Archean Greenstone Belts: Archean greenstone belts are an important component of Archean crustal provinces. They are sequence of volcanic and sedimentary rocks, in varying states of deformation which occur as sublinear belts in Archean cratons. They have striking physiographic and geological contrast to the granitoids plutons and gneisses, which most commonly surround them. The name greenstone is derived from the low-grade metamorphosed basalts that have a green hue imparted by the typical green colour of the metamorphic minerals like chlorite, epidote, actinolite, and green amphiboles. These belts are sometimes known as ‘supracrustal belts’ or as ‘schist belts’ because of their state of deformation. They are important source of metals, particularly gold and nickel and silver, copper, zinc and lead also. An ideal greenstone belt has three major litho-units – - Volcanic rocks in the lower part - Volcanic rocks in the middle part - Sedimentary rocks in the upper part The belts have synclinal structure, mostly intruded by granitic magma and cut by thrust faults. Greenstone belts formed in between 3.8-2.5 Ga, although they are not unique to the Archean and many are reported from the Proterozoic and Phanerozoic times. Most of the greenstone belts have short span of history – nearly 100Ma, sometimes < 50Ma and very few preserve a long history. For example, Barberton greenstone belt in South Africa preserved over 300Ma of Earth’s history. Significance – Archean greenstone belts contain regions of low strain and low grade metamorphism which provide critical information about the early Earth. One of the most studied of these is the Belingwe greenstone belt in Zimbabwe. Greenstone belts basalts and ultramafic lavas give important insights about Earth’s early mantle. The rocks formed as direct mantle melts and provide best window into the Archean mantle. They give significant clues to the nature of Earth surface processes. Sedimentary rocks record information about Archean weathering processes in the form of erosion surfaces and paleosols. Chemical sediments like evaporites, limestones and cherts provide information about ocean chemistry. Banded iron formations reflect hydrothermal input into the oceans Greenstone Belts have great significance in studying the Archean Tectonics, Early atmosphere, volcanism, crustal evolution, geothermal gradient, mantle dynamics and many informations related to early geologic history. It is also a very important source of economic deposits of chromite, magnetite, sulfides, PGE, gold and silver. Thus worldwide distribution of Greenstone Belt helps to understand the early geologic processes in a very significant way for future prospects as well as ancient dynamics Grenvillian Orogeny: Orogenesis is defined by extended periods of mountain building, usually resulting from the convergence of tectonic plates. The Grenvillian Orogeny is named after the village “Grenville” in Quebec, Canada. It is postulated to have been one of the largest orogenic events in Earth’s history and the last major orogeny in Laurentia, the Precambrian cratonic core of North America. Grenvillian Orogeny took place during the Late-Mesoproterozoic to EarlyNeoproterozoic. Evidences are preserved at the erosion surface in the Grenville Province, in inliers within, west of the Appalachians, and in the adjoining subsurface where Grenvillian rocks are covered under younger sedimentary sequences. This period of orogeny includes – 1. An early accretionary stage at 1.3-1.2 Ga 2. An interval of widespread magmatism at 1.18-1.08 Ga 3. A period of continent-continent collision at 1.08-0.98 Ga that was rapidly followed by uplift and exhumation of the orogenic core. The age of Grenvillian Orogeny, ca. 1090–980 Ma, usually based on the metamorphic signature. The orogeny marked the continent-continent collision between Laurentia and another craton, inferred to be Amazonia and finally the assembly of the Mesoproterozoic supercontinent of Rodinia. This orogeny is characterized by province wide occurrence of reworked gneisses with upper amphibolite to granulite facies and local eclogite facies of Archean age. The tectonic events in this region occurring within the 1.3-1.0Ga time frame record subduction of the oceanic crust, closing of an ocean basin, accretion of magmatic ores (Elzevirian