Parallelism of composition’s changeability of ultramafic-mafic and salic magmatic associations in the conditions of “tension – contraction”
Sveshnikov Kirill Igorevich
Russian People’s Friendship University, Moscow, Russia
It is well known that magmatic rocks form genetically interconnected combinations in the Nature. Usually such combinations are named as magmatic complexes, but the practice of this term usage shows that volumes of complexes, composed of similar rock sets distinguished by different researchers may, differ essentially even at a partition of the same rock sets in one area. A paramount task is the objective distinguishing of the divisions corresponding to sets of genetically or paragenetically connected rocks, arisen as a result of one completed magmatic process. Such combinations of rocks are steadily repeating in space irrespectively of other rock combinations and may be regarded as elementary or stable. Elementary magmatic associations may be united in certain systematic groups, differing by sets of rocks, mineral paragenesà and petrochemical features. It is very important to underline that members of different groups can not belong to one age generation of magmatic bodies (for example, the massives of normal and alkaline granitoids or tholeitic volcanites and orthopyroxene-bearing gabbroids cannot arise from the same portions of magmatic melts) and can not be spatial variations of the same complexes. At the same time members of one systematic group usually form lateral (spatial) variations of the same magmatic complexes. Elementary associations, belonging to one systematic group, often differ between themselves by average quantity of SiO2 (for example, qurtzdiorite-granodioritic – granodiorite-granitic – granitic associations). In this respect they are resembling the isomorphic mineral lines. Investigations of elementary associations and their groups open new opportunities for detection of some new petrological regularities.
Some of such ultramafic-mafic groups of elementary associations are known for a long time as magmatic series (tholeitic, calc-alkaline, subalkali basic, alkali basic). Other groups were not allocated up till now (gabbronoritic, anorthozitic, clinopyroxenitic, orthopyroxenitic). Each of these groups differ by an individual petrochemical trend on the "*FeO - MgO - CaO" diagrams . Salic associations form two big groups - low-calcic and high-calcic. In associations of low-calcic group the attitude of calcium to iron changes from 1:2 up to 1:3. This group includes 4 different types of associations: the two-mica granitoid, granitic, granosyenitic, alkaligranitoid. Associations of these types differ by main mineral paragenesa and by position on the different petrochemical diagrams (“relative aluminiferocity vs silica", K2O/Na2O, *Feo/MgO). Groups of ultramafic-mafic and salic elementary associations in the systematic relation may be located in a sequences from peraluminous to alkaline (table 1). As it is seen from the table, there is a certain parallelism in changes of many important features in both groups (that is, in mantle’s and crust’s derivatives). It is possible to assume, that the composition of the fluids participating in magmatic processes is the unique factor, which equally operates in conditions of the Earth's crust and the mantle. All petrologists believe, that the increasing of alkalinity in magmatic melts is connected with the increasing of CO2 saturation in fluids. The generalization of the literary data (F.A. Letnikov et all., 1977) has shown, that the "Ñ/Í" ratio grows in both sequences from peraluminous up to alkaline groups of associations (table 2).
Table 1. The parallelism of main features in systematic sequences of basic and granitic groups of associations
It is known, that alkaline associations usually arise in the conditions of a stretching, while calk-alkali or Andian type (mafic-salic) - in stress conditions. Representatives of tholeitic group may arise in various conditions - oceans and continents, in conditions of a stretching (middle ocean ridges) and stress (island arches). The smaller amount of the data is known about a geodynamic position of peraluminous granitoids and gabbroids. For example, peraluminous granitoids in Himalayas are considered syncollisional . Very similar Early Proterozoic two-mica granites in the southern part of the East European platform basement also have some spatial links with zones of collision though they are located outside of them. Numerous gabbronorite bodies also associate spatially with the same zones . These data allow to suppose, that the systematic sequence from peraluminous to alkaline mantle’s and Earth crust’s magmatic associations corresponds to gradual change of geodynamic conditions from compression up to a stretching.
Table 2. Ratio CO2/H2O in elementary magmatic associations by F.A. Letnikov et all., 1977
In all cases the quantities of H2O in fluids are higher then qauntities of CO2. So, traditional explanation, that magmatic melts, saturated by H2O, can not form volcanic eruptions, is not true. The capacity for volcanic flows depends on the relative quantity CO2 in fluids. These data allow to suppose, that the connections of magmatic melts containing low quantity of CO2 with stress conditions and the connections of melts with higher quantities of CO2 with stretch conditions may depend on relative sizes of atoms “H” and “C”. As atoms of “C” have much larger radius then “H”, magmatic melts, saturated with “C”, have more difficulties in penetrating in stress zones. That is why alkaline melts are attracted towards stretch zones.
The age sequence of magmatic associations (complexes, series), emerged during one tectonic-magmatic stage, usually corresponds to the trend from "neutral" (tholeitic or granitic, occupying medial position in above-mentioned systematic sequences), to peraluminous or alkaline (occupying extreme positions in the same sequences). Associations of the two last mentioned types (that is peraluminous and alkaline) form forbidden paragenesa - they never arise during the same tectono-magmatic stage. So, it may be supposed, that in the Nature exist H – and C – tectonic structures.
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