SummaryThe charnockite series consisting of pyroxene granulites and charnockites are not formed by magmatic differentiation, although both units had their antecedents in igneous material. The solid-state recrystallization of pyroxene granulite and the liquid-state crystallization of charnockite are inferred from the field criteria. The pyroxene granulites with the associated paragneisses are formed from the original sedimentary-volcanic sequence under granulite facies of metamorphism. The magmatic charnockites emplaced into these rocks resulting in the development of intermediate varieties by the partial assimilation and incorporation of the pyroxene granulites. The stability of the minerals is not affected by this migmatization, which is therefore thought to have taken place under granulite facies conditions. The evolutionary trend of the charnockite series is one of retrogression from the earlier granulite facies to the later almandine-amphibolite facies, caused by the subsequent granite invasion.

SUMMARY. The potash feldspar in the charnockites is orthoclase and they are inferred to have been formed around 7oo ~ A twin-law study of the plagioclase indicates an igneous origin of the charnockites and pyroxene granulites. The K D values of the coexisting pyroxenes in the granulites are appropriate to equilibrium at 67o ~ A substantial substitution of A1 for Si in the Z group in all mafic minerals studied also indicates relatively high temperatures of formation. Almandine is pyroperich, a result of high pressure. The charnockites and pyroxene granulites are inferred to be formed under conditions of high pressure and temperature.THE Amaravathi area (16 ~ 2o'-I6 ~ 35' N., 80 ~ 21'-8o ~ 28' E.) is a part of the highgrade metamorphic terrain of the Eastern Ghats in Andhra Pradesh. In this area there are rocks of the khondalite and charnockite series, granites, and dolerites. The charnockite series is represented by gabbro, hornblende gabbro, biotite gabbro, norite, hornblende norite, garnetiferous hornblende norite, biotite norite, and garnetiferous biotite norite in the basic division (pyroxene granulites) and by granitic, syenitic, granodioritic, and dioritic types in the acid division (charnockites). The field relations indicate that the khondalite series of rocks and the pyroxene granulite are the basement rocks into which charnockites, granites, and dolerites were successively emplaced. The present paper deals with the mineralogy of the charnockite series.The bluish-grey colour of the quartz together with the greenish-blue colour of feldspars makes the charnockite look more basic than it actually is. A satisfactory explanation to the colour of the quartz is yet to be found, although some attribute it to the randomly oriented inclusions and some to some chemical element such as titanium.Potash feldspar is untwinned and invariably perthitic. Of IOO extinction-angle measurements, over 8o % of the grains exhibit straight extinction and the rest oblique extinction up to a maximum of 3 ~ with the (OLO) cleavage trace on (ooi). 125 out of 15o grains have 2Va 5o to 65 ~ indicating orthoclase (Harker, 1954) and only 25 have 2Va 66 to 780 indicating transitional microcline.Five representative samples of potash feldspar from Amaravathi, two each from Kondapalli and Chimakurti (all areas in the Eastern Ghats) were X-rayed; none showed any separation of the 131 and 13o reflections, indicating zero obliquity. Murty (1965) has also shown the potash feldspar of Visakhapatnam (in the Eastern Ghats) to be orthoclase. These studies prove that the potash feldspar from the charnockites not only of Amaravathi, but of part if not the entire region of the Eastern Ghats is orthoclase. Nevertheless in the type Madras area it is microcline. In the light 9

SUMMARY. The chemical analyses, structural formulae, and optical data of seven orthopyroxenes and six clinopyroxenes, all from the charnockite series of Amaravathi, are presented. The orthopyroxenes are in the hypersthene-ferrohypersthene range while the clinopyroxenes are in the sahlite range. The aluminous nature of the pyroxene is believed to be the result of high pressure. The clinopyroxenes have the granulite trend on the Wo-En-Fs plot. The tie-line projections and the KD values for the pyroxene pairs indicate conditions of chemical equilibrium around 67o ~ for the charnockite series of Amaravathi.DURING the course of geological mapping of about 250 sq. km area around Amaravathi (16 ~ 20' to 16 ~ 35' N., 80 ~ 2I' to 80 ~ 28' E.), paragneisses, a charnockite series consisting of pyroxene granulites (basic) and charnockites (acidic), granites, and dolerites are noticed. The interbanded paragneisses and pyroxene granulites of the basement rocks were successively emplaced by charnockites, granites, and dolerites. This paper is a concise account of ortho-and clino-pyroxenes from the charnockite series.Orthopyroxenes. Seven orthopyroxenes were chemically analysed and proved to be hypersthene or ferrohypersthene. The chemical and optical data and the atomic ratios of both ortho-and clino-pyroxenes are given in table I.The pleochroism of the orthopyroxene is ~ pink, p yellow, 7' green, the intensity varying from section to section, from grain to grain within the section, and from portion to portion within the grain.The range of composition of the major oxides of the pyroxenes of Amaravathi is well within that of the pyroxenes of Kondapalli and Madras (Leelanandam, 1967), excepting that TiO2 is less in pyroxenes of the present area. CaO is generally low in the orthopyroxenes and related to exsolution c/inopyroxene lamellae.The lower limit of A120~ in orthopyroxenes from Amaravathi is 2.56 ~ whereas the upper limit of Al~O~ in orthopyroxenes of igneous rocks is about 2 % (Muir, I95I). High A1203 in pyroxenes is attributed to the host-rock chemistry by Howie 0965) and Leelanandam (1967), to host-rock chemistry and to the nature of the coexisting phases (more aluminous pyroxenes coexist with more sodic plagioclase) by Binns (1965), to their formation from aluminium-bearing minerals like hornblende, biotite, and garnet by Murty (t965), and to high pressures by Eskola. Those parent rocks that have O