Geology / petrology: Classification of Granitoids - Research Paper Example

Granitoids and its ifications Introduction Granitoid is a rock which has texture and resemblance to granite rock and many composition like quartz, plagioclase and K- feldspar.A granitoid need to have two criteria that it should be a plutonic rock and needs to contain between 20 percent and 60 percent quartz. The granite is different from granitoid as the latter contains many composites. Granitoides includes all rocks that are rich in plagioclase and quartz. The most identifiable mafic mineral found in granitoids are biotite and hornblende. Hornblende is regarded as amphibole. Other minerals present in granitoids are apatite, Zircon , Fe-Ti oxides ,pyrite, tourmaline,monazite,xenotime and almandine. Granitoids are rich in iron and magnesium which are dark minerals. Granitoid is an assortment of rock which is coarse in texture and predominantly contains feldspar and quartz. Granitoid is classified in to different types according to mineral composition, chemical and isotopic characteristics, petrographical feature and their field. Granitoid can be loosely termed as range of felsic plutonic rocks. The granitoids are found in large volume in continental crust that has been thickened by orogeny. Many might have taken tens of millions of years. The granitoids needs some thermal disturbance to form. There are many classification of granitoids however the most common one is SIAM classification devised by two Australians Bruce Chappel and Alan White Granitoid formation The evolution of granitoid is depended on the magma source composition, tectonic settings, reaction of the magma rocks, time of crystallization , fractionation process at the crystallizatiom time and depth of emplacement. The mineral content of the granitoid depends on the temperature and composition of magma source.. Granitoid has white and black bands of feldspar and mica minerals and is medium to coarse grained. As per (Pandey,2012,pg 1-4 ) “Petrographically the rock consists of potash feldspar (Microline orthoclase, microperthite) bleb like quartz, granet,biotite with accessory apatite” Granitoid is formed when magma or molten rock is pressed between other rocks in the earth’s crust. It when cooled gets crystallized in the deep underground and low cooling process makes large crystals to form. Granitoids generally form large intrusion in the core of mountain ranges and they are generally are surrounded by metamorphic rocks. These rocks are formed due to pressure or heat. . Granitoid is the most abundant form igneous rock found on earth’s surface. These are classified by visual inspection and according to source of origin. Granitoid is called by petrologic what might be for normal people granite because of their mineral composition. Magma or molten rock is that become granitites. As per (Murphy,2007.pg .21-34 ) “Magmas with intermediate compositions can also be produced by mixing at depth of mafic and felsic end-member compositions”. SIAM Classification of granitoid The SIAM classification has S,I,A,M types of granitoids S- Type granitoids S- type granitoids are derived from sedimentary rocks in a phenomenon called anatexis or ultrametamorphis.S – type have a reddish brown color when it comes to appearance. They also have intense pleochroic haloes. The S- type granitoids have the characteristics of being Xenolithe type or minerals not absorbed fully during anatexis.The S – type granitoids have low sodium content and have a MOL ration of greater than 1.1. S- type also have various isotope composition. Their mineralogy is different from other types and they have high AI but no hornblende. They have biotite, muscovite, cordierite and garnet. In the source rock there is high Rb. The initial S r ration is > 0.710 . S- type granitoids are predominant in quartz rich adamellites and grandiorites . S- type is mostly early in the intrusive. S- type granitoids are formed by the partial melting of met sedimentary source rocks in a phenomenon called anatexis or ultrametamorphism. The S- type granites have a high initial 87Sr/86Sr, due to the enrichment