DYNAMIC AND KINEMATIC CONDITIONS OF VARIOUS TYPE GRANITE FORMATION IN AREAS OF TECTONO-MAGMATIC ACTIVIZATION

Shevchuk V.V.

Kiev State University named after Taras Shevchenko, Kiev, Ukraine, shevchuk@univ.kiev.ua

The two main aspects are clearly observed in long history of “granite problem”. These are: (a) matter aspect that is related to searching the petrogenetic ways of extremely mixed association, in structural-textural respect, of largely quartz-feldspar rocks united by 'granite' concept, and (b) dynamic (tectonic, structural) one for determining the factors of force and deformational schemes that ensure formation itself. The necessity of discussing the numerous questions of this big problem is explained both by complexity of the object under investigation and uncertainty of concept 'granite' itself. Denoting the acid holocrystalline rocks of exclusively magma origin, i.e. those ones crystallized from melt by this concept, we do not take into consideration large masses of granitoid formations that did not pass the stage of homogenization, from the one hand. From the other hand, we should still recognize various ways of granitoid magmatism, missing at the same time premagmatic and late-postmagmatic transformations, being sometimes more important for granite formation than melting itself.

Hot discussions, particularly, opposition between magmatists and metasomatists, first of all, demonstrates unperspectivity for search of single mechanism, and necessity to differentiate possible ways of granitoid composition rocks formation (and they are numerous and scales of their occurrence are various) and the main mechanisms of mass granite formation that result in development of the planet granite layer. The geodynamic and dynamic-kinematic aspects are particularly important just for such large scale petrogenesis. Mass granite formation has quite definite spatial-temporary addressing. It is referred to so called areas of tectonic-magmatic activization (TMA) that are named by various terms in papers (V.A.Obruchev, E.Argan, G.F.Mirchinok, E.V.Pavlovsky, M.I.Nikolaev, M.P.Kheraskov, L.King, M.S.Nagibina, Yu.V.Komarov, P.M.Khrenov and many other researchers).

Granites of TMA areas are traditionally referred to the plutono-metamorohic and volcano-plutonic associations. The genetic reconstructions on example of East Transbaikalian fragment of the TMA Mesozoic belt allowed to identify granitoids of both associations as respectively palingeno-metasomatic and metamagmatic ones (Korzhinsky 1972, 1973; Shevchuk 1990). These and those granitoids appear to be differentiated at crust level fluid products of stable potassium-silicium specialization with hard crustal substrate in the first case or largely basite melts of various depth – in the second case. Due to global scales of fluid flows shown by the areal-belt distribution of various type granites in fields of various age TMA, their critical masses likely formed, in view of transmantle migration possibility, on the core – mantle border and made structures under effect of rotational regime in the corresponding stage.

Greatly different depth of some components of both autochthonous palingeno-metasomatic granites and allochthonous miasmatic ones at the exclusively uppercrust level of granite large mass formation complicates the problem of their geodynamic belonging. A necessity of analyzing the geodynamics of core that generates necessary volumes of fluids, geodynamics of mantle that ensures fluid migration and, finally, lithospheric dynamic-kinematic conditions of granitoid petrogenesis and structural formation that is conjugated with it. Traditional reference within geosynclinal concept of granite formation to the late geosynclinal, subsequently early and late orogenic development lost the suggested universality in TMA view, and was added by the ideas of activizational (epigeosynclinal, epiplatform) formations. An establishment of anorogenic complexes that are likely not associated with geosynclinal development became usual together with establishment of orogenic ones. Determination of geodynamic setting that controls mass granite formation (island arc, collisional granites) got actual after conceptual recognition of lithospheric plate tectonics. At the same time, a majority of facts indicate that aureoles of granitoid distribution in the TMA areas are not limited by the only setting, constantly comprising geostructures with various geological pre-history. In addition, practically any geodynamic setting is characterized by combined appearance of various strain-deformational states of respective extent (contraction, extension, shift), including those ones that control certain mechanisms of granitoid petrogenesis and magmatogene structural formation.

The inversional fields of stresses with subvertical orientation of minimal compression axes that are practically responsible for lens-like batholith form in section with structure of granite-gneiss domes and allochthonous plutons as well. As the calculations show, formation of such stress fields gets inevitable near the earth's surface that is free of stresses (Shevchuk 1996, 2002). The reason of their origin can be both tectonic lateral compression in various converegent zones, and positive volumetric effects of thermostability and phase transformations in columns of anomalous thermofluidal flows in no dependence from character of lithospheric structures. Thus, thermofluidal flows appear both the reason of matter transformations that correspond to their geochemical specialization, and one of the efficient factors of force of lithospheric structural formation. Overlapping the mosaics of lithospheric plates of certain age, they coincide with various geostructural zones and geodynamic settings, though they are together with them controlled by dynamics of deep geospheres.

Therefore, wide-scale granite formation, being an attribute of the TMA periodical occurrences, is agreed to planetary differentiational processes and stages of lithospheric plate systematic reconstruction. It realizes due to functioning of the transmantle themofluidal flows only in the upper crust conditions, where formation of inversional stress fields that ensure decompression and increases of volume, being necessary for mass formation of quartz-feldspar associations on the background of thermofluidal orogeny by its nature.

References

Korzhinsky D.S. (1972) Flows of transmagmatic solutions and processes of granitization // Magmatism, formations of crystalline rocks and deapths of Earth. Paper collection of IV All-Union Petrographic Meeting. Part I. M., Nauka, P.144-153.

Korzhinsky D.S. (1973) Metamagmatic processes // Izvestia of USSR Acad. Sci., geol. Series, N12, p.3-6.

Shevchuk V.V. (1990)About origin of porphyry- and rapakivi-granites // Min. paper collection, issue 2, N44, P.104-111.

Shevchuk V.V. (1997) Mesozoic tectonics and magmatism of East Transbaaikalia, a fragment of East Asian orogenic belt // Tectonics of Asia. M., GEOS, P.249-252.

Shevchuk V.V. (2002) Evolution of stress fields during formation of Phanerozoic granite-gneiss domes // Geophys. Journal, v.24, N6, P.220-229.

Shevchuk V.V., Likhachev V.V. (1996) Mathematical model of stress field caused by heat anomaly in resilient media. // Geophys. Journal, v.18, N6, P.74-80.