Migmatite and metamorphism
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This document discusses migmatisation and metamorphism. Migmatisation is the process where a metamorphic rock like gneiss partially melts and the melt recrystallizes, forming a mixture. Migmatites contain both metamorphic and igneous parts. They often form under extreme heat during metamorphism. Metamorphism transforms rocks through heat, pressure and fluids. Pelitic rocks like shales and mudstones are important in metamorphism as they develop distinctive minerals and commonly produce migmatites at high grades. Migmatites are found widely in Precambrian terranes and the Himalayas.
Migmatite and metamorphism
- 2. CONTENTS Migmatisitation Migmatite Structures of Migmatites Metamorphism Association of metamorphism of pelitic rocks with Migmatites Occurences Conclusion Reference
- 3. MIGMATISATION The process of formation of a migmatite is called migmatisation. The term “migmatite” was introduced by Sederholm(1908) to designate certain gneissic rocks which looked like mixed rock . It is created when a metamorphic rock such as gneiss partially melts, and then that melt recrystallises into an igneous rock, creating a mixture of the unmelted metamorphic part with the recrystallised igneous part. Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre- existing rocks. Many migmatites probably represent the partial fusion of the metamorphic host during extreme metamorphism. Migmatite also can form near large intrusions of granite when some of the magma is injected into the neighbouring metamorphic rocks.
- 4. Mehnert classified the following parts that can generally be distinguished in insitu migmatites: 1) Paleosome-unaltered or slightly modified parent rock or country rock i.e., a gneiss. 2) Neosome-newly formed rock portion. a) Leucosome-Containing more light minerals with respect to the paleosome. b) Melanosome-containing mainly dark minerals such as biotite, hornblende, cordierite, garnet, sillimanite and others . 3) Mesosome- Rock portion of a migmatite,that is intermediate in colour between leucosome and melanosome. If present, the mesosome is mostly a more or less unmodified remnant of the parent rock (protolith) of the migmatite.
- 6. MIGMATITE Migmatite is a rock that is a mixture of both metamorphic and igneous rock. Migmatites often appear as tightly incoherently folded(ptygmatic folds) dikelets, veins and segregations of light colored granitic composition called leucosome, within dark colored amphibole and biotite rich material called the melanosome. Migmatite is a rock composed of a metamorphic (altered ) host material that is streaked or veined with granite rock. The name means mixed rock. Such rocks are usually gneissic (banded) and felsic rather than mafic in composition. They may occur in regional scale in areas of high grade metamorphism.
- 7. STRUCTURES OF MIGMATITES DIETZONITIC(NET) STRUCTURE Paleosome is interlocked by net-like narrow veins of neosome. Two sets of Granite veins combine to form a net-like pattern. Broken Hill area, New South Wales, Australia.
- 8. SCHOLLEN(RAFT) STRUCTURE Paleosome occur as raft-like fragments partially dissolved in the neosome and showing distinct borders. The rafts are paleosome which occur in a coarse-grained, light coloured host. Ashuanipi subprovince, Northern Quebec, Canada.
- 9. PHLEBITIC(VEIN) STRUCTURE The paleosome is irregularly traversed by veins of neosomes hence resulting in a veined structure. Veins of light coloured neosome traversing dark coloured paleosome. Nemiscau, North Quebec, Canada.
- 10. STROMATIC (LAYERED) STRUCTURE The light coloured layers of leucosomes and dark layers of neosomes generally occur parallel. The neosomes are not entirely even,but thicken and thin out irregularly. The thickest leucosome is at the bottom and it has the largest grain size. Glenelg River Complex, Victoria, Australia.
- 11. SURREITIC (DILATION) STRUCTURE Dilatent sites are produced as crystals in the paleosome were pulled in a direction parallel to the foliation in the host. Tabular segments of leucosome which are oriented parallel in mafic paleosome. Georgian Bay, Grenville Province, Canada.
- 12. AGMATIC (BRECCIA) STRUCTURE: Agma means fragment. Here fragments of paleosome are surrounded by narrow veins of neosome. Brecciation is due to simple fracturing of paleosome. Dark coloured paleosome surrounded by veins of neosome Quetico Subprovince, Canada.
