GRANITE FORMATION WITHIN CONVERGENT AND DIVERGENT BOUNDARIES OF

LITHOSPHERIC PLATES (ON EXAMPLE OF CENTRAL ASIAN FOLD BELT)

Gordienko I.V.*, Kiselev A.I.**

*Geological Institute SB RAS, Ulan-Ude, Russia, gord@pres.bscnet.ru

**Institute of the Earth's Crust SB RAS, Irkutsk, Russia, akiselev@crust.irk.ru

At present, it has been revealed that mechanism of lithospheric plate tectonics in its modern form works since Late Proterozoic. It is suggested that different conditions where tectonics of small and thin plates caused mostly by plume tectonics, existed on the Earth in the Archean and Early Proterozoic (Khain 1995).

The period of the Rodinia supercontinent split (750-630 Ma) and Paleo-Asian ocean start of formation (Gordienko, 2006) were an important event of the Siberian Platform fold belt formation in the Late Proterozoic. Due to the age of numerous ophiolite zones, the boundary 570-550 Ma, when divergent, convergent and transform borders of lithospheric plates being fixed in many structures of Central Asian Fold Belt (CAFB) in place of the Paleo-Asian Ocean (PAO) became the most significant epoch in ocean formation.

Within the divergent borders in conditions of spreading or riftogenesis, both the formation of the middle-oceanic ridges (MOR) with extended rift valleys and intercontinental rift systems of the East African type with dominance of extension strains, crust's thickness being decreased and maximal heat flow, occurred. Not volumetric granite formation took place during development of ophiolite association within both the slowly spreading MOR and bimodal volcano-plutonic series at continental riftogenesis. In the first case, granite melts are represented by plagiogranites of the tholeiite series (Tauson, 1977; Antipin, Makrygina, 2006). Within the PAO, similar rocks have been found in ophiolite complexes of Mongolia (Khan-Taishiri, Shishkhid), Altai-Sayan area (West and East Sayan), Dzhida and Baikal-Muya zones of Transbaaikalia. Thus, plagiogranite petrotype of tholeiite series has been studied in the Mainskiy intrusive complex (550-515 Ma) that is analogues of Fiji island acid tonalities in the Pacific Ocean by petrological and geochemical characteristics. The Mainskiy plagiogranites are differed by very high silica content and low alkalinity at rather low contents of K (o.5%), F and most lithophile elements and elements of iron group. Similar and very low values of initial ⁸⁷Sr/⁸⁶Sr ratios both in plagiogranites and preexisting gabbroids indicate their mantle origin. The similar plagiogranites have been described in ophiolites of the East Sayan Denzhigur complex, Dzhida sequence in the Dzhida caledonide zone (interfluve of Dzhida and Tsakirka rivers), Muya complex in the Baikal-Muya zone.

More volumetric granitoids of largely alkaline granitoid composition form in riftogenic intercontinental volcano-plutonic belts. Granophyres, granite- and syenite-porphyries, alkaline granites and syenites, comendites and pantellerites above spreading rift zone of Islandia, East African rift system (Nigerian type), Kergelen oceanic plateau, etc. are modern analogues of similar rocks. However, they are most widely spread within the Paleozoic and Mesozoic fold belts, particularly, in the fold rim of southern Siberian platform, where riftogenic volcano-plutonic belts of various ages (East Sayan D_1,2, Selenga-Vitim or Mongolo-Transbaikalian C₂-P-T₁, Central Mongolian C₂-P₂, South Mongolian C₂-P₁, etc.) existed. Alkaline agpait granitoids in these belts are closely associated with subalkaline and alkaline rhyolitoids and basaltoids in single bimodal series that are of mantle origin. Most researchers refer them to the interpolate formations that are associated with mantle plumes. It is suggested that alkaline and rare metal granitoids of riftogenic structures were formed in similar way in rear of the Andian continental margin (Altiplano, Kito and Santjago grabens), as well as the transformic type granitoid batholiths of the California margin in North America.

Processes of granite formation are mostly observed within the convergent borders of lithospheric plates. In these conditions, granitoids form in the following geodynamic settings: 1) at subduction or collision of oceanic plate with island arcs of various maturity (ensimatic or ensialic); 2) at collision of oceanic plate with continent (Andian type of active continental margins); 3) at collision of oceanic plate with microcontinent; 4) at collision of island arcs or microcontinents with each other; 5) at continental subduction or collision of continents (Alpian or Indo-Himalayas types of margins). Various types of collisional and accretionary-collisional processes, when granitoids of andesite and calc-alkaline geochemical types mainly form, proceed in all these settings. The acid magma melting occurs under the following conditions: 1) in process of compression and melting the subdued oceanic crust (slab) with formation of andesite type supra-subduction granitoids; 2) in conditions of slab extension and detachment (rupture)with formation of slab window, penetration of asthenospheric material into this window and melting the lower crust with formation of I type granites; 3) as a result of intensive collision (compression), exfoliation (delamination) of low lithosphere, its subsidence into asthenospheric layer, subsequent melting and introduction of crust-mantle melts with formation of large calc-alkaline geochemical type granitoid massifs; 4) tectonic clustering with simultaneous under- and overthrust of plates, their thermal heating with origin of palingenic stress-granites of crustal origin, occurs in collision of large continental plates (Indo-Himalayas type).

Within CAFB, nearly all enumerated types of geodynamic settings existed. Here, granitoids of various genetic types and volumes, including large granitoid batholiths (their characteristics will be presented in the report) formed.

The study has been carried out with financial support of RFBR (grants 08-05-00290, 08-05-00225) and intergrational projects ONZ RAS 7.10.1 and 7.10.2.

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