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Gemstones and Earth history: A Gondwana perspective M. Santosh Division of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan Tel. and Fax: +81-88-844-8278. E-mail: [email protected] ; [email protected] Abstract The Solid Earth can be considered to comprise a plate tectonics domain with broadly horizontal motion in the upper mantle, plume tectonics dominated by vertical movements in the lower mantle region and an ‘anti-plate tectonics’ zone characterized by horizontal movements at the bottom of the mantle. Large plumes rising from the core-mantle boundary act as pipes connecting the core to the surface, and play an important role in material transfer within the Solid Earth. The history of evolution of our planet witnessed the assembly of continental blocks within large land masses called supercontinents at different periods. The formation of mineral resources, including various types of

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Gemstones and Earth history:

A Gondwana perspective

M. Santosh

Division of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho

2-5-1, Kochi 780-8520, Japan

Tel. and Fax: +81-88-844-8278. E-mail: [email protected];

[email protected]

Abstract

The Solid Earth can be considered to comprise a plate tectonics domain with broadly

horizontal motion in the upper mantle, plume tectonics dominated by vertical

movements in the lower mantle region and an ‘anti-plate tectonics’ zone characterized

by horizontal movements at the bottom of the mantle. Large plumes rising from the

core-mantle boundary act as pipes connecting the core to the surface, and play an

important role in material transfer within the Solid Earth. The history of evolution of our

planet witnessed the assembly of continental blocks within large land masses called

supercontinents at different periods. The formation of mineral resources, including

various types of gemstones, is broadly related to the assembly, evolution and break-up

of supercontinents and associated thermal events. Among the supercontinents in Earth

history, the Gondwana assembly at around 540 million years ago comprises continental

fragments which are the treasure chests of a variety of mineral deposits including

gemstones. A typical example is southern India, which formed the central piece of

Gondwana, where gemstone belts provide marker belts for continental correlations. The

Karur-Kangayam belt and the southern Kerala belt host a variety of precious and semi-

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precious stones including ruby, sapphire, emerald, aquamarine, chrysoberyl,

kornerupine, topaz, zircon, garnet, tourmaline, amethyst, among others. The

gemstones are hosted by both metamorphic rocks as well as magmatic and related

pegmatitic/hydrothermal phases. Understanding the formation of these belts provide

valuable insights into the tectonics of the region and correlation with global processes.

Gemstones have played an important role in the history of human society. Public

awareness programmes on gemstones would not only aid in promoting gem industry,

but would also help in a proper understanding of Earth history. This would also be a

befitting appreciation of Mother Nature’s fascinating expressions through precious and

semi-precious stones meticulously crafted at different times over the 4.6 billion year

history of the Earth.

Key words: Gemstones, Earth history, supercontinents, Gondwana, southern India.

1. Introduction

The outer layer of the Earth is like a cracked egg shell and is constituted of a number of

lithospheric plates. These plates are in constant motion, moving away from each other,

sliding past or colliding. The horizontal motion of the lithospheric plates on the surface

of the Earth is explained by the concept of plate tectonics and contributes to the

generation of new crust at spreading centers which is transported and eventually

destroyed at subduction zones (Fig. 1). The subduction zones are also production

centers where hydrous melting generates arc magmas which rise up and build the arc

crust above. In convergent zones, the subducting plate moves vertically down like a

curtain-like sheet until it reaches the mantle transition zone between 410 to 660 km

depth where the material accumulates as ‘subducted slabs’ (Fig. 2). Some, but not all,

of the subducted slabs may descend vertically as blobs, reaching the core-mantle

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boundary (Zhao, 2004; Maruyama et al., 2007, and references therein) where they

accumulate as ‘slab graveyards’. This material provides a potential trigger as well as the

essential fuel for the generation of superplumes either through heating from the core, or

radioactive heat generated from enriched basaltic slab remnants, or both. Superplumes

rise from the core-mantle boundary to the uppermost mantle, penetrating the mantle

transition zone, eventually giving rise to hot spots. Thus plumes act as pipes connecting

the core to the surface, and play an important role in material transfer within the Solid

Earth (Santosh et al., 2009a) (Fig. 3). In the present day Earth, two hot upwellings

corresponding to superplumes are identified, the Pacific and African, and one cold

downwelling (e.g., Maruyama et al., 2007).

The subducted slab material, inferred from seismic tomography, moves horizontally at

the core-mantle boundary. The implication of horizontal movement at the base of the

mantle has been referred to as “anti-plate tectonics”, in many respects analogous to

lithospheric plate tectonic processes operating in near-surface regions. Accordingly,

through time, it is conjectured that continents gradually develop at or close to the outer

earth surface while concomitant ‘anti-continents’ would be generated at the core-mantle

boundary (Maruyama et al., 2007). Thus, geodynamic processes can be viewed on a

whole as a combination of ‘plate-’, ‘plume-‘, and ‘anti-plate-’ tectonics (Fig. 4). In other

words, melts and fluids between upper- and lower-most levels of the mantle may play a

profound role in the process of global material circulation. Thus, the Solid Earth can be

considered to comprise a plate tectonics domain with broadly horizontal motion in the

upper mantle, plume tectonics dominated by vertical movements in the lower mantle

region and an ‘anti-plate tectonics’ zone characterized by horizontal movements at the

bottom of the mantle (Maruyama et al., 2007; Santosh et al., 2009a). Recent models

propose that a third layer of continental crust might have accumulated over time on the

mantle transition zone, predominated by subducted granitic crust.

