Introduction: The term Archaean was introduced by J.D Dana for the ancient rock succession of Canadian Shield. Later this succession was divided into two units. The lower and the older unit made up of highly deformed and metamorphosed rocks referred to as Arc Archaean group. The upp ppeer unit nit consis nsisti tin ng of less def deformed and less metamorphosed rock succession was named as the “Proterozoic Group”. The term Pre-Cambrian Pre-Cambrian was rather loosely used for all rock formations formations older than Cambrian age. But, now it is restricted to the Proterozoic Group only and the Archaean are excluded from the definition of the Pre-Cambrians. Archaean are the oldest known rocks on the earth’s surface dating back to about 3600my before present. These are found to be occurring in the central portions of all the shield areas of the world and form the basement for all the stratified stratified formations , meaning unfossiliferous. They also, of younger ages. These rocks are totally “azoic” form the cores of the great mountain chains of the world. These rocks being the most most anci ancien entt have have un unde derg rgon onee thor thorou ough gh recr recrys ysta tall lliz izat atio ion n du duee to inte intens nsee metamorphism metamorphism and have also, suffered suffered multiple multiple episodes of structural structural deformation. deformation. As a result of this the rocks show well-defined foliated structures i.e. gneissosity and schistoc schistocity ity and at places places even even granul granular ar nature nature like igneous igneous rocks rocks i.e. granulo granulose se structure. Hence these rocks are often referred as ancient recrystalline rocks. They are highly folded and faulted due to multiple episodes of structural events. They are also intruded by numerous plutonic igneous bodies. All these above cited events have rendered these rocks extremely complex. For this reason the Archaean are called “Basement Complexes or Fundamental Complexes”.
Origin or Mode of formation of Archaean: The rocks of Archaean age are structurally disturbed and metamorphosed to such an exten extentt that that most most of the their ir origi original nal chara characte cters rs are more or less less destr destroye oyed d or altered. Hence, it is difficult to decipher their mode of formation properly. The possible mode of formation of various units making up the Archaean is as follows. 1. The lowest stage has a strong influence of basic rocks. This unit is believed to represent the relics of the primordial crust i.e. the part of the first formed crust crust of the earth earth resul resultin ting g thr throu ough gh the the conso consolid lidati ation on of the gaseo gaseous us or molt molten en plan planet et.. Th Thes esee rock rockss are are pres preser erve ved d as un undi dige gest sted ed mass masses es or “xenoliths” in a vast country of granitic gneisses. 2. The graniti graniticc gneisses gneisses forming forming the largest largest masses masses of Archaean Archaean were were formed formed either by differentiation of the original basaltic magma or by metasomatic transformation of preexisting rocks. 3. At the third third stag stagee protog protogeo eosyn syncli clines nes were were forme formed d over over the basem basemen entt of granitic rocks. A thick pile of sedimentary rocks along with the products of submar submarine ine volcani volcanicc outburst outburstss were deposite deposited d in these these geosync geosyncline lines. s. The composition of hydrosphere and atmosphere were probably much different than than pre presen sentt day. day. Th These ese volca volcano no sedim sedimen ents ts contai contain n some some of the riche richest st
deposits of iron and manganese that were deposited as chemical precipitates [chemogenic sediments]. 4. The These se protogeosy protogeosyncli nclines nes were subjected subjected to orogenic orogenic movemen movements ts and granitic granitic plutons were emplaced in them. As a result of this stage, conspicuous fold mountain ranges came into existence. This fourth event marked the closure of the Archaean era. The four major units of Archaean represent each of these four events.
Distribution of Archaean in India : The various units of Archaean rocks form an enormous extent of the surface of India. They cover nearly two thirds of the peninsular India. They are very well developed in south India, Aravalli hill ranges of Rajasthan, Bundelkhand region of Madhya Pradesh, Singhbhum area of Bihar and Orissa and also along the Eastern Ghats. In the extra-Peninsular India, the Archaean rocks occur along the whole length of the Himalayan Mountain forming the bulk of the high ranges. This portion of the Himalayas is known as “Central Crystalline axis” and runs as a broad central zone from Kashmir to Assam.
