The Earth originated about 4.5-4.6 billion years ago. It is characterized by a crust enveloping the interior (mantle) of the planet with a sharp break (Mohorovicic discontinuity) in between. The crust is thickest (30-80 km) in the continental parts of the Earth. The continent represents about 79% of the total volume of the crust. It is not yet precisely known when the continental crust began to form. The oldest in situ continental crust dated in Slave Province of Canada is 3.96 Ga. Scarcity of ancient crust on present Earth’s surface is presumably due mainly to recycling of the old crust into the mantle or due to remelting during later process of crustal development.
The first 50-100 Ma of Earth’s history is characterized by extensive melting in the outer part of the mantle. During this period, a thin komatiitic crust was formed following devolatilization of the mantle. Partial melting of the komatiitic crust produced tonalitic magmas constituting the oldest continental crust. The process of crustal growth incorporated interaction of early crust with the mantle and redistribution of the crustal constituents by sedimentary and tectonic processes involving several evolutionary mechanisms. There is a general agreement that bulk of the continental crust was developed during 2.7-3.0 Ga. To sum up, the ancient Earth crust owes its origin to magmatic processes. The oldest recognizable continental crust is composed mainly of tonalitic gneisses containing fragments of komatiite and basalt, part of which belongs to early oceanic crust.
An internationally agreeable scheme of time stratigraphic classification of the Precambrian is yet to emerge. According to a broad classification, the period 4.5-4.2 Ga is considered as pre-geological period. The Precambrian era has been provisionally subdivided into three major periods, viz. the Kata-archaean (>3.5 Ga), the Archaean (3.5-2.5 Ga) and the Proterozoic (2.5-0.65 Ga). The Proterozoic has further been divided into Palaeoproterozoic (2.5 to1.6 Ga), Mesoproterozoic (1.6 to 0.8 Ga) and Neoproterozoic (0.8 to 0.65 Ga). The lower and upper limits of these classified periods are not precisely defined.
In the late sixties, two distinct tectonic domains were recognized in Precambrian viz. cratons (>2.5 Ga) and encircling mobile belts (< 2.5 Ga). The cratons are mostly Archaean in age and are generally composed of low-grade greenstones and related metasediments (greenschist – amphibolite facies) with voluminous granitoids belonging to the tonalite-trondhjemite-granodiorite (TTG) suites. Many granite-greenstone terrains of major cratonic blocks show evidence of stratigraphic cyclicity. The deformation in cratonic domains are now recognized to be largely due to extensive granite diapirism. The mobile belts, on the other hand, comprise medium – to high-grade supracrustals, granitoids, gneisses along with a wide array of plutonic suites. The mobile belts generally bear evidence of repeated reactivation and reworking. The stratigraphic relationships in mobile belts are often obliterated due to polyphase tectonothermal events.
Framework of Indian Precambrian
About two-thirds of the Indian Peninsula comprises Precambrian rocks (Fig.1). These rocks in parts are covered by Phanerozoic sedimentary suites and by the Deccan plateau basalts. The Indo-Gangetic plain occurring between the Precambrian rocks of the Indian Peninsula and the highly deformed suites of the Himalaya conceals northward dipping Precambrian rocks of which little is known. Precambrian rocks of the Himalaya are highly tectonised due to collision of the Indian Peninsula with the Asiatic mainland and are separated from the southern shield by a northward dipping boundary thrust plane. The continuity of Precambrians across this boundary thrust below the Indo-Gangetic alluvium is, therefore, difficult to establish.
Geological Map of India (click to view large image)
The Indian Peninsula is traditionally considered to be a monolithic continental shield constituted by crystalline rocks. Later, the Precambrian rocks of India were divided into distinct segments based on principal orogenic trend, viz. Dharwar, Eastern Ghat, Aravalli and Satpura. Several genetic classifications have been attempted reflecting divergent interpretation of the tectono-stratigraphic make-up of different parts of the Peninsular shield.
There is serious difficulty in adopting a uniform concept-based genetic classification because such concept may change as new data and more acceptable alternate understanding become available at a later date. The subdivisions of the Precambrian rocks of India are not, therefore, strictly based on tectonic status of the different crustal segments either as cratons and mobile belts, or blocks and orogenic belts, etc. Instead, the (Peninsular) Indian Precambrian crust could be divided into geologically well defined geographical parts (Fig.2). Such geographical parts have been identified taking to consideration that their boundaries, in many cases, are marked by some fundamental crustal dislocations and/ or by vast expanse of Phanerozoic rocks. Two such fundamental crustal dislocation zones in the Indian Precambrian are the Godavari graben and the Mahanadi graben. The portion of the Indian Peninsula lying south and southwest of the Pranhita-Godavari graben, can be called the Southern part of the Peninsula. Similarly, another important part lying between Mahanadi graben and the Godavari graben is the Central part of the Peninsula. The combined Eastern and Northeastern part of the Peninsula, north and northeast of the Mahanadi graben with northern boundary formed by the Quaternary landscape of the Indo-Gangetic Plains and the Brahmaputra valley. The North and Northwestern part of the Peninsula lies to north of the tectonic zone between the rocks of the Vindhyan Supergroup and the Bundelkhand-BGC-Aravalli massif.
