Lab 7:

Intermediate and silicic igneous rocks; determining plagioclase compositions

Goals:

Further practice at determining textures, modes, and complete names for intermediate and silicic igneous rocks
An understanding of the chemical and mineralogical constituents of intermediate and silicic igneous rocks
To learn how to determine plagioclase compositions in thin section using extinction angles

Exercises

Examine the rocks and thin sections during lab, but be sure to answer the following questions too. They provide useful background information.

Part I.

What is the silica content (wt. %) of an intermediate composition igneous rock?
Of a silicic igneous rock?
What minerals are common in intermediate igneous rocks? (Give specific varieties of solid solution minerals).
What is a pyroclastic rock?
What minerals are common in silicic igneous rocks? (Give specific varieties of solid solution minerals).

Part II.

Examine the following hand samples and corresponding thin sections, if present. For each, give a complete name and answer any other questions.

hs = hand sample; ts = thin section

6-31 (Black Tusk, B.C.) andesite (hs; ts)
Give a brief statement of the origin of 6-31.
7-45 rhyolite, Udy Creek, B.C. (hs,ts)
- What textural terms apply to this sample?
- Name the phenocrysts and give their percentages.
7-48 welded tuff, west of Burns, Oregon (hs,ts)
List the textural terms that apply to this sample.

See Raymond Ch. 8 for information on how and where welded tuffs form.
821 aplite, Albion Range, Idaho (hs)
What is an aplite?
6-36 quartz monzonite, locality unknown (hs)
7-41 syenite, Mt. Fleet, Paul Lake, near Kamloops,B.C (hs,ts)
- What type of feldspar is present?
- What is the primary mafic mineral?
7-42 pegmatite, locality unknown (hs,ts)
- Name the dark colored mineral.
- What does its chemical composition tell you about the volatile composition of the melt from which this rock crystallized?

Part III.

Plagioclase is perhaps the most abundant of the rock-forming minerals commonly seen in thin section. It forms a complete solid solution series ranging in composition from pure albite (NaAlSi₃O₈) to pure anorthite (CaAl₂Si₂O₈). Compositions of plagioclase are usually expressed in terms of the percentage of the anorthite molecule in the plagioclase (e.g. An₅₇ = 57% anorthite and 43% albite).

Several optical properties vary systematically with plagioclase composition. These variations can be used to determine the composition of plagioclase to within a few percent. We will use variations in extinction angle and refractive index (relief) to determine plagioclase compositions.

Determine the composition of plagioclase in granitoid 202 and at least 1 of the following thin sections, using the Michel-Lévy technique (p. 272-273 Nesse; also MA p. 57-58 has a brief explanation).
For each of approximately 10 correctly oriented grains within a single thin section, measure the extinction angle of each set of albite twin lamellae and take the average. (The extinction angles for each set must agree to within 4 degrees or the grain is of no use). Give both the composition and the name of the plagioclase (e.g. An₅₇, labradorite).
- granitoid 202(thin sections in Lab 1 tray)
- Thin sections containing plagioclase: 1300 to 1350, MG1000, MG1002, SWC-1, SWC-2; also sample 77-11-0000 (Lab 6)

Note that parallel extinction for plagioclases occurs in the middle of the compositional range of oligoclase.

What other optical property does one need to determine in order to unambiguously ascertain the composition of "low" plagioclase with an extinction angle of < 20 ?

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since August 6, 1997.
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Geology 202
Introduction to Petrology
University of British Columbia