GRANITE FORMATION IN THE EPIOCEANIC OROGENS (ON EXAMPLE OF THE URALS)

Fershtater G.B.

Institute of Geology and Geochemistry UrB RAS, Ekaterinburg, Russia, gerfer@online.ural.ru

Ultramafic-mafic rocks are an important component of the epioceanic orogens basement, such as the Urals. This peculiar feature of deep structure has a strong effect on the magmatic rocks, especially on granitoids that are the subject of this report.

The development of the Paleozoic magmatism of the Urals includes three main stages, from the mantle island-arc magmatism (460-390 Ma) to the mantle-crustal continental marginal subduction-related magmatism (360-300 Ma) and then to the continental crustal collision granite magmatism (292-250 Ma). This “continentalization” of the magmatism including higher level sources changes the general compositions of rock series with increasing the part of acid rocks. Such general trend was disturbed by the formation of rifts and continental arcs that controlled localization of the specific magmatic complexes as gabbro-granitoid and monzodiorite-granite rock series of higher alkalinity.

The products of mantle magmatism are represented by volcano-plutonic high level gabbro-granite rock series in which granitic rocks were produced mainly by fractionation of basic magma. They make up not more than 10% of the intrusive concentrating in the upper part of magmatic bodies.

The mantle crust magmatism produces the mesoabyssal above-subduction gabbro-tonalite-granodiorite-granite batholiths belonging to continental marginal geodynamic setting. Mantle component in these associations is represented by hornblende (Hbl) gabbros and diorites formed by crystallization of hydrous basic magma. These rocks are the source of heat and protolith for granitoids forming mainly in the process of gabbro and diorite partial melting under influence of the subduction related fluid. Underplating of Hbl gabbro increased the continental crust thickness, and the most part of tonalite and granodiorite liquids were formed at the depth of 35-30 km (Р_tot = 6-8 кb = 1.3-1.1Р_Н2О). A great part of granites are the products of secondary anatexis at the depth of 20 - 15 km (Р_tot = 4-5 kb = 1.2-1.0 Р_Н2О) having the tonalite and granodiorite as a protolith. Most of massifs have tonalite-granodiorite-granite composition at the modern erosion level. The whole time of large and composite batholith formation is in the range of 100-90 Ma and includes three main stages: 1) mantle hydrous magmatism (Hbl gabbro and diorite) with duration about 50 Ma, 2) partial melting (anatexis) of gabbros and diorites and tonalitie and granodiorite formation 30-40 Ma later after the beginning of basic magmatism, 3) anatexis of tonalities and granodiorites with duration of 20-30 Ma that produced granites. In the South Urals they occupy the time interval 400-300 Ma and in the Central Urals – 370-280 Ma.

Main granite plutons of the Central and South Urals were formed by collision-related crustal magmatism having as protolith the Precambrian basement of Uralian orogen or the newly formed Paleozoic crust. 290 Ma old granites are localized in the paleocontinental zone of the South Urals among the metamorphic rocks of amphibolite facies with thickness more than 10 km.

The beginning of magmatism which produces large migmatite-plutons is synchronous with the peak of metamorphic events (about 360 Ma). The main stage of granite magmatism has the age of about 290 Ma. The massifs are surrounded by zonal metamorphic aureole. The last episodes of magmatism are represented by numerous adamellite and granite dikes and minor intrusions (260-250 Ma) which induced skarnification and feldspathization of metamorphic rocks. These magmatic and metamorphic events were accompanied by the general uplift of the collision area for no less than 10-15 km with decrease in the depth of emplacement of granitic complexes from 20-25 to 6-8 km

Isotope properties of granites (⁸⁷Sr/⁸⁶Sr_ini = 0.7043-0.7049, Nd₂₉₀ = 0.8-1.6) allow to assume that the crust which was the protolith of the granites had mainly Paleozoic age and was formed by redeposition and granitization of the oceanic and island arc crust with low Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios.

The large and composite massifs have long history and several magmatic sources. A good example is provided by the large and multiphase Aduy massif closely associated with various rare metal deposits. Detailed studying and dating the zircons in combination with the field data show that substratum of Aduy granites is represented both by the rocks of Pre-Paleozoic basement (older than 1300 Ma) and by Late Paleozoic tonalities and granodiorites. Two peaks of magmatic activity (about 290 Ma and 260-255 Ma) are observed. The first is associated with migmatization and partial melting of tonalities and granodiorites which remain hot up to this time and thus can easily be melted. The second magmatic episode is associated with partial melting the Pre-Paleozoic metamorphic rocks from the basement. The time necessary for heating this basement up to the melting temperature averages 35-30 Ma. Partial melting of the basement was the main process of Murzinka massif formation. The age of the Murzinka granites (K-Ar, Rb-Sr, Pb-Pb and U-Pb) is equal to 255-250 Ma.

Tectonics that controls magmatism in the Urals belongs to the global system of cyclic movements as demonstrated by good agreement between well known fluctuations of Paleozoic sea water ⁸⁷Sr/⁸⁶Sr ratio and main magmatic events in the Urals. Epochs of mantle magmatism correspond to minimum of ⁸⁷Sr/⁸⁶Sr ratio of the sea water reflecting the higher entry to the surface of mantle material with low ⁸⁷Sr/⁸⁶Sr ratio, whereas mantle-crustal and crustal magmatism coincides with maximum of these values, because of abundant addition of the crustal material with high ⁸⁷Sr/⁸⁶Sr ratio.