Metamorphic And Rock Fall Lecture

Metamorphic Rocks

• Isochemical! 

- Minerals are transformed – Texturally or mineralogically distinct

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Contact and Regional Metamorphism 1. regional: burial, transformation, and exhumation of entire regions 2. contact: transformed by contact with an igneous intrusion.

N o te : T h e ro cks exp o se d in B o u ld e r C a n yo n w e st o f to w n a re a llre g io n a lly m e ta m o rp h o se d Pre C a m b ria n ro cks. T h e a g e o f m e ta m o rp h ism d e te rm in e d fro m ra d io m e tric d a tin g is a b o u t 1 . 8 b illio n ye a rs

• Contact metamorphism – Locally and adjacent to igneous intrusions – Along fractures in contact with hot fluids (hydrothermal) • Mineral crystals precipitate along fractures

– Caused by low P, high T (from magma or fluids) – Time scale: days  kyr – Intensity greatest at contact between parent and intrusive magma or fluids • Decreases rapidly over short distances

Check out fig. 7.1 in LM!

Lava flow bakes mud beneath it!  Shale!

• Regional Metamorphism – Large intrusions, tectonism, widespread hydrothermal fluids – High P, Lower T – Usually results in rocks that are strongly foliated – Widespread hydrothermal migration

The Cascade Range Ocean Ridges

Mineralogical Composition -Kinds and abundances within the rock -Composition can change OR stay the same (texture changes) Recrystallization = crystals of one mineral  fewer, larger crystals of same mineral! Neomorphism = changes mineral composition! Recrystallizes minerals and form different minerals from same elements! Metasomatism = Chemicals are added or lost, can gain new elements! (Minerals of a totally different composition)

Textures • Foliated – layering, parallel alignment of platy mineral crystals (i.e. micas)

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– From pressure and shearing of crystals 1. Slaty (low grade) – closely spaced shear planes, flat foliated 2. Pyllite texture (intermediate) – wavy foliation, metallic luster 3. Schistosity (intermediate-high) glittery layering or linear alignment of crystals, breaks on wavy foliations 4. Gneissic Banding – layers of light and dark, medium-coarse grained

Slaty

Schistosity

Phyllite Texture

Gneissic Banding

• Nonfoliated – no obvious layering 1.Crystalline texture (nonfoliated) – intergrown, usually equal size visible crystals 2.Microcrystalline texture – finegrained, intergrown microscopic minerals (i.e. sugar cube) 3.Sandy texture – medium-coarse grained, resemble sandstone, fused sand 4.Glassy texture – no visible grains or structures, breaks along glossy surfaces