- 13. FOLDED STRUCTURE It is formed by compressional pressure acting on a rock. Here the incompetent layers are bent in contrast to competent layers thus resulting in buckling and bending of layers. The light coloured neosome are bent in contrast to grey coloured paleosome. Colorado Front Range, U.S.A.
- 14. PTYGMATIC STRUCTURE Here The folds are highly disharmonic which thicken and thin out irregularly. The fold at the bottom is thickened and at the top it is thinned out. Saint Fulgence, Grenville provine, Canada
- 15. OPTHALMIC(AUGEN)STRUCTURE Here neosome material is distributed within the paleosome in the form of augen (eye-like) structures. chitradurga schist belt, karnataka.
- 16. STICTOLITHIC (FLECK) STRUCTURE This is a rare structure. Here the mafic minerals are concentrated in flecks leaving around them a mantle poor in mafic minerals. Orthopyroxene flecks. Southern closepet region , karnataka.
- 17. METAMORPHISM Metamorphism is the process by which pre-existing rocks are transformed or altered in a solid state under relatively high pressure, temperature, and/or hot circulating fluids. The effects of metamorphism are the formation of new minerals, changes in shape and size of mineral grains, and the development of new structures in the rock.
- 18. ASSOCIATION OF THE METAMORPHISM OF PELITIC ROCKS WITH MIGMATITES Pelitic rocks are derived from clay-rich sediment and are of particular importance in metamorphism because they develop a wide range of distinctive minerals. The term ‘pelitic rocks’ is used to signify all slaty or schistose rocks. They are usually alumina rich rocks such as shales, mudstones. Migmatites are form in siliceous metasediments , metabasic rocks etc but are best developed in pelitic rocks . The chemical composition of pelites can be represented by the system K2O-FeO-MgO-Al2O3-SiO2-H2 The chemistry of pelitic metamorphic rocks change with increasing metamorphic grade. Fe, Mg and Ca contents increase, and K content decreases with increasing metamorphic grade.
- 19. • Most pelitic rocks are foliated type in composition with quartz and feldspars. These are formed due to the regional metamorphism Eg: Slate, Phyllite, schist , gneiss.
- 20. OCCURENCES The bulk of the earth’s migmatites occur in extensive terranes, often of Precambrian age. Eg: Eastern Ghats, Orissa. The higher Himalayan crystalline in Sikkim consist of pelitic migmatites interlayered with calc- silicate rock. Migmatised amphibolite, a few metres are traceable in the supracrustal rocks and in major part of peninsular gneiss of Archean dharwar craton of southern india. Cu can be found in migmatitic terrain at Kalyadi and Aladhalli in Karnataka. Kolar district consist of immense expanse of migmatitic gneiss. Migmatites and gneisses are found in Thirthahalli taluk, Shimoga district .
- 21. CONCLUSION The process of formation of a migmatite is called migmatisation. Paleosome, Neosome, Mesosome are some important terms used in migmatites. Metamorphism is the process by which pre-existing rocks are transformed or altered in a solid state under relatively high pressure, temperature, and hot circulating fluids. The term ‘pelitic rocks’ is used to signify all slaty or schistose rocks. They are usually alumina rich rocks such as shales, mudstones. The chemistry of pelitic metamorphic rocks change with increasing metamorphic grade. Most pelitic rocks are foliated type in composition with quartz and feldspars. These are formed due to the regional metamorphism.
- 22. REFERENCE Helmut G. Winkler, 1976, Petrogenesis of metamorphic rocks, springer-verlag New York Inc. Pp 278-282. Myron G. Best, 1986, Igneous and Metamorphic petrology, W.H. Freeman and company, New York, Pp 120,393, 425, 533. ASHWORTH, J.R. (1985): Migmatites. Ashworth, J. R. (editor). Blackie, Glasgow, 302 pp. MEHNERT, K.R. (1968): Migmatites and theorigin of granitic rocks. Developments inPetrology 1. Elsevier, Amsterdam, 393 pp. www.angelfire.com/sc3/farooqs/askot/migmatites.html. en.wikipedia.org/wiki/peninsular-gneiss. en.wikipedia.org/wiki/pelite