorthite (An,,) in zoned spinel-amphibole-orthopyroxenites; exsolution rods or blebs of nearly pure K-feldspar (Or& in the high calcic plagioclase ( A n d ; twinned plagioclase (An,?) exsolution lamellae in the orthopyroxene (En4,) of deformed “unusual” enderbites; coronal garnet in rare high Fe-gabbros; inter- and intra-granular compositional variation and different zoning patterns in chromite of the chromitites and ultramafics; and finally, dense networks of fine exsolution lamellae of Al- and Mg-rich chromite in Fe’+- and Fe’+-rich host chromite. The coexisting pyroxenes from the ultrabasic, gabbroic and anorthositic rocks of the KLC, as well as those from the enclosing charnockites, yield similar K, values and P (6-8 kbar) - T (830-950°C)estimates. It is suggested that the KLC has intruded dry country rocks at great depths (lower to middle crustal levels), probably in the period immediately following or coincident with the highest temperature metamorphism of the country rock charnockites. Extensive subsolidus re-equilibration of the KLC has taken place, with all the rocks (KLC and regional charnockites) retaining signatures of nearly identical physical conditions of formation characteristic of the granulite facies metamorphism.

Key words: Adcumulates, layering, two magmas, slow cooling, granulite facies.

Introduction The continued interest in layered intrusions (Irvine,1982) and origins of magmatic layering (Parsons, 1987), among earth scientists, has intensified in recent years as evidenced by the publication of special issues of Economic Geology (v.80, no.4,1985), Geological Magazine (v.122, no.5, 1985) and Journal of Petrology (v.30, no.2, 1989) on Bushveld, Rhum and Skaergaard, respectively. The more recent special issues of South African Journal of Geology (v.97, no.4, 1994) and Journal of African Earth Sciences (v.21, no.4,1995) manifest the current trends of research on layered complexes.Thepresent writer has made, during the last three decades, systematic studies on the Kondapalli Layered Complex (KLC) in the Krishna district of Andhra Pradesh (Fig. l), and the present communication presents a summary account of the field

relations, petrographic features and chemical data acquired on the various components of the KLC and on their constituent minerals. The occurrenceof high-Ca anorthosite (Leelanandam, 1965) in the Kondapalli charnockitic region (Leelanandam, 1961) was first reported by the present writer (Leelanandam,1967a) who has also made passing references to it, without any particular emphasis, in some later publications (Leelanandam, 1967b, 1969a, 1970). The KondapalliLayered Complex (KLC) in its entirety, was first recognised by Leelanandam (1972) who also high-lighted its spectacular layered characters of variable styles and sizes, and recognized that the chromitites (with the associated ultramafics) formed an integral component of the KLC. The KLC represents a discontinuous stratiformtype complex enveloped by a much greater volume of charnockite and granulite, and its disrupted fragments contain

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Fig. 1. Geological map of the Kondapalli hill ranges (simplified after Nanda and Natarajan, 1980,1982). The inset shows the locations of the Chimalpahad (C), Kondapalli (K) and Sittampundi (S) layered complexes in south India; the Cuddapah basin (ruled area) is outlined. Gondwana Research, V I , No. I , 1997

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different components in disproportionateamounts. These early findings formed an impetus for further investigations by other workers in subsequent years (Bose and Bose, 1977; Nanda and Natarajan, 1980,1982; Bose and Bose, 1982,1989). Although a considerable amount of chemical data were available in the literature on the ultramafic rocks and chromitites (and their constituent minerals) of the Kondapalli-Gangineni region, many workers did not recognize that these rock types constitute an integral part of the layered complex (see, for example, Mall and Rao,1970; Ma11,1973; Rao,1978,1980). However, Chakravarti and Mukherjee (1 971), and Sinha and Mall (1974) realised that the chromite-bearing ultrabasics (stratiform deposits) are (chemically) different from the “ultrabasic charnockites” and bear no genetic relation with charnockite series. The KLC is the only one of its type in thr:entire Eastern Ghats mobile belt of Peninsular India (Leelanandam and Narasimha Rcddy, 1988; Leelanandam, 1990).

Field Relations The KLC consists dominantly of gabbroic and anorthositic rocks, with subordinate ultramafic rocks (orthopyroxenites, websterites, clinopyroxenites, dunites and harzburgites) which contain chromitites (with orthopyroxene, clinopyroxene or amphibole but not with olivine). In any exposed part of the KLC, a particular group of rocks is prominently displayed usually to the near exclusion of the other groups. The KLC is variably deformed and is of only minor areal extent compared with the enclosing mass of charnockites and granulites (see Fig. I). The different components of the KLC occur as isolated sheets, bands or lenses of various dimensions. The rugged Kondapalli hill ranges, according to Nanda and Natarajan (1980), may represent the denuded remnants of a

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major southerly plunging synformal structure, and the intrusions are emplaced in the N-S trending prominent shear zones located along the axial regions of the dominant antifomal F, folds in the charnockitic rocks (Fig. 1). Features characteristic of post-emplacement deformation of the KLC, and the overprints of terminal phases of granulite facies metamorphism, are recorded by Nanda and Natarajan (1980, 1982). Rao (1978) infers that the “chromite orthopyroxene nodules” are confined to the cores of southeasterly plunging overturned isoclinal anticlines. These structural complexities introduce an element of ambiguity in establishing the magmatic stratigraphy of the KLC. The contacts of the components of the KLC with the enclosing granulites are generally concealed, but are assumed to be tectonic in nature and modified by some shearing and also probably by hybridization. Along these contact (marginal) zones of the complex with the enclosing granulites, occur the acidic members of trondhjemitic, tonalitic or even “enderbitic” affinity along with quartzo-feldspathic veins. They may represent a contaminated and “mixed” group of rocks but are not interpreted as late differentiates (cc Bose and Bose, 1989). Chilled margins and contact metamorphic effects of the KLC are not preserved. The KLC is cut by rare 851+28 Ma (wholerock K/Ar age; unpublished work of the writer) metadolerite dykes. Although most rocks of the KLC exhibit vestiges of layering, it is only the anorthositic and gabbroic group of rocks that occasionally display well developed layering and banding on different scales and styles (Fig.2a-h). Small-scale slump and folded (Fig.2g) structures, imperfect low-angle local unconformities, cross lamination, flow folds (due to synconsolidation deformation), and modally graded layers (Fig.2e) are extremely rare in the KLC. Chromitites commonly exhibit

Figs.2a-h. Field and hand specimen photographs showing different types of layering in the JUC.

a.

Alternating niafic and felsic isomodal (uniform) layers with rather sharp contacts. Note the slight deformation of layering (to the right of lens cap) and tapering of some thin mafic layers (to the left of lens cap).

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Relatively thick anorthositic layers alternating with mafic layers which taper out towards left. Local (slight) cross lamination can be seen in the top left portion. Scale in cms.

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Opposing cut surfaces ol'a hand specimen exhibiting fine scale rhythmic phase layering.

g.

Folded structuie showing the thinned limbs and thickened hinges due to buckling and subsequent flattening.

h.

Quartz and feldspathic veins in chroniitite. Note the phlogopite-rich layer at the top portion of the specimen, and indistinctly visible minor alteration along the contacts. Scale in cnis.

Extrernely thick laycrcd lithologics with inconstancy of layers across the outcrop. Note the stt-eakcd out mafic layers in thc anorthosite zone (central poilion) giving a schlieren structure, and anorthositic laminae (of non-uniforni thickness) in the mafic zone (left portion). Scale in inches. Same outcrop as shown in photo 5 of Nanda and Natarajan (1980).

d.

e.