Orogeny), sedimentation (Filton Group and related meta-sedimentary successions), AMGG magmatism, Himalayan-style continentcontinent collision (Ottawan Orogeny), and uplift of the orogenic belt. Considering the larger cratons that were proximal to Laurentia, most Rodinia configurations place Australia-East Antarctica, Laurentia and Baltica on one side of the Grenville Orogen within the supercontinent, and Kalahari, Rio de la Plata, and Amazonia on the opposite side defining the major cratonic players of the collisional Grenvillian Orogeny. Geological Significance of Grenvillian Orogeny with reference to India – The Grenvillian orogeny, a significant geological event occurring around 1.3 to 1 billion years ago during the Proterozoic Eon, has profound implications for the geological evolution of India. This mountain-building episode, although more commonly associated with North America, played a crucial role in shaping the geological framework of the Indian subcontinent. Here, we delve into the geological significance of the Grenvillian orogeny in India. Formation of the Aravalli Range: The most conspicuous geological consequence of the Grenvillian orogeny in India is the formation of the Aravalli Range, one of the oldest mountain ranges on Earth. The Grenvillian orogeny initiated the collision and subsequent convergence of continental fragments, leading to intense tectonic activity and uplift along the present-day Aravalli region. The Aravalli Range is predominantly composed of ancient rocks such as granites, gneisses, and schists, which underwent intense metamorphism during the orogeny. By studying these rocks and their structural characteristics, geologists gain insights into the timing, nature, and magnitude of tectonic forces involved in the orogenic process. Tectonic Framework and Crustal Evolution: The Grenvillian orogeny played a pivotal role in the crustal evolution of India, influencing the tectonic framework and structural architecture of the region. The collision and amalgamation of continental blocks during the orogeny led to the formation of thickened crustal segments and uplifted mountain belts. The Aravalli Range represents a segment of such crustal thickening and mountain-building, providing a window into the tectonic processes that shaped India's geological history. Furthermore, the orogeny influenced the development of regional fault systems and shear zones, which continue to affect the tectonic dynamics of the Indian subcontinent. Sedimentary Basin Development: In addition to mountain-building, the Grenvillian orogeny influenced the development of sedimentary basins adjacent to the emerging mountain belts. The intense tectonic activity associated with the orogeny led to the subsidence of crustal blocks, creating accommodation space for the accumulation of sediments. These sedimentary basins, such as the Delhi Supergroup, contain a diverse array of sedimentary rocks, including conglomerates, sandstones, and shales, which record the environmental conditions and paleogeography of ancient India during the Grenvillian orogeny. Studying the stratigraphic sequences and fossil assemblages within these basins provides invaluable insights into the depositional environments and tectonic history of the region. Mineralization and Economic Resources: The Grenvillian orogeny played a significant role in the generation of mineral resources in the Indian subcontinent. The metamorphic processes associated with mountain-building events resulted in the formation of economically significant mineral deposits within the Aravalli region. Pan - African Orogeny: • The Pan-African Orogeny refers to a series of major magmatic, metamorphic and orogenic activities of Neoproterozoic to early Paleozoic age that included the opening and closing of several large oceans, collisions of continental blocks, arc and microcontinent accretion and final suturing of continental fragments, finally leading to the formation of the supercontinent Gondwana. • This Orogeny is also known as the Pan-Gondwanan or Saldanian Orogeny. • The term Pan-African was first introduced by W Q Kennedy in 1964 to describe continent-wide thermo-tectonic events which culminated in the structural differentiation of Africa at ca. 500 Ma - a number of mobile belt formed between much older cratons. • Earlier, other terms were used