In the early 2 billion year history of the Earth, there were no large continents and the

surface of our planet was dominated by an oceanic realm with island arcs. Arc-arc

collision and formation of composite arcs led to the development of embryonic

continents which subsequently amalgamated to form larger continental masses (Fig. 5)

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(Santosh et al., 2009b). The continental fragments joined together to form

megacontinents and these in turn were welded together into supercontinents.

Supercontinents are large landmasses formed by the convergence of multiple

continental fragments carrying ancient cratons, together with accreted terranes. The

first coherent supercontinent is thought to have been assembled by around 2.0 Ga,

although it was much smaller in size as compared to that of some of the younger

supercontinents. The configuration of supercontinents and pseudo-supercontinents

which shaped the globe during various periods in Earth history as often found in

literature are the hypothetical assembly Ur (3.0 Ga), Kenorland (2.7-25. Ga), Columbia

(1.8-1.9 Ga), Rodinia (1.1 Ga), Gondwana (0.54 Ga) and Pangea (0.25 Ga) (e.g.,

Rogers and Santosh, 2004 and references therein; Santosh et al., 2009b). A future

supercontinent Amasia is speculated to form at around 250 million years from now.

Supercontinents once assembled are not stable, because heating beneath the large

land mass, either through radiogenic input (thermal blanket effect) or through the

impingement of superplumes, eventually breaks them apart, and the crustal fragments

move away to cooler regions on the globe to reassemble into newer configurations.

The dynamics of supercontinents also impact the origin and extinction of life as well as

surface environmental changes. Large scale flow of material and energy through mantle

downwelling and upwelling associated with supercontinent assembly and breakup is

thought to affect the Earth’s dynamo which would lead to catastrophic environmental

changes, sometimes even triggering mass extinction of life forms. When a rising plume

impinges the base of a supercontinent, the resultant continental rifting, formation of large

igneous provinces and volcanic emissions might lead to the initiation of a plume winter,

the aftermath of which would be mass extinction and long term oceanic anoxia.

Supercontinent tectonics in relation to mantle dynamics thus provides a key to evaluate

the history of evolution and destruction of the continental crust, to understand the history

of life, and to trace the major surface environmental changes of our planet (Santosh,

2010).

Supercontinent formation and destruction and associated geodynamic processes have

been fundamental to the generation of the various types of mineral deposits in our planet,

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including the formation of precious and semi-precious stones. As mentioned above, the

material transfer on a whole-earth scale controls these factors, with elements transported

from the core to the surface through plumes and recycled back through plate tectonics.

Magmatism, metamorphism and fluid processes associated with supercontinent

dynamics critically control the formation of mineral resources. In this contribution, I

provide an overview of the gemstone occurrences in one of the principal crustal

fragments that was part of the supercontinent Gondwana and discuss the implications in

relation to geodynamics. This contribution is a partial summary of my inaugural address

delivered at the Annual Meeting of the Indian Gemmological Society held in Munnar

(Kerala) during 2009, and does not claim to be a comprehensive review.

2. Gemstones in relation to tectonic setting

Rocks are aggregates of minerals and the formation conditions and stability of the

mineral assemblages in different rock types are dictated by the tectonic environment of

their genesis. The primary growth of minerals occurs from melts and/or fluids associated

with magmatic, metamorphic and sometimes even sedimentary environments. The

nature of magma/melt, the compositional characteristics of the associated fluids, the

depth of formation (which essentially translates into pressure-temperature conditions),

among other parameters, dictate the type of mineral assemblage. The duration of

crystallization is also an important factor, particularly In the case of large and well-formed

crystals. Shorter duration or intermittent tectonic disturbances result in smaller grain size

or malformation. Whereas diamonds form in deep-seated kimberlitic magmas generated

in the mantle, minerals like emerald (a variety of beryl), and amethyst form in shallow

levels from hydrothermal fluids emanating from the residual stage of crustal magmas or

circulating fluids in the upper crust.

We now briefly consider the gemstone ‘factories’ of the dynamic Earth from a layman’s

point of view. Minerals like agate, amethyst, opal, turquoise, and malachite/azurite

precipitate from percolating water charged with different types of elements in the near-

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surface environment. If this water emanates from a magma emplaced at depth, then it

would have higher temperatures and would be concentrated with various elements that

are scavenged from the residual magma leading to a variety of rich hydrothermal

deposits including minerals like beryl (emerald, aquamarine), tourmaline, chrysoberyl,

among others. At much deeper levels, within the cooling magma chambers, a variety of

high temperature minerals are produced such as for example, zircon and topaz. Further

inside the bowels of the earth in the deep mantle, diamonds crystallize in rare magmas

like kimberlites and lamproites which form by the partial melting of the upper mantle. The

gas expansion within these magmas propels them to rise upward at rates of 10 to 30 km

per hour, taking just 4 to 15 hours to reach from mantle to the surface.

When magmatic rocks and sedimentary rocks are subjected to metamorphism, a variety

of minerals are formed, a typical example being garnet. Some of the other popular

minerals of gemstone family such as sapphire and ruby (corundum), jadeite, sillimanite-

kyanite, iolite (cordierite), kornerupine etc. are also formed in metamorphic rocks.

Depending on the tectonic setting and pressure-temperature conditions of

metamorphism, the mineral assemblages vary widely to the extent that even diamonds

(mostly micro-diamonds) can also crystallize in metamorphic rocks, such as in the case

of eclogites generated by ultrahigh-pressure metamorphism involving deep subduction of

crustal materials in major continent-continent collision zones.

In gem industry, alluvial stones carry prominent importance because they are easily

recoverable and also due to their better quality, devoid of cracks and other abnormalities

as they are naturally sorted with only the better lot surviving the long fluvial transport

before deposition. These minerals preserve a long history of their formation in

magmatic/metamorphic environment prior to their departure from the source through

weathering of the host rocks and transport in fluvial medium and final accumulation within

placer deposits. The mechanisms of formation of placer deposits are linked to

geomorphologic parameters which in turn are ultimately related to regional tectonics.

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A cartoon illustration of the different tectonic environments of formation of a few popular

gemstones is shown in Fig. 6.

3. The Gondwana gemstone treasury

Among the various supercontinents that shaped the surface or our globe from time to

time, the formation of the Gondwana supercontinent during the Late Neproterozoic-

Cambrian (ca. 540 Ma) marks a turning point in the history of the Earth as it coincides

with major environmental changes including the first appearance of the modern

(metazoan) life forms. The Gondwana supercontinent comprises a large assembly of

continental masses including India, Antarctica, Australia, Madagascar, Sri Lanka, South

Africa and South America, and formed the southern half of Pangea (Rogers and

Santosh, 2004, and references therein; Meert and Lieberman, 2008) (Fig. 7). It was later

rifted and fragmented, firstly by the eastward rifting of Africa at 160 Ma to separate from

North America, followed by the Cretaceous opening in the central and southern Atlantic,

together with the separation of India at 120-100 Ma, and finally Australia at 60 Ma. India

moved rapidly northward and collided against Asia at 50 Ma, giving rise to the mighty

Himalayan Mountain Belt.

The Gondwana crustal fragments are our planet’s treasure chests of a variety of

economic mineralization and gemstone deposits. Gold, base metals, diamond, iron,

manganese, iron-ores, tin-tungsten, molybdenum, tantalum-niobium, platinum-group

metals, uranium and graphite are among the variety of economic resources in these

regions. High quality gemstones are found in India, Madagascar, Sri Lanka, Brazil and

East Africa including diamond, ruby, sapphire, emerald, aquamarine, chrysoberyl, zircon

garnet, amethyst, tourmaline and a host of other semi-precious minerals.

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4. Southern India as a fragment of Gondwana

The Indian Peninsula comprises two major tectonic units: the South India and the North

India crustal blocks which were welded along the Central India Tectonic Zone (the

Narmada-Tapti River course) during the Paleo-Mesoproterozoic times. Each of these

major units incorporate Archean cratons (such as Dharwar, Singhbhum, Bastar and

Bundelkhand) and surrounding younger belts (such as the Southern Granulite Terrain,

the Eastern Ghats Belt, the Aravalli-Delhi belt etc.). The different units belonged to

different supercontinents in the past including the earliest supercontinent Ur at 3.0 Ga,

Columbia at 1.9 Ga, Rodinia at 1.0 Ga and Gondwana at ca. 0.54 Ga. A variety of

mineralization including gemstones occurs in the different blocks associated with

magmatic and metamorphic processes attending various thermal regimes during different

periods in Earth history. In this contribution, I will focus attention on the Gondwana-

related gem mineralization in southern India.

Southern India occupies a central position in the Late Neoproterozoic-Cambrian

Gondwana supercontinent assembly. Southern India comprises two major tectonic units:

the Archean Dharwar Craton to the north and the Proterozoic granulite blocks to the

south. The Proterozoic mosaic of southern India comprises a collage of crustal blocks

dissected by Late Neoproterozoic-Cambrian crust-scale shear/suture zones (Fig. 8).

Among these, the Palghat-Cauvery Suture Zone (PCSZ) has been identified as the trace

of the Cambrian suture representing Mozambique Ocean closure during the final phase

of amalgamation of the Gondwana supercontinent.

In a recent plate tectonic model, Santosh et al. (2009c) proposed the various stages

associated with the final assembly of southern India within Gondwana (Fig. 9). The early

part of the cycle witnessed a continental rifting stage and formation of the Mozambique

Ocean basin probably at around 900-800 million years ago which extended all the way from

East Africa through Madagascar, India and Sri Lanka into East Antarctica. The charnockite

massifs and associated granitoid suites covering a width of over 200 km in the Madurai

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Block to the south Palghat-Cauvery Suture Zone in southern India, and their extension into

adjacent continental fragments within Gondwana suggest that a large ocean must have been

present, perhaps similar to the present day Indian Ocean, and that these arc massifs were

generated by the southward subduction of an oceanic crust. The closure of the Mozambique

Ocean was marked by the collision of the Archean Dharwar craton in to the north to create a

Himalayan-type Cambrian orogenic belt, the trace of which is the Palghat-Cauvery Suture

Zone, accompanied by regional high grade metamorphism. The final stage is marked by

post-orogenic extension and emplacement of minor late stage intrusive plutons widely

occurring in various blocks and shear/suture zones. Post-collision Barrovian hydration is

also widely noticed, particularly in the development of lower grade assemblages dominated

by hydrous minerals. A composite schematic illustration proposed by Santosh et al. (2009c)

for the final amalgamation and generation of a Cambrian “Himalaya” in southern India is

shown in Fig. 10. This model envisages two ocean closures, one along the Palghat-Cauvery

Suture Zone and the other along the Achankovil Suture Zone further south during the final

assembly of southern India within Gondwana.