Each of these parts can conveniently be sub-divided into provinces on the basis of distinct lithological, structural, metamorphic, geochemical characteristics and geochronology. The boundaries of the provinces are usually marked by shear/ fault zone or in some cases by a transition zone. However, genetic terms like craton, mobile belt, cratonic block etc. cannot be entirely avoided because of long usage of these terms in the existing literature.
Map Showing distribution of Geological Provinces in India (click to view large image)
Salient characteristics of Precambrian geology of India
The Precambrian geology of the geographical segments and the provinces therein can be summarised as follows:
Southern part of the Peninsula
The region falls south of Central India Tectonic Zone and southwest of the Godavari rift. Only a small part of the Eastern Ghats occupies the northeastern part. The Precambrian rocks of the southern part of the Indian Peninsula are dominantly granulites, high-grade gneisses with migmatites, greenstones, ancient supracrustal rocks, post-tectonic granites and cover sequences viz. Cuddapah, Kurnool, Bhima, Badami, Kaladgi, Pakhal and Sullavai.
The southern part of the Peninsula comprises two provinces. They are (i) Dharwar Province and (ii)Southern Granulite Province. The Dharwar Province falls north of the Southern Granulite Province and the two are separated by Palghat-Kavery shear zone.
The Dharwar Province is essentially a granite-greenstone terrain characterized by a number of NNW-SSE trending belts of schistose rocks separated by granitic terrains. The Province is divisible into western and eastern parts along a major shear zone west of the Closepet Granite.The supracrustal schistose rocks (metavolcanics) belong mostly to Dharwar Supergroup (3000-2600 Ma), but relics of an earlier supracrustal sequence (Sargur Group; ?3400 Ma) have been recognized which predate the granitic gneisses. The tectonometamorphic status of the older and younger schist belts and the gneisses is not very clear.
The Southern Granulite Province is made up chiefly of charnockites, mafic granulites and khondalites and is intersected by several shear zones. The province is, however, not solely composed of rocks of granulite facies, but gneisses and supracrustal rocks of amphibolite facies are also abundant. Radiometric dates obtained from this province vary between 3.0 and 2.0 Ga indicating the ages of the parent rocks, their transformation into granulites and also their subsequent retrogression. As a result, this province includes 2.5 Ga old Nilgiri-Madras belt as well as even 550 Ma old charnockites of Madurai and Trivandrum area. This province is often focused in recent literatures in an attempt to reconstruct the East Gondwanaland by a possible Madagascar-India fit.
Central part of the Peninsula
The central part of the Peninsular Shield forms broadly a rectangular area bounded by the Eastern Ghats, the Mahanadi and Godavari rifts, and the Narmada-Son Lineament (North). This part is made up dominantly of Archaean to Middle Proterozoic basement complexes and Late Proterozoic to Early Palaeozoic platformal cover rocks. The greater part of the area is occupied by granites and gneisses with engulfed and overlying supracrustals of metasedimentary and metavolcanic rocks of Sausar, Sakoli, Bengpal, Bailadila and other suites. In this ancient terrain, a number of Late Proterozoic basins were developed (e.g. Vindhyan, Indravati, Chhatisgarh, Baster, etc.) which are now occupied by unmetamorphosed sediments.
The central part has been broadly divided into three provinces, viz. Bastar Province, Satpura Province and Eastern Ghats Province.
The Bastar Province represents major part of the Central Peninsula and includes the rocks of Sukma, Amgaon, Bengpal, Bailadila, Dongargarh and Sakoli belt. Some of the granitoids in the Bastar Province are intrusive into the supracrustal rocks while a number of them appear to form the basement and, thus, are older than the supracrustals. The oldest granitoid in the Bastar province is a felsic gneiss occurring in Sukma area which is 3.0 Ga old (Pb-Pb age). More recently generated date (U-Pb from single zircon crystals) from the basement (tonalite gneiss) of the Kotri belt indicates an age of 3562 ± 2 Ma.