Mafic and felsic layers of equal thickness, a rare feature, with rather diffuse contacts. Scale i n cms.

Close-up view of a mafic layer showing mineral grading which is truncated by a local shear plane (towards right).

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Figs. 3a-I. Colour photomicrographs showing textural features of rocks from the KLC. The length of the photos is given within [ ]

a.

Nearly idiomorphic olivines (Fog?)in orthopyroxene (En,,)-bearing dunite adcumulate (R44). Note the planar grain boundaries and deformation lamellae, XPL, [3.2mm].

b.

Tectonized dunite ( M 2 ) showing networks of serpentinized microfractures and irregular crystals of intergrain chromite, XPL, [3.2 mm].

c.

Colour photomicrograph of orthopyroxene (En,,) in a vein in olivine (Fo,),) adcumulate dunite (95). Note larger chromite grains in fractured areas outside, and very much smaller octahedra within, the orthopyroxene with two sets of cleavage, XPL, [2.8 mm].

d.

Perfect equilibrium granular texture with 120" triple junctions in orthopyroxenite adcumulate (R 135). Note the minor tine-grained amphibole, XPL, [4.3mm].

e.

Cumulus orthopyroxene (En,,) CVstals with m m ~ t and h straight grain boundaries and intergrain chromite in orthopyroxenite (48). XPL, [ I .8 mm].

f.

Orthopyroxene crystals showing typical schiller texture with clear margins in orthopyroxenite ( R 103). Note the smooth curved grain boundaries and intergranular Fe-Ti oxides, PPL, (3.2mml.

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g.

Coarse orthopyroxene (En,,) crystals with rather irregular &rain boundaries, kink bands and exsolution lainellae in orthopyroxenite (323). The intercumulus clinopyroxene (Ca,,Mg,,Fe,) with exsolution lamellae and twinned plagioclase are noticeable, XPL, [4.3 mm].

h.

Clinopyroxenite (R85)with intercumulus plagioclase, opaques and amphibole rimming clinopyroxene: note the incipicnt granulation, XPL, [4.3 mm].

i.

Mottled area of mafic gabbro (R147) consisting of clinopyroxene and amphibole. Note the difference in grain size of plagioclase within and outside the blotchy area, XPL, [4.3 mm]; see also Fig. 4a.

j.

Meta-gabbroic dyke (R149) showing the subhedral (or anhcdral) and inequigranular clinopyroxene, amphibole and plagioclase Note the twinning in clinopyroxene and zoning in plagioclase, XPL, 14.3 mm]; see also Fig. 4b.

k.

High-Fe gabbro (H17) showing coronal garnet around plagioclase (An,5.65)in contact with clinopyroxene (Ca,,Mg,,Fe,,) with lamelhe, amphibole and orthopyroxene (En,,) with intervening quartz, XPL, [1.8 mm].

1.

Gabhro (R 47) showing plagioclase and orthopyroxene; note the larger grain size of plagioclase than pyroxene, XPL, [4.3 mm].

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mincral layering and banding along with the silicate. Chroniitites, as a rule, are not associated with (and not found in) anorthositic rocks. The occurrence of mafic pegmatites (and also phlogopite) associated with the KLC chromitites containing sulphides suggests the possibility of the presence of the platinum group elements (PGE) (see Mathez,1989; Naldrett ct al., 1990; Roberts et al., 1990). Quartz veins and quartzo-feldspathic veins [not acid charnockites of Mall (1973)l are sporadically, and rather unusually, present in chromitites (Fig.2h) and also in the enclosing ultramafic rocks. Both chromitites and ultramafic rocks display effects of intense tectonization. The chromitebearing ultramafic rocks (Chakravarti and Mukherjee, 1971) have becn initially subjected to lzydrotheriiinl alteration producing serpentine and talc rocks and subsequently to widespread potash rrzetusonzntism resulting in the development of perthite and biotite.

Petrography Most rocks of thc KLC comprise plagioclase, orthopyroxene and clinopyroxene in different combinations, with varying proportions and textural relationships; see Figs. 3a-31, and Figs. 4-6. Dunites and harzburgites are the rarest rock types and are totally devoid of clinopyroxene and plagioclase; occasionally they arc traversed by orthopyroxenite veins. Olivine grains in somc dunites are cut by serpentinized microfractures; olivine

Fig. 4b. Zoned and twinned intercumulus plagioclase (Ans,~,J i n mediumgrained hornblende meta-gabbroic dyke (R 149). Note the exsolution larnellae in pyroxene (upper portion), XPL. See also Fig.3j.

Fig. 4a. A cluster of fcrrornagnesian minerals showing the texture in hornblende-bearing mottled rnafic gabbro (R147). Note the shapes of interstitial plagioclase (An,,.,,,) grains, PPL. See also Fig.3.

rarely shows deformation lamellae or banded undulatory extinction (Figs. 3a-c). Among the ultramafic group, orthopyroxenites (Figs. 3d-g) dominate over websterites and clinopyroxenites (Fig. 3h); minor amount of intercumulus plagioclase, sometimes weakly zoned, is not uncommon and hornblende is more than a subordinate mineral in somc members including mafic- and meta-gabbros (see Fig. 3i and 3j; Fig. 4a and 4b). Some rare green spinel-hornblendeorthopyroxenites contain extremely calcic plagioclase (An,,. ,oo)and a trace of garnet. In some ultramafic rocks, pyroxenes exhibit severe cataclastic effects with undulatory extinction, bent cleavage, bent lamellae and “kinktxinds” (see Fig. 3g). In the anorthositic rocks, plagioclase is the cumulus phase (Figs. 5 and 6), and ortho- and clino-pyroxenes are intercumulus-interstitial phases, often showing optical continuity between one interstice and the next. The overall shape of the interstitial matrix mimics that of the pre-exiiting melt and has taken on the melt shape to give rise to “melt pseudomorph” (Harte et al., 1993). Hornblende is not rare, and may be locally abundant whereas biotite is extremely scarce. Garnet is virtually absent, though its presence is detected in some hornblende high Fe-gabbros (as coronas; Fig.3k) and in feldspathic ultramafic rocks containing hornblende. Cr-spinels occur as intercumulus between (and also as inclusions within) the cumulus olivine (in dunites) and pyroxenes (in pyroxenites). In chromitites, chromite is not only enclosed by, but also encloses, pyroxene and amphibole.

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Fig. S. A very low niagnification photomicrograph ofthe entii-c t h i n section of thc anoilhosite (R SO), showing the overall equilibrium granular texture due to thermal annealing, XPL. The ~~iutual interference boundaries and granuloblastic (polygonal-granoblastic)shapes i n the plagioclase (An,,) adcuniulate can bc clearly seen. Thc insct shows a inagniticd view ofthe rclationship between thc plagioclasc grains (with smooth grain boundaries and different sets of twin laniellnc) from a siiiall poition close to thc ccntrc of main photomicrograph, XPL.

Small euhcdra of chroniitc (often forming clusters or chains by coalescence) arc poikilitically enclosed by hornblende (Fig.7), and oval-shaped hornblende and orthopyroxene “inclusions” (Fig.8a) occur i n chromite. Regions of monomineralic chromite, seen in reflected light, are composed of polygonal grains with rather smooth boundaries and triple junctions (Fig.%), and hence the appearance o f a silicate phase as an “inclusion” (whcn observed under transmitted light i n 2D) may be deceptive. Plagioclase is extremely rare and is poikilitic to euhedral chromite in hornblende chromitites. The

range of chromite textures suggests that it crystallized over an extended time period and possibly over an extended range of temperatures. Some Fe-rich chromites (host) in the KLC contain dense networks of fine lamellae (Figs. 8b and 8c) of AI-, Mg- and Cr-rich chromitc. Cumulate textures (Figs. 3-6) are well prcscrved throughout the KLC, and adcumulus growth was the dominant form of post-cumulus enlargement. Most plagioclases do not show any significant optical zoning, and no attempt has been made to distinguish primocrysts from post-cumulus overgrowth; for Gorldwma Reseorcli, V I , N o

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Fig. 8a. Net-textured chrornite around oval-shaped orthopyroxene (En,,) grains (R 167), PPL. See also Figs. 8b and 8c.