Within various domains in the Palghat-Cauvery Suture Zone, high pressure and ultra-high

temperature granulite facies rocks occur with diagnostic mineral assemblages of extreme

metamorphism (e.g., Shimpo et al., 2006; Santosh et al., 2010; Nishimiya et al., 2010) (Fig.

11). Recently, Santosh and Kusky (2010) proposed a ridge subduction model to explain the

paired high pressure and ultrahigh-temperature metamorphic rocks commonly occurring

within the Palghat-Cauvery Suture Zone (Fig. 12). The development of a wide accretionary

belt with typical features of ocean plate stratigraphy associated with the closure of the

Mozambique Ocean in the late Neoproterozoic has been recently recorded from this zone

(Santosh et al., 2009c). The nature and distribution of the rock types in this zone with

charnockites and granites at the higher crustal level followed by mafic/ultramafic rocks and

HP-UHT paired sequences towards the deeper level broadly corresponding with the

southward polarity of ridge subduction and possible slab window opening. Thus, one of the

possible scenarios for the HP-UHT conditions in a CO2-dominated fluid regime recorded from

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these rocks would be a model where the slab window was placed against a hot

asthenosphere.

5. The gemstone province in southern India

The Karur-Kangayam gemstone belt in Tamil Nadu, southern India is spread over an

area of approximately 70 x 70 sq. km and is located at the southern margin of the

Palghat-Cauvery Suture Zone. The gemstone belt occurs within a system of shear

zones passing through Karur, Dharapuram,Oddanchathram and Palani areas. The belt

is located at a critical junction of contrasting crustal blocks, and within a major suture

zone where ocean closure, deep subduction, accretion and continental collision

occurred accompanied by magmatism, metamsomatism and high grade metamorphism.

The varied gemstone mineralization in this belt are related to different lithologic

horizons, and apparently belong to different genetic environments, although their

formation can be linked to processes accompanying the latest prominent thermal event

in the region associated with the final amalgamation of the Gondwana supercontinent.

Geochronological data (summarized in Santosh et al., 2009c) suggest ca. 540-520 Ma

ages.

Santosh and Collins (2003) summarized the general features of the gemstone

occurrences in the Karur-Kangayam belt. A number of semi-precious stones occur in

this belt associated with various lithologies (Fig. 13) These include star ruby and

sapphire varieties of corundum together with cordierite of iolite type (Lachmanapatti,

Malapatti and Kiranur), yellow topaz (Dindigul), aquamarine and emerald varieties of

beryl (Manikapuram), alexandrite (Chinnadharapuram), bronzite with cat’s eye

(Pulampatty), apatite (Edapadi), spinel (Manavadi, Kunjampalayam), kornerupine with

cat’s eye (Edappadi, Ponnambalam), black tourmaline (Ottani), labradorite

(Oddanchathram), amethyst (Kadavur) and garnet (Oddanchathram), among others.

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At the Lachmanapatti ruby mine, corundum crystals of various sizes ranging up to 5 cm

occur. Large crystals of deep purple cordierite (iolite) occur in intimate association with

corundum. Cordierite also occurs as symplectites in association with needle-like crystals

of sapphirine (Tsunogae and Santosh, 2003). Symplectitic moats of cordierite form

corona around corundum. Hercynite spinel occurs in between the sapphirine-cordierite

symplectite and moats of cordierite. Coarse sapphirine-corundum-cordierite-biotite

assemblages also occur in this locality. At Malapatty, another cordierite (iolite) and

corundum (sapphire) mine, tiny bluish sapphirine crystals occur enclosed within

plagioclase (Tsunogae and Santosh, 2003). Corundum here occurs as coarse crystals

surrounded by plagioclase, with no direct contact with sapphirine. Corundum-bearing

samples in Malapatty also contain gedrite and biotite. Star-ruby variety of corundum

also occurs in other localities in the Karur-Kangayam belt, including Landakottai and

Tantoni. An exclusive variety of a blend of red ruby and blue sapphire within single

crystals occur at Sevitturangampatti, a location from where Shimpo et al. (2006)

reported high pressure and ultrahigh-temperature metamorphic conditions derived from

garnet and corundum bearing rocks.

Coarse cordierite crystals at Lachmanapatty and Kiranur are associated with feldspar of

sunstone variety. The feldspars contain numerous oriented and thin, elongate inclusions

of brown biotite which impart bright reddish brown glitter in reflected light. Composite

grains of iolite and sunstone from these localities are used to make single stones with

coupled deep blue and orange-brown hues. The moonstone variety of feldspar in the

Karur-Kangayam belt shows various hues including pearly white, grey, bamboo green,

light brownish pink and black. The gem varieties are characterized by a milky white

sheen that moves across the crystal when rotated against the light, and is identical to

the chatoyancy observed in cat’s eye chrysoberyls from southern Kerala (see Menon et

al., 1994). A similar chatoyancy effect is also shown by some varieties of other semi-

precious stones from Karur-Kangayam belt such as bytownite, kornerupine and spinel.

Some of the exclusive varieties of feldspar with multiple sheen are locally termed as

rainbow stones.

