Precamdrian study on southern granulite terrane of pan-african rejuvenation

PRESENTED BYSANDIP SAHADIPANKAR BURAGOHAINLISA KOLEMEERA RAJANSOHINI BHATTACHARYAMEHEBUB –a-ALAM

CONTENTS1.Introduction : Geological Settings2.Geological Attributes of the Granulites Blocks3.Structural set up of SGT4.Metamorphic history of SGT5.Pandyan Mobile Belt6.Models and Evolutions7.Discussion8.Intracratonic Purana Basins in Peninsular India Cuddahpah Basin Kaladgi Basin Godavari Basin Vindhyan Basin

Introduction: Geological SettingsThe Southern Granulite Terrain(SGT) is known as Pandyan Mobile Belt , The name is given by Ramkrishnan(1993, 1988)

SGT is situated to the south of the E-W trending Palghat-Cauvery Shear Zone(PCSZ).

According to Fermor(1936), this terrain is a part of the large “”Charnockite Province”” located to the south of the Orthopyroxene-in isograd,delineated along a line straddling the Join Mangalore-Mysore-Bangalore-Chennai.

India’s Southern Granulite Terrain Consists of three late Archean to Neoproterozoic,high-grade Metamorphic blocks, joined together.

The whole terrain is divided into two distinct crustal blocks-1.The Northern Granulite Terrain(NBSGT)2.The Southern Granulite Terrain(SBSGT)

The Northern Block (also known as the Salem Block) of the Southern Granulites consists of a granulite massif at the southern edge of the Dharwar Craton. The block is located between the ‘Fermor line’ and the Palghat-Cauvery Shear Zone . Lithologies present in the Salem include, pyroxene-bearing granites(charnockites), granite gneisses, and migmatites (Devaraju et al., 2007; Clark et al., 2009).

Geological Attributes of the Granulites Blocks

This terrain is dissected by Proterozoic shear zones that separate the Granulite rocks into four regions or blocks---1.Madras Block2.Nilgiri Block3. Madurai Block4.Trivandrum Block

Darwar Craton

Northern Block

Madras Block

Nilgiri Block

Madurai Block

Trivandrum Block

ACSZ

PCSZ

MBSZ

ASZ

GRANULITE TERRAIN

Granite GreenstoneTerrane

Granites,Gneisses &Supercrustals

Legend

Sketch map of the Southern Granulite Province blocks and associated shear zones after Naqvi and Rogers (1987) and Ramakrishnan and Vaidyanadhan (2008

Structural set up of SGTThe multiple deformations experienced by the Archaean-Proterozoic rocks have affected them differently in different sectors.

According to the Geological Survey of India (Sugavanam andVidyadharan, 1988), the granulite terrain is characterized by isoclinal and Asymmetrical F1 folds with NNE-SSW axial planes, observed in both 2.6–2.5 Ga old charnockites and supracrustals. The superposed F2 folds with WNW-ESE trend affected migmatites, granulites and 2450 Ma old granites. The F3 are shear folds with N-S trend.

According to Mukhopadhyay et al. (2003),the superposition of F2 on F1 folds produced interference patterns in the Gneisses and granulites.

The sheared rock

The major fault and fracture systems of Tamil Nadu are oriented in the following directions-1.Nearly E-W to WNW-ESE2.NNE-SSW to NE-SW3.ENE-WSW4.N-S5.NW-SE

Vijaya Rao and Prasad (2006) proposed southwardsubduction of Dharwar Craton beneath the SGT and proposed collision along the MBSZ inthe late Archaean followed by crustal delamination.

Sanjeev et al . (2009) considered both MBSZ and PCSZ as parts a suture zone between the Dharwar Craton and the Madurai Block that formed in the Neoproterozoic-Cambrian during which UHT granulites developed. However, the available geochronological and structural data do not support thisinterpretation.

Metamorphic history of SGTThe SGT, India represents a very unique opportunity to study crustal process at depth(Newton and Hansen, 1986). Modern studies with textural imprints, mineral chemistry have improved our understanding of the metamorphic evolution of the SGT.

UHT metamorphism in the south block of the SGT (Madurai Block) is shown by the occurrence of sapphirine bearing granulite enclaves within the gneisses (Prakash et al., 2010).

Madurai Block shows high grade metamorphic features including preservation of excellent texture textures. Lal et al. (1984) reported enclaves of Mg-Al granulites within the gneisses.

In Madurai Block wide variations in mineral assemblages are found in different granulites facies rocks, especially Mg-Al granulites

Tectonic map of Southern India, simplified after Drury and Holt (1980)showing the major metamorphic and structural subdivisions

Pandyan Mobile BeltThe Pandyan mobile belt (PMB), according to Ramakrishnan (1993), is the geological domain between the PCSZ in the north and the AKSZ in the south.

Ramakrishnan (2003) divided the enlarged Pandyan Mobile Belt into Three zones.These zones are: (1) Northern Marginal Zone (NMZ), (2) Central Zone (CZ), and (3) Southern Zone (SZ).

The NMZ lies just north of the E-W trending Moyar-Bhavani Shear zone (MBSZ).This zone is characterized by the charnockite massifs of Shevaroy, Javada, BRHills and Nilgiri.

The Central Zone, CZ (2) is located between the MBSZ and Noyil-Cauvery shear zone (i.e. PCSZ). This zone consists of mainly high-grade relics of older greenstone (Sargur Group), namely Satyamangalam Group.

The area lying to the south of the PCSZ is the Southern Zone (SZ). It Contains charnockite-khondalite rocks. The SZ of the PMB, according to Ramakrishnan(2003), contains two sub-units on either side of the AKSZ. First is the Kerala Khondalite Belt (KKB) to the south of the AKSZ and is predominantly formed of khondalite-leptynite suite with subordinate charnockite. Second sub-unit is the Madurai Block to the north of the AKSZ. It consists of quartzo-feldspathic gneisses;and Charnockites.

With the basic information given above, the two different evolutionary models of the Pandyan Mobile Belt are discussed…..

1. Subduction-Collision Model (Ramakrishnan, 1993, 2003, 2004)2. Accretion Model (Radhakrishna and Naqvi, 1986)