Petrology of the Archaean system: In all the localities of occurrence of Archaean rocks of India, the most common rock is gneiss. This rock characteristically shows constant banded and foliated structure i.e. gneissosity. The mineral composition varies from granitic to gabbroic. There also, is a wide variation in fineness and thickness of layers. The grain size also varies from fine felsitic to coarse crystalline. The rock is called gneiss regardless of the mineral mineral compos compositio ition, n, but owing owing to prevaili prevailing ng gneisso gneissose se structur structure. e. The mineral mineralss present present in common common granitic granitic gneiss gneiss is orthocla orthoclase, se, oligocl oligoclase, ase, microc microcline line,, quartz, quartz, muscovite, biotite, hornblende etc with numerous minor accessories minerals. The next abundant rock in Archaean is schists showing recrystallised foliaceous structur structure. e. Mica schists schists are most most common common and other other varietie varietiess hornblen hornblende, de, talc, talc, chlorite, epidote, sillimanite and graphite. In addition to these most common rocks, the Archaean also contain bands layers and lenses of slates, phyllites, granulites, marbles marbles,, dolomi dolomites tes and banded banded fer ferrugi ruginous nous quartzi quartzites tes etc. The Archaea Archaean n rocks rocks generally have undergone high-grade regional or dynamic metamorphism since they have have witne witnesse ssed d thre threee to four four cycl cycles es of oroge orogenic nic movem movement entss and also also igneo igneous us emplacements. At the same time there are also some areas, which show feeble [lowgrade grade]] meta metamo morph rphism ism.. Th Thus, us, the these se rocks rocks show show all all vari variati ations ons from from low-gr low-grad adee regional metamorphism to high-grade plutonic metamorphism.
Archaean succession of Karnataka :
Introduction: The geolo geologic gical al histo history ry of Karn Karnata ataka ka is large largely ly confin confined ed to the oldest oldest eras, eras, the Archaean and the Proterozoic. The rest of the periods from Cambrian to Recent are hardly represented except some minor sediments of recent age exposed along the
coastal margin to the west. A substantial part of the north Karnataka is covered by Deccan traps, representing a phenomenal outburst of volcanic activity that occurred at the dawn of the Cenozoic era. The Karnataka region forms the type area for the study of Archaean rocks. For the first time R.Bruce Foot studied this region in the last century.
Mode of occurrence of Archeans of Karnataka: The type area of Archaean is made up mainly of gneisses granites and schists. Of these, gneisses occupy the largest area. The schists occur as bands within the vast country of gneisses. There are about eleven large schists bands reported along with a number of small schistose outcrops. The schistose rocks are isoclinally folded exhibiting exhibiting a steep dip towards east. The region is nearly peneplain due to prolonged prolonged erosion. Only the synclinal parts of the isoclinal folds could survive through the denudation and these exists here and there within the vast stretch of gneissose country rocks. The regional strike of schistose rocks is north northwest to southsoutheast in the northern region and this trend changes over to north south and even even to north northeas eastt south southwe west st near near the south southern ern bord border er of the state state.. Rece Recentl ntly y distinction has being made between various schistose bands and some have being assigned an older age then the gneisses.
Classification Number of workers have studied these Archaean rocks of Karnataka and have suggested different schemes of classification and succession. Views drastically differ regarding their origin and hence order of superposition of these rocks. Earlier workers regarded the entire succession as of igneous origin. Later these views were modified in the course of time and at least some members of this succession were assigned a sedimentary origin. Following are the few attempts of classification of Archaean of Karnataka. Classification by W.F. Schmith This is one of the earliest attempts and considers Dharwar system as one of the oldest formation of Karnataka. In his opinion the schists and gneisses have being produced produced due to metamor metamorphis phism m of pree preexist xisting ing igneous igneous country country rocks. rocks. Sch Schmith mith classified the Dharwar system on the basis of degree of metamorphism. He has divided the dharwar system into lower hornblendic hornblendic division made up of hornblende hornblende schist, calc granulites and some magnetite. The upper chloritic division consists of chlorite and mica schists, quartzites, marbles, conglomerates, banded ferruginous quartzites etc.
