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Mineralogy of INDIAN CHARNOCKITES Ortllopyroxenes Are the Most Characteristic and Important Ferromagnesian Minerals in Rocks Of

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Mineralogy of INDIAN CHARNOCKITES Ortllopyroxenes Are the Most Characteristic and Important Ferromagnesian Minerals in Rocks Of JOUR. GEOLOGICAL SOCIETY OF INDIA Vol. 20. June 1979, pp. 257 to 276 MiNERALOGY OF INDIAN CHARNOCKITES C. S. PICHAMUTHU Abstract Considerable amount of investigation has recently been done on the minerals commonly present in the charnockites of India. In this paper a very concise account is given of the optical properties and chemical composition of orthopyroxene, clino­ PYroxene, hornblende, biotite, garnet, plagioclase, and potassium feldspars. Mention is made of some of the minor minerals such as monazite, magnetite, ilmenite, pyrites, PYrrhotite, apatite, zircon, spinel, and sapphirlne. The work done on the chemistry of coexisting pyroxenes, and the cation distri­ bution in the coexisting silicate minerals is also briefly reviewed. ORTHOPYROXENES OrtllOpyroxenes are the most characteristic and important ferromagnesian minerals in rocks of the charnockite series, in which they occur exclusively or in asso­ ciation with diopside-salite clinopyroxenes, brown hornblende, biotite, or garnet. Composition: The orthopyroxenes in charnockites range in composition from eulite EnZ5 to hypersthene En64' They are ferrohypersthenes and eulites in enderbitic rocks, and typical hypersthenes in syenites and charnockites (Subramaniam, 1959). This varia­ tion in the FejMg ratio is not entirely in line with the behaviour of an igneous differentiation series (Howie, 1964a). As pointed out by Bhattacharyya (1977), though there is a general increase of Fe and decrease of Mg in the orthopyroxenes with increasing acidity of the host rocks, more acidic rocks often contain less Fe-rich orthopyroxene as in the Madras (Howie & Subramaniam, 1957) and Amaravathi (Ramaswamy & Murty, 1973) areas. This was one of the points made by Subramaniam (1959) to show that the mafic charnockite in the type area is not genetically related to his' charnockite suite'. The belief that the pyroxenes in chamcckitic and granulitic rocks contain high alumina is not generally true (Bhattacharyya, 1968; Sen, 1974). The alumina content of metamorphic pyroxenes appear to be related to the bulk chemistry; other things being equal, higher pressure increases their alumina content. According to Leelanandam (1967a) the pyroxenes in the ultra-mafic charnockites of Kondapalli are more magnesian than those in the Madras region (Howie, 1955; Subramaniam, 1962). Pleochtoism : Hypersthene is generally strongly pleochroic in shades of pink and green. According to Howie (1955), the pleochroism is not directly related to the content of ferrous or ferric iron, to the amount of manganese present, or to any particular trace element, though a combination of chemical factors may have some influence. He considers that the explanation of Kuno (1954) that there is some relationship between pleochroism and titanium content does not hold for the Madras rocks since in the most pleochroic bronzites the Ti02 content is very low; the pleochroism may be related to the oriented exsolution lamellae and schiller inclusions. The intensity of pleochroism is not controlled by the FejMg ratio but can be tentatively correlated with decrease in the cell dimensions, and with the entry of appreciable aluminium to I 258 C. S. PICHAMUTHU the octahedral position in the structure replacing the larger Mg and Fe2+ ions (Howie, 1964b). A variety of views, often contradictory, have been expressed regarding the causes for the pleochroism exhibited by orthopyroxenes as can be seen from the following: (l) Iron content is not responsible for the intensity of pleochroism (Turner, 1948; Howie, 1955; Subramaniam, 1959; Eskola, 1957); (2) Pleochroism is dependent on Ti02 content (Kuno, 1954; Hess, 1960; Murty, 1964a, b); (3) Pleochroism is un­ related to Ti02 content (Howie, 1955; Lovering, and White, 1964); (4) Pleochroism can be correlated with the presence of Ni or Cr (Subramaniam, 1962); (5) Pleo­ chroism is related to the A1203 content (Binns, 1962; Howie, 1964a, b); (6) Pleo­ chroism is not directly related to anyone particular clement but to a combination of chemical factors (Leelanandam, 1967e); (7) Intensity ofpleochroism can be correlated satisfactorily with Ti02 content (Bhattacharyya, 1969). Origin: Whether charnockites were formed by magmatic or metamorphic and meta­ somatic processes, hypersthene is the most stable phase that can exist in a •dry' system. The real problem is to determine whether the pyroxene is primary and formed during magmatic processes, or secondary due to the breakdown of a pre­ existing mafic mineral. Both processes are possible, and it is difficult to distinguish them. Hess (1952) has classified an orthopyroxene in a granulite from the type area of Pallavaram as metamorphic. Wilson (1955) considers that