Sausar Group Research Articles

Earth observation approach for targeting stratiform deposit of manganese in central India

The stratiform manganese deposits, hosted within the Sausar Group of rocks, in Madhya Pradesh state of India, has undergone polyphase deformation and metamorphism. The region is an enterprising geosite to understand the lithological and tectonic control on manganese mineralization. We have carried out a conjugate remote sensing based analysis using Visible, Near Infrared and Short Wave Infrared bands (VIS-NIR-SWIR) of optical imagery from multiple sensors supplemented with analysis of L-band microwave data over the Balaghat district of Madhya Pradesh to delineate prospective areas for manganese exploration, supported by the lab and field-based evidence.The enhanced image products (i.e.band ratios, principal components) and spectral anomaly map derived from Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) and Sentinel-2B multispectral bands are used to delineate the lithological contrast of the host rock and indicate key locales of mineralization. The geological structures interpreted using the merged product of Advanced Land Observation Satellite Phased Array L-band SAR (ALOS PALSAR −1), Landsat 8 PC bands (2 & 3) and the processed Horizontal Gradient Magnitude (HGM) from advanced satellite gravity model (EIGEN-6C4), illustrates the structural control on the mineralization. The petrological details derived from the host rock samples shows microstructures like micro-folding, foliations, quartz grain elongation etc., thus representing the microscale deformations within the rock units occurring in the area.The lab-based spectral signatures of manganese-bearing host rock was resampled to image spectra and spectral anomaly map using ConstrainedEnergyMinimization(CEM), which shows good correlation with the existing mine locations.The information from remote sensing and lab analysis has been integrated thematically using the knowledge-based Weighted Sum Method (WSM) with existing mine locations as the training dataset. The derived mineral prospect map indicates several new areas of manganese prospectivity which can serve as a guide for detailed future explorations in the region .

"Hydrothermal Veins and Not Hydrothermal Exhalations-Responsible for Deposition of Manganese Ores in the Precambrian Sausar Group of Rocks of Madhya Pradesh, India"

After the discovery of Manganese deposits associated with the Precambrian rocks of Sausar Group of Bhandara district by P.N.Dutta in 1893-94, it was Sir Fermor LL [1] who brought out a detailed account of Sausar rocks and the associated manganese ore deposits of Central India.

Comments on: “The current status of orogenesis in the Central Indian Tectonic Zone: A view from its southern margin” by Bhowmik, S. K. (2019). Geological Journal 54, 2912–2934

A three‐stage evolution model proposed for the Central Indian Tectonic Zone (CITZ) by Bhowmik is an incomplete model for the zone and has flaws regarding (a) the status of the Tirodi Gneiss vis‐à‐vis the Sausar Group, (b) the time of granulite metamorphism with respect to the deposition of the Sausar Group, and (c) the style of orogeny, direction of subduction, and position of suture. The author needs to clarify some of the regional structural and stratigraphic relations to support the conclusions drawn in the paper.

Reply to discussion of the current status of orogenesis in the Central Indian Tectonic Zone: A view from its southern margin

Bhowmik suggested two stages of orogenesis at the southern margin of the Central Indian Tectonic Zone (CITZ): (a) an accretionary orogenesis type at ca. 1.6–1.5 Ga that formed a proto‐Greater Indian Landmass (GIL) through the assembly of arc, back‐arc and continental margins, (b) a Himalayan‐style continental collision event between 1.06 and 0.93 Ga that finally stitched the North and South India blocks to produce the final configuration of the GIL and (c) an intervening phase of extension between 1.5 and 1.06 Ga that led to the opening up of the Sausar Basin and deposition of the Sausar Group of rocks.

Hierarchy Amongst Rocks of Sausar Group - Evidences from Seoni-Balaghat Area, Madhya Pradesh, India

Hierarchy Amongst Rocks of Sausar Group - Evidences from Seoni-Balaghat Area, Madhya Pradesh, India

Rare earth element characteristics of Paleoproterozoic cap carbonates pertaining to the Sausar Group, Central India: Implications for ocean paleoredox conditions

