METAMORFISME

METAMORPHISM MET AMORPHISM AND MET METAMORPHIC AMORPHIC ROCKS

BATUAN METAMORF Oleh : Agus Hendratno, MT. Dr.Ir. I Wayan Warmada


I. PENDAHULUAN PENDAHULUAN Batuan metamorf merupakan batuan hasil malihan dari batuan yang telah ada sebelumny sebe lumnya, a, ditunjukkan dengan adanya perubahan komposisi mineral, tekstur, dan struktur batuan yang terjadi pada fase padat (s o l i d s t a t e ) akibat adanya perubahan temperatur, tekanan, dan kondisi kimia di kerak bumi (Ehlers & Blatt, 1982). Macam metamorfosa (Jackson, 1970) : 1. Metamo Metamorfo rfosa sa isoki isokimia mia (siste (sistem m tertu tertutup tup); ); tidak tidak melibatkan atau hanya sedikit melibatkan perubahan komposisi kimia batuan. 2. Metamorfosa allokimia (sistem terbuka); melibatkan perubahan komposisi kimia batuan secara nyata, tipe metamorfosa ini sering disebut metasomatisme.


I. PENDAHULUAN PENDAHULUAN Batuan metamorf merupakan batuan hasil malihan dari batuan yang telah ada sebelumny sebe lumnya, a, ditunjukkan dengan adanya perubahan komposisi mineral, tekstur, dan struktur batuan yang terjadi pada fase padat (s o l i d s t a t e ) akibat adanya perubahan temperatur, tekanan, dan kondisi kimia di kerak bumi (Ehlers & Blatt, 1982). Macam metamorfosa (Jackson, 1970) : 1. Metamo Metamorfo rfosa sa isoki isokimia mia (siste (sistem m tertu tertutup tup); ); tidak tidak melibatkan atau hanya sedikit melibatkan perubahan komposisi kimia batuan. 2. Metamorfosa allokimia (sistem terbuka); melibatkan perubahan komposisi kimia batuan secara nyata, tipe metamorfosa ini sering disebut metasomatisme.

METAMORPHISM MET AMORPHISM AND MET METAMORPHIC AMORPHIC ROCKS Faktor Faktor penyebab penyebab terjadiny terjadinya a metamorfosa : perubahan perubahan temperatur, tekanan dan adanya aktifitas kimia fluida atau gas (Huang, 1962). Perubahan temperatur dapat terjadi karena pemanasan akibat intrusi magmatik dan perubahan gradien geothermal. Tekanan yang menyebabkan terjadinya suatu metamorfosa bervariasi besarnya. besarnya . Fluida aktif yang banyak berperan adalah air, karbondioksida, asam hidroklorik, dan hidrofluorik; umumnya bertindak sebagai katalis atau solven serta bersifat membantu reaksi kimia dan penyetimbangan mekanis (Huang, 1962).

The term metamorphism means to change. Most of us think of the metamorphosis that occurs when a caterpillar becomes a butterfly. While not as dramatic, similar changes can occur in rocks. Rocks will alter their form and appearance to suit new conditions Unfortunately, metamorphism is a slow process that occurs deep within the Earth. We cannot directly observe the process, but we can see the end result, metamorphic rocks. Metamorphic rocks result from mineralogical and structural adjustments of solid rocks to physical and chemical conditions differing from those under which the rocks originally formed. Changes produced by surface conditions such as compaction are usually excluded. The most important agents of metamorphism are temperature, and pressure. Equally as significant are changes in chemical environment that result in chemical recrystallization where a mineral assemblage becomes out of equilibrium due to temperature and pressure changes and a new mineral assemblage forms. David Jessey is a Professor of Geology at Cal Poly-Pomona. He specializes in mineralogy and ore deposit geochemistry. He has written many articles on ore deposits in the Mojave and lead numerous field trips to the Mojave and eastern Sierra Nevada. Don Tarman is a Professor of Geology at Cal Poly-Pomona. He specializes in stratigraphy and structural geology. He has authored many guidebooks to Mojave geology.

How Metamorphic Rock Is Formed ?

TABLE 1: FOLIATED (banded) ROCK CLASSIFICATION

Metamorphic Environment

50-300ºC

300-450ºC

Above 450ºC

Metamorphic Grade

Low

Intermediate

High

SLATE

SCHIST

GNEISS

Minerals not visible with the naked eye or with a hand lens, rock shows slaty cleavage, is usually dark-colored. A product of low-grade metamorphism of shale or mudstone.

