Mesoproterozoic Nepheline Syenite Complexes
of the Prakasam Alkaline Province, Andhra Pradesh, India
 
J. Ratnakar*, Victor Zaitsev**, M. Srinivas*, K. Vijaya Kumar*** and K. Rathna***

*Department of Geology, Post-Graduate College of Science, Osmania University,
 Saifabad, Hyderabad-500 004, Andhra Pradesh, India.
** Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Russia.
*** School of Earth Sciences, Swami Ramanand Teerth Marathwada University,
Nanded-431606, Maharashtra, India.

 

In the Indian subcontinent, the Archaean protocontinental core composed of the Dharwar, Bastar and Singhbhum cratons, is skirted, on east and south, by the Precambrian charnockitic granulite terrain. The Prakasam alkaline province (PAP) (Leelanandam, 1989; Ratnakar, 2006) in Andhra Pradesh is evolved in a sutured contact zone between these two terrains of contrasting grades of metamorphism i.e. the core of amphibolite facies and the granulite terrain (Ratnakar and Leelanandam, 1989).

 

There are three main nepheline syenite complexes (NSCs) in the PAP, namely are Elchuru, Purimetla and Uppalapadu intrusions. The alkaline rocks and carbonatites of the PAP are emplaced during the Mesoproterozoic rifting during the period ~1.3-1.2 Ga (Ratnakar and Leelanandam, 1989; Ratnakar, 1995). They are subsequently deformed during the Grenvillian and/or Pan-African. These events have been resulted also in overprint of mineral isochrons and partly in zircon ages (Leelanandam et al., 2006; Upadhyay et al., 2006).

 

The Elchuru nepheline syenite complex, dated as 132117 Ma by U-Pb method on zircon, and 124233 Ma by Rb-Sr whole-rock isochron method (Upadhyay et al., 2006, Subba Rao, et al., 1989) occupies an area of 16 sq km and is roughly an oval- or skull-shaped intrusive, oriented in NE-SW. The complex is composed of predominant nepheline syenite. Subordinate ijolite, malignite, shonkinite, nepheline diorite and melanocratic nepheline syenite occur as elongate lenses up to several tens of meters in mesocratic and leucocatic nepheline syenite (Czygan and Goldenberg, 1989). At places the mafic rocks are intruded by apophyses and vines of nepheline syenite. Rare quartz-bearing syenite forms the subvertical dyke in the nepheline syenite. The complex is traversed by lamprophyre dykes. Sovite carbonatite was reported in the contact zone of the complex (Madhavan et al., 1989).

 

The 136928 Ma old Purimetla nepheline syenite complex covers an area of 7 sq km and is a distorted spindle-shaped body oriented in NE-SW. The intrusive is chiefly composed of nepheline syenite. The hornblende syenite is closely associated with amphibolite xenoliths in the pluton. Minor quartz syenite occurs in the massive nepheline syenite and also outside the pluton (in amphibolite and in granite gneiss). Rare malignite occurs as a autholiths, traversed by the bands and veins of nepheline syenite. Several ocellar lamprophyre dykes also occur (Ratnakar and Leelanandam, 1986).

 

The 134841 Ma old Uppalapadu nepheline syenite complex spreads over 50 sq km and is roughly triangular in plan view and oriented in NE-SW. The body is chiefly composed of nepheline syenite which contains inclusions of granite gneiss, gabbro and horblende syenite; hornblende syenite carries inclusions of biotite schist, gabbro, quartz syenite and ferrosyenite. Quartzite and migmatite masses occur along the contacts of nepheline syenite-hornblende syenite, hornblende syenite-ferrosyenite and, ferrosyenite- hornblende syenite-gabbro. There are no mafic members but a few dolerite dykes cut across the complex (Krishna Reddy et al., 1998; Vijaya Kumar et al., 2007). The field relations suggest that oldest country rocks (amphibolite, granite gness, schist, quartzite and migmatite) are intruded firstly by gabbro and ferrosyenite, followed by hornblende syenite and nepheline syenite in that order (Krishna Reddy et al., 1998); the place of quartz syenite in this order is not clear.