The Paleoproterozoic era is marked by significant changes in the level of oxygen in the atmosphere and chemistry of the ocean, and the presence of world-wide glaciation. Geochemistry of chemogenic sediments provide clues to these changes. The Sausar Group (2478–2250Ma) of Central India is one of the few places in the world where Paleoproterozoic glacial unit overlained by a cap carbonate horizon has been identified. Major, trace and rare earth element (REE) contents of Paleoproterozoic cap carbonates of the Sausar Group have been analysed for their implications for ocean redox conditions during interglacial periods of the Huronian glaciation. The La/La∗, Gd/Gd∗, Lusn/Lasn and Y/Ho ratios of the cap carbonate reflect their pristine marine nature. Conspicuous Ce anomaly (Ce/Ce∗ up to 1.75), accompanied by the enrichment of Fe, Mn, Zn and U, provides insight into the redox state of the ocean at the time of deposition of the cap carbonate succession. Based on these observations, we infer anoxic depositional conditions in the basin during the interglacial period of the Paleoproterozoic glaciation before the Great Oxidation Event (GOE), and the level of oxygen in the atmosphere fluctuating between 10−5 and 10−2 of the present atmospheric level (PAL).

Geochemistry of a paleosol horizon at the base of the Sausar Group, central India: Implications on atmospheric conditions at the Archean–Paleoproterozoic boundary

A paleosol horizon is described from the contact of the Sausar Group (∼2400 Ma) and its basement (Tirodi Gneiss; >2500 Ma) in Central India. Physical evidence of pedogenesis is marked by the development of stress corrosion cracks, soil peds, corestone weathering and nodular rocks. XRD and SEM-EDX data indicate the presence of siderite, ankerite, uraninite, chlorite, alumino-silicate minerals, ilmenite, rutile and magnetite, in addition to quartz, feldspar and mica. The chemical index of alteration, the plagioclase index of alteration, and the chemical index of weathering show an increasing trend from parent rock to the paleosol and indicate a moderate trend of weathering. The A-CN-K plot indicates loss of feldspars, enrichment in Al2O3 and formation of illite. Different major element ratios indicate baseloss through hydrolysis, clay formation, leaching of some elements, and more precipitation with good surface drainage. The paleosol is depleted in HREE in comparison to the parent rock indicating high fluid-rock interaction during weathering. The paleosol samples show flat Ce and Eu anomalies, low ΣREE, and high (La/Yb)N, indicative of a reducing environment of formation. Reducing condition can also be inferred from the concentration of elements such as V, Co, Cu, Pb, and Zn in the paleosol profile. Although enriched in Fe and Mg, the overall geochemical patterns of the paleosol indicate oxygen deficient conditions in the atmosphere and development by weathering and leaching processes associated with high precipitation and good surface drainage at the time of development of this paleosol during the Archean–Paleoproterozoic transition.

Reply to the discussion on: “Carbon and oxygen isotope systematic of a Paleoproterozoic cap-carbonate sequence from the Sausar Group, Central India” by Anupam Chattopadhyay (2015) published in Palaeogeography, Palaeoclimatology, Palaeoecology 433, 156–157

Abstract The discussion on our paper on Paleoproterozoic glaciation arises out of the failure of previous geological mapping in the Sausar belt to identify many features, which were reported earlier, and to examine the outcrops critically for evidence of volcanism and glaciation in Central India. The details of these are given in this reply.

Discussion on: “Carbon and oxygen isotope systematic of a Paleoproterozoic cap-carbonate sequence from the Sausar Group, central India” by S. Mohanty, A. Barik, S. Sarangi and A. Sarkar (2015) published in Palaeogeography, Palaeoclimatology, Palaeoecology 417, 195–209

Abstract The recent proposition by Mohanty et al. (2015) that the lower part of Sausar Group of rocks in central India has a glacial origin, and represents a major Paleoproterozoic super-glaciation event, contradicts some well-established geological and geochronological data on these rocks put forward by other workers in recent past. The merits of this conclusion are discussed here.

Syn- and post-tectonic granite plutonism in the Sausar Fold Belt, central India: Age constraints and tectonic implications

Sausar Fold Belt (SFB) in central India forms the southern part of the Central Indian Tectonic Zone (CITZ) – a crustal scale Proterozoic mobile belt dissecting the Indian craton, whose tectonothermal history and age is important for understanding the Proterozoic crustal history of the Indian craton. SFB comprises a gneissic basement (TBG: Tirodi Biotite Gneiss) overlain by a supracrustal sequence of quartzite–pelite–carbonate (SSG: Sausar Group). SSG and TBG are deformed and metamorphosed in greenschist to amphibolite facies. Two phases of granite intrusion are observed in the SSG – a syntectonic foliated granite and a post-tectonic massive granite, with clear structural relationship with the host rocks. Monazite chemical dating (U–Th–total Pb) of the foliated and massive granites yield Neoproterozoic (ca. 945–928Ma) ages that contradict many earlier geochronological interpretations. Foliated granites and the immediately adjacent TBG show monazite grains with ca. 945Ma mean age, interpreted as the timing of D2 deformation and amphibolite facies metamorphism of SSG. The post tectonic granites intruded these rocks around 928Ma, and were largely undeformed. A terminal thermal overprint is found in some monazite grain rims at ca. 785Ma age. The younger Sausar tectonothermal events have overprinted the adjacent high-grade granulites of Ramakona–Katangi Granulite (RKG) belt, and should not be considered as parts of the same tectonothermal event representing different depth sections only.

