Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Representation of chemical distinction of potassium alkaline rocks, kimberlites, lamproites, and lamprophyres based on RHA information language

Krasnova N.I. *, Burnaeva M.J. **

* St.-Petersburg State University, Russia

** FGUP “VNIIOkeangeologia”, St.-Petersburg, Russia

nataly_krasnova@rambler.ru

Using the traditional approaches to a problem of classification of many magmatic rock types we had shown that, e.g., the alkaline rich rocks, far not always allow distinguishing and identifying them unequivocally. When chemical composition of different groups of rocks is displayed on TAS diagrams, their fields very often strongly overlap each other, and use of the consecutive scheme for classification of magmatic rocks, according M. Le Bas and A. Streckaisen (1991) and recommendations of IUGS and the Petrographic Code approved by Russian IPC, also causes some questions. So, it is not clear why some borders between groups of compositions are appointed on some "round" figures (10, 50, 75, etc. %%) of concentrations of any components (oxides, minerals in modal, and sometimes normative volume %, or their incorporated groups – e.g., foids – F, or mafic minerals – M). As other classification principle, distinctions of rocks by their composition and texture, agpaite factor are used also, etc. In existing classifications, the general and uniform system of description, ordering, and also an opportunity of comparison with known objects of new compositions have not been incorporated.

Use of RHA information language-method developed more than 40 years ago (Petrov, 1971) has allowed to begin drawing up on its basis of catalogues of chemical composition of rocks (see file CollStart_En-2012 with data for more than 5,650 inputs) and mineral composition in volumetric % (Vol. %) according to calculation of normative contents of minerals (601 analyses of lamprophyres + 475 of alkaline rocks) and modal compositions (457 analyses). All these datasets represented in Excel tables, are available at web site: http://www.geology.pu.ru/, further “Method RHA”. We shall remind that the first part of the composition description – R – is a sequence of system components on decreasing of their concentration in this system. As a result of such ordering, the rank formulas (Rmin) – sequences of minerals like AlbtOrtcNeph are received, that corresponds to volume mineral parts (Vol. %) in rock Albt> Ortc> Neph>.…. Sets of rank formulas are ordered under a certain "alphabet". As the “alphabet”, the sequence of these minerals received in their chemical R-classification of all minerals (R-Min-Catalogue-2012.rar file at the same Internet address) serves. The description of the method is resulted both in the Internet, and in many publications (Petrov, Krasnova, 2010; Petrov, Moshkin, 2012). Thus, the offered method allows ordering unequivocally all compositions without any exception (see table, the right column), that known classifications do not provide, despite many attempts on there detailed elaboration and refinement.

Examples with data both on chemical and mineral compositions of the same rock samples are given extremely rarely, that complicates correct comparison of the received rank formulas (Rmin). Based on revision of Rmin sets received on recalculation of normative and modal mineral composition of different rock groups using Petros program, it is possible to draw the following conclusions. 1) For the majority of magmatic rocks, even on their normative compositions it is possible to define more or less authentically their classification group. An exception is made with rocks, the main components of which are water-bearing or other minerals, that are not involved in the program of CIPW recalculation (for example, micas, amphiboles, melilite, etc.). 2) At calculation of norms, in many cases virtual components, such as K₂SiO₃, Na₂CO₃, corundum in granites, larnite in ultrabasic and basic rocks, etc. (in the table they are italicized) are obtained. 3) Modal compositions allow to define unequivocally not only a group of rocks, but also to allocate in their limits the varieties differing in the content of minor and/or accessory minerals (their symbols are in the right column of the table on 3-4 ranks).

In Petros computer program used by us for data processing, composing of various diagrams (including traditional, such as TAS) is also rather convenient. The more informativeness of the dependences made for chemical rock compositions on ratios of elements, instead of oxides, has been revealed. Therefore, on the diagram reflecting the composition of potassium-bearing rocks in coordinates of atomic % K/Al relation, fields of points for different rock types are allocated individually enough. The majority of compositions of dikes from the Kola Peninsula and picrites from Spitsbergen correspond on these diagrams to composition of lamprophyres and gabbroids.

After critical revision of such full collections of RHAmin data for different rock groups from different regions of the world, it will be possible to develop a rational nomenclature for them. As a specification of the composition description in the form of a rank formula, values of complexity (H or En) and purity (A or An) of the composition, taking into account the degree of rock monominerality can be used.

Creation of a representative RHA-catalogue of such full data for all rocks will provide an opportunity of their unequivocal identification on the real mineral composition, opened for addition of new, previously unknown types. In the future, such classifications on RHA method can become a perspective task for different groups of geologists all over the world.

References

Le Bas M., Streckeisen A.L. The IUGS Systematics of igneous rocks. // J. Geol. Soc. London. 1991. 148, 825-833.

Petrov T.G. Substantiation of a variant of the general classification of geochemical systems. // Vestnik LGU. 1971. Ser. 18. No 3. 30-38. (In Russian).

Petrov T.G., Krasnova N.I. R-dictionary-catalogue of chemical compositions of minerals. S.-Petersburg: Nauka Press. 150 p. (In Russian).

Petrov Tomas G., Moshkin Sergey V. RHA – System for Coding of Discrete Distributions and Their Alteration Processes. // Proc. The 3rd International Multi-Conference on Complexity, Informatics and Cybernetics IMCIC 2012.2012 pp. 12-16.