Succession of Archaean according to Schmith is as follows. Pre Cambrian Basic dykes --------------------- Eparchaean interval -----------------Felsite and porphyry dykes Closepet Granite ARCHAEANS Charnockites Peninsular Gneiss Champion Gneiss ---------------------- Eruptive Unconformity ----------------Upper Chloritic Division Dharwar system Lower Hornblendic Division
Classification by B Rama Rao: After a careful and detailed fieldwork B Rama Rao concluded that the Dharwarian schists were originally sedimentary rocks. His field observations observations included remnants of curr current ent bedd bedding ing,, rippl ripplee marks marks,, grade graded d bedd bedding ing and simila similarr sedim sedimen entar tary y structures partially obliterated by the process of metamorphism and diastrophism. The interbedded lava flows suggest the volcanic eruption in submarine conditions. On the basis of conglomerate horizons he divided the Dharwarian succession into three divisions. Succession of Archaean according to B. Rama Rao is as follows: --------------------- Eparchaean Interval ----------------Felsite porphyry dykes Closepet Granite Charnockites Archaean Peninsular Gneisses Champion Gneisses Upper Dharwars Dharwar System Middle Dharwars Lower Dharwars -------------------- Base not known ----------------
Classification by Radhakrishnan and Vasudev: This is more recent and accepted classification because it is based on radiometric age data of various units and also on degree of metamorphism. In this classification distinct distinction ion is made made between between older older schistos schistosee rocks rocks and younger younger schistos schistosee rocks. rocks. Dharw Dharwar ar Su Super pergr group oup is consi conside dere red d as meta metamo morph rphose osed d sedim sedimen ents ts that that were were deposited in basins formed over the basement basement of peninsular peninsular gneiss and also of older schi schist stos osee rock rocks. s. Both Both olde olderr and and youn younge gerr schi schist stos osee rock rockss are are rena rename med d as Greenstones.
Succession of Archaean according to Radhakrishnan and Vasudev is as follows: Radiometric age 2100my 2380 my
Felsite and porphyry dykes Closepet Granite Chitradurga Group – Greenschist facies of metamorphism and gentle to strong deformation. deformation . -------------------- Unconformity --------------------------Dharwar Supergroup Or Younger Greenstones Bababudan Group – Greenschist – Greenschist to lower amphibolite facies of metamorphism of metamorphism --------------------- Unconformity -------------------2600 my Peninsular Gneissic Complex – Migmatitic – Migmatitic and amphibolite facies of metamorphism. 3200 my Sargur Schist Belt – Upper – Upper amphibolite to OR granulite facies of Older Greenstones metamorphism.. metamorphism -------------------------- Basement not known ----------------------
Lithology: Lithology of various units of Archaean of Karnataka is as follows
Sargur schist Belt or Lower Greenstones: The oldest rocks dated so far in Karnataka area are grey gneisses and included schists. These schistose bands probably represent the relics of the primordial crust i.e. the remains of the first formed earths crust. The rocks of these schsist belts contain usually high magnesium. The name “ancient Supracrustal” is given to these inten intensel sely y metam metamor orpho phosed sed remn remnan ants, ts, whic which h occur occur within within the the grey grey gnei gneisse ssess as enclaves. In older literature the same has been called “Sargur Schist Belts”. These are regarded as metamorphosed remnants of granulitic rocks with Kyanite, Sill Sillim iman anit ite, e, Graph raphit itee and and othe otherr miner ineral alss sugg sugges esti tive ve of inte inten nse ther therma mall metamorphism. These enclaves along with enclosing gneisses are together called ancient ancient supracr supracrusta ustals ls since since these these together together are forming forming the basement basement for a well well developed group of schistose rocks of younger age i.e. ‘Dharwar Schist Belt”.
Older Gneissic Complex: Large Large part part of Karn Karnata ataka ka and and the India Indian n peni peninsu nsula la in gener general al are are cove covered red by granit granites es and and gneiss gneisses es of differ differing ing ages. ages. A varie variety ty of age age det deter ermi minat nation ion value valuess rangin ranging g from from 3400 3400 to 2000m 2000my y have have been been obtai obtained ned for for the these se rocks rocks of gnei gneissi ssicc complex. Majority of ages determined for these rocks lying within the Archaean nucleus have given the age values around 3000my suggesting suggesting a major event of crust forming around this period. These gneisses have been described in earlier literature
“Peninsular sular Gneiss Gneisses” es” . It is infact a complex containing in it several cycles of as “Penin schist schist develop developmen ment, t, graniti graniticc intrusio intrusion n and migmati migmatitic tic transfor transformat mation. ion. This This is large largely ly compos composed ed of amph amphibo ibolit litee facie faciess of gneiss gneisses es of Tona Tonalit lite-G e-Gran ranod odior iorite ite composition. These gneisses show Na 2O /K 2O ratio more than ONE while younger complexes are potassic in nature with Na 2O/K 2O ratio less than ONE.