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Aquamarine variety of beryl from Karur-Kangayam belt occurs in various sizes, and

some crystals are up to a few centimetres in length. Black tourmaline, amethyst and

crystal quartz also occur commonly. Another major attraction of this gem belt is the

multiple variety of rutile-bearing quartz and amethyst. The elongated needles of rutile,

often black or sometimes in golden hue associated with hematite, offer spectacular

samples for collectors.

Geologically, the different semi-precious stones in the Karur- Kangayam belt are related

to different lithologies and were derived by different processes in a wide range of

pressure-temperature-fluid conditions. The semi-precious varieties of corundum (star

ruby and sapphire), spinel and kornerupine are related to metamorphic processes, and

are seen within aluminous metasedimentary horizons occurring as thin slivers indise

host orthogniesses. For example, at Lachmanapatty, a 3 m thick garnet and

phlogopite/biotite-bearing metasedimentary layer is intercalated within biotite and

hornblende bearing orthogneisses. A coarse pegmatitic zone rich in feldspar and bluish

cordierite occurs between the orthogniess and metasedimentary layer. The

orthogniesses in the area contain deformed and boudinaged mafic enclaves of various

dimensions ranging from centimeter to decimeter scale. The corundum-bearing zone

occurs metasedimentary unit. At Malapatty, corundum and cordierite-bearing

amphibole-rich gneisses are intruded by N-S striking coarse pegmatite which also

contains cordierite. The sunstone variety of feldspar occurs in intimate association with

iolite variety of cordierite within pegmatites. These pegmatites are often associated with

metasedimentary layers carrying corundum. The ruby iolite-sunstone associations at

Lachmanapatti and Kiranur are typical examples. On the other hand, the various

varieties of moonstones, such as those mined near Onamparai, 15 km west of

Kangayam, occur within thick pegmatitic dykes almost entirely composed of feldspar

and minor quartz, in the absence of cordierite and corundum. Evidently, the moonstone

variety of feldspar comes from late residual phase of granitic magmas, the evidence for

which is abundant in the area, such as the pink granite of Vattamali near Karur.

Pegmatite emplacement and related hydrothermal activity also gave rise to various

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other semi-precious minerals in the area including gem variety of beryl (aquamarine),

topaz, tourmaline, amethyst and crystal quartz.

Apart from the gemstones, the zones described are also of fundamental importance to

petrologists as they often associate a variety of diagnostic indicator minerals for

extreme metamorphic conditions. For example, many of the Mg-Al granulite layers in

these zones associate the blue mineral sapphirine, often in direct association with

quartz, suggesting metamorphic temperatures in excess of 1000oC (e.g. Nishimiya et

al., 2010). Similarly, low Zn spinel in association with quartz is also found in some

locations suggesting extreme metamorphic temperatures. A variety of other associated

minerals including garnet, gedrite, kyanite, sillimanite, cordierite, feldspars and others

have aided petrologists working in this region to reconstruct the metamorphic and

tectonic history. In addition, the finding of high pressure assemblages within mafic

granulites have aided in a better understanding of the subduction-collision tectonics in

this area.

6. Gemstone belts as marker beds for continental correlations

The Karur-Kangayam gemstone occurrences provide a key belt to investigate the

correlation between India and Madagascar within the Gondwana supercontinent

assembly. One of the key gemstones is ruby. Extensive new finds of gem-quality

corundum have been reported from the central-eastern coast and from the north-central

region of Madagascar. Both these localities lie along the Betsimisaraka suture and

therefore support correlation of the Betsimisaraka suture in Madagasacar with the

Palghat-Cauvery Suture Zone system in India. In Madagascar, several other sapphire

and ruby occurrences are also known. Gem variety of red corundum (ruby) from the

Ejeda-Fotadrevo area in southwestern Madagascar are associated with basic/

ultrabasic complexes of the Precambrian Vohibory unit metamorphosed to granulite

facies (Mercier et al., 1999) and have been correlated with similar occurrences in

Tanzania.

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In a previous study, Menon et al. (1994) synthesized the occurrences and

characteristics of semi-precious stones in the chrysoberyl belt of southern Kerala.

Southern Kerala region in South India is among the historically important regions within

Gondwana fragments for gemstone exploration and exploitation, with recovery of a

variety of gemstones continuing for over a century. Among the important gemstones

found in this region are the peacock's eye (alexandrite) and the cat's eye (chrysoberyl).

Several semi-precious stones including translucent chrysoberyl, ruby, green beryl,

aquamarine, topaz, kornerupine, green and golden zircons, blue spinel, almandine and

pyrope garnets, blue and green apatites, feldspar (moonstone), smoky quartz, amethyst

and cordierite, among other minor varieties, have also been recovered from this region.

Menon et al. (2004) proposed a gemstone province in the East Gondwana assembly

compiling similar occurrences in southern Kerala, Sri Lanka and Madagascar. In

another study, Santosh et al. (2002) reported pink sapphire from the Achankovil Shear

Zone and Trivandrum Block in southern Kerala. The mineralization is sporadic and

associated with granulite facies aluminous supracrustals. The available radiometric age

of around 513 Ma for gem quality zircon associated with pink sapphire in the Melankode

locality confirms that the mineralization is of late Pan-African age. Pink sapphires have

been widely reported from a number of localities in southern Madagascar including

Betroka, Illakaka, Antranondambo and Ambossary. Sapphires of various hues also

occur in the Ratnapura gem district in the southwestern part of Sri Lanka. The pink

sapphire occurrences in southern Madagascar and southern Kerala provide strong

evidence for India-Madagascar-Sri Lanka juxtaposition in the Gondwana assembly with

the Ranotsara Shear Zone in southern Madagascar extending into the Achankovil

Shear Zone in southern India.

