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

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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, plagioclase, biotite and hornblende. Magnetite, sphene and 2. Geological setting zircon occur as accessory constituents. Both orthoclase 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 within carbonatites. moderate to large magnitude in mostly NNE-SSW, Under microscope, the alkali pyroxenite, in general, NE-SW, ENE-WSW and NNW-SSE directions, apart show hypidiomorphic granular texture and at places from deep-seated regional fractures such as Kaliyani exhibits poikilitic character. It is essentially composed lineament and Sarhed fault (Nag et al 1993). of aegirine augite (80-90%), biotite, phlogopite and The magmatic alkaline rocks of SAC particularly nepheline (10-5%) which often show alteration to occur in the eastern, northern and western margins turbid cancrinite. Aegirine augite is usually subhedral depicting an arcuate pattern without any appreciable to anhedral, commonly zoned with diopsidic core and dip component and thus possibly representing part of often alters to hornblende. Biotite is pleochroic in a ring dyke/cone sheet structure and mode of emplace- shades of green (chromium-rich), often bleached in ment at the nodal points of subvertical mega-joints central parts, phlogopite is usually pleochroic from (shear-joints). colourless to yellowish green and occurs in clusters, poikilitically enclosing the clinopyroxene. Calcite is ubiquitous in this rock, being mostly late-introduced. 3. Lithologic Spectrum--disposition and Apatite, sphene, magnetite and zircon form the petrographic resume accessories. Acmite is rarely developed in this rock. The ijolite-melteigites are usually associated with 3.1 Country rocks syenitic fenite in the peripheral part of the complex The host rocks, which are exposed in the immediate and form a partial screen between the fenites and vicinity of the alkaline complex, are represented by the country rocks. Megascopically, the rocks are 36 S Nag et al

Table 1. Volumepercentages of minerals from alkaline rocks of Samchampi, District Karbi Anglong, Assam, India. Serial Nos. 1 2 3 4 5 6 7 8 9 10 11 Clinopyroxene 45 72 85 63 55 57 63 11 28 15 17 Amphibole 7 ...... Biotite 31 7 - - 8 8 ..... Alkali feldspar - 16 - - 25 - 12 79 65 80 82 Nepheline 3 5 12 18 7 33 21 8 4 4 - Apatite 1 1 2 - 2 1 - 1 2 1 1 Sphene 2 .... 1 - - 1 - - Calcite - - 1 9 1 1 1 .... Opaque 11 1 1 - 2 - 3 .... Serial Nos. 12 13 14 15 16 17 18 19 Olivine ..... 5 2 17 Clinopyroxene 14 22 22 13 11 1 - - Amphibole ...... Biotite ..... 6 - 1 Alkali feldspar 56 62 75 61 80 - - - Nepheline 13 15 3 ..... Apatite 1 - 1 - 1 5 1 1 Sphene ...... 1 Calcite 17 .... 71 79 80 Opaque - - - 1 - 2 4 1 Others ..... 11 8 - 1-2: Alkali pyroxenite; 3-5: Melteigite; 6-7: Ijolite; 8-12: Syenitic fenite; 13-16: Nepheline syenite; 17-19: Carbonatite. medium-grained, light green to dark green in appear- are common because of fenitisation by late carbona- ance. Numerous veins of carbonatite and syenite are titic (sovite) liquid, the details of which are discussed seen to cut across these rocks, resulting in a hybrid later on. Calcite usually occurs as veinlets and in the variant rich in carbonate. It has been observed that intergranular spaces, being late in the crystallisation the melteigite grades to ijolite and then to syenite. history. Under microscope, the melteigite consists essen- The petrographic nomenclature alkali pyroxenite, tially of clinopyroxene (diopsidic augite to aegirine melteigite and ijolite are amply justified as can be augite constituting about 75% of the total volume, verified through Nepheline-Mafics-Alkali feldspar table 1) with the average grain dimension ranging (NMA) diagram (Streckeisen 1967). The alkali pyroxe- from 0.5 to 0.40 mm. The other essential constituent is nite, melteigite and ijolite carrying notable amounts nepheline which mostly occurs as large, anhedral of alkali feldspar (table 1) are plotted in j acupirangite, grains subophitically enclosing clinopyroxene. Theo- ijolite, feldspar-bearing melteigite and alkali gabbro retically, alkali feldspar is absent in ijolite-melteigite fields respectively (figure 2). rocks, but they can carry a notable amount of alkali feldspars (up to 10% by volume) as has been noted 3.3 Magnetite i perovskite • apatite (MPA ) rock from igneous alkaline rocks of Eastern Ontario (Appleyard 1974), ijolitic complexes of East Africa The present work revealed four isolated bodies of this (LeBas 1977) and also in ijolite-melteigites from lithotype within the complex (figure 1). Megascopi- Proterozoic alkaline complex of Elchuru, Andhra cally, the rock is hard, massive, very coarse grained Pradesh (Nag 1983; Nag et al 1984). A similar feature and deep chocolate brown/dark brown in colour. It is has been noted in ijolite-melteigites of Samchampi predominantly composed magnetite with accessory (table 1). The K-feldspar phase present in melteigite perovskite, apatite and phlogopitic biotite. Ore mine- usually encloses aegirine augite. Apatite, calcite and ragraphic studies reveal the dominance of magnetite, magnetite form the accessories. often martitised along with laths and lamellae of The ijolite, west of Samteran, shows hypidiomor- hematite which are produced as secondary alteration phic granular texture and at places develops poikilitic after magnetite. texture. It consists essentially of aegirine and aegirine augite which together constitute about 60% by volume 3.4 Granitic fenite (table 1). The clinopyroxenes are strongly pleochroic, twinned and commonly zoned. Nepheline, being an Small isolated outcrops of this lithotype are discern- essential constituent, occurs in intergranular spaces ible within the complex as shown in figure 1. The rock and poikilitically enclosed in clinopyroxene. Sphene shows imprints of deformation and silicification and and magnetite occur as accessories. Phlogopitisation correspond to quartz syenite in composition. In gene- along fracture and grain boundaries of clinopyroxenes ral, the rock is coarse to medium grained, leucocratic Alkaline rocks of Samchampi, Assam, India 37

20 80

Juvite 60 40

Molicjnite 40 ,,o ,

30 70 Nepheline Syenite

80 el3 _ A3//~/,,b"-~q" ~/--..-_~ ~,e'/ Melanoc, ratic malignite ~ol2

~-- ~/ e2 Alkali (jabbro "/v ~ v v v v v ~, \~ v \/ v v, V / 2o 3o 40 5o 6o / ro so 9o Jacupirangite / Me$ocraticMe$ocr( nepheline-beoring $yeni~ /---Melanocratir nepheline- beoring syenite

