Basic Charnockites Research Articles

Review: the charnockite problem, a twenty first century perspective

Beginning of the twentieth century was marked by coinage of a new rock name, Charnockite, first described as a hypersthene-bearing granite from Southern India. Since then charnockites have been described from most of the conti-nents and mostly restricted to high-grade belts. Later half of the last century saw a lively debate over an igneous versus metamorphic origin. However, two factors acted as deterrents for the resolution of the debate. First, charnockites and associated rocks occur in a variety of different structural setting and display diverse field rela-tions, attesting to possible different mode of origin. Second and possibly more important is the lack of consensus on the nomenclature of charnockites and associated rocks and this is commonly linked with the metamorphic versus magmatic perspective. Scanning the literature of this period makes one believe that both metamorphic and magmatic hypotheses are valid, but applicable to different field setting only. Before critically evaluating individual cases, it is imperative that a uniform approach in nomenclature should be agreed upon. It is proposed that name charnockite be adopted for any quartzofeldspathic rock with orthopyroxene, irrespective of its mode of occurrence, struc-tural setting and mode of origin. The associated more mafic varieties, be better described as mafic granulite, rather than basic charnockite. For the patchy charnockites of east Gondwana (including parts of Peninsular India, Sri Lanka and Antarctica), metamorphic transformation from amphibolite facies gneiss, by two different mechanisms: CO2 ingress from deep level, and drop in fluid pressure, has been proposed. However, all such patchy occurrence is not amenable to explanation by metamorphic trans- formation. In some instances, migmatisation of older charnockitic rocks is evident. Also pro- gressive charnockitisation relating patchy char-nockite to banded variety could be argued against on two counts: grain-size relation and time-relation. Larger bodies or bands have been explained as magmatic, but in many instances, from geochemical consideration alone. The compositional variation, commonly encoun-tered in many high-grade belts, if not described in terms of field relation, may lead to wrong no-tions of magmatic differentiation of mantle-de- rived melts. Crustal melting of dry granulite fa-cies source rocks has been proposed from geochemical and isotopic data of charnockitic intrusions. This model proposes high-tempera-ture melting of previously dehydrated and dry granulite source rocks. However, tectonic per-turbation subsequent to granulite facies meta-morphism that might have been responsible for such high temperatures, is not well constrained in this model. Finally, with advent of high- pressure dehydration-melting experiments in the nineties, dehydration-melting of mafic to intermediate composition, syn-kinematic with granulite facies metamorphism has been pro-posed.

Geochemistry of calcic horizons in relation to hillslope processes, southern India

The study concerns chemical and mineralogical properties of calcic horizons in relation to their physical environment such as position in the soil profile and the terrain. The carbonates were calcitic in upslope positions but dolomite was abundant in valley sites. The content of fluorine increased downslope. The distribution of the composition in the terrain indicates a sequential precipitation of calcite, dolomite and fluorite from soil water and groundwater as the water flows downslope to discharge areas. Carbonates in Vertisols formed from the more basic charnockites were of the low Mg calcite type, while dolomite occurred only in Entisols and Alfisols formed from biotite-hornblende gneisses. Manganese in calcic horizons increased as the groundwater level was approached. Black intercalations in laminated petrocalcic horizons contained elevated concentrations of Mn, K, Al and Si. This may indicate periods of higher soil moisture and clay formation. The presence of low-Mg calcitic carbonates overlying dolomitic ones in valley sites is likely to be due to deposition of calcite-rich material from upslope. The amount of soil eroded from upslope positions was between 0.2 and 2 m in four investigated profiles.

The relationship between the geology and the lateritic engineering soils in the northern environs of Akure, Nigeria

Two contiguously occurring brownish to reddish residual soils in the northern environs of Akure, Nigeria, which show definite relationships in their physical and engineering properties to their parent rocks, have been studied. The parent rocks are coarse porphyritic granite and fine, to medium grained basic charnockite respectively. For each soil, the profile development, clay mineralogy, texture, consistency and the compaction characteristics are investigated. The test results show that the differences in physical and engineering properties are conditioned fundamentally by the parent rock, with drainage and other environmental conditions being practically identical. Although the two soils are both usually described as “lateritic”, the engineering properties of the soils considered on a comparative basis have established the coarse granite derived soil as superior to the charnockite derived soil as road base of fill material generally.

L'orthoferrosilite et les roches associées de la région des Gour Oumelalen (N.E. Ahaggar, Algérie)

Pyroxenite, almost entirely composed of iron-rich orthopyroxene with very little fayalite and quartz, forms the extremity of a lens of pyroxenites and basic charnockites. This lens is one of a group of numerous bodies of basic and ultrabasic rocks belonging to the charnockitic suite of the Gour Oumelalen (Ahaggar, Algeria), which are surrounded by garnet-sillimanite gneisses and marbles. This orthopyroxenite gives place in few decimeters to a clinopyroxenite with orthoferrosilite and fayalite. Basic charnockite is a norite with brown amphibole and biotite. The interaction between a vein of hypersthene pegmatite and the clinopyroxenite has produced secondary minerals, amphibole (hastingsite) and garnet (almandine). Physical properties of the iron-rich OPX from the "orthoferrosilitite" are given. Microprobe analyses of ferromagnesian silicates in these rocks permit some estimates, based upon experimental data, of temperature and pressure. The composition of coexisting clinopyroxene-orthopyroxene suggests that temperatures ranged from 800 °C to 860 °C, The composition of orthoferrosilite (OFS 87-92) coexisting with fayalite (Fa 93 -95) applied to various experimental data indicates pressures of 8. 1-8.7 kbar at 860 °C (8-9 kbar at 800 °C). Temperatures estimated from garnet-biotite geothermometry in gamet-sillimanite gneisses are between 760 °C and 850 °C. The metamorphic pressure, measured from the garnet-plagioclase-sillimanite-quartz equilibrium, is close to 8.5 kbar at 790 °C. These diverse P-T estimates define the final metamorphic crystallisation conditions as 760 °C-860 °C and 8-9 kbar, which agree well with those recorded from other Proterozoic granulite facies series.

A Petrological Study of Charnockites and Associated Rocks East of Chipurupalle, Srikakulam District, Andhra Pradesh

Abstract Geological mapping of a 300 sq km area forming part of the Eastern Ghats, east of Chipurupalle, supplemented by modal analyses, optical petrographic data, and chemical analyses of seven whole-rock samples and a garnet, show that (1) basic charnockites of the area represent igneous rocks of noritic composition metamorphosed to granulite facies, and (2) some rocks of the area that resemble intermediate charnockites are metamorphosed hybrid rocks derived from assimilative reaction between basic magma and pelitic sedimentary rocks.

Rocks of unusual chemistry in the charnockitic terrains of India, and their geological significance

SummaryThe paper reports the occurrence in the 2.9–3.1 B.Y. old charnockitic terrains of Southern India of some basic charnockites that are strikingly similar, chemically, to basaltic komatiite from the 3.4 B.Y. old Barberton greenstone belt of Southern Africa, and suggests that the pre-metamorphic evolution of Archaean charnockitic terrains followed the same trends that characterized the development of Archaean greenstone belts. The main events in the evolution of one such terrain, the ‘Madras granulite belt’, before it was subjected to granulite facies meamorphism some 2.6 B.Y. ago, is outlined in Table 2.

Structural State of Plagioclases from Basic Charnockites

Structural State of Plagioclases from Basic Charnockites

Coexisting plagioclases from basic charnockites of India

Plagioclases of widely different compositions are found to coexist in basic charnockites of India. This may be due to structural discontinuities or difficult and slow substitution of Si/Al in tetrahedral sites. The presence of hydrous environment in making plagioclases homogeneous is discussed.

Myrmekite from the Charnockitic Rocks of the Eastern Ghats, India

SummaryThe myrmekite occurring in acid to basic charnockites of the area around Garbham and Shikaruganji, Srikakulam district, Andhra Pradesh is described. Myrmekite, whenever present, occurs only at the contact of plagioclase and potash feldspar. The textures and paragenesis of minerals suggest that potash feldspar reacted with plagioclase giving rise to myrmekite under favourable stress condition, higher stress favouring the formation of myrmekite. The difference in the stress factor or the microstrain pattern in the rock can explain the presence of myrmekite at some contacts of plagioclase–potash feldspar and absence in others.

Orthopyroxenes from the kondavidu charnockites, guntur district, andhra pradesh

The chemical analyses of 3 orthopyroxenes from the Kondavidu charnockites are presented along with their structural formulae calculated on the basis of six oxygens. The atomic ratio 100 Mg/(Mg + Fe+2 + Fe3+ + Mn) of the analysed orthopyroxene, varies from 44·7 to 51·6. The high and variable nature of the alumina content of orthopyroxenes is discussed. The optical properties of orthopyroxenes of ultrabasic, basic, intermediate, and acid charnockites are given. The composition, as deduced from optical properties, is expressed in terms of end-member molecules and varies from En83Fs17 to En26Fs74. The D (x) values, calculated according to the method outlined by Dobretsov (1968), indicate low-temperature formation for the Kondavidu orthopyroxenes.

Pyroxene and biotite from the charnockitic rocks of Garbham area, Srikakulam district, Andhra Pradesh, India

SummaryChemical analyses and optical properties of ten orthopyroxenes, two clinopyroxenes, and six biotites (partial analyses) are presented from basic to intermediate charnockites of the Garbham area. The orthopyroxene is hypersthene to ferrohypersthene and the clinopyroxene is sahlite. Biotite has secondarily formed from pyroxene, particularly orthopyroxene, as a result of injection of potassic fluid in the basic charnockite. By combining the data of Srikakulam with those from other areas of the world it is shown that during the transformation of pyroxene to biotite in charnockitic rocks, in general, Mg and Fe 2+ attain near-equilibrium distribution between orthopyroxene and biotite as well as between clinopyroxene and biotite. The magnesium distribution coefficient (KD) of coexisting orthopyroxene and biotite is found to increase with the increase of temperature of biotite formation. A tentative geothermometric scale has been proposed plotting temperature of biotite formation against magnesium distribution coefficient of coexisting orthopyroxene and biotite. According to this scale, biotite in basic and semibasic charnockites from Garbham has formed between 285 and 465 °C.

Porphyritic plagioclase–hornblende–pyroxene granulite from charnockitic rocks of Chipurupalli, Visakhapatnam district, Andhra Pradesh, South India

SummaryAn unusual occurrence of porphyritic plagioclase-hornblende-pyroxene granulite (porphyritic basic charnockite) has been observed within the ‘Charnockite region’ of the Eastern Ghats of the Precambrian formations of India. From the X-ray and chemical data the phenocrysts are considered to be augite phenocrysts. Based on petrographic and chemical evidence it is inferred that the basic charnockites of Chipurupalli area in Visakhapatnam district are formed from tholeiitic magma as a gabbroic mass under deep seated plutonic conditions. The gabbroic mass has been recrystallized as plagioclase-hornblende-pyroxene granulite due to later metamorphism, leaving most of the phenocrysts unaffected.

Distribution of elements in coexisting minerals and the problem of chemical disequilibrium in metamorphosed basic rocks

Distribution of elements in coexisting minerals—biotite, hornblende, augite, hypersthene and plagioclase in charnockitic rocks of West Uusimaa Complex, Finland, is mostly orderly indicating a close approach to chemical equilibrium. The distribution of iron and magnesium in coexisting hornblende and pyroxenes of basic charnockites and other rocks of granulite facies from several different areas is also orderly but the variation in the fugacities of H2O and H2 may cause a disorderly distribution locally in some rocks. The probable oxidation or reduction reactions are discussed on the basis of thermochemical and mineralchemical data.

Zoned plagioclase from the charnockites of Kondapalli, Krishna district, Andhra Pradesh, India

SummaryZoning in plagioclase is reported from the charnockitic rocks of Kondapalli, Krishna district, Andhra Pradesh, India, for the first time. Electronprobe studies indicate that the zoned plagioclases consist of homogeneous sodic cores mantled by thin calcic shells, which show continuous zoning of the inverse type. Partial chemical analyses of twelve density fractions of the zoned plagioclase from a basic charnockite exhibit a wide variation in An content from 54·80 to 72·39 %; X-ray studies suggest that they possess ‘fixed’ transitional structural state.

