Prof. Tejas S Patil Dept Of Geology M.J.College.
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
Metamorphism Is the process that occur in rocks due to the effects of High temperature High pressure Chemically active fluids
Temperature 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. > 100 0 C and pressure > 1000 s Mpa. Low-grade metamorphism: Occurs at about 100 0 C to 500 0 C. High-grade metamorphism: Occurs at > 500 0 C
Pressure UNIFORM PRESSURE - increases with depth due to increase in overburden. - acts vertically downwards and affects the volume of both liquid & solids. - high temperature is also associated with (due to depth factor) - Lithostatic pressure- due to overburden DIRECT or Differential PRESSURE - increases with depth upto some extent, effective in the upper part of the crust. - acts in all direction and affects only on solids resulting into deformation of shape and change in mineral composition - high temperature is not always associated. to depth factor) - Stress- due to tectonic forces
min max inter Differential Stress Uniform Stress
Granite Granite-Gneiss
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
PHYLLITES - 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 minerals show preferred orientation - grain size is the main difference between the slaty structure and schistos structure.
GNEISSIC STRUCTURE - usually associated with high-grade regional metamorphism (where differential stress prevails I.e. tectonic forces) - where the sheet silicates and other minerals like quartz/feldspars/hornblende/pyroxene are segregated in distinct bands in the rocks- known as gneissic banding.
Classification of Metamorphic rocks based on texture/structures 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 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
SCHIST -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 gneisses
QUARTIZITE -it comprise equidimensional minerals viz. quartz and feldspars Non foliated; show GRANULOSE STRUCTURE
Type of Metamorphism Cataclastic Metamorphism This type of metamorphism occurs mainly due to direct pressure eg. when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along the zone, and the rocks tend to crushed and pulverized due to the sliding. 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 know as METAMORPHIC AUREOLE. 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 SLATE PHYLLITE SCHIST GNEISS NON-FOLIATED Quartzites and hornfels
Importance of Metamorphic rocks- SLATES 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 GNEISS 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 Load perpendicular to foliated planes gives more stronger foundation
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. SCHIST 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
QUARTZITE SANDSTONE (composed of quartz/feldspars/feldspathoid minerals) when under go metamorphism result into Quartzite. Granulose texture/structure (Granoblastic) makes them most competent rock amongst all other metamorphic rocks. Because metamorphism of Sst. Result disappearance of cementing material, bedding planes, fossil content etc. Quartzites are compact, hard and strong; very less porous and less permeable than the parent Sst. Predominance of Quartz makes the rock very hard and suitable for road metal; can be used as concrete aggregate etc. Acts as strong foundation for any CE structure.
MARBLE Latin word Marmor Shining stone. 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