F I ~6a. . Cumulus plagioclase (An,) in anorthosite (61) with straight or slightly curved grain boundaries with I 2 0 triplejunctions. Significantly some srnallcr plagioclase grains (not seen in the photomicrograph) exhibit tapering of twin lainellac and weak zoning. Note the typical shapes (triangular grains) of the intercumulus pyroxenes, XPL. 6b. Deformed anorthosite (R 162) in which the large original plagioclase crystals (porphyroclasts) show rccrystallization along their grain boundaries to form chains of neoblasts. The porphyroclasts exhibit hcnding of thcir twin larnellae, XPL.

Fig. 7.

Poikilitic amphibole (oikocryst) in chromitite (R178) enclosing a large nuinher of smaller euhedral cumulus chrornite crystals (of different dimensions) which show coalescence, PPL.

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Fig. 8b. A high magnification photomicrograph (under reflected light) showing annealed texture with well developed 120" triple junctions between the chromite grains in chrornitite (R 167). The patchy development of exsolution lamellae in chrornite can be clearly seen.

Fig. 8c. An extremely high magnification photomicrograph (under reflected light) showing different sets of complex lamellae of Al- and Mg-rich chromite in Fe-rich chromite in chromitite (R 167).

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a reconsideration of the cumulate paradigm, see McBirney and Hunter (1995). The monomineralic cumulates of the KLC show excellent solid state textures with regular triple junctions and 120”grain boundaries (Figs. 3d-30, typically indicative of hightemperature annealing [see Vernon (1970); Raedeke and McCalluni (1984); Frost et al., 1993)l. With slow cooling and grain boundary adjustment, the cumulus mineral grains show granuloblastic (polygonal-granoblastic) shapes (Harte et al., 1993). Typical magmatic textures of adcumulates are indeed indistinguishable from those obtained due to (static) recrystallization (Ashwal, 1993, p. 115) under P-T conditions of the (dry) granulite facies metamorphism. Symplectitic and coronitic textures [such as those described by Ballhaus and Berry (1991) for the Giles Layered Igneous Complex] are typically absent in the KLC. Most of the obscrvcd textures in the KLC were formed d u r i n g h i g h - t e ni pera t u re post - c um u 1 u s to sub so 1 i d u s (deformation and) recrystallization. Variable deformational and metamorphic textures are of restricted nature and of localized occurrence. Based on the ideas of George (1978), and Himmelberg and Loney (1980), preservation of igneous textures i n the deformed sequences may be attributed to syntectonic magmatic sedimentation and crystal-mush flow where thc post-cumulus material was largely liquid during deformation and accommodated much of the strain. In some unusual quartz-bearing anorthositic rocks, plagioclase shows intense defonizatiorzal effects and severe zoning (An,, ,J; furthermore, the high-Ca plagioclasc (An is strongly antiperthitic (Figs.9a and 9b) with blebs or rods of nearly pure K-feldspar (Orgl,). Some of the deformed rocks which contain a greater amount of modal quartz and lesser amount of (antiperthitic)plagioclase (An,,, ,) are grouped as “unusual enderbites” (Leelanandam, 1969b). They contain rare garnet and minor orthopyroxene (En,? ,) with exsolved lamellae of plagioclase (Figs. 10a and

lob) which shows distinct lamellar twinning (Leelanandam, 1967b). The plagioclase lamellae are no more calcic than the tl iscrete coarse-grained plagioclase. The acidic members, probably corresponding to rocks of trondhjemitic or tonalitic affinity (see Bose and Bose, 1989), contain perthitic K-feldspar, quartz, plagioclase, orthopyroxene, (? clinopyroxene), myrmekite, biotite and garnet. They exhibit severe cataclastic and deformational textures. The rare metadolerite dykes which cut the KLC contain intensely clouded plagioclase (An,,), and an insignificant amount of garnet. The plagioclase in general is so heavily clouded (due to the presence of innumerable particles of Fe-Ti oxide minerals) to such an unusual extent that it looks nearly black (except along the thin margins) under the microscope. Bent twin lamellae and wavy extinction in curved plagioclase laths are common; zoning (An44-6,)is not uncommon. Clinopyroxene (with clouding and zoning), subordinate orthopyroxene and relict olivine are present; secondary hornblende and biotite are ubiquitous. Subophitic texture is more prevalent than porphyritic texture in these meta-dolerites.

Mineral Chemistry The electron microprobe (EMP) analyses of minerals (4 olivines, 13 orthopyroxenes, 10 clinopyroxenes, 7 amphiboles, 2 micas, 9 plagioclase feldspars, 13 chromites and 1 green spinel) from the KLC were obtained by the writer at the Geochemisches Institut, Gottingen (formerly of FRG) during the period 1972-74, at the Department of Earth Sciences, Cambridge (U.K.) in 1988, and at the Department of Geology and Geophysics, Edinburgh (U.K.) in 1988 and 199 I , following the standard procedures which were i n vogue i n those laboratories. A few mineral analyses (6 orthopyroxenes, 4 Fig. 9a. Photomicrograph of the quartz-bearing anoithosite (55). Note the wavy extinction and zoning (AnG,.,,?)in some antiperthitic plagioclase grains, while a few others are twinned and devoid of zoning and of exsolution blebs, XPL. Fig. 9b. A magnified view of the top left portion of Fig. 9a. Zoned and untwinned antiperthite showing exsolved orthoclase (under extinction; 0r8,,.
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Fig. I0a.Unusual cnderbite (B4) showing thc inequigranulnr texture and cffects of crushing. Brush extinction and bent lamcllae in plagioclase (An,:) can be clearly seen, XPL.

Fig. I Ob. A high magnification photoniicrograph showing the exsolved lamcllac of plagioclase (with twinning; An,,) in the orthopyroxene (En JJhost, unusual cnderbite (B4), XPL.

clinopyroxenes, 4 amphiboles, 2 micas and 9 plagioclase feldspars) earlier obtained by the classical chemical (wet) inethods (Lcclanandam, 1965) were also utilized in the present study. Chemical compositions and structural formulae of only sonic representative analyses, which display the enormous diversity and great spectrum of overall chemical variation, are presented in Tables 1-4. In general, there is a compositional break among the constituent minerals between the chromitites and ultramafic rocks, and a far more significant break between the ultramafic rocks and gabbroic-anorthositic rocks.

