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Alkaline Rocks of Samchampi-Samteran, District Karbi-Anglong, Assam, India

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Alkaline Rocks of Samchampi-Samteran, District Karbi-Anglong, Assam, India Alkaline rocks of Samchampi-Samteran, District Karbi-Anglong, Assam, India S NAG 1, S K SENGUPTA 2, R K GAUR 1 and A ABSAR 1 1 Geological Survey of India, Lucknow 226 024, India 2 Geological Survey of India, Guwahati 781 021, India The Samchampi-Samteran alkaline igneous complex (SAC) is a near circular, plug-like body approximately 12 km 2 area and is emplaced into the Precambrian gneissic terrain of the Karbi Anglong district of Assam. The host rocks, which are exposed in immediate vicinity of the intrusion, comprise granite gneiss, migmatite, granodiorite, amphibolite, pegmatite and quartz veins. The SAC is composed of a wide variety of lithologies identified as syenitic fenite, magnetite -4- perovskite • apatite rock, alkali pyroxenite, ijolite-melteigite, carbonatite, nepheline syenite with leucocratic and mesocratic variants, phonolite, volcanic tuff, phosphatic rock and chert breccia. The magnetite -4- perovskite d: apatite rock was generated as a cumulus phase owing to the partitioning of Ti, Fe at a shallow level magma chamber (not evolved DI = O1). The highly alkaline hydrous fluid activity indicated by the presence of strongly alkalic minerals in carbonatites and associated alkaline rocks suggests that the composition of original melt was more alkalic than those now found and represent a silica undersaturated ultramafic rock of carbonated olivine-poor nephelinite which splits with falling temperature into two immiscible fractions--one ultimately crystallises as alkali pyroxenite/ijolite and the other as carbonatite. The spatial distribution of varied lithotypes of SAC and their genetic relationships suggests that the silicate and carbonate melts, produced through liquid immiscibility, during ascent generated into an array of lithotypes and also reaction with the country rocks by alkali emanations produced fenitic aureoles (nephelinisation process). Isotopic studies (51So and 513C) on carbonatites of Samchampi have indicated that the 5L~C of the source magma is related to contamination from recycled carbon. 1. Introduction 1992). The SAC is composed of a wide spectrum of lithologies identified in order of abundance as syenitic The Samchampi (26~ 93~ fenite, magnetite + perovskite =t= apatite rock, phos- (26~ 93~ alkaline igneous intrusion is phatic rock, ijolite-melteigite, carbonatites of varied a roughly circular, plug-like body, approximately dimensions, chert breccia, nepheline syenite, alkali 12 km 2 in area, cropping out in the Precambrian gneis- pyroxenite, phonolite and volcanic tuff (Nag et a11993). sic terrain of the Karbi Anglong district of Assam. The magnetite =t=perovskite ~ apatite rock (here- The alkaline rocks, in and around Samchampi- after called MPA-rock) occupies the central part of Samteran, are part of an alkaline province of the the complex and also in the western and southern Assam-Meghalaya belt (e.g. Samchampi, Barpung and parts. The ijolite-melteigite series of rocks occur as Jasra in Assam and Sung in Meghalaya; figure 1 inset). discontinuous dykes and veinlets along the peripheral These rock types indicate a major alkaline magmatic part of the complex with NW-SE, N-S, NE-SW and episode, largely in the Cretaceous (Chattopadhyay E-W trends in accordance with several major and and Hashmi 1984; Acharyya et al 1986; Kent et al minor joints traversing the area. The syenitic fenite Keywords. Alkali pyroxenite; ijolite-melteigite; carbonatite; phonolite; aegirine augite; aegirine; sodi-calcic amphibole; pyrochlore; monazite; phlogopite; scapolite; major element chemistry; 518 O and 513 C isotopic studies; liquid immiscibility; fractional crystallisation. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 108, No. 1, March 1999, pp. 33-48 Printed in India 33 34 S Nag et al + 4. 4- + SAMCHAMPI'I'm-~ + S0~ SC A L E N + + ;.~---~.<1- + m.250 0 250 5OO ?,~Om. r r ~ ~ 'I" 4" +/ ,~" F r~ + + + l / + ,' ' 4. + + + / + +~ ,IZ i.c .-"J ~ ~ ~ ~'"~.TJITIITr. + + + 4. _, ~'~ ,'~,, ".' "-- ,, . ~-~l/l',,~ + + 4. + . I X/ - "--- ,~Jl'~,, + + + + "e I /Y I" _~1 ",4 IX % / /(0/ -:". ! "~,IX + + .--~+ + § + ~ Al," - IE ,=, ""-+-J ~ 'k + + ~1 t I. -' ,~' q~'~ ~l I;, ~ \ ~ r ~, + I- .'~ II I .-. ./OF" ~ J"~V 4: ~ :,(;Fi \ r If, § 1. + ~ ~-~ /lY ~:' " ~ 4. , cU/,s I~/ + § , P t + .'~ __/2 ,~oo q"~ ~y/F F ) C/ (i) ~1, , + § IF f SAMTERAN ~ ,,F F .4"~ 11] ./'.~ 5"* 4. ;., ,1/t d.k~' ,'?t,'Y ~ / + + 4-~ ". z'l" ,~) ,';'~ ,,/~'4~r n + 4. 4- § F F F._~I / F.1: v, . - '# _-... " / ~.4l'r NS F / + + 4. F F x ,.,i"~ / /1~ / ~. \ - - ~ /~t ''~*'~ ,'-~ " F A/ill F / + 4' ~J~ ",7 " r . " ./I M, / § +-""" "'-2 " _,-rm~ ///r /rTr r/" 4. + +~,,, - ,f[LLL~r__ .s/ ,l~' :..v . , >_ .t _..,r,,llll', .-.. , _.r" + . + 4.