Geoscience Reference
In-Depth Information
9
Recording features of
metamorphic rocks
Tom W. Argles
9
The environments in which metamorphic rocks form are a long
way from our everyday experience: crushing pressures, baking
temperatures, and stresses that stretch and distort rocks
weakened by the high temperatures. Studying the textures and
mineralogy of the rocks that have been exhumed to the Earth's
surface is a fascinating window into these alien environments,
and gives critical insights into how the Earth works, the
processes operating in active mountain belts and subduction
zones, and how crustal and mantle materials are recycled.
Metamorphic processes play a key role in concentrating a
number of useful elements into economically viable mineral
deposits, so metamorphic studies are critical in the exploration
and exploitation of such resources.
This chapter will consider common metamorphic textures and
minerals, before examining how we can extract information
about the timing of different metamorphic processes from
metamorphic rocks in the fi eld.
9.1 Basic skills and equipment for
metamorphic fi eldwork
Metamorphic study in the fi eld needs little more than the basic
equipment (Table 2.1, p. 4). You will also fi nd simple aids to
mineral identifi cation (e.g. a scratch/streak plate, a steel
penknife and dilute acid) from the optional equipment in Table
2.3 useful. Field study of metamorphic rocks mainly requires
keen observation at all scales, in particular of fi ne-scale
textures (Section 9.2) using the hand lens, and mineral
identifi cation (Section 9.3).
9.1.1 Field relations and context
Before focusing in detail on an exposure, consider its wider
context. Is the rock layered, massive or patchy in appearance?
Are there cross-cutting relationships or contacts between