anorthositic groups (En,? The compositional range foi clinopyroxene is far more restricted in the corresponding rock groups. The major chemical variations of both ortho- and clinopyroxenes are best illustrated in the conventional pyroxene quadrilateral (Fig. I I). The overall compositional range is Ca,,3 I p g p 33 5 0 P e , 47 for OrthoPYroxene,and Ca,, 3 $, ,Mg,?, 76,,Fe3,, ' ) 5 for clinopyroxene. The [Mg] values [=100xMg/ (Mg+Fe+Mn)] range from 94.2-50.8 for orthopyroxene, and from 93.3-64.4 for clinopyroxene (see Table I ) . Most of the samples described as UBC (ultrabasic charnockites) in an earlier study (Leelanandani, 1967b), may belong to the ultramafic group ofthe KLC. The orthopyroxenes of this group contain high AI,O, contents (upto 5.40%); the orthopyroxene from the green spinel-bearing pyroxenitc 287 (Table 1) contains even a higher amount of A1,0, (5.82%). Similarly the clinopyroxenes also contain high A1,0, upto ~ 5 % the ; clinopyroxene always contains more AI,O, than the coexisting orthopyroxene. The K,, [= (FeO'/MgO)opx/(FeO1/MgO)L,,] values for 14 coexisting pyroxene pairs from the ultramafic, gabbroanorthositic and charnockitic rocks a r e virtually indistinguishable, and 10 out of 14 values are confined to an

Olivirie: It is highly magncsian and has a rather restricted composition ( F O , ~ . ~ As ~ ) .olivines arc rarely more magnesian than Fo,,, in both stratiforni and Alaskan-type complexes (Jan and Windlcy, 1990), the KLC olivines arc exceptionally magncsian. The crystallization of KLC olivine, as in other layered complexes (see also Irvine, 1970, 13.443, was almost invariably accoinpanied by precipitation of small amounts of chromitc, which shows a considerable compositional range. Pyroxerie: The composition of orthopyroxene in chromitites (Enc14-8s) and ultramafic rocks (En,,.,?) is quite variable depending on the rock type and its habit (Table I ) . However, its composition is very much restricted in the gabbroic and

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Fig. I 1. Ca-Mg-Fe’ compositions of clinopyroxenes, amphiboles and orthopyroxenes. The variations in Cr* ( 100Cr/R3+)of chromites, Fo of olivines and An of plagioclases are separately shown. Chromitites (solid squares), ultramafic rocks (open squares), gabbroic rocks (triangles), anorthositic rocks (circles) and contaminated rocks (plus signs) are identified.

extremely narrow range of 1.6-1.9, indicating that all attained chemical equilibrium at about the same temperature. The pyroxene pairs from all the five anorthositic rocks with magmatic textures yield K,, values of 1.75-1.93, characteristic of the granulite facies metamorphism.

gabbro-anorthositic rocks (Na,O = 1.0-1.5%, 6 0 < 1%). The total alkali contents in the amphiboles from Kondapalli basic charnockites are relatively very high (Na,O = 1.4-1.7%, 6 0 = 1.9-2.6%) and K,O > Na,O (see Leelanandam, 1970) unlike those from the KLC.

Amphibole: It exhibits considerable variations in SiO, and A1,0,, FeO and MgO, and Na,O and K,O contents (Table 2). The extremely low amount of A1 ,03(5.55%)in the amphibole from the chromitite R207 is striking. The amphibole in a similar sample from Gangineni (R178), having a similarAl,O, content (6.37%), coexists with another variety of amphibole containing 14.21% A1,0, (and lesser amounts of SO,, MgO and CaO than the former). These features are very much akin in their complexity to those of the metamorphosed Messina complex (southern Africa), (Hor et al., 1975, p.306). Three amphiboles from green spinel-bearing orthopyroxenites (J22,59 and 287) contain very high A1,03 (14.66-16.05%), Na,O (2.12-2.57%), and TiO, (1.11 -1.54%), and very. low K,O (0.37-0.68%) contents. The [Mg] values of the KLC amphiboles range from 92.9-52.2, and the overall compositional range is

Plugiocluse: The calcic nature of plagioclase is the hall-mark of the KLC (Leelanandam, 1967a, 1972). Although the plagioclase is highly calcic in many ultramafic rocks (Table 3A), it is sodic (An,,) in some clinopyroxenites (see Fig. 11). The intercumulus plagioclase in hornblende-(zoned) green spinel-bearing orthopyroxenites (J22,59 and 287) is in general unusually calcic (An,,,), and in some, it is a pure Ca endmember of the plagioclase series (An,,); see Table 3A. The plagioclase is monotonously uniform in composition in most of the gabbroic and anorthositic rocks (Table 3B); see also Fig.11. Eight out of the eleven analyzed plagioclases from these two groups of rocks have a narrow range of composition (An7,*,), only slightly different from the average composition of plagioclase (An78+,)from the Stillwater anorthosites (Czamanske and Bohlen, 1990). The KLC plagioclases are chemically comparable to those from the nearby metamorphosed layered anorthosite complex of Chimalpahad in the Khammam district of Andhra Pradesh (Leelanandam and Narsimha Reddy, 1983, 1985).

Ca27.4-,0.3Mg,7.S-,7.~Fe~.0-~4.2’

Na,O and K,O contents are distinctly different in the amphiboles from chromitites (Na,O < 1%, K,O = 0.15%), ultramafic rocks (Na,O = 2.0-2.5%, K,O = 0.4-1.6%), and

Gondwana Research, Y l , No. I , 1997

I 07

KONDAPALLI LAYERED COMPLEX, INDIA

Table I Chemical comoositions and structural formulae of ortho- and clino- wroxenes. M4

SiO, Ti02 Alp, Fe20, FeO MnO MgO CaO Na,O K,0

cv?, TOTAL Si All" 2

Al"' Ti Fe" Fe" Mn Mg Ca Na K Cr Xations [Mgl' Atomic % Ca Mg Fe'

R 167

287

61

R 209

1OOb

323

R 48

61

OPX

OPX

OPX

OPX

OPX

CPX

CPX

CPX

CPX

58.23 0.02 0.7 I 4.5 1 0.13 36.8 1 0.13 0.13

55.52 0.04 I .77 9.97 0.2 I 32.07 0.24 0.0 I 0.0 I 0. I4

5 I .76 0.29 5.82 17.03 0.89 25.95 0.28 -

5 I .24 0.26 2.16 1.14 24.75 0.45 19.64 0.60 0.02 0.02 -

5 I .40 0.04 0.98

54.59 0.22 1.88 4.44

50.80 0.69 4.96 I .97 3.53 0.10 16.67 20.8 I 0.78

0.05 -

5 I .30 0.19 I .57 10.49 0.15 13.33 22.30 0.27 0.05

5 I .07 0.48 2.87 0.92 8.90 0.19 13.04 22.26 0.4 I 0.02 -

100.67

99.98

100.36

99.65

100.16

I .975 0.025 2.000 0.003 0.00I 0.017 0.1 I I 0.004 1.864 0.005 0.003 4.008 93.39

1.947 0.053 2.000 0.020 0.001 0.028 0.265 0.006 1.679 0.009 0.00 I 0.004 4.0 I3 84.88

102.02 100.28 102.09 99.88 Number of cations on the basis of 6 oxygens 1.845 1.938 1.950 I .972 0.155 0.062 0.044 0.028 2.000 2.000 1.994 2.000 0.090 0.034 0.052 0.00 I 0.006 0.008 0.007 0.044 0.007 0.049 0.015 0.920 0.127 0.459 0.802 0.01 8 0.027 0.0 I4 I .38 1 1.109 1.015 1.025 0.01 I 0.024 0.033 0.749 0.00 I 0.002 0.029 0.00I 4.025 4.008 4.028 3.996 72.08 57.16 50.82 88.44

