Metamorphism and metamorphic rocks

GEOL115 Alexander Lusk Outline: • Metamorphic rocks – Definion and major types of metamorphism – Rock cycle • Metamorphic processes • Deformaon and development of a foliaon/ schistosity/cleavage • Progressive (prograde) metamorphic series • Environments of metamorphism • Metamorphic facies Metamorphic rocks If pre-exisng rocks (e.g. igneous, sedimentary, metamorphic) are buried within in the Earth, new minerals can grow in the solid rock through recrystallizaon and the rock can deform in response to tectonic forces.

This rock may have started off as a sediment of mud or clay, which later become a sedimentary rock such as mudstone or shale …

What minerals are present? Recall the rock cycle: Recall where different rock types form: Types of metamorphism Regional metamorphism: as rocks are buried deeper and deeper within the earth they are subject to increasingly higher pressures and temperatures, which result in recrystallization. The directed pressures (stresses) produce planar structures called foliation. Contact metamorphism: rocks within the crust that are intruded by magma are recrystallized by the magma’s high temperatures. These rocks are commonly not deformed (foliated).

Processes involved: 1. Neomineralizaon – growth of new minerals replacing minerals that become unstable.

2. Grain growth – growth of new or exisng mineral grains.

3. Deformaon – creates a schistosity or foliaon defined by the orientaon of platy or elongate mineral grains. Neomineralizaon Porphyroblasts: Porphyroclasts: Grain growth: Exisng crystals grow:

Quartz overgrowths

Original quartz grain Rock deformaon: Cleavage and schistosity

Foliaon – repeve layering and parallel alignment of platy (flat) minerals, such as micas, by shearing and/or differenal pressure

Examples of Foliaon

Slaty rock cleavage – low grade metamorphism Examples of Foliaon

Phyllic texture – intermediate grade metamorphism Examples of Foliaon

Schistosity – intermediate to high grade metamorphism Examples of Foliaon

Gneissic banding – high grade metamorphism Metamorphic Textures Environments of metamorphism:

1.Subducon zones

2.Connental collision zones

3.Magmac environments Tectonic sengs of metamorphism Rock cycle in subducon zone context

contact metamorphism

regional metamorphism Subducon zones: • Sedimentary and volcanic rocks from the ocean floor carried down subducon zones experience substanal increase in pressure, modest increase in temperature. • Rocks are commonly broken and disrupted: tectonic melange • High pressure / low temperature metamorphism: – blueschist facies – eclogite facies Tectonic melange – California coast ranges Microscopic view of blueschist – Catalina Islands Collisional environments

• Where crust is thickened by connental collision rocks become buried and heated; may start to melt. Regional metamorphism • Rocks become ducle and start to flow • Sedimentary rocks are converted into slate or schist; – abundant well-oriented mica – porphyroblasts of minerals like garnet • If rocks start to melt, produce gneiss. – abundant quartz and feldspar, strong foliaon

Magmac environments

• Large intrusions (plutons) rise into the crust, heat surrounding rocks. • Temperature increases, but pressure does not. Contact metamorphism • Lile or no deformaon • Neomineralizaon, but no fabrics – hornfels • Magmac environments include: – mid-ocean ridges – plutons related to subducon zone magmasm – plutons related to collision-related melng.

Zone of contact metamorphism

Contact metamorphic rocks in the roof of a granite pluton, Sierra Nevada batholith Metamorphic Facies Mineral assemblages indicave of a certain range of P-T condions

Using minerals to get a rock’s history Garnet as a tape recorder

Green “island”: 1 Ga garnet growth. White: 450 Ma garnet growth aer earlier garnet cracked up.

Red: 360 Ma garnet growth.

Every me that Africa and North America collided this garnet grew and recorded the P-T condions. Andalusite replaced by kyanite and staurolite. Kyanite is the high P form of Andalusite.

Kyanite is being deformed. This rock records burial and deformaon of the rock.