Lab 7: Metamorphic Rocks

Objectives:

- Two types of metamorphism

- Identification of metamorphic rocks

- Infer parent (protolith) rocks

- Review major rock types and the rock cycle

- Identification of the three rock types

Introduction:

Metamorphic rocks: rocks changed from one form to another by intense heat, intense

pressure, or the action of watery hot fluids

Protolith: the parent rock that was metamorphosed

Figure 7.1, pg 134

Metamorphic grade: the intensity of deformation

Metamorphic minerals:

Quartz, feldspars (plagioclase and potassium), muscovite, biotite, chlorite, garnet,

tourmaline, calcite, dolomite, serpentine, talc, kyanite, sillimanite, and amphibole

Metamorphic Processes:

Contact metamorphism:

- occurs locally, adjacent to igneous intrusions

- hydrothermal metamorphism: occurs along fractures that are in contact with watery hot (hydrothermal) fluids

- days to thousands of years

- intensity decreases rapidly over a short distance

- zone of metamorphism millimeters to tens of meters thick

Regional metamorphism:

- very large areas (regions), deep within the cores of rising mountain ranges, accompanied by folding of rock layers

- large igneous intrusions

- thousands to tens of millions of years

- extreme pressure and heat from deep burial or tectonic movements

- widespread migration of hot fluids along fractures and pore space

Most major intrusions are preceded by contact meta. and followed by regional meta.

Mineralogical composition: description of the kinds and relative abundances of mineral

crystals that comprise the rock (same definition again)

- May change from a parent rock to a metamorphic rock

Recrystallization: small crystals of one mineral will slowly convert to fewer, larger

crystals without melting

Neomorphism: minerals not only recrystallized, but also form different minerals from the

same chemical elements

Metasomatism: chemicals are added or lost to form new minerals

Textures:

Texture: description of its constituent parts and their sizes, shapes and arrangements

Foliated textures:

Foliations: layering and parallel alignment of platy (flat) mineral crystals

Slaty rock cleavage: very flat foliation developed along closely spaced shear

planes (microscopic faults) in tightly folded clayey or mica-rich rocks

- low-grade metamorphism

Phyllite: wavy and or wrinkled foliation of cryptocrystalline (too small to be

seen) platy minerals that give it a metallic luster

- intermediate-grade metamorphism

Schistosity: scaly glittery layering of visible platy minerals and or linear

alignment of long prismatic crystals (tourmaline, hornblende, kyanite)

- intermediate to high-grade metamorphism

Gneissic banding: alternating layers or lenses of light and dark medium to coarse-

grained minerals

- light and dark bands from mafic and felsic minerals

- high-grade metamorphism

Non-foliated textures:

Crystalline: medium to coarse-grained aggregate of intergrown, equigranular

visible crystals

Microcrystalline: very fine-grained aggregate of intergrown microscopic crystals

Sandy: medium to coarse-grained aggregate of fused sand-sized grains

Glassy: homogeneous texture with no visible grains or other structures

Other features:

Stretched or sheared grains: deformed pebbles, fossils or mineral crystals

Porphyroblastic: arrangement of large crystals (porphyroblasts) set in a finer-

grained groundmass

Hydrothermal veins: fractures filled by minerals that precipitated from

hydrothermal fluids

Folds: bends in rock layers that were initially flat

Lineations: lines on rocks at the edges of foliations, shear planes, slaty cleavage,

folds or aligned crystals

Identification of Metamorphic Rocks:

Figure 7.15, pg 143

1. Determine textural features

2. Determine the mineralogical composition

3. Use Figure 7.15

4. Infer the protolith of the rock