Dharwars or Younger Schist Belt: . These In recent literature these formations are described as “Younger Greenstones” are prominent schistose rocks of Karnataka and have been given a Supergroup status. They are Archaean in age and belong to the age group of 2900 to 2600 my. Recent studies consider these rocks as representing the transition from Archaeozoic to Proterozoic, and also suggest that the name “Dharwar Type” should be restricted to “Karnataka Craton” alone.
Two main divisions are recognized in this Supergroup. The older of the two is mainly igneous in character and is named as “Bababudan Group” and hosts the main Iron Ore Formation. This group is made up of alternating bands of basalt and quartzites. On top is the banded Iron Formation. These rocks are comparatively more metamorphosed than the upper formations. The group above the Bababudan “Chitradurga radurga Group” . Th Group Gr oup is calle called d the “Chit This is is the the more more extens extensive ive group group of schistos schistosee rocks rocks largely largely sedimen sedimentary tary in charact character er compose composed d of conglom conglomera erates, tes, quartzit quartzites, es, limesto limestones, nes, greywac greywackes kes and associat associated ed magnife magniferous rous and ferr ferrugin uginous ous cherts. This formation is given the name “Chitradurga Group” after the township of Chitradurga in Karnataka. The most recent account of the mode of formation of the “Dharwar Supergroup” which differs from the earlier views is as follows. Since recently some clear-cut evidences of biogenic structures have been found in younger greenstones, these are thought to mark a transition period from Archaean to Proterozoic era. The end of earlier greenstone cycle [Sargur schists] witnessed the invasion of Tonalite-granodiorite Tonalite-granodiorite material [Peninsular Gneiss] on a very large scale leading to the formation of a relatively thick Sialic crust of continental dimension. Folding, faulting and rifting of this old crust gave rise to long linear and oval-shaped basin basins. s. In the these se basin basinss were were depo deposit sited ed thick thick serie seriess of volc volcani anicc produc products ts and sediments. By 2600my deposition within these linear basins had come to an end. The cratonic blocks made up of older gneisses lying on either sides of the depositional basi basins ns conv conver erge ged, d, ther thereb eby y comp compre ress ssin ing g and and tele telesc scop opin ing g the the volc volcan anic ic and and sedimentary material [volcano-sediments] accumulated in these basins, to form subparallel parallel linear linear belts belts of schist’s schist’s and gneisse gneisses. s. Actually Actually,, these these linear linear schist schist belts belts represent the sites of Archaean crustal sutures.
Younger [Clospet] Granites: A striking feature in the geological map of Karnataka is the occurrence of a long linear belt of a granitic body extending in NS direction for nearly 500 km keeping an average width of 20 kms. This granitic body runs more or less parallel to schist
belts. This granitic batholithic body demarcates the Archaean nucleus of Karnataka made up of older gneisses and schists lying on the west form the younger block to the east. The trend of these granites probably marks the line of collision between two distinct blocks and probably formed a major shear- zone. This belt is not a single mass of granite but a cluster of individual individual plutons of differing sizes, emplaced emplaced all along the collision zone. The most characteristic rock type is coarse-grained porphyritic granite with large sized phenocryst of pink or grey potash feldspars. These have been named after the town of Closepet [Present day Ramnagaram]. These are radiometrically dated as 2528+- 5 my. These are the youngest unit of the Archaean complex of Karnataka, hence known as younger granites. These granites are believed to have originated originated by both both igne igneou ouss intr intrus usio ion n and and also also by meta metaso soma mati ticc alte altera rati tion on.. Th Ther eree is stil stilll a considerable controversy regarding the origin of these granitic bodies. The great era of Archaean came to an end with a large-scale orogenic activity. As a result, Dharwarians were isoclinally folded and converted into one of the largest and loftiest mountain chains. These ranges were subjected to prolonged erosion and have been peneplained before the sedimentary basin came into existence during the Prote Protero rozoi zoicc era. era. Th This is time time perio period d of erosi erosion on and and non-de non-depos positi ition on is kn know own n as “Eparchaean Unconformity” in the Indian startigraphy.
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