Mantle Plume

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MORDEN VIEWS ON MANTLEPLUME AND AFFILIATED

MINERALIZATION

Introduction-

Theory of mantle plume is initially used to explain mid-plate (hot spot)volcanisms or linear chains of volcanoes that grow older in direction of platemotion, erupting far from the usual site along plate boundaries(ridge

volcanism). as This was proposed, initially to explain string of volcanicislands exhibiting a trend of age progression, such as the Hawaii-

Emperor chain in the Pacific Ocean (mid-Pacific plate) andsubsequently applied to explain the ChagosLaccadiveReunion chainin the Indian Ocean (Indian Plate). mantle plumes are columns of hot,solid material that originate deep in the mantle, probably at the coremantle boundary New mantle plumes are predicted to consist of alarge head, 1000 km in diameter, followed by a narrower tail. Initialeruption of basalt from a plume head should be preceded by ~1000 mof domal uplift. Studies of Mesozoic and Cenozoic igneous provincessome of the best opportunity for detail characterization of mantleplumes and related geological event.

Content-

1) Brief history2) Hot spots3) Large igneous provinces4) Mantle plumesa) Generationb) Ascent

c) Entrainmentd) Eruption5) Evidence for the theorya) Linear volcanic tracksb) Noble gas and other isotopesc) Geophysical anomaliesd) Geochemistry6) Ore deposit association


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7) Opponent of mantle plume theory

Brief history-

hot spots-

Hotspots are topographic swells with a relief of 500-1000m and typical width1000-2000km.it capped by active or volcanism recently active volcanism with

extinct volcanic chain.

Hot, solid rock rises to the hot spot from greater depths. Due to thelower pressure at the shallower depth, the rock begins to melt, formingmagma. The magma rises through the Pacific Plate to supply the activevolcanoes. The older islands were once located above the stationary hot spotbut were carried away as the Pacific Plate drifted to the northwest.


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Some examples of hot spots-

Hawaiian-Emperor seamount chain (Hawaii hotspot)

Louisville seamount chain (Louisville hotspot)

Walvis Ridge (Gough and Tristan hotspot) Kodiak-Bowie Seamount chain (Bowie hotspot)

Cobb-Eickelberg Seamount chain (Cobb hotspot)

New England Seamount chain (New England hotspot)

Anahim Volcanic Belt (Anahim hotspot)

Mackenzie dike swarm (Mackenzie hotspot)

Great Meteor hotspot track (New England hotspot)

St. Helena Seamount Chain - Cameroon Volcanic Line (Saint Helena hotspot)

3 Large igneous provinces

LIPs are voluminous occurrence of dominantly mafic igneous rock notdirectly related to plate tectonic process and dominated by thick, laterallyextensive basalt flows, some which have areal distribution of more than 105


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km2 and volume greater than or equal to 106 km3.they are found on earth inform of

a. Oceanic plateaub. Continental flood basalt

c. Passive margin volcanicd. Ocean-basin flood basalte. Submarine ridgesf. Giant dyke(and sill) swarmsg. Some large layered intrusions.

Distribution- LIP occurs in both continental and oceanic setting in interplatelocation, on present and former plate boundaries, and within and along the edge ofcontinents.

Fig. distribution of large igneous provinces formed in last 250 Myr. After Coffin and Eldolin(1994)

LIPs Duration (Ma) Age (Ma)

1 Siberien trap 9 250

2 Karoo 17 178-195

3 Parana Etendeca 11 127-138

4 Deccan 1 64.5-65.5

5 Ontong Java 30 88-93, 120-124

6 Columbia river 3 14.5-17.5


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7 North Atlantic 20 40-61

8 Iceland 15 0-15

9 Ethopia / East Africa 15 15-23 , 28-32

10 Kergulene 4 110-114

11 Bushveld complex 0.06 20-60

Table showing Age, Duration, Large igneous provinces compared with Oceaniccrust

4) Mantle plume-

Mantle plumes are columns of hot, solid material that originate deepin the mantle,probably at the coremantle boundary. Laboratory and numerical models replicatingconditions appropriate to the mantle show that mantle plumes have a regular andpredictable shape that allows a number of testable predictions to be made. Newmantle plumes are predicted to consist of a large head, 1000 km in diameter,followed by a narrower tail. Initial eruption of basalt from a plume head should bepreceded by ~1000 m of domal uplift. High-temperature magmas are expected todominate the first eruptive products of a new plume and should be concentratednear the centre of the volcanic province. All of these predictions are confirmed byobservations.

a) Generation-

We generally considered that mantle is a Newtonian fluid, which is a material

whose rate of deformation is proportional to applied force.

