A COLLISION OROGENY AND FORMATION OF GRANITOID BATHOLITHES

Rusin A.I.

Institute of Geology and Geochemistry UrB RAS, Ekaterinburg, Russia, rusin@igg.uran.ru

When paradigm of Earth contraction was predominant, the regional metamorphism, granite batholith intrusions and alpine-type folding was considered as interrelated and sometime independent manifestation of orogeny. The plate tectonics introduced the new understanding of orogeny as a consequence of oceanic and/or continental plate collision. That interpretation allowed to demonstrate variety of collision environments (Miyashro et al., 1982) and to confine the using of actualism in paleodynamic reconstruction as well as relationship between magmatic, metamorphic and deformation processes in orogenic belts. It was made a conclusion, that high- and ultra-high-pressure eclogite and glaucophan-schist metamorphism are characteristic for collision orogens and formation of granite belts relates to high-gradient middle- and low-pressure metamorphism (Rusin, 2007). Their conjunction clearly manifests in the surface exhibition of subduction zones - island arcs and continental margins. It is believe that the deep plunge of hydration oceanic crust is the main reason of above-subduction andesite volcanism but there are disageements in understanding of tonalite-trondhjemite series generation.

The west margin of American continent with huge number of basic granitoid batholiths is a standard example of above-subduction granitoid belt conjugated with volcanic rocks. In recent models their formation is connected with under-plating, following by low-crust anatexis in condition of maximum fluid-saturated. Such genesis is supposed for Uralian margin-continental tonalite-granodiorite massifs conjugated with belt of after-subduction granites (Fershtater, 2001).

Stopping of subduction and continent-continent collision leads to crust shortening and disturbance of crust thermal structure. The estimate of collision duration on the base of real valuation of meeting plate speed and orogen thermal evolution showed that thermal relaxation is much more prolonged then collision. First of all the quartz-bearing rocks is softening and switches on the post-collision stretch process and only later the lifting of geoisotherms leads to metamorphism and granite generation. The fluid deficit (which sours may be hydro-content minerals and/or porous water in sediment series) stipulates the generation of water under-saturated granite magma and it is agrees with nature and experiment data (Wyllie, 1977). We are sure, that the reason of crust anatexis is high-temperature metamorphism, but analysis of Urals orogen metamorphic migmatite complexes demonstrated that there isn’t the granite melt formation horizon on the modern erosion level. Thus, the present-day general idea about autochthonous granite with root zones migmatite is only traditional tribute.

The concept on polycyclic evolution of Phanerozoic belts is founded on the event recurrence postulate. Our idea of total geodynamic cycle (Ivanov, Rusin, 1986) allows affirming, that granite generation in collision orogens and in Late-Precambrian rifts of their previous stage have great difference. In the present-day science level we can suppose that the reason of periodical activization of Riphean and Vendian endogenic process was pulse activity of deepest plumes (“superplumes”) and connected earth crust uplifts (“ensialic orogenies”). Such orogenies don’t terminate, but precede to sediment basins formation. All signs of folded orogenesis (discordances, molasses, metamorphism, granites) can be found in rifting belts. Now it is known, that angular discordances and molasses are the indispensable elements of stretch-links sediment basins, rift metamorphism has a self specific (Ivanov, Rusin, 1997) but the nature of Late-Precambrian granites in Phanerozoic orogens is the object of debates. Nobody disputes their belonging to anorogenic A-type, but in connection with popularity of “super-continental cycles” there are opinions that great batholiths of anorthosite-rapakivi-granite formation together with high-pressure granulites contain batholiths can register the stages of sialic mass confluence (e.g. “Rodinia”). That point of view, in our opinion, is disputable and need of continued discussion.

High-temperature high-pressure metamorphism is not believed as indisputable evidence of collision processes. In those belts the relic association of middle-pressure granulites are constantly discovered and new-forming paragenesises point to their forming in connection with ductile (brittle-ductile) deformation, representative for continental rift deep zones (Ivanov, Rusin, 1997). The disposal of anorthosite-rapakivi-granite formation in areas with high thickness Earth’s crust can be connected with plume underplating, forming the substratum and supplying the energy for melting of dry unorogenic magmas. At this interpretation we can say that Precambrian rifting is not only destruction, but crust-forming process, whereas the subduction plays the main role in new-forming of Earth’s crust in Phanerozoic.

References

Ivanov S.N., Rusin A.I. Continental rift metamorphism // Geotectonics, 1997. V. 31. N 1. P.3-15.

Miyashiro A., Aki K., Senor A.M.C. Orogeny. Chichester – New-York. J. Wiley and Sons lmt. 1982. 242p.

Rusin A.I. Orogenic (collision ) metamorphism in the Urals. Ekateriburg: UB RAS, 2007. P.63-74. (in Russian).

Fershtater G.B. Granitoid magmatism and formation of Earth’s crust during evolution of Urals orogen // Lithosphere, 2001. № 1. P.62-85. (in Russian).

Ivanov S.N., Rusin A.I. Model for the evolution of the linear folds in the continents: example of the Urals // Tectonophysics, 1986. N127. P.383-397.

Wyllie P.J. Crustal anatexis: an experimental review // Tectonophysics, 1977. V.43. P.41-71.