Figure 2. Nepheline(N) - Mafics(M) - Alkali feldspar (A) diagram (Streckeisen 1967) illustrating modal variation plots for the alkaline rocks of SAC. Explanations same as in table 1. and shows relict gneissosity similar to country rock and often occurs as inclusion in K-feldspars, apart from granite gneisses. Under microscope, it is essentially a few independent stubby crystals. Biotite, mostly comprised of K-feldspar (mostly clouded and turbid stubby, subhedral are distinctly pleochroic from light microperthitic orthoclase), quartz, albite, diopsidic brown to green (chromian biotite). augite, aegirine augite and biotite. Quartz is mostly anhedral, often fractured. The dominant clinopyroxene 3.5 Alkaline felsic rocks phase identified as diopsidic augite shows weak pleochroism in shades of green and often changes to Syenitic fenite and nepheline syenite (colour index = aegirine at the margins. Development of aegirine augite 12-32) are considered under this clan. The syenitic in granitic fenite is due to the influx of alkali (Na >K) fenite, being the predominant rock type of the during metasomatic processes as both aegirine and complex, occurs intermittently along the margins of aegirine augite are not essential constituents of the the complex and also in the inner part of it. The rock, granitic country rock. Aegirine augite is strongly pleo- on megascopic characters, appears to be leucocratic, chroic in green (X = deep green, Y = greenish yellow, coarse grained and often containing streaks and Z = green to light green; X > Z > Y; Z A c = 50-56 ~ clots of mafics. It shows predominance of orthoclase 38 S Nag et al microperthite, aegirine augite, aegirinc and albite in accordance with several major and minor joints along with sphene, apatite, magnetite and monazite traversing the area. Two types of carbonatite occur at as accessories. The alkali feldspar phase is often SAC. The older, on megascopic characters, can be altered and encloses aegirine augite. The presence of differentiated into tiny, subhedral albite crystals, in some cases, indicates (1) mesocratic type, mostly coarse grained crystalline subsolidus colling history. The rock, in general, shows with impersistent mafic-rich bands, often contain- inequigranular porphyritic texture. ing small rounded xenoliths of alkali pyroxenite Refenitisation of syenitic fenite by carbonatite (marie digested carbonatite) and magma has been noted from the rocks exposed to the (2) predominantly leucocratic, coarse grained, mas- north of Samteran. Under microscope it generally exhi- sive rock showing homophanous nature as the bits porphyritic texture. Fine aggregates of orthoclase, mafics have drawn out appearance. The second microcline, nepheline and calcite usually form the generation carbonatite is mostly in the form of groundmass and encloses megacryst of orthoclase veins cutting across the carbonatites of earlier microperthite, suggesting that the megacrysts predate parentage, as observed in the Langbrik river, the deformation and metasomatism which produced south of the Samchampi village. the fabric of the groundmass. Refcnitisation of earlier formed fenites by carbonatite magma suggests poly- On the basis of essential mineralogical constituents phase intrusion and different stages of fenitisation in the first generation carbonatite can be categorised as the area (Sengupta et al 1996). These rocks are (i) olivine-phlogopite sovite and (ii) sovite with pyro- distinctly peralkaline as evidenced from the presence chlore. The second generation carbonatite is grouped of aegirine and normative acmite. as olivine-biotite sovite. Isolated outcrops of nepheline syenite (hypersolvus) Under microscope the sovites are holocrystalline occur near northeast of Ganjang and southwest of with dominant calcite, phlogopite, olivine, aegirine Samteran (figure 1). On megascopic characters, the augite, biotite, apatite with minor magnetite, pyro- rock is usually leucocratic, coarse grained and massive. chlore, monazite and sphene. Calcite occurs as sub- Under microscope, it is composed essentially of ortho- hedral to euhedral interlocking grains, often clase microperthite, aegirine augite, nepheline with intergrown with phlogopite, poikilitically enclosing minor albite and arfvedsonite. Phlogopite, titano- pyroxene, olivine and apatite..The calcite is often magnetite, apatite and scapolite occur as accessories. ferroan in nature and have exsolved lamellae and In some braided type microperthites, the exsolved patches of dolomite (as evident from staining with sodic blebs appear as the tortuous nature of segrega- Alizarin Red-S and potassium ferrocyanide solutions). tion from the potassic host at a certain stage. Excellent Trails of ferromagnesian minerals such as pyroxene, developmeY~t of sodic rims in contact with nepheline is phlogopite, biotite and magnetite impart a crude the result of the ultimate stage of unmixing at a foliation to the rock. Apatite is stubby, anhedral to relatively low temperature and these sodic blebs are subhedral and sometimes with aggregates of smaller bodily connected with the microperthites. grains. Phlogopite is pleochroic from colourless to light The leucocratic variant of nepheline syenite is brown and shows bent cleavages and microfolds. exposed in the Langbrik river near Samteran where it Biotite is strongly pleochroic from brown to dark intrudes into the fenitised ijolite. Megascopically, the brown (Fe-Ti rich) and often replaces clinopyroxene. rock is grey, coarse grained and composed essentially Pyrochlore is deep cinnamon brown and feebly of orthoclase. Similar rock has also been recorded in isotropic. Carbonatite, occurring close to alkaline tributary drainage to the west of Samteran, indicating mafic rocks, contain xenocrysts of hornblende and southeasterly extension of the same. Under micro- olivine. scope, the rock shows the presence of stubby, subhedral orthoclase commonly altered to kaolin and 3.7 Phosphatic rock and chert breccia constituting 60-80% of the rock by volume. Nepheline usually occurs as large anhedral to subhedral grains, Four isolated bodies of phosphatic rock are exposed often altered to cancrinite and subophitic relationship at (1) Samteran, (2) West of Samteran, (3) NE of with orthoclase. Minor amount of aegirine occurs as Ganjang and (4) East of Ganjang (figure 1). In discrete laths. Tiny grains of pyrite and magnetite general, the rock is yellowish grey with a pronounced form the accessories. Calcite occurs as thin veins development of the oolitic and pisolitic structures mostly late crystallised, a feature commonly found in of chert and phosphatic material. Fragments of alkaline complexes (Heinrich 1966). chert and veins of secondary silica in phosphatic matrix are often present. Kumar et al (1989) reported as high as 38% P205 in this rock. The phosphatic 3.6 Carbonatite rock, being a member of diverse litho-assemblage This lithounit commonly occurs as discontinuous of SAC, is presumably a product of secondary dykes and veinlets with NW-SE, N-S, NE-SW trends enrichment as a result of widespread hydrothermal Alkaline rocks of Samchampi, Assam, India 39

