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Magnesio-Hastingsite from the Anorthosites of Oddanchatram, Palni-Dindigul Area, Tamil Nadu

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Magnesio-Hastingsite from the Anorthosites of Oddanchatram, Palni-Dindigul Area, Tamil Nadu Proc. Indian Acad. Sci., Vol. 83 B. No. 3, 1976, pp. 103-108 Magnesio-hastingsite from the anorthosites of Oddanchatram, Palni-Dindigul area, Tamil Nadu P. NARASIMHA RAO Department of Geology, Annamalai University, Annamalainagar 608101 (Communicated by Prof. V. Baliah, F.A.Sc.) MS received 24 July 1975; after revision 10 December 1975 ABSTRACT The characteristic mafic co~stituent of the anorthosites of Odda~- chatram is clinoamphibole. The chemical composition, optical properties and cell parameters indicate it to be magnesio-hastil~gsite. It is suggested that the mineral is of primary origin and precipitated early from a hydrous gabbroic altorthosite magma that gave rise to the ar~orthositic suite of rocks of the area. 1. GEOLOGIC SETTING OF THE ANORTHOSITES TIaE anorthositic suite of rocks round about Oddanchatram, Tamil Nadu, comprises anorthosites, gabbroic anorthosites, gabbros and norites. They display a dominant anorthosite facies concentrated in the central portions of the masses and relatively subordinate gabbroic anorthosite and gabbroic rocks segregated towards the marginal portions. Structural studies in the area have shown that the anorthosites constitute a phacolithic pluton emplaced in a NNE plunging anticlinal fold of metasediments, basic granulites and charnockites. A domical form is suggested for the mass by the outward dipping foliation all around. 2. MODE OF OCCURRENCE OF THE MINERAL Clinoamphibole is a more characteristic marie constituent of the anortho- sites than the pyroxenes. Megascopically the amphibole is of medium grain size and greenish black in colour. In some outcrops of the anorthositic rocks, a pegmatitic facies is found which contains coarse amphibole crystals measuring 4 cm to 6 cm in length (Kulandeivelappan Hill. 1381 and Sundari- puram). In thin sections of the anorthosite, the mineral is deeply pleoohroic from pale yellow to dark green. It occurs both as discrete crystals and also enclosed within plagioclase exhibiting euhedral forms (figure 1). The plagio- clase is of labradorite composition (Anho to An60 ) and is also zoned in places (figure 2). !03 104 P. NARASIMHARAO 3. RESULTS OF EXPERIMENTAL STUDIES The clinoamphibole was separated from the anorthosite and its chemical composition, optical properties and cell parameters were studied. The results obtained are presented below : Mineral chemistry The chemical composition of the clinoamphibole as determined by the method of Shapiro and Brannock 1 in comparison with amphiboles of ana- logous occurrences is shown in table 1. The numbers of ions on the basis of 24 (O, OH, F) as recalculated from the analysis are as follows : Si 5"941 8-00 A1 2.059 AI 0" 560' Ti 0.388 Fe+3 0.253 Mg 2.213 5.229 Fe +2 1- 806 Mn 0.009 Na 0- 343) Ca 1.734~ 2.474 K 0.397J OH 1.806- The structural formula for the mineral may be written as Cal. ~ (Na q- K)0.74 (Mg + Mn q- Fe+2 -? Ti ÷ Al+3 -k Fe+3)s.2 AI~ Sis O2z (OH)2. The value of 100 Mg/Mg-? Fe+2 q-Fe+3 -k Mn is equal to 51.7. In terms of the sub-divisions of hastingsite made by Billingsz the mineral with a FeO/MgO ratio Of 0.82 becomes femahastingsite. The structural account in formula unit satisfies the criteria of Berman 3 for hastingsite with X=4; Y=I; Z=Si: AI=6:2 and Ca inW<2. As per theelassi- fication of Sundius~ the formula of the mineral may be written as (Na, K)0.T Cal.7 (Mg, Fe+~)4 Alo.6 ALz.l Si.~.~ O,~.~ (OH)I. 8 According to his criteria, the amphibole with two tetrahedraUy coordinated A1 atoms per formula unit, fits into the pargasite ferrohasti.ngsite series. The plots of the mineral in the diagrams of Deer etaL 5 which show chemical variations of calcium-rich amphiboles expressed as the numbers of (Na, K) and (A1L) atoms and as the numbers of (A1e + Fe+3 + Ti) and (AI~) atoms per formula unit, reveal that the mineral belongs to pargasite-- forrohastingsite series. The plot of the mineral made in the diagram of Deer etal. 5 which shows the relationship between the chemical composition and optical properties and density of pargasite--ferrohastingsite series indicates a composition of 51.7% pargasite and 48"3~o ferrohastin/~site for the mineral. P. Narasimha Rao Proc. Indian Acad. Sci., Plate I Vol. 83 B, No. 3, 1976, pp. 103-108 Figure 1. Euhedral crystal of amphibole enclosed in a coarse plagioc!ase grain of the anorthosite" Crossed nicols, x 50. Figure 2. Zoned plagioclase crystal in the anorthosite. Note the presence of twinning in zoned crystals also and intergrowth of anhedral hypersthene with plagioclase. Crossed nie,ols, x 50. ( facin~ page 104) Magnesio-hastingsite of the anorthosites 105 Table 1 1 2 3 4 SiO~ 39"50 42"32 42'91 45"91 TiO2 3"38 0"27 0"93 0"05 AI~O~ 14"78 15"62 12"45 18"25 Fe2Oa 2-20 4.22 4"34 .. Cr20~ ...... 