Chapter 11 Palaeoproterozoic supracrustals of the Bastar Craton: Dongargarh Supergroup and Sausar Group

Abstract The Bastar Craton of India is composed of Archaean nuclei of tonalite–trondhjemite–granodiorite gneisses, enveloped by an older granite–greenstone belt (>3000 Ma) with banded iron formation (BIF), and an auriferous younger granite–greenstone belt with BIF. Available geological, geochemical and geochronological data indicate multiple episodes of orogeny with high-grade metamorphism at 3200–3300, 2600–2700, 2100–2200, 1900–2000, 1800–1850, 1500–1600 and 1400–1450 Ma, and continental rifting and basin development marked by emplacement of mafic dyke swarms at c. 2900 (subalkaline mafic dykes; BD-1A), 2480 (high-Mg mafic dykes; BD-1B), 2100 (Fe–tholeiite dykes; BD-2A), 1880 (Fe–tholeiites dykes; BD-2B), 1776 and 1422 Ma. Associations of extensive bimodal volcanics and riftogenic sediments are found in the Neoarchaean and Palaeoproterozoic basins of the craton. Evidence of Palaeoproterozoic (Huronian) glaciation and associated ‘cap carbonate’ followed by deposition of fine clastics with manganese ore is found in the Palaeoproterozoic Sausar Group. The lithological association of the Sausar Group is comparable to the carbonate–tillite association of the Huronian Supergroup, Snowy Pass Supergroup, Transvaal Supergroup and Turee Creek Group. The geological evolution of the Bastar Craton matches that of Western Australia and South Africa. Such similarities can be analysed to develop a unified Palaeoproterozoic assembly for these provinces.

Carbon and oxygen isotope systematics of a Paleoproterozoic cap-carbonate sequence from the Sausar Group, Central India

Well-preserved sedimentological features of the Paleoproterozoic age Sausar Group of central India indicate glaciogenic origin for the diamictite in the lower part of the unit. Deglaciation was responsible for a marine transgression and the deposition of proximal glaciomarine facies and a cap-carbonate, followed by fine clastics with manganese ore. Analyses of δ13C and δ18O contents in the carbonate unit overlying the diamictite, together with Sr and Ba contents and REE data, indicate the preservation of primary geochemical signatures. The average δ13Ccarb content ranges between −3.1‰ and +0.1‰ V-PDB, having peak negative δ13C excursions of up to −7.4‰ V-PDB, similar to Paleoproterozoic and Neoproterozoic cap-carbonates elsewhere. High Sr and Ba contents indicate the presence of aragonite and barite precursors, similar to well-studied Neoproterozoic cap-carbonates. A single continuous carbonate unit shows δ13C excursions from −4.4 to +2.6‰ V-PDB. The lithological association and chemostratigraphy are comparable with the carbonate–tillite association of the Huronian Supergroup of Canada, the Snowy Pass Supergroup of the USA, the Transvaal Supergroup of South Africa, and the Turee Creek Group of Australia.

Tectonic evolution of the Satpura Mountain Belt: A critical evaluation and implication on supercontinent assembly