for similar orogenic events on other continents, i.e., Brasiliano in South America; Adelaidean in Australia; and Beardmore in Antarctica. • Later, when plate tectonics became generally accepted, the term Pan-African was extended to all of the supercontinent Gondwana, but is rarely applied to Neoproterozoic crust-forming episodes of similar age in North America or Eurasia. • Meert (2003) proposed that the Pan-African could be subdivided into three different orogenic episodes: 1. The earliest activity, from ca. 900 to 600 Ma consisted largely of compression and consumption of oceanic lithosphere, primarily in the Arabian–Nubian shield and farther south along the East African Orogen. 2. This was followed by a period of extension and intrusion of post-orogenic granites for around 500 Ma centered around 570 Ma, designated as the Kuunga Orogeny. 3. The final episode consisted largely of thermal resetting of many areas which lasted up to 450 Ma. • Within the Pan-African domains, two broad types of orogenic or mobile belts can be distinguished: a. One type consists predominantly of Neoproterozoic supracrustal and magmatic assemblages of juvenile (mantle- derived) origin, with structural and metamorphic histories similar to those in Phanerozoic collision and accretion belts. - These belts expose upper to middle crustal levels and contain diagnostic features such as ophiolites, subduction- or collision-related granitoids, island-arc or passive continental margin assemblages as well as exotic terranes. Such belts include the (1) Arabian-Nubian shield of Arabia and north-east Africa. (2) Damara- Kaoko-Gariep Belt and Lufilian Arc of south-central and southwestern Africa (3) West Congo Belt of Angola and Congo Republic (4) Trans-Sahara Belt of West Africa (5) Rokelide and Mauretanian belts along the western Part of the West African Craton b. The other type of mobile belt contains poly-deformed high-grade metamorphic assemblages, exposing middle to lower crustal levels, whose origin, environment of formation and structural evolution are more difficult to reconstruct. Well studied examples are: (1) Mozambique Belt of East Africa, including Madagascar with extensions into western Antarctica (2) Zambezi Belt of northern Zimbabwe and Zambia Indian Context: In India, signatures of the Pan African orogeny are found only in the Southern Granulite Terrain, which forms the southern tip of the Indian Peninsula. It predominantly consists of Late Archaean to Paleoproterozoic gneisses and granulites, deformed and metamorphosed during the Pan-African event and sutured against the Dharwar Craton. The SGT is demarcated from the Northern Granulite Terrane by a group of E-W trending shear zones including the Moyar-Bhavani-Salem-Attur Shear Zone (MBSASZ) and Palghat-Cauveri Shear Zone (PCSZ). The northern granulite continues northward into Dharwar granite-greenstone terrane which is separated from the SGT by the Fermor Line (F.L). Towards the south of the Fermor line, large charnockitic massifs are found. The charnockites found in the marginal zone (from F.L to PCSZ) have an age of 2600 to 2500 Ma. However, the ones on the southern part of the PCSZ are of Neoproterozoic age (550-500Ma). This stretch of relatively younger rocks is known as the Pandyan Mobile Belt (PMB). The predominant lithological association in the PMB and the boundary shear zone (PCSZ) abutting against the Dharwar Craton is characterized by presence of Quartz Carbonate Pelite (QCP) suites, which suggests its origin by collision of the Dharwar Craton with another craton in the East Gondwana (possibly the Congo Craton) during the Pan-African age. From recent isotopic dating, a series of magmatic, metamorphic and deformation events that occurred in the PMB-shearing along PCSZ at ~800 Ma were identified. This was followed by a granite emplacement event at 550-600 Ma along shear zones and a 500-600 Ma event of charnockitisation in the PMB and a final thermal resetting event in the Kerela Khondalite Belt at 480 Ma. Komatiites:- Banded Iron Formation :- Archean – Proterozoic Boundary :- The Archaean-Proterozoic boundary represents a critical period in Earth's history, marking a transition from the ancient Archaean eon to the more modern Proterozoic eon. This boundary, spanning from approximately 3000 to 2500 million years ago, is characterized