Use of the RHA information languagefor systematisation of alkaline and some other rock types on their modal mineral composition Krasnova N.I.* & Burnaeva M.Yu.** *Saint-Petersburg State University, Saint Petersburg, Russia ** FGUP “BNIIOkeangeologia”, Saint Petersburg, Russia 2012
Using the traditional approaches to a problem of classification of many magmatic rock types of we had shown, that, e.g., the alkaline rich rocks, far not always allow distinguishing and identifying them unequivocally. By displaying of chemical composition of different groups of rocks at TAS diagrams their fields very often strongly overlap each other.
synnyrites Khibina alkaline rocks lamproites minettes Kola dikes gabbro kimberlites The chemical composition of these rock groupssome better shows this diagram. K > Al Al > K K at.% Composition of some K-bearing rocks at diagram K / Al at.%
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, not clearly why some borders between groups of compositions are appointed on some "round" figures (10, 50, 75, etc. %%) 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 composition, agpaite factor are used also, etc. in existing classifications the general and uniform system of the description, ordering, and also an opportunity of comparison with known objects of new compositions have not been incorporated. In existing classifications the general and uniform system of the description, ordering, and also an opportunity of comparison with known objects of new compositions have not been incorporated.Use of information language-method RHA developed more 40 years ago (Petrov, 1971) has allowed to begin drawing up on its basis of catalogues of a chemical composition of rocks (see file CollStart_En-2012 with data for more than 6410 inputs) and mineral composition in volumetric % (Vol. %) according to calculation of normative contents of minerals (601 analyses of lamprophyres + 475 of alkaline rocks) and now modal compositions (860 analyses).This offered method allows ordering unequivocally all compositions without any exception. The address for our data: http://geology.spbu.ru/department/scientific/rha-language-method
Alphabet forRHA Table Fragment ofRHA Table of Modal Mineral rock composition (total an. = 865)
Fragment ofRHA Table of Modal Mineral ultrabasic rock composition Fragment ofRHA Table of Modal Mineral acidic rock composition Real comp. Microcline Microcline Except of rank formula(ranks 1-4 + 5-6), the RHA description includes the calculation of complexity (H orEn), which shows the degree of monominerality composition,and purity (A orAn) of rock’s analysis.
Fragment ofRHA Table of Modal Mineral urtite-ijolite-melteigite series rock composition
Modal mineral composition ofurtite-melteigiterock series at Neph-Aegn diagram Neph-68.9, Aegn-20.8, Titn-4,Vol.% ijolite Neph-69.6, Aegn-21.2, Titn-5.6 urtite Aegn Vol.% IUGS: Urtite has more than 70% of Neph; ijolite has 30-70% of Neph. For points shown by arrows will be better more common name urtite-ijolite. The names for rock compositions, which overlap each other, should be revised more careful.
So, on the presented material it is visible, that different on mineral composition of group of rocks evidently differusing the RHA system, that, naturally, reflects their visual diagnostics. Difficulties arise only at incorrect, or uncertain diagnostics of rockformingminerals. Thus, for instance, often authors specify only presence in rock of plagioclase, without definition of its number, and also pyroxeneor amphibole, not explaining to which group it is necessary to its attribute. Specification of these data will demand attraction of different experts – petrographers and mineralogists.
Comparison ofRHA Tables of Modal and Normative Mineral rock composition, calculated using Norm-4 1. The first (dominating) minerals are mostly similar by calculation both of modal and normative mineral composition for different rock types. Sometimes the minerals in normative composition are rearranged as compared with modal rank formula. 2. Some virtual components, as leucite – Lect, K2SiO3, Na2CO3, corundum, larnite etc., are received sometimes at norms calculation. 3. The best conformity of modal and normative mineral composition is observed for gabbroid rock series, but it is senseless to calculate the normative composition of such rocks as dolomite-bearing carbonatites or phoscorites, melilite-bearing and some other rock types.
After critical revision of such full collections RHAmin data for different groups of rocks from various regions of the world, it will be possible to work out for them the rational nomenclature. Both tables of RHA-data, and the various plots constructed on their basis, help to diagnose correctly rocks on the mineral composition determined in volumetric %, to reveal erroneous diagnostics of some of rocks. Creation of the 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 new, before unknown types. In the future such classifications on method RHA can become a perspective task for different groups of geologists of all world.
Thank you for attention! Natalia Krasnova Marina Burnaeva