0"05 FeO 14.36 6-78 10.89 6.07 MnO 0.11 0.15 0.15 0.08 MgO 9-80 13.68 11.34 12.10 CaO 10"77 11.78 12-04 15.42 Na20 1.19 2.4l -20 0.32 K~O 2.11 0-34 1.49 0.05 I-IzO+ 1.80 2-13 1"67 0.54 I-I~O- 0.02 0.11 0.08 0.15 F, Cl .. 0.05 0.I1 0"02 TOTAL 100"02 99'86 99'60 99"01 1. Clinoamphibole (magnesio-hastingsite) from the anorthosites of Oddanchatram (Nara- simha Rao). 2. Clinoamphibole (edenite) from the anorthite gneiss, Sittampundi, Salem District (Subramaniam, 1956 a) 9. 3. Hornblende from anorthosite gneiss, South Russell, Adirondacks 7 (Buddington 1952) 7 4. Amphibole from anorthite gneiss, Sittampundi (Nehru 1955) 8. Thus the mineral under discussion is found to belong to pargasite ferro- hastingsite series and can be considered to be of magnesio-hastingsite compo- sition. Optical and Physical properties =1.670; /3: 1.683; y= 1.690; n.y--n,,=0"02 2V,~=68 ° to 70 ° ; z A c = 20 ° Pleoehroism s-Pale yellow; -Brownish green; y-Dark green D = 3.26 106 P. NARASIMHA RAO Cell parameters The x-ray diffraction pattern of the amphibole was taken on CuK,, radiation using nickel filter. The related data and the calculated cell para- meters are shown in table 2. The values of the cell parameters obtained for the mineral show a near agreement with those of the synthetic magnesio-hastingsite of Colville et al. 6 4. DISCUSSION ON MINERAL PARAGENESIS The amphibole of the anorthosites of Oddanchatram is very distinctive in its characters and differs significantly from the amphiboles described by Buddington,7 Nehru 8 and Subramaniam? Buddington1° states that the hornblende of Adirondack anorthositic rocks is rarely observed with the relation of a primary magmatic mineral and that it is a common constituent particularly in the border facies. Nehru ~ describes an amphibole from the anorthite-gneisses of Sittampundi, Salem District as a calciferous aluminous variety with predominant tschermakite and ferro-tschermakite molecules. He finds ambiguity in the nature of the mineral and so does not name it specifically. Subramaniam9 finds the clinoamphibole from the anorthite gneisses of Sittampundi complex to be of edenite composition. He attri- butes its formation to the mineralogical transformation of the gravity strati- fied complex under regional metamorphism. The same author 11 also records the occurrence of calciferous amphiboles in the anorthositic rocks of Kadavur, Trichy District. He explains that this mineral was derived from the primary- pyroxenes by the action of late stage mineralizers of the parent gabbroic anorthosite magma. Table 2. X-ray diffraction pattern and calculated cell parameters of amphibole. d(A) 1/11 hkl Calculated cell parameters 3.275 34 240 a A 9- 855 3.118 81 310 b A 18.020 2.803 44 331 2.712 29 151 c A 5" 383 2.378 30 350 /3 105 ° 30' 2.348 29 351 2.159 25 332 a sin/3 9.495 2.021 26 351 c sin/3 5"187 1" 679 28 172 1.668 26 480 Magnesio-hastingsite of the anorthosites 107 The following are some of the important points to be taken into account with regard to the genesis of the amphibole of the anorthosites of Oddan- chatram : (i) The anorthosific rocks of Oddanchatram, particularly the central portions of the mass, show many igneous parameters like cumulate textures, normal zoning of plagioclase, occurrence of magmatic twin laws of plagioclase, towage structures and euhedralism of some constituent minerals. This reveals that the suite of rocks are magmatic and also did not undergo much of post-emplacement deformation and metamorphism. It is probable that the parent gabbroic anorthosite magma was emplaced as a late stage oro- genic pluton. (ii) The amphibole is found in thin sections of the core anorthosites as fresh and discrete crystals. It exhibits euhedral forms in places and also is enclosed by labradorite indicating the later crystalliza- tion of the plagioclase. (iii) The pegmatitic facies of the anorthositic rocks composed of very coarse crystals of amphibole and plagioclase noticeable in some of the outcrops is suggestive of the role played by the volatiles during the evolution of the rocks. (iv) In the presence of elevated water pressures the amphibole may become the primary silicate phase and precede the plagioclase and also coexist with pyroxone and olivine stably over a limited range of temperature. I~ (v) The plagicalse of the anorthosites shows in places zoning in thin sections. Though calcic, the plagioclase is free from antiper- thitic intergrowths.
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