Praktikum Petrologi

KLASIFIKASI BATUAN BEKU SECARA MEGASKOPIS

Berdasarkan Klasifikasi IUGS (1999) • Golongan faneritik• Golongan Afanitik

1. Golongan Faneritik

• Batuan bertekstur faneritik, dapat teramati secara megaskopis (mata biasa), berbutir sedang – kasar (lebih besar dari 1mm)

• Golongan faneritik dapat dibagi atas beberapa jenis batuan• Dasar pembagiannya adalah kandungan mineral kuarsa

(Q), atau mineral feldspartoid (F), feldspar alkali (A), serta kandungan mineral plagioklas (P)

• Cara menentukan nama batuan dihitung dengan menganggap jumlah ketiga mineral utama (Q + A + P atau F + A + P) adalah 100%

Q

A P

F

60 60

Granitic-rocks

20 20

10 10

60 60

Syenitic-rockDioritic-rockGabbroic-rockAnorthositic-rock

Foid-syenitic- rock

Foid-dioritic-rock

Foid-gabbroic-rock

Foidolitik-rock

P

Px+Ho Ol

90 90

10 10

40

VIII

IV

X+XI IX

Ol

Px Ho

IX

X XI

Q --- quartz

A --- alkali feldspar

P --- plagioclase

F --- foid

Px --- pyroxene

Ho --- hornblende

Ol --- olivine

Quartz-rick-coarse-grained- rock

VIII = Anortositic-rock IX = Peridotic-rock X = Piroksenitic-rockXI = Hornblenditic-rock

Digunakan untuk Phaneritik

I = Granitoid

II = Syenitoid

III = Dioritoid

IV = Gabroid

V = Foid Syenitoid

VI = Foid Dioritoid & Gabroid

VII = Foidolit

VIII = Anortosit

IX = Peridotit

X = Piroksenit

XI = Hornblendit

Untuk II dan IV, “ foid bearing” digunakan bila feldspartoid hadir

2. Golongan Afanitik

• Batuan beku bertekstur afanitik, mineral-mineralnya tidak dapat dibedakan dengan mata biasa atau menggunakan loupe, umumnya berbutir halus (< 1mm), sehingga batuan beku jenis ini tidak dapat ditentukan prosentase mineraloginya secara megaskopis.

• Salah satu cara terbaik untuk memperkirakan komposisi mineralnya adalah didasarkan atas warna batuan, karena warna batuan umumnya mencerminkan proporsi mineral yang dikandungnya, dalam hal ini proporsi mineral felsik (berwarna terang) dan mineral mafik (berwarna gelap). Semakin banyak mineral mafik, semakin gelap warna batuannya.

• Penentuan nama atau jenis batuan beku afanitik masih dapat dilakukan bagi batuan yang bertekstur porfiritik atau vitrofirik, dimana fenokrisnya masih dapat terlihat dan dapat dibedakan, sehingga dapat dibedakan jenis batuannya. Dengan menghitung prosentase mineral yang hadir sebagai fenokris, serta didasarkan pada warna batuan/mineral, maka dapat diperkirakan prosentase masing-masing mineral (Q/F, A. P), maka nama batuan dapat ditentukan.

Q

A P

F

60 60

Rhyolitic-rock

20 20

10 10

60 60

Trachytic-rock

Andesitic- rock

Basaltic-rock

Phonolitic-rock Tephritic-rock

Foiditic-rock

Ol Px

Ultramafitic

Q --- quartz

A --- alkali feldspar

P --- plagioclase

F --- foid

Px --- pyroxene

Mel --- melilite

Ol --- olivine

Mel

Dacitic- rock

Digunakan untuk Afanitik

Hal – hal utama yang perlu dicatat dalam deskripsi batuan beku :

1. Warna, sebagai petunjuk awal, untuk memperkirakan komposisi kimia dan mineral dari batuan

2. Tesktur, besar butir dan kemas, yang mana hubungan dengan sejarah dan cara kejadian batuan, serta kecepatan dan urutan pertumbuhan kristal.

3. Mineralogi, sebagai petunjuk untuk identifikasi batuan, biasanya di dalam batuan beku terdapat antara 2 – 4 mineral utama.