Charnockites of Manbhum and the Charnockite Problem

Abstract The ultrabasic rocks and norites of Manbhurn are not to be included in the charnockite series. They belong to the earlier sedimentary-igneous sequence that underwent intense deformation and recrystallization before the charnockites (sensu stricto) came into being. The basic charnockites (pyroxene granulites) are derived as a result of metamorphic convergence at granulite facies from cafemic sediments and norites. These are isofacial with diopside-calc-granulite, khondalite and leptynite. Influx of granitic material converted the pyroxene granulites to a series of rocks of intermediate composition and ultimately to a granite. A part of all these varieties are hypersthene bearing with the features characteristic of the group. The charnockite habit, including the index mineral, was impressed on the rocks by the pre-granite metamorphism, retained in the successive products of granitization, and obtained by the intrusive granite by crystallization under identical PT environment. All hypersthene granites in this typical milieu, and the granulite facies rocks are charnockitic, and have a dry mineral assemblage association together with other members of the charnockite series. In the term 'charnockite' is to stay, it should be used as a synonym of hypersthene granite in the typical set-up and association. The term 'charnockite series' should be dropped.

Isotopic ages of the basic charnockite and khondalite from Kondapalli, Andhra Pradesh, India

SummaryThe isotopic ages of three biotites from the Kondapalli area, Andhra Pradesh, India, have been determined by the 40Ar/40K method. The results (440–524 Myr) indicate that the basic charnockite and khondalite of pre-Cambrian age have been subjected to at least one subsequent metamorphism the age of which must be 440 Myr or younger. The significance of the results is discussed in the light of the available geochronological evidence from Peninsular India.

Age measurements on pegmatites and a basic charnockite lens occurring near Lützow-Holm Bay, Antarctica

Rb 87-Sr 87 age measurements were carried out on large biotite crystals from three granitic pegmatites and a basic charnockite lens occurring near Lützow—Holm Bay, Queen Maud Land. The mean age obtained (∼ 515 million years) probably refers to the last major metamorphic episode in this region. The age data are also significant for the interpretation of palaeomagnetic results reported from this area.

Charnockite-Quartzite Association in the Rangala-Madulkele Area, Ceylon

Abstract Basic charnockites several feet thick and completely contained in quartzites of the Khondalite Series are common in the Rangala-Madulkele area of Ceylon. Recently however, extremely Darrow bands of basic charnockites, 1/3 to 1 1/2 inches thick, in quartzite, have been noted. The nature of the field occurrence and the petrographic characteristics of these locks are described. It is concluded that the significant features of these occurrences can be best explained on the assumption that the basic charnockites, at least in this area, are the metamorphosed equivalents of sediments which formed part of the Khondalite Series.

Radioactive ages from the Precambrian rocks in Australia

The central nucleus of the Australian Precambrian shield lies in the western half of the continent and is characterized by striking similarities in structure and age over a vast area. Ages in the vicinity of 2,700 m.y. have been determined at many localities by the Rb/Sr and K/A methods. Flanking and cutting the shield are much younger belts of Precambrian rocks which have been fused onto the older shield. On the south‐eastern side, the deep‐seated Fraser Fault separates the Goldfields rocks (2,700 m.y.) from pegmatized basic charnockites (1,300 m.y.). In this region the shield rocks also show a metamorphism at 2,400 m.y. A similar structure separates the nucleus on the southern side from the east‐trending rocks of the south coast which were pegmatized about 1,400 m.y. ago. The western margin of the shield bounded by the Darling Fault shows evidence of vigorous recrystallization (about 650 m.y.) and regional magmatic activity (900 to 1,100 m.y.). A long E.‐W. 1,000 m.y. belt in central Australia may continue westward to divide the central nucleus of the shield. So far no basement rocks with ages in excess of 2,000 rn.y. have been found on this continent outside Western Australia. Granites in northern Australia appear to be at least 1,650 m.y. old, with a uranium mineralization at 500 m.y. In South Australia two periods of uranium mineralization have been recognized at 500 and 1,500 m.y. The 500 m.y. event is particularly well documented at Myponga, where there is agreement between the U/Pb, K/A and Rb/Sr ages of minerals from the main uranium lode channel. Lead mineralization at Broken Hill and Mount Isa also appears to have occurred in the vicinity of 1,500 m.y. ago. The major age divisions in Canada and Australia appear to be closely comparable.

Amphibolite-granulite facies charnockites

The Cheri Hill in the Madras type area of charnockites, shows on careful mapping the folding of bands of leptynite and basic charnockites. The mineralogy of the basic types gives striking evidences of the metamorphic origin of charnockites. The rocks have been subjected to some little retrograde metamorphism and are ascribed to the almandine-diopside-hornblende subfacies of the amphibolite facies. A quantitative idea of the influx and efflux of ions is worked out on the lines of Barth, in the process of replacement of the norite to the associated garnetiferous-diopside-granulite.