7. Need for public awareness

Gemstones are the flowers of Mother Earth crystallizing from magmas, melts and fluids.

Some of the minerals like zircons date back from the early history of earth, more than

4000 million years ago. With their rarity, durability and beauty, gem stones have figured

in various phases of the cultural history of human society. However, precious and semi-

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precious stones, apart from their attraction as ornamental stones, have not been

adequately recognized for their geologic complexity and cultural heritage among the

common public, particularly in developing countries. The recovery and marketing of

gemstones is often riddled with problems and local issues including regulations imposed

by administrative authorities. Gems provide a better alternative against the mad rush

for gold, with the ever-increasing demand for the yellow metal particularly in developing

countries. Mineral markets, gem clubs and gem hunting expeditions should be

organized to create public awareness which would open up immense possibilities for

development and growth of the gem industry in the coming years.

Acknowledgements

I would like to sincerely thank Dr. K.T. Ramachandran, Hon. Secretary of the

Gemmological Society of India, and the organizers of the 10 th IGS in Munnar, Kerala for

inviting me to inaugurate the ceremony and for asking me to deliver a lecture. I am

particularly thankful to Dr. Ramachandran for inviting this manuscript and for the

immense patience and patronage in waiting for it. I would like to thank my colleague

Mr. A.K. Salim and “Karur Babu” without whose help and assistance, I would not have

had the chance to study the gemstone belt of Karur-Kangayam area. I also express my

sincere thanks to many of my research collaborators, particularly Dr. T. Tsunogae

(Japan), with whom I carried out a number of research studies on the metamorphic

mineral assemblages in the gemstone belt of Karur and brought out several joint

research publications in the recent years. I had benefitted much from discussions as

well as joint publications with Prof. S. Maruyama (Japan) regarding Earth history and

mantle dynamics, the concepts from some of which are included in this manuscript.

References

Maruyama, S., Santosh, M., Zhao, D., 2007. Superplume, supercontinent, and post-

perovskite: Mantle dynamics and anti–plate tectonics on the core-mantle

boundary: Gondwana Research 11, 7–37.

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Meert, J.G. and Lieberman, B.S., 2008. The Neoproterozoic assembly of Gondwana

and its relationship to the Ediacaran-Cambrian radiation. Gondwana Research,

14, 5-21.

Menon, R.D., Santosh, M. Yoshida, M., 1994. Gemstone mineralization in southern

Kerala, India. Journal Geological Society of India 44, 242-252.

Mercier, A., Rakotondrazafy, M., Ravalomiandrinarivo, B., 1999. Ruby mineralization in

Southwest Madagascar. Gondwana Research 2, 433-438.

Nishimiya, Y., Tsunogae, T., Santosh, M., 2010. Sapphirine + quartz corona around

magnesian (XMg ~ 0.58) staurolite from the Palghat-Cauvery Suture Zone, southern

India: Evidence for high-pressure and ultrahigh-temperature metamorphism within

the Gondwana suture. Lithos 114, 490-502.

Rogers, J.J.W., Santosh, M. 2004. Continents and Supercontinents. Oxford University

Press, USA, 289p.

Rino, S., Kon, Y., Sato, W., Maruyama, S., Santosh, M., Zhao, D., 2008. The Grenvillian

and Pan-African orogens: World’s largest orogenies through geologic time, and

their implications on the origin of superplumes. Gondwana Research14, 51–72.

Santosh, M. 2010. A synopsis of recent conceptual models on supercontinent tectonics

in relation to mantle dynamics, life evolution and surface environment. Journal of

Geodynamics, in press.

Santosh, M., Collins, A., 2003. Gemstone mineralization in the Palghat-Cauvery shear zone

system (Karur-Kangayam Belt), southern India. Gondwana Research 6, 911-918.

Santosh, M., Sajeev, K. 2006. Anticlockwise evolution of ultrahigh-temperature

granulites within continental collision zone in southern India. Lithos 92, 447-464.

Santosh, M., Katori, R., Yoshikura, S., Higashi, S., Salim, A.K. 2002. Pink sapphire from

southern Kerala, S. India: implications on India-Madagascar correlation within

Gondwana assembly. Gondwana Research 5, 894-901.

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Santosh, M., Kusky, T. 2010. Origin of paired high pressure-ultrahigh-temperature

orogens: A ridge subduction and slab window model Terra Nova 22, 35-42.

Santosh, M., Maruyama, S., Omori, S. 2009a. A fluid factory in Solid Earth. Lithosphere

(Geological Society of America), doi: 10.1130/L2.1

Santosh, M., Maruyama, S., Sato, K. 2009b. The making and breaking of

supercontinents: some speculations based on superplume, superdownwelling and

the role of tectosphere. Gondwana Research 15, 324-341.

Santosh, M., Maruyama, S., Sato, K. 2009c Anatomy of a Cambrian suture in

Gondwana: Pacific-type orogeny in southern India? Gondwana Research 16, 321-

341.