activity which is a part of the alkaline magmatic associated with chert and shows crude banding with episode in this terrain. pyrite. Bouldery outcrop of chert breccia is found to occur Under microscope it exhibits phenocrystal apatite, in the southeastern margin of the complex. This magnetite, plagioclase, K-feldspar, biotite and vermi- breccia zone, intermittently exposed over a length of culite set in a groundmass of plagioclase microlites, 2 km in NE-SW direction is in conformity with the actinolite (pleochroic from colourless to pale greenish, linear fracture pattern which is subparallel to Kaliyani mostly as slender needles and elongation parallel lineament, shows an average width of 10m and extinction --10-13~ apatite, K-feldspar and glass represents a fault brecciated zone. Based on mega- (partly devitrified). Both apatite and magnetite occur scopic characters the rock can be described as very in equal amounts and largely dominate over the other hard, compact, greenish grey to wheatish grey, mostly phenocrystal phases. Swerving of groundmass consti- fine grained and at places containing fragments of tuents around phenocrystal grains are common. chert, quartz and feldspar. Under microscope, it Apatite, magnetite along with groundmass material consists essentially of quartz and chalcedony along show strong preferred orientation defining flow struc- with fragments of chert in cryptocrystalline ground- ture. Ore mineragraphic studies reveal the presence of mass. Quartz grains are often fractured and shattered pyrite, magnetite and a few grains of idiomorphic indicating that they have undergone deformation. uraninite. Pyrochlore has also been recorded as an accessory constituent. 4. Geochemistry 3.8 Phonolite and volcanic tuff Table 2 shows the average chemical analyses of A small outcrop (5.0m x 0.30m) of phonolite is carbonatite and associated alkaline rocks of Sam- exposed in the northern periphery of the complex, champi along with some selected average values from about 1.75km NNW of the Samteran-Samchampi other areas for comparison. The major elements were foottrack. Megascopically the rock is greenish, hard, analysed by rapid methods (Shapiro and Brannock massive, fine grained and possesses randomly oriented 1962). Total alkalies were determined by flame feldspar phenocrysts. photometric techniques. The analytical work was Under microscope, the rock exhibits fine grained carried out at the Chemical Laboratory, Geological porphyritic texture and essentially consists of pheno- Survey of India, Shillong. Altogether five selected crystal sanidine, aegirine augite, aegirine, alkali samples of carbonatites were analysed for oxygen and amphibole and nepheline set in a fine grained ground- carbon isotopic studies at the Physical Research mass of nepheline, aegirine, iron oxides, calcite and Laboratory, Ahmedabad. The details of analytical leucite. The coexistence of leucite, alkali amphibole techniques in measuring 5180 and 513C has been and their stability relations are mentioned in sub- discussed elsewhere (Sengupta et al 1997). sequent discussion. Aegirine augite and aegirine The chemical analyses and petrographic characters usually occur as stubby, subhedral to euhedral of the rocks of SAC indicate that there exists a wide crystals, strongly pleochroic in shades of green and range of heterogenity in composition of alkaline mafic often zoned. Sodicalcic amphibole phase present in rocks, syenitic fenite, carbonatite and other rock types. this rock is strongly pleochroic from brown to reddish Excepting the ijolite, MPA-rock and mafic digested brown, commonly zoned and can be termed as carbonatite, all other rock types have agpaitic coeffi- barkevikitic amphibole (Leake 1978), although wide cient less than unity and hence designate a miaskitic variations in pleochroic colours occur owing to wide character. variations in chemical compositions (Deer et al 1963). From table 2 it appears that total alkalies (Na20 + Nepheline occurs as microphenocrysts, mostly anhe- K20) do not reveal any marked difference between dral to subhedral and at places rimmed by cancrinite alkali pyroxenite and melteigite, whereas from mel- (owing to marginal alteration). Leucite present in this teigite through ijolite to syenitic fraction an increasing rock shows occasional development of fine lamellar trend is discernible. Although imprint of widespread twinning and its characteristic association with fenitisation is evident, yet the geochemical variations aegirine augite, aegirine (and also nepheline) distin- that exist within the complex has been deciphered guishes from analcite which is paragenetically asso- against D.I. which show a more or less increasing trend ciated with Ti-augite, nepheline and leucite (see from alkaline mafies to syenitic fractions. Excepting a Pichler and Schmitt-Riegraf 1997). Perovskite occurs few, a majority of the rocks show higher K20/Na20 as an accessory constituent in this rock. ratios (cf. table 2). The alkali-lime index (Peacock A small isolated outcrop of volcanic tuff is exposed 1931) of the complex is 47 indicating its strongly about 1 km SW of Ganjang. Megascopically, the rock alkaline nature. is grey, fine grained and consists of phenocrystal mag- Plots of SiO2 against D.I. reveal scattered plots netite, mica and felsic minerals. The rock is intimately (figure 3A) yet a trend can be established from alkaline 40 S Nag et al

Table 2. Chemical analyses of carbonatites and associated alkaline rocks of Samchampi, District Karbi A nglong, Assam, India.

Serial Nos. 1 2 3 4 5 6 7 8 9 10 11

SiO2 44.09 45.20 43.63 53.66 52.48 55.95 26.98 5.48 3.52 35.64 53.19 TiO2 1.00 0.90 1.25 1.27 0.98 0.45 0.80 0.46 1.62 1.00 1.00 A1203 14.90 3.20 13.45 9.12 16.80 14.85 0.30 0.50 5.21 12.80 13.40 Fe203 10.89 5.80 5.38 10.06 3.65 5.90 8.43 3.92 72.64 3.74 4.00 FeO 0.70 5.91 2.75 3.27 3.67 1.91 0.17 2.41 1.01 1.36 1.12 MnO 0.12 0.38 0.11 0.05 0.11 0.07 0.12 0.20 0.17 0.02 0.23 MgO 4.48 14.80 8.04 3.47 4.11 4.26 13.28 5.63 2.80 4.98 9.16 CaO 20.27 16.58 11.33 13.02 6.07 7.02 30.84 46.32 3.58 19.47 15.30 Na20 1.93 1.62 4.35 2.03 4.48 3.68 0,39 0.13 0.05 4.79 0.63 K20 2.05 2.51 4.21 2.13 4.66 2.94 2.21 0.16 0.08 4,92 0.67 H20 + n.d. 0.71 1.74 0.97 n.d. n.d. 2.18 1.96 2.52 3.55 n.d. H20- 0.14 0.I0 0.43 0.50 n.d. n.d. 0.43 0.10 n.d. n.d, n.d. P205 0.10 0.62 0.40 0.10 0.17 0.65 0,75 0.90 1.55 Tr Tr CO2 n.d. 0.84 1.84 n.d. n.d. n.d. 15.00 31.45 n.d. 0.54 Tr LOI .... 1.63 1.55 - - 5.38 6.71 1.48 Total 100.67 99.17 98.91 99.65 98.81 99.22 99.40 99.62 100.0 99.52 99.97 D.I. 25 20 44 44 60 55 04 01 45 19 A.I. 0.02 1.71 0.87 0.63 0.74 0.62 9.83 0.80 2.60 1.02 0.13 Na20+K20 3.98 4.13 8.56 4.16 0.14 6.62 2.60 0.29 0.13 9.71 1.30 K20/Na20 1.06 1.55 0.97 1.05 1.04 0.80 5.66 1.23 1.66 1.03 1.06

