Aravalli Rocks Research Articles

Contribution to the Mineral Chemistry of the Proterozoic Aravalli Mafic Meta-Volcanic Rocks from Rajasthan, NW India

Field, petrological and mineral chemistry for meta-volcanic rocks from the Aravalli sequence (Aravalli Craton, India) are presented. Field evidence such as volcanic flows and suspect pillow lava structures, dominant Fe-tholeiite lava flows intercalated with quartzites and argillaceous sediments, indicate rift tectonic environment. Primary mineralogy was obliterated during post-magmatic processes such as metamorphism corresponding to the greenschist to lower amphibolite facies conditions. The rock’s mineral composition was overprinted by plagioclase–chlorite–amphibole–epidote assemblage. The relicts of clinopyroxene were observed. The P-T estimation indicates a temperature of 550–600 °C for the pressure ranging from 3.0 to 7.0 kbar for the majority of amphiboles and 8.0–10.7 kbar for the minority. Geochemically, these rocks are komatiitic (picritic) and high-Fe tholeiitic basalts with 45.06−59.2 wt.% SiO2 and MgO content from 5 to 22.4 wt.% and Mg# of 17 to 71. They show large-ion lithophile elements (LILE) and light rare-earth elements (LREE) enrichment. Chondrite normalized rare-earth elements (REE) patterns for the Aravalli lava are moderately enriched with (La/Sm)N = 1.1−3.85, (La/Yb)N from 1.49 (komatiites) to 14.91 (komatiitic basalts). The trace element systematics with the negative Nb, P and Zr anomalies reflect their derivation from enriched sub-continental lithospheric sources, although minor crustal contamination cannot be ruled out. Aravalli rocks are considered to represent the transition from continental rift magmatism to shallow submarine eruption.

Strain History of Aravalli Rocks Exposed Around Village Maruwas, Udaipur District, Rajasthan

Detailed structural analysis of the area suggests that the Precambrian rocks under investigation revealed a complex deformational history. The area has suffered one major and two minor phases of deformations. Out of which the second deformation (AF2) is most dominant structural event, which has controlled the topography. Fold produced during first deformation (AF1) are tight to isoclinals in nature and suggesting a reclined geometry. While folds of second generation (AF2) exhibit open, upright fold geometry, the third deformation is less penetrative. Analysis of shape and orientation of the AF1 and AF2 folds is suggestive of their initiation by buckling and subsequent modification by flattening resulting in to flattened parallel folds (Class 1C) geometry. Geometrically, some of the folds do not fall distinctly into one class but show a combination of two to three classes. This may be due to original layer irregularities or may be due to differential flattening. The strain ratio varies from 0.1 to 1, which indicates that different folds underwent varying amount of flattening. Stratigraphically, the area under investigation comprises rocks of Middle and Upper Aravalli Group of shelf as well as deep-sea faces.

BNa rich Palaeoproterozoic Aravalli metasediments of evaporitic association, NW India: a new repository of gold mineralization

We report field observations and preliminary petrochemical and stable isotopic compositions of the host rock and sulfide minerals for a recently discovered sulfide-hosted gold deposit from the Palaeoproterozoic Aravalli rocks in northwestern India. Gold occurs associated with FeAsCu (±Co) sulfides in lithofacies of diverse mineralogical composition, dominated by amphibole-bearing dolomitic marble and albite-rich quartzo-feldspathic rocks (QFR). The presence of vugs, believed to be after dissolution of halite patches, coupled with the development of scapolite-bearing assemblages in dolomite marble, occurrence of tourmaline-rich mineralized rocks and very high Na content of carbonates and QFR indicate hypersaline depositional environment for the host rock. A narrow spread of δ 34S CDT values from +10.5 to +12.8‰ for stratiform arsenopyrite and remobilized pyrrhotite, corroborates hypersaline to evaporitic depositional environment. The δ 13C values for gold-bearing carbonates show bimodal distribution; one population (mean −0.4‰ V-PDB) is close to the sea water carbonate values while the other (−3.l‰) indicates probable mantle affinity. The δ 18O values for carbonates (mean=+18.4‰ V-SMOW) are not discriminative as they appear to have been modified by subsequent metamorphism and hydrothermal activity. Interpretation of mantellic source for carbon is borne out from the absence of biogenic activity and incompatibility of significantly negative δ 13C values in the evaporitic setting of deposition of Bhukia rocks. We suggest that NaB rich fluids, channeled through rift-related faults, were responsible for creating hypersaline conditions that eventually evolved into an evaporitic environment. Mantle carbon, through some still deeper faults, was probably responsible for the gold transport and its subsequent deposition in an otherwise uncommon (evaporitic) repository of carbonates and QFR.

