A note on the occurrence of carbonatite in the Nellur Schist Belt, near Vinjamur, Udayagtrt Taluk, A.P.

D. VASUDEVAN, T. M. RAO AND C. KOTA REDDY

Abstract About 80 km north-north-west of , in the northern portion of the Nellore schist belt (referable to the Dharwar and older metamorphics) linear bands of rock of considerable strike length made up of Calcite, dolomite, siderite, with concentrations of baryte and phlogopite, Occur as conformable sheets within a cogenetic volcanic sequence of intermediate to acid rocks.' The volcanic sequence into which the carbonatites are emplaced is part of a group of rocks, locally called the Udayagiri group. The Carbonatites occurring here can be classified essentialIy into two groups: (I) Fine grained siderite-rich rock exhibiting flow banding, vugs and other features suggestive of flow. This variety is rich in baryte and euhedral phlogopite plates; (2) Sheets of carbonate rich rock emplaced in conformity with the litho-layering trends of the enclosing meta-volcanics. These are medium to coarse grained, deep brownish rocks made up of siderite, clacite, dolomite, baryte and soda-amphibole, with magnetite and apatite in lesser amounts. Also found are micro-layers of a green mineral which has a characteristic fluorescence, and is strongly suspected to be a cerium-lanthanum rare earth mineral. The first type is seen about 1 km ssw of Bandakindipalli (14°52'30": 79°44') while the second is near Kodandarama baryte mine at 4.5 km wsw of Vinjamur (14°50': 79°35').

Introduction About 80 km NW of Nellore, and in close proximity to Vinjamur, carbonatite has been located in the form of flows and sheets showing conformable relationship to the host rocks, which are metamorphosed intermediate to acid volcanics, tuffs and agglomerates. This is the first report from an early Precambrian belt and is interest­ ing on account of its probable contemporaneity with the associated volcanics and its enrichment in barium.

Geological set-up High grade pelitic schists, namely staurolite-kyanite-biotite schist, showing migmatisation in various stages, is the oldest rock found at Vinjamur (14°50' : 79°35') and is designated as the Vinjamur schist complex and considered as equivalent of the Sargur schist complex of Karnataka. It extends from Vinjamur in the south to Uttukuru in the north-east. In association with these high grade schists on the western side are a group of metavolcanics, which range in composition from acid to intermediate volcanics (andesite, rhyolite and rhyodacite, acid to intermediate tuffs and agglomerate. This passes gradually upwards in the sequence into the low grade greenstones (actinolite schist, metabasalts, chlorite schist). Both the high grade pelitic schist and the low grade metavolcanics, show a regional foliation trend which varies between N-S and NNW-SSE and a dip of mode­ Tate angle to the wsw. Minor folds in the high grade pelites and the metavolcanics show south plunging axes with local northerly plunges. Pronounced NE-SW trend­ ing cross-fold axes are found superimposed on the earlier NNW-SSE axes. A long shear zone, probably a thrust, slicesthrough the low grade metavolcanics (Figure 1). Rocks close to the thrust show extensive silicification, and mylo- 516 RESEARCH NOTES nitisation along foliation planes. Mullions developed in the rocks show westerly plunges and slickensides too show movement to the west. The low grade metavolcanics are further faulted, the faults trending almost in an E-W direction. As shown in Fig. 1 rocks made up essentially of carbonates occur as isolated bands of varying strike continuity within these volcanics, and, are always found showing parallelism in their litholayering to the enclosing volcanic rocks.