the hypersthenes of magmatic charnockites are constantly more ferriferous than those of gneissic char- TABLE r. Chemical analyses of orthopyroxenes 2 3 4 5 6 7 8 9 -- ---------- -------- --~--- - Si02 46.17 51.17 49.50 49.76 49.60 50.81 51.69 50.08 48.77 AI203 3.22 1.97 2.01 1.56 2.51 3.20 2.51 1.34 1.06 Ti02 1.78 0.15 0.11 0.13 tr 0.38 0.43 0.12 tr FC203 1.16 0.59 1.31 1.66 1.78 0.14 1.29 0.75 FcO 33.17 23.01 32.60 30.96 27.48 24.36 21.41 35.44 34.55 MgO 12.24 20.7S 13.74 13.10 15.27 19.51 22.29 10.81 13.61 MnO 0.75 0.87 0.59 0.76 0.53 0.60 0.38 0.93 1.08 CaO 0.04 0.90 0.16 2.18 2.77 0.64 0.53 1.37 0.60 Na20 0.52 0.11 0.17 0.16 0.49 0.11 0.02 0.24 0.21 K20 0.34 0.53 0.07 0.03 0.03 002 H20+ n.d. n.d. 0.01 0.01 0.19 0.10 n.d. H20- 0.31 0.14 0.02 0.05 0.03 --- ~- - --- --~-_.<_-- Total 99.70 100.19 100.29 100.31 100.65 99.92 100.58 100.33 100.63 Original number, host rock. locality, and reference 1. 6436 Felsic charnockite, Meenambakam, Anal: R. A. Howie (1955) 2. 2270 Intermediate charnockite, Salem, Anal: R. A. Howie (1955) 3. 4642 A Mafic charnockite, Pallavaram, Anal: R. A. Howie (1955) 4. 2 Maficcharnockite, Type area, Madras, (Ray & Sen, 1970) 5. 4 Mafic charnockite, Type area, Madras, (Ray & Sen, 1970) 6. 28 Mafic charnockite, Kondapalli, Anal: (C. Leelanandam, 1967e) 7. DI4 Ultramafic charnockite, Kondapalli, Anal: (c. Leelanandam, 1967c) 8. 98 Mafic granulite, Saltora, W. Bengal, (Sen & Manna, 1976) 9. 335 Mafic granulite, Saltora, W. Bengal, (Sen & Manna, 1976) MINERALOGY OF INDIAN CHARNOCKITES 259 nockites (whether felsic or mafic). According to Rao (1967), hypersthenes do have a variable composition, but the variation appears to be dependant on temperature, and so may not reveal their true origin. Bhattacharyya (I97Ia) found that plots of weight percentage of (MgO + FeO + Fe203) vs weight percentage of Ah03 form the demarcation between igneous and metamorphic orthopyroxenes. Analyses of 12 orthopyroxenes from the ultramafic rocks of Gangineni and Kondapalli, and 29 orthopyroxenes from charnockites (18 from Kondapalli and 11 from Madras) were plotted on such a diagram by Sinha and Mall (1974). It was found that they fall into two distinct groups-the orthopyroxene analyses from the chromiferous ultramafics fall in the igneous field, and those from charnockites in the metamorphic field. It is obvious, therefore, that the ultramafic rocks of Kondapalli and Gangineni do not bear any genetic relationship to the associated charnockites. CLINOPYROXENES The elinopyroxenes generally occur in irregular light green grains with faint pleochroism, sometimes with lamellae parallel to (100). The composition is typically in the salite or augite range. In the Ca-Mg-Fe compositional plot given by Leela­ nandam (1967e), the Kondapalli elinopyroxenes fall in the fields of endiopside, diop­ side, saIite, and augite. Unlike orthopyroxenes which occur throughout the charnockite series, the elino­ pyroxenes are normally confined to the less acid rocks such as basic granulites and pyroxenites. In the ultramafic rocks of the Madras charnockite series, the clino­ pyroxene has a diopsidic composition. Although in the less mafic rocks the clino- 260 c. S. PICHAMUTHU pyroxene is an augite and in the intermediate charnockite of Tirunelveli it is a ferro­ augite, the clinopyroxenes of this series are all characterised by a high calcium content, and their compositions lie close to the diopside-hedenbergite join (Howie, 1955). According to Howie (1955, Fig. 6), the clinopyroxene trend line for the char­ nockite series in the type area is displaced upwards nearer the diopside-hedenbergite composition line in comparison with typical igneous series i.e., these clinopyroxenes are richer in lime, and that this indicates that the Madras rocks were held at a relatively higher temperature over an extended period of time, allowing the two pyroxene phases to become almost completely exsolved. The low calcium content of the orthopyroxenes appears to support this. HORNBLENDE In the charnockite rocks of Madras, hornblendes are common in the ultramafic and mafic rocks, but are less frequently present in the intermediate varieties, and are almost entirely absent in the felsic rocks of this region. No pyroxene granulite is completely free from hornblende in the type area (Sen, 1970; Ray, 1970). De Waard (1967) reports that in the Adirondack Highlands, another classic granulite facies area, pyroxene granulites free from hornblende and/or biotite, are rare. Hornblende can be considered as an essential phase of basic granulites. Hornblende in charnockite is optically olive-brown to green in colour, and strongly pleochroic. Sometimes there are fine lamellae parallel to (100). Composition: The mineral, often surprisingly rich in fluorine, is almost always present in mafic and ultramafic charnockites. It is usually more aluminous than the hornblendes of normal igneous rocks. There is a general decrease of Ah03 with the decrease of basicity of the rocks of the charnockite series.
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