1.859 0.141 2.000 0.073 0.019 0.085 0.078 0.003 0.91 I 0.817 0.055 0.002 4.044 84.59

1.939 0.06 1 2.000 0.009 0.005 0.061 0.27 I 0.005 0.752 0.904 0.020 0.00 I 4.029 69.05

1.915 0.085 2.000 0.042 0.0 I4 0.045 0.26 I 0.006 0.730 0.896 0.030 0.00 I 4.023 70.06

0.25 93.34 6.41

0.45 84.76 14.79

39.26 53.72 7.02

43.20 48.17 8.62

45.47 37.83 16.70

46.38 37.78 15.84

-

0.57 72.68 26.74

I .23 56.87 41.90

-

30.34 0.56 17.92 0.82 0.03 -

-

1.64 50.45 47.9 1

-

19.01 19.32 0.42 -

[Mg] = 100 x Mg I (Mg+Fe+Mn) Table 2. Chemical compositions and structural formulae of amphiboles. R 207

Si Al" 2

AP Ti Fe" Fe2+ Mn Mg

Y

D 14

R 209

41.42 53.7 1 42.2 I I .82 0.22 1.54 14.34 5.55 16.05 2.99 10.15 2.84 8.42 0.07 0. I9 0.11 15.56 12.25 2 I .25 I 1.99 12.02 12.26 0.77 2.12 2.39 0.10 0.39 0.82 96.53 98.74 98.28 Number of cations on the basis of 23 oxygens

44.04 1.16 10.80 17.87 0.17 1 I .03 IISO I .32 0.93 98.82

7.467 0.533 8.000 0.378 0.023 0.33 I 0.008 4.4 I0 5.150

287

6028 1.972 8.000 0.733 0.166 0.380 0.627 0.023 3.317 5.246

Gondwann Research. c! 1. No. I , I997

6.113 1.887 8.000 0.61 I 0.202 0.330 1.245 0.014 2.699 5.101

6.546 1.454 8.000 0.44 1 0.130 0.298 1.926 0.021 2.447 5.263

Table 2 contd. Ca Na K X CCations [Mgl* Name' Atomic % Ca Mg Fe'

1.793 0.208 0.018 2.019 15.169 92.68 t.h.

1.879 0.588 0.07 1 2.538 15.784 76.30 P.

1.899 0.685 0. I55 2.739 15.840 62.7 I f.p.

1.834 0.38 I 0.177 2.392 15.655 52. I5 e.h.

27.44 67.49 5.07

30.29 53.47 16.24

30.68 43.61 25.7 I

28.19 37.62 34.19

*[Mg] = 100 x Mg/(Mg+Fe+Mn) Name after Leake (1978) t.h. = tremolitic hornblende

p. = pargasite f.p. = ferroan pargasite e.h. = edenitic hornblende

The antiperthiticplagioclases in unusual enderbites B4 and 322 (with 65.90% and 71.48% SiO, respectively in the host rocks) are relatively calcic (Or,Ab,,An,, and Or,Ab,,An,, respectively) when compared to those in normal enderbites (Leelanandam, 1969b). The most unusual antiperthitic (zoned and untwinned) plagioclase comes from a quartz-bearing anorthositicrock (55). While 77 EMP spot analyses measured

I08

C. LEELANANDAM

Table 3 A. Plagioclases from ultramafic rocks.

SiO, AI?O,, CnO Na,O K,O w t % Or Ab An

281

287

59

322

472

D14

44.27 36.41 19.56 0.00 0.00 0.00 0.00 100.00

44.03 36.23 18.60 0.28 0.00 0.00 2.50 97.50

43.86 36.61 19.97 0.18 0.00 0.00 1.51 98.49

n.d. n.d. 18.30 0.93 0.20 1.18 7.88 90.93

n.d. n.d. 17.69 1.31 0.07 0.41 11.16 88.42

n.d. n.d. 11.06 5.36 0.13 0.76 44.91 54.33

Table 38. Plagioclases from gabbroic and anorthositic rocks.

R208

R48

61

R209

R 174

R80

46.96 34.83 17.62 Na,O I.05 K,O 0.00 Wt. %Or 0.00 Ab 9.22 An 90.77

48.53 33.79 16.55 2.2 1 0.06 0.3s 18.48 81.17

n.d. n.d. 16.14 2.24 0.13 0.77 18.99 80.24

49.68 32.82 15.49 2.77 0.09 0.53 23.25 76.22

49.07 32.59 15.44 2.65 0.16 0.95 22.43 76.62

48.93 32.51 15.40 2.66 0.12 0.71 22.59 76.69

SO, AI,O, CaO

Table 3C. Antiperthitic zoned plagioclase from quartz-bearing anorthositic rock (55). Bulk n.d. SiO, n.d. AW., CaO 15.84 Na,O 2.06 0.33 K,O Wt. %Or I .99 Ab 17.79 An 80.2 I

Host

44. I9 34.34 18.02 0.95 0.00 0.00 8.25 91.75

47. I8 33.27 16.44 2.04 0.03 0.18 17.44 82.38

Lamellae

5 1.56 30.97 13.52 3.76 0.17 I.oo

31.85 67. I5

63.20 18.94 0.28 0.5 I 14.23 93.65 4.80 I.55

63.10 19.54 I .22 0.5 1 13.93 88.82 4.65 6.53

on 6 grains range from An,, to An,,, the bulk (wet) analysis of the plagioclase (Leelanandam, 1965) gives a composition of Or,Ab,,An,,, (Table 3C). The most extra-ordinary feature of this sample is that the exsolved K-feldspar rods, as described in the petrography section (Figs. 9A and 9B), are nearly pure orthoclase (Or,J while the adjacent portion of the host plagioclase is nearly pure anorthite (An,,). Chuornite: It is not compositionally homogeneous in many specimens and exhibits different patterns of rimmed or zoned textures. The extreme variations in AI,O3, FeO' and Cr,O, (and also considerable variation in MgO) in different chromites are shown in Table 4; see also Fig. 11. Chromite compositions are probably related to textural and mineralogical environments. In the diagrams (not given here) showing the compositional relationships (Loferski et al., 1990; Jan and Windley, 1990), the KLC chromites fall in the fields assigned to stratiform-type complexes of the world. The erratic values obtained in 13 spot analyses in the chromite R167 (Table 4), from Gangineni, are due to the fact that the host (with more FeO') contains unusual lamellae rich in Cr,O,, AI,O, and MgO. The FeO and Fe,O,(calc.) contents are 29.74% and 40.16% in the host, while they are 19.81% and 10.77% (respectively) in the lamella; see also Figs. 8a-8c. The mode of occurrence and compositions of the host and lamellae, are similar to the Type B textural type of the exsolved chromite from Montana (Loferski and Lipin, 1983; Loferski et al., 1990). The iron-rich nature of the Gangineni chromites is well known (Mall and Rao,1970; Rao,1978), and in fact Mall and Rao (1970) have identified (atleast in one sample) two generations of chromite, one "replacing" the other. Contrary to the observations of Rao (1978), the writer's studies

Table 4. Chemical compositions and structural formulae of chromian spinels.

