ORIGIN and EMPLACEMENT OFTHE Mylllem GRANITE, KHASI HILLS, MEGHALAYA, INDIA *

Revista Brasileira de Geociências 17(4):660-662, dezembro de 1987

ORIGIN AND EMPLACEMENT OFTHE MYLLlEM GRANITE, KHASI HILLS, MEGHALAYA, INDIA *

S. RAHNAN**

INTROOUCTION The Mylliem granite occurs as a the growing surfaces of the microcline grains in the grani te rectangular pluton covering nearly 90 km2 around the village (Hibbai"d 1965). Mylliem in the Khasi hills of lhe state of Meghalaya (Fig. I). The zircons of the Mylliem granite are mostly euhedral It is a normal, coarse, porphyritic biotite granite rich in with an average rounding index 22.2%. Coloured zircons K-feldspar. The granite is intru sive in nature. The Rb-Sr show compositional zoning.The elongation dating of lhe Mylliem granite has given an age of 765 ± 10 Ma frequency-Iengthlbreadth (X/Y) ranges between 2.0 and 3.0. (Crawford 1969). The origin of the granite is discussed were Such zircons possibly indicate their free growth in the grani te mainly on the basis of petrological characters and structural melt (Rahman & Borah 1982). relations of the granite and the associated country rocks. Geochemistry Major element analyses of seven samples of the Mylliem granite (G l-G7), their average (A) and one GEO LOGIAL SETTING The Mylliem granite is well more analysis (B) for comparison, are given in table 1. The known in the Precambrian stratigraphy of the Shillong plateau average composition of the granite (A) compares well with (Meghalaya plateau) , NE lndia. The plateau stands on ancient lhe 72 Calc-alkaline granites (B) of Nockolds (1954). gneisses and schists forming lhe basement complex. This is overlain unconformably by the argillaceous and arenaceous Table J - Chemical analyses (Weight percent) and CJ.P.W. sediments of the Shillong Group (Medlicott 1869). First nonn ofthe Mylliem granite intrusive into these sediments were the doleritic rocks (Khasi Samplc G I G2 G3 G4 GS G6 G7 A B 5i02 71 .72 72 .66 72 .66 73.21 69 .89 71.8 1 70.36 7 1.75 72 .08 greenstone) which along with the sediments suffered regional TiO: 0 .53 0 .32 0.36 0.38 0.45 0 .7 1 0 .55 0.47 0.37 metamorphism prior to the intrusion of lhe granite. The AhOJ 13.88 13.93 13.7 8 13.4 8 14.23 13.93 13.83 13.86 13.86 F C20l 0 .90 1. 14 0.51 0.73 1.88 0 .90 1.70 1.10 0.8 6 granite is the youngest member of the Precambrian rocks FeO 1.57 0 .97 1.29 1.4 3 1.38 1.37 2.0 1 1.43 1.67 MoO - - - 0.0 4 - - 0 .04 0.06 occurring in lhe Shillong plateau. MgO 0.50 0.30 0 .52 0.47 0.48 0.56 0 .52 0.4 7 0.52 C.O 1.58 0.61 1.05 1.40 1.1 7 1.49 1.56 1.26 1.33 The Mylliem granite is surrounded on all sides by the Na20 3.70 4.55 4.10 3. 30 4.10 3.25 3.00 3.7 1 3.08 K. O 4.72 5.00 5.1 0 5. 15 5.85 5.20 5.65 5.23 5.46 metasediments (quartzites and phyllites) of the Shillong " 20 + 0. 42 0 .29 0.35 0 .40 0.6 1 0 .50 0.36 0 .41 0.53 Gro up. Metadolerite bodies also occur within lhe quartzites at P20 S 0. 15 0.16 0.17 0.23 0.12 0. 11 0.12 0. 15 0.18 Tota l 99.67 99.93 99.89 100.18 100.20 99.83 99.66 99.88 00.00 varying distances from lhe granite. However, lhe basement Q 27.87 25.64 26.15 30.64 21.05 29.03 26.90 0' 27.89 29.54 30.13 30.43 34.56 30.72 33.38 rocks are not exposed here. eb 31.29 38.48 34.68 27.9 1 34.68 27.49 25.31 ao 6.86 1.98 4.10 5.44 3.16 6.61 6.95 c 0. 