Rock is medium to coarse grained with visible grains of mica or other metamorphic minerals. Often shiny due to reflection of mica on foliation planes. Product of intermediate grade metamorphism of shale, slate, phyllite, basalt or granite.

Rock is coarse grained and usually banded with alternating layers of light and dark minerals. Foliation bands may be folded. Product of high grade metamorphism of shale, schist, granite or many other rock types.

Rock Name

Rock Description

TABLE 2.

NONFOLIATED (not banded) ROCK CLASSIFICATION

Mineral(s) Description

MARBLE

QUARTZITE

ANTHRACITE COAL

calcite Coarse-grained recrystallized limestone or dolomite. Typically harder than the protolith. May have dark bands due to organic impurities.

quartz Rock has intergrown quartz grains, thus is massive and hard. Protolith is sandstone. Intermediate to high grade metamorphism.

crystalline carbon Hard, black shiny coal; product of low-grade metamorphism of bituminous coal.

METAMORPHIC ROCKS TEXTURE SLATY Finely crystalline rock in which minerals, such as mica, are aligned paralle to one another, which means that the rock splints readlily along the mica cleavage planes (e.g. slate). •

SCHISTOSE Minerals such as mica, chlorite and hornblende are aligned in easily visible parallel bands and, because of their platy alignment, the rock splits easily (e.g. schist). •

GNEISSOSE Characterised by a coarse foliation with individual bands several centimetres across -- indeed, the foliation may wrap aound larger crystals, as in Augen gneiss -- and all the minerals are coarsely granular and readily identifiable (e.g. gneiss). •

GRANOBLASTIC Mainly large mineral grains that have crystallised at the same time, and therefore, penetrate each other, the grains reaming large enough to be identifiesd easily. (e.g. grauwacke or greywacke). •

HORNFELS Compact, finely grained rock that shatters into sharply fragments (e.g. hornfels). •

BANDED Components occur in well-defined bands (e.g. gneiss). •

METAMORPHISM AND METAMORPHIC ROCKS

II. TIPE METAMORFOSA Macam metamorfosa berdasarkan tatanan geologinya (Bucher & Frey, 1994) :

Terjadi pada daerah yang sangat luas, dibedakan menjadi : a. Metamorfosa orogenik b. Metamorfosa b u r i a l c. Metamorfosa dasar samudera ( o c e a n - f l o o r )


II.2. Metamorfosa Lokal Terjadi pada daerah yang sempit berkisar antara beberapa meter sampai kilometer saja, dibedakan menjadi : a. Metamorfosa kontak b. Pirometamorfosa/metamorfosa optalic/kaustik/thermal c. Metamorfosa kataklastik/dislokasi/kinematik/dinamik d. Metamorfosa hidrothermal/metasomatisme e. Metamorfosa impact f. Metamorfosa retrograde/diaropteris


REGIONAL METAMORPHISM Alters rocks over a large geographic region. Appalachians of New England North Cascades of Washington-British Columbia


REGIONAL METAMORPHISM BURIAL METAMORPHISM Occurs deep in sedimentary basins. Requires depths > 10 km At these depths and greater, lithostatic pressure and geothermal heat drive metamorphic reactions and recrystallization. Does not require a tectonic process. Does not create mountain belts. Generally no foliation because no directed pressure.


REGIONAL METAMORPHISM BURIAL METAMORPHISM


REGIONAL METAMORPHISM DYNAMOTHERMAL METAMORPHISM Affected by pressure (squeezing) and heat (magma). Occurs when converging plates squeeze rock caught between the plates. Crust generally thickens. Surface rocks get taken deep into the Earth. Rocks are subjected to high temperatures, lithostatic pressure and directed pressure. Rocks become foliated.


REGIONAL METAMORPHISM DYNAMOTHERMAL METAMORPHISM Produced metamorphic rocks found in the Alps, Himalaya, and Appalachian Mountains. Also produced the greatly eroded metamorphic rocks in the Great Lakes region. These rocks rocks are exposed by uplift (faulting) and rapid r apid erosion. The process doesn’t reverse itself.


