Some Problems Pertaining to The Peninsular Gneissic Complex

Abstract

Since the terms Peninsular gneiss and Dharwar schist have recently been used by some writers without reference to the original meaning ascribed to them by the geologists who first coined those terms, attention is drawn here to their exact scope and significance. In Karnataka, 'Peninsular gneiss' comprised all granites and gneisses except those of Closepet, Chitaldrug, Hosdurga, Arsikere, Banavar, and Saulanga; and 'Dharwar schist' included all the crystalline schists (metavolcanics and metasediments) as opposed to the felsic gneisses. peninsular gneiss was considered to be intrusive into the Dharwars. No early geologist in India was entirely of the opinion that the Dharwars were younger than the gneisses.

The Bababudan Syncline contains the oldest Dharwar sequence composed of mafic and ultramafic volcanics and intrusives (some of komatiitic affinity) intercalated with iron formations. Limestones are entirely absent.

The actual base of the Dharwars has nowhere been definitely proved. Vertical shafts more than 3km deep in the Kolar Gold Fields have not touched the bottom. The fact that in some places the schists are seen 'resting on gneiss' does not mean that they constitute the lowest Dharwars.

A general progressive metamorphism in the schist belts in southern Karnataka can be noticed from north to south, culminating in high grade granulite facies rocks. There is no field evidence which lends support to the presumption that the chamockites are overthrust on the Dharwars.

Numerous examples are available throughout Karnataka to prove the intrusive relationship between Peninsular gneiss and Dharwars. There are probably some schistose rocks which are older than the Dharwars, but, so far, they have not been definitely identified. The suggestion that the Sargur schists of southern Karnataka are pre-Dharwars is not based on precise stratigraphical, petrological, geochemical, or geochronological data. These highly metamorphosed isolated strips of schists could weIl be constituents of the lowest Dharwar sequence.

The problem of the nature of the primordial crust, as elsewhere in the world, has not been satisfactorily resolved in southern India. The occurrence of very old migmatitic gneisses suggest a probable primitive simatic crust.

As in many precambrian shield areas, the granites and gneisses of Karnataka can be generally grouped into three types which were formed at different times in varying tectonic environments.

While the granulite facies rocks in the most ancient rock complexes are probably related to a higher thermal regime of the earth in early Precambrian time, it must be realised that the existence of pyroxene granulite facies assemblages alone is insufficient evidence to infer that such rocks have had an exclusive origin in the lower crust. It is now known not only in south India but in many parts of the world that gneisses have been transformed into charnockites. There are charnockites of different geological ages and of different modes of formation. The prevalence of enderbites in southern India is probably due to the conversion of early tonalitic rocks in the high grade metamorphic terrains.