Chemographic projections and normative calculations for silica-undersaturated igneous rocks

Classification of silica-undersaturated igneous rocks represents of one long-standing problems in igneous petrology. Mineralogical classification of foid-bearing rocks has been formally set [1,2] but no continuously applicable systematics and universal classification criteria have been developed for alkaline ultramafic rocks including melilite-bearing varieties and special rock groups [3,4,5]. Chemical classification is primarily based on the total-alkali – silica (TAS) diagram [2], but condensation of multiple components (CaO, MgO, MnO, FeO, Fe₂O₃, Al₂O₃, TiO₂, P₂O₅) in the diagram’s origin precludes its effective use for descriptive classification and petrogenetic interpretation involving mafic and ultramafic rocks. Universal availability and accuracy of whole-rock geochemical data together with fine-grained or glassy character of many volcanic rocks on one hand and historical origin of petrographic classifications in mineral mode and involvement of mineral-melt relations in magma evolution and crystallization on the other hand require consistent and universal link between the chemical and mineralogical approaches. This is a component transformation problem, which can be approached from several different perspectives: (i) component transformation sensu stricto preserving the composition space dimensionality, (ii) reduction of space dimensionality involving projecting or condensing components, usually for graphical applications or for condensation of complex natural compositions into simplified synthetic (e.g., experimental) systems, and (iii) subsection of the space leading to multiple combinations of new components; this approach is embodied in norm calculations. The widely applied tool – the CIPW norm [6,7] – suffers from several inadequacies when applied to silica-undersaturated rocks: (i) persistence of anorthite to critically undersaturated state, (ii) absence of melilite or its end-members, and (iii) incomplete or incorrect feldspar-foid compatibility relations. In this contribution we develop a condensed composition space to represent principal chemical variations in silica-undersaturated rocks. The condensation offers uniform treatment of diverse heteromorphic relations in dependence on temperature, pressure or water activity. The breakdown of plagioclase to aluminous clinopyroxene with decreasing silica activity and subsequent transformation of clinopyroxene to melilite is visualized in chemographic projections via olivine and nepheline, involving thermodynamically based phase relations as a function of silica activity. Finally, we define intermediate members of feldspar, nepheline, clinopyroxene and melilite solid solutions and develop a more comprehensive, quasimodal normative calculation for anhydrous silica-undersaturated igneous assemblages. This approach offers successive, rigorous steps for (i) overall classification and interpretation of chemical variations independently of mineral assemblages, (ii) projective analysis for comparison of chemical variations with experimental or thermodynamic phase relations, and (iii) algorithm for normative calculation approaching modal associations. This provides a uniform basis for both descriptive classification as well as genetic interpretation of silica-undersaturated magmas and rocks.

References: [1] Streckeisen A., 1965. Geol. Rundsch. 55, 478-494; [2] Le Maitre R.W., ed., 2002. Igneous Rocks. A Classification and Glossary of Terms, Cambridge Univ. Press; [3] Woolley A.R., et al., 1996. Can. Mineral. 34, 175-186; [4] Dunworth E.A., Bell K., 1998. Can. Mineral. 36, 895-903; [5] Tappe S., et al., 2005. J. Petrol. 46, 1893-1900; [6] Cross W., et al., 1902. J. Geol. 10, 555-690; [7] Janoušek V., et al., 2016. Geochemical Modelling of Igneous Processes, Springer.