Metamorphic rocks (2)

SLATY CLEAVAGE- usually formed during the early stage of Low-grade Metamorphism due to lithostatic stress.. - hence, the sheet minerals as well as foliation will no longer be parallel t

Metamorphic rocks

 When rocks are baked by heat of molten magma or squeezed by the movements of huge tectonic plates or by the pressure of overlying thick succession of rocks

 They are altered or changed beyond their recognition

 i.e change in Chemical composition, texture and

structure

Metamorphic rocks

Is the process that occur in rocks

due to the effects of

 High temperature

 High pressure

 The source of temperature is either from magma or due to the depth factor

 Metamorphism usually result into change in min comp and texture of rocks (Ig and Sed.) which are subjected to temp >

1000 C and pressure > 1000’s Mpa.

UNIFORM PRESSURE

Pressure

- increases with depth due to

increase in overburden

- acts vertically downwards and

affects the volume of both liquid

& solids

- increases with depth upto some extent, effective in the upper part

of the crust

DIRECT or Differential PRESSURE

- acts in all direction and affects only

on solids resulting into deformation

of shape and change in mineral composition

- high temperature is also

associated with (due to depth

factor)

- high temperature is not always associated to depth factor)

STRUCTURES IN METAMORPHIC ROCKS

 Foliation: when platy, lamellar or flaky minerals (eg sheet silicate minerals the micas: biotite and muscovite, chlorite, talc, and serpentine), occurring in rock orient themselves parallel to one another (i.e perpendicular to the direction of maximum pressure or stress).

Random

orientation

Of minerals

Preferred orientation

Of minerals

 Lineation: when prismatic or rod-like minerals (eg Hornblende, tourmaline etc.) occurring in a rock orient themselves parallel to one another (perpendicular to direction of maxi Pressure or stress)

SLATY CLEAVAGE

- usually formed during the early stage of Low-grade Metamorphism due to lithostatic stress.

- New sheet-structure minerals tends to be

parallel to the bedding planes during metamorphism.

- however, further deep burial along

the continental margin; compressional forces will cause deformation (folding).

- hence, the sheet minerals as well as

foliation will no longer be parallel to the bedding planes, such type of foliation in fine grained rocks is

called slaty cleavage.

Shale Slate

- usually associated with intermediate grade of metamorphism; where the mineral grains grows large in size as compare to that seen in slates

-This develops a pronounced foliation where the preferred oriented

minerals are seen.

SCHISTOSE STRUCTURE

- usually formed during intermediate and high grade metamorphism

- Grain size increases and can be seen by naked eye; grains tends to enlarge with increasing grade of metamorphism; the coarse grained sheet-structure

Classification of Metamorphic rocks based on

texture/structures

PHYLLITE

-similar to slate, but slightly coarser phyllosilicate grains

-grains can be seen in hand specimen, giving silk appearance to cleavage surfaces

-often cleavage planes less perfectly planar than slates

SLATE

-strongly cleaved rock

-cleavage planes are developed due to orientation of fine phyllosilcate grains

eg Muscovite, biotite, chlorite etc.

-individual grains too fine to be visible with naked eye

-overall dull appearance

-parallel alignment of moderately coarse grains (fabric=schistocity)

-grains are visible by eye

-mainly phyllosilicates and other minerals such as hornblende, kyanite etc

GNEISS

-coarse grained rock (grain size several millimetres) and

-foliated (planar fabric: either schistosity or compositional layering)

-tendency for different minerals to segregate into layers parallel to foliation (gneissic layering): typically quartz and feldspar rich layers tend to separate from micaceous layers

Varieties:

Orthogneiss: rocks formed from Igneous rocks

paragneiss: rocks formed from Sedimentary rocks -metasedimentary

 QUARTIZITE

-it comprise equidimensional minerals viz quartz and feldspars

Non foliated; show GRANULOSE STRUCTURE

 Cataclastic metamorphism is mere mechanical breakdown of rocks without any new mineral formation, however, sometime due to intense shearing few new minerals are formed.

 Contact Metamorphism -

 This type of metamorphism occurs locally adjacent to the igneous intrusion; with high temp and low stress

 There is little change in bulk composition of the rock

 Area surrounding the intrusion (Batholith) is heated by the magma; metamorphism is restricted to a zone surrounding the intrusion, this zone is

 The rocks formed are non-foliated fine-grained rocks called as

HORNFELS.

 Regional Metamorphism -

 metamorphism occurs covering larger area, which is subjected to intense deformation under direct or differential stress

 Rocks formed under such environment are usually strongly foliated, such

as slates, schists, and gniesses

 The differential stresses result from tectonic forces,

 eg when two continental masses collide with one another resulting into mountain building activity Compressive stresses result in folding of the rock

Types of Metamorphic Rocks

FOLIATED

The common foliated rocks in the order of increasing grain size are

NON-FOLIATED

 Importance of Metamorphic rocks -

 Fine grained impermeable, cleavable and soft

 Incompetent; cannot withstand great loads

 But since they are impermeable and split easily; thin large sized slabs of uniform thickness can be extracted for roofing purpose

 Economic importance: Since they are bad conductor of electricity– used in electrical industries for switch board base

 Gneissic rocks are rich in SILICA i.e predominantly Quartz and Feldspars along with garnet, pyroxene, Hornblende etc

 Non-porous and impermeable nature increases the strength of the rock

 Foliated character to some extend improves workability

If mineral assemblage is more or less similar to

Granite (with less % mafic minerals) then:

It is used as building stone

As aggregate for making concrete

As road metals etc

Mainly composed of prismatic or platy minerals, which contributes in

development of Schistose Structure Eg Hornblende, tourmaline, sillimanite etc (prismatic); chlorite, muscovite, biotite, talc, kyanite etc (platy)

Cleavable nature of Schists is the main reason for their weakness; they are incompetent

 MARBLE

 Calcareous metamorphic rock

 Though it shows granulose structure it is not as hard as Quartzite because of its Calcareous composition; but can withstand reasonable load

 Due to its pleasant colour and brilliant appearance when polished it is extensively used as building stone

Calcite