Mineralogy of Gold and Associated Alloys in Sulfides of Bhukia Gold Prospect in Southeastern Rajasthan, Western India

Abstract Primary gold occurs with sulfides in Paleoproterozoic Aravalli rocks in the Bhukia area of Banswara District, Rajasthan. It occurs as microscopic grains of various shapes, mainly in arsenopyrite and lollingite. With a combined approach of ore petrologic and EPMA studies, we report high purity gold, native bismuth and alloys of gold-bismuth and bismuth-tellurium which best approximate to the rare minerals maldonite and wehrlite respectivety. The fineness of gold ranges between 934 and 995. Preliminary estimates on temperature using arsenopyrite composition coupled with rare presence of spherulitic graphite indicate the temperature of equilibration of around 550°C for the coexisting mineral phases of pyrrhotite, arsenopyrite and lollingite.

Structural Geometry and Strain History of the Early Proterozoic Aravalli Rocks of Gorimari, Udaipur District, Rajasthan

Abstract Detailed mapping and analyses of the structural elements from Gorimari revealed a complex structural geometry and deformation history of the rocks. Only two prominent planar structural elements have been observed i.e., the bedding schistosity (S0S1) and cleavage (S2). The earliest AF1 and the later AF2 folds on every scale are suggestive of reclined geometry. The strain analyses of AF1 and AF2 folds suggest their initiation by buckling and subsequent flattening resulting into class I C geometry. Some of the folds show a complex geometry which may be due to original layer irregularities or may be due to differential flattening because of the variation in thickness of single layer. The values of compressive apparent strain ratio varies from 0.1 to 1.0. Stratigraphically these rocks are equivalent to the Early Proterozoic rocks of Bagdunda. The grade of metamorphism is oflower amphibolite facies.

Influence of differential basement mobility on contrasting structural styles in the cover rocks: An example from Early Precambrian rocks east of Udaipur, Rajasthan

Detailed structural and lithological mapping of the Aravalli rocks overlying the Mewar Gneiss in the area east of Udaipur, Rajasthan, suggests presence of blocks bounded by faults, showing a contrasting structural pattern. The contrast is reflected in the differential development and in the orientation of AF1, AF2 and AF4 folds in different blocks. In the central Umra block, the rocks constitute a virtually homoclinal sequence showing one dominant orientation of bedding and axial planar schistosity. Fold axes, lineations andβ orientations indicate presence of reclined folds of AF1 generation. AF2 folds are either absent or have developed only locally. The two other blocks which border the Umra block show development of large AF2 synforms and local minor antiforms having N-S or NNE-SSW trend. The folds interfere with AF4 folds producing irregular domes and basins in the western Kanpur-Kalarwas Block and minor plunge reversals in Bagdara-Dhamdhar Block. It is argued that the constituents of the different blocks which formed a collage of rift basins and horsts during sedimentation, responded differentially to deforming forces because of differential mobility of the underlying basement.

