GEOGACETA, 66, 2019 A chemical approximation to the modal QAPF and normative Q' (F')-ANOR classification of the igneous rocks based on their SiO2-CaO-K2O content Una aproximación química a la clasificación modal QAPF y normativa Q' (F')-ANOR de las rocas ígneas, basada en su contenido en SiO2, CaO y K2O Pere Enrique and Sergi Esteve Departament de Mineralogia, Petrologia i Geologia Aplicada. Facultat de Geologia. Universitat de Barcelona. C/ Martí i Franquès s/n, 08028 Barcelona, España; [email protected], [email protected] ABSTRACT RESUMEN Magmas can form plutonic or volcanic rocks of the same chemical com- Los magmas pueden dar origen a rocas plutónicas o volcánicas de la position. The modal composition of the plutonic rocks can be quantified but misma composición química. La composición modal de las rocas plutónicas in the case of volcanic rocks this is not always possible, because of its puede ser cuantificada pero en el caso de las rocas volcánicas no suele ser aphanitic texture and presence of glass. These characteristics prevent a min- posible por su textura afanítica y presencia de vidrio. Estas características eralogical classification in equivalent diagrams for both types of rocks. To re- impiden una clasificación mineralógica en diagramas equivalentes para solve this problem, the IUGS recommends the use of chemical TAS diagram ambos tipos de rocas. La IUGS recomienda el uso del diagrama químico TAS in cases where obtaining the modal composition is not possible, in an at- en los casos en que la obtención de la composición modal no sea posible, tempt to adjust its nomenclature to that represented in the QAPF modal di- tratando de ajustar su nomenclatura a la del diagrama modal QAPF. Una agram. A best approximation is obtained with the normative diagram mejor aproximación se consigue con el diagrama normativo Q'(F')-ANOR en Q'(F')-ANOR in which the anorthite and the orthoclase are used as a dis- el que se usa la relación anortita-ortosa como factor discriminante. En este criminating factor. In this work, the chemical SiO2-100·CaO/(CaO+K2O) di- trabajo se presenta el diagrama químico SiO2-100·CaO/(CaO+K2O) que agram is presented, which largely reproduces the aforementioned normative reproduce en gran parte la clasificación normativa citada pero de forma sim- classification but in a simplified form since it uses only three discriminating plificada, utilizando solamente tres componentes químicos discriminantes chemical components (SiO2, CaO and K2O). (SiO2, CaO y K2O). The delimitation and nomenclature of the fields has been undertaken La delimitación y nomenclatura de los campos se ha realizado de forma empirically using as an example the normative diagram and plotting the empírica tomando como ejemplo el diagrama normativo y la representación analyses of typical igneous rocks. The approximate limit of silica saturation de análisis de rocas ígneas características. El límite de saturación en sílice has been drawn as a straight line joining the feldspathic compositions, be- aproximado se ha dibujado como una recta que une las composiciones fel- tween orthoclase and anorthite despáticas, comprendidas entre la ortosa y la anortita. Key-words: classification of igneous rocks, SiO -100·CaO/(CaO+K O) diagram, 2 2 Palabras clave: Clasificación de rocas ígneas, Diagrama SiO2-100·CaO/(CaO+K2O), QAPF diagram, Q'(F')-ANOR diagram, igneous petrology. Diagrama modal QAPF, Diagrama normativo Q'(F')-ANOR, petrología ígnea. Geogaceta, 66 (2019), 91-94 Recepción: 1 de julio de 2018 ISSN (versión impresa): 0213683X Revisión: 25 de abril de 2019 ISSN (Internet): 2173-6545 Aceptación: 24 de mayo de 2019 Introduction to determine the exact composition of the to their modal mineral content that could rock through the usual microscopic study. be represented in the double QAPF triangle. Since igneous rocks are constituted by In order to unify criteria in the classifi- In the case of volcanic rocks, the first mineral matter, their study and classification cation and nomenclature of igneous rocks, problem was to decide whether their clas- have been based on the precise determina- an international commission (IUGS Subcom- sification should be based on mineralogy or tion of the minerals and their textural rela- mission on the Systematics of Igneous chemistry given the difficulty, or even the tionships. However, a large number of rocks, Rocks) was created and began its work in impossibility, of obtaining a representative especially volcanic and hypabyssal, are lar- 1970 (Le Maitre et al. 2002). As regards modal composition. It was also considered gely made up of glass. This characteristic plutonic rocks it was agreed that they complex to obtain a calculated mineral prevents the quantification of the minerals should be named and classified according composition that reproduced a realistic Copyright© 2019 Sociedad Geológica de España / www.geogaceta.com 91 GEOGACETA, 66, 2019 P. Enrique and S. Esteve composition of the rocks. Finally, a