OTHER TYPES OF METAMORPHISM HYDROTHERMAL METAMORPISM Chemical alteration of pre-existing rocks rocks by hot water. water. Occurs primarily beneath ocean floor near divergent plate boundaries. Creates water-rich minerals. Includes serpentine, talc, and chlorite In addition, when steam hits ocean water and cools rapidly, rapidly, valuable metals metals such as Cu, Ni, Fe and Pb are precipitated.


OTHER TYPES OF METAMORPHISM FAULT ZONE METAMORPISM Produced by rocks grinding past each other at a fault. Produced by directed pressure and frictional heat.


III. MINERALOGI Mineral-mineral yang terdapat pada batuan metamorf : 1.

Mineral yang umumnya terdapat pada batuan beku dan metamorf, ex. kuarsa, feldspar, muskovit,dll.

2.

Mineral yang umumnya terdapat pada batuan sedimen dan metamorf, ex. mineral-mineral lempung, kalsit, dolomit,dll.

3.

Mineral indeks batuan metamorf, ex. garnet, andalusit, kianit, silimanit,dll.

Major Metamorphic Rock Types Temp C

Temp F

Coal

Limeston e

Sandston e

Basalt

Shale

Index Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite Bituminous

300

500

Anthracite

600

Graphite

Marble

700 800 500

1100 1200

700

Quartzite

900 1000

600

Greenstone

Staurolite

Amphibolite Gneiss

Kyanite Sillimanit e Melting Begins


Temp F

Coal

Limeston e

Sandston e

Basalt

Shale

Index Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite Bituminous

300

500

Anthracite

600

Graphite

Marble

700 800 500

1100 1200

700

Quartzite

900 1000

600

Greenstone

Staurolite

Amphibolite Gneiss



Temp F

Coal

Limeston e

Sandston e

Basalt

Shale

Index Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite Bituminous

300

500

Anthracite

600

Graphite

Marble

700 800 500

1100 1200

700

Quartzite

900 1000

600

Greenstone

Staurolite

Amphibolite Gneiss



Temp F

Coal

Limeston e

Sandston e

Basalt

Shale

Index Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite Bituminous

300

500

Anthracite

600

Graphite

Marble

700 800 500

1100 1200

700

Quartzite

900 1000

600

Greenstone

Staurolite

Amphibolite Gneiss



Temp F

Coal

Limeston e

Sandston e

Basalt

Shale

Index Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite Bituminous

300

500

Anthracite

600

Graphite

Marble

700 800 500

1100 1200

700

Quartzite

900 1000

600

Greenstone

Staurolite

Amphibolite Gneiss


What About Other Rocks?

Polymorphism Al2SiO5 

Andalusite



Kyanite



Sillimanite

Calcite - Aragonite Quartz 

- Tridymite Cristobalite (increasing temperature)



- Coesite - Stishovite (increasing pressure)

Ice - 6 high pressure forms Diamond Graphite

Metamorphic Facies Depth\Temp

300C

400C

5 km

Zeo l it e

C o n tac t Met am o r p h i s m - Andalusite forms

10 km - 3 kb 15 km

500 C

600 C

Greenschis Amphibolite Garnet, B l u e s c h i t Chlorite, Staurolite, s t

Biotite form Kyanite form Slate Schist Greenstone Amphibolite Quartzite Quartzite Marble Marble Gneiss

20 km - 6 kb 25 km 30 km - 9 kb

•

•

•

•

•

•

•

•

700 C

Granulite

Sillimanite forms Muscovite breaks down to K-feldspar Partial Melting Gneiss •

•

35 km 40 km - 12 kb

Not Found

800 C

E c l o g i t e (Mantle)

Mantle Rocks

IV. FASIES METAMORFIK Diperkenalkan oleh Eskola, 1915 (Bucher & Frey, 1994). Sebuah fasies metamorfik merupakan kelompok batuan yang termetamorfosa pada kondisi yang sama yang dicirikan oleh kumpulan mineral yang tetap. Tiap fasies metamorfik dibatasi oleh tekanan dan temperatur tertentu serta dicirikan oleh hubungan teratur antar komposisi kimia dan mineralogi dalam batuan.

V. STRUKTUR BATUAN METAMORF V.1.

Struktur Foliasi

Terjadi karena adanya penjajaran mineral menjadi lapisan-lapisan (gneissosity ), orientasi butiran (schistosity ), permukaan belahan planar (cleavage) atau kombinasi dari ketiga hal tersebut (Jackson, 1970). a.