The Satpura Province represents a Proterozoic mobile belt (including Mahakoshal belt, Sausar belt. Bilaspur-Raigarh-Surguja belt and Betul-Chindwara belt) trending ENE-WSW to the west of the Chhattisgarh basin and north of the Amgaon, Sakoli and Nandgaon supracrustals. The rocks in the province are extensively deformed and metamorphosed during the Early to Middle Proterozoic.
Eastern and Northeastern part of the Peninsula
This part is separated from the rest of the Indian Peninsula by the Mahanadi rift graben bounding it in the west and south. Different provinces included in this segment are Singhbhum-North Orissa Province, Chhotanagpur Province and Meghalaya Province.
The Singhbhum North Orissa Province is characterized by an Archaean continental nucleus (> 3.0 Ga) bounded by the arcuate Singhbhum shear / thrust zone in the north and northwest and Sukinda thrust in the south. This Archaean basement is overlain to the north by Early Proterozoic supracrustals (North Singhbhum Mobile Belt) trending roughly E-W. This belt of supracrustals is bounded by the Chhotanagpur Province, which occurs further towards north. The youngest Precambiran supracrustal sediments in the province occur in the Kolhan basin showing a synclinal structure. Striking characteristics of the Singhbhum-North Orissa Province include wide spread occurrences of Early Archaean tonalite-trondhjemite gneiss and greenstone belts with prominent Banded Iron Formation.
The Chhotanagpur Province represents a sea of composite granitoids with enclaves of metasedimentary rocks, granulites, subordinate mafic/ ultramafic schists and minor anorthosites. Rocks of the Chhotanagpur belt range in age (K/ Ar dates) from 1500 to 800 Ma. The terrain has experienced a series of tectonothermal events spanning over more than a billion years.
The Meghalaya Province is bounded to the south by the Dawki lineament and its northern edge is covered by alluvium of the Brahmaputra River. The Precambrian rocks of the Khasi Hills representing a part of the province, have been subdivided into the Gneissic Complex, non-porphyritic granitic rocks, the supracrustals of the Shillong Group, the Khasi Greenstone and porphyritic plutons. The province is a basement reactivated terrain with a Proterozoic tectonothermal history comparing well with the Chhotanagpur terrain. The Gneissic Complex has yielded two Proterozoic dates viz. 1700 Ma and 1150 Ma with the intrusive porphyritic granite plutons yielding ages around 800 and 550 Ma.
North and Northwestern part of the Peninsula
This part is bounded to the south by the Son-Narmada Tapti megalineament. Though the western and northern India is treated together here, these two segments are separated from each other by a mega thrust/ fault known as the Great Boundary Fault. Provinces identified in this part are Bundelkhand Province, BGC Province and Aravalli-Delhi Province.
The Bundelkhand Province is broadly a triangular segment composed of granite-granodiorite, quartzo-feldspathic gneisses and enclaves of metasedimentary and metavolcanic rocks, quartz-reefs and dykes of dolerite. The existing geochronological data indicate that the rocks were cratonised mainly in the Late Archaean period.
The BGC Province is composed of an assemblage (BGC/ Bhilwara Supergroup) of high grade gneisses and granulites, vestiges of greenstones and low grade supracrustals, syn-orogenic granites showing a wide variety of ages (3500 to 2000 Ma) that overlap, the period of deposition of overlying Aravalli-Delhi Supergroup of rocks. The contact between the BGC and the overlying supracrustal suites is a well defined unconformity. Part of the Bhilwara Supergroup appears to represent a continental rift, later developed into an ensimatic orogen.
The Aravalli-Delhi province is composed of Proterozoic supracrustal sequences classified as the Aravalli Supergroup and the Delhi Supergroup. In addition, a host of granitic, basic and ultrabasic intrusives and acid extrustives of different ages also occur in the Province. Deposition of the Aravalli rocks (dominated by granite derived sediments, carbonates and volcanics) took place in fault-troughs under eugeosynclinal-miogeosynclinal environment. The Delhi basins opened in close proximity to the Aravalli province following the evolution of the Aravalli Supergroup, where thick piles of volcano-sedimentary rocks were deposited.
Late Proterozoic igneous and sedimentary rock assemblages (Trans-Aravalli Sequences) grouped under Malani Igneous Suite and Marwar Supergroup respectively, occupy a wide area to the west of the Aravalli Mountains. The Malani and Marwar rocks are not deformed indicating that the episodes of the Precambrian magmatism/ sedimentation in this part of northwestern India are post-tectonic.