The Satpura Mountain Belt (also referred as Central Indian Tectonic Zone in recent literature) forms an important morphotectonic unit in the central part of India. Some of the recent workers have reported an orogenic event at ∼1000–900 Ma (termed “Sausar orogeny”) which led to amalgamation of the North Indian Block and the South Indian Block and formation of the Satpura Mountain Belt. In this model the stratigraphic relations of two important lithostratigraphic units on either side of the Satpura Mountain Belt (the Sausar Group in the south and the Vindhyan Supergroup on the north) are suggested to be revised from previously held ideas. Critical analyses of available published work in the region to assess the status of the Sausar Group vis a vis the Vindhyan Supergroup was carried out. It is found that the ideas proposed by the recent workers stem from an earlier interpretation that the Sausar Group has monocyclic evolution and the earliest fabric in the Sausar Group is marked by a schistosity with EW strike. Re-mapping of the gneissic rocks and adjacent matasedimentary rocks of Khawasa, Deolapar, and Kandri–Mansar areas revealed presence of gneissic rocks and granulites of two generations, and of four phases of superposed deformations in the metasediments and gneisses. The older gneisses and granulites constitute the basement over which the rocks of the Sausar Group were deposited; and the younger gneisses developed by metamorphism and migmatisation of the rocks of the Sausar Group. The latter types are found in the Khawasa–Ramakona areas. Contrary to the belief of the recent workers that no volcanic activity is present in the Sausar Group, volcanic rocks marked by amygdular basic flows and tuffs have been mapped from different parts of the Sausar Group. Migmatisation and metamorphism of these volcanic rocks (of the Sausar Group) have given rise to amphibolites and granulites in Khawasa and Ramakona areas. Therefore, the use of fabric patterns in these areas to suggest that the granulite facies metamorphism in the Ramakona–Katangi granulite domain was pre-Sausar in age is debatable. Available geochronological data of the Satpura Mountain Belt and its eastward continuation into the Chhotanagpur Gneiss terrain indicate that the basement and cover rocks of these areas were subjected to multiple deformation and metamorphic episodes of similar style and nature. The earliest deformation and metamorphism of the rocks of the Sausar Group and its equivalent rocks to the east took place at ∼2100–1900 Ma. The regional EW trend of the belt developed during the second deformation at ∼1800–1700 Ma and again at ∼1600–1500 Ma. This deformation was accompanied by migmatisation and granulite facies metamorphism in the northern domain of the Sausar Belt and in the Chhotanagpur Gneiss region. Late phase low intensity deformations in the region were associated with thermal events at ∼1100–1000 Ma and ∼900–800 Ma. The ∼EW trending fabric, referred as “Satpura orogenic trend” in Indian literature marks a major compressional tectonic event, developed during the second deformation of the Sausar Group. This has its counter part in Western Australia as the Capricorn orogeny (∼1780–1830 Ma). The development of the Satpura Mountain Belt during the Grenvillian orogeny is ruled out from the synthesis of event stratigraphic data of the region and from its comparison with the Western Australian Craton.

Polyphase deformation and garnet growth in pelitic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast-matrix relationship

Polyphase deformation and metamorphism of pelitic schists of Chorbaoli Formation of Sausar Group in and around Ramtek area, Nagpur district, Maharashtra, India has led to the development of garnet and sataurolite porphyroblasts in a predominantly quartz-mica matrix. Microstructural study of oriented thin sections of these rocks shows that garnet and staurolite have different growth histories and these porphyroblasts share a complex relationship with the matrix. Garnet shows at least two phases of growth — first intertectonic between D1 and D2 (pre-D2 phase) and then syn-tectonic to post-tectonic with respect to D2 deformation. Growth of later phase of garnet on the earlier (pre-D2) garnet grains has led to the discordance of quartz inclusion trails between core and rim portion of the same garnet grain. Staurolite develops only syn-D2 and shows close association with garnet of the later phase. The peak metamorphic temperature thus coincided with D2 deformation, which developed the dominant crenulation schistosity (S2), regionally persistent in the terrain. The metamorphic grade reached up to middle amphibolite facies in the study area, which is higher than the adjoining southern parts of Sausar Fold Belt.

Nambulite, (Li,Na)Mn4Si5O14(OH), in the Sausar Group of rocks in Central India

Manganese carbonate-silicate rocks in the Sausar Group, India are often traversed by pegmatite veins. Nambulite, a lithium- and sodium-bearing hydrous manganosilicate, is formed by the reaction of a hydrothermal solution with the host mineral, rhodonite. Despite the similarity of natronambulite and nambulite, the chemical composition shows a variation in the concentration of Na+ in the empirical formula: (Li0.94-0.60Na0.06-0.40)Σ1.00(Mn3.57-3.82Ca0.13-0.07Mg0.36-0.07Fe0.03-0.01)Σ3.96-3.86Si5.01-4.98O14(OH)1.00. This is the first report of the sodium-rich variety from India. Powder X-ray diffraction studies reveal the unit cell parameters of nambulite in the triclinic system as a =7.55 ± 0.06 A, b = 11.77 ± 0.07 A, c = 6.68 ± 0.04 A, α = 91.3° ± 0.8°, β = 95.4° ± 0.5°, γ =107.2° ± 0.7° and V= 563 ± 6 A3.