95 -

21.91 15.29 0.35 14.26 46.70 98.5 I

R 137

0.16 12.52 22.56 0.47 9.46 56.87 102.04

R 163

R 173

-

-

20.29 40.36

11.49 51.10

-

-

9.18 27.57 97.40

5.23 30.75 98.57

Range (1 3 pts)

R 167 Host

Lamella

0.42-2.17 25.03-3.00 33. I 1-71.46 0.25-0.21 8.73-I .92 31.07-16.05 98.61-94.8I

2.58 4.24 65.87 0.34 2.70 19.43 95.16

0.32 27.49 29.49 0.32 10.49 29.67 97.78

0.662 1.703 5.232 10.292 8.470 0.097 1.369 27.825 13.91 75.44 30.37 59.74 9.88

0.062 8.301 6.009 2.076 4.242 0.070 4.004 24.764 48.56 42.02 36.67 12.67 50.66

Number of cations on the basis of 32 oxygens Ti Al Cr Fe" Fe" Mn Mg Xations I OOMg/(Mg+Fe'+) 100 Cr/(Cr+Al) 100 Cr/CR'+ I00 Fe'+/ER'+ IOOAl/~RZ+

-

6.433 9.198 0.492 2.693 0.074 5.295 24. I85 66.29 58.84 57.05 3.05 39.90

0.03 I 3.833 I 1.680 0.566 4.335 0.103 3.663 24.2 II 45.80 75.29 72.64 3.52 28.84

-

-

6.597 6.013 4.52 I 4.790 3.775 25.696 44.07 47.68 35.10 26.39 38.5 I

4.038 7.250 6.283 6.461 2.325 26.357 26.46 64.23 41.26 35.76 22.98

0.082-0.577 7.69 1-1.25I 6.40.5-4.490 2.317-12.13I 4.924-9.014 0.055-0.063 3.392- 1.013 24.866-28.539 40.79-10.10 45.44-78.21 39.02-25.I2 14.12-67.88 46.86-6.99

Gondwanci Resenrch, Y l , No. I , 1997

KONDAPALLI LAYERED COMPLEX,INDIA

on the polished sections reveal complex exsolutionsand zoning. Favourable kinetics may permit exsolution (ofAl-richCr-spinel in Fe-rich Cr-spinel), but it may be too fine to detect under the optical microscope (see also Loferski et al., 1990). The four olivine-spinel pairs from the KLC yield a temperature range of 7O0-81O0C, adopting the method of Fabries (1 979). These temperature estimates are much lower (480-625OC; 890”)if the method of O’Neill and Wall (1987) is employed. Olivine-spinel pairs may equilibrate to lower temperatures than pyroxenes, and the calculated temperatures only reflect subsolidus re-equilibration (see Henry and Medaris, 1980; Himmelberg and Loney, 1980). In fact, olivine-spinel is one of the most easily reset thermometers in plutonic rocks (Prof. B.R.Frost, p e n . Comm.).

Whole-Rock Chemistry A total of 45 whole-rock XRF analyses (15 ultramafic rocks and mafic gabbros, 9 gabbros and anorthositic gabbros, 12 gabbroic anorthosites and anorthosites, 2 quartz-bearing anorthosites, and 7 acidic rocks) were obtained by the writer at the Geochemisches Institut, Gottingen (formerly of FRG)

109

during the period 1972-74 following the standard procedure which was in vogue at that time. For each sample, two pellets (fused discs) were made, and each pellet was measured twice, and the values quoted here are the average of four readings thus obtained. Some whole-rock analyses, obtained by classical (wet) chemical methods in the Department of Mineralogy and Petrology, Cambridge (U.K.), during the period 1962-65 (Leelanandam, 1965), are also utilized in the present investigation. The typical chemical analyses from each of the major rock types of the KLC are presented in Tables 5-7 and the overall chemical variations in the rocks of the KLC are represented in Figs.12a and 12b. The large chemical variations observed in the ultramafic group (Table 5 ) correlate well with the modal mineralogies of the rocks. As can be expected in the monomineralic cumulates (dunite R44, orthopyroxeniteR165 and clinopyroxenite R85), the rock composition virtually represents the composition of the cumulate phase. In the three rocks R135,522 and R149, variations in CaO, A1,0, and Na,O contents are attributed to the variable contents of clinopyroxene and/or hornblende. The rock 522 contains considerable green spinel (and Fe-Ti oxides) and hence shows especially high contents of A1,0,, FeO and

Tabe 5. Analyses and norms of the ultramafic rocks.

SiO, TiO, AP, FeP, FeO MnO MgO CaO Nap K,O PPS Total

R44

R165

R135

J22

R149

R85

39.68 0.04 0.48 3.27 5.68 0.13 47.52 0.2 I 0.54 0.10 0.02 97.67

52.67 0.15 3.32 2.94 10.92 0.26 27.56 0.96 0.35 0.10 0.02 99.25

50.70 0.33 5.07 2.79 4.28 0.10 22.59 11.40 0.86 0.17 0.02 98.3 I

40.75 I .42 15.07 5.11 11.85 0.37 17.85 5.67 0.90 0.21 0.12 99.32

49.93 0.68 9.92 2.10 9.88 0.09 16.22 9.09 I .so 0.51 0.10 100.12

48.85 0.40 6.90 2.43 7.28 0.17 12.37 19.66 0.86 0.18 0.0 I 99.1 1

3.01 12.68 18.81 20.57 10.68 7.13 2.76 25.66 18.50 7.16 14.72 10.31 4.40 3.04 1.29 0.24

1.07 3.59 14.56 67.24 34.98 23.77 8.50

Q C or ab an di wo en fs hY en fs 01

fo fa mt il

aP %An (Plag) En (CPX) En (OPX) Fo (01)

0.5 I

0.95 0.59 2.98 4.67

0.73 0.39 0.32 0.02

9 I .45 84.69 6.76 3.65 0.08 0.05

83.75 66.45 17.29 2.44 I .89 0.54 4.29 0.29 0.05 61

44 93

Gondwana Research, V l , No. I . 1997

79 77

I .02 7.40 9.63 37.53 19.94 16.10 I .49 26.81 24.54 2.27 12.80 11.62 1.18 4.11 0.64 0.05 57 43 92 91

3.36 1.25 7.67 27.53

2 I .47 15.82 5.65 28.26 20.27 7.98 7.46 2.72 0.29 78 74 72

60 35 72 70

7.15 5.13 2.02 3.55 0.77 0.02 80 35 -.r

‘ I

72!:;

C. LEELANANDAM

I10

Field of Composition

Na+K

Field of Composition

(Fe” + Fe” + Mn)

Figs.12a & 12b. Cation percentage plots of rocks from the KLC. Symbols same as in Fig. 11. Fields of composition of the Bushveld and Stillwater complexes (after Bowes et al., 1970)are shown for comparison.

Fe,O,, and TiO,. The ultramafic rocks show a rather regular reciprocal relationship between the (MgO+Fe,O,‘) and (CaO+Al,O,) contents, and a similar relationship exists in the gabbroic and anorthositic rocks also (Leelanandam, 1994). In the gabbroic group, a small variation in A1,0, is accompanied by considerable inverse change in Fe,O,+FeO. The chemical changes in the rocks of either the gabbroic or anorthositic group, are minor, gradational and subtle (Table 6); but, between the two groups, the variations are marked. Specifically, the changes in A1,0, and (MgO+FeO) contents reflect the highly significant changes in modal mineralogies between the two groups. The chemical hiatus between the two groups is of course less marked than that observed between the ultrarcafic and gabbroic groups (see Tables 5 and 6). It is unlikely that these two hiatuses are due to sampling inadequacies. These chemical gaps or breaks are also noticed in other variation diagrams (not given here). There is an overall petrochemical similarity between the Kondapalli, Bushveld and Stillwater complexes.The general correspondencebetween the fields of composition and magmatic trends of these complexes can be seen in Figs. 12a and 12b (see also Bowes et al., 1970). The Upper Zone of the Bushveld Complex (with considerable Fe enrichment followed by progressive enrichment in Na + K)is not represented in the IUC, and this is evident from Figs. 12a and 12b. In the absence of any chilled border facies, the composition of the parent magma cannot be determined directly. In the quartz-bearingrocks (Table 7), the quartz-anorthosite 55 is distinctly different from the rest and is more akin to the rocks represented in Table 6, and it is this rock which contains the unusual K-feldspar lamellae in the host plagioclase. The unusual enderbites (B4 and 322), trondhjemites (R134 and R140) and tonalite (R144) can easily be distinguished taking into account their relative CaO, Na,O and K,O contents. The MgO and FeO contents (and their ratio) of the rock R134 are more comparable to those of the anorthosite R120 (Table 6) than to any of the rocks in Table 7. The high normative En (opx) contents of the felsic rocks R134, R140 and R144 (with

Table 6. Analyses and norms of the gabbroic and anorthositic rocks.