Convection in fluids is driven by buoyancy anomalies that originate in thermalboundary layers. Earths mantle has two boundary layers. The upper boundary layeris the lithosphere, which cools through its upper surface. It eventually becomesdenser than the underlying mantle and sinks back into it, driving plate tectonics.The lower boundary layer is the contact between the Earths molten ironnickelouter core and the mantle. High-pressure experimental studies of the melting pointof ironnickel alloys show that the core is several hundred degrees hotter than theoverlying mantle. A temperature difference of this magnitude is expected toproduce an unstable boundary layer above the core which, in turn, should produceplumes of hot, solid material that rise through the mantle, driven by their thermal

buoyancy. Therefore, from theoretical considerations, mantle plumes are theinevitable consequence of a hot core


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Fig. Convection model of Earth a thick zone deep in the mantle that isconvectively isolated from middle and upper mantle. Mantle plume may beproduced in D layer at the Core mantle-interface and at the upper boundaryof deep mantle layer at local high spots. Modified after Kellogg et al.(1999)

b) Ascent-

After generation mantle plume rises. The material in the lower boundarylayer will be lighter than the overlying mantle, but before it can rise at asignificant rate, it must gather enough buoyancy to overcome the viscosity ofthe mantle that opposes its rise. As a consequence, new plumes have a largehead followed by a relatively narrow tail.

Experimental study show that the viscosity of injected fluiddetermine the diameter of conduit required to carry a give plume flux,whereas viscosity of surrounding determine the rate of rise and size of plumehead. When plume head reaches the top, it is nearly symmetric and spreadhorizontally beneath the surface layer (analogous to the lithosphere in earth.


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Photograph of a laboratory model of a starting thermal plume (A) mid-way through its ascent and (B) after thehead flattens at the top of its ascent. The dark fluid represents hot material from the plume source and the lighter

fluid is cooler entrained material. White arrows show motion within the plume and black arrows the direction ofmotion in the boundary layer adjacent to the plume; the boundary layer has been heated by conduction so that itsdensity is approximately the same as that of the plume (after Griffiths and Campbell

c) Entrainment in mantle plume-

Both experimental and numerical modeling of plume shows that they entrainmaterial from the surrounding mantle as they rise. this is because hot, buoyantplumes transfer some of their heat surrounding ambient mantle, which increases itsbuoyancy and lower its viscosity, hence , plumes may sample not only the sourcematerial in D layer but also other mantle geochemical domain as they rise to thebase of lithosphere

Plume consisting of material in which viscosity is stronglytemperature dependent, the plum head grow as it become mushroom shaped. Asthe plume rise it entrain surrounding mantle in head, but very little mixing occur inthe plume tail. This indicates that geochemical signature coming from head may becontaminated with ambient mantle, where as plume tail should carry a sourcesignature. Thus flood basalt should reflect mixed mantle source and oceanic islands,relatively pure sample from plume source.

The amount of entrainment is critically dependent on the ratio of plume viscosityto ambient mantle viscosity. As expected the closer the viscosity of the plume is to that of surroundingmantle, the greater the degree of entrainment.

d) Eruption-

the continental lithosphere is thinned by extension, the surface subsides tomaintain isostatic balance.


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It is now widely agreed that melting in mantle plume result from adiabatic decompression

during their rise neat the base of the lithosphere. A numerical plume melting modle by

Ribe and Christensen suggest that meltig occure at two depth levels in a plume; majormelting occure in primary melting zone in plume head or at shallow level in the plume

tail, and secondary melting occure in plume tail at a depth of 300-500 km with the two

melting zone separated by a region of no melting.begins in the hottest, central part of theplume, and the temperature continues to rise. This high temperature lead to large degree

of melting, produsing picrite and high Mg theolite magmas.

Most of magma will collect in central part of a plume and, upon eruption, will form

shield volcanoes (small volume of magma) or oceanic plateaus and flood basalt (largevolume of magma).

5) Evidence for theory-

a) LINEARVOLCANICTRACKS-

The apparent linear, age-progressive distribution of the Hawaiian-Emperor seamount chain is explained in

this context as a result of a fixed, deep-mantle plume impinging into the upper mantle, partly melting, and

causing a "track" as the plate moves with respect to the plume source.