4. Inklusi material asing (sebagai tambahan dalam membantu identifikasi batuan). Inklusi ini kadang ditemukan dalam batuan beku dan harus dideskripsi terpisah, inklusi penting ketika kita ingin menilai cara kejadian dan asal tubuh batuan beku.

sampel

Warna : Hitam bintik-bintik putih/putih kemerahan dll (warna yang representatif)

Struktur : Masif/vesikuler/amigdaloidal/kekar akibat pendinginan dll

Tekstur

Granulitas/Besar butir

Sedang 1-5mm, Kasar 5mm – 3cm, sangat kasar > 3cm

Halus < 1mm

Faneritik Afanitik

Derajat Kristalisasi

Holokristalin HolohyalinHipokristalin/hipohyalin

Keseragaman butir/kristal

Equigranular Inequagranular Porfiritik/Vitrofirik

Panidiomorfik Granular (Euhedral)

Hipidiomorfik Granular (Subhedral)

Alotriomorfik Granular (Anhedral)

Fenokris Komposisi mineral : Kuarsa (..%), ciri-cirinya dll

Nama Batuan : Granitoid/Syenitoid/Dioritoid dll

GRANIT

• granite [petrology] (a) In the IUGS classification, a plutonic rock with Q between 20 and 60 and P/(A+P) between 10 and 65. (b) A plutonic rock in which quartz constitutes 10 to 50 percent of the felsic components and in which the alkali feldspar/total feldspar ratio is generally restricted to the range of 65 to 90 percent. Rocks in this range of composition are scarce, and sentiment has been growing to expand the definition to include rocks designated as adamellite or quartz monzonite, which are abundant in the U.S. (c) Broadly applied, any holocrystalline, quartz-bearing plutonic rock. Syn: granitic rock.—Etymol: Latin granum, “grain”.

RIOLIT• rhyolite (rhy'-o-lite) (a) In the IUGS

classification, a volcanic rock defined in the QAPF diagram as having Q/(Q+A+P) between 20 and 60% and P/(A+P) between 10 and 65%, and in the TAS diagram by a field, extending to higher silica and alkali contents from a vertical line with its lower end at 69 wt% SiO2, 8% alkalis, and a line sloping from 69 SiO2, 8 alkalkis to 77 SiO2, 0 alkalis. (b) A group of igneous rocks, typically porphyritic and commonly exhibiting flow texture, with phenocrysts of quartz and alkali feldspar in a glassy to cryptocrystalline groundmass; also, any rock in that group; the fine-grained equivalent of granite. Rhyolite grades into rhyodacite with decreasing alkali feldspar content and into trachyte with a decrease in quartz. The term was coined in 1860 by Baron von Richthofen (grandfather of the World War I aviator). Etymol: Greek rhyo-, from rhyax, “stream of lava”. Syn: liparite. Cf: quartz porphyry.

SYENIT• syenite (sy'-e-nite) (a) In the IUGS

classification, a plutonic rock with Q between 0 and 5, and P/(A+P) between 10 and 35. (b) A group of plutonic rocks containing alkali feldspar (usually orthoclase, microcline, or perthite), a small amount of plagioclase (less than in monzonite), one or more mafic minerals (esp. amphibole), and quartz, if present, only as an accessory; also, any rock in that group; the intrusive equivalent of trachyte. With an increase in the quartz content, syenite grades into granite. Its name is derived from Syene, Egypt. A. G. Werner in 1788 applied the name in its present meaning to rock at Plauenscher Grund, Dresden, Germany; the Egyptian rock is a granite containing much more quartz.

TRACHIT• trachyte (tra'-chyte) (a) In the IUGS

classification, a volcanic rock defined in the QAPF diagram by Q/(Q+A+P) between 0 and 5% and P/(P+A) between 10 and 35%, and in the TAS diagram by a field partly bounded by points with SiO2 and total alkali coordinates: 57.6, 11.7; 61, 13.5; 63, 7; and 69, 8. The field is bounded at high silica contents by a vertical line with its lowest end at 69, 8. In addition, normative quartz is <20%. Cf: trachydacite. (b) A group of fine-grained, generally porphyritic, extrusive rocks having alkali feldspar and minor mafic minerals (biotite, amphibole, or pyroxene) as the main components, and possibly a small amount of sodic plagioclase; also, any member of that group; the extrusive equivalent of syenite. Trachyte grades into latite as the alkali feldspar content decreases, and into rhyolite with an increase in quartz. Etymol: Greek trachys, “rough”, in reference to the fact that rocks of this group are commonly rough to the touch.

MONZONIT• monzonite (mon'-zo-nite) (a) In the

IUGS classification, a plutonic rock with Q between 0 and 5, and P/(A+P) between 35 and 65. (b) A group of plutonic rocks intermediate in composition between syenite and diorite, containing approximately equal amounts of alkali feldspar and plagioclase, little or no quartz, and commonly augite as the main mafic mineral; also, any rock in that group; the intrusive equivalent of latite. With a decrease in the alkali feldspar content, monzonite grades into diorite or gabbro, depending on the composition of the plagioclase; with an increase in alkali feldspar, it grades into syenite. Syn: syenodiorite.—The name, given by de Lapparent in 1864, is for Monzoni, Tyrolean Alps.