of Rb relative to Sr in the pelitic rock. S-type also contain xenolith type minerals which are partially absorbed by the anatexis.In this type, restite milky quartz inclusion is common.One of the example of S- type granite are Sikait leucogranite and Azna granitoid..However SIAM classification of S – type granitoids can be confusing about the granite setting. Anorgenic is related to magmatic event and it is not related to orogenic event. So any magmatic response due to thermal event in preorogenic and pstorogenic is anorogenic.Hence anaorgenic is not alkhaline in all cases. If there is a mantle uprise after orogenic compression may produce positive thermal anomaly which can melt fertile rocks of the crust. In case the metapelitic rock go under anatexis it will give rise to peraluminous s type rocks in anorogenic context so can it be A – type? Fig 1; S type granitoid I – Type granitoid The color of I- type is of chocolate to straw color while the S – type has red brown color. I – type are found in subduction zones and continental margins.They have hornblende rich inclusions and the source region is poor in Rb.. Their initial Sr isotope ratio is less than 0.708. The s- type and I – type are one – stage genetic types because they are directly derived from putative source. I – types can be both metaluminous and peraluminous.The Sr is less radiogenic in I –types and there is a low proportion of sedimentary material in this type. ( Villaseca, 2009,pg1-15) “Biotite isa good discriminant for I-type granites because it plots in subaluminous fields, when compared to biotite from more peraluminous”.This type of granitoids is found in the subduction zone continental margin. They have Ca and Na content in high quantity. It contains hornblende and sphene. They are formed due to the melting of deep crustal igneous rock. These granitoids are formed by partial melting of igneous rocks that have metaluminous nature. I – type granitoids are commonly found in Kosciusko Batholith of southeast AustraliaThe other commonly found I – type granitoids are granodiorite and gabbro. They gabbroic rocks have composition of apatite,zircon, magnetite and granodiorite have magnetite, allanite,apatite and zircon. Fig 2 ; I – type granitoid A – Type granitoid They have source of igneous rocks and have anaorogenic origin. These have mineral compositon same as alkhaline granites. It is believed that they are found in mantle region of earth’s crust. The minerals found in this granite are biotite,annite, and fluorite. A – type granite also are comprise of hypersolves granite. The S and I type granitoids accessory mineral composition saturate earlier than A – types.They are found in non – orogenic setting. They are subaluminous to perkalkhaline and sometime peraluminous. They have iron rich malfic silicates and perthitic feldspars. The A – type granitoid is derived from igneous rockThe A – types are normally pink in color owing to the large volume of brick red _ - feldspar. But there are also cream – white A – types present. According to (King,1996,pg71-91 ) “A-type magmas have been inferred to have low H2O contents, on the basis of crystallization of annite interstitial to abundant feldspar and quartz”. The A – type granitoids found in Lachlan Belt have connection with gabbroic rocks. One example of A- type granitoid is the Pertek granitoid in Taurus orogenic belt in Turkey. Fig 3 ;A – type granitoid M - type granitoids M – type granitoids are mantle derived and can be found in island arc region generally. Hornblende, biotite andK- feldspar and pyroxene are often seen in them. These rocks have porphyry copper and gold mineralization. The granitoids are classified on different basis and their chemical composition, source and mienrology all depicts the types of granitoid. There is other classification of granitoids and it depends on the experts who have classified it and what characteristics they found in different granitites that defined them. The fractioned mantle melts create this granitoids. The M- type granitoids are formed by mantle sources mostly with least contamination from crust. As per ( Mondal,2011,pg 