Smaller plumes, arguably called petitspots, are also common within intraplate areas. For instance, tracks

of ocean island basalts are found within the Indian Plate, namely the Marshall Islands hotspot.

Continental flood basalt in Oregon and Washington and the Yellowstone caldera-forming event are also

used as evidence for mantle plumes, with the voluminous flood basalt envisaged as a product of the

vigorous mantle plume head, and the hot 'tail' to the plume driving a progressively younger series of

caldera events as the North American continental mass tracks above it.

Smaller series of intracontinental volcanic rocks are also ascribed to small plumes or petitspots. These are

notably the Glasshouse Mountains in Queensland (Cohen et al. 2004), which are the oldest Tertiary (25

Ma) members of a progressively younger trend of basaltic and intraplate volcanic cones and plugsculminating in the maars and small peridotitic basalts of the Newer Volcanics in Victoria of 40,000 years

ago, far to the southeast.

It is notable that these volcanic features become younger in the same vector as the motion of the

Indo-Australian Plate, and matching the trend of the intraplate ocean island basalts in the Indian

Ocean.

b) NOBLEGASANDOTHERISOTOPES-

One of the most important observations in oceanic basalt is thattheir helium isotopic ratio differs according to tectonic setting.There are two isotopes of Helium; He3, which is primordial isotopes
http://en.wikipedia.org/wiki/Hawaiian-Emperor_seamount_chainhttp://en.wikipedia.org/w/index.php?title=Petitspots&action=edit&redlink=1http://en.wikipedia.org/wiki/Indian_Platehttp://en.wikipedia.org/wiki/Glasshouse_Mountainshttp://en.wikipedia.org/wiki/Queenslandhttp://en.wikipedia.org/wiki/Maarshttp://en.wikipedia.org/wiki/Peridotitehttp://en.wikipedia.org/wiki/Hawaiian-Emperor_seamount_chainhttp://en.wikipedia.org/w/index.php?title=Petitspots&action=edit&redlink=1http://en.wikipedia.org/wiki/Indian_Platehttp://en.wikipedia.org/wiki/Glasshouse_Mountainshttp://en.wikipedia.org/wiki/Queenslandhttp://en.wikipedia.org/wiki/Maarshttp://en.wikipedia.org/wiki/Peridotite


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that was incorporated in the earth as it accreted, and He4, anisotope produced by radioactive decay of Uranium and thoriumisotopes. Plume related basalt in oceanic areas have relatively highHe3\He4 ratio often more than 20 times that of air (R\RA=20),whereas MORB generality has R\RA value of 7-9.

Two classes of model have been suggested to explain high He3\ He4

reservoir in the mantle. Some investigator interprets the high ratioof reflected recycled oceanic lithosphere in the deep mantle. Suchlithosphere should have high He3\ He4 ratio partial melting atoceanic ridges extract almost all the U and Th from the mantlesource. Because these elements are responsible for accumulationof He4 over time, causing the He3\He4 ratio to decrease in thesource. Without U and Th the depleted oceanic lithosphere wouldacquired a high He3\He4 ratio. Alternative model call upon primitive,unfractionated sources deep in the mantle that still retain theiroriginal high He3\He4 ratio

a) GEOPHYSICALANOMALIES-

Diagram showing a cross section though the Earth's lithosphere (in yellow)with magma rising from the mantle (in red). The crust may translate relativeto the plume, creating a track.

Geophysical anomalies associated with hotspots and plumes include thermal,seismic, and geodetic. Thermal anomalies are inherent in the term "hotspot."Thermal anomalies are reflected in high heat flow values at the Earth's surface andexcess volcanism. Thermal anomalies also produce anomalies in the travel times ofseismic waves.

Seismic anomalies are identified by measuring spatial variations in the time it takesseismic waves to travel through the earth. A fluid body with a lower density (e.g., a
http://en.wikipedia.org/wiki/Lithospherehttp://en.wikipedia.org/wiki/Magmahttp://en.wikipedia.org/wiki/Mantle_(geology)http://en.wikipedia.org/wiki/File:Hotspot(geology)-1.svghttp://en.wikipedia.org/wiki/Lithospherehttp://en.wikipedia.org/wiki/Magmahttp://en.wikipedia.org/wiki/Mantle_(geology)


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hot mantle plume or wetter mantle) exhibits lower seismic velocity compared tosurrounding mantle. Observations of regions where seismic waves take longer toarrive are used as evidence for regions of anomalously hot mantle, as is observedunderneath Hawaii.Other indicators of plumes would be from the dynamic uplift ofthe surface and an elevated heat flow.

