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2011 |
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Òåçèñû ìåæäóíàðîäíîé êîíôåðåíöèè |
Abstracts of International conference |
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About the possible mechanism of formation of massifs of anorthosite-rapakivi-granite association in the East-European platform(EEP).Galetskiy L.S. *, Remezova Å.À.*, Komskiy N.Ì.** * Institute of Geological Sciences of the National Academy of Sciences of Ukraine; geos@geolog.kiev.ua; remezova-e@mail.ru; **State geological commission of experts for projects and budgets of Ukraine; nkom@i.ua
Utillizing the information of geochemical treatment more than 670 silicate analyses of rocks from Korosten’ and Korsun'-Novomirhorod plutons, the model of formation of massifs of anorthosite-rapakivi-granite association is developed. In the basis of the examined model there is the cumulative nature of plagioclase; as a result of evolution of melts density there was layering in massifs and the dynamic specialization on titan was revealed.
The massifs of anorthosite-rapakivi-granite association form an enormous belt, associated to the western margin of EEP. Korosten’ Korsun'-Novomirhorod, Salminskiy, Vyborg, Riga and other the massifs are known today. There are different opinions to the origin of these massifs. V.S.Sobolev examined Korosten’ pluton as a complicated multi-phase intrusive body, where two phases are selected: basic rocks and granites. The last one includes three groups of rocks: 1) granites of main phase - rapakivi, biotite- amphibole rapakivi-like and biotite; 2) endocontact and contaminated granites and veins in basic rocks; 3) veinstones are in granites. According to his the opinion, the second phase is divided into the stages, and the magma of rapakivi has high ferruginousity and alkalinity[3]. A.A.Polkanov selected in composition the basic rocks of pluton the layered texture and described the crystals of plagioclase, which have signs of the free moving in melt. Thus he marked the consonantal bedding of granites and basic rocks with wide development of rhythmically layered intermediate varieties of rocks(monzonites, quartz monzonites, quartz diorites, diorites, small-ovoid rapakivi). In addition, granites often cut anorthosites with formation of intrusive contacts. The separate varieties of granites cut each other in a certain sequence: grey rapakivi is cut by rose biotite-hornblende rapakivi, and those, in turn - by regular-graned biotite granites. On this basis three phases of granitoids of Korosten’ pluton are selected[1,2]. I.e. the previous researchers marked contradictory correlations of rocks in pluton, which are hardly explained by a polyphase. Monographers (1978) also adhered to opinion of polyphase of plutons, by an initial magma for a anorthosite-rapakivi-granite formation they considered an intermediate type – between tholeitic and alkaline basalts. On the calculated composition it corresponds with plagioclase porphirite. E.V.Sharkov considers Korosten’ pluton as large layered massif. Similar opinions exist also for Korsun'-Novomirhgorod pluton[5]. According to our researches the main paradigm of Korosten’ block magmatism consists in the following. Magmatism within the limits of Korosten’ pluton is related to the collision of Sarmatian and Fennoscandian plates, the final stage of which was signed on becoming of structures and magmatism of north-western part of the Ukrainian shield and took place in a time of a 1.83 -1.78 milliard of years. This process was accompanied both a compression and tension of lithosphere, which on some time interrupted pressure of plates one on other, or took place right after their consolidation. To such periods of tension the forming of riftogenious structures is related on the EEP, and row of manifestations of magmatism. Collision sutures is expressed as Volyn’-Dvina of volcanic-plutonic belt, prolate in north-eastern direction on more than 2000km. In the south-western part of this megastructure the Osnitsa-Mikashevichy belt and boundary Suschany-Perha tectonic-magmatic and tectonic-metasomatic zone of activation are selected The collision phenomena are the precursors of super-plume magmatism. On its existence indirectly high rock ferruginousity of Korosten’ complex, exposed cells of crust-mantle mixture under pluton and to the west of it can indicate. Enriching of magmas on iron took place due to the processes on a border crust-mantle, where plum is forming. Enriching on water and forming of fluid streams is also related to the area of collision, where a contact with salt waters of the heated material was. The fluid streams are concentrated on the proper ÐÒ-levels and change mechanical properties of matter. Such levels are L and N horizons of overhead mantle(accordingly under North Eurasia they are located on depths 80-100 and 180-240êì). They are thin layered areas with alternation of high and low speeds of Vp seismic waves. The presence of fluids is fixed after high conductivity. Such changes of mantle‘s matter are conditioned by the partial melting and metasomatism of mantle’s material due to the presence of fluids, and faults are the original «trigger mechanism» of this process[7]. In the north of Ukrainian shield such complicated geodynamic system is the North Ukrainian zone of tectonic-magmatic activation selected by L.S.Galetskiy with which the manifestations of activation structures (in particular, alkaline-granit, leucogranitic, alkaline metasomstites with rare metal and complex ore mineralization) are connected[6]. Suschany-Perha zone also arose up in connection with the collision of afore-mentioned plates. The regulator