CARBONATITE IN THE . ~~-",~~~~_' ' ~::".::..~ NELLORE SCHIST BELT NEAR VlNJAhllJR, ~~--:1Ir-~--" ;'.~" , A . P . ... IIETItU 1000 soo 0 lOOOMU'JIli '--':""-"'"~--35 :\,- tl I,, ",,===~~~~~ ~~ ~\- ~~ ~ ~ ~ __-::...... -l\- - - -,-ll - I: ~ 35~.. ,I!l-"-'-fl-fl-II-It-~-" s...-.=Il""",='=-'~.:"'\ 0-,,- tt - • - " -- t - • - • _ • "'-~--';:;='i""\~'=-lr"Jnl'".'.'OIiI II=-" ~ I- t ­ ,:--",---"",',!-- n-. -t- 1 - a - I_a 'r-~~-...... ,.\ . • -a-,-~ -,-l-t ~-n.'l[--_\~,,' - t -. -. -~,~ ~ -" "~lo-',.:---~--.:... , - tt -l' - II ~ It ~1'. "', HH ... 0 ~ -, "-It -II-I--f!:_. 14 O Chlor.t, actinolit. 'Chl't...... HH ... " '0' l"':"::"::"l ... \ lil- t' - l\ - tI: _l' - • .,.0 ~ a.c,tO·Rhyodoclt, (Gorn.t1feroul ',\M \ ~ H .-.. ... •_ - n _ n _ 'Jd ~ H ~ ,-I' r-a--l Cluor1z:blotitllChll", H 50 ~ ,.,to ondlll''',Tuffl ,A;olc)l'nIralts \ \. .. H - - l -It - ~ [~ Quartzite ,H ~ a. __, __ \ , ._t ~ \ HHH " ,'-_'~' _~~ • Hematile·Mo;n,me QUO'" foci. \ , ""'H ' . ~ , " II '\. F~=-""'~.~~ ~ ~ Corbonatite 3~~ .r-: ~ J,<-- ~ Lay.rR bary" with minor maQrl.llt. one! .Id.rll....' ~\{" \ ~,,~ § ""0 ac le! voicanici (Quartz • • rl:~:r~~t l~at:)" \ ~ ~ , [o... ~ Quart.lt. ,, (-- . ~ ~ Staurallt.~t(yanih "hi.,. '\\' D. VASUDE VAN [~ Thrust OR sheor 10'" \ \ ~ T .. . RAD 1:.I'.0't4 R o ~oult \ G StTfh 6 dip or folio'ion 79° >d Figure 1.

Types of Carbonatites Three distinct types of carbonatites are found: I) Carbonatite flow: A brown rock showing aphanitic to porphyritic texture and megascopically identified to be composed of siderite phenocrysts with laths and plates of phlogopite in a fine groundrnass of carbonates and baryte (occurring as distinct blebs). This variety occurs within the intermediate volcanics, south-south­ west of Bandakindipalli (14°52'30" : 79°44') and is characterised by its richness in siderite, ankerite, carbonate and baryte. It is silicified. 2) Carbonatite sheet: Medium grained sideritic carbonatites are found (often showing banding) associated with meta,rhyolite and magnetite-hematite-quartz rock, about t km east of Venkatadripalem (14°48'30": 70°31'30'/). This rock is highly silicifiedevidently having been subjected to extensive shearing movements. 3) Sovite: Rock made up essentially of carbonate, biotite and apatite with subordinate amounts of dolomite and euhedral magnetite. Outcrops are seen on the Udayagiri- road near milestone-50. RESEARCH NOTES 517

Petrography and Petrology A grab sample of the carbonatite collected from the Bandakindipalli area has been analysed for trace elements at the AMD Labs., Hyderabad. This revealed the presence of Ce, La, Nb and Ba in proportion more than normal for a limestone. {Table I). TABLE J Composition of average '" Abundance of trace abundance of Trace Elements found in Elements in Limestones Vinjamur Carbonatite Gold (963) (in ppm)

Ce 11.5 45 La 20 Nb 0.3 50 Ba data not available 510 Sr " 30

'"Average chemical composition of Carbonatites. Econ. Geol. 58,1963 Carbonates: Fairly persistent bands of almost pure carbonates described in previous literature as crystalline limestone, are now considered as the probable early phase of carbonatite activity in this area represented as sovite. Sovite from this area has analysed high calcium content as much as 52% CaO. It is made up of calcite and dolomite, forming a mosaic and biotite as the predominant accessory. Magnetite and apatite occur as subordinate accessories. Magnetite shows euhedral forms and is often found embayed by carbonate. Magnetite: Highly characteristic of the carbonatites which occur as flows and conformable sheets within the metavolcanics, is the presence of accessory to essential magnetite. It occurs as disseminated euhedra, sometimes even along flow banding where it shows distortion. Embaying and veining of magnetite by calcite is common. No primary niobium mineral could be seen so far and it is presumed that niobium

Reference HEINRICH, E. WM., (1963) The Geology of Carbonatites. Econ. Geology, v, 58.

Address ofthe authors D. VASUDEVAN, T. M. RAo and C. KOTA REDDY, Geological Survey of India, Hyderabad,