By deploying a dense network of seismometers and a technique known as seismictomography, scientists can construct 3-d images of seismic velocities to try andidentify vertical plume like structures. This is referred to as seismic tomographybecause it uses techniques similar to medical tomography

. Seismic waves generated by large earthquakes are used to determine structurebelow the Earths surface because they can be detected far from the earthquakeepicenter. Far-travelled seismic waves (also called teleseismic waves) are especiallyuseful for seismic tomography because they have steep travel paths that samplesmaller longitudinal domains. Density differences between a mantle plume andcooler material that surrounds it enable researchers to distinguish between the two.Seismic waves slow down when they travel through low-density (hotter) material,and speed up when traveling through denser (cooler) material. Density differencesmay also arise from compositional differences between the plume material and thesurrounding mantle.

By analyzing pressure pulses, or P-waves, a group of scientists at Princeton haveidentified 32 regions throughout the world where P-waves travel slower thanaverage. They conclude that these areas are mantle plumes. The team usedanalysis ofS-waves, another type of seismic wave generated by earthquakes, todetermine that those plumes extend to the core-mantle boundary.

Geodetic anomalies are reflected in topographic bulges above the plume location,

and in positive geoid anomalies. The geoid is a potential surface that reflects thetheoretical height to sealevel if mass was distributed uniformly within the Earth.Positive geoid anomalies reflect excess mass associated with uplift and doming overa thermal plume. The Yellowstone plume has a positive geoid anomaly of around+15 meters at its center, and over 1000 km in diameter.

Computer modeling of the mantle plume theory shows that changes of temperatureand chemical composition of rising plumes can lead to plumes of varying contoursas opposed to the early conceptualization that plumes developed as ahomogeneous mushroom shape.

b) GEOCHEMISTRY-

Basalts associated with hotspots or mantle plumes are geochemically distinctfrom mid-ocean ridge basalts and from lavas associated with island arcvolcanoes. In major elements, hotspot basalts are typically higher in iron (Fe)and titanium (Ti) than mid-ocean ridge basalts at similar magnesium (Mg)contents, reflecting their higher temperatures of formation. In trace elements,
http://en.wikipedia.org/wiki/Seismologyhttp://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/wiki/S-waveshttp://en.wikipedia.org/wiki/Geoidhttp://en.wikipedia.org/wiki/Geoidhttp://en.wikipedia.org/wiki/Seismologyhttp://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/wiki/S-waveshttp://en.wikipedia.org/wiki/Geoidhttp://en.wikipedia.org/wiki/Geoid


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hotspot basalts are typically more enriched in the light rare earth elements thanmid-ocean ridge basalts. Compared to island arc basalts, hotspot basalts arelower in alumina (Al2O3) and much higher in the immobile trace elements (e.g.,Ti, Nb, Ta).

The significance of these differences among ocean island basalts(hotspots), mid-ocean ridge basalts, and island arc basalts rests onprocesses that occur during subduction of oceanic crust and mantlelithosphere. Oceanic crust (and to a lesser extent, the underlying mantle)typically becomes hydrated to varying degrees on the seafloor, partly as theresult of seafloor weathering, and partly in response to hydrothermalcirculation near the ridge crest. As oceanic crust-lithosphere subduct, wateris released by dehydration reactions, along with water-soluble chemicalelements and trace elements. This enriched fluid rises to metasomatize theoverlying mantle wedge and leads to the formation of island arc basalts. Thesubducting slab is depleted in these water-mobile elements (e.g., K, Rb, Th,Pb) and thus relatively enriched in elements that are not water-mobile (e.g.,Ti, Nb, Ta) compared to both mid-ocean ridge and island arc basalts.

Ocean island basalts, which represent the volcanic product of mantleplumes, are also relatively enriched in the immobile elements relative to thewater-mobile elements, leading to the conclusion that subducted oceaniccrust plays a major role in their origin.

Superplume event and continental growth-

Super plume is large mantle plume that spread at the base of lithosphere, flatting theplume head to 1500 to 3000 km in diameter.

Single superplume typically give rise to large erupted volume of mafic magma

(>500000km ) in period of time less than 3 Ma.

superplume event is event (


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Superplume event at 1.9 Ga

Minor events are-

Late proterozoic (280 Ma)

Mid Cretaceous (100 Ma) Ordovician (480 Ma)

Equal area projection of the continental showing the distribution of juvenile continental crust

produced in four time window Modified after Condie.


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Islay and arbbott(1990) have used the distribution of Komatite flood basalt, mafic

dyke swarm, and layered mafic intrusion in the geological record to identity superplumeevent in the Precambrian.

Time series analysis of data shows major superplume event at 2.75, 2.70, 2.45,and 2.0,

1.9 Ga and several minor or possible event between 2.5 and 1.75 Ga.

Frequency distribution of juvenile Continental crust. After Condie (1998)

Time series global distribution of global mantle plume related igneous rocks in the geologicalrecord . After Isley and Abbott (1999)


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6) Ore deposit association

Magmatism and crustal rifting associated with the ascent of anomalously

hot and chemically distinctive material from the deep mantle are potentiallyimportant ore forming processes and are of obvious interest to economicgeologist.

Coffin and Eldholm propose that some Ophiolites are the product ofmantle plume related large igneous provinces.

One implication is that associated with Alpine Peridotites is may beplume generated.

Schissel and small Endorse previously proposed link between mantleplume and Noilsk Copper-Nickel-PGE deposit and Kimberliteic diamond

field. These authors also consider link between a mantle plume and Carlin

gold deposit; and also briefly consider the Kidd Creek massive sulphidedeposit of the Abitibi belt and nickel copper deposit of Yilgarn cratonbut mainly from the perspective that the deposit constitute evidencefor specific Archean mantle plume.

8) Opponent of mantle plume theory-

a. Large volume of melt, considered typical of plumemagmatism, is also now questioned. In terms of lateralspread, volume and duration of eruption as well as from fluiddynamical calculations for athermal mechanisms formagmatism8, the magnitude of plate boundary volcanism(ridges and island arc basins) arising from mantle upwellingfar exceeds plume eruptions. High volume of melt can alsoarise at normal mantle temperature under the oceanic crustand such melts with lower mantle geochemistry, consideredtypical of plume derivation, can be generated at much

shallower depths in the upper mantle itself from the meltingof recycled crustb. One of the basic tenets for basaltic flooding by mantle

plume heads is their rapid eruption. Yet, this fails in the caseof the Deccan eruption, one of the largest CFBs, which lastedfor 89 million years, barring a few minor flows erupting for ashort 0.51 million years; also, the eruptions forming theKerguelen Plateau, the second largest oceanic flood basalt


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formation, lasted for 130 million years.Removal of blockagesto the upward progress of magma, plate reorganization,mantle convection changes, partially molten asthenosphere,midplate mantle melting due to continental insulation canalso lead to sudden expulsion of large volume of magma8. In

the same token, Reunion plume dynamics for the uplift of thewest coast of India (Western Ghats) is dismissed as the latteris now ascribed to combined surface erosion and magmaticunderplating processes12. The Yellowstone (USA) example isnow ascribed to mantle convection and regional tectonics.

c. Even though all LIPs are claimed to beproducts of plume heads, opponents ofplume model have drawn attention to absenceof such plumes for Ontong-Java,Fijior Siberian Traps and they also doubt the suggested genetic links to remote

Louisville hotspot (for Ontong-Java, FijiLIPs) and Hawaii and Jan Mayen hotspots(for the Siberian Traps). Likewise,several hundred seamounts distributed inthePacific Ocean having hot spot derivedchemistry do not have hot spots beneaththem and are more likely to have shallowlevel melt-source in lithosphere

d. The observed bend midway in the Hawaii- Emperor chain inthe Pacific Plate and the shift in the ChagosLacccadiveReunion chain in the Indian Plate. According to the plumemodel, these changes reflect the shift in direction ofmovement of the concerned plate with respect to thehotspot fixed in the mantle below. But plume opponents explainthat forces on plates arise from combined effects of alldriving and resisting forces and hence changes to platemotion are bound to be too slow for the abrupt shiftsnoticed. On the other hand, local stresses inside the plate,influenced by the subduction geometry, can rapidly alterfracture trends in the plate thereby shifting the eruptionalong the new direction. Such fracture tectonics operating inthe Pacific plate, around 43 m.y. ago, resulted in Hawaii-Emperor chain bend8. Likewise, the Reunion plumeconnection is rejected for the shift of Chagos-Lacadive-Reunion Island track, which instead is attributed to thesouthward deviation of crack propagation through oceaniclithosphere.

e. Another bastion of plume theorists to come under theonslaught of plume opponents is the elevated 3He/4He ratioscited as strong evidence for the origin of several plumederived rocks from an undegassed 3He-rich lower mantlereservoir retaining primordial composition. The opponentsconsider this improbable as earths pre- and post core-formation periods were noted for high incidence of bolideimpacts, including a major one that formed the moon, all ofwhich would have extended early earths hot magma oceanphase long enough for the escape of primordial gases.


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Secondly, it is now argued that such high ratios can resultalso from the helium present in CO2 fluid inclusions inolivines and from U, Th retained in the mantle melt. Theymay also be contributed from old caught up olivine-gabbroicrocks in the upper mantle involved in partial melting or

during shallow-mantle partial melting of recycled, radiogenicand nonradiogenic regions of different ages8f. Superplume triggered continental breakup and development of new ocean

basins6, especially during the last one billion years, are also discredited in

view of the earths decreasing mantle potential (Raleigh number).

Superplume tectonics do not seem to have operated during the growth ofRodinia and Gondwana, two major supercontinents in earths history, judged

from volumetrically minor juvenile crust production during Grenvillian

(Rodinia) and Pan-African (Gondwana) periods14. Likewise, superplumeevents were absent also when Australia and Antarctica separated from

Gondwana. Alternatively, non-plume agencies like plate boundary driving

forces, top-down plate tectonic dynamics or combination of latter andmantle upwelling could as well have brought about these episodes8

g. Though plume model has been extensively applied for explaining midplatemagmatism, recent seismological and other studies have come up with

alternate nonplume models. Anderson, California Institute of Technology,

Pasadena, rejects the idea of the ascent of magma plumes from CMB on the

grounds that the pressure, viscosity, coefficient of thermal expansion,thermal conductivity, interatomic distances at these depths forbid such a

mechanism of magmatism8. He argues that the. high pressure and viscosity

here suppresses heat flow from the core and slows down generation ofmantle convection cells at the thermal boundary near CMB, which in turn

impedes buoyancy effects for initiating plumes. Further, the high mantletemperature theorized for plume involvement for the Precambriankomatiites, picrites and other rocks are not supported by heat flow data or

petrology and in fact, calculations indicate that the early mantle was merely

120C hotter than now (1300C) and hence these rocks could as well haveformed by partial melting of upwelling mantle accompanying passive rifting.

References -


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1. Armstrong, R. L., Philos. Trans. R. Soc., London, 1981, A301, 443472.

2. Taylor, S. R. and McLennan, S. M., Rev. Geophys., 1995, 33, 241265.

3. Condie, K. C., Tectonophysics, 2000, 322, 153162.

4. Stein, M. and Hoffmann, A. W., Nature, 1994, 372, 6368.

5. McCulloch, T. M. and Bennet, V. C., Geochim. Cosmochim. Acta, 1994, 58,

47174738.

6. Sylvester, P. J., Campbell, I. H. and Bowyer, D. A., Science, 1997, 275, 521

523.

7. Condie, K. C., Des Marais, D. J. and Abbot, D., Precambrian Res., 2001, 106,

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8. Condie, K. C., J. African Earth Sci., 2002, 35, 179183.

9. Condie, K. C., Earth Planet. Sci. Lett., 1998, 163, 97108.

10. Rogers, J. J. W. and Santosh, M., Gondwana Res., 2003, 6, 357368.

11. Rogers, J. J. W., J. Geol., 1996, 104, 91 107.

12. Rogers, J. J. W. and Santosh, M., Gondwana Res., 2002, 5, 522.

13. Zhao, G., Cawood, P. A., Wilde, S. A. and Sun, M., Earth Sci. Rev., 2002, 59,

125162; Zhao, G., Suna, M. and Wildeb,S. A., Precambrian Res., 2003, 122, 201233;

Piper, J. D. A., Mallik, S. B.,Bandyopadhyay, G., Mondal, S. and Das,A. K., PrecambrianRes., 2003, 121,185219; Rosen, O. M., Russian J. Earth Sci., 2002, 4, 16. 14. Mishra, D.

C., Chandrasekhar, D. Ch., Venkata Raju, V. and Vijaya Kumar, V., Earth Planet. Sci.Lett., 1999, 172, 287 300.

15. Dalziel, I. W. D., Geology, 1991, 19, 598601.

16. Sankaran, A. V., Curr. Sci., 1997, 73, 901903.

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