Bea, F.

Corretgé, L. G.

Fershtater, G. B.

Repositorio da Universidade da Coruña

Cadernos Lab. Xeolóxico de LaxeCoruña. 2000. Vol. 25, pp. 147-149A Systematic typology of granitoid rocksfrom major element composition. I: The Upper Silica RangeTipología sistemática de granitoides a partir de su composición en elementos mayores. I: El rango más alto de SiF. BEA (1); L.G. CORRETGÉ (2); G.B. FERSHTATER (3).(1) Department of Mineralogy and Petrology. University of Granada. Campus Fuentenueva.18002 Granada, Spain.(2) Department of Geology. University of Oviedo. 33005 Oviedo, Spain.(3) Institute od Geology and Geochemistry of the Acad. Sciences of Russia; Pochtovi per. 7, Ekaterinburg, 620151, Russia.The composition of granitic rocks is acomplex function of many parameters,among which the nature of source mate-rials, P-T-Xvolatiles  conditions of partialmelting, and the efficiency of melt segre-gation are of remarkable importance. Allthese factors, in turn, are closely related tothe geotectonic environment where grani-tes originated. The recognition of suchrelationship gave a great impulse to theunderstanding of granitic rocks variety(Pitcher, 1982), and is the basis of granitetypology as it is currently understood(Pitcher, 1979; Pitcher, 1982), which esta-blishes the following fundamental granitetypologies: I- and S- types (Chappell andWhite, 1974, A-type (Loiselle and Wones,1979) and M-type (Pitcher, 1983). This S-I-A-M scheme reflects in a simple waybroad genetical concepts concerning thesource rocks in spite of the, sometimestroublesome, indetermination in their cha-racteristic and definition (Miller, 1985;Clarke, 1992 ). The classification focuses aswell attention on granite types, tectonicsetting, source mineralogy and sometimes,metallogenic specialization regardless ofwhether or not the ore specialization ofgranites depends upon other criteria(Hannah and Stein, 1990).This paper is the first result of a world-wide survey on granite chemistry underta-ken in order to recognize the differentcompositional types of granites of the S- I-A- M- types on a statistical basis. It focu-ses on the silicic margin of the granitoidscompositional spectrum. A data set of4,200 samples with SiO2 > 70 wt. % fromall over the world has been worked out.Only major elements from this data setwere used in the classification. We testedthree methods of distiguishing betweengranites: The raw major elements, theirnormative components and various petro-chemical parameters. The best results havebeen obtained by using four parameters: (a)Aluminium Saturation Index, ASI= mol.(Al2O3/[CaO+Na2O+K2O]); (b) Calc-Alkaline Index, CAI = mol.(CaO/[CaO+Na2O+K2O]); (c) Potassium-Sodium Index, KNaI = mol.(K2O/[K2O+Na2O]); and (d) Iron-Magnesium Index, FeMgI= mol.(FeO/[FeO+MgO]). In order to reduce thedimensionality of the problem, we perfor-med a principal components analysis onthese four parameters. Scores of samples onthe Principal Components are given by:FD1=10*(0.1601*ASI-0.0354*CAI+0.048*KNaI-0.026*FeMgI); [1]FD2=10*(0.0061*ASI-0.0434*CAI+0.0145*KNaI+0.1236*FeMgI); [2]The first principal component is domina-ted by the Aluminium Saturation Index andaccounts for almost 50 % of the total varian-ce of the system. The second principal com-ponent is dominated by the Iron-MagnesiumIndex and accounts for nearly 30% of thetotal variance. The plane defined by bothprincipal components is used as the maintool for classification. The plot of a subset ofsamples whose typology is well knownallows one to split this area into five regions,in which to plot different granite types: (a)Primitive-types; (b) A-types; (c) I-types; (d)SC-types (low Fe/[Fe+Mg] S-types); (e) SG-types (high Fe/[Fe+Mg] S-types).Discrimination between primitive-types, isalso possible by means of a K2O vs Na2Oplot, in which fields for M-types, I-types,Magnitogorsk-type granites and ophioliticplagiogranites are clearly identified.148 F. Bea, et al. CAD. LAB. XEOL. LAXE 25 (2000)XXas Jornadas S.E.M. A systematic typology of granitoid rocks 149Figura 1.BIBLIOGRAFÍACLARKE, D. B. (1992). Granitoid Rocks. London,Chapman & Hall. CHAPPELL, B. W. AND WHITE, A. J. R. (1974).Two contrasting granite types. Pac. Geology. 8:173-174.HANNAH, J. L. AND STEIN, H. J. (1990).Magmatic and hydrothermal processes in ore-bearing systems. Ore-bearing granite systems,petrogenesis and mineralizing processes. H. J.Stein and J. L. Stein. Geological Society ofAmerica. 246: 1-10.LOISELLE, M. C. AND WONES, D. R. (1979).Characteristics and origin of anorogenic grani-tes. Geol. Soc. Am. Abst. Prog. 11: 468.MILLER, C. F. (1985). Are strongly peralumimousmagmas derived from pelitic sources? Journal ofGeology. 93: 673-689.PITCHER, W. S. (1979). The nature, ascent andemplacement of granite magmas. J. Geol. Soc.Lond. 136: 627-662.PITCHER, W. S. (1982). Granite types and tecto-nic environment. Mountain Building Processes(ed. K. Hsu). Academic Press, London: 19-40.PITCHER, W. S. (1983). Granite: typology, geolo-gical environment and melting relationships.Migmatites, Melting and Metamorphism (ed.:M.P. Atherton & C.D. Gribble). Shiva Pub.Ltd., Cheshire: 277-285.WHALEN, J. B.,CURRIE, K. L. ANDCHAPPELL, B. W. (1987). A-type granites:geochemical characteristics, discrimination andpetrogenesis. Contrib. Mineral. Petrol. 95: 407-419.

Tipología sistemática de granitoides a partir de su composición en elementos mayores. I: El rango más alto de Si F. BEA

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Tipología sistemática de granitoides a partir de su composición en elementos mayores. I: El rango más alto de Si F. BEA

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