Metamorphism and metamorphic rocks. GEOL115 Alexander Lusk

Transcription

1 Metamorphism and metamorphic rocks GEOL115 Alexander Lusk

2 Outline: Metamorphic rocks Defini>on and major types of metamorphism Rock cycle Metamorphic processes Deforma>on and development of a folia>on/ schistosity/cleavage Progressive (prograde) metamorphic series Environments of metamorphism Metamorphic facies

3 Metamorphic rocks If pre-exis>ng rocks (e.g. igneous, sedimentary, metamorphic) are buried within in the Earth, new minerals can grow in the solid rock through recrystalliza>on 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?

4 Recall the rock cycle:

5 Recall where different rock types form:

6 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).

7 Processes involved: 1. Neomineraliza-on growth of new minerals replacing minerals that become unstable. 2. Grain growth growth of new or exis>ng mineral grains. 3. Deforma-on creates a schistosity or folia2on defined by the orienta>on of platy or elongate mineral grains.

8 Porphyroblasts: Neomineraliza>on

9 Porphyroclasts:

10 Grain growth: Exis>ng crystals grow: Quartz overgrowths Original quartz grain

11 Rock deforma>on:

12 Cleavage and schistosity Folia-on repe>>ve layering and parallel alignment of platy (flat) minerals, such as micas, by shearing and/or differen>al pressure

13 Examples of Folia>on Slaty rock cleavage low grade metamorphism

14 Examples of Folia>on Phylli-c texture intermediate grade metamorphism

15 Examples of Folia>on Schistosity intermediate to high grade metamorphism

16 Examples of Folia>on Gneissic banding high grade metamorphism

17 Metamorphic Textures

18 Environments of metamorphism: 1.Subduc(on zones 2.Con(nental collision zones 3.Magma(c environments

19 Tectonic sezngs of metamorphism Rock cycle in subduc>on zone context contact metamorphism regional metamorphism

20 Subduc>on zones: Sedimentary and volcanic rocks from the ocean floor carried down subduc>on zones experience substan>al increase in pressure, modest increase in temperature. Rocks are commonly broken and disrupted: tectonic melange High pressure / low temperature metamorphism: blueschist facies eclogite facies

21 Tectonic melange California coast ranges

22 Microscopic view of blueschist Catalina Islands

23 Collisional environments Where crust is thickened by con>nental collision rocks become buried and heated; may start to melt. Regional metamorphism Rocks become duc>le 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 folia>on

24 Magma>c environments Large intrusions (plutons) rise into the crust, heat surrounding rocks. Temperature increases, but pressure does not. Contact metamorphism Li^le or no deforma>on Neomineraliza2on, but no fabrics hornfels Magma>c environments include: mid-ocean ridges plutons related to subduc>on zone magma>sm plutons related to collision-related mel>ng.

25 Zone of contact metamorphism Contact metamorphic rocks in the roof of a granite pluton, Sierra Nevada batholith

26 Metamorphic Facies Mineral assemblages indica2ve of a certain range of P-T condi2ons

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28 Using minerals to get a rock s history

29 Every >me that Africa and North America collided this garnet grew and recorded the P-T condi>ons. Garnet as a tape recorder Green island : 1 Ga garnet growth. White: 450 Ma garnet growth aeer earlier garnet cracked up. Red: 360 Ma garnet growth.

30 Andalusite replaced by kyanite and staurolite. Kyanite is the high P form of Andalusite. Kyanite is being deformed. This rock records burial and deforma>on of the rock.