Indian shield Rocks

The concept of Indian Shield along with its stability and boundaries has been an interesting topic of discussion among the students of South Asian geology. Some workers in Pakistan also have their reservation to the use of Indian Shield elements for the Kirana and Nagar Parkar complexes.
             Literally, the shield is defined as continental block of the earth's crust that has been relatively stable over a long period of time and has undergone only gentle overlapping in contrast to the strong folding of bordering geosynclinal belts. They are essentially Precambrian rocks and practically synonymous with craton.
             The earliest concept of Indian Shield had been attributed to the ancient crystalline rocks, extremely contorted and largely intruded by plutonic rocks, with well-defined foliated structures. These rocks are of Archaean age and often they are called “Fundamental Complex” and “Basement Complex” (Wadia, 1957). Some workers have included rocks of Middle Proterozoic age within the sphere of “Basement Complex” which includes all the rocks of “Aravalli Super Group”, “Delhi Super Group” and the overlying “Erinpura Granite” and ultrabasic intrusives present in the western Rajasthan, (Paliwal, 1992). Some others have included Vindhyans to make Indian Shield stratigraphically spanning up to the end of Precambrian Era, where great unconformity exists before the advent of great marine transgression of Cambrian.
            A. M. Heron in twenties and thirties and of the last century mapped Rajasthan area and worked out its Precambrian history in detail. His four-fold classification of the pre-Vindhyan rocks of the area included Banded Gneissic Complex (Bundel-khand gneiss), Aravalli System, Raialo Series and Delhi System. This assemblage continued northwards from Peninsular India and formed the northwestern part of Indian Shield with Kirana and Nagar Parkar igneous complexes as deeply eroded outliers of Aravalli chain.
           Workers of South Asian geology have questioned the status of Indian Shield many times, and it has not yet been completely resolved. A short history narrated by Varadan (1976) is quoted here. “In recent years certain questions have been raised regarding the stratigraphic status of the Banded Gneissic Complex, correlation of gneissic rocks of isolated areas. Doubts have also been expressed on the validity of maintaining a separate stratigraphic status of the Raialo Series. However, remapping in the Udaipur area enuring sixties by Geological Survey of Indian (GSI), has confirmed the existence of a basement of theAravalli supracrustals as postulated by Heron, but the geographic spread of this basement may be much restricted than what had been envisaged by Heron. The recognitionÿ the gncissic rocks of different orogcnic cycles isone of the crucial problems, not only in the Precambrian of Rajasthan, but also elsewhere in the Shield areas. Revision of geological mapping carried out in western Rajasthan since 1960's (by GSI) has indicated that the so-called Trans-Aravalli Vindhyans overlying the Malani Rhyolite arc not homotaxial with the Vindhyans of eastern Rajasthan and Madhya Pradesh. Geo-chronological dates have ascribed an age of about 1400 Ma for the Vindhyan and Chitorgarh-Bundelkhand, whereas the Trans-Aravalli Vindhyans
(West of Aravalli) appear to be younger than 700 Ma by virtue of their overlying the Malanics, which
have been dated at about 700 Ma.

           An important contribution made by the Geological Survey of India in recent years is the discovery of stromatolites, phosphorite and syngcnitic sedimentary sulphides of possible biogenic derivation in theAravallics, hitherto,considered being barren of fossils.

          According to Varadan (1976), “Auden's classic work on the Vindhyans of Son Valley has served as a modal for subsequent studies. Recent radiometric dating of glauconite and kimberlitic intrusives into them has indicated an age of about 1200 to 1400 Ma for the base of the Vindhyan in central India. However, a younger age of less than 700 Ma is ascribed to the Trans Aravalli Vindhyans. The age date has necessitated a rethinking on the history of stabilization and subsequent fracturing of the Indian Shield”.