consen- elements that serves to obtain an appro- turation line on silica is approximate. All the sus was reached on the basis of two princi- ximation to the QAPF nomenclature of granitoids and rocks with quartz are above ples: a) if modal compositions can be common igneous rocks is proposed. The this line, whereas the rocks with feldspa- obtained, volcanic rocks should be classified underlying idea is to take directly the dis- thoids and olivine are located below it. and named according to their position in criminating major elements used in the The delimitation of the lines of the the QAPF diagram; b) if modal analyses are normative diagram Q'(F')-ANOR and ana- fields was carried out empirically using a li- not available, chemical parameters that give logous parameters to obtain an empirical mited number of analyses of characteristic a result comparable to the mineralogical compositional equivalence, maintaining, rocks taken from the bibliography. QAPF classification should be used. After as far as possible, the original modal no- The provenance of the samples used considering several chemical diagrams, it menclature. The major components used is as follows: a) Cox et al. (1979, pp. 402- was decided to use the TAS (alkalis-silica) are SiO2, CaO and K2O. Unlike the TAS dia- 406), some characteristic plutonic and diagram of Le Maitre (1984), slightly modi- gram, which uses SiO2, Na2O and K2O, this volcanic rocks; b) Carmichael et al. (1974, fied by Le Bas et al., (1986). However, this diagram takes into account the calcium p. 499), East African alkaline rock series; diagram does not strictly comply with the that is part of all the plagioclase in the c) Wager and Brown (1967), plutonic tho- second principle agreed by the commission modal QAPF diagram and avoids sodium, leiitic intrusion from Skaergaard, East Gre- itself since neither the fields obtained nor which is part of both plagioclase and al- enland; d) Enrique (1990), Ferrés and the nomenclature are consistent with those kali feldspar. Enrique (1996), hercynian batholith from of the QAPF classification. Thus, terms such Catalan Coastal Ranges, NE Iberian Penin- as basaltic andesite, trachydacite, tephri- Methodology sula; e) Ferré and Leake (2001), plutonic phonolite, picrobasalt, etc. do not have an Mg-K series from Corsica; f) Carmichael et equivalent nomenclature in the modal QAPF In order to establish the classification al. (1974, p. 405), alkaline volcanics from diagram. For these reasons, volcanic rocks fields of the igneous rocks (non-ultramafic) Tenerife, Canary Islands; g) Carmichael et (classified in the TAS) cannot be translated simulating the normative diagram Q'(F')- al. (1974, p. 414), Hawaiian lavas; h) Car- to a simulated QAPF classification and com- ANOR, a new diagram was constructed in michael et al. (1974, p. 400), Galapagos pared with the plutonic rocks of equivalent which the silica is placed on the ordinate tholeiitic lavas; i) Enrique and Toribio composition. In addition, to correctly na- axis and the proportion (2009), alkaline lavas from Olot, NE Ibe- ming the rocks in certain fields, it is neces- 100·CaO/(CaO+K2O) on the abscissa axis. rian Peninsula. sary to obtain the CIPW normative The silica content is taken directly from the composition (Le Maitre et al. 2002). By con- analytical result and distributes the rocks in The SiO2-100·CaO/(CaO+K2O) trast, the normative Q'(F')-ANOR diagram acid, intermediate, basic and ultrabasic (Le diagram of Streckeisen and Le Maitre (1979) Maitre et al. 2002). The percentage ratio (Q'=quartz/quartz+feldspars, F'=feldspa- between the CaO and the K2O of the abs- As a result of the study of the graphic re- thoids/feldspathoids+feldspars) constitutes cissas establishes an approximation of the presentation of the samples mentioned a very remarkable approximation to the proportion between the anorthite and the above, a series of compositional fields were QAPF modal classification, both in the sum of normative orthoclase and anorthite. established in the SiO2-100·CaO/(CaO+K2O) choice of discrimination parameters and in The agreement is not identical since in the diagram that largely reproduce the modal terminological equivalence. Its conceptual norm, the calcium of the anorthite is distin- classification of the double triangular QAPF scheme is almost identical, given a) the in- guished from that which is part of the mafic diagram of Le Maitre et al. (2002). The re- compatibility of quartz and feldspathoids, minerals, which it is not possible in the pro- presentation in rectangular form of the and b) the relative proportion between al- posed diagram. However, one advantage of same fields using normative minerals (Strec- kali feldspar (in this case normative ortho- the diagram is that it can classify the rocks keisen and Le Maitre, 1979) is the one that clase) and plagioclase (represented by the with only three oxides of the major ele- has been used as a model in the construc- anorthite).