high technology (BEST 1982, WILSON 1989). ... BARAGAR 1971, BEST 1982, WILSON 1989, Le MAITRE et al. 1989, ROLLINSON 1993). FROST et al. (2001) ...
Using the Rittmann Serial Index
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N. Jb. Miner. Abh. 2007, Vol. 184/1, p. 95–103, Stuttgart, August 2007, published online 2007 © by E. Schweizerbart’sche Verlagsbuchhandlung 2007
Using the Rittmann Serial Index to define the alkalinity of igneous rocks Xue-Ming Yang With 2 figures and 1 table
Abstract: The Rittmann Serial Index [σ = (Na2O + K2O)2/(SiO2 – 43), units in wt.%] (Rittmann 1957, 1962) is used as a convenient petrochemical parameter to discriminate igneous rock series on the basis of alkalinity: calcic- (σ < 1.2), calcalkaline- (1.2–3.5), alkaline- (3.5–8.8), and peralkaline-series (> 8.8). For silicic (SiO2 > 70%) rocks with Na2O > K2O, (2 x Na2O) is used to replace the total alkalis in the above equation for σ calculation. Based on a simple calculation of the σ value, each whole-rock sample corresponds to a specific igneous series. The discriminant results are compatible to the assignment for igneous rock series originally defined by the Peacock Index (Peacock 1931), and the method presented here is numerical and relatively easy to handle. Although this method is applicable only to igneous rocks with SiO2 higher than 43 wt.%, it can cover most igneous rocks ranging from basic through intermediate to acid rocks. The alkalinity of an igneous rock series is not only related to tectonic setting, but also it may be used as an indicator for mineralization. Therefore, the Rittmann Serial Index may be utilized as a supplementary tool in aiding tectonic setting identification and mineral exploration, and in particular, applied to exploration for mineral deposits associated with granitoid intrusions and (or) complexes. Key words: alkalinity; the Rittmann Serial Index (σ); Tectonic setting; mineralization; exploration; granite
1. Introduction The chemical composition of igneous rocks, in particularly their alkalinity, has long been recognized to reflect their sources and tectonic settings, and can be used as an important parameter for mineral exploration. For instance, calcalkaline rocks are usually associated with subduction processes (volcanic arc and/or continental arc) and may be associated with porphyry copper (gold) deposits, whereas alkaline and peralkaline rocks are commonly related to an extensional regime, within plate environment and may produce rare earths, niobium and tantalum minerals, which are the essential materials used in modern high technology (Best 1982, Wilson 1989). The alkali-lime index of Peacock (1931) and the AFM diagram (cf. Best 1982, Thompson 1982) are of little use in identification of the alkalinity of granitoids, because they considered only a few chemical parameters and because the granitoid rocks have a narrow variation in composition (Stussi 1989). Stussi (1989) adopted the classification scheme of De La Roche (1980) to divide DOI: 10.1127/0077-7757/2007/0082
granitoid rocks from the French Variscan into calcalkaline, subalkaline, and alumino-potassic suites (series). However, this classification is too complicated, and the distinction between calcalkaline, subalkaline, and alumino-potassic series is equivocal on the multiple-cationic plots. Stussi’s alumino-potassic suite actually represents a mixture of peraluminous and potassic (atomic K > Na) granitoid rocks, causing confusion of the original definition on the alkalinity of igneous rocks (Peacock 1931). Petrochemically, igneous rocks can be divided into two general groups, namely, alkaline and subalkaline series on the basis of total alkalis versus silica (TAS) diagram; the subalkaline series is further subdivided into calcalkaline and tholeiitic (calcic) series (Miyashiro 1970, Irvine & Baragar 1971, Best 1982, Wilson 1989, Le Maitre et al. 1989, Rollinson 1993). Frost et al. (2001) modified the Peacock Index (Peacock 1931) and established a new empirical diagram (MALI vs. SiO2) to determine the alkalinity of granitoids based on a large database of granitoids that are mainly from North America. The MALI defined in their classification is equal to (Na2O + K2O – CaO) (unit 0077-7757/07/0082 $ 2.25 © 2007 E. Schweizerbartsche Verlagsbuchhandlung, D-70176 Stuttgart