GRANODIORIT• granodiorite (gran-o-di'-o-rite) (a) In the

IUGS classification, a plutonic rock with Q between 20 and 60, and P/(A+P) between 65 and 90. (b) A group of coarse-grained plutonic rocks intermediate in composition between quartz diorite and quartz monzonite (U.S. usage), containing quartz, plagioclase (oligoclase or andesine), and potassium feldspar, with biotite, hornblende, or, more rarely, pyroxene, as the mafic components; also, any member of that group; the approximate intrusive equivalent of rhyodacite. The ratio of plagioclase to total feldspar is at least two to one but less than nine to ten. With less alkali feldspar it grades into quartz diorite, and with more alkali feldspar, into granite or quartz monzonite. The term first appeared in print in 1893 in a paper by Lindgren and was applied to all rocks intermediate in composition between granite and diorite. The term has the connotation that the rock is a diorite with granitic characteristics, i.e. with quartz and a certain amount of alkali feldspar (Johannsen, 1939, p. 254).

DASIT

• dacite (da'-cite) (a) In the IUGS classification, a volcanic rock defined in the QAPF diagram as having Q/(Q+A+P) between 20 and 60 and P/(P+A)>65, and in the TAS diagram as falling in the field bounded by points with the SiO2, total alkali coordinates: 63, 0; 63, 7; 69, 8; and 77, 0. (b) A fine-grained extrusive rock with the same general composition as andesite but having a less calcic plagioclase and more quartz; according to many, it is the extrusive equivalent of granodiorite. The name, given by Stache in 1863, is from the ancient Roman province of Dacia (now part of Romania).

DIORIT• diorite (di'-o-rite) (a) In the IUGS

classification, a plutonic rock with Q between 0 and 5, P/(A+P) greater than 90, and plagioclase more sodic than An50. (b) A group of plutonic rocks intermediate in composition between acidic and basic, characteristically composed of dark-colored amphibole (esp. hornblende), sodic plagioclase (oligoclase, andesine), pyroxene, and sometimes a small amount of quartz; also, any rock in that group; the approximate intrusive equivalent of andesite. Diorite grades into monzonite with an increase in the alkali feldspar content. In typical diorite, plagioclase contains less than 50% anorthite, hornblende predominates over pyroxene, and mafic minerals total less than 50% of the rock. Etymol: Greek diorizein, “to distinguish”, in reference to the fact that the characteristic mineral, hornblende, is usually identifiable megascopically. Cf: dolerite; gabbro. See also: diabase.

ANDESIT• andesite (an'-de-site) (a) In the IUGS classification, a

volcanic rock defined modally by Q/(Q+A+P) between 0 and 20% or F/(F+A+P) between 0 and 10%, P/(A+P)>65%, and M<35%. Because modes are difficult to estimate for these rocks, andesite is also defined in the TAS diagram as rock falling in the area bounded by points with the SiO2 and total alkali coordinates: 57, 0; 57, 5.9; 63, 0; 63, 7. (b) A dark-colored, fine-grained extrusive rock that, when porphyritic, contains phenocrysts composed primarily of zoned sodic plagioclase (esp. andesine) and one or more of the mafic minerals (e.g. biotite, hornblende, pyroxene), with a groundmass composed generally of the same minerals as the phenocrysts, although the plagioclase may be more sodic and quartz is generally present; the extrusive equivalent of diorite. It was named by Buch in 1826 from the Andes Mountains, South America. andesite (an'-de-site) (a) In the IUGS classification, a volcanic rock defined modally by Q/(Q+A+P) between 0 and 20% or F/(F+A+P) between 0 and 10%, P/(A+P)>65%, and M<35%. Because modes are difficult to estimate for these rocks, andesite is also defined in the TAS diagram as rock falling in the area bounded by points with the SiO2 and total alkali coordinates: 57, 0; 57, 5.9; 63, 0; 63, 7. (b) A dark-colored, fine-grained extrusive rock that, when porphyritic, contains phenocrysts composed primarily of zoned sodic plagioclase (esp. andesine) and one or more of the mafic minerals (e.g. biotite, hornblende, pyroxene), with a groundmass composed generally of the same minerals as the phenocrysts, although the plagioclase may be more sodic and quartz is generally present; the extrusive equivalent of diorite. It was named by Buch in 1826 from the Andes Mountains, South America.

