Although for ultrabasic and basic magmas a plethora of tectonomagmatic diagrams have been used, with the exception of one bivariate diagram for refined tectonic setting of orogenic andesites, none is available for highly abundant intermediate magma. We present 3 sets of discrimination diagrams obtained from the correct statistical methodology of loge-ratio transformation and linear discriminant analysis. All major element log_e-ratio variables in 3664 samples, only immobile major and trace element log_e-ratio variables in 1858 samples, and immobile trace element log_e-ratio variables in 1512 samples were used. These diagrams with probability-based tectonic field boundaries and high success rates (about 69%-96%, 63%-100%, and 64%-100%, respectively, for diagrams based on all major elements, immobile major and trace elements, and immobile trace elements) were first tested for fresh and highly altered rocks. The expected tectonic setting was indicated from our diagrams. The probability-based decisions and total percent probability estimates can fully replace the actual plotting of samples in the diagrams. The probability calculations were then used for tectonic discrimination of 7 case studies of Archean to Proterozoic rocks. An island arc setting was indicated for the Wawa greenstone belt (Canada), implying the existence of plate tectonic processes during the Late Archean, for western Tasmania (Australia) during the Cambrian, and for Chichijima Island (Bonin Islands, Japan) during the Eocene. Similarly, an arc setting (indecisive island or continental type) was obtained for south-central Sweden during the Paleoproterozoic and for Adola (southern Ethiopia) during the Neoproterozoic. A within-plate setting was inferred for the Neoproterozoic Malani igneous complex, Rajasthan, India. A collision setting was indicated for the Alps (France-Italy-Switzerland) during the Late Carboniferous. Modeling of likely as well as extreme processes indicates that these diagrams are robust against postemplacement compositional changes caused by analytical errors, element mobility, Fe-oxidation, alteration, and petrogenetic processes.
VERMA, SURENDRA PAL and VERMA, SANJEET K.
"First 15 probability-based multidimensional tectonic discrimination diagrams for intermediate magmas and their robustness against postemplacement compositional changes and petrogenetic processes,"
Turkish Journal of Earth Sciences: Vol. 22:
6, Article 3.
Available at: https://journals.tubitak.gov.tr/earth/vol22/iss6/3