ИСТИНА

Typical magmas of subduction zones have normal alkalinity and sodium specification (Na/K>1). Their origin is connected with water fluid released at the dehydration of the descending oceanic slab. In some occasions the potassic subalkaline or alkaline magmas appear at the active continental margins. Decade and half ago no clear source of such magmas was proposed (Price et al., 1999). Recently role of CO2 in the potassic magmas formation is becoming accepted (Gupta, 2015). We perform experiments that directly demonstrate process of the potassic melt assembling on the olivine- Spl matrix from the components transported by the dry reduced CO2 fluid from the basaltic source at the crustal pressures 2-5 Kbar and T=900-1000oC. Novel technique was applied. Carbonic fluid was generated during experiment by the decomposition of the (Fe,Mg)CO3 according to the reactions: FeCO3=Fe3O4+CO+CO2 MgCO3=MgO+CO2 Natural siderite was placed into the small open capsule while source material (spilitized basalt) was placed into the main capsule hosting small one. Natural siderite aggregate contains quartz and feldspar grains. In the experiment these grains become the centers of the melting. Fluid transfer dissolved components from source basalt towards oxide matrix. As a result of this transfer olivines and spinels (predominantly MgFe2O4) form. Melt domains expand by consuming first of all K2O and Al2O3 from the dry reduced carbonic fluid. Compositions of all 115components of the studied system are characterized by ICP-MS (siderite), XRF (basalt) and SIMS (experimentalproducts). a) b) Fig. 1. Composition of the fluid modified oxide matrix a) potassium enrichment relative albite and basalt sources b) composition of the glass + crystals mixture with SIMS normalized on the initial oxides matrix composition. Experiments with basalt and albite sources demonstrate that melt formed in the oxide matrix has K/Na ratio that is 25-28 time higher than in the source basalt. Fluid mediated melt has rather high Rb, LREE contents. Ba, Sr, Yb enrichments relative source are moderate. Ti shows the least mobility among studied elements. Since HFSE (Zr, Hf) contents in the spilite appears to be below high determination limit for of ICP-MS method source of these elements in the matrix melt is unclear. Level of Zr content (no more than 50-60 ppm) is substantially lower than usually registered in the high-K basalts (200-350 ppm). We explain this fact by the involvement of fluorine in the HFSE transport. As shown by the results of our thermodynamic modeling and petrologic data reduced carbonic fluid would accumulate fluorine from the melt.Gupta (2015) declared universal connection of the high potassic low silica magmas with contamination of the mantle with crustal carbon. At the routine smooth subduction only small fraction of the sediments is transported with descending slab. Large fraction of carbon sinks into the lower mantle due to slab heating slower than pressurization that keeps carbonates stable. At the same time continent arc collision and accretion events creates dynamic irregularities that greatly enhance mantle contamination with the crustal material. In the numerical study (Simakin, 2014) it was shown that temporary reversal of the subduction direction can create and push large pieces of the continental of island crust under the mantle wedge. Fig. 2. Break of the accreted terrain edge during subduction restarting (to the right island, to the left oceanic plate),numerical modeling from (Simakin, 2014). Results of the analogous modeling from (Boutelier and Chemenda, 2011) are in the inset. We anticipate that Kronotsky paleo-arc accretion to Kamchatka 5-10 Mys ago put carbonates and organic material (coal) under mantle wedge. Several kilometers blocks of carbonaceous and siliceous rocks in the mantle are gravitationaly unstable and have extremely low relative viscosity that empowers their pluming and dyking propagation upwards. For the mechanical failure at the zone of collision it is essential that locking of subduction is local and slab continues to descend aside. Alkaline melts produced in the carbonatized mantle under Klyuchevskaya group of volcanoes can contribute to the basalts erupted in Tolbachik 2012-2013 voluminous fissure eruption. In the described by Pirce et al.(1999) case with high-K volcanism in North Island (New Zealand) direction of subduction is reversed along the New Zealand Plate Boundary. Pacific plate subduction propagates westwards and delamination of Australian crust is anticipated (Furlong, 2015) similar to N.Kamchatka case. 116 Work was supported by the RFBR grants № 13-05-00397а and № 13-05-00397a. References: Boutelier, D. and Chemenda, A. Physical Modeling of Arc - Continent Collision: A Review of 2D, 3D,Purely Mechanical and Thermo-Mechanical Experimental Models // In D.Brown and P.D. Ryan (Eds.), Arc-Continent Collision, Frontiers in Earth Sciences. 2011. Gupta, A.K. Origin of Potassium-rich Silica-deficient Igneous Rocks. Springer, India. 2015. 536 P.de Hoog, J.C.M. and van Bergen M.J.. Volatile induced transport of HFSE, REE, Th and U in arcmagmas: evidence from zirconolite-bearing vesicles in potassic lavas of Lewotollo volcano (Indonesia) //