Structural Framework of Deolapar Area, Central India and its Implications for Proterozoic Nappe Tectonics

A controversial ‘nappe-like’ structure (‘Deolapar Nappe’) in the intensely deformed and polymetamorphosed Precambrian rocks of the Sausar Fold Belt around the Deolapar area of Central India has remained the subject of a long- standing, unresolved debate. While some workers interpreted it as the inverted limb of a large-scale recumbent fold which has been thrusted onto the adjoining part of the supracrustal sequence (Sausar Group), others discounted the very existence of the nappe because of the lack of structural evidence in favour of the recumbent fold-thrust structure. Recent structural study by the present authors around the Deolapar area and also in the adjoining parts of the Sausar Fold Belt has looked back into the controversy surrounding the Deolapar Nappe. The study reveals that the Deolapar Nappe is constituted of an allochthonous block of metasedimentary rocks and crystalline gneiss-migmatites, placed over the metacalcareous rocks (Lohangi Formation) of the Sausar Group along a low angle thrust plane, marked by a sheared fibrolite-biotite schist. The whole thrust sheet has been folded by later (F 2) upright folds. However, careful structural mapping confirmed that there is no evidence of any map-scale F 1 recumbent fold in the whole area, although small-scale, thrust-related reclined (F 1) folds are occasionally found. Also, there is not enough field evidence suggesting regional inversion of stratigraphic succession so as to indicate the lower limb of a recumbent fold. The Deolapar Nappe can therefore be best interpreted as a thrust-nappe. Details of the existing controversy, the new observations and a new tectonic model of the Deolapar Nappe is presented here.

Evolution of the Bhandara-Balaghat granulite belt along the southern margin of the Sausar Mobile Belt of central India

The Bhandara-Balaghat granulite (BBG) belt occurs as a 190 km long, detached narrow, linear, NE-SW to ENE-WSW trending belt that is in tectonic contact on its northern margin with the Sausar Group of rocks and is bordered by the Sakoli fold belt in the south. The Bhandara part of the BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulite body that is of gabbroic composition and preserves relic igneous fabric. The main part of the belt in Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites) and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbanded with charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves within migmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically, basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continental affinity and show evolved nature. Mineral parageneses and reaction textures in different rock compositions indicate early prograde, dehydration melt forming reactions followed by orthopyroxene stability with or without melt. Coronitic and symplectitic garnets have formed over earlier minerals indicating onset of retrograde IBC path. Evidences for high temperature ductile shearing are preserved at places. Retrogressive hydration events clearly post-date the above paths. The present study has shown that the BBG belt may form a part of the Bastar Craton and does not represent exhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belt have undergone an earlier high-grade granulite metamorphism at 2672 ± 54 Ma (Sm-Nd age) and a post-peak granulite metamorphism at 1416 ± 59Ma (Sm-Nd age, 1380 ± 28Ma Rb-Sr age). These events were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogeny between 973 ± 63Ma and 800 ± 16Ma (Rb-Sr ages).

GRANITIC PEGMATITES OF KORADI-KOLAR SECTOR, NAGPUR DISTRICT, CENTRAL INDIA: FIELD, PETROGRAPHIC AND MINERALOGICAL FEATURES

Several bodies of pegmatites have been located within the rocks of Sausar Group (Precambrian) occurring in the Kolar valley, north of Koradi, in Nagpur district of Central India. The XRD and the chemical data have shown that these pegmatites are mostly potassic (K2O= 15.58 to 16.31 % and Na2O=2.50 to 3.12 %) and are barren of economic mineral deposits. The pegmatites range from a few cm to more than 50 m in width with a maximum extension of about one kilometre. Several mappable pegmatite bodies trend roughly EW and NS, and many good outcrops are encountered in the Kolar Valley and Suradevi Hills. Based on the field and petrographic studies, the pegmatites of this area are characterised in terms of size, shape and mode of emplacement. They are mostly tabular, lenticular, branching and irregular bodies, localized along bedding, foliation, fault planes, tension joints and fold axis. Structurally they can be subdivided into concordant and discordant types on the basis of their relationship with the country rocks. Mineralogically, pegmatites are grouped into (1) Simple Pegmatite, containing quartz, feldspar, biotite, apatite, garnet and (?) monazite, and (2) Complex Pegmatite comprising in addition muscovite, tourmaline and epidote. However, common mineral variation is observed at different places within the same body of pegmatite. In these pegmatites, muscovite, quartz, tourmaline and feldspar seem to have developed mainly in two distinct generations, for example, the muscovite of earlier generation forms tiny books while later crystals occur as small disseminated flakes in the groundmass of less coarser quartz. The earlier quartz is strongly deformed, while the later occurs as veins of variable dimensions. The black tourmaline of earlier generation is represented by schorlite and of later origin has been identified as dravite. It may be concluded that these pegmatites were formed by the influx of fluids rich in silica, Al and K that filled the fractures and other planes of weakness (foliation) in the host rocks. The alkali components while migrating towards the wall of the country rocks, crystallised as muscovite and feldspar. In larger pegmatite bodies of Suradevi Hills, the development and concentration of muscovite along their contact with the mica schist is probably due to the migration of Al and K and volatiles towards the border of the intrusion. This crystallization of muscovite in preference to potash-feldspar may be due to the presence of high contents of volatiles.

GRANITIC PEGMATITES OF KORADI-KOLAR SECTOR, NAGPUR DISTRICT, CENTRAL INDIA: FIELD, PETROGRAPHIC AND MINERALOGICAL FEATURES

Several bodies of pegmatites have been located within the rocks of Sausar Group (Precambrian) occurring in the Kolar valley, north of Koradi, in Nagpur district of Central India. The XRD and the chemical data have shown that these pegmatites are mostly potassic (K 2 O= 15.58 to 16.31 % and Na 2 O=2.50 to 3.12 %) and are barren of economic mineral deposits. The pegmatites range from a few cm to more than 50 m in width with a maximum extension of about one kilometre. Several mappable pegmatite bodies trend roughly EW and NS, and many good outcrops are encountered in the Kolar Valley and Suradevi Hills. Based on the field and petrographic studies, the pegmatites of this area are characterised in terms of size, shape and mode of emplacement. They are mostly tabular, lenticular, branching and irregular bodies, localised along bedding, foliation, fault planes, tension joints and fold axis. Structurally they can be subdivided into concordant and discordant types on the basis of their relationship with the country rocks. Mineralogically, pegmatites are grouped into (1) Simple Pegmatite, containing quartz, feldspar, biotite, apatite, garnet and (?) monazite, and (2) Complex Pegmatite comprising in addition muscovite, tourmaline and epidote. However, common mineral variation is observed at different places within the same body of pegmatite. In these pegmatites, muscovite, quartz, tourmaline and feldspar seem to have developed mainly in two distinct generations, for example, the muscovite of earlier generation forms tiny "books" while later crystals occur as small disseminated flakes in the groundmass of less coarser quartz. The earlier quartz is strongly deformed, while the later occurs as veins of variable dimensions. The black tourmaline of earlier generation is represented by schorlite and of later origin has been identified as dravite. It may be concluded that these pegmatites were formed by the influx of fluids rich in silica, Al and K that filled the fractures and other planes of weakness (foliation) in the host rocks. The alkali components while migrating towards the wall of the country rocks, crystallised as muscovite and feldspar. In larger pegmatite bodies of Suradevi Hills, the development and concentration of muscovite along their contact with the mica schist is probably due to the migration of Al and K and volatiles towards the border of the intrusion. This crystallization of muscovite in preference to potash-feldspar may be due to the presence of high contents of volatiles.

Structural Patterns in the Sausar Group Around Mansar, Nagpur District, Maharashtra

Abstract The Precambrian rocks of the Mansar area, Nagpur district, Maharashtra, belong to the Sausar Group and the Tirodi Gneiss. Both these lithological units show an unconformable relation in the area, marked by a conglomerate at the base of the Sausar Group. The metasediments of the Sausar Group bear imprints of polyphase deformation. The folds of the first generation (F1) are isoclinal with high amplitude to wavelength ratios and pervasive axial planar cleavage (S1). The structures of the second generation are represented by WNW-ESE trending upright isoclinal folds (F2) and axial planar crenulation cleavage (S2). Gentle upright folds with NE-SW striking axial planes represent structures of the third generation (F3) Superposition of F2 folds on F1 folds is marked by mushroom shaped interference pattern in small-scale. Because of upright isoclinal nature of F2 folds on WNW-ESE striking axial planes, the metasedimentary bands extend in E-W direction. Slight arcuate geometry of the metasedimentary bands is the result of superposition of F3 folds on F1/F2 folds.