by significant geological changes and evolutionary milestones that have shaped the planet's crustal evolution. During the Archaean eon, the Earth's crust underwent intense deformation and metamorphism, leading to the formation of highly deformed rocks. Stable cratons were rare during this period, with most rocks showing signs of tectonic activity and metamorphic alteration. In contrast, the early Proterozoic eon witnessed the emergence of more stable continental plates, characterized by flat-lying or less deformed sedimentary-volcanic sequences. These sequences, such as the Pongola, Witwatersrand, and Huronian basins, were deposited unconformably on large, rigid Archaean continental areas. One of the key features of the Archaean-Proterozoic boundary is the presence of a major unconformity separating the highly deformed Archaean rocks from the relatively undeformed early Proterozoic cratonic basins. This unconformity signifies a shift in tectonic processes and crustal stability, with the early Proterozoic showing evidence of rigidity and stability in the form of aulacogens, dyke swarms, kimberlites (diamonds), and linear/arcuate erogenic belts bordering older cratons. The transition from the Archaean to the Proterozoic also marked a change in the distribution and characteristics of rock units in geological belts. Early Proterozoic belts exhibited a distinctive polarity and structural organization similar to modern belts formed by plate tectonic processes. This regular polarity was not commonly found in Archaean belts, indicating a shift towards more modern tectonic settings and processes by the early Proterozoic. Despite the emphasis on the Archaean-Proterozoic boundary as a significant turning point in crustal evolution, it is important to note that certain geological features, such as greenstone belts and granulite-gneiss belts, continued to form throughout the Proterozoic, albeit in lesser abundance. Understanding the tectonic environment and mode of occurrence of these belts is crucial for interpreting the origin of similar Archaean belts and their evolution across the boundary. In conclusion, the Archaean-Proterozoic boundary represents a pivotal period in Earth's history, marked by profound changes in crustal evolution, tectonic processes, and the development of stable continental plates. The transition from the Archaean to the Proterozoic eon brought about significant geological transformations that continue to shape our understanding of the planet's early history and evolution. Purana Basins:The Purana Basins of India, represent a group of Proterozoic sedimentary basins that are integral to understanding the geological evolution of the Indian subcontinent. These basins, including the Vindhyan, Cuddapah, Chhattisgarh, Khariar, Ampani, Indravati, Bayana, Marwar, Gwalior, and Bijawar Basins, are characterized by their ancient sedimentary deposits and unique tectonic histories. Each of these Purana Basins exhibits distinct geological features, sedimentary sequences, and structural characteristics that provide valuable insights into the tectonic processes and paleoenvironmental conditions prevalent during the Proterozoic era in India. The basins serve as geological archives that offer a glimpse into the early Earth's history, including the evolution of sedimentary basins, crustal movements, and tectonic events that shaped the Indian Plate. Researchers have conducted detailed studies on these Purana Basins to unravel their depositional histories, stratigraphic successions, and tectonic settings. By analyzing the sedimentary records and structural complexities of these basins, scientists aim to reconstruct the geological evolution of the Indian subcontinent and gain a better understanding of the processes that influenced the formation and development of these ancient basins. In the context of the basins discussed in the paper, the Purana Basins of India play a crucial role in elucidating the geodynamic processes, sedimentary dynamics, and tectonic interactions that have shaped the Indian Plate over millions of years. Studying these basins provides valuable information about the ancient Earth's history, the evolution of sedimentary environments, and the tectonic evolution of the Indian subcontinent during the Proterozoic period. A Crustal Province refers to a region