3. Lamprophyres and Ultrabasic Rocks

A. Lamprophyre

B. Melilite-rich rocks

C. Carbonatites

D. Kimberlite

E. Peridotite

A. Lamprophyre• Lamprophyre : merupakan istilah yang digunakan untuk menyebut suatu kelompok batuan beku basic-ultrabasic, berwarna gelap, yang pada umumnya terbentuk pada dyke dangkal.

• Umumnya kaya akan unsur alkali dan ferro-magnesian.

• Ciri utamanya adalah tekstur, dimana sebagian besar lamprophyre bertekstur porfiritik (fenokris biotit, amphibole, augit dan groundmass feldspar). Baik fenokris maupun groundmass berbentuk euhedral (oleh beberapa ahli disebut tekstur panidiomorfik).

Sayatan tipis Lamprophyre

• Ada 2 kelompok lamprophyre :a. Calcalkaline(biotite and

hornblenda lamprophyre)b. Alkaline (ultrabasic lamprophyre)

a. CALKALKALINE- SiO2 content : 50%-54%- Colour Index : 35-50- Mineralogi : biotite, green-brown

hornblende, diopsidic augite, feldspar, with or without quartz or altered olivine.

• Anggota :

i. Biotite lamprophyre

a.minette (biotit, orthoklas) b.kersantite(biotit,plagioklas)

ii. Hornblende lamprophyre a.vogesite(hornblenda,orthoklas) b.spessartite(hornblenda,

plagioclas)

• b. Alkaline lamprophyre- SiO2 content : 38-42%- Mineralogi : titaniferous augit, brown alkaline

amphibole. Olivin + biotit sering hadir sebagai fenokris.

- Anggota :i : camptonite (groundmass : plagioclase)ii: monchiquites (groundmass :glass)

B. MELILITIC ROCK

i. Alnoites- SiO2 content :<25%- Asosiasi :nepheline syenite, ijolite- Mineralogi : fenokris biotit (with or without

olivine) in groundmass of melilite, carbonate, perovskite

ii. Melilites- SiO2 content : 30-35%- Volcanic rock, with abundant of melilite and

olivine, but lack of feldspar.

C. CARBONATITES- Merupakan batuan beku yang terutama tersusun

olehlebih dari 50% mineral karbonat (calcite, dolomite, ankerite)

- Umumnya terbentuk sebagai small plugs pada kompleks intrusi alkalin, atau sebagai dyke, sill, breccia dan vein.

- Umumnya terdapat pada continental rift zone tectonic setting.

- Asosiasi : with basic or ultrabasic alkaline rock (ijolite and alkaline pyroxenite)

- Mineralogi : calcite, dolomite, ankerite, with biotite, olivine, alkali feldspar, sodic pyroxene

Karbonatit

carbonatite

D. Kimberlites

• Kimberlite is a gas-rich, potassic ultramafic igneous rock that contains the minerals olivine, phlogopite, diopside, serpentine, calcite, and minor amounts of apatite, magnetite, chromite, garnet, diamond, and other upper mantle minerals.

• Upper mantle xenoliths are found in some kimberlite and provide clues to the magma's origin.

• The source depth for kimberlite magmas is estimated at 200 km, more than twice as deep as the source region for most magmas.

• at depth of 200 km the pressure is 60.000 times greater than the surface and the temperature is about 1500 C.

• Kimberlite magmas are rich in carbon dioxide and water which brings the magma quickly and violently to the surface.

• Kimberlite was named for the rock associated with diamonds in Kimberley, South Africa.

E. Peridotite and allied rocki. Peridotite- Umumnya bertekstur holocrystalin,

anhedral-granular, medium to coarse grain.- Mineralogi : olivin, pyroxene (enstatite and

diposidic augite),with plagioclase, hornblende, biotite, garnet.

- Peridotit biasanya dijumpai dalam sekuen OPHIOLITE. Ophiolite merupakan suatu seri batuan kerak samudra yang muncul ke permukaan karena spreading.

ii. Dunite- Tersusun oleh olivin

iii. Pyroxene peridotit- Wehrlite : olivine and diposidic augite (3:1) assesori : enstatite, picotite, chromite, hornblende- Harzburgite : magnesian olivine and orthopyroxene

(enstatite, bronzite). Asesori : chromite, iron-titanium oxide, diopsidic augit, calcic plagioclas

- Lherzolite :Lherzolites contain over 60% olivine, and some clinopyroxene and orthopyroxene. Lherzolite is a major constituent of the mantle and the source of most basaltic magmas.

lherzolite

OL

OPx

CPx

iv. Hornblende peridotitev. Garnet peridotitevi. Mica peridotitevii. Plagioclas peridotitviii.Komatiites

4. FELDSPATHOIDAL FELSIC ROCK

A. Volcanic Division : Phonolite Family

It consists mainly of nepheline and alkali feldspar (sanidine or anorthoclase). Other minerals occuring in phonolite are soda-pyroxenes, soda-amphiboles, leucite, alcime and sodalite-group minerals (sodalite, nosean, hauyne). It is the volcanic equivalent of nepheline syenite.

• B. Plutonic Division : Feldspathoidal Syenite family

- Intermediate SiO2 content- Alkali feldspar & feldspathoid sebagai

penyusun utama- Kaya unsur sodium (dalam feldspathoid

berupa nephelin, analcime, sodalite, nosean, dalam sodic feldspar, dan mafic mineral :augit, aegirin)

syenite

Plg

Px

Bi

Ne