- I,'_ f P '~lr ~'1 I I I I I I I I I/1 o/'-- .,- l;;" + + - ~ . ~xlO /1111111111J/ r t 4. 1. + ~ - . r ~ ~o /, A/" + § + "~,~ --_.' ./, ./ + + + ~"~-'~ / /"A ,.~"l + + 4. 4. u ~__-.~.>~ + + 1. I N D E X 1. + + I, -~- Ijolite MelteiQile Suite Carbonofite b Magnetite AP Rock ~ Pllonolite 20 O 20 40 (;em~l~0~ II,,Z|] m Syenillc Fenite ~ Volcanic Tuff q 5. ,g~. // Nogaon /. ~v~- /,1 P7 | /t/.u'$omr hoOr Guwahoti,..~ -- ~ / " / j / ~Foidal Syenite/NS -'] Granitic Finite l~ Chert- Breccia / ~'~ Phosphatic Rock /'@. p- / _ Shear-Zone --]Mostly Soll Covered Gronltold Gneiss With Minor Outcrops of ----'-J Syenite / Flnites ..,4o.o..-" Sung t Figure 1. Geologicalmap of Samchampi-Samteran alkaline complex, Karbi Anglong District, Assam, India. Inset: Locations of alkaline complexes in respect of major lineaments of Assam-Meghalayabelt. occupies the major part of the complex. Although a An area of about 12km 2 of alkaline rocks at majority of the area is hidden under thick alluvial Samchampi were mapped and field relationships and cover, yet excellent exposures of different litho units laboratory studies were used to establish: are discernible along intense drainage of the area, apart from a few exposures in high mounds The structural geometry of the complex. and hillocks. The lithologic variation within the complex and the Alkaline rocks of Samchampi, Assam, India 35 mineralogical and petrographic variations within pink granite gneiss, migmatite with enclaves of amphi- the various units. bolite and later intruded by granitoids, diorite, meta- The temporal relationship between the units. basics, pegmatite and quartz veins. The effect of The geochemical variations existing within the suite. fenitisation in gneisses is best developed near the contact with alkaline rocks. These data were used to develop a model for the The granite gneiss, under microscope, show pre- magmatic evolution of the complex. dominance of orthoclase, microcline, quartz, plagio- clase, biotite and hornblende. Magnetite, sphene and 2. Geological setting zircon occur as accessory constituents. Both ortho- clase and microcline show perthitic intergrowth. Exposures of amphibolite are rare in the area and The SAC is emplaced into Precambrian gneissic rocks only a limited outcrop (5.0 m • 2.0 m) was found about which are exposed as deeply eroded segments of Mikir 2.5 km SSW of Samteran village. Megascopically, the Hills. The gneissic complex is characterised by highly rock is hard, massive, coarse grained and well jointed. deformed granite gneiss, migmatite with enclaves of Under microscope the rock is essentially composed of amphibolite. These rocks are later intruded by basic hornblende, biotite, diopsidic augite, plagioclase with dykes and granitoids. In certain areas the enclaves in magnetite and sphene as accessories. Hornblende is gneissic rocks are represented by granulites. Intrusive dark green, associated with clinopyroxene and con- granitoid pluton with diverse shapes are reported in tains tiny magnetite inclusions. Marginal alteration of this terrain. hornblende to biotite is common. Biotite is strongly Linear fracture patterns along the Assam-Megha- pleochroic from green to greenish brown and inti- laya belt, directed NE-SW and ENE-WSW and mately associated with alkali feldspar which occur as subparallel to Kaliyani lineament and Sarhed fault are anhedral aggregates and often clouded with magnetite the loci of major alkaline emplacements, earthquake schillers. Plagioclase show partial replacement by K- activity, etc. The Kaliyani lineament, north of feldspar and contains trails of clinopyroxene which Samchampi, possibly merges with the deep-seated usually occur as stubby, subhedral grains often show- regional fracture (Barapani lineament) further to the ing alteration to aegirine. west of it and continues right up to Sung valley alkaline complex in a NE-SW direction. A set of subparallel mega-joints of moderate magnitude, directed NE-SW 3.2 Alkaline mafic rocks and NNE-SSW, occur to the south and east of Sung The assemblage alkali pyroxenite, melteigite and perhaps formed syntectonically with the large scale ijolite broadly constitute this clan. The outcrops of lineaments. The rocks of SAC display a number of sub- alkali pyroxenite are extremely limited at the present vertical to vertical joints of greater magnitude in E-W, level of erosion and their buried nature at depth can be N-S, NNW-SSE, NE-SW and ENE-WSW directions surmised as small patches and lenses of less than a and are in conformity with the regional mega-joints. meter in diameter are exposed within ijolite-meltei- Thus the entire stretch from Sung to Barpung (figure 1 gite. It also occurs in the tributary northwest of inset) had witnessed repeated tectonic activity a~s Ganjang, 1.5km southwest and south of Samteran evident from numerous lineaments and mega-joints of and also as xenoliths
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