characterized by distinct geological features, such as specific types of rocks, structures, and tectonic history, which differentiate it from surrounding areas. Crustal provinces can vary widely in size, ranging from small areas to entire continents. They are typically defined by their geological history, including processes such as plate tectonics, mountain building, and the deposition and deformation of rocks over time. Examples – Canadian Shield, North American Shield etc. Different Components of Archean Crustal Province – 1.) Greenstone Belts 2.) Granite- Gneiss Terranes 3.) Cratons 4.) Sedimentary Basins The East African Orogen (EAO) is a Neoproterozoic–early Cambrian mobile belt that today extends south along eastern Africa and western Arabia from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south. These belts were part of an orogenic cycle spanning the period between breakup of Rodinia (870–800 Ma) and final amalgamation of Gondwana (∼500 Ma). Extending ∼6000 km N–S, the EAO forms the largest continuous Neoproterozoic–Cambrian orogen on Earth, and is considered a “Transgondwanan super mountain range” whose formation and destruction had important implications for atmospheric oxygenation and thus for the evolution of higher organized organisms on Earth. The sedimentary deposition from this mountain chain, known as the Gondwana Super-fan, lasted for 260 million years and coincided with the Cambrian explosion, the sudden radiation of animal (Metazoan) life c. 550 Ma. These unprecedented sedimentary depositions probably made the diversification of early animal life possible. Traditionally, the EAO is subdivided into the Arabian–Nubian Shield (ANS) in the north, composed largely of juvenile Neoproterozoic crust, and the Mozambique Belt (MB) in the south comprising mostly pre-Neoproterozoic crust with a Neoproterozoic–early Cambrian tectonothermal overprint. Formation of supercontinent Rodinia through worldwide orogenic events occurred between ∼1300 Ma and 900 Ma. At ∼825 Ma Rodinia began breakingup in association with widespread continental rifting. By the end of the Neoproterozoic to early Cambrian the supercontinent Gondwana was formed by multiple closures of oceanic realms. East African Orogeny resulted from the collision of amalgamated arc terranes of the ANS with the Sahara and Congo–Tanzania Cratons to the west and the Azania and Afif terranes to the east. In one model, the EAO evolved from an accretionary orogeny involving the amalgamation of arcs and evolved into a collisional orogeny when the Neoproterozoic continent Azania collided with the CongoTanzania-Bangweulu Block at c. 640 Ma. In another model, the assembly of East Gondwana c. 750 to 530 Ma was a multiphase process which included two main periods of orogenesis: the older EAO (c. 750 to 620 Ma) and the younger Kuunga Orogeny (c. 570 to 530 Ma). Giant Dyke Swarms – Mafic dyke swarms are an important feature of the Proterozoic and in parts of some stabilised Archean cratons may be significant geological event in 2 G.a, these are potentially important time markers. Proterozoic swarms tend to be voluminous in terms of the number, width and length of dykes. The genesis of each swarms clearly constitutes a major thermal event affecting the Earth’s mantle. More-over because dyke swarms are often parallel to major shear zone and the dykes may be subsequently affected by these shear zones, these clearly an important tectonic control on their genesis. Recent U-Pb dating indicate that emplacement of any particular swarm took place over a very restricted time period. Not only it has large amount of thermal energy to be concentrated in order to meet the mantle, but the energy has to be delivered quickly and then apparently shut off The majority of Proterozoic dykes worldwide are aphyric, iron rich tholeiites comprising augite, plagioclase and titanomagnetite and minor hypersthene. Most are over-saturated quartz tholeiites but they range to olivine tholeiites that are still Fe-rich. The noritic and picritic rock types represent an important magma type in the late Archean-early Proterozoic where as more alkali dykes appear in the later Proterozoic. The high Mg picritic and noritic suites are dominant in early Proterozoic and are petrologically similar to many of the large layered intrusions such as Stillwater, Bushveld, Great