Slaty cleavage, batuannya disebut slate (batusabak)

b.

Phylitic, batuannya disebut phylite (filit)

c.

Schistosic, batuannya disebut schist (sekis)

d.

Gneissic/Gneissose, batuannya disebut gneis

V.2. Struktur Non Foliasi Terbentuk oleh mineral-mineral equidimensional dan umumnya terdiri dari butiran-butiran (granular). a.

Hornfelsic/Granulose, batuannya disebut hornfels (batutanduk).

b.

Cataclastic, batuannya disebut cataclasite (kataklasit)

c.

Mylonitic, batuannya disebut mylonite (milonit)

d.

Phyllonitic, batuannya phyllonite (filonit)

VI. TEKSTUR BATUAN METAMORF Tekstur merupakan kenampakan batuan yang berdasarkan pada ukuran, bentuk dan orientasi butir mineral individual penyusun batuan metamorf (Jackson, 1970).

Foliated, very fine grained-no visible minerals

METAMORPHIC ROCKS Interlocking nonhomogenous crystalline texture commonly with a preferred orientation to the mineral grains

Dull - passes "tink test"

Slate

Foliated, shiny due to increased size of micaceous minerals (almost see them)

Phyllite

Foliated - medium to coarse grain

Individual mineral grains visible. Major mineral(s) included as name modifiers

Color banded

Alternating layers of light (felsic) and dark (mafic) minerals

Gneiss

Distinct layering often highly folded and contorted

Alternating layers of felsic igneous rock (light) and mafic gneiss (dark)

Migmatite

Soft - passes fizz test

Marble

Hard - fails fizz test

Quartzite

Interlocking hornblende crystals

Amphibolite

Non-foliated with nonoriented grains

Schist (ex. Mica Schist)

Metamorphic textures are either granular or foliated. Here we examine only the foliated types. Foliation - any planar set of minerals, or banding of mineral concentrations, especially the planar structure that results from flattening of the mineral grains, like micas.

Slaty cleavage- a pervasive, parallel foliation (layering) of fine-grained platy minerals (chlorite) in a direction perpendicular to the direction of maximum stress. Produces the rocks slate and phyllite. Scistosity - the layering in a coarse grained, crystalline rock due to the parallel arrangement of platy mineral grains such as muscovite and biotite. Other minerals present are typically quartz and feldspar, plus a variety of other minerals such as garnet, staurolite, kyanite, sillimanite. Mineral Banding (Gneiss) - The layering in a rock in which bands or lenses of granular minerals (quartz and feldspar) alternate with bands or lenses in which platy (mica) or elongate (amphibole) minerals predominate.

Slaty Cleavage A pervasive, parallel foliation (layering) of fine-grained platy minerals (chlori te) in a direction perpendicular to the direction of maximum stress. Found in the rocks slate and phyllite.

During the earliest stages of low grade metamorphism, most pressure is from the weight of overlying rock. Therefore the new sheet structure minerals, such as clay, tend to parallel the bedding planes of the sedimentary rock being metamorphosed. With folding the sedimentary clay layering folds with the rock so that the layering still runs parallel with the bedding planes. At this point the rock is still sedimentary. With deeper burial or under the influence of compression, metamorphism begins. The sedimentary clay minerals are converted into the mineral chlorite, that has flat basal cleavage like a mica. But the chlorite is growing in a stress field that is not always running parallel to the bedding. In the drawing to the right we can clearly see the bedding, but the parallel lines running vertically is the slaty cleavage. In the link to slaty cleavage we can see how the cleavage does not run parallel to the bedding. Low grade metamorphic rocks are so fine-grained that the new mineral grains are not visible with the unaided eye. Under a polarizing light microscope, the foliation can be seen. However, the slaty cleavage produces a very distinct layering in the rock that often runs at an angle to the bedding. Practically we see this in the rock slate, often used as roof shingles or as paving stones. The slate easily splits into thin sheets with smooth, flat surfaces.