Santosh, M., Tsunogae, T., Shimizu, H., Dubessy, J. 2010. Fluid characteristics of

retrogressed eclogites and mafic granulites from the Cambrian Gondwana suture

zone in southern India Contributions to Mineralogy and Petrology 159, 349–369.

Shimpo, K., Tsunogae, T. and Santosh, M. 2006. First report of garnet-corundum

rocks from southern India: implications for prograde high-pressure (eclogite

facies?) metamorphism. Earth and Planetary Science Letters 242, 111- 129.

Tsunogae, T. Santosh, M. 2003. Sapphirine and corundum bearing granulites from Karur,

Madurai Block, southern India. Gondwana Research 6, 925-930.

Zhao, D., 2004. Global tomographic images of mantle plumes and subducting slabs:

insight into deep earth dynamics. Physics of the Earth and Planetary Interiors,

146, 3-34.

Figure captions

Fig. 1 Schematic illustration of divergent and convergent plate tectonics showing the

birth of oceanic plate at the mid oceanic ridge and its subduction at the trench. The

principal magma production factories are at the ridge and also in the subduction zone;

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in the latter case hydrous partial melting of the subducting slab generates arc magmas.

MORB- Mid oceanic ridge basalt.

Fig. 2 P-wave tomography of the Western Pacific region showing slab graveyards (after

Zhao, 2004). The vertical cross-section is shown for a profile passing through northeast

China and central Japan. The velocity perturbation scale is shown at the bottom. The

flat-lying stagnant slabs in the mantle transition zone extends for over 2000 km. The low

velocity anomalies under the Pacific slab in the upper and topmost lower mantle, if

representing a compositional anomaly, might indicate subducted felsic material.

Fig. 3 (a) Cartoon speculating the fluid distribution from surface to the core of the Earth

(after Santosh et al., 2009a and references therein). Mid oceanic ridge basalt (MORB)

subducted at the trench sinks through the mantle transition zone and finally drops down

to the core-mantle boundary (CMB). The MORB components are then heated up by the

outer core leading to partial melting. The dense iron rich melts accumulate on the

bottom of the D” prime layer. The remaining restite MORB with dominant andesitic

composition rises upward to form superplume. The subducted slab from the trench is

hydrous, and is heated up by the surrounding mantle which releases the water in the

mantle wedge and enhances the viscosity. The origin of a superplume at the core-

mantle boundary is discussed in Maruyama et al. (2007). The vertically rising

superplume enters into the upper mantle, transforms to horizontal and branches out into

several hot spots. These hot spots cause the rifting of the continent, and deliver the

mantle fluid (mainly C-O-H-S) to the surface. Surface CO2 has been selectively

transported into the mantle in the Hadean to the Archean. After the Neoproterozoic,

surface water started to be transported into the mantle transition zones (410 to 660

km). For the major part of the Earth’s fluid history, the fluid transport was mostly one

way- from the outer core to the surface. The return flow of water is thought to have been

initiated probably after 750 Ma, and has not yet entered into the lower mantle. MOR-

Mid ocean ridge.

(b) Thermal structure of the Earth. (after Maruyama et al., 2007). The data indicate two

hot upwellings (Pacific and African superplume) and one cold downwelling (Asian cold

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plume).

Fig. 4 Cartoon illustrating the process of material circulation on a whole-earth scale

controlled by plate, plume and anti-plate tectonics (after Maruyama et al., 2007;

Santosh et al., 2009a). The subducted slab material, as inferred from seismic

tomography, may be plausibly considered to reach the core-mantle boundary. This

recycled oceanic lithosphere is viewed as a potential trigger of, and contributor to, the

superplume rising from the core-mantle interface to the uppermost mantle, penetrating

the mantle transition zone and eventually giving rise to hot spots. Radioactive heat

generated in ‘enriched’ basaltic slab remnants, and the heat given off by post-perovskite

– perovskite phase transformation may contribute to generating small-scale plumes

which ultimately coalesce into a superplume (see Maruyama et al., 2007 for details).

The implication of horizontal movement at the base of the mantle has been referred to

as ‘anti-plate tectonics’, in many respects analogous to lithospheric plate tectonic

processes operating in near-surface regions. Accordingly, through time, it is conjectured

that continents gradually develop at or close to the outer earth surface while

concomitant ‘anti-continents’ would be generated at the CMB (Maruyama et al., 2007).

Also shown are tectosphere-bearing surface continents and water subduction into the

mantle boundary layer (mantle transition zone) where rising hydrous plumes are

predicted.

Fig. 5 A generalized cartoon to show subduction or collision-amalgamation of intra-

oceanic arcs (after Santosh et al., 2009b). Top figure shows vertical or orthogonal

collision of arcs with a cross section of subducting arc crust. B shows the parallel

collision of arcs to increase the continental crust effectively on the earth by collision-

accretion of continental crust. The buoyancy of the continental crust is a function of its

volume as discussed in the text. Orthogonal collision has a small cross section,

whereas parallel collision has a big cross section. The Archean is dominated by

tectospheric mantle domain, but in the Modern oceanic domain, there is no tectospheric

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mantle, although the tectonic settings are relatively similar. In the continental region, the

tectospheric mantle and oceanic mantle are mixed now, one of the reasons that can

explain the Wilson cycle, because tectosphere plays a dominant role in driving the

Wilson cycle. The volume of continental volume is relatively too small; for example, a 15

km thin felsic upper curst is anchored by 200-300 km thick tectosphere. Therefore,

tectosphere behaves as a buoyant mass for continent dispersion and amalgamation.