Stratigraphic Relations of the Precambrian Rocks in the Salumbar Area, Southeastern Rajasthan

Abstract Around Salumbar in the Udaipur district, Rajasthan, the Banded Gneissic Complex (BGC). the Aravalli metasedimentary rocks, the Dudar Gneiss and the Lakapa Granite Gneiss have been involved in the same style and sequence of muhiple deformation. The oldest rocks are the migmatites forming the BGC, which underlie the Aravalli metasediments with an unconformity marked by a conglomerate. Rare relict pre-Aravalli structures in the BGC, and the conglomerate containing gneissic pebbles provide incontrovertible evidence for a part of the BGC being the original basement. Identical structural style and sequence of the basement gneisses wilh those of the metasedimentary cover rocks, however, point to mobilization so that they could take part in the earliest deformation affecting the metasediments in a ductile manner. The Dudar Gneiss has formed by migmatization of the Aravalli rocks synkinematically with the first deformation, whereas the Lakapa Granite Gneiss has been emplaced as a syn-to late-tectonic intrusion.

The Ahar River Granite, Its Stratigraphic and Structural Relations with the Early Proterozoic Rocks of South-Eastern Rajasthan

Abstract The Ahar River granite near Udaipur city, considered by Heron as intrusive into the Aravalli rocks of early Proterozoic age, is a coarse-grained, massive, leucocratic rock except along the outcrop margins. The granite ranges in composition from alkali granite to granodiorite and tonalite, with about 5% ferromagnesian minerals. The enveloping Aravalli metasediments form a continuous sequence starting with greenschist (metavolcanics) and quartzite (locally conglomerate) along the granite contact. The stratigraphic position of the Ahar River granite in relation to the surrounding metasediments suggests that the granite is the basement rock. The granite characteristically shows post-crystalline deformation of the constituent grains, specially quartz and feldspar; and the granite-cover contacts are marked by ductile shear zones developed during the earliest deformation of the Aravalli rocks.

Photogeological mapping for regional studies of the Vindhyan rocks around Bundi area, Rajasthan

An area of 2300 sq km s has been mapped around Bundi based on aerial photo-interpretation, with selected field checks at key areas. The oldest rocks exposed are Aravallis comprising a thick pile of argillaceous sediments with thin interbeds of quartzite. The Aravalli rocks have been subjected to two phases of folding and have undergone a low grade of metamorphism.

The problem of the Precambrian basement in Rajasthan, western India

The Banded Gneissic Complex (BGC) of Rajasthan, considered to form the basement underlying the Precambrian (Proterozoic) Aravalli metasediments, shows an erosion surface marked by a conglomerate and an angular unconformity, with gneissic foliation crossing the metasedimentary bands at only a few places. The BGC is a composite gneiss, evolved by extensive migmatization of metasedimentary rocks of diverse composition, and possibly metaigneous rocks. The contact between the BGC and the Aravalli rocks is a gently curved surface, whereas the gneissic foliation, as well as the large-scale metasedimentary enclaves within the gneissic complex, show all the intricate patterns of super-imposed folding traceable in the Aravalli rocks. This implies that the “basement” gneisses have been involved in ductile deformation with the Aravalli rocks, the migmatization being synkinematic with the first deformation in the latter. All these apparently conflicting lines of evidence can be resolved if the gneisses, as we see them now, represent not the original, but the mobilized basement, with the BGC-Aravalli boundary representing, for a large part, a migmatite front, rather than the original basement-cover interface. Only at a few places was there a chance of the original basement escaping mobilization and thus, little chance of identifying this original interface.

Evolution of lower proterozoic epicontinental deposits: Stromatolite-bearing Aravalli rocks of Udaipur, Rajasthan, India

Abstract The Aravalli rocks (> 2060 Ma old) which crop out around Udaipur, Western India, comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement, known as the Banded Gneissic Complex (ca. 2585 Ma old). The rocks have undergone a very low grade of metamorphism, and display a complex structure resulting from two major and several minor episodes of folding. There are two distinctly different ‘facies sequences’ in the Aravalli rocks, indicating deep-sea and nearshore shelf environments. The stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs (altered to greenschists) and quartizites with arkosic conglomerate. In the next sequence carbonates predominate in association with orthoquartzites, carbonaceous phyllites, phyllites, and stromatolitic rock-phosphate. The carbonate sequence passes upward into greywacke-phyllite-lithic arenite in the distal parts and conglomerate-arkose-orthoquartzite in proximal areas. The youngest sequence comprises orthoquartzite with silty arenite. The distribution of different facies, particularly that of dolomite with stromatolitic rock-phosphate, is controlled by sea-floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals. Sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault-controlled troughs along craton margins. Terrigenous debris was deposited in newly-developed troughs with contemporaneous volcanicity along trough margins. With the erasing of the ephemeral relief in the provenance, carbonate deposition was initiated. The environment encouraged algal growth and formation of stromatolitic rock phosphate. Carbonaceous phyllites developed in areas of restricted circulation. Rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke-phyllite and lithic arenite in the deeper parts of the epicontinental sea, and conglomerate-arkose-orthoquartizite in the marginal areas. The final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling. The nature of the terrigenous clasts indicates a predominantly granitic source of sediments. Supply of sediment was mainly from the continent to the east and partly from a landmass to the west. The cycle of sedimentation noted in the epicontinental Aravalli sea is broadly similar to the model of tectonic stages suggested by Krynine (1942).

Calcite Fabric in Relation to the Tectonic Setting of the Precambrian Rocks Around Karera, District Bhilwara, Rajasthan, India

Abstract Subfabric studies of calcite from an Aravalli marble have been carried out by measuring [0001], e-lamellae, r-cleavages, and a-axes in each grain in the rock-slide. The coaxes maximum is located nearly normal to the regional foliation (S2-surface), thus conforming to the results of the experimental deformation of calcite. The orientation of [0001] is attributed to the syntectonic recrystallization accompanying the first deformation D1; while the (0112) orientation appears to be related to the late phase of D1 or possibly to the second deformation, D2, which refolded the Aravalli rocks and the underlying Banded Gneissic Complex. The two deformation phases have been established on the basis of different structural elements in the rocks of the investigated area.

Superposed Folding in Nepheline Syenites and Associated Metamorphites Around Kishangarh, Rajasthan, India

Abstract Near Kishangarh in Rajasthan, Delhi conglomeratic quartzite and quartz mica schist overlie unconformably Aravalli metasediments. The foliation in the concordant nepheline syenites is parallel to the bedding plane foliation of the Aravallis. The Aravalli metasediments and the nepheline syenites show exactly similar development of superposed folds, with the axial surface of early isoclinal, inclined to reclined folds by nearly N-S striking upright folds. Evidence of such superposition is seen in all scales from hand specimens to maps. Spectacular outcrop patterns characteristic of superposed folds are developed. The effects of both the deformations can be seen only in the older Aravalli rocks and the nepheline syenites, while the younger Delhi rocks, in contrast, show only the later folds of the Aravalli rocks, which can be tied up with the first folding of the Delhi rocks.

Modification of Flexural Folds by Flattening Process-Examples from the Aravalli Rocks of Zawar Mine Area, Rajasthan

Abstract Study of two broadly similar folds from the Aravalli rocks of Zawar Lead-Zinc Mines area south of Udaipur in Rajasthan, shows that the folds are tight to isoclinal with wavelength/amplitude ratio between 0.35 to 0.37. The thickened hinges and the stretched limbs (v?2/?1=0.3) of the folds, and the weakly convergent pattern of the isogon lines suggest that the folds belong to the flattened parallel type. The axial plane cleavage in the rocks is a crenulation cleavage formed by microfolding and migration of quartz from the limbs, which make an angle less than 40° with the axial surfaces of the microfolds. Although some slip has occurred along the cleavage after it formed, this slippage has not altered the shapes of the folds to any great extent. The folds are thought to have been modified by post-buckling homogeneous strain aided by migration of quartz from the limbs to the hinges of the folds.

Thermal History of Rajasthan Pegmatites of India as Revealed by Fission Track Studies: Discussion

Nagpal and Nagpaul (1974) have inferred a cooling rate of m.y.-' for Rajasthan pegmatites and related this to ,the last major metamorphic event, that is, Delhi orogeny (860-735 m.y.) (p. 522). They bracket my work (Crawford 1970) with that of Holmes (1949), Holmes (1955), Sarkar et al. (1964), and Vinogradov et al. (1964), and imply that we all agree that ages of gneissic complex are of order of 2600 m.y., Aravalli orogeny from about 1500 m.y. to about 950 m.y., and Delhi from about 860 m.y. to about 735 m.y. We do not so agree. I attempted to show that picture is far more complex, that gneissic complex contains rocks of various ages, that Aravalli rocks must be younger than about 2500 m.y. and older than about 2000 m.y., and that Delhi rocks were first intruded by granites dated between 1700 and 1650 m.y. In other words, dates show that sequences are very much older than previous workers believed and the several orogenic cycles postulated by Holmes (1955) in his valuable pioneer study cannot be substantiated . (Crawford 1970, p. 108). I agree with Fermor (1950) who, in questioning conclusions Holmes drew about age of Delhi Systems from pegmatite mineral ages, stated that they gave information only about those pegmatites. Nagpal and Nagpaul have obtained fission-track ages for muscovites with a mean value of 787 + 42 m.y. It is relevant to state here that total-rock Rb-Sr analyses of Mount Abu granite and Idar granite fit an isochron giving an age almost exactly that of Malani Rhyolite and its associated granites, 745 + 10 m.y.

Early Precambrian stratigraphy of central and southern Rajasthan, India

Abstract Structural, stratigraphic and petrologic studies between Amet and Sembal in the Udaipur district of southcentral Rajasthan indicate that all the rocks belonging to the Banded Gneissic Complex, the Aravalli Group and the Raialo Formation have been involved in isoclinal folding on a westerly trend, co-axial refolding, and upright folding on a north to north-northeast trend. There is neither an unconformity nor an overlap between the Aravallis and the Raialos. The conglomerates supposed to mark the erosional unconformity above the Banded Gneissic Complex near Rajnagar is a tectonic melange of folded and torn quartz veins in mica schist within the Aravalli Group. The Aravalli—Raialo metasediments have been migmatized synkinematically with the first folding to give rise to the Banded Gneissic Complex; the gneissic complex does not have any separate stratigraphic entity. By contrast, there is an undoubted erosional unconformity between the type Aravalli rocks and the underlying Sarara granite to the south. These relations, coupled with the continuity of the Aravalli rocks of Udaipur northward to the metasedimentary rocks of the Sembal—Amet area along the strike, and a comparable structural history, point to granitic rocks of at least two generations in the Early Precambrian of central and southern Rajasthan. Preliminary radiometric dating of rocks of known stratigraphic—structural relationship seems to confirm the presence of granitic rocks of two ages in the Early Precambrian, and of a considerable interval between the deposition of the Aravalli—Raialo rocks and the Delhi rocks. The Udaipur granite, post-dating the first deformation but preceding the upright folding on the northerly trend, provides evidence for granitic activity of a third phase before the deposition of rocks of the Delhi Group.

Significance of Variable Plunge and Trend of Small-Scale Upright Folds in the Type Aravalli Rocks around Udaipur, Rajasthan (Western India)

Research Article| May 01, 1972 Significance of Variable Plunge and Trend of Small-Scale Upright Folds in the Type Aravalli Rocks around Udaipur, Rajasthan (Western India) ASHIT BARAN ROY ASHIT BARAN ROY Department of Geology, University of Rajasthan, Udaipur, India Search for other works by this author on: GSW Google Scholar Author and Article Information ASHIT BARAN ROY Department of Geology, University of Rajasthan, Udaipur, India Publisher: Geological Society of America Received: 01 Nov 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1972, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1972) 83 (5): 1553–1556. https://doi.org/10.1130/0016-7606(1972)83[1553:SOVPAT]2.0.CO;2 Article history Received: 01 Nov 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ASHIT BARAN ROY; Significance of Variable Plunge and Trend of Small-Scale Upright Folds in the Type Aravalli Rocks around Udaipur, Rajasthan (Western India). GSA Bulletin 1972;; 83 (5): 1553–1556. doi: https://doi.org/10.1130/0016-7606(1972)83[1553:SOVPAT]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The variable axial trend and plunge in the folds of the Aravalli rocks of early Precambrian age (2,500 to 2,000 m.y.) around Udaipur (lat 24° 35′ N.; long 73° 41′ E.) in southern Rajasthan, has been interpreted as being the result of interference of two periods of folding. The later folds with approximately north-south, steep axial planes are thought to have developed over an early set of overturned folds. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Superposed Folding in the Aravalli Rocks of the Type Area around Udaipur, Rajasthan

Abstract Around Udaipur, the Precambrian rocks belonging to the Aravalli System are represented by feldspathic greywacke grading to phyllite and mica schist, calc-arenite, and ferruginous and carbonaceous quartzites. The rocks display a high degree of structural complexity resulting from the impress of three distinct periods of folding (F1, F2 and F3). The earliest folding movement (F1) produced tight to isoclinal folds with drawn-out hinges. The F1 folds are accompanied by a set of very well developed axial plane schistosity. These folds are presumed to have been initially recumbent with E-W to NW-SE axial trend. The second fold movement (F2) refolded the F1 folds to upright, open to occasionally isoclinal folds. The F2 folds have variable plunge and an overall northerly axial trend. The third fold movement has resulted in the formation of tight to isoclinal folds (F3) of small and intermediate scales on the limbs of larger F2 folds. The F3 folds are characterized by a set of axial plane crenulation cleavages. The superposition of the three fold systems has resulted in complex interference patterns on different scales.

Evolution of the Banded Gneissic Complex of central Rajasthan, India

The Banded Gneissic Complex of central Rajasthan, the only gneissic basement in India considered to underlie an early Precambrian sedimentary suite unconformably, comprises composite gneisses formed by extensive migmatization of metasedimentary rocks of diverse composition. The migmatites and the metasedimentaries maintain a structural continuity in a plan of superposed deformations, with the migmatite front involved in the early folding but transgressing the stratigraphic boundaries. Structures in the metasedimentary palaeosomes within the gneisses match in their entirety those in the migmatite host and the metasedimentary bands outside. On a smaller scale of microsections, migmatites show para tectonic crystallization with reference to the first deformation. The Banded Gneissic Complex thus loses its unique position in the Indian Precambrians as older than the earliest decipherable sedimentary series, but is older than the Aravalli rocks of the type area, the partially migmatized metasedimentaries belonging to an earlier series.

Petrology of the Alkalic Rocks of Kishangarh, Rajasthan, India

Alkalic rocks are emplaced as conformable plutons in the Precambrian Aravalli rocks. These are generally associated with a gabbro. Petrographically, these coarse-grained alkalic rocks with a porphyroblastic texture contain a plagioclase (An 5–30 ) as the dominant feldspar, a subordinate amount of microcline, an amphibole (magnesioriebeckite), and nepheline-sodalite-cancrinite. The occurrences show a, bilateral symmetry with a medial granitoid type, followed out on both sides by a foliated type merging into the peripheral layered impregnation gneiss. A nepheline-free diorite gneiss is generally present along the junction with country rocks. Foliation inside the alkalic rocks is parallel to the borders, to the foliation of the country rocks, and to foliation inside the inclusions, the inclusions having undergone no apparent rotation. Internal structures of the alkalic gneisses are identical with those of country rocks. Structural, petrographical, mineralogical, and geochemical evidence indicates that the nepheline gneisses originated by in situ replacement of the gabbro. The process was essentially a type of alkali metasomatism in the course of which sodium, potassium, and alumina were added. The “alkalization” process was a phase of the regional granitization. Both these processes were syntectonic to start with, but their later stages outlasted tectonism. Long after emplacement, the alkalic rocks along with the country rocks were modified by the superimposition of Delhi structure and metamorphism.