PAIRED METAMORPHIC BELTS

CONTENTS INTRODUCTION METAMORPHISH TYPES OF METAMORPHISH PAIRED METAMORPHISM ORIGIN OF THE PAIRED METAMORPHIC BELTS PAIRED METAMORPHIC BELTS AND PLATE

TECTONICS OCCURRENCE OF PAIRED METAMORPHIC BELT CONCLUSION REFERENCE

METAMORPHISM

• Metamorphism (meta-change, morphism-form or character)

• The process by which rocks are

altered in composition, texture, or internal structure by extreme heat, pressure, and the introduction of new chemical substances.

INTRODUCTION

TYPES OF METAMORPHISM

Contact Metamorphism Regional Metamorphism Shock Metamorphism High Strain Metamorphism Paired Metamorphism

CONTACT METAMORPHISM• Metamorphism in which the mineralogy and texture of a body of rock are changed by exposure to the pressure and extreme temperature associated with a body of intruding magma. •Contact metamorphism often results in the formation of valuable minerals, such as garnet and emery, through the interaction of the hot magma with adjacent rock.

REGIONAL METAMORPHISM• A type of metamorphism in which the mineralogy and texture of rocks are

changed over a wide area by deep burial and heating associated with the large-scale forces of plate tectonics.

• In regional metamorphism, rocks that form closer to the margin of the tectonic plates, where the heat and pressure are greatest, often differ in their minerals and texture from those that form farther away.

SHOCK METAMORPHISM

• It exclusively occurs locally around impact craters and possibly around some diatremes. • characterized by extremely high P/T conditions (tens of hundred of kilobars) over very short time spans. • These conditions result from the passing of a shock wave through the rocks.

HIGH STRAIN METAMORPHISM• High-strain metamorphism deforms rocks with only minor

thermal effects and occurs when the strain rate exceeds the ability of the rock to deform plastically.

• It is sometimes also called cataclastic metamorphism.

PAIRED METAMORPHIC BELT• Paired metamorphic belt juxtaposed zones of metamorphic rocks a

high pressure low temperature (Blueschist facies) zones which was parallel to a low pres-high temp(Greenschist facies)zones. Paired metamorphic belt were first recognized in Japan.

• In 1961, Miyashiro noted that  belts of high-pressure / low-temperature metamorphism on the oceanic side in the Circum-Pacific region were associated with belts of high-pressure / high-temperature metamorphism on the continental side.  He termed such occurrences Paired Metamorphic Belts. 

ORIGIN OF THE PAIRED METAMORPHIC BELTS

• The origin of the paired structure of metamorphic zones is interpreted in terms of subduction at the relatively cool rocks of the oceanic crust beneath the continental crust or at the abduction (translation of the continental crust on the oceanic)

• A narrow zone of high-pressure facies thus develops in the lower plate in the thintarland the heat flow increases (in island –arc zone it amounts of 2.5 cm3 on the average) and descending rocks being rich in water undergo melting.

• As result of dehydration the volume of rocks is reduced which accounts for the origin of earthquakes at the plate boundaries.

PAIRED METAMORPHIC BELT

PAIRED METAMORPHIC BELTS AND PLATE TECTONICS

• In 1961 , before plate tectonics had been conceived . A . Miyashiro (1973) recognized that in the mesozoic-cenozoic terranes of the circum –pacific region.

• The high P/T belt with its associated tectonically juxtaposed ophiolites and trench sediments generally lies closer to the ocean and adjacent to the oceanic trench .

• The formation of paired regionally extensive metamorphic belt with mineral assemblages characteristic of contrasting P/T gradients can be rationalized in the light of global plate tectonics

• In region of ocean – continental plate convergence there are two, but not necessarily distinct and separable , region of potential metamorphism which may be appreciated by examination

occurrence• Paired metamorphic belt occur in many parts of the

circum-pacific regions • A pair is composed of two contrasting belt running

parallel a high-pressure metamorphic belt which probably beneath a trench zone

• The former and a low-pressure metamorphic belt which probably formed a volcanic chain in the adjacent island arc or continental margin

• the latter belt are accompanied by basic-ultrabasic rocks and by granitic-andesitic rhyollitic rocks respectively.

Conclusion • The term metamorphism is the changes in

mineralogical composition and structure in rocks

• In regional metamorphism, rocks that form closer to the margin of the tectonic plates, where the heat and pressure are greatest, often differ in their minerals and texture from those that form farther away.

• high pressure low temperature (Blueschist facies) zones which was parallel to a low pres-high temp(Greenschist facies)zones. Paired metamorphic belt were first recognized in Japan.

References

• Petrology of igneous and metamorphic rocks by Donald W. Hyndman, (1985). Page no.27

• Igneous and metamorphic petrology by Myron G.Best(1986) .page no.432,433,434• Metamorphic texture by Bhaskar Rao.• http://earthds.info/pdfs/EDS_19.PDF• http://www.cms.fu-berlin.de/geo/fb/e-

paired_belts_metamorphism

Indian Examples• Gondwanaland, based on geological arguments,

paleomagnetic data and best fit of continents have derived two possible models of reassembly

• The most popular places India–Ceylon adjacent to East Antarctica the other places it next to Western Australia

• That Precambrian granulite facies, paired metamorphic belts can be traced throughout the basement rocks of Gondwanaland

• The significance of these paired belts in terms of plate tectonics during the Precambrian is also discussed.

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