17 0.31 0.02 0.48 - 0.51 0.23 OISCUSSION ON OR IGIN ANO EMPLACEMENT wo -- - - 0.18 - - di - -- - 0.11 - Field evidence The junction between the granite and the hy 2.50 1.06 2.64 2.51 0.80 1.99 3.67 rnt 1.30 1.65 0.14 1.06 2.73 1.30 2.46 quartzite (country rock) is sharp and well defmed. Dykes and iI 1.01 0.61 0.68 0.72 0.85 1.35 1.04 veins of the granite traverse lhe country rocks. The grani te ep 0.36 0.38 0.40 0.55 0.28 0.26 0.28 show cross-cutting relation with the quartzite. Xenoliths of O I (gn:y) 46 41 31 293 the quartzites and metadolerites are found inside the granite. 02 (pink) 65 23 76 254 g~'a~ ~ir:l :~::~t~~:=t: Petrol09Y The Mylliem granite occur in two distinct 4S6 and 7 : > Locations shown in figure I types - grey and pink. They are composed essentially of ~ : ~::~:: ~:::::~: ::::=;~:: ~~ ::: ~~:ik~~~e· ~ilcs (Nockolds 1954) K-feldspar with subordinate oligoclase (An 20-34)' quar tz and AnaJyst: Or. C.C. Bhanadwjec, Glesgow Universily Leborat ory . biotite. K-feldspar occurs as phenocrysts (maximum size 3 em The normative Q-Ab-Or values of the granites fall within x 5 em), The modal composition is Kvfeldspar ' 44% the closed contours (Fig, 2a, after Tuttle & Bowen 1958, (microcline 38%, orthoclase 6%), plagioclase 22.4%, quar tz figure 42), representing the concentration of graniti c rocks of 23.1%, biotite and other accessories 10.5%. The granite is magmatic origino The superimposed cotectic line P-E'5 classified as "normal granite" as per QAP plots for plutonic representing lhe variation in position of the minimum melting igneous rocks (Streckeisen 1976). point in the system NaAISi30a - KAISi30a - Si02 - H20 The freq uencies of the twin laws in the plagioclase of the (Tuttle & Bowen op; cit.; figure 38) is extended to the granite are : Albite 36.7, Carlsbad 22.9, Albite-Carlsbad 29.4, quartz-orthoclase join to ilIustrate the dose relationship Albite-Ala B 2.7 , Pericline 5 and Acline or Manebach-Ala 3.7 between temary rnaxirnum for low water vapour pressures (Rahman 1984). Dominance of complex twins of C-twins and the normative composition of lhe grani tes. It is seen that (Tumer 1951) in the granite suggest its plutonic igneous lhe plots of lhe granites falI dose to this line within lhe character. pressure range below 0.5 and 4.0 kbar and are therefore The inclusions of plagioclase, biotite and quartz are consistent with a rnagmatic origin (Winkler 1976). The invaria bly zonally arranged inside lhe host microcline crystallization of the granite seems to have started around phenocrysts. The plagioclase inclusions do not represent an 700°C. exsolved phase as indicated by associated inclusions of biotite The composition of the Mylliem granite is also seen to and quartz paralleling plagioclase orientatio n (Higgins & correspond with composition in the low temperature trough in Kawachi 1977). It is thought that the early formed minerals lhe Q-An-Ab-Or system (Kleeman 1965). Plots of normative that were in suspension in the liquid melt were drawn along feldspars of the granite within the thermal trough (Fig. 2b)

