GL23C IGNEOUS & METAMORPHIC PETROLOGY LAB 4

STUDENT NAME ____________________________________________________________

Granite-Diorite-Gabbro Association

Describe thoroughly the thin sections provided by your instructor, noting texture, mineralogy, mineral size and shape, and any other special features. Sketch the thin section and label the diagram.

In your petrographic descriptions, you must identify and describe in detail the optical properties of the different minerals present in the rock and estimate the modal mineralogy for each. Name the rock, using the IUGS classification.

In your thin section description you should devote a paragraph to the crystallization history or sequence of crystallization of the minerals (this can come after the detailed mineralogical descriptions). In order to do so, you need to follow some basic rules (that are not always infallible).

  1. If one mineral is surrounded by another, the enclosing mineral is younger.
  2. Early formed crystals/minerals are generally euhedral (or nearly so), compared to later crystals/minerals.
  3. If both large and small minerals occur together, the larger ones developed first.

    N.B. THERE ARE EXCEPTIONS TO THE RULE.

Your description of each thin section should be readable and logically constructed, and should pay regard to the proper use of technical language, and avoid disjointed phrases.

List of Plutonic Rocks to be Examined in Lab 4

  1. Biotite Granite
  2. Alkali Granite
  3. Tonalite
  4. Norite

N.B. Ensure that when you have completed the description you place your thin section into the correct holder i.e. the number on the thin section corresponds to the number on the holder/container.

The microscopic description, that is provided with each slide, should supply you with background information on the mineral composition of the rock. However you must expand on the description given.

Read ATLAS OF IGNEOUS ROCKS AND THEIR TEXTURES by MacKenzie, Donaldson & Guildford.