Mechanisms of mantle-crust interaction at the late stages of evolution of Hercynian Altai collision system

Hercynian Altai collision system was formed in Late Paleozoic as a result of collision of Siberia and Kazakhstan continents. At late stages if its evolution (from 300 to 280 Ma) the huge and different gabbro and granitoid magmatism occurred in this territory. A variety of igneous associations prove the significant thermal gradient in lithosphere, the essential role of mantle and the active manifestation of the mantle-crust interaction processes. In gabbro-granite intrusions the specific relationships between mafic and felsic rocks are observed. It allows to classify them as a result of magma-mingling and magma-mixing processes. Based on petrological and geochemical data there was determined that the consistent active interaction between mafic magmas and crustal substrates were happened during forming of intrusive massifs. The mutual contamination of mafic and felsic magmas occurred at low-crustal level. The active interaction between different magmas with mingling-structure forming occurred at middle-crustal level. When intrusions have cool the mafic magmas intruded at upper-crustal levels as dikes. Kalba-Narym granite batholith was formed in close interval, no more than 20 Ma (296276 Ma). There are two stages of its forming: 1) main volume of batholith presented by rocks of granodiorite-granite association (S-type) formed from 296 to 286 Ma; 2) second stage corresponds to formation of granite-leucogranite association (A-type) from 284 to 276 Ma. Granites of S-type formed by partial melting of mixed substrate (metapelitic and metabasic). Granites of A-type formed by partial melting of metapelitic substrate with the assistance of juvenile F-riched fluids. Rare-metal granites form a series of large dikes. They have high content of rare lithophilic elements and fluorine, it allows to classify the rocks as ongonites. Results of mineralogical, petrological and thermobarogeochemical investigations confirm that rare-metal specialization was characteristic for the parental melts. Rare-metal magmas were formed as a result of differentiation of granitic magmas in large chambers under conditions of filtration of specific ore-concentrated fluids (F, P₂O₅ etc.) and enriched them in rare-metal elements. Thus at the late stages of evolution of Hercyni-an Altai Collision System the processes of mantle-crust interaction occurred synchronously but in two different ways. First case - direct interaction of mantle magmas with crustal substratum and anatectic melts. The process suggests contamination of mafic magmas by crustal material, chemical interaction of magmas and formation of hybrid rocks (magma-mixing) or physical interaction of magmas and formation of magma-mingling structures. The second case - thermal and fluid impact of mantle magmas on crustal substratum. The process suggests fluid infusing of special juvenile rare elements in crustal chambers of granitic magmas or effect of juvenile fluids on processes of anatexis and processes of differentiation of granitic magmas. The huge endogenous activity corresponds to Tarim Large Igneous Province formed in Late Paleozoic as a manifestation of Tarim mantle plume.

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