Abstract
Due to the unavailability of suitable software, a new computer program TecD (Tectonomagmatic Discrimination) was written in Visual Basic for using four sets of new discriminant function based diagrams published during 2004-2011. This bilingual (both in English and Spanish) program evaluates igneous rock geochemistry data in 20 different multi-dimensional diagrams (4 sets of 5 diagrams each), automatically counts the number of samples plotting in different tectonic fields, computes success rates (%) for a given area or set of rock samples, provides scalable vector diagrams that can be opened and modified in different commercial software, and presents a synthesis of this application in a final report. Three examples are presented to highlight the use of TecD. Ocean island setting is inferred for ~56 Ma basic rocks from Faroe Islands (Atlantic Ocean), mid-ocean ridge for ~2700 Ma Archean Abitibi greenstone belt (Canada), and arc setting for ~2950 Ma Mallina basin (Australia). Additional criteria for the interpretation of these diagrams are also briefly discussed.
Highlights
Tectonomagmatic discrimination diagrams have been in use in igneous petrology almost since the advent of plate tectonics theory (Rollinson, 1993; Verma, 2010)
The first diagrams for igneous rocks were proposed by Pearce and Cann (1971, 1973) and since there have been many proposals (e.g., Wood, 1980; Shervais, 1982; Pearce et al, 1984; Cabanis and Lecolle, 1989; Vasconcelos-F. et al, 1998, 2001)
Except for the first set (Agrawal et al, 2004), which was obtained from linear discriminant analysis (LDA) of adjusted major-element concentrations from SINCLAS computer program (Verma et al, 2002), these diagrams are based on LDA of natural logarithm-transformed element ratios
Summary
Tectonomagmatic discrimination diagrams have been in use in igneous petrology almost since the advent of plate tectonics theory (Rollinson, 1993; Verma, 2010). Because of complex arithmetical calculations involved, their use is likely to be cumbersome and less frequent as compared to simple bivariate or ternary diagrams All these simple diagrams are, plagued by incoherent statistical treatment of compositional data; besides, the tectonic field boundaries in them were drawn by eye (e.g., Aitchison, 1986; Agrawal, 1999; Aitchison et al, 2000; Thomas and Aitchison, 2005; Agrawal and Verma, 2007; Verma, 2010; Verma et al, 2010). Deficiencies in several existing discrimination diagrams for sedimentary rocks have been documented (Armstrong-Altrin and Verma, 2005) From this discussion, it is clear that only the new discriminant function based, multi-dimensional diagrams obtained from LDA comply with all statistical requirements and provide satisfactory answers to the need of tectonic discrimination. A suitable computer program could be helpful for an efficient use of such discriminant function based diagrams
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