GABRO• gabbro (gab'-bro) (a) In the IUGS

classification, a plutonic rock with Q between 0 and 5, P/(A+P) greater than 90, and plagioclase more calcic than An50. (b) A group of dark-colored, basic intrusive igneous rocks composed principally of calcic plagioclase (commonly labradorite or bytownite) and clinopyroxene (augite), with or without olivine and orthopyroxene; also, any member of that group. It is the approximate coarse-grained equivalent of basalt. Apatite and magnetite or ilmenite are common accessory minerals. Gabbro grades into monzonite with increasing alkali-feldspar content. According to Streckeisen (1967, p. 171, 198), plagioclase with more than 50% anorthite distinguishes gabbro from diorite; quartz is 0-20% of the light-colored constituents, and the plagioclase/total feldspar ratio is 90/100.—The name, introduced by Buch in 1810, is apparently after the town of Gabbro in Tuscany, Italy.

BASALT• basalt [petrology] (ba-salt', ba'-salt) (a) In

the IUGS classification, a volcanic rock defined modally by Q/(Q+A+P) between 0 and 20% or F/(F+A+P) between 0 and 10%, P/(A+P)>65%, and M>35%. Because modes are difficult to estimate for these rocks, basalt is also defined in the TAS diagram as rock falling in the area bounded by points with the SiO2 and total alkali coordinates: 45, 0; 45, 5; 52, 0; 52, 5. (b) A general term for dark-colored mafic igneous rocks, commonly extrusive but locally intrusive (e.g. as dikes), composed chiefly of calcic plagioclase and clinopyroxene; the fine-grained equivalent of gabbro. Nepheline, olivine, orthopyroxene, and quartz may be present in the CIPW norm, but not all simultaneously: nepheline and olivine can occur together, as can olivine and orthopyroxene, and orthopyroxene and quartz, but nepheline does not coexist with orthopyroxene or quartz, nor quartz with nepheline or olivine. These associations and incompatibilities are discussed by Yoder and Tilley (1962) and by Muir and Tilley (1961). Cf: basaltic rocks.

DIABAS

• diabase (di'-a-base) In the U.S., an intrusive rock whose main components are labradorite and pyroxene and which is characterized by ophitic texture. As originally applied by Brongniart in 1807, the term corresponded to what is now recognized as diorite. “The word has come to mean a pre-Tertiary basalt in Germany, a decomposed basalt in England, and a dike-rock with ophitic texture in the United States and Canada” (Johannsen, 1939, p. 248). Cf: trap [ign]. Syn: dolerite.

ANORTOSIT• anorthosite (an-or'-tho-site) (a)

In the IUGS classification, a plutonic rock with Q between 0 and 5, P/(A+P) greater than 90, and M less than 10. (b) A group of essentially monomineralic plutonic igneous rocks composed almost entirely of plagioclase feldspar, which is usually labradorite but may be as calcic as bytownite or as sodic as andesine or oligoclase, and little or no dark-colored minerals; also, any rock in that group. Anorthosites occur as large nonstratiform plutonic bodies and as stratiform intrusions; they have been identified in lunar rock samples. Syn: plagioclasite; plagioclase rock.

PIROKSENIT

• pyroxenite (py-rox'-e-nite) (a) In the IUGS classification, a plutonic rock with M equal to or greater than 90 and ol/(ol+opx+cpx) less than 40. (b) An ultramafic plutonic rock chiefly composed of pyroxene, with accessory hornblende, biotite, or olivine. Syn: pyroxenolite.

PEGMATIT• pegmatite (peg'-ma-tite) An

exceptionally coarse-grained igneous rock, with interlocking crystals, usually found as irregular dikes, lenses, or veins, esp. at the margins of batholiths. Most grains are one cm or more in diameter. Although pegmatites having gross compositions similar to other rock types are known, their composition is generally that of granite; the composition may be simple or complex and may include rare minerals rich in such elements as lithium, boron, fluorine, niobium, tantalum, uranium, and rare earths. Pegmatites represent the last and most hydrous portion of a magma to crystallize and hence contain high concentrations of minerals present only in trace amounts in granitic rocks. The first use of the term “pegmatite” is attributed to Haüy, who used it in 1822 as a syn. of graphic granite. Haidinger in 1845 introduced the present meaning. Cf: pegmatoid; symplectite. Adj: pegmatitic. Syn: giant granite.

OBSIDIAN

• obsidian (ob-sid'-i-an) A black or dark-colored volcanic glass, usually of rhyolite composition, characterized by conchoidal fracture. It is sometimes banded or has microlites. Usage of the term goes back as far as Pliny, who described the rock from Ethiopia. Obsidian has been used for making arrowheads, other sharp implements, jewelry, and art objects. Syn: Iceland agate.