of fluid streams is an astenosphere which is examined as a self-regulating system, processes in which are conditioned the speeds of matter import to the roof and outflow combined in natural way as magmas and over- astenospheric fluids. Their composition depends on pressure on the roof of astenosphere and concentrations of components in melts. In this direction, forming of Korosten’pluton and subsequent magmatism within the limits of its framing is laid. It should be noted that rock alkalinity grows towards the area of collision. In this direction there is a change of formations from gabbro-anorthosite, ore gabbro, gabbro -syenite to syenite with granosyenites, by subalkaline and alkaline-granit final, members. Utillizing the information of geochemical treatment more than 670 silicate analyses of rocks from Korosten’ and Korsun'-Novomirhorod plutons, the model of formation of massifs of anorthosite-rapakivi-granite association is developed. In the basis of the examined model there is the cumulative nature of plagioclase; as a result of evolution of melts density there was layering in massifs and the dynamic specialization on titan was revealed. In the first phase of formation anorthosites(vas’kovichy type) were intruded. Introduction of the second portion of melt(anorthosites of II phase) took place then. The processes of differentiation took a place in a magmatic chamber, and gradually as a result of this process the layers appeared on the scheme of layered intrusions: layers of anorthosites, gabbro and most melanocratic varieties, formed from remaining melt after floating of plagioclase. The petrochemical researches showed that forming of Korosten’ pluton is related to cumulative plagioclase. Magnesium-ferrous trend begins very poorly to show up only in gabbro - anorthosites, where it changes «cumclative- plagioclase » gradually, and further, in transition to leucogabbro and gabbro, increases. That for the last differences of rocks cumulative minerals are become by ferrous - magnesium. Bimodality of Korosten’ complex, as follows from researches, specifies on two differentõ types of magma - acid and basic composition. Processes of both contrasting magma generation can have only one reason, even if they, being connected spatally, flow on different depths (maternal melt for the of Korosten’ complex granites arose up within the limits of low-medium crust reservoir). Delay, «standing» of magma in a crust foresees the formation of granitic melts. Assumption of two parental magmas does not set up requirements of their initial composition difference from the prevailing rock types of complex - from rapakivi, similar to them granites and from gabbro - anorthosites. As composition of initial magma - parental for all of rocks of gabbro – anorthosite formation of complex, we are accept middle composition, probably it is the most widespread variety of gabbro - anorthosites from all of group of these rocks. It is composition of variety, with the content variations of alumina within the limits of 18.7-22 % that corresponds to high-aluminous basalts. Activity of plagioclase component goes down at growth of pressure, there is melting of feldspar and composition of magma aspires to high-aluminous. Appearing, afterwards, on more high levels – in the layer of crust, in the situation of considerably more low pressures, a high-aluminous magma will become unsteady and will throw down surplus of plagioclase in cumulate. For remaining melts an accumulation of ferrous - magnesium minerals and ore(ilmenite, titaniferous magnetite, apatite) will be characteristic. The formation within the pluton’s limits of specific ore-concentrating structures with forming of industrial deposits of titanium and phosphorus – layered intrusions(Stremihorod, Fedorivka, Kropivenka in Korosten’, Nosachiva in Korsun'-Novomirhorod plutons and others) promotes the re-distribution of ore minerals. In relation to the origin of hybrid rocks of plutons, it should be noted that the hypothesis of melts fractionating does not explain their origin sufficiently full. The transitional rocks were formed by mixing of two contrasting magmas - acid and basic. The substantial for understanding of correlations between granites and basic rocks of complex there is considerable distinction of their soliduses. Such distinction means that acid and intermediate on composition melts will “live” substantially longer than basic ones. Formed by them the late phases of acid rocks, such, for example, as Lezniky granites, or intermediate composition – monzonites and syenites of Bol'sheviska complex, in relation to gabbro - anorthosites and gabbro, already distinctly more late. Outliving thermosetting of gabbroids the acid melt, directed in appearing in these coolings off gabbroids the cracks, will give secant veins and apophysis. Similar secant veins, apophysis and other small bodies, do not specify on more young, than for gabbro - anorthosites age of rapakivi-granite formation on the whole. So the problem of age-dependent correlations in the association of rocks of gabbro – anorthosite and rapakivi-granite formations for the decision requires a concretization, with an accent on comparison of separate constituents of these formations – rocks and, even, separate areas of rocks. I.e. the relations betwen acid and basic rocks are more intricate: possibly, it concerns about alternation of granites and gabbro intrusions. The model developed by us can be specified and utillized for explanation of origin and other massifs within the limits of the EEP, in particular, mostovsky and grodno complexes on territory of Byelorussia and others.
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