Schistosity The layering in a coarse grained, crystalline rock due to the parallel arrangement of platy mineral grains such as muscovite and biotite. Other minerals present are typically quartz and feldspar, plus a variety of other minerals such as garnet, staurolite, kyanite, sillimanite. At intermediate and high grades of metamorphism the chlorite breaks down and recrystallizes to form quartz, feldspar, and mica. The grain size also increases and individual mineral grains can be seen with the unaided eye. Foliation in coarse grained metamorphic rocks is called SCHISTOSITY. In a hand sample the foliation can be easily seen, and ususally runs planar through the rock; that is, it all runs the same direction. In larger specimens, however, the foliation may be folded. Schistosity is derived from the Latin schistos meaning cleaves easily. Schistosity differs from slaty cleavage in both grain size and mineral content.

Gneissic Mineral Banding The layering in a rock in which bands or lenses of granular minerals (quartz and feldspar) alternate with bands or lenses in which platy (mica) or elongate (amphibole) minerals predominate. The most intense form of foliation is mineral banding. At the highest grades of metamorphism, minerals begin to segregate into separate bands. The micaceous minerals separate from the quartz and feldspars.

Migmatite A rock in which metamorphic textures (schistosity or mineral banding) are intermixed with igneous textures (coarse grained igneous rocks). At this stage we are leaving the realm of metamorphism and entering the realm of igneous rocks. Only the rock has not yet completely melted - it has fractionally melted. Click image for more explanation.

VII. PENAMAAN DAN KLASIFIKASI BATUAN METAMORF 

Didasarkan pada kenampakan struktur dan teksturnya.



Dipergunakan kata tambahan yang menunjukkan ciri khusus batuan: keberadaan mineral pencirinya, ex. sekis klorit. nama batuan beku yang mempunyai komposisi yang sama, ex. granite gneiss. mineral penyusun utamanya, ex. kuarsit. fasies metamorfiknya, ex. granulit

METAMORPHIC MINERALS Metamorphic Rocks Amphibolite Blueschist Eclogite Gneiss Granulite Greenschist Greenstone Hornfels Marble - limestone Marble - dolomite Migmatite Phyllite Quartzite Schist Serpentinite Slate Soapstone

Special Metamorphic Minerals Actinolite Chlorite Corundum Epidote Garnet Graphite Kyanite Serpentinite Sillimanite Staurolite Talc

Common Metamorphic Minerals Quartz Orthoclase Plagioclase Amphibole Pyroxene Biotite Muscovite Chlorite

Metamorphic Agents and Changes 

Temperature: typically the most important factor in metamorphism

Figure 1-9. Estimated ranges of oceanic and continental steadystate geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al . (1980), Earth. Rev. Geophys. Space Sci., 18, 269311.

Metamorphic Agents and Changes Deviatoric stresses come in three principal types: Tension Compression Shear 

Tension: 3 is negative, and the resulting strain is extension, or pulling apart strain original shape

ellipsoid

1

3

Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. a. Tension, in which one stress in negative. “Tension fractures” may open normal to the extension direction and become filled with mineral precipitates. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Compression: s1 is dominant, homogenous flattening

folding or more

3

1

Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. b. Compression, causing flattening or folding. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.



Foliation is a common result, which allows us to estimate the orientation of 1 1

· s1 > s2 = s3

foliation and no lineation

· s1 = s2 > s3

lineation and no foliation

· s1 > s2 > s3

both foliation and lineation

Figure 21-3. Flattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrix flows with progressive flattening, and the flakes are rotated toward parallelism normal to the predominant stress. Winter

Metamorphic Agents and Changes Shear motion occurs along planes at an angle to

1

1

Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. b. Shear, causing slip along parallel planes and rotation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.



Batuan metamorf lain yang banyak dikenal antara lain : Amphibolit;

butir sedang-kasar,mineral utama penyusunnya

amfibol dan plagioklas, menunjukkan schistosity bila mineral prismatiknya terorientasi. Serpentinit; komposisi mineral hampir semuanya mineral kelompok serpentin. Marmer; komposisi mineral karbonat (kalsit atau dolomit), bertekstur granoblastik. Kuarsit; mengandung kuarsa lebih dari 80%. Soapstone; komposisi mineral utama talk. Eclogit;

butir sedang-kasar, mineral utama penyusunnya

piroksin ompasit (diopsid kaya sodium dan aluminium) dan garnet kaya pyrope. Rodingit; komposisi calc-silikat yang terjad akibat alterasi metasomatik batuan beku basa di dekat batuan beku ultrabasa yang mengalami serpentinisasi.

METAMORFISME

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