1138-1140) “The definition of M-type, like I–A–S, is also not without flaw. M-type granitoid has not been defined explicitly and it merges technically with the definition of the I-type. The ore – type deposit of M- type granites is porphyry deposit. Apart from SIAM classification the granitoids are also identified on the basis of tectonic settings. Fig 4 ;M – type granitoid Table 1 – SIAM classification Table 2 – Geography and mineral assemblage of granitoid samples ( Speciale,2013.pg 15-19) Sample Location Rock type Mineral assemblage TA01A 40°05′10.7″N, 31°54′35.2″E Quartz monzonite Hbl + Ttn + Ap TA01B same as TA01A Granite Bt + Ap KO01B 40°02′47.3″N, 31°56′44.5″E Granodiorite Hbl + Ttn + Ap KO02B 40°03′00.1″N, 31° 56′41.3″E Quartz monzonite Hbl + Ap KO03 40°03′30.6″N, 31°56′57.2″E Granodiorite Hbl + Ttn + Ap + Cal KO04A 40°03′50.5″N, 31°57′12.4″E Quartz monzonite Hbl + Ttn + Ap KO04B same as KO04A Quartz monzonite Hbl + Ttn + Bt + Ap KO05 40°05′14.0″N, 31°54′ 05.5″E Quartz monzonite Hbl + Ttn + Bt + Ap Table –3 (McClenaghan,1982,pg .1-30) Specimen Rock Type K-feldspar . Plagioclase Quartz Biotite Muscovite Nos of counts Grid size(mm) Grain size (mm) ( Annite) ( Zinnwaldite) (mm) 802640 1 43.45 23.92 25.68 0 6.86- 1998 5 2 742518 1 32.82 23.75 35.94 6.20- 1.24 1764 5 3 \742528 1 37.98 24.84 30.95 5.54- 0.70 1522 5 3 73-694t 1 31.53 30.00 32.23 3.80- 0.80 3000 0.33 0.4 J74-8 1 33.16 29.70 34.64 1.08- 1.34 5000 0.33 0.4 802647 2 39.36 21.56 34.92 3.12- 1.04 1498 5 2 J74-16 2 38.1 23.9 31.8 5.4- 0.6 5000 0.33 0.4 BT3-41 3 29.06 32.04 33.22 5.67 0 2993 5 5 735702 3 35.52 30.69 28.20 5.60 0 1360 5 2.5 735703 3 32.86 30.86 34.96 1.32 0 3686 5 3 802646 4 33.54 22.17 32.49 8.7 3.1 1619 5 3 BT3-45 4 34.26 27.06 36.47 1.46 0.75 1722 2 5 802628 5 41.38 22.85 30.58 3.46 1. 73 3816 2 0.7 802619 5 33.80 29.48 31. 29 3.26 2.17 2154 2 0.7 t includes 1.6% topaz Rock type 1-Equigranular alkali.-feldspar granite 2-Phyric alkali feldspar granite 3-Phyric coarse-grained adamellite 4-Phyric medium grained adamellite 5- Phyric fine-grained adamellite Tectonic classification of granitoid On the basis of the platonic setting the granitoids are classified as follows; orogenic granitoids and anorogenic granitoids. Orogenic granitoids can be further classified as follows- a) Island Arc Granitoids - The island arc granitoids are formed during the subduction of one oceanic plate underneath another oceanic plate. According to ( Patwardhan,262) “ If the modal percentage of quartz, plagioclase and alkali feldspars is only considered, then the Island Arc Granitoids can be viewed also as quartz diorite, quartz monzodiorite, tonalite and granitoids”. In the island arc environment the magamatic process happens only on the upper plate and it is mainly extrusive and balsatic. The Island arc granitoids has however received less attention from scientific world. b) Continental Arc Granitoide – The contintental arc granitoide is placed in the continental or volcanic arc form within the continental crust by subduction of an oceanic plate underneath a continent plate. They can take the form of granitoids, tonalite and granite with alkali feldspar or plagioclase less than 2.0. According to ( Shen,2009,pg.20652-57) “Continental arc crust can thus be derived from magmatic differentiation of juvenile magmas or by recycling of ancient crust that is, remelting of preexisting crustal basement that may have been previously modified by chemical weathering”. c) Continental collision granitoids - The continental collision granitoids are granitic intrusion during the continent to continent collision. Like it has happened in the case of Himalayan or in the case of the Eastern Ghats Region. This granitoid have alkali feldspar or plagioclase less than 2.0 d) Post – orogenic Granitoids - This type of granitoids intruded during the last phase of an orogeny usually after the deformation of the region has stopped. Every class of orogenic granitoid consists of rocks with two different feldspars – the alkali feldspar is perthitic and the plagioclase contains oligoclase. The above mentioned granitoides that is CAG,CCG has