Si02 TiO, A1,q Fe, 0, FeO MnO MgO CaO Na,O K20 P,O, Total

R 208

R94

R 47

R 129

R 120

R 58

61

R 174

48.43 I .07 16.17 1.38 10.56 0.18 9.23 11.61 1.10 0.10 0.16 99.99

49.29 0.13 20.19 I .29 5.88 0.14 8.84 10.12 2.27 0.38 0.02 98.55

48.94 0.82 15.18 1.69 11.28 0.20 7.39 12.05 1.57 0.25 0.06 99.43

52.36

48.46 0.18 26.14 0.94 4.32 0.10 4.46 12.24 2.17 0.18 0.02 99.2I

47.79 0.13 29.2I 0.17 2.98 0.06 2.73 15.24 1.86 0.20 0.03 100.40

0.59 9.31

2.28 19.49

1.49 13.36

49.34 0.16 27.72 0.92 3.44 0.07 3.09 12.45 2.33 0.27 0.03 99.82 1.09 1.03 1.60 19.75

47.94 0.I4 29.27 1.62 1.84 0.05 2.28 13.92 2.20 0.10 0.01 99.37 0.70 0.26 0.59 18.73

Q

1 .oo

15.23 1.55 8.84 0.16 6.32 9.26 3.08 0.63 0.25 98.68 1.28

C or ab

3.77 26.4I

0.17 1.07 18.51

1.18 15.68

Gondwam Research, V l , No. 1, 1997

KONDAPALLI LAYERED COMPLEX, INDIA

111

Table 6. contd. an di wo

en

fs hY en

fs 01

fo fa mt il aP

%An (Plag) En (CPX)

En (OPX) Fo (01)

44.42 5.16 2.67 1.72 0.77 17.82 12.34 5.48 8.64 5.80 2.84 I .90 0.25 0.05 70 33 69 67

38.89 14.47 7.38 4.09 3.00 3 I .30 18.08 13.22 1.03 0.57 0.46 2.00 2.03 0.38 81 28 58 55

33.83 2 1.43 10.81 5.32 5.30 24.03 12.04 11.99 I .70 0.81 0.89 2.46 1.57 0.14 72 25 50 48

26.22 15.38 7.80 4.07 3.51 22.15 11.88 10.27

2.28 1.92 0.60 50 26 54

6 I .08

6 I .68

15.03 9.20 5.83 2.37 1.40 0.97 I .37 0.34 0.05 77

13.14 7.7 I 5.43

61 59

59

I .34 0.30 0.07 76

70.48 3.80 I .93 I .05 0.8 I 3.37 1.90 1.46 4.94 2.67 2.27 0.25 0.25 0.07 82 28 56 54

69.43

7.63 5.72 1.91

2.36 0.27 0.02 79 75

Table 7. Analyses and norms of the quartz-bearing rocks.

SiOz TiOz Ap2 Fez 0, FeO MnO MgO CaO Na, 0 K$ P20, Total

Q C or ab an di wo

en fs hY en fs mt il

aP %An (Plag) En (CPX) En (OPX)

55

R 134

R 140

R 144

B4

322

53.00 I .09 18.50 0.88 9.50

62.8 I 0.28 16.20 0.67 4.00 0.08 4.35 4.75 4.01 I .oo 0.17 98.32 16.81 0.30 6.01 34.5 I 22.84

63.82 0.57 14.26 1.32 4.80 0.09 3.61 4.32 3.32 2.32 0.11 98.54 19.19

70.04 0.45 13.26 2.49 1.92 0.04 0.70 2.16 2.07 4.70 0.15 100.01 35.1 I 1.22 28.35 17.88 9.94

65.90 0.09 17.00 0.57 3.35 0.06 1.59 5.89 3.74 1.09 0.03 99.3 1 23.23

7 I .48 0.66 12.98 3.23 3.63 0.05 0.75 4.74 2.33 0.36 0.12 100.33 44.10 0.43 2.12 19.65 22.66

0.15

5.19 9.74 I .05 0.38 0.24 99.72 I I .07 3.25 8.91 44.76 I .76 0.88 0.40 0.47 27,32 12.56 14.76 1.28 2.08 0.57 83 23 46

17,61 I I .02 6.59 0.99 0.54 0.4 I 40 63

high SiO,) are close to those from the anorthositic group; although the felsic rocks contain low normative An (plag) contents,they are interpreted as contaminatedmixed rocks with charnockites, but not as late evolved differentiatesof the KLC. One can come to the same conclusion for the quartz-bearing anorthosite ( 5 5 ) and unusual enderbites (B4 and 322) also, which contain relatively lower En(opx) and higher An (plag) values, compared to the members in the correspondinggroups. As one would expect, there is no coherent relationship between the SiO,, En(opx) and An (plag) contents of these "mixed" rocks. Gondwana Research. V l , No. I , 1997

13.91 28.5 I 17.41 2.96 1.51 0.82 0.63 14.72 8.30 6.42 1.94 1.10 0.26 38 28 56

6.49 31.87 26.56 2.23 1.11 0.46

2.60 1.78 0.82 3.68 0.87 0.36 36 68

0.66 8.55 3.53 5.03 0.83 0.17 0.07 45 21 41

4.85 I .86 2.99 4.67 1.25 0.28 54 38

Discussion and Conclusions The occurrence of dunite, harzburgite, orthopyroxenite, websterite, clinopyroxenite, gabbro and anorthosite (and the absence of wehrlite, lherzolite and troctolite) in the KLC implies that the most common order of crystallization was 01, ol+opx, opx(kplag), opx+cpx (kplag), cpx(kplag), opx+cpx+plag, plag(+opx+cpx). The predominance of orthopyroxene(rather than olivine) as an early cumulate phase in the ultramafic zones can be attributed to moderate-high pressure of crystallization of a tholeiitic magma.