        Detailed work on Vindhyans Super Group of Peninsular India has shown that the Vindhyans are not only known as Precambrian, but also the upper part of it is clearly Phanerozoic, apart from other organic remains, spores of vascular plants, fossil wood of primitive organization and other microfossils, the stromatolites reported from the Vindhyan are usually regarded as early to middle Riphcan age. The Jodhpur Group of Trans Aravallics, western Rajasthan correlated with the uppermost Vindhyan unconformably overlying Malani volcanic is dated as 745 Ma (Acharaya, 1976).According to him, the upper most Vindhyan exposed in theTrans-Aravalli region is similar to the eastern extension of the Cambrian sea of Salt Range, which is well correlated by the occurrence of lithological similarity,occurrence of salt pseudomorphs,gypsum-anhydrite-halite succession and above all “Cambrian-Late Palaeozoic Hiatus” in both areas. Paliwal (1992), defining the basement stated, that besides the main Aravalli Range and isolated hills to the west, deformed Aravalli and Delhi metasediments cropout at several places, forming the basement. He further stated that at Khatu, sandstone and shale sequence directly overlies the deformed Aravalli slates with the intervening Malani suite of rocks missing completely. Here the unconformity is marked by the presence of a conglomerate horizon. Granites and granitic gneisses belonging to the “Banded

Gneissic Complex” arealso encountered at a number of places along the western flank of the Aravalli
Mountain Range”.

           La Touche (1902) and Heron (1932) have grouped Malani rhyolite, igninbarite and associated welded tuffs under Malani suiteof rocks. According to Paliwal (1992), this acid volcanic, representing an early eruptive phase of the Malani suite, arc spread over a large area (55000 km㎢) along the western flank of the Aravalli Range, and extend subsurface up to Tosham hills of Haryana. and Kirana hills of Pakistan. Paliwal quoting Kochhar (1973, 1983) stated that the presence of ring dykes and vertical lines of some flows in rhyolites “indicates a zone of extension and the release in stress associated with the Aravalli-Delhi orogenic movements” which began in 2000 Ma and ended between 1500 to 1700 Ma, giving final shape to the Aravalli Mountain Building, which produced linear zones of crustal weakness along which the Malani suite was triggered by mantle plumes. MacDonald (1974) considered such ring dykes as continental representatives of mantle plumes and hot spots occurring in cratonic areas of crustal extension.

           All that happened long after the Aravalli orogeny had ended and according to Paliwal “a tcnsional tectonic setting preceded this early fragmentation of Pangaea in Late Proterozoic period to initiate the process. During the period, a number of linear zones of crustal weakness developed in the northwestern part of the Indian Peninsular Shield, parallel to the Aravalli trend. Because of these linear faults and high heat flow, the magmatismofthe Malani suite was triggered by mantle plumes (Kochhar, 1983) about 745 ± 10 Ma. This became the site of volcanic activities and later depositional basin of a shallow epicontinental sea west of Aravalli Range”. On the other hand, Chaudhry ct al. (1999) postulated that in the post Aravalli period at about 1000 Ma rifting started in thearea.Thiswas due to mantle plume, which resulted in widespread igneous activities in Rajasthan, Haryana and Kirana. He further added, “Extension of the crust not only resulted in the widespread volcanism and plutonism but also the subsequent deposition of sedimentary packages of Marwar SuperGroup in Indiaand Machh SuperGroup in Pakistan”.

           In conclusion, Chaudhry ct al. (1999) recorded that the Kirana complex of Late Proterozoic rocks, exposed in Indus Basin including rocks of Kirana, Nagar Parkar in Pakistan and Proterozoic exposures at Jodhpur, Malani, Tosham, Mount Abu and Erinpura in Indian, are composed of , volcano-plutonic and sedimentary packages. According to them, they represent a distinct “Cratonic rift assemblage” which was deposited in an extensional basin formed because of rising of the mantle plume around 1000 Ma. They named this entire outcrop area as “Malani-Kirana Basin” and ascertained that this basin neither belongs to Vindhyan System nor it is a part of Aravallics. Previously, however, it was believed that Kirana hills are highly eroded outliers of theAravalli chain (Wadia,1957) and wasincluded in the IndianShield.