Tectonically the Indian Peninsular Shield is a composite mosaic of several Precambrian crustal blocks characterized by cratons, mobile belts, terrains of basement reactivation etc. along with cover rocks of Proterozoic and Phanerozoic age and accumulated in intracratonic sags and rifts. The joins in between the blocks are mostly thrusts/ rifts, exact nature of which is often uncertain. It seems that most of the evolutionary processes took place broadly simultaneously and sequentially in the different blocks. The present day configuration of the Peninsular India took place mainly during the Middle Proterozoic period. Deposition of platformal sediments in the sags and rifts was followed by legation of the blocks and it continued from Middle Proterozoic to Early Palaeozoic.
The Middle Proterozoic rocks constitute complexly folded mobile belts along the margins of the cratonic blocks. These belts are associated with certain major tectonic elements indicating major zones of dislocation. Some of these dislocation surfaces in Indian subcontinent are i) Phulad lineament (suture zone) in north-western India, ii) E-W trending Central Indian Shear in central India, iii) Singhbhum Shear Zone showing a curvilinear trend in eastern India, iv) E-W trending Palghat-Kaveri shear system in south India and v) Mega thrusts (MBT, MCT) showing an average WNW-ESE trend in Extra-peninsular India.
The Extra-peninsular region forms the northern border of the Indian subcontinent and is occupied by the Himalayan mountain range. The Precambrians of the Extra-peninsular region have been described under two subdivisions viz. i) Eastern Himalaya and ii) Western Himalaya. The segment of the Himalayan belt, which passes through Sikkim, Darjeeling and Arunachal Pradesh, falls in Eastern Himalaya while the Western Himalaya consists rest of the belt. The eastern and western segments broadly show identical geological features.
The average trend of the belt in the Eastern Himalaya is E-W while that the western Himalayan is nearly NW-SE. The bulk of the Himalayan range is composed of Proterozoic rocks that have undergone thrusting and reactivation during Meso-Cenozoic orogenesis.
The Himalayan ranges can be subdivided longitudinally into four regions. These are (from south to north) i) Sub-Himalayas (foothills belt), ii) Lesser (lower) Himalayas, iii) Great (central) Himalayas and iv) Tibetan (Tethyan) Himalayas. The Sub-Himalayas are made up dominantly of Tertiary and Quaternary sediments, which were derived from northern mountains and accumulated along the Main Boundary Thrust bordering the Sub-Himalayas on the north. The Lesser Himalayas consists largely of nappes that override autochthonous Precambrian sedimentary suites. The Great Himalayas override the Lesser Himalayan belt along the Main Central Thrust. It is made up dominantly of high grade Precambrian metamorphites overthrust by Phanerozoic rocks. In addition, it contains Cenozoic granites. Multiple deformation and metamorphism, thrust-nappe tectonics, migmatisation and granitic emplacement have made this belt a complex tectonic unit. The Tibetan Himalayas are made up of supracrustal assemblage belonging to controversial theories of one-stage Tethys or two-stage Tethys evolution.
Though a theory of continental collision is related to the evolution of the Himalayas, the belt bears stamps of geological activities spanning over a period from Early Proterozoic to the present. According to the plate-tectonic model, the Indian shield was subducted below the overriding Tethyan suite and a sedimentary wedge contained below the two converging plates. It is believed that the Himalayan mountain chain is a part of the global Meso-Cenozoic mobile belt extending from the Atlas-Alpine mountain belts in the west to the Indonesian arc in the east. It is also thought that the northerly advancing Indo-African plate has subducted below the Eurasian plate leaving behind trails of ophiolite sutures. The drifting and legation of India with Eurasia was preceded by rifting of several continental blocks from the northern Indo-Australian margin of the erstwhile Gondwana supercontinent and these tectonic processes took place during a period ranging from Silurian to Cretaceous.
Precise classification of the Precambrian crust in general and that of the Indian shield in particular as adopted and described in the preceding paragraphs, are based on several important aspects such as major cycles of sedimentation, events of deformation, metamorphism, magmatism etc. during the Precambrian period. In spite of the hindrances in unraveling the history of geological evolution, the present state of knowledge on the subject is at an advanced stage through accumulation of new objective data, revolutionary changes in the understanding of global geological processes and interpretation of earlier and newly generated data which provided a comprehensive picture of this fascinating area of Indian geology. It is, however, needless to mention that the present idea on the scenario is not a complete one since concepts are still evolving with incremental enrichment of knowledge.
Source: A Manual of the Geology of IndiaVol. 1, Pt. 1, Spl. Pub. No. 77,Geol. Surv. Ind., 2006, 572p
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