SiO2'–CaO/(CaO + K2O) (S'CK) Diagram as a Petrochemical Tool for Deciphering the Tectono-Magmatic Characterization of Granitoid Suites

Abstract

The SiO₂'–CaO/(CaO + K₂O) (S'CK) diagram is an empirically derived major element-based equivalent to the modal IUGS alkali feldspar–quartz–plagioclase classification scheme for granitoids. It employs the content of SiO₂ and CaO/(CaO + K₂O) ratio to approximate the IUGS classification diagram and a normative-based Q'–ANOR plot. Four trends have been superimposed onto the SiO₂'–CaO/(CaO + K₂O) diagram based on published datasets from the Peninsular Ranges (calcic: C), Tuolumne (calc–alkalic: CA), Sherman (alkali–calcic: AC), and Bjerkreim-Sokndal (alkalic: A) batholiths, which were employed to constrain the positions of the C–CA, CA–AC and AC–A suite boundaries on the SiO₂ versus (Na₂O + K₂O – CaO) (or modified alkali–lime index, MALI) granitic classification diagram. A merit of the SiO₂'–CaO/(CaO + K₂O) plot is identifying rock types comprising a suite and their relative abundances. The distinguished projections of five typical granitoid assemblages, which are summarized by Bonin et al. (2020), demonstrate the ability of SiO₂'–CaO/(CaO + K₂O) diagram to decipher their petrogenesis. The SiO₂' –CaO/(CaO + K₂O) plots for the plutonic suites of ‘known’ tectonic settings can reveal their evolution paths and the lithological statistics. Accordingly, it is suggested that the SiO₂'–CaO/(CaO + K₂O) plot can distinguish the tectonic environments of plutonic suits by comparing the plutonic suites or batholiths of ‘unknown’ tectonic context to the published datasets from granitoid suites formed within ‘known’ tectonic settings. The modified SiO₂'–CaO/(CaO + K₂O) diagram links the bulk chemical composition of granitoid suites to the likely source, magmatic evolution, and tectonic setting; thus, it may be a useful tectono-magmatic classification scheme for granitoid suites.