CIPW Norms

Q - - - 13.50 - 7.23 .... 9.85 Or 12.23 - 24.12 12.79 27.80 17.24 - - 0.44 - 3.96 Ab 7.86 1.36 0.55 17.29 25.33 30.92 - - 0.42 - 5.33 An 25.85 - 4.73 9.17 11.95 15.43 - - 1.95 - 31.76 Lc ...... 1.28 - - 21.17 - Ne 4.54 0.40 19.58 - 6.73 .... 20.83 - C ...... 4.32 - - Kp - 8.52 ...... 1.18 - Ac - 9.94 .... 2.91 - - 1.84 - Wo ...... Di 24.19 56.25 25.45 29.58 13.74 16.97 27.18 - - - 35.13 Hy ...... 6.52 - O1 - 12.57 6.26 - 4.16 - 14.98 - 4.90 8.89 - Mt 2.78 3.48 7.89 10.67 5.34 5.08 3.52 - 3.71 1.55 0.78 Hm 8.96 - - 2.72 - 2.42 - - 69.12 2.04 3.54 Ilm - 1.67 - - 1.82 0.85 1.52 - - 1.90 1.90 Ap 0.34 1.48 1.00 0.33 0.40 1.56 1,78 - 3.70 - - Ct - 1.90 4.20 - - - 35.25 - - 1.23 - Sp 1.63 - 2.18 2.18 .... 2.72 - - Dicalcium silicate 12.21 ..... 4.73 - - 28.84 -

Serial Nos 12 13 14 15 16 17 18 19 20 21 22

SiO2 40.40 37.92 38.65 56.97 50.12 54.49 55.70 0.29 0.37 0,69 n.f. TiO2 2.83 4.53 3.52 0.83 2.31 1.41 1.50 0.02 0.05 0.49 0.19 A1203 4.02 8.39 10.46 20.47 16.05 19.01 17.66 0.09 0,39 0.31 1.09 Fe203 4.05 7.04 6.52 2.52 3.59 2.52 2.82 0.39 0.82 1.46 3.90 FeO 11.35 8.28 6.94 1.31 5.18 2.41 3.15 1.28 2.83 3.14 9.30 MnO 0.23 0.22 0.23 0.06 0.17 0.10 0.16 0.45 2.11 0.64 4.20 MgO 23.61 9.15 7.03 0.57 2.82 1.03 1.39 16.19 4.81 1.83 10.22 CaO 10.81 15.94 15.62 1.13 5.46 3.02 3.00 32.54 45.46 48.44 28.00 Na~O 0.59 3.00 4.47 9.14 5.45 6.91 6.69 0.56 0.42 0.26 0.15 K20 0.26 2.40 2.59 6.25 6.47 7.69 6.71 0.02 1.55 0.10 0.03 H20 + ...... 1.00 2.07 0.23 H20- - ...... P205 0.18 1.89 2.93 0.16 0.59 0.23 0.30 7.39 2.01 3.37 1.59 CO2 ...... 39.21 36.01 28.63 LOI 1.67 1.04 1.02 0.59 1.79 1.48 0.92 40.76 - - 3.00 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 99.98 101.0 98.81 90.43 D.I. 06 26 35 93 69 85 83 .... A.I. 0.32 0.90 0.97 1.06 1.00 1.04 1.03 10.45 5.87 1.70 0.25 Na20+K20 0.85 5.41 7.06 15.39 11.92 14.60 13.40 0.58 1,97 0.36 0.18 K20/Na20 0.44 0.80 0.58 0.68 1.19 1.11 1.00 0.06 3.69 0.38 0.20 (Continued) Alkaline rocks of Samchampi, Assam, India 41

Table 2. (Continued.)

Serial Nos 12 13 14 15 16 17 18 19 20 21 22

CIPW Norms

Q ...... Or 1.67 7.02 11.11 36.93 38.36 45.59 39.47 Ab 3.70 - - 25.16 13.30 14.34 25.67 An 7.37 2.22 0.83 .... Lc - 5.61 3.06 .... Ne 0.74 13.77 20.45 24.56 17.78 21.76 15.05 Kp ...... Ac - - - 6.03 - 3.23 2.77 Wo - - - 0.23 - - - Di 28.63 38.82 35.45 7.84 12.58 6.87 6.79 Hy ...... O1 45.32 10.69 6.44 - 5.65 1.88 3.35 Mt 5.80 10.21 9.51 0.63 5.10 2.09 2.78 Hm ...... Ilm - - - 1.58 - - - Ap 0.33 4.37 7.06 0.38 1.34 0.67 0.67 Ct ...... Sp 4.26 7.75 5.98 - 3.94 2.45 2.58

Serial Nos 23 24 25 26 27 28 29 30 31 32 33

SiO2 2.56 5.67 3.34 3.36 2.00 44.20 43.28 37.29 41.90 45.10 42.50 TiO2 0.50a 0.50 0.20 0.30 0.19 0.80 0.92 3.46 2.21 0.85 1.41 A1203 0.25b 1.77 0.72 1.69 0.66 15.02 13.47 14.41 12.20 6.46 18.46 Fe203 3.96 7.59 2.97 6.13 2.44 6.99 8.41 4.23 6.41 6.76 4.01 FeO - - 1.03 2.99 2.06 2.93 2.00 6.10 4.32 8.82 4.19 MnO 0.61 0.78 0.27 0.31 0.29 0.14 0.12 0.32 0.22 0.36 0.20 MgO 1.17 6.10 0.64 3.10 2.25 9.81 12.15 4.15 5.45 7.17 3.22 CaO 47.70 37.06 50.10 44.35 48.72 3.00 2.72 13.69 16.60 16.80 11.38 Na20 0.15a 1.09 0.88 0.04 0.02 7.75 4.82 5.61 5.10 3.63 9.55