• Extended abstract ** Department of Geology, Gauhati University. Guwahati , 781 014, Assam, India Revista Brasileira deGeociências, Volume 17, 1987 661

NE INDt A 91°50' E GREY MY LLlEM --r- FOLl ATlON tBfi PINK GRAN ITE o ME TADOLER ITE "-. FLOW U NE5 G 5HILLONG GR OUP -<' FLOW LAYER5 A- ANDA LU5 1TE \ 5LATY CLEAVAGE (52) o S- 51 LLI MAN ITE O, o~71!o o o

I o o o o o o o V / fi fi 45° \ --- 60° o ~o / o o o o l-;; o o V V ...... 32° J/-..t.- fi fi 50° o o / I o o o /' VVV V -- V V V v/ fi fi fi fi ri V o VV V v/ fi Iv VV V V VV o • MYLLI EM fi / I G7 / o V V V V V VV V V r, G3 / fi 40~ fi fi fi \ I G2 J G4 ~ V V / VVVV VV I" fi V/ \O I " " / V O / I V fi / VVV V V V V VV / ./ I " " 25° _/ O O -r , ---V VV V V V V V VV VV V /~ " V V V VVVV V VV V O " O V VV V VV V V V I O OO " -- V V VV V V V VV O O " -<'" OO V V VV V VV 60° V G6 O OO O V VV V O O 1MILE O O -- O O lkm 0 O O O 65 O ~ O «>: OO O OO O O 36° O

Figure 1 - The Mylliem granite pluton

indieate magmatie erystallization at a temperature range due to the presence of pink eoloured mieroeline. Careful between 6500C and 69()OC. examination of the exposures of the Mylliem granite pluton Trace element eontents (Tab. 1) of the granites show that has shown that the ratio of grey to pink is approximately 3:1 in bulk eomposition the grey granite (G 1) is rieher in Ba, Ce, (see figure 1). The pinkish eolour seems to be due to higher La, Y and Sr while Cu, Ni, Pb, Rb and Th eontents are higher proportion of ferric oxide impurities in the mineral. The in the pink granite (G2). There is no primary museovite in Fe20 a eontent in the pink granite (G2 and G5) is these granites, hence alI the Sr in both the varieties is likely to eomparatively higher than in the grey granite . Assuming that be eoncentrated in the plagioclase, If this is so, there should most of the Fe in K-feldspars is trivaIent, the entry and exit have been proportionately still lesser Sr in the pink granite of Fe in K-feldspars is perhaps related to the bulk Fe-eontent (see table 1). However, the reverse relationship (CaO: Sr = and oxidation state of the parent magma (Smith 1974). Hence 1.58:360 in G 1 and 0.61:302 in G2) exists. This indicate that for a favourable oxidation eondition, it is thought that the Sr enriehment has taken place in G2. Thus the pink granite is pink granite is a late fraetion of erystallization of the grey believed to the latter thanthe grey granite (Rahman 1985). granite. This is also supported by the faet that there is.a progressive inerease in the amounts of K20 and Na20 and deerease in Effects on the country rocks The quartzites (quartz CaO and MgO in the pink variety. 90%, serieite 10%) and phyllites (quartz-muscovite-serieite) It may be noted here that the pink eolour of the granite is of the eountry rocks, within a zone of 200 to 400 meters 662 Revista Brasileira de Geociências, Volume 17, 1987

Q a A n b t i 50

50 Ab 50

Figure 2 - e.Normative Q-Ab-Or p/ots ofthe My/liem granite (dots). The closed contours represent the distribution of571 ana/ysed p/utonic rocks that carry 80% normative Q-Ab-Or (after Tutt/e & Bowen 1958); b. P/ots ofnormative An-Ab-Or proportions ofthe My/liem granite in the Q-Ab-An-Or system (after K/eeman 1965) around the granite body, show pronounced thennal effects. the granite body have the same southerly dip, being directed The sericite of the quartzite is recrystallized to muscovite and towards the granite on the N and directed away from it on the quart z acquires polygonal crystal outlines displaying S. This can be taken as an evidence in support of granoblastic-polygonal texture. The phyllite is coverted into downwarping of the rocks of the Shillong Group at this part andalusite homfels with sillimanite (fibrolite) at places. to fonn a synclinal basin. The hydrous low grade assemblage: actinolite-chlorite­ epidote-albite of the regionally metamorphosed dolerites is CONCLUSIONS From the preceding discussion, it is recrystallized to homblende and oligoclase at the proximity of evident, that the Mylliem granite possess ali the characters of the granite (Rahaman 1981). intrusive rocks. It is a nonnal two -feldspar- biotite granite . The xenoliths of the older rocks show varying degrees of The zonally arran ged inclusions of early fonned minerals in hybridization due to assimilation by the granitic material the microcline phenocrysts, predominance of complex twins (Rahman 1972). (C-twins) in plagioclase, the morphology of the zircons, and also the hybridization of the xenoliths, ali point to the Structural setting The long axis of the feldspar crystallization of the rock from a granite melt. phenocrysts representing lineation in the granite trend The petrochemistry supports a magmatic on grn of the roughly E- W except along the margins of the granite body granite, crystallized between 650°C and 700°C. The pink where they follow the trends of the wall rock. The foliations granite is a late fraction of crystallization of the grey granite. or platy flow layers in the granite, along a N-S section dip A thennal aureole (about 400 meters wide) showing high 550N and 800S on the extreme northem and southem margin temperature and low pressure metamorphism can be traced of the granite body respectively. In the central part the flow around the Mylliem granite pluton. layers dip moderately towards N and S with local variations. The granite magma is believed to have intruded along the The orientations of the flow layers show that the granite trough of the tectonically controlled synclinal basin and pluton represent an inverted bowl with an undulating roof. exerted lateral pressure on the wall rocks, which in tum wrap The regional trend of the Shillong Group of rocks is around the granite body. Ali evidences c1earlysuggest that the NE-SW, dippíng moderately (350 -400 ) towards SE. But Mylliem granite is a post-tectonic pluton. nearer the granite , the trend of the rocks follow the margin of the granite body and from ali sides díp towards the granite . Acknowledgments The author is grateful to Dr. c.c. This confonn to the shape of a synclinal basin, trending E- W, Bhattacharjee, Professor of Geology, Gauhati University for the central part of which is occupied by the Mylliem granite. the trace element determinations. The slaty c1eavage in the phyllite, both on the N and S of

