Experience of classification on mineral composition using of the RHA-method of lamprophyres –
the dyke rocks in consolidated areas of the earth's crust
Burnaeva M.Yu.*, Krasnova N.I.**
*FGUP “BNIIOkeangeologia”, St.-Petersburg, Russia, ** St.-Petersburg State University, Russia
Lamprophyres are the group of widespread igneous rocks, which have no plutonic analogues. They form dikes, silles and various sort extrusions in folded areas of consolidated mobile belts of an Earth's crust or in areas of a platform mode at transition to ascending character of tectonic movements. Some geologists unite the lamprophyres with kimberlites and lamproites in so-called «lamprophyre clan» by their resembling textural-structural features and presence phenocrysts of mafic minerals. Correct diagnostics of all these often externally similar rocks is rather actually. Interest to rocks of this group is supported in connection with found in them of deep xenoliths and diamonds; besides the existential connection between mesothermal deposits of gold and lamprophyres is established, that has led to creation of a hypothesis of gold transportation by lamprophyres from mantle depths (Rock, 1991). There is often uncertain a division of lamprophyres to subgroups, what is underlined in decisions of the Subcommittee on systematisation of igneous rocks IUGS (Le Maitre, 2002).
In the present work, the attempt to classify lamprophyres not only on their chemical composition (see Burnaeva, etc., 2007), but also by results of recalculation of analyses on CIPW-normative minerals is made. For ordering results of recalculations of a databank of rock’s chemical analyses has been used the RHA-method (Petrov, Krasnova, 2010). Competency of such approach to an opportunity to use the normative mineral rock’s compositions for their express identification and classification earlier has been shown on an example on composition of granites, diorites, gabbro and some other igneous rocks, which demonstrate the distinct differences (Tab. 1). The full version of these results displays the file RHA-Min comp_alkaline rocks.rar on its separate pages, alongside with the R-classified mineral composition of alkaline rocks (Krasnova et al., 2009). Earlier the possibility of creation of unequivocal rock’s ordering on their mineral composition with use of rank formulas (R) was shown on an example phoscorites and carbonatites – the rocks for which the standard nomenclature has not been developed (Krasnova et al., 2004).
Abbreviations hereinafter: Ab – albite, An – anortite, Ands – andesine, Ap – apatite, Cal – calcite, Crn – corundum, Di – diopside, Hyp – hypersten, Ilm – ilmenite, Ksl – kalsilite, Lbr – labrador, Lct – leucite, Mag – magnetite, Ne – nepheline, Ol – olivine, Ort – orthoclase, Pl – plagioclase, Prv – perovskite, Qtz – quartz.
Normative compositions of 601 chemical analyses of lamprophyres, taken from many references have been calculated with the help of program Norm-4. Results are represented in volumetric % (Vol. %), that most corresponds to visual definition of rock. The full version of these data is available on a site: <http://geology.pu.ru/>, file RHA-Min comp_lamprophyre rocks.rar. The RHA-catalogue, where each analysis of rock is presented in the rank formula form – R, in which minerals are located in decreasing order of their molecular quantities, has been as a result compiled. The rank formulas, as peculiar "words", are ordered under certain "alphabet". As the alphabet, the sequence of minerals in their chemical R-classification serves (Petrov, Krasnova, 2010). Three predominant minerals, occupying the first ranks are used only (tab. 2) in the given preliminary analysis of such lamprophyre mineral compositions classification. Let us note, that plagioclases in these displayed tables have not been dismembered, whereas in the full version on the above mentioned web site the plagioclase number everywhere is specified. The main phenocrysts minerals are marked with the bold font designates and in brackets the character of quantitative content of different feldspars – Ort and Pl – is shown after the name of a lamprophyre variety.
Comparison of the received rank formulas of lamprophyre’s mineral compositions, expressed in Vol. %, allows to draw following conclusions which we have tried to compare as to recommendations IUGS (Le Maitre, 2002), and Russian IPC (Petrogr …, 2008). Two groups are possible to allocate among all diversity of lamprophyre mineral compositions. The first is with prevalence of Cal, Ol and feldspathoid Ne or Lct, to which apply the damkjernites, alnoites and polzenites. These rocks are depleted in silicium, which naturally shows the set of normative minerals; besides presence of calcite in the first ranks is one of diagnostic features for these lamprophyres. The second group has the prevalence of feldspars: Pl, Ort and Hyp, what is typical to malchites, spessartites, sannaites, camptonites, monchiquites, minettes, vogesites and kersantites. The majority monchiquites is similar on composition with camptonites, differing only glassy character of a groundmass. In this II group the prevalence Ort above Pl for minettes and partly for vogesites is characteristic, and essential role Qtz is visible by malchites. It has appeared, that some normative compositions lamprophyres do not correspond to modal at all, known inquiry data – they are marked by italics. Probably, that these rocks have been wrongly specified.
If to compare the rank formulas received for lamprophyres (tab. 2) to those for igneous rocks of normal and alkaline suites (tab. 1), the affinity of some rank formulas for malchites and spessartites to R for granites (Pl-Qtz-Hyp, Pl-Qtz-Ort) and diorites (Pl-Hyp-Qtz) is found out. The R’s of camptonites and monchiquites are similar to that of diorites (Pl-Hyp-Ort, Pl-Ort-Ol) and gabbro (Pl-Di-Ol, Pl-Di-Hyp). This circumstance enables to lead parallels between igneous rocks of normal suite and rocks of lamprophyre group, specifying their identification. In future undoubtedly, it is necessary to use not CIPW mineral norms, but minerals objectively existing in these rocks, considering presence of micas, amphiboles, melilites etc., for correct distinction of lamprophyre types by the RHA-method. Mineral species of mafic minerals and number of plagioclase are necessary to specify for definition of lamprophyre type. Creation of the representative RHA-catalogue of such data sets for lamprophyres will provide an opportunity to identify these rocks unequivocal by their real mineral composition. The classification of all existing rock types – igneous, metamorphic and sedimentary –could be a perspective theme for different groups of geologist worldwide with using this RHA-method.
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