Igneous petrology EOSC 321

Laboratory 4:

Felsic and Intermediate Volcanic 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) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke)

Introduction:

This lab introduces intermediate and felsic volcanic rocks ranging in composition from andesite to rhyolite. For their classification you should plot quartz, plagioclase and alkali feldspar modes in the "Volcanic" triangle (attached below). Classification of these rocks is difficult because viscous silicic magmas tend to solidify into glasses rather than crystallize. You can examine such glassy rocks in T/s's 302, 581 (pumices) and in T/s 765 (vitrophyric andesite). Later devitrification of these glasses produces fine-grained aggregates of felsic minerals. In such rocks where it is impossible to determine mineral abundances of phases in the groundmass, a rock classification must be based on phenocrysts. The IUGS recommends that rocks identified in such a manner be called PHENOTYPES (like “pheno-latite). If based on phenocrysts, the position of a rock on classification triangles will be biased to more mafic and basic rocks and usually erroneous for the rock as a whole. 

In intermediate-silicic volcanics, plagioclase is usually the most abundant phenocryst phase and generally displays complex oscillatory zoning. Quartz is the next most common felsic mineral, whereas alkali feldspar is not common in calc-alkaline volcanic rocks and is restricted only to the alkaline series.

In volcanic felsic rocks you can come across 2 varieties of K-Fsp: sanidine and anorthoclase. The latter is very rare. Sanidine can be identified by conoscopy (it has a low 2V angle in contrast to higher 2V's in microcline and orthoclase). Sanidine may have simple Carlsbad twinning (as in T/s 899) or may have no twinning at all. As a simple rule of thumb, you should call any volcanic K-Fsp a sanidine.

Amphiboles in andesites are strongly pleochroic hornblendes frequently showing intense opaque reation rims due to low-pressure instability. When reddish-brown they are termed oxyhornblende. They are also distinguished from hornblende by their higher birefringence, and lower extinction angles. Oxyhornblende is not a distinct mineral species, but a hornblende in which a substantial amount of the iron has been oxidized to the Fe³⁺ state and (OH)^- is replaces by O₂^_ to balance the charge. There is a continuous range of compositions and properties between members of the hornblende and oxyhornblende. Conventional hornblende can be converted to oxyhornblende by heating in an oxydizing environment, and the process may be reversed by heating in a reduced environment.

Make sure you see rhyolite with spherulitic textures (T/s 866) characteristic of felsic volcanics. Spherulites are spheroidal bodies in a rock composed of an aggregate of fibrous crystals of one or more minerals radiating from a nucleus, with glass or crystals in between. The most common occurrence of spherulitic texture is a radiating aggregate of acicular alkali feldspars with glass between them, though quartz or other mineral may also be present, resulting in the intergrowth texture. The colour variation in the spherulites is caused by variations in the density of fibres.

Make sure that you see the following minerals and textures in the Reference Collection:

q       Oxyhornblende

q       Sanidine, with or without simple twinning

q       Trachytic, vitrophiric, spherulitic, perlitic amd pumiceous (in the order of importance)

In the third hour you will receive a thin section of an unknown granitoid for your independent examination. Write its petrographic description and give a rock name. Determination of the plagioclase composition is a necessary part of the petrographic description. Your petrographic report should be completed and handed to the TA by the end of the Lab.       

Reference collection:  Felsic and Intermediate Volcanic rocks

Thin Section Description:

Texture: Porphyritic. Texture of the groundmass is concertal, i.e. with notched or serrated boundaries between grains.

20%     Phenocrysts:

12%     Hornblende, strongly pleochroic from yellow to green-blue, zoned, euhedral, many grains are simply twinned, extinction angle 22^o

8%       Plagioclase, euhedral, zoned, N > N Balsam, An15, with simple Calsbad twinning, and polysynthetic twinning, severely altered to clay minerals and saussurite

1%       Augite, euhedral, pale olive to colorless.

Few grains        K-Fsp? Grains with no Calsbad or polysynthetic twinning

80% groundmass:

68%     Fine-grained felsic mineral with N<N Balsam. Could be either albite or K-Fsp.  Tentatively identified as K feldspar (sanidine) as it lacks polysynthetic twinning.

10%     Hornblende, in small green high-relief grains

1%       Augite, in small colourless high-relief grains

1%       Apatite

Opaque mineral

Secondary Minerals: Clay minerals and Saussurite after Plagioclase

Thin Section: 1245                                        

Sample: P 631

Rock Type: Andesite

Location: Mt. Shasta, Fort Ebbutt

Thin Section: 765                  

Sample: P 1580

Rock Type: Vitrophyric Andesite

Location: Mt. Lassen

  Opaques + Plag + Bi + Hb

Thin Section   1006

Sample Number         P242

Rock Name:               Silicitized Trachyte

Location: Chaffee, Colorado

10%     Vesicles of elongate irregular shapes marking flow directions. The vesicles are filled with a fine-grained aggregate of secondary quartz.

5%       Quartz of secondary origin filling out fractures and veins

Comment: An Alkali feldsar in the groundmass cannot be identified. However, this rock should be classified as "quartz-rich rhyolite" irrespectively of the type of the alkali felspar in it.

  Modal proportions of the minerals are unknown. Also present apatite and opaque mineral.

Thin Section: 722                              

Sample: P 514

Rock Type: Andesite

Location: Mt. Shasta

Thin Section Description:

Texture: Pumiceous - Frothy vesicular

                                    Glassy (holohyaline)

Groundmass:

50%     oval and irregularly shaped vesicles marking flow structures and flow turbulence

50%     glass with N << N Balsam, clear in T/s 302 and grey-brown in T/s 581, contain tiny crystallites of opaque and light minerals.

Comment: The rock is classified as "rhyolite" based on the very low refractive index of the glass ( << N balsam) typical of acid and silicic glass.

97% groundmass:

92%     Fine-grained intergrowth of felsic minerals. They include sanidine with N<N Balsam growing in feather-like microlites with undulose extinction; quartz, and yellow fibrous crystobalite. Perlitic fractures  and textures are very pronounced. The cores in perlitic round structures are composed of yellow fine-grained crystobalite, whereas rims crystallised in less finer-grained quartz and sanidine.

5%       Spherulites - spheroidal bodies composed of an aggregate of fibrous crystals of crystobalite (yellow), quartz and K-Fsp.

Secondary Minerals: Carbonate in the groundmass and around spherulites.

Comment: The rock name "rhyolite" is tentative since it is impossible to determine a mineralogical mode of the groundmass. The rhyolitic nature of this rock is suggested by: 1) the presence of spherulites typical only for acid and silicic volcanic rocks; 2) the presence of crystobalite in characteristic yellow-grey fibrous radiating aggregates; 3). The presence of sanidine identified by N<N Balsam.