The archean system in india

geology of India

1. The Archean system in India it is made of ancient rocks like gneisses, schist and granites. General characters 1. They are unfossiliferous. It shows that there was no life at the time of formation. 2. Generally at high metamorphosed. 3. Contorted and subjected intense folding and faulting. 4. Intruded by igneous rocks to ultrabasic composition They are also called as “fundamental gneiss” or basement complex due to their complex structure and composition. The Archean gneisses are typically magmatic gneisses that contains alternate bands of amphibolites and tonalities.

2. Gneiss: Gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than  schist . It is formed by the metamorphosis of  granite , or  sedimentary  rock. Gneiss displays distinct foliation, representing alternating layers composed of different minerals. However, unlike  slate  and schist, gneiss does not preferentially break along planes of foliation because less than 50% of the minerals formed during the metamorphism are aligned in thin layers. Because of the coarseness of the foliation, the layers are often sub-parallel, i.e. they do not have a constant thickness, and discontinuous. Gneiss is typically associated with major mountain building episodes. During these episodes, sedimentary or felsic  igneous  rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such episodes. Gneisses from western Greenland comprise the oldest crustal rocks known (more than 3.5 billion years old). Gneiss is an old German word meaning bright or sparkling. Foliation: distinct plane of weakness in the rock. Foliation is caused by the re-alignment of  minerals  when they are subjected to high pressure and temperature.

3. Schist: Schist is medium grade metamorphic rock, formed by the metamorphosis of  mudstone  / shale, or some types of  igneous  rock, to a higher degree than  slate , i.e. it has been subjected to higher temperatures and pressures. Granite:  is a common type of  felsic   intrusive   igneous rock  that is granular and  phaneritic  in texture. Granites can be predominantly white, pink, or gray in color, depending on their  mineralogy .

4. Felsic: In   geolog y ,  felsic  is an adjective describing  igneous rocks  that are relatively rich in elements that form  feldspar  and  quartz . [1]  It is contrasted with  mafic  rocks, which are relatively richer in  magnesium  and  iron . Felsic refers to  silicate minerals ,  magma , and  rocks  which are enriched in the lighter elements such as  silicon ,  oxygen ,  aluminium ,  sodium , and  potassium . Felsic magma or lava is higher in  viscosity  than  mafic  magma/lava. Felsic rocks are usually light in color and have  specific gravities  less than 3. The most common felsic rock is  granite . Common felsic minerals include quartz,  muscovite ,  orthoclase , and the sodium-rich  plagioclase  feldspars ( albite -rich). A  phanerite [1]  is an  igneous rock  whose  microstructure  is made up of  crystals  large enough to be distinguished with the unaided  eye . (In contrast, the crystals in an  aphanitic  rock are too small to be seen with the naked eye.) Phaneritic   texture  forms when  magma  deep underground in the  plutonic  environment cools slowly, giving the crystals time to grow. Phanerites are often described as  coarse grained  or  macroscopically  crystalline .

5. Distribution of Archean rocks in India the Archean rocks cover nearly two thirds of the peninsular India Exposure: South India, M.P, Bihar & Orissa Gujarat & Rajasthan Assam plateau & Central Himalayan region Zones of Archean rocks of the Peninsular India. Archean rocks of south India. Archean rocks of Eastern Ghat . Archean rocks of M.P & Maharastra . Archean rocks of Bihar & Orissa. Archean rocks of Rajasthan.

6. Thank you This is joseph R Lalfakawma