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Comprehensive Overview of Metasomatism in Geology: Processes and Rock Types

Detailed exploration of metasomatism, including processes like diffusion and infiltration, geological types, and characteristic metasomatic rocks such as fenite, greisen, skarn, and propylite.

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METASOMATISM Nayana Geogy C BSc Geology Christ College (Autonomous) , Irinjalakuda
!’ Metasomatism is defined as metamorphism accompanied by changes in whole-rock composition. It is the bulk chemical composition change which happens in solid state. !’ Metasomatism is most dramatically developed in situations where rocks of highly contrasting composition arejuxtaposed and elements move easily. !’ It can be divided into two on the basis of: i) Metasomatic Process ii) Geological Process
i) Metasomatic Process !’ The two principle process ofmass transfer in rocks : - Diffusion - Infilitration !’ These two processes are probably end-members ofa continuous spextrum of combined mechanisms.
Diffusion !’ Diffusion is the process by which components move through another medium, either a solid or a stationary fluid. Diffusion is driven by chemical potential (¼ ) gradient. Element mobility is limited. !’ Diffusional metasomatic rocks form rather thinly zoned bodies (rims) along cracks, veins and contact surfaces. !’ The composition ofminerals may vary gradually across each metasomatic zones. eg : Formation ofskarns
!’ Infiltration refers to passive mass transfer ofsolute species carried in a moving fluid medium. Whereas diffusion occurs in response to internal chemical potential gradients, infiltration is driven by external fluid pressure gradients that cause the fluids to move. !’ Pervasive fluid flow through a porous-permeable medium is described by Darcy’s law. !’ It operates at larger scale. !’ It produce zoned alteration haloes. !’ It requires permeability. eg : Sodic Metasomatism Infiltration
ii) Geological Process - Autometasomatism It is a type ofmetasomatism that occurs at the top ofmagmatic bodies during post magmatic stage. eg : Albitisation in granitic plutons and serpentinisation ofultramafic rocks - Contact Metasomatism It is a type ofmetasomatism that occurs at or near to the contact between a magmatic body and another rock. It may occur at various stages in the magmatic zones.
Endocontact zones develop by replacement ofthe magmatic rocks and exocontact zones are formed by replacement ofthe host rock. - Boundary Metasomatism It is a type ofmetasomatism that occurs at the contact between two rock layers. - Bimetasomatism It is a variety ofthe contact metasomatism, which causes replacement ofboth the rocks in contact due to two-way diffusion ofdifferent components across the contact. - Near vein Metasomatism It is a type ofdiffusional metasomatism, which forms symmetrixal metasomatic zonation on either side ofan infiltrational metasomatic veins (or a vein filling).
- Regional Metasomatism It occupies great areas developing in various geological situations. It commonly forms alkaline metsomatic rocks during the magmatic and postmagmatic stages. Regional metasomatic rocks at moderate and even shallow depths from outer zones of Metasomatic rock associations accompanying ore deposits such as greisens, quartz sericite rocks, propylites and some others.
Metasomatic Rocks FENITE - High temperature, metasomatic rock - It is characterised by the presence ofAlk-feldspar, sodic amphibole and sodic pyroxene. - They are formed mainly or completely during the ragmatic stage by silica-undersaturated alkaline magmatis flieds- eg: Melasomatic feldspar rocks (potash feridas) associated co the fluorite deposits are carbonatites ofAmba Dergar, Gujarat, India
GREISENS - Medium termperature metasomatic rock. - It is characterized by the presence ofquartz and white mica, commonly with topaz, fluorite and fourmaline. - It may host Be, W, Mo, Sn and Tamineralisation. - In most cases, it is formed inside the granitic pluton itself. eg: Degana tungsten deposit, Balda tungsten deposit and Sewariya tungstenn deposit in Rajasthan.
SKARN - Skarn is a metasomatic rock formed at the contact between silicate rock and a carbonate rock. - Skarn alteration systems are typically zoned. - Skarn genesis essentially involves isochemical contact metamorphism and metasomatism (prograde skarns). eg: Major deposits ofwollastonite are found in Sirohi, Udaipur Dungarpur and Ajmer districts ofRajasthan..
GUMBEITE - Medium to low temperature metasomatic rock. - It is mainly composed ofquartz, orthoclase and carbonate. - It forms as an alteration ofgranodiorite or syenite and is closely associated with W-Cu or Au-U bearèng veins.
PROPYLLTE - Low to medium temperature metasomatic granofels. - It is formed by the alteration ofbasic volcanic rocks; low temperature varities are principally composed ofalbite, calcite and chlorite; high-temperature varities are composed ofepidote, actindite and biotite.
BERESITE - Low temperature metasomatic rock. - It is characterized by quartz, sericite, carbonate (ankerite) and pyrite assemblages resulting from the replacement ofboth igneous and sedimentary protoliths -
Reference ' PRINCIPLES OF IGNEOUS AND METAMORPHICPETROLOGY ' - JOHN D. WINTER