biotite while POG has both biotite and muscovite as an minerals and that too in generous amount. Hornblende is also present in all of these classes. Anorgenic granitoids The Anorgenic granitoids are alkali natured granitoids which does not imply to specific source or mode of origin. They are Ca poor but has Sr, Ba and Rb in abundant quantity. According to ( Clemens,1986.pg.17-24) “Anaorgenic type granitoids form subvolcanic plutons or occur as lava flows; ash flows are less common” These are of different source that orogenic granitoids. a) Continental Epeirogenic Uplift Granitoids – The CEUG granitoids has bimodal distribution and and they from epeirogenic and are found in the uplifted region on continental arcs They include granite alkali granite, quartz alkali syenite and quartz syenite.They are alkali or peralkaline. b) Post Rifting Granitoids – PRG are found among rifts. These granitoids have a mineral composition of alkali granite, quartz alkali syenite and quartz monozonite . c) Oceanic Plagiogranites – The felsic plutonic rocks which are found among mid – ocean ridges and ophiolites paleocollisional zones are oceanic plagiogranites. According to ( Silantyev,2010,pg.369-383)“The origin of oceanic plagiogranites is widly discussed in world scientific community. The condition of formation of these rocks that from thin vein bodies and dikes in the gabbro – peridotite substrate of slow – spreading mid – ocean ridges imply the participation of aqueous fluid in their parental magmatic system”. Fig 5 - Granitoid Fig 6 – Magma melting Conclusion Granitoid is a rock consisting quartz with a granular structure and crystals and in come with different size and colors . They granitoids are classified in SIAM by Chappel and white after studying them exclusively. The granitoids are classified based on their source of origin, appearance and chemical and mineral composition. The granitoids are also calssified according to their tectonic settings. As per ( Ephiram,2012,pg994-1007) “ Field and petrographic characteristics confirm that the granitic parent rock most likely had magmatic origin, and thatone of the mechanisms of emplacement of the parent granitic rocks may have been the process of piecemeal stoping”.The granitoid need to be studied more by geologists as there is more to be explored about them. Granitoids takes place on collision belts and they are recognized by their spatial association and mineral deposit. Research is still going on regarding the granitoid types and its sources. As time progress, more classification will arise as more is discovered about this magma rocks. Bibliography Clemens, J D. (1986)Origin of an A-type granite: Experimental constraints. American Minerologist 71 317-24. Ephraim, B. E. (2012). Granitoids of the older granite suites in Southeastern Nigeria. , 3(2), 994-1007. King, P. L. (1996). Characterization and Origin of Aluminous A-type Granites from the Lachlan Fold Belt, Southeastern Australia. Oxford Journals, 38(3), 371-391 McClenaghan. M.P, (1982)DISTRIBUTION AND CHARACTERIZATION OF GRANITOID INTRUSION IN THE BLUE TIER SEA.Pap Geol Survey Tasm 4 1-30. Mondal, M. A. (2011). Classification of granitic rocks: a march from alphabet soup to. Current Science, 100(8), 1138-1140 Murphy, B. (2007). Igneous Rock Associations 8. Geoscience Canada, 34(1), 21-34. Patwardhan, A. M. (2010). The Dynamic Earth System (2nd ed., p. 162). New Delhi: PHI Learnin Private Limited. Pandey, A. K. (2012). Geo chemical analysis and petrogenesis of granite gneisses around. Golden Research Thoughts, 1(10), 1-4. Speciale, P. A. (2013). Zircon ages from the Beypazarı granitoid pluton (north central Turkey): tectonic implications. Taylor & Francis, 10(4), 15-19. Shen, B. (2009). The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation. PNAS, 106(49), 20652-20657. Silantyev, S. A. (2010). Ocean plagiogranites as a result of interaction between magmatic andhydrothermal system in the slow spreading mid- ocean ridges. Petrology, 18(4), 369-383. Villaseca, C. (2009). Multiple crustal sources for post-tectonic I-type granites. Department of Petrology, 7(2), 1-15. Read More