REFERENCES

CRA WFORD, A.R . - 1969 - lndia, Ceylon and Paki ston: New age data hills, Meghalaya. J. Geol. Soc.India, 26:356-359. and comparisons with Australia. Nature, 223:380-383. RAHMAN,S. & BORA H, A.K. - 1982 - Zirco ns from the Mylliem HIBBARD, M,J. - 1965 - Origin of some alkal i feldspar phen ocrysts and South Khasi granites and associated gneiss, Khasi hills, and their bearing on petrogenesis. Am. J. Sei.. 263:245-261 . Meghalaya. Quart , J . Geol, Min. Soe. India, 54:80- 87. HIGGINS. N.C . & KAWACHI, Y. - 1977 - Microcline megacrysts SMITH, I.V. - 1974 - Feldspar Minerais - ehemieal aOO textural from lhe Green Lake granod iorite , Eastem Fiordland, New prop erties. 2 ed., New York, Spri nger-Verlag , 690 p. Zealand, New Zealand J. Geol. Geophys., 20(2):273-286. STRECKEISEN, A. - 1976 - To each plutonic rock , its pro per name. KLEEMAN, A.W. - 1965 - The origin of granite magmas. J. Geol. Earth Sei. Rev.• 12(1):1-33. Soe . Aust•• 2(1):35-52. TURNER . F,J. - 1951 - Observations on twinning of plagiocl ase in ME DLICOTT. H.B . - 1869 - Geological sketch of lhe Shillong plateau metam orphic rocks. Am. Mineral., 36:58 1-5 89. in North-Eastem Bengal. Mem. Geol. Sur.India, 7(1):197-207. T UTTLE n F. & BOW EN, N.L. - 1958 - Origin of granite in the light NOCK OL DS, S.R. - 1954 - Average chemical composition of some of ..pen.. -ntal studies in lhe system NaAISi30a - KAIS i30a ­ igneous rock series . Bull. Geol , Soe . Am., 65:1007-1032. S'J2 - H20. . 'em. Geol. Soc.Am., 74,153 p. RAHMAN, S. - 1972 - Petrology of lhe xenoliths in lhe Mylliem WIN .LER, G.F. - 1976 - Petrogenesis of metamorphie rocks. 4 ed., granite, K & I hills , Meghalaya. J. AssamSci. Soc. , 15(1):76-81 . New York , Spn iger-Verlag, 334 p. RAHMAN, S. - 1981 - Petrology and petrochem istry of lhe Khasi greenstones occurring around Mylliem granite, Khasi hills , Meghalaya. BulJ./OO. Geol. Assoe. , 14(2):133-144. RAHMAN, S. - 1984 - The feldspars of lhe Mylliem granite, Khasi MANUSCRITO 505 hills, Meghalaya. J. Assam Sei. Soe., 26(2):43-49. Recebido em 04 dejunho de 1987 RAHMAN, S. - 1985 - Petrochemistry of lhe Mylliem granite, Khasi Revisão aceita em 29 de setembro de 1987