Magmatism and metal concentration along deep-rooted discontinuities in the granulite terrain of Tamil Nadu, India
published: Jun 28, 2001
Open Access (paper can be downloaded for free)
The Indian subcontinent on a closer study reveals itself as being made of different crustal blocks geologically unrelated to each other which have been brought into juxtaposition and sutured together during different periods of evolution of lithosphere. About eight tectostratigraphic terranes were distinguished for the Indian subcontinent with definite boundaries demarcated by fault or shear zones. The Tamil Nadu form part of the well known southern Tamil Nadu-Kerala granulite terrane. It is extensively made up of charnockites, pyroxene granulite and gneisses. Other rocks include khondalited and associated metasediments, layered metaanorthosites, massif anorthosites, ultramafics, late potassic granites and post alkaline rocks, comprising carbonatites, syenites, nepheline syenites and mafic alkaline dykes and dolerites. The charnockites form hills such as St. Thomas Mount, Kolli Hills, Nilgiri Hills, Palani Hills, Kodai Hills and Anaimalai Hills in Tamil Nadu in striking contrast to the gently rolling topography of the gneissic terrane. Regional structural studies carried out by many workers reveal deep-rooted fracture zones or shear zones or lineaments in the parts of Tamil Nadu acted as loci for igneous intrusions of different kinds. There are three distinct fracture patterns trending nearly N-S, NE-SW, and EW directions. Parallel to these lineaments or along these fracture zones magmatism and mineralisations were noted. The E-W trending lineament or Cauvery-Palaghat suture zone and Bhavani lineament found to host deep-rooted magmatic bodies which are chiefly layered meta-anorthosite suite of rocks. (Sittampundi complex and Bhavani complex). These meta-igneous suites (>2.9 B.Y to 3.4 B.Y) found to host chromite mineralisation as early magmatic crystallisation product. The massif type anorthosite body of Kadavur found emplaced in the dome and basin parallel to the NE trending Easternghat lineament zone of mAdurai aulocogene belt indicate con centration of Fe-T1 oxide mineral concentration as early crystallisation product. The NNE to N-S trending fault zones in the parts of Dharamapuri and Salem districts of Tamil Nadu were acted as loci for the emplacement of syenite-carbonatite complexes which have got concentration or atomic minerals like pitchblende etc. as also the formation of magnesite and barite mineral deposits. The nearly N-S trending Elagiri lineament or fracture zone in the parts of Uttangarai and adjoining area in Dharmapuri district, the only molybdenum mineralisation in India is identified. In North Arcot district near Mamandur, the nearly NE-SW trending fracture zone has acted as loci for the emplacement of multisulphide (Pb-Zn-Cu) mineralisation. Thus the distribution of metalliferous deposites in the parts of Tamil Nadu clearly examplify the concentrations of metals in the form of stable minerals controlled by magmatism through deep-rooted zones of discontinuity or shear zones or fault zones. The deep-rooted zones of discontinuity have been well identified by recent techniques like remote sensing and magnetic and gravity anomaly studies in the parts of Southern Peninsula with which Tamil Nadu form part of it.