The nepheline syenites of the province are typically miaskitic and the K2O/Na2O ratio varies between 2:1 and 1:2 (Ratnakar, 2007a). The NSCs are characterized by the association of nepheline syenite with comagmatic quartz syenite that portrays liquidus relations of the petrogenys residua system (PRS) (Ne-Ks-Qz) (Hamilton and MacKenzie, 1965). The cospatial quartz syenite in the NSCs is considered to originate as an off-shoot from the middle stage of the main differentiation trend by fractionation of ferrohastingsite and biotite. In such a case the liquidus relations in PRS are soon shifted to those of Ab-Or join (the critical plane of Si-saturation). For example, in the case of Purimetla alkaline complex, Ratnakar and Leelanandam (1986) have proposed that Si-oversaturated rocks could form as an off-shoot products by branching differentiation mechanism, from a part of the differentiating liquid, at the middle (syenitic) stage of differentiation by fractional crystallization of ferrohastingsite and biotite that leads to accumulation of silica oversaturated residues that eventually migrate to the oversaturated part of the residua system. In the case of Elchuru alkaline complex, Ratnakar and Vijaya Kumar (1995) have advocated liquid immiscibility between the liquids of Si-undersaturation and oversaturation in the PRS. However, recent studies of trace element chemistry especially of REE on the Purimetla complex (Ratnakar et al., 2006) proved crustal contamination of middle stage liquids to increase the levels of silica saturation whereby the residues crystallized as quartz syenite (Ratnakar, 2007b).

 

Isotopic studies on some of the NSCs also suggest signatures of crustal contamination. The initial 87Sr / 86Sr ratios of the NSCs of southern India also suggest that parental basic magmas were derived primarily from mantle with seemingly possible contamination with lower crust. It is now considered that the generation of quartz syenites in the NSCs is due to assimilation fractional crystallization (AFC) rather than simple fractional crystallization (FC) as shown earlier (Ratnakar and Leelanandam, 1986, Ratnakar, 2007).

 

The NSCs display strongly fractionated, LREE enriched and HREE depleted, patterns with flat, or positive, but rarely negative Eu anomalies. The NSCs are enriched in large ion lithophile elements (LILE) reflecting differentiation or enrichment of the mantle source before melting.  It is considered that LILE enrichment in the sub-continental lithospheric mantle (SCLM) is due to amphibole, phlogopite and apatite that exist due to influx of F, Cl and CO2 by mantle metasomatism.  Small degree of partial melting of such a metasomatically enriched mantle has yielded alkaline basic (lamprophyric) liquids that differentiated to form the Elchuru, Purimetla and Uppalapadu nepheline syenite complexes in the PAP (Ratnakar et al., 2008a, 2008b).

 

References

Czygan W., Goldenberg G. Petrography and geochemistry of the Alkaline complexes of Sivanmalai, Elchuru and Upallapadu, India. Geol.Soc.India Memoir 15, pp. 225-240.

Hamilton, D. L. and MacKenzie, W. S. (1965). Phase equilibrium studies in the system NaAlSiO4(nepheline)-KAlSiO4(kalsilite)-SiO4(quartz)-H2O. Min. Mag., v. 34, pp. 214-231.

 

Krishna Reddy, K. K., Ratnakar, J. and Leelanandam, C. (1998) A petrochemical study of the Proterozoic alkaline complexof Uppalapdu, Prakasam alkaline province, Andhra Pradesh. Jour. Geol. Soc. India, v. 53, pp. 41-52.

Leelanandam, C. (1989) The Prakasam alkaline province in Andhra Pradesh. Jour. Geol. Soc. India, v.53, pp. 41-52.

 

Leelanandam, C., Burke, K., Ashwal, L. D. and Webb, S. J. (2006) Proterozoic mountain building in Peninsular India : an analysis based primarily on alkaline rock distribution. Geol. Mag., v.143, pp.195-212.

 

Madhavan V., Mallikhavrjuna R., Subrahmanyam K., Krishna S.G., Leelanandam, C. (1989) Bedrock geology of Elchuru Alkaline Pluton, Prakasam Disreict, Andhra Pradesh. Geol.Soc.India Memoir 15, pp. 189-205.

 

Ratnakar, J. (2008) Nature of Precambrian alkaline magmatism in southern India : The nepheline syenite complexes. Jour. Appl. Geochem., v. 10, pp. 60-75.

 

Ratnakar, J. (2007a) Lithosphere control of the of the Proterozoic alkaline magmatism : the miasktic nepheline syenites of southern India. Gond. Res. (IAGR) Memoir No. 10, pp. 109-121.

 

Ratnakar, J. (2007b) What made the Proterozoic alkaline magmas in southern India to deliver nepheline syenites at one time and quartz syenites at the other, in the Ne-Ks-Qz system ? International Seminar on Crustal Evolution, Sedimentary Processes and Metallogeny, Dharwad, India. Abstracts, pp. 38-40.

 

Ratnakar, J. (2006) Precambrian alkaline complexes of southern India. International Seminar on Alkaline Igneous Systems: Dissecting Magmatic to Hydrothermal Mineralizing Processes. Canada, Abstracts, p. 126.

 

Ratnakar, J. (1995) Structural loci of the Proterozoic alkaline plutons of southern Peninsular India. Jour. Indian Acad.Geosci., v. 38, pp. 23-31.

 

Ratnakar, J. and Leelanandam, C. (1986) A petrochemical study of the Purimetla alkaline pluton, Prakasam District, Andhra Pradesh, India. Neues Jahrbuch Mineral.Abh., v. 156, pp. 99-119.

Ratnakar, J. and Leelanandam, C. (1989) Petrology of the alkaline plutons from the eastern and southern Peninsular India, Geol.Soc.India Memoir 15, pp. 145-176.

.

Ratnakar, J. and Vijaya Kumar, K. (1995) Petrogenesis of quartz-bearing syenite occurring within nepheline syenite of the Elchuru alkaline complex, Prakasam province, Andhra Pradesh, India. Jour.Geol.Soc.India, v. 46, pp. 611-618.

 

Ratnakar, J., Vijaya Kumar, K. and Rathna, K. (2006) The Purimetla alkaline complex : a DARC from the Precambrian suture zone, Prakasam alkaline province, Andhra Pradesh, south India. National Seminar on Active and Fossil Suture Zones, Dehra Dun, India. Abstracts, p. 122.

 

Ratnakar, J. (2007) Lithosphere control of the of the Proterozoic alkaline magmatism : the miasktic nepheline syenites of southern India. Gond. Res. (IAGR) Memoir No. 10, pp. 109-121.

 

Ratnakar, J., Vijaya Kumar, K. and Rathna, K. (2008a) Geochemical investigation of the alkaline mafic dykes in the environs of the Prakasam alkaline province, Eastern Ghats belt, India. In : Indian dykes: Geochemistry, Geophysics and Geochronology, Editors: Rajesh K. Srivastava, Ch. Sivaji and N. V. Chalapathi Rao, Narosa Publ., New Delhi, India, pp. 291-308.

 

Ratnakar, J., Vijaya Kumar, K. and Rathna, K. (2008b) Meso-Proterozoic rifting and Grenville / Pan-African collision in the Prakasam alkaline province, Andhra Pradesh, southern India : Evidence from nepheline syenite compexes. International Conference on Tectonics  of the Indian Subcontinent (TOIS), Intern. Assoc. Gond. Res. Conf. Ser., No. 5, pp. 194-196.

 

Subba Rao T.V., Bhaskar Rao Y.J. Sivaraman T.V., Gopalan K. Rb-Sr age and petrology of the Elchuru alkaline complex: Implication to the alkaline magmatism in the Eastern Ghat mobile belt.// Memeirs Geol. Sci. of India, No 15, 1989, pp. 207-223

 

Vijaya Kumar, K., Frost, C. D. Frost., R. B. Frost. and Chamberlin, R. (2007) The Chimakurthi, Errakonda and Uppalapadu plutons, Eastern Ghats belt, India: an unusual association of tholeiitic and alkaline magmatism. Lithos, v.97, pp. 30-57.

 

Upadhyay, D., Raith, M. M., Mezger, K. and Hammerschimdt, K. (2006) Mesoproterozoic rift-related alkaline magmatism at Elchuru, Prakasam alkaline province, SE India. Lithos, v. 86, pp. 447-477.


" "