Dyke etc. Tholeiitic magma type seems to have been available contemporaneously with the noritic type and indeed may have been locally present as one of the inhomogeneous magma pulses that characterises these large intrusions. Conversely alkaline magma types are quite rare in early Proterozoic dyke suites Archean TectonicsDuring Archean Earth’s mantle was hotter and lithosphere was thinner and weaker, more prone to necking and breaking. Asthenosphere must have been melted more extensively and hence was less viscous and more bouyant. Hence subduction was difficult. Rather Drip and Plume were common. Abundant mafic outpourings loaded the thin felsic proto-crust. Increase in density due to eclogitization stimulated sinking. Detached sinking diapirs formed. This was sagduction or drip tectonics. In Archean time only weak plate boundaries can form during mantle convection. Hence, subduction may occur on a locally confined scale associated with lithospheric mobility, whereas intraplate magmatism may occur elsewhere due to passive mantle upwelling. On secular cooling, as subduction became more widespread, intraplate magmatism waned, and subduction-related magmatism increased during a transition to an active lid mode. OphiolitesOphiolites are slices of oceanic lithosphere that have been thrust onto continental crust during tectonic processes. They provide valuable insights into the structure and composition of oceanic crust, which is otherwise mostly inaccessible due to being submerged beneath the ocean. Typically, an ophiolite sequence consists of several layers: 1. Sedimentary Layer: At the base of an ophiolite sequence, there may be a layer of sedimentary rocks, including shale and limestone, which accumulated on the ocean floor. 2. Mafic Layer: The mafic layer consists of rocks such as basalt and gabbro, representing the oceanic crust. Basalt forms from the solidification of lava flows at the surface, while gabbro forms from the cooling and crystallization of magma beneath the surface. 3. Ultramafic Layer: Beneath the mafic layer, there is often an ultramafic layer composed of rocks like peridotite. These rocks originate from the mantle and represent the deeper levels of the oceanic lithosphere. 4. Sheeted Dykes: Sheet-like intrusions of basalt known as sheeted dykes cut through the mafic layer. These dykes formed as magma intruded into fractures in the oceanic crust and cooled rapidly, creating a network of interconnected basaltic veins. 5. Pillow Lavas: Pillow lavas are bulbous structures formed when lava erupts underwater and quickly cools. They are characteristic features of the uppermost layer of oceanic crust and are often preserved in ophiolite sequences. OMTG – The OMTG occupies an area of about 900 sq. km and occurs as mega-xenoliths Within the vast Singhbhum granice, near Champua, Seraikala, Onlajodi, Rairangpur and ThakuITPunda, where they extensively intrude the OMG. The structural concordance between OMG and OMTG shows their syn-kinematic Development The OMTG is predominantly of tonalite-trondhjemite-granodiorite (ITG) composition, with subordinate granodiorite, qu_artz diorite and monzodiorite. The gneisses are well foliated and locally banded, and contain plagioclase, quartz, biotite and subordinate K-felspar, with accessory zircon, sphene and apatite. Garnet is rare. In overall composition, they are comparable to the Archaean TTGs of the world. Tlie gneisses show high Sr, low Ba/Sr ratio and Variable K/Rb. They-are moderately enriched in LR.EE with a steeply sloping pattern without Eu anomaly. They may have been formed by 40% partial melting of OMG-type amphibolites at 590-770°C and 5kb, corresponding to depths of 718 km. The gneisses define a Pb-Pb isochron of 3378±98 Ma and 3405±53 Ma, along with Sm-Nd age of -3.29 Ga and Rb-Sr isochron of 3280±130Ma. In situ Pb-Pb zircon age and U-Pb TIMS zircon age cluster around -3.45 Ga. Older ages of about 3600-3800 Ma that were initially obtained, were retracted through later studies. The age of these gneisses can therefore be bracketed between 3300 and 3450 Ma. The OMG, together with OMTG, were probably involved in a widespread later metamorphic event at 3100-3200 Ma. IOG – - lies unconformably over Besoi Granite - mildly metamorphosed, predominantly - ferrugenous sediments associated with basic volcanics - occurs in three belts- Noamundi-Jamda-Koira, Gorumahisani-Badampahar and Tomka-Daitari - in all the three belts the BIF is closely associated with lavas and tuffs. - IOG forms a synclinorium in the Jamda-Koira valley. - age: 3145 My - sedimentation started before the intrusion of 3120 My old Singhbhum Granite Type B. Intraplate Subduction -N Sarkar and A.K Saha (1983) visualise the Singhbhum shear zone as the suture between the Singhbhum craton and Chotonagpur Gneiss (CGC). The stages in the evolution of the ·suture zone are described below – (i) Formation of ocean around the craton at 2.2 Ga in which sediments of North Singhbhum orogen are deposited. (ii) Northward subduction of the craton below CGC and folding and metamorphism of Singhbhum Group. (iii) Tensional phase of Dhanjori-Dalma volcanism and emplacement of gabbro-anorthosites. (iv) Renewed subduction and partial melting of the cratonic plate to produce - granites and granophyres (Arkasani, Chakradharpur, Mayurbhanj, Kuilapal etc.). (v) Renewed compression, folding and transcurrent / thrust faulting along the Singhbhum shear zone, forming the suture. The difficulties with the model are the absence of petro-tectonic assemblages in the suture (which is a common drawback in most Precambrian models), lack of evidence for large-scale transport across the Singhbhum shear zone and the problem of a major intervening tensile phase associated with extensive oceanic volcanism in an overall compressive regime. Ensialic Orogeny The essential features of this model by A. Gupta and A. Basu (2000) are the riftrelated volcanism on the craton (Dalma, Dhanjori) with concurrent development of sedimentary basins. The compressional regime that followed was triggered by delamination and sinking of subcrustal lithosphere leading to ensialic orogeny. This model gains limited support from the geochemistry of basic lavas, which may be related either to back-arc or Continental rift settings. The model visualises a single continuous landmass of singhbhum craton together with Chhotanagpur Gneiss (CGC) for which there is no gecchronological evidence of older Archaean ages in CGC. Plate and Plume Tectonic Model P.K. Banerji and S.K. Ghosh (1994) propose two events of plume activity, the first one forming a series of lavas (Simlipal, Ongarbira) involved in seafloor spreading followed by orogeny and the second one forming another Series of lavas (Dhanjori) after cratonisation. The difficulty with the modei is the longlived plume in the area despite one earlier orogeny, lack of lavas of Intermediate composition like andesites, and presence of conspicuous arenite interbeds in the lavas of Simlipal basin. Collision of Microplates A.N. Sarkar (1988) proposed three cycles of micro platic collision between Singhbhum craton and Chhotanagpur Gneiss. The first cycle saw the evolution of Dalma lavas as ophiolites, the second cycle resulted in the development of rotation of plates, intense folding and transcurrent shear zones. The third cycle resulted in the abduction of the continental lithosphere. The collision of microplates spans a period of about 300 m.y. required too slow motion of plates in Precambrian. This model also does not account for differences in age of Singhbhum craton and Chhotanagpur Gneiss. Back-arc Marginal Basin Model M.K. Bose (1994) proposed that North Singhbhum orogen was a back-arc marginal basin accreting to the Singhbhum craton. A mid-basin rift was formed at the site of Dalma volcanism. Chhotanagpur Gneiss represented the root zone of island arc. The complex evolution of CGC described in the section on Satpura mobile belt negates the simplistic island arc-back arc model for CGGC. Eastern Ghat Mobile Belt The Proterozoic Eastern Ghat Mobile Belt (EGMB) extends for about 1000km in a NE direction from Ongole in Andhra Pradesh in the south to Brahmi River in Odisha in the north, with a maximum width of about 300km, tapering from north to south. The mobile belt occurs along the eastern margin of Deccan protocontinent, consisting of the Archean, Dharwar, Bastar and Singhbhum Cratons. It is overlain by Phanerozoic sediments along the East Coast and is bordered by the Bay of Bengal. The belt is dissected by two prominent grabens of Mahanadi and Godavari. Geographically, the term Eastern Ghats includes the Nallmalai Hills of Cuddapah basin and the charnockite massifs of Southern Granulite Terrane, but geologically it is restricted to the mountain belt close to the East Coast and is essentially a granulite belt. It is also referred to as the Eastern Ghats Granulite Belt. Geological Importance of EGMB: 1. Importance of the belt lies in the assembly of Rodinia supercontinent, in which EGMB forms part of the SWEAT (south west United states and East Antarctica). WGMB got separated from Antarctica during the break up on Gondwana sediments. 