International Journal of Innovative Studies in Sciences and Engineering Technology (IJISSET) ISSN 2455-4863 (Online) www.ijisset.org Volume: 6 Issue: 7 | 2020 Ultrapotassic rocks from Carbonatite Complex of Tiruppattur, Tamil Nadu, India R. Ramasamy Former Deputy Director, Department of Geology and Mining, Chennai, Pavanar, Street, Thirty Feet Road, Polichalur, Chennai, The data for ultrapotassic rock emplaced in Jogipatti Abstract: Ultrapotassic rock occurs volumetrically basin is extremely high in molar content of minor in the carbonatite complex of Tiruppattur (N12o K O/Na O>3 and wt% K O/Na O>2. It occurs in 15-30’ E78o 25’-30’), Tamil Nadu, India. The rock occurs 2 2 2 2 southwestern end of the area (Fig. 1- 4). as a small plug and dyke. It is composed of >40% normative foids against k-feldspars. However, no 2. METHODOLOGY feldspathoids are seen in this rock. Field and Most wet-gravimetric chemical analyses of rock petrographical features reveal that a discontinuous samples were carried out between 1967 and 1973 by continental half graben rift zone (>3000x>200km) the author for his Ph.D. work [3] in geochemical caused fragmented segments from Arunachal Pradesh to laboratory of the Presidency College, Chennai. Some Tamil Nadu extending several tens of km depth below were carried out by geochemists of geochemical which numerous hairline fractures might have been laboratory, Moscow State University, Russia. Additional developed up to mantle. Deep extension of such hairline analyses were carried out in the geochemical fractures into mantle of shonkinite / phlogopite- laboratory in the Department of Geology and Mining, pyroxenite might have been subjected a low degree of Chennai. Field and laboratory studies were carried out partial melting. The melt has been raised and filled in an for the past 40 years. Rittmann’s norms were intermittent magmatic chamber and has been resulted calculated for these rocks [4] and included in these to continuous differentiation and fractionation leading tables. to successive serial order of emplacements of co- magmatic alkali syenites and carbonatites in two 3. FIELD STUDIES adjacent Sevvattur basin and Jogipatti basin. In middle Figs.1 to 4 is prepared based on textural and of continuous magmatic activities in serial order in mineralogical variation present in rock in the field. The Sevvattur basin, the Jogipatti basin might have been ultrapotassic rock is very fine-grained white colored uplifted, rotated and shifted to series of continuous rock showing vitreous glossy texture. It was mined for emplacements of potassic syenites in it. Shonkinite dyke building stone till 1970, latter mining was banned. It is occurs at contact between ultrabasic rock and granite found at contact between Mg-Al rich phlogopite- gneiss. A large body of an arc like Mg-Al rich lamproite pyroxenite (lamproite) and garnet sanidine syenite. (phlogopite-pyroxenite) exposes in southwestern part of Vermiculite pockets are seen in ultramafic rock in the basin. It has similar trace-element content to Sevvattur and in Jogipatti basins. Field and ultrapotassic rock which is enriched with lithophile petrographical studies reveal that alkali syenites elements, PGE, Au, Ag, Mo, Cu, Zn, Cr, Ni, Pb, REE, F, CO 2 associated with carbonatites are subjected to and SO . Ultrapotassic rock is an exploration tool for 3 prolonged process co-magmatic differentiation, several mineral deposits. fractionation and sequential emplacements in serial Keywords: Ultrapotassic rock, melanite-microcline order by tectonic setting in the two adjacent structural syenite, shonkinite, phlogopite-pyroxenite, lamproite. basins [5-9]. 1. INTRODUCTION The Sevvattur basin is comprised with sequences of porphyritic oligoclase syenite which rimmed with Ultrapotassic rock is classified when molar contents of massive fine-grained hornblende / augite syenites. In bulk rock composition of K2O/Na2O>3 [1]. Wide spread inner portion feldspar platelet of maximum size 10x8x1 global geochemical characterization weight percent cm is seen. It exhibits normal zoning (an28-24 ab72-76) [3- of composition K2O/Na2O >1 is potassic rock and it 9]. Such zoning is visible to naked eye. Cumulates of exceeds over K2O/Na2O >2 is discriminated as feldspars and mafic minerals show magmatic flow ultrapotassic rock [2]. There exists vast geochemical orientation. The grain size of mafic and feldsic minerals difference by using these two factors [1, 2] for increases towards south. Further, hornblende has interpreting data (Tables 1, 2, 3 for the alkali complex imperceptive gradation into augite enriched with of Tiruppattur). It is most intriguing and perplexing sodium content. Thin veinlets of acmite-magnetite- problem in solving petrogenesis of the rocks. Detailed quartz syenite