K20 - 0.87 0.26 0.50 0.32 1.82 3.31 4.22 2.66 0.66 2.55 H20 + - - 1.66t 0.30t 0.20 3.30t 2.98t 0.69 0.87 0.75 0.55 H20- - - - 0.04 - - - P205 2.97 1.73 1.68 3.26 3.38 0.44 0.47 0.77 1.24 0.33 1.52 CO2 38.29 32.16 35.64 32.80 36.22 4.20 5.20 3.29 0.82 1.30 0.38 LOI - 1.42 ...... Total 98.16 96.74 99.39 99.13 98.75 100.4 99.85 98.27 100.0 99.01 100.0 D.I. - .... 59.5 52 44 37 25 58 A.I. - 1.55 2.39 0.35 0.57 0.98 0.86 0.96 0.92 1.04 1.00 Na20+K20 - 1.96 1.14 0.54 0.34 9.57 8.13 9.83 7.76 4.29 12.10 K20/Na20 - 0.80 0.30 12.50 16.00 0.23 0.69 0.75 0.52 0.18 0.27

CIPW Norms

Q ...... Or 10.56 19.46 3.89 6.70 3.89 10.00 Ab 29.41 23.45 - - 10.15 - An - 5.28 3.89 2.50 - - Lc - - 16.57 7.40 - 3.90 Ne 19.56 9.44 23.28 23.30 10.40 43.70 Kp ...... Ac .... 0.92 - Wo - - 3.13 - - 18.50 Di 15.12 14.92 22.46 41.50 57.29 10.40 Hy ...... O1 0.35 - - - 1.04 - Mt 7.66 4.41 1.86 8.10 9.28 5.80 Hm 1.76 5.28 2.88 0.80 - - Ilm 1.52 1.67 6.54 4.30 - 2.70 Ap 1.01 1.01 2.02 2.90 0.67 3.60 Ct 9.50 11.80 7.50 1.80 2.90 0.90 Sp .... 1.50 - (Continued) 42 S Nag et al

Table 2. (Continued.) Serial Nos. 34 35 36 37

SiO2 44.01 39.07 40.29 47.01 TiO2 2.38 3.86 2.90 1.22 A1203 15.25 12.82 11.32 17.53 Fe203 6.61 8.75 4.87 4.80 FeO 3.39 6.39 7.69 2.80 MnO 0.16 0.26 0.22 0.24 MgO 3.50 6.14 13.28 1.65 CaO 16.36 14.28 12.99 6.91 Na20 7.10 4.09 3.14 8.68 K20 3.10 2.07 1.44 4.36 H20 + 0.64 1.59 1.08 2.55 H20- 0.07 - - 1.03 P205 0.88 0.76 0.78 0.36 CO2 0.56 - - 0.69 LOI .... Total 100.0 100.0 100.0 99.41 D.I. 47 30 28 71 A.I. 0.99 0.70 0.60 1.08 Na20+K20 10.20 6.16 4.58 13.04 K20/Na20 0.44 0.51 0.46 0.50 CIPW Norms

Q .... Or 0.69 9.31 1.57 25.57 Ab - - 12.51 6.16 An 0.56 10.56 6.54 - Lc 13.84 2.29 - - Ne 32.38 18.74 13.63 32.45 Kp .... Ac - - - 6.47 TWO 29.81 - -- 9.86 Di - 33.72 29.77 7.55 Hy .... O1 6.06 2.65 21.19 - Mt 3.94 12.76 6.96 3.71 Hm 4.16 - - - Ilm 4.50 - - - Ap 2.02 1.68 1.68 0.67 Ct 1.30 - - 1.60 Sp - 6.53 4.90 2.04

n.d.: Not determined. Tr.: Traces. n.f.: Not found. a: average of 4 analyses. b: average of 5 analyses. t: total H20 + with normative H1-0.59, Th-0.43 and Pr-4.48. D.I: Differentiation Index (normative Q§247247247 A.I: Agpaitic Index (Na20§ mol. Prop. 1-11: Alkaline rocks of Samchampi-Samteran, District Karbi Anglong, Assam. 1: Alkali pyroxenite. 2: Average 2 analyses melteigite. 3: Average 2 analyses ijolite. 4: Average 3 analyses melasyenite. 5: Average 5 analyses syenite. 6: Average 4 analyses granitic fenite. 7: Mafic digested carbonatite. 8: Average 5 analyses carbonatite (sovite). 9: Average 4 analyses MPA-rock. 10: Phonolite. 11: Volcanic tuff. 12--19: Alkaline rocks of Juquia, Sao Paulo, Brazil (Beccaluva et al 1992) 12: Average 3 analyses olivine clinopyroxenite. 13: Average 2 analyses melteigite. 14: Average 2 analyses ijolite. 15: Phonolite. Alkaline rocks of Samchampi, Assam, India 43

16: Average 4 analyses nepheline syenite of Group I (least differentiated variety). 17: Average 4 analyses nepheline syenite of Group II (differentiated variety). 18: Average 2 analyses nepheline syenite of Group III (most evolved). 19: Average 4 analyses carbonatite. 20-22: Carbonatites of Tomtor, Eastern Anabar region, Yakutia (Bagdasarov 1997) 20: Early carbonatites. 21: Pyrochlore-bearing carbonatite. 22: Fe-dolomite-carbonatite. 23: Average 8 analyses carbonatite (sovite) from Amba Dongar, Gujarat, India (Viladkar and Wimmenauer 1992). 24: Typical carbonatite (Gold 1966). 25: Average 3 analyses sovite from East African occurrences (LeBas 1977). 26: Sovite from Hydro's quarry, Fen area, Norway (Brogger 1921 in Barth and Ramberg 1966). 27: Sovite from Cappelen's quarry Fen area, Norway (ibid.) 28: Average 4 analyses nephelinite from Amba Dongar, Gujarat, India (Viladkar 1984). 29: Average 2 analyses phonolitic nephelinite from Amba Dongar, Gujarat, India (ibid.). 30: Fine grained ijolite, Magnet Cove, Arkansas (Erickson and Blade 1963). 31: Average 9 analyses melteigite (Nockolds 1954) 32: Melteigite (nephelinised pyroxenite) from Siroko, Budeda (Sutherland 1966). 33: Average 11 analyses ijolite (Nockolds 1954). 34: Average 41 analyses ijolite (LeBas 1977). 35: Average 8 analyses nephelinite (Nockolds 1954). 36: Average 21 analyses olivine nephelinite (ibid.). 37: Average 14 analyses nephelinite (LeBas 1977).