Fig. 6 Cartoon illustration of two different types of geological settings of formation of

representative gem minerals. (a) Near-surface environment. (b) Magmatic and

pegmatitic/hydrothermal environment.

Fig. 7 The configuration of Gondwana supercontinent at ca. 540 Ma (after Rino et al.,

2008).

Fig. 8 Geological framework of southern India (after Santosh and Sajeev, 2006)

showing the Archean Craton in the north and the various Proterozoic granulite blocks

and the two major shear/suture zones in the south.

Fig. 9 Cartoon cross-sections illustrating the rifting stage and Pacific-stage to explain the

pre-collisional history of the southern Indian Gondwana fragment (after Santosh et al.,

2009c). Judging from the width of Pacific-type TTG belts, the Mozambique Ocean must have

been over 3000 km, similar to the present day Indian Ocean. Note the presence of

tectosphere under the Dharwar Craton and passive continental margin deposits which are

ultimately incorporated in collision with the evolved arc at 540 Ma, regionally

metamorphosed at mantle depth, and returned back to the surface. The birth of the

Mozambique Ocean is speculated to be between 1 billion to 750 million years ago. Bimodal

volcanics interlayered with clastics and impure platform carbonates with conformable

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relationship must have prevailed in a sequence quite different from the accretionary complex

in the Pacific-type orogen.

Fig. 10 Cartoon illustration showing a composite plate tectonic model for southern India from

North (Archean Dharwar Craton) to South (Pacific-type orogen) with two major collisional

orogens in between (after Santosh et al., 2009c). The Palghat-Cauvery Suture Zone

represents the main suture developed through Mozambique Ocean closure. The cross

section also covers Achankovil Suture Zone and Trivandrum Block up to the Nagercoil Block

at the southern tip. The Madurai Block represents a wide magmatic arc in between. This

model envisages the two Cambrian “Himalayas” in southern India, one major belt along the

Palghat-Cauvery Suture Zone and another minor one along the Achankovil Suture Zone.

Fig. 11 Combined P-T diagram showing data from mineral phase equilibria and fluid

inclusions in high pressure granulites from the Palghat-Cauvery Suture Zone (after Santosh

et al., 2010). The ranges of isochores for the two major fluid inclusion categories (high

density and low density) are shown by shaded regions. The star denotes the peak P-T

conditions estimated for the Sittampundi eclogite. The decompression P-T path (thick broken

line with arrow) is also shown. The thick rectangular box represents the P-T conditions for

high pressure and ultrahigh-temperature granulites within the Palghat-Cauvery Suture Zone.

Fields of extreme crustal metamorphism (shaded regions) include ultrahigh-pressure,

ultrahigh-temperature, and very low temperature conditions. Abbreviations are UHP:

ultrahigh-pressure; HP- high-pressure; UHT: ultrahigh-temperature; VLT: very low

temperature; AM: amphibolite; Amp-EC: amphibolite–eclogite; BS: blueschist; EA: epidote

amphibolite; EC: eclogite; Ep-EC: epidote–eclogite; GR: granulite; GS: greenschist; HGR:

high-grade granulite; and Lw-EC: lawsonite–eclogite. The figure shows that the rocks in this

region have been subjected to extreme metamorphic conditions at high pressures and

extreme temperatures.

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Fig. 12 Ridge subduction model to explain the paired HP-UHT metamorphic rocks within

the Palghat-Cauvery Suture Zone in the Cambrian Gondwana collisional suture of

southern India (after Santosh and Kusky, 2010). (a) Cartoon illustrating the pattern of

ridge subduction and slab window opening (after Bradley, 2003). (b) Part of the central

Gondwana assembly showing the trace of Late Neoproterozoic-Cambrian suture formed

by the closure of Mozambique Ocean. The location of the Palghat-Cauvery Suture Zone

(PCSZ) is also shown. (c) Cartoon illustrating the subduction-collision-accretion

tectonics along the PCSZ with a southward polarity of the subducting oceanic plate. The

major location of accretionary belts, dismembered ophiolite and exhumed HP-UHP

orogens are also shown. (d) Geological map of the PCSZ area showing the locations of

HP-UHT granulites. The proposed slab window is shown by thick dashed lines.

Fig. 13 Photographs of representative gemstones from the Karur-Kangayam belt of the

southern Indian Gondwana fragment. (a) Corundum (ruby) in association with garnet

and gedrite at Sevitturangampatti. (b) A large ruby crystal from the Lachmanapatti mine.

(c) and (d) Different varieties of moonstones and ‘rainbow stones’ from Kangayam belt.

(e) Diopsidite from Edappadi known in the local market as black cat’s eye. (f) Rutile-

bearing quartz and amethyst from Karur. (g) Beryl (including aquamarine variety) from

Karur. (h) Sunstones from Karur. (i) Ultrahigh-temperature metamorphic rock from

Sengal-Sakkarakkottai belt showing the association of sapphirine. (j) Garnet-kaynite-

corundum-sapphirine-gedrite-cordierite ultrahigh temperature metamorphic rock from

Panangad. Recent petrologic studies (Nishimiya et al., 2010) revealed equilibrium

sapphirine+quartz in these rocks indicating metamorphic temperatures exceeding

1000oC.

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