are seen in syenite bodies. In this basin, structural tectonic and petrographical studies might have been shed light on tectonic setting of this complex. © 2020, IJISSET Page 48 International Journal of Innovative Studies in Sciences and Engineering Technology (IJISSET) ISSN 2455-4863 (Online) www.ijisset.org Volume: 6 Issue: 7 | 2020 younger syenites are found sequentially in inner system extending >3000 km in length and >200 km portion. width intersecting NE-SW and N-S rift zones extending to Cap Comorin [5, 10]. The deep narrow fracture The southwestern Jogipatti basin is relatively younger, might have been produced hairline fractures below uplifted, rotated [10-20] and shifting with sequential faulted segments of several tens of meters down to the emplacements co-magmatic series of rocks. A serial Earth to release pressure causing low degree of partial order of co-magmatic miarolitic riebeckite potassic melting of mantle source. Shonkinite / phlogopite- pegmatite-aplite, scapolite-wollastonite aplite, garnet pyroxenite might have been present at that mantle sanidine syenite, melanite-microcline ultrapotassic source as they occur in the field. The low degree of syenite and zircon syenite are seen. Riebeckite- partial melt appears to be filled in an intermittent richterite sovite, garnet sovite, beforsite, ferro- cavity large enough to be a magmatic chamber where carbonatite, benstonite carbonatite, magnesite, barite the melt might have been differentiated and and apatite-ilmenorutile veins are seen in Mg-Al rich fractionated into different series of alkaline magma, lamproite. Ring dyke of carbonatite is seen emplaced periodically emplaced in sequential order in these amidst syenites at the center of the basin near Jogipatti basins. The reactivations of Precambrian rift system village [3-5]. might have been continued by effusions of carbonatitic The syenites and carbonatites are enveloped by an arc lava and ash flows and silicate perovskite directly from of ultramafic rocks steeply plunging inwards inner mantle source lying below 2000 km during Early sides of the basins. The ultramafic rocks include Pleistocene in Tamil Nadu [13-20] and continual members of kimberlite, lamproite, phlogopite- reactivations till Recent. pyroxenite, porphyritic-pyroxenite, cavernous 4. PETROGRAPHY AND GEOCHEMISTRY pyroxenite, ilmenorutile-pyroxenite, peridotite and dunite [21-31]. Apatite-ilmenorutile veinlets 100x10x5 Syenites having molar (K+Na/Al >1.2 exhibits agpaitic cm extending from 3 to 5 m length are found about 1 texture due to formation of early crystallization of km SW of Onnakarai village. Ultramafic rocks have feldsic minerals before the crystallization of barite, benstonite and carbonatite veinlets. An arc like melanocratic minerals [32]. The molar ratio (K+Na /Al skarn is seen at the contact between alkali syenite and < 1.2 is miaskitic rock in which mafic mineral phlogopite-pyroxenite and numerous folded [22] ultra crystallize earlier than feldspars. basic nodules varying in sizes from >1cm to >30cm are seen in the skarn rock Thin veins of calcite (>1mm width) alternatively is released from ultramafic rock . Over turned folded ultramafic nodule is comprised with ellipsoidal or spherical tightly folded dome and swelled basin. Native Cu, Ag, galena, bornite, sphalerite, phyrrhotite, pyrite, covellite, chalcocite, malachite, and azurite are seen on the broken surfaces of such nodules. Coarse-grained garnet wollastonite syenite and fine- grained garnet bearing aplite are seen within syenite complex. In wollastonite rock, wollastonite and para- wollastonite syenite occur together [3]. Syenites vary in bulk chemical composition from potassic to ultrapotassic in Jogipatti basin. They are enriched with Fig. 1 Simplified geological map of carbonatite-alkaline sanidine / anorthoclase and high temperature complex, re-classified on the basis of bulk chemical maximum microcline (ferrian feldspar) as essential composition. N-S trending ultrapotassic rock occurs in minerals. The accessory minerals are zircon, magnetite, SW corner of the map [1, 2, 3 ]. phlogopite, richterite, magnesioriebeckite, garnet, scapolite and wollastonite. Veins of garnet syenite in Sevvattur basin has lower K2O/Na2O content than garnet-sanidine syenite in Jogipatti basin. A small body of zircon syenite occurs 1 km SW of Samalpatti village is a potassic syenite having euhedral metamict zircon as essential accessory mineral and sanidine as essential mineral. These two basins set amidst in Alangayam rift- zone spreading hundreds of km in NE-SW towards Arabian Fig.2 Field exposure of ultrapotassic rock is seen north Sea on length and width of tens of km NW-SE [11]. The rift zone crosscuts a major Precambrian paleo-rift of coarse-grained garnet syenite.