mafic rocks to syenitic rocks. The silica-total alkali D.I. = 01 signifying that the rock was not evolved as relationship shows that from alkali pyroxenite, ijolite- compared to the alkaline mafic rocks (D.I. = 20-44) melteigite to syenitic fractions, SiO2 sharply increases and syenitic fractions (D.I. = 44-60). (table 2) but the ratios (Na20+K20)/SiO2 and The first order chemical variance involving A1203/SIO2 are too low and thus undersaturation CaO + Fe203 and MgO + FeO (Bailey 1974) indicates occurred in respect of silica as evidenced from the that the melteigite and ijolite of SAC bear a more or presence of normative leucite, nepheline, acmite and less close resemblance with that of Juquia, Brazil olivine. The presence of modal aegirine and normative (Beccaluva et al 1992), Siroko (Sutherland 1966) and acmite suggests the peralkaline character of the rocks. average values of Nockolds (1954) and LeBas (1977). Figure 3(B) shows that the nature of alumina When total alkali content of the alkaline rocks of SAC variation is highly scattered but an increasing trend are plotted against SiO2 following Currie (1976), it is can be established from melteigite to ijolite to syenitic clear that the alkali pyroxenite and melteigite fall fractions. As the rate of cooling is related to the within the alkali basalt family as that of olivine clino- crystallisation of Al-poor mafics in the younger pyroxenite of Juquia (Beccaluva et al op cit) and mel- members, the slight increase in A1203 is reflected in teigite from Siroko (Sutherland op cit) whereas the modal increase of alkali feldspar and nepheline in ijolite fall within the nephelinite family. The fine syenitic fractions. The melasyenite shows lower con- grained ijolite of Magnet Cove, Arkansas (Erickson centration of total alkalies (average 4.16%) as and Blade 1963) ijolite-melteigite of Juquia and compared to the later fractions (average 9.14%). average ijolites of Nockolds (op cit) and LeBas Figure 3(C) depicts that the nature of total alkali (1977) fall within the nephelinite family along with variation increases from alkaline mafic rocks to the average nephelinites of Nockolds (1954), LeBas syenitic fractions. (1977) and from Amba Dongar, Gujrat (Viladkar The influence of MgO, CaO and combined iron 1984). While the melasyenite and volcanic tuff fall in (FeO + Fe203), when plotted against D.I. (figure 3D, the field subalkaline rocks, the syenitic rocks fall E and F) show depletion in amount with fractionation within alkali basalt family because of significantly indicating that the residual melt became enriched in lower concentration of total alkali content. The phono- total alkalies at the expense of lime and ferromagne- lite and nepheline syenites of Juquia, Brazil (Becca- sian oxides and this is clearly reflected in the modal luva et al 1992) fall within miaskitic syenite family (vol%) depletion of mafic minerals in the late crystal- whereas the phonolite from Samchampi is plotted in lised syenftes. Calcite is an ubiquitous accessory the nephelinite family (figure 4). constituent in almost all the alkaline rocks of SAC, except the carbonatite where it occurs in profusion. The alkali pyroxenite and volcanic tuff contains 5. Discussion appreciable normative anorthite. The early formed cumulate represented by MPA- Petrographic studies of varied lithotypes of SAC rock is also undersaturated in respect of silica as reveal that the pyroxene phase in alkaline mafic rocks evident from the presence of normative olivine. It has are diopsidic augite Cat core) rimmed by aegirine 44 S Nag et al

m a. 30 (B) 4- l 2O 5 O I 6 2 I 4 5 II ,oJ ~ 3 60 F) 3e,lO oil '~10 o ,o e4 0 < ,9 2 @ I 0 .7 40 (E) 17

(A) Oo 20 it olO o Ilee2 e4 6O *s.5 II 5, =605 I 0 , 2 el 5 40 el0 T 40 (0) o e7

o 20 2 20[, 9 am e"f =E il 5e .' 4.,o e .5 0 L ! t I l ! , 1 0 ,9 i | t I I i 20 40 60 20 40 60 D.I. ~.-.- O I. L

Figure 3. Diagram showing variations of different oxides as a function of D.I. for the alkaline rocks of Samchampi. Explanations are the same as in table 2. augite whereas in phonolite, syenitic fenite and nephe- rocks (alkali pyroxenite, melteigite and ijolite), as line syenite the same is represented by aegirine augite evident from their detailed petrographic studies and aegirine indicating progressive alkali enrichment. (mentioned earlier), can be explained as due to Mg- Amphibole (hornblende/barkevikite and arfvedso- Fe ratio of the pyroxenes in the host rocks during K- nite), likewise shows alkali enrichment from a domi- rich fenitising fluids from sovitic carbonatite magma. nant ealcie type. Two types of biotite occur: The Mg-content of the pyroxenes in these rocks coupled with the Mg-content from the intruding (1) a green variety having colourless to dark green sovitic carbonatite (with average 5.637o by weight pleochroism (chromian biotite) and MgO) favoured the phlogopitisation of pyroxenes by (2) a brown variety which is pleochroic from straw K-metasomatism. It is noteworthy to mention that yellow to brown/dark brown (Fe-Ti rich). the clinopyroxene phase (mostly aegirine augite) in Alkali feldspar (micro-to crypto-perthitic orthoclase association with abundant K-feldspar in syenitic and microcline microperthite) and nepheline are the fenite does not show development of phlogopite after dominant felsic phases present in the alkaline rocks. pyroxene and or K-feldspar even during refenitisation Olivine commonly occurs in carbonatites and is rare in of fenites by carbonatite magma, suggesting that it other rock-types. Phlogopitisation of clinopyroxenes possibly failed to react with abundantly available K- (diopsidic augite to aegirine augite) in alkaline marie feldspar to produce phlogopite. As in Samchampi, Alkaline rocks of Samchampi, Assam, India 45

20 Agpoitic syenite

fomily Mioskitic syenite fomily

IS 15 Nephelinite fomily "17

,18 0 A v § ~ IO ",, 0 IO. .30 \\ .5 o \\ Z s, /z? 33/ Atkoli besolt fomily \% ,4 iS" ~ -oC \

e~ 36 u !/ ? J: -0 "Gep Suleolkoline rocks u. 0

@ i ,(| A A 30 40 50 GO SiO 2 >

Figure 4. Na20+K20 versus SiO2 diagram (Currie 1976) for the alkaline rocks of Samchampi. Explanations are the same as in table 2. similar behaviour of phlogopitisation of pyroxene by According to Gupta and Yagi (1980), coexistence of carbonatite magma were reported from Kaiserstuhl, leucite, phlogopite and alkali amphibole (in phonolite) Germany (Wimmenauer 1966), Hogenakal, Tamil indicate crystallisation below 3Kb but with little Nadu (Srinivasan 1977) and Amba Dongar, Gujrat water (4% by wt. or lower) in the isobaric (10Kb) (Viladkar 1991). The presence of scapolite in nephe- diagram of the system K20.6 MgO.A1203.6SiO2.H20 line syenite at SAC indicates that the nephelinisation (Yoder and Kushiro 1969), leucite (and also kalsilite) process was coupled with widespread hydrothermal may be stable up to 10Kb even at 1200~ The activity which was part of alkaline magmatic episode, equilibrium crystallisation of phonolite above 5 Kb a feature similarly observed in alkaline rocks of the pressure can be justified through its nature of plots in Bancroft area, Canada (Appleyard and Williams (SiO2+A1203)-(Na20+K20)-Cafemics system at 5 Kb 1981). and 1250~ (Kjarsgaard and Hamilton 1988; figure 5). 46 S Nag et al

No20 + K20 .o/ 6 o

"I0

-... ~0

/ ",,'.'7 ",:- _ _

/0/ -- .,,r "~ ~ Alkali - Poor gO .:, -. _. c...... ,,,. / -, ,. ., ,,'--

Si02+AItO 3 90 80 70 60 50 40 30 20 I 0 Cafemics

Figure 5. (SiOt+A1203)-(Na20+KtO)-Cafemics diagram (Kjarsgaard and Hamilton 1988) for the alkaline rocks of Samchampi. Explanations are the same as in table 2.

In the system Diopside-Nepheline-Sanidine at 1 atm. plotted in this system, show that the carbonatites pressure, Platt and Edgar (1971) had demonstrated maintain distinct identity (sl. nos. 7 & 8 in figure 5) in that the field of leucitess is related to the incongruency relation to the silicate rocks. The crystallisation of sanidiness and thus the former should react with behavionr of carbonatite (no. 19 in figure 5) and silica rich liquid at low temperature to form alkali associated alkaline rocks of Juquia (130-135 Ma, K/ feldspar. This system suggests that a pyroxene-rich Ar age; Games et a11990), Sao Paolo, Brazil bear close nepheline phonolite can be produced from a nepheline resemblance in this respect (cf. table 2). leucitite melt, which is a derivative of a melilite- The predominantly felsic liquid (enriched in Ca, nephelinite melt from an initial olivine-melilite-nephe- alkalies and volatiles), as a result of liquid immiscibility linite magma. split, generated a carbonated liquid by inheriting 513C LeBas (1977, 1987) concluded that carbonatite of the original source magma as the 513C values of complexes form a petrographic association with the carbonatites show 2%o enrichment compared to olivine-poor nephelinite and phonolite or their pluto- the average mantle (~I3CpD B ~-- 5.5%0). Sengupta et al nic equivalents. The fact that liquid immiscibility can (1997) have shown that the average (~13CpDB and occur between carbonate and silicate melts (nepheli- 5]8OsMow of these carbonatites are -3.5 + 0.3%0 and nite or phonolite) was experimentally demonstrated 7.2 -9 0.2%0 respectively. It is believed that the mantle by Verwood (1978) and Freestone and Hamilton source region for Samchampi carbonatite was already (1980). Earlier experiments on the phenomenon of enriched in ~13C (by 2%0 than the normal mantle) and liquid immiscibility at low P-T condition (1 Kb and this enrichment could be by contamination from 650-950~ related to nepheline-CaCO3 system was recycled carbon. Phanerozoic pelagic carbonates and made by Koster Van Gross and Wyllie (1966, 1968, secondary carbonatites in altered oceanic basalt gen- 1973). Lee and Wyllie (1992) studied the liquid erally have ~13CpD B values of 0-2%o (Deines 1989). If immiscibility gap in the system NaA1Si3Os-CaCO3 such an oceanic crust gets recycled into the mantle at 10Kb and 25Kb pressures. In both isobaric then with a mixture of 27-36%0 of this recycled carbon sections they found that the miscibility gap closes with the mantle carbon can generate the expected with increasing Ca-Na ratio in carbonate liquid. values of the carbonatite source region. However, such Although they accept that the evidence of liquid an effect may not be detected in 51So as the reservoir immiscibility is strong, yet they argued that it is not for oxygen is much larger than carbon. acceptable for all carbonatites. The fact that the carbonatites at SAC was formed as a result of liquid immiscibility is amply justified through figure 5. The 6. Conclusion solvus touching point in the system (SiO2+A1203)- (Na20+K20)-Cafemics (Kjarsgaard and Hamilton In the most favoured model of carbonatite genesis, 1988) is for 8 Kb and the rocks of Samchampi, when LeBas (1977, 1987, 1989) proposed that the genetic Alkaline rocks of Samchampi, Assam, India 47

scheme which best explains most occurrences begins The phosphatic rock, being a member of the with alkali-rich carbonatite melt separating immisci- alkaline complex, indicates that it was formed as bily from nephelinitic and phonolitic magmas which supergene secondary enrichment and hydrothermal are themselves derived from olivine nephelinite activity, being a part of the alkaline magmatic acti- magma from mantle partial melt source. He concluded vity, has played a major role in their formation as that at the initial stage carbonated nephelinitic adequate development of vermiculite, limonitic box- magma is formed by < 1% partial melting of astheno- work have been found in this rock. sphere. The resulting magma segregates and on ascent at or near t-he base of the crust it pauses to form a magma chamber. In a ternary diagram Acknowledgements involving (Na20 + K20)-(SiO2 + A1203)-CaO, LeBas The authors (SN and SKS) express their deep sense of (1989) tried to demonstrate the liquid immiscibility gratitude to Dr. S K Acharyya, the then Sr. Dy. relations between silicate and carbonate magmas. As Director-General, Geological Survey of India, North- the silicate magma fractionate to phonolite along the eastern Region, Shillong and presently Director- solvus (c. 10Kb and c. ll00~ increasingly alkali General, Geological Survey of India for sincere help rich carbonate magma separates immiscibly. and inspiration in carrying out the field work during The carbonatites that we investigated do not 1991-92. The authors are also grateful to Sri Ravi contain significant amount of alkalies (average 0.19~; Shanker, Sr. Dy. Director-General, Geological Survey table 2) because during upwelling of carbonatite of India, Northern Region, Lucknow for his generous magma, it nearly lost both the alkalies to the help and the facilities provided while writing the wall rock producing alkali pyroxene, alkali amphibole (arfvedsonite), nepheline and predominantly K-rich paper. We also express our sincere gratitude to two anonymous reviewers for their useful comments in feldspars and as a result of K-metasomatism refeniti- improving the text. sation of earlier formed fenites were produced at the upper level of intrusion. Treiman and Schedl (1983) have shown that carbo- References natite magma has low density (2.2) and low viscosity and it is therefore likely that dense minerals such as Acharyya S K, Mitra N D and Nandi D R 1986 Geol. Surv. apatite, magnetite, olivine and pyrochlore would sink Mem. 119 6-12 effectively in carbonatite magma, a feature character- Appleyard E C 1974 Lithos 7 149-169 Appleyard E C and Williams S E 1981 TMPM Tschermaks istically observed in carbonatites of SAC. The highly Min. Pet. Mitt. 28 81-97 alkaline hydrous fluid activity indicated by the Bagdasarov Yu A 1997 Geochim. International 35(1) 7-16 presence of strongly alkalic minerals in carbonatites Bailey D K 1974 Nephelinites and Ijolites. In: The Alkaline and associated alkaline rocks suggests that the Rocks (ed) H Sorensen (London: John Wiley Interscience) composition of original magma was more alkalic than 53-66 Barth T F W and Ramberg I B 1966 The Fen Circular those now found and may represent a silica under- Complex. In: Carbonatites (eds) O F Tuttle and J Gittins saturated ultramafic rock of carbonated olivine-poor (London: John Wiley Interscience) 225-257 nephelinite (because mantle type xenoliths are abso- Beccaluva L, Barbieri M, Born H, Brotzu P, Coltorti M, Conte lutely scarce, the presence of which indicates olivine A, Garbarino C, Comes C B, Macclotta G, Morbidelli M, rich nephelinite magma) which splits with falling Ruberti E, Siena F and Traversa G 1992 J. Petrol. 33(6) 1371-1404 temperature into two immiscible fractions, one ulti- Chattopadhyay N and Hashmi S 1984 Rec. Geol. Surv. India mately crystallises as alkali pyroxenite/ijolite and the 113 24-33 other as carbonatite. The close proximity of alkali Currie K L 1976 Geol. Surv. Can. Bull. 239 228pp silicates and carbonatites and their genetic relation- Deer W A, Howie R A and Zussman J 1963 Rock-forming ships suggest that the silicate and carbonate melts, Minerals 2 Chain silicates. (London: Longmas) 379pp Deines P 1989 In: Carbonatite Genesis and Evolution (ed) K produced through liquid immiscibility, during ascent Bell (London: Unwin Hyman) 301-359 generated an array of lithotypes and also reaction with Erickson R L and Blade L V 1963 US Geol. Surv. Prof. Paper the country rocks by alkali emanations produced 325 95pp fenitic aureoles. The carbonatites and associated alka- Freestone I C and Hamilton D L 1980 Contrib. Miner. Petrol. line rocks of Juquia, Brazil (Beccaluva et al 1992) and 73 105-117 Gold D P 1966 Miner. Soc. India IMA Volume (eds) M N the carbonatites of Tomtor, Yakutia (650-750 Ma, K- Viswanathiah and P R J Naidu 83-91 Ar age; Bagdasarov 1997) show similar behaviour of Gomes C B, Ruberti E and Morbidelli L 1990 J.S. Am. Earth low alkali enrichment (cf. table 2) as that of SAC. Sci. 3 51-63 The partitioning of Ti, Fe and Ca at shallow level Cupta A K and Yagi K 1980 Petrology and genesis of Leucite magma chamber favoured the formation of MPA-rock bearing rocks, (Berlin: Springer Verlag) 252pp Heinrich E W 1966 The Geology of Carbonatites. (Chicago: as cumulus phase and it being not evolved (D.I. -- 01) Rand McNally) 555pp got emplaced into the complex during the upwelling of Kent R W, Storey M and Saunders A D 1992 Geology 20 magma. 891-894 48 S Nag et al

Kjarsgaard B A and Hamilton D L 1988 Mincr. Mag. 52 43-55 Pichler H and Schmitt-Riegraf C 1997 Rock-forming Minerals Koster Van Gross A P and Wyllie P J 1966 Amer. J. Sci. 264 in Thin Sections (London: Chapman & Hall) 220pp 234-255 Platt R G and Edgar A 1971 J. Geol. 80 224-236 Koster Van Gross A P and Wyllie P J 1968 Amer. J. Sci. 266 Sengupta S K, Rehman S and Nag S 1996 Regional Seminar on 932-967 Development of Geological Research in Northeast India. Koster Van Gross A P and Wyllie P J 1973 Amer. J. Sei. 273 Guwahati University, April 1996 (Abs.) 465-487 Sengupta S K, Ray J S, Rehman S and Nag S 1997 Inter- Kumar D, Mamallan R, Srinivasan B, Jain S K and national Symposium on Isotopes of Solar System. PRL, Krishnamurthy P 1989 Explo. and Res. for Atomic Minerals Ahmedabad, November 1997 (Abs.) 2 183-199 Shapiro L and Brannock W W 1962 lrS Geol. Surv. Bull. 1141- Leake B E 1978 Can. Miner. 16 501-520 A 56pp LeBas M J 1977 Carbonatite-Nephelinite Volcanism, (London: Srinivasan V 1977 J. Geol. Soc. India 48 598-604 Wiley Interscience) 347pp Streckeisen A L 1967 N. Jb. Miner. Abh. 107 144-240 LeBas M J 1987 Geol. Soe. London Spl. Publ. 30 53-83 Sutherland D S 1966 Proc. Geol. Soc. London 1629 20-23 LeBas M J 1989 Mem. Geol. Soc. India 15 137-144 Treiman A H and Schedl A 1983 J. Geol. 91 437-447 Lee W and Wyllie P J 1992 29th IGC, Kyoto, Japan 11-8-10 0- Verwood W J 1978 Annual Rep. of Geophys. Lab. Washington 9 1296 (Abs.) 77 767-774 In: LeBas M J 1987 Nag S 1983 N. Jb. Miner. Abh. 147 217-227 Viladkar S G 1984 India Mineralogist 130-135 Nag S, Chakravorty P S, Smith T E and Huang C H 1984 Geol. Viladkar S G 1991 N. Jb. Miner. Abh. 162(2) 201-213 J. 19 57-76 Viladkar S G and Wimmenaner W 1992 Chem. Erde. 52 277-291 Nag S, Sengupta S K and Chawade M P 1993 Rec. Geol. Surv. Wimmenauer W 1966 The eruptive rocks and carbonatites of India 126(4) 24-26 Kaiserstuhl, Germany. In: Carbonatites (eds) O F Tuttle and Nockolds S R 1954 Bull. Geol. Amer. 65 1007-1032 J Gittins (London: John Willey Interscience) 183-204 Peacock M A 1931 J. Geol. 39 54-67 Yoder H S and Kushiro I 1969 Amer. J. Sci. 267 558-582

MS received 6 September 1997; revised 11 November 1998