Mafic Rocks (Non Feldspathoidal)

8/10/2019 Mafic Rocks (Non Feldspathoidal)

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MMAFICAFIC RROCKSOCKS

(Non(Non feldspathoidalfeldspathoidal))

Oktober 2009

Nurcahyo I. BasukiTeknik Geologi ITB


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Mafic rocks:

- (Hand specimen) Dark in color

- (Hand specimen) Color index: > 40

- Silica content: 45 52%

- Two groups: 1. Calc alkaline (non-feldspathoidal)

2. Feldspathoidal (usually with nepheline)

- Two groups (based on textures): 1. Basalt - diabase

2. Gabbro


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Basalts (+ diabases)

Based on level of undersaturation in SiO2

:

1- Tholeiites:

a. slightly oversaturated normative Q (quartz) and abundant hy

(hypersthene)

b. saturated abundant normative hy

2- Olivine tholeiites, undersaturated normative ol (olivine) and hy

3- Olivine basalts, undersaturated normative ol ; hy insignificant or

absent

4- Alkali olivine basalts, strongly undersaturated normative ol ; minorne (nepheline)

5- Basanites, strongly undersaturated normative oland significant ne

Incr

easing

undersatura

tion

Remember:

normative (or norm) does notnecessarily correspond to

modal mineralogy

These rocks are

feldspathoidal mafic rocks;will be discussed later


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Based on mineralogical data:

1- Tholeiitic basalts pyroxene and plagioclase (labradorite) are the

main principal minerals. Mg-olivine is almost present, especially

as phenocrysts. Mg-pyroxene may be present as pheno and/or in

groundmass. SiO2 may be concentrated in the interstitial glass, or

can be present as fine-grained interstitial residue.

2- Tholeiitic olivine basalts diopsidic augite, Mg-olivine

(microphenocryst and groundmass) and plagioclase (labradorite).

3- Alkali olivine basalts Mg-olivine, augite, nepheline rarely present inbasalts, but interstitial analcime may be present in diabases. Little

K-feldspar (rimming plagioclase) or brown biotite may be present

in the groudmass.

4- Picrite basalts abundant phenocrysts of Mg-olivine (cummulates)

or augite, or both. Contain NO feldspathoid minerals.



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Textural characteristics:

- Basalts fine grained ; diabases medium-grained

- Most are holocrystalline (pheno + groundmass), except some parts

of groundmass are glass (intersertal texture)

- Porphyritic and intergranulartextures are common in basalts

- Ophitic and sub-ophitic textures are typical of diabases



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Ophitic texture. A single pyroxene envelops several well-developed plagioclase laths.

Width 1 mm. Skaergrd intrusion, E. Greenland. John Winter and Prentice Hall.


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Ophitic texture - plagioclase grains are partly to completely enclosed by pyroxene, indicating that

the minerals crystallized out of the magma to in a certain order: first plagioclase, then pyroxene

(http://www.cas.usf.edu/~jryan/rocks.html)


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Sub-ophitic - Sub-ophitic texture , Chilcotin Group Basalt, B.C..

Note clinopyroxenes partially surrounding plagioclase laths.


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Amygdaloidal

The oval feature in this photomicrograph is an amygdule: a formerly open vesicle which has been

filled with a secondary mineral(s), probably a zeolite mineral.


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The photos above show chlorite filling a vesicle in basalt.

Plane polarized light on the left, crossed polarizers on the right.


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Gabbros, Norites, Troctolites, Anorthosites, Pyroxenites

Textural characteristics:

- Plagioclase: more calcic than An50

- Holocrystalline, coarse to medium-grained

- Subhedral to anhedral granular textures

- Ophitic and poikilitic textures may be present


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Based on mineralogical data:

1- Gabbros plagioclase and diopsidic augite; olivine and hornblende

are the main principal minerals.

2- Norites plagioclase plus hypersthene, with subordinate augite;

olivine may be present.

3- Troctolites predominant plagioclase plus olivine.

4- Anorthosites predominant plagioclase (An35 An80), minor

hypersthene or augite are common.

5- Magnesian and calcmagnesian pyroxenites Mg-orthopyroxene or

diopsidic clinopyroxene.

Gabbros, Norites, Troctolites, Anorthosites, Pyroxenites


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Gabbro plagioclase, olivine and pyroxene dominate this rock.


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Gabbro - plagioclase and hypersthene (orthopyroxene) dominate this rock.

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Mafic Rocks (Non Feldspathoidal)