GEOPHYSICAL AND GEOCHEMICAL ASPECTS OF GRANITOID BATHOLITH FORMATION IN TIN ORE AREAS OF SOUTH FAR EAST

Kopylov M.I.

FGUP “Dal'geophysics”, Khabarovsk, Russia, kopylov@dalgeoph.ru

General direction of magmatism evolution

An association of magmatism and sources of mineralization with deep structure of lithosphere is one of radical problems in modern geology. The new data recently obtained on deep structure and matter composition, distribution of igneous rock natural series in various structures of the Pacific Belt significantly enlarge our ideas of magmatic process possible associations with the Earth structure.

General direction of magmatism evolution when passing from oceanic stage of earth's crust formation to platform one is expressed by relative decrease in mantle-basalt role and progressive increase in anatectoidal magmatism one that is associated with total thickening the sialic crust and sharp dip of asthenospheric layer toward continent. The subsequent alkaline-basaltoid magmatism of final stages in development of mobile belts associated with cleavage of the crust that are directed into mantle is an exception. In this respect, it can be suggested that progressive increase in K contents of initial basaltoidal magmas during the Earth evolution is caused both by processes of sialic contamination and deeper subsidence of asthenospheric layer under the continent.

Geochemical aspect of tin ore mineralization

The geochemical associations of some elements and their associations with magmatic, sedimentary and metamorphic rock complexes of the South Far East have been analyzed. The femic rocks that determine general specialization of tin and accompanying elements in deep parts of the Earth's crust and upper mantle are included in the first group. The second group comprises sialic rocks that characterize migration of elements in formations of sialic envelope (metamorphic basement, sedimentary and magmatic rocks, various metasomatites). The study of tin and accompanying element femic cycles has been carried out on comparison of the element contents in ultrabasites and basaltoids of the Far East region and partial Pacfic Belt. The highest content of dispersed tin is observed in Cretaceous granites (7-12 ppm), particularly, in the late stage of their formation (10-15 ppm and more). The analysis of the above data, results in the fact that tin high contents can be only in certain physical and chemical conditions in the basaltoid melts of mantle origin, whereas its concentration is high in acid rocks. Based on the mentioned association of tin with potassium that belongs to petrogenic elements being most easily extracted from substrate, it can be concluded that the most intensive tin subtraction into basalt melt from mantle peridotite is likely to occur with the increasing T in zone of partial melting or asthenospheric layer. In this case, the process of tin subtraction by potassium is a reply to the question, why high contents of tin lack in the region of oceanic crust development. Here, the asthenospheric layer occurs at small depths of 40-50 km. Within the continental crust, blocks of substrate are less involved in debaltization and degassing and have significant thickness of lithospheric layer up to 120-150 km.

Geophysical aspect of tin ore system (OS) formation

By data of deep studies (GSZ, MOVZ, MTZ, ∆g, ∆T) carried out, one of the important conditions of tin large contrasting formation is the presence of thick continental crust. An increase in crustal thickness is associated with collisional and accretionary processes that occur in the geodynamic setting of lithospheric plate convergent borders. During such processes, significant involvement of crustal deep matter and lithosphere that are responsible for intensive occurrences of magmatogenic and tectonic phenomena, takes place with formation of plutonic, volcano-plutonic belts of magmatism calc-alkaline type. Development of such belts is possible both in convergent (zones of collision, active continental margins) and divergent (interplate riftogenesis) settings. As a result of underthrusting the one terrane (block) under the other one, the fusion of anatectic granites of S-types occurs in the underthrusted block. The overlying block is deformed with formation of linear fold belt because of collision. The most specific example is the Main Kolyma batholith belt, its formation being associated with the collision of the Kolyma-Olonoiskiy supperterrane and North Asian craton. As a result of friction along zones of large scale overthrusts, great quantity of heat is released that is significant for selective fusion of crustal rocks involved in process of collision. A slightly melted material is forced in front of mobile plate area and pressed out into the upper structural levels, causing the formation of S-type intrusive massifs. By calculations, the thickness of slightly melted rock layer could reach 8-9 km, and depth of magma generation should equal 25-30 km, if initial water content in the rocks is near 1% that by calculations, corresponds to the depth of initial magmatic hearth origin for granodiorite-granite intrusions. With respect to Fe₂O₃/FeO < 0.5, granitoid complexes belong to ilmenite series. Their early phases correspond, as a rule, to the first type with value of ratio Al₂O₃/CaO + Na₂O ₊ K₂O_mol < 1.1 and their final ones – to the S-type with Al₂O₃/CaO + Na₂O ₊ K₂O_mol ≥ 1.1(Chappell, 1974).

In setting of active margin, “growth” of continental crust is provided by processes of plate convergence with formation of subduction zones and accretionary prisms followed by the phenomenon of underplating, and lateral “slipping” of plates relatively each other with development of lithospheric plate transform border setting. As an intermediate variant, tangential subduction that combines the elements of both frontal and lateral interaction of plates can be considered.

Discussion of the study results

In recent years, large material on mantle metasomatism has been accumulated. The mantle metasomatism is most clearly observed at alkaline magmatism that occurs in the tin ore regions. The following regularities are noted in the light of the experimental data (Kogarko, 2005) of studying the metasomatic interaction of melt-fluid and mantle material of pyrolite-CO₂-H₂O type. Highly dense magnesial silicates can be present in mantle lherzolite together with magnetite at depths more than 220-300 km. At the same depths, steam that is rich in water occurs, therefore highly dense magnesial silicates and amphibole are not stable in this field. The water role increases in the mantle gaseous phase at lesser depths (180-100 km). CO₂ is the main component of mantle gaseous phase at depth 80 km and more. Near –solidus carbonate melts are rather mobile in the intergranular space of mantle rocks and, thus, they can be active agents of mantle metasomatism.

References

Chappell B.W., White A.J. (1974) Two contrasting granite types // Pacif. Geol. N8. P.968-970.

Kogarko L.N. (2005) Role of deep fluids in genesis of mantle heterogeneities and alkaline magmatism. // Geology and Geophysics. V.46, N12, P.1234-1245.