Evolution of the meimechite magmas by the data of the microprobe research meimechite tuffolavas.

Asavin A.M., Senin V.G.

Institute of Geochemistry and Analytical Chemistry, 117945, Moscow, Kosigina str.19 aalex06@inbox.ru

The origin of primary magmas which closely connected with the forming of large ultrabasic alkaline intrusions, until now, remains the most interesting problem of geochemistry and petrology. In particular great interest is a question of forming and evolution of the high-Mg meimechite and picrite effusive rocks from the Maymecha-Kotuyskoi province (North of Siberia). To this problem is dedicated a number of geochemical works (Sheynmann 1968, Sobolyev et al 1972; Vasil'yev, Zolotukhin 1975; Arndt et al, 1995; Basu et al. 1995; Kogarko of & Ryabchikov 2000, Kamo et al, 2003 and other). In recent years appeared experimental studies (Elkin-Tanton et al 2007).

These works showed the high-temperature nature of meimechite melts (1550-1600áC), and according to the data of experimental works occurs at the depths more than 200 km at pressures 5.5 GPa. This model of the formation of meimechite magma assumes extremely quick ascent to the surface of the large volume of melts, with the very weak heat loss. Until now the majority of data was obtained for the dyke. The rate of cooling of subvolcanic bodies, apparently is sufficiently significant, and liquid were crystallized during the sufficiently wide interval of time. The observed of the hydrodynamic separation of the crystals of olivine into the dikes in particular testify about this. However, together with the subvolcanic bodies in the region are widely exposed the effusive varieties of meimechite revealed in the escarps of the rivers Maymecha and Delkan. In 1987 we have observe the thicknesses of tuff and lava flows of meimechite in the estuary of the Delkan.

These are the fresh rocks of black color with a small quantity of phenocrysts of olivine and spinel. Petrographical and geological signs indicate the surface explosive forming flows. There are forms in section of the sharp pieces pyroclastic suspension, variations in the textures, fiemme pyroclastic breccia inferred to be a result of a fluidization. We observe the presence of the tracks of lava flow and, more dense crusts on the rounded sections of tuff with the concentric structure, as any volcanic bombs. A study of these products of explosive activity they will make it possible to more correctly estimate the temperature conditions of the being raise magmatic column. Great interest presents comparison with meimechite dykes and the search of the special features of geochemistry of mineralogy tuff-lavas flows. There is a great interest of the estimation of the temperature conditions of their formation. Data by the compositions of several tuff and lava flows are published in the table.

Table Concentration of main elements by the RFA methods.

* total iron calculate as FeO.

** measure by the microprobe CAMEBAX 100 defocused  beam  100mm.

In spite of the absence of a significant quantity of phenocrysts of olivine in the species effusion rocks remain extremely magnesia. It is interesting to note that handmade sorting under binocular microscope groundmass without olivine phenocrysts show the closely to total rock composition. There is the same result carry out the analysis of the sections of bulk by the defocused wide ray by the microprobe. The difference did not descend the content of magnesium in these analysis no more then several wt.% MgO. In the comparison with meimechite dykes the content of MgO in them is considerably higher. The content of rare and rare-earth elements is compared with meimechite dykes. Enrichment is noted by the light rare earth together with the high content of siderophilic trace elements. The contents Sr, Th are very high (Fig). The composition of olivine about Fa _5-12 with high content Ni and Cr. In contrast to dyke varieties in tuff meimechite is observed the wide interval of the compositions of both the changed glass and the olivine. The presence of the large number of spinel phenocrysts is essential peculiarities also. Phenocrysts spinel large frequently fused and crushed. There is the evidence about the forming tuff under the conditions for the active mixing of liquids from different level sections of the magmatic camera.

A study of these species will make it possible to more fully present information about spectrum of the fusions existing in the deep camera and cumulative process. There is the cumulus rocks type association can be attributed phenocrysts of spinel.

The simulation of the crystallization of compositions according to the program MELTS (Ghiorso M. S et al 2002) showed that the equilibrium phase composition of phenocrysts and liquid, closely to the conditions of high pressure and temperatures. However, the reaction of the congruent melting of olivine and spinel occurs on the surface and are formed phenocrysts of clinopyroