Paper Number: 337
Neo-Proterozoic alkali mafic magmatism in Betul belt, Central Indian Tectonic Zone
Mohamed Shareef1*, Ramachandra H.M2., Suresh, G3., Dora M. L3., Amitava Kundu4, Atif Raza5, Parwez Akhter5 and V. P. Malviya6
1.Geological Survey of India, NCEGR, Bengaluru, India; Email: shareefgeo@gmail.com
2. Kumarswamy Layout, Bengaluru, India
3. Geological Survey of India, Training Institute, Hyderabad, India
4. Geological Survey of India, RTI, Shillong, India
5. Geological Survey of India, Central Region, Nagpur, India,
6. Geological Survey of India, Northern Region, Lucknow, India. _________________________________________________________________________
In this paper we are reporting Neo-Proterozoic alkali mafic magmatism from Betul belt in Central Indian Tectonic Zone (CITZ). Alkali gabbroic rocks are occurring as dykes and circular to subcircular bodies in the eastern part of the belt. They are least affected by deformation and metamorphism. Field evidence such as mixing, mingling with host granite suggest there emplacement in the veining stages of granite cooling. Petrographic studies indicate two distinct mineral assemblages. Aegirine augite, alkali amphiboles is present in one assemblage, whereas olivine, titan augite and enstatite are noticed in the other with phlogophite, apatite, monazite, chevkinite, baddeleyite and huttonite as common minerals (Fig.1). Aegerine augite is poikilitically enclosed by biotite indicating alkali lineage. Intergranular and fretwork texture is noticed, where in biotite is developed in the intergranular spaces of plagioclase. Olivines are Fe rich having mg # varying from 32 to 61. Trace elements concentration is high i.e. high Zr (203 to 397 ppm), Rb (53 to 105 ppm), Sr (599 to 604 ppm) and Ba (951 to 1540 ppm).
Figure.1. Photomicrograph of alkali gabbro
showing intergrowth of phlogopite and
aegirine augite
Chondrite normalized REE pattern show moderate differentiation pattern with enriched LREE with no Eu anomaly and depleted to nearly flat HREE pattern. They are characterized by high ∑REE i.e. 280 to 492 ppm. Geochemically these rocks show distinct enrichment of LILE and HFSE characteristic of alkaline rocks formed from a metasomatized mantle source [1]. Monazite chime dating of alkali gabbros has yielded their emplacement around 912+39 Ma representing the last phase of the Grenvillian Orogeny in CITZ [2] emplaced in an intra-arc rift basin developed in a convergent setting.
References:
[1] Bailey, D. K., (1987), Geol. Soc. Spl. Publ. 30: 1 – 14.
[2] Roy, A., and Prasad, H. M., (2003), Jour. Asian Earth Sci., 22:115-129.