Igneous petrology EOSC 321

Laboratory 2:

Mafic and intermediate plutonic rocks

Duration: This lab is to be completed within the lab period. The results will be evaluated and returned to you in the following lab period.

Material Needed: a) Microscope, b) Classification triangles and instructions on determination of plagioclase composition included with lab handout; d) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke)

Introduction: This lab introduces mafic and intermediate plutonic rocks and teaches you how to determine plagioclase composition in a thin section. The importance of this petrographic skill cannot be overestimated. The very classification of rocks depends on  a correct determination of the composition of plagioclase. A clinopyroxene-plagioclase rock could be called gabbro if the plagioclase is Ca-rich or diorite if the plagioclase is more sodic (i.e. andesine).

Mafic rocks consist mainly of clinopyroxene and plagioclase (labradorite) with accessory orthopyroxene, olivine, hornblende and biotite. Typical alteration products of these primary phases are chlorite, sericite (fine-grained muscovite), talc, serpentine, amphibole and mica. Mafic plutonic rocks include gabbro, norite, troctolite, anorthosite, and diabase. Diabase is the North American name used for medium-grained basic rocks (grains 1-5 mm) consisting essentially of labradorite, augite and ore minerals, i.e. equivalent of basalt and gabbro.  The rest of the world uses the term Dolerite in preference to Diabase to denote the same rock. Diabase has smaller grains than other plutonic mafic rocks and often is formed in a hypabyssal environment.

A most abundant intermediate plutonic rock is diorite. This is a coarse-grained rock consisting of andesine plagioclase and one or more mafic minerals clino-and ortho-pyroxene, hornblende, and biotite. A little quartz, K-feldspar and olivine may be present. Diorite is a coarse-grained equivalent of andesite.

Assignment: In the first half-hour of the Lab you will practice optical determination of plagioclase composition. Then, after this skill is mastered, you will examine reference thin sections of mafic-intermediate rocks. Each thin section has a brief petrographic description to assist you in the identification of minerals, textures and rock classification. Please make sure you understand the reference petrographic descriptions and can find all minerals mentioned. There is also a separate box with reference thin sections for rock-forming minerals that can be helpful in this respect. You should also be able to understand why a rock is given a particular name. Look through thin sections of different rock types so that by the end of the second hour you will have examined all of them. In the third hour you will receive a thin section of an unknown mafic rock for your independent examination. Write its petrographic description and give a rock name. Your petrographic report should be completed and handed to the TA by the end of the Lab.                                         

Reference collection:  Mafic and intermediate plutonic rocks

            Thin Section Description:

Texture: Hypidiomorphic

60%     Plagioclase. Euhedral to subhedral shapes, An₆₀ , but zoned!

15%     Orthopyroxene. Euhedral to subhedral shapes. Shows lamellae. Low birefringence, parallel extinction. 2V ~65^o. Rimmed by late amphibole

5%       Clinopyroxene. Euhedral to subhedral shapes (+) 2V~ 60^o, inclined extinction. Rimmed by late amphibole

10%     Amphibole, pleochroic from darker to lighter green (Hornblende?)

2%       Opaque mineral, anhedral

Comments: Amphibole is late magmatic (deuteric?) or secondary

            Thin Section Description:

Texture: Ophitic. This texture is typical of mafic intrusive rocks. It is created by elongate chadacrysts partly enclosed by the oikocryst. In gabbros chadacrysts of plagioclase are partly enclosed by oikocrysts of pyroxene and olivine.

70%     Plagioclase, euhedral to subhedral. Combined Carlsbad-Albite twins give extinction angles of 20 and 31 => An₆₁

15%     Clinopyroxene. Anhedral

8%       Olivine in subhedral grains now almost totally replaced by a secondary  aggregate of  serpentine, fine-grained black opaque hematite and talc.

4%       Orthopyroxene. Anhedral to subhedral shapes. Slightly pleochroic

1%       Opaque mineral, anhedral

Sample:P3393                                                                                                            7

Thin Section: 1849

Rock Type: Gabbro

Location: Troodos, Cyprus

Thin Section Description:

Texture: Panidiomorphic, all minerals are auhedral

80%     Plagioclase

15%     Clinopyroxene- High birefringence, inclined extinction, (+)2V~60^o.

5%       Orthopyroxene- parallel extinction, lamellar exsolution, low birefringence, pleochroic.

Thin Section Description:

Texture: Allotriomorphic: all minerals are anhedral

96%     Plagioclase , anhedral to subhedral

3%       Olivine, anhedral, interstitial

1%       Amphibole, forms rims on olivine, pleochroic from light to dark green , 2V large, cleavage at 60o (hornblende?)

few grains of black opaque minerals

Secondary Minerals: Epidote or clinozoisite, characterized by moderate relief, anomalous blue interference colour and twinning. Found in large auhedral elongate laths, after Plag or Px?

Thin Section Description:

Texture: Ophitic: euhedral Plag partially penetrates anhedral Px.

84%     Plagioclase , subhedral, slightly zoned, based on complex Carlsbad-Albite twinning its composition is An85

10%     Orthopyroxene, grey-greenish, anhedral, often in intergrowths with Cpx, sometime has lamellar structures ( Twinning? Exsolution lamellae?), altered along fractures and cleavage cracks

5%       Clinopyroxene, grey-greenish, anhedral, often in intergrowths with Opx, sometime has lamellar structures ( Twinning? Exsolution lamellae?) altered along fractures and cleavage cracks

1%       Biotite, pleochroic from colourless to brown, occurs around opaques

few grains of black opaque minerals

Thin Section Description:

Texture: Ophitic

 73%    Plagioclase- Euhedral, ~An₄₂. based on albite twins.

10%     Clinopyroxene, grey, from subhedral to anhedral, often twinned.

7%       Opaque mineral or minerals. Some grains are euhedral elongate (Ilmenite?), some are anhedral.

5%       Hornblende, pleochroic from dark to light khaki, with cleavage at 60o

4%       Apatite in small euhedral crystals hexagonal or elongate.

1%       Quartz

Secondary Minerals: Biotite, chlorite

Thin Section Description:

Texture: Ophitic

 70%    Plagioclase- Euhedral to subhedral, ~An₈₇. based on albite twins.

27%     Clinopyroxene, grey, subhedral

3%       Olivine, subhedral, one grain is surrounded by a reaction rim made of orthopyroxene. Another grain is replaced by green serpentine

1%       Quartz

Secondary Minerals: Talc after Cpx, Serp after Ol