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C. LEELANANDAM

The post-intrusive metamorphic over-printing on the KLC is not pervasive. The parental magma for the KLC must have intruded the dry country rocks at great depths (lower to middle crustal levels) under moderately high pressure, probably in the very early period of the post-peak metamorphic stage of the country-rocks (charnockites/granulites), and cooled slowly together with the enclosing charnockites and granulites. The substantial subsolidus re-equilibration that is witnessed in the KLC must have taken place under the same physical conditions as those which prevailed during cooling i n the enclosing charnockitic/granulitic rocks. This is particularly well documented by the identical K,, values and P-T estimates (T= 830-950°C; P = 6-8 kbar) obtained from the coexisting pyroxenes in the rocks of the KLC and the enclosing granulites. In fact the pressure estimates pertain to the granulites, and similar P-T ranges were earlier obtained by Grew( 1982) using the data given in Leelanandam (1967b). The temperature values given were obtained by utilizing the thermometer of Wood and Banno (1973). Values obtained after the method of Wells (1 977) are practically the same, though they are higher in some cases. Some clinopyroxenes from the ultramafic rocks yield unusually high temperatures in the range 1100-1200°C (method by Lindsley, 1983). Mall (1973) has clearly recognized that the ultrabasics of the Gangineni area (a part of the KLC) were subjected to “metamorphism under deep-seated conditions in relatively dry environment” and “the various mineral phases in ultrabasics equilibrated together along with associated charnockites under granulite facies conditions during or after the emplacement of acid charnockites”; the orthopyroxenes from ultrabasics surprisingly retain their original (igneous) “chemical character” in spite of the fact that the K,, values for the pyroxene pairs are characteristically metamorphic. Bose and Bose (1 989) have suggested that the orthopyroxenes (coexisting with clinopyroxene) fall within the metamorphic compositional field, and that there are several chemical evidences (including the K,, values) strongly favouring a metamorphic regime for the KLC pyroxenes (especially from the ultrabasics); however, they continue to prefer their original idea (Bose and Bose, 1982) that the KLC is largely post-tectonic, and is undeformed and unmetamorphosed as “there is no textural or distinctive mineralogical criterion or deformational history to suggest even incipient recrystallization in the rocks”. Obviously, the association of igneous textures with metamorphic P-T conditions has led to some confusion in the minds of the researchers on the KLC regarding the relationships between the textural features, chemical characteristics and K,, values of the paired pyroxenes. One should remember that the K,, value is not a particularly good discriminant for slowly cooled plutons (Dr. S. Harley, pers. comm.). The writer has not encountered any late differentiates such as ferro-diorites, granophyres, etc. (see also Nanda and Natarajan, 1980). However, Bose and Bose (1982) claim that the felsic rocks [tonalite-leucotonalite (trondhjemite)- rare quartz diorite] occur towards the top of the sequence [in between

the gabbro-norites and granolites (country rocks)] and, represent fractionated or evolved parts of the KLC. These rocks show no trace of any magmatic textures, but have intenSe deformational and granulitic textures, and also contain orthopyroxene as the sole mafic mineral (Bose and Bose, 1982, p.154), and generally without any trace of hornblende and clinopyroxene which are constant constituents of the gabbroanorthosites. The present writer interprets the felsic rocks as “mixed” (contaminated) rocks occurring in tectonized junction zones between the KLC and enclosing charnockitic rocks. These rocks (containing rare garnet) are more akin to the charnockitic group (than to the KLC) retaining the calc-alkaline signature. However, contamination is constantly conceived by Bose and Bose (1977,1982,1989), not to account for these rocks, but to explain the occurrence of spinel (of igneous origin) in some ultramafic rocks. Nanda and Natarajan (1980) envisaged a parental basic magma fractionating in two major pulses to account for the KLC, whilst Bose and Bose (1989) advocated an unmodified fractionation history of a tholeiitic magma (with a late stage calc-alkaline stamp). The present writer tentatively prefers two parental magmas: ( 1 ) a magnesian liquid to yield a variety of the ultramafic rocks; and (2) an alumina-rich tholeiitic liquid to give gabbroic (to high Fe-gabbroic) and anorthositic rocks. These two magmas may have related origins. The KLC gabbros are compositionally similar to high-A1 gabbros from several Proterozoic anorthosite complexes (see Mitchell et al., 1995); the former expectedly contain slightly higher amounts ofAl,O, and CaO, and lower amounts of N 3 0 , than the latter. Some of the Kondapalli mafic granulite (basic charnockite) dykes and lenses in khondalites (Leelanandam, 1961, 1969c) contain antiperthitic high-Ca plagioclase (An 2 80) and minor garnet (Leelanandam, 1965, 1967~);they are, in general, chemically similar to the KLC gabbros and also to the mafic granulite massifs of the area. The relationship, if any, between the melts from which mafic granulites (all types) and gabbros are derived, is yet to be investigated. The major layered intrusions (Bushveld, Stillwater etc.) are interpreted in terms of complex magma mixing models involving at least an ultramafic and a basaltic component (e.g., Hatton and Gruenewaldt, 1990). On the other hand, Czamanske and Bohlen (1990) infer that the major anorthosite zones of the Stillwater complex are difficult to explain either by in situ fractionation of mafic magma or by popular models for mixing of two magma types (Irvine et al., 1983). Instead the Stillwater anorthosite zones, according to Czamanske and Bohlen (1990), represent concentrations of plagioclase emplaced as crystalladen mushes into a vast staging magma chamber near the crust/ mantle interface which accommodates extensive magmatic underplating. This is an attractive proposition especially for those who invoke basaltic underplating (instead of continental collision) for the evolution of the Eastern Ghats mobile belt. The occurrence of different rock associations in several detached and dismembered portions of the KLC, together with the presence of rare garnet-bearing metadolerites (with Gondwana Research, V l , No. I , 1997

KONDAPALLI LAYERED COMPLEX, INDIA

intensely clouded plagioclase) which cut the complex, negate any simple, straightforward and unmodified magmatic history for the KLC. The writer suggests that slow cooling of the KLC at great depths (moderate-high pressures) under granulite facies conditions has resulted in : ( 1 ) preservation of many “magmatic” textures in most of the rocks, whilst at the same time, (2) registering signatures of chemical equilibrium in the constituent phases (especially in the coexisting pyroxenes) typical of high-grade metamorphism, and, (3) local generation o f exceptional mineralogical, textural and chemical peculiarities.

Acknowledgements Most of the data presented in this paper were obtained by the writer at the Geochemisches Institut, Gottingen (formerly of FRG) during the tenure of Alexander von Humboldt Senior Research Fellowship ( 1 972-74), and the facilities offered by Prof.Dr.K.H.Wedepoh1 are gratefully acknowledged. Some chemical data incorporated in this paper were obtained by the writer in the course of his Ph.D. work under the supervision of Pmf.W.A.Deer, F.R.S. at the Department of Mineralogy and Petrology, University of Cambridge (England) during the tenure of a Commonwealth Fellowship (1962-65). A limited amount of mineral data, especially from the chromitites, was obtained by the writer during his short scientific sojourns to Cambridge (1988) and Edinburgh (1988 and 1991) in U.K. Mr.S.R.Sarma (Hyderabad) has accompanied the writer on numerous visits to the Kondapalli area. Prof. Ben Harte (Edinburgh) has critically examined several thin sections, and particularly discussed the aspects pertaining to the textural equilibrium in the Kondapalli rocks. Prof.Dr. S. Koritnig (Gottingen), Dr. K. R.Gill (Edinburgh) and Dr. N. Krishna Rao (Byderabad) have carefully observed many polished sections; some of the photographs incorporated in this paper were taken by-them and Mr. S.R.Sarma. All the colour photomicrographs were taken by Dr. N. Krishna Rao at a very short notice. The writer expresses his grateful thanks to all of them. Mr. G. Ramatheertha Rao and Mr. B. Ananta Sharma have done Some preliminary field and petrographic work. Mr.Suresh Pate1 ‘(Laramie) has obtained several geothermobarometric data; Profs. B.R. Frost (Laramie) and D. H. Lindsley (Stony Brook) have derived the P-T estimates for some Kondapalli rocks. Dr. J. Ratnakar and Mr. E.V.S.S.K. Babu have done m o s t o f t h e computations and drawn the diagrams; Mr.T.S.Sunilkumar has brought out computer print-outs of several versions of the manuscript. The writer sincerely thanks all of them. The writer expresses his gratitude to Profs. L.D. Ashwal (Rand Afrikaans University, Johannesburg), B.R. Frost (University of Wyoming, Laramie), S. Harley (University of Edinburgh), B. Harte (University of Edinburgh) and R.Kretz (University of Ottawa) for critically reviewing the manuscript and suggesting improvements. Gondwana Research. V l , No. 1. 1997

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