2. They are well-known for ultrahigh temperature metamorphism imprints – nearly about 1000°C at a depth of ~32-33km and 9-10kbar pressure. 3. Granulite facies rock charnockite, khondalite are dominant rock types found here. 4. Well known resource of bauxite, small deposits of Mn, sillimanite, graphite and gemstone. 5. The belt is the main provenance of Th and Ti beach placers along the East coast of India. 6. Presence of layered anorthosite, nepheline, syenite. Tectonic Evolution of EGMB: Age Events 550 – 650 Ma Exhumation and Stabilitisation in Pan-African age. 800 – 850 Ma Emplacement of anorthosite massifs, some alkaline rocks, younger granitoids and charnockites. 950 – 1150 Ma ▪ Main Eastern Ghats orogeny (equivalent to Grenvillean orogeny) 950 – 1150 Ma ▪ Main Eastern Ghats orogeny (equivalent to Grenvillean orogeny) ▪ Khondalite Group – Garnet-sillimanite-graphite gneiss with minor cordierite-sapphirine-spinel, calc-silicate rocks and rare marbles; Quartzites (garnet + sillimanite) Emplacement of alkaline rocks along the rift margins ▪ Evolution of platform (‘Purana’) basins like Cuddapah, Chhattisgarh, Indravati etc. ▪ Evolution of Nellore-Khammam Schist Belt Charnockite and gneisses of the basement (WCZ) ~ 1500 Ma 1600 – 1800 Ma 2600 – 2800 Ma ▪ Khondalite Group – Garnet-sillimanite-graphite gneiss with minor cordierite-sapphirine-spinel, calc-silicate rocks and rare marbles; Quartzites (garnet + sillimanite) Based on Rock types – Lithological division of EGMB in 1998 (Ramakrishnan et al.) EGMB was divided into provinces based on isotopes (Rickers et al. 2001) and geology (Dobmeir and Raith, 2003) Ramakrishnan et. al 1998 Rickers et al. 2001 WCZ Domain IA and IB (2.9-3.9 Ga) Krishna Province and Domain IV (2.5-2.9 Ga) (Nellore-Khammam Schist Belt) Domain II (2.2 – 2.5 Ga) Jeypore-Rengali Province WKZ CMZ EKZ Domain III (1.8-2.2 Ga) Dobmeir and Raith, 2003 Eastern Ghat Province Different Tectonic Zones of EGMB: WCZ: The WCZ is seen all along the margins of EGMB except between Junagarh and Sambalpur, where WKZ and CMZ tectonically overlie the cratonic gneisses. WCZ consists essentially enderbites with bands and enclaves of mafic granulites, which gives total it ages of 2600-2800 Ma that correspond to the major orogenic culmination in the cratons. The WCZ exposes the mid to lower crustal level of the craton that is now involved in the Eastern Ghats orogeny. CMZ: The CMZ occurs in the middle of EGM be sandwiched between the two Khondalite Zones: WKZ and EKZ. The characteristic feature of this zone is extensive anatexis and migmatization of all rock types, and prominent development of structural domes and basins. Porphyritic charnockite with megacrysts of perthite are common in shear zones, and as separate intrusive bodies. Charnockite and Khondalite enclose lenses and bands of pyroxene granulite which may be basic sills or flows. Khondalite occurs as enclaves in granitic rocks. WKZ: Khondalites dominate the areas near Koraput and Kalahandi, immediately to the east of the WCZ. bedded manganese cherts associated with calc-silicate rocks and pelite are well developed in WKZ, in a 150 km long belt. Marbles are however, rare. Numerous basic-ultrabasic bodies intrude into the Khondalites between Riamal and Angul. EKZ: The zone continues to the north near the Chilka Lake and disappears under the phanerozoic sediments - calc-silicate rocks, marbles and manganese silicate rocks. Structural studies in WKZ and EKZ show that the first episode of folding along the axial planar foliation is contemporaneous with granulite facies metamorphism and conforms to the main Eastern ghats trend. The second episode of folding produced C-axial tight folds resulting in hook shaped folds as seen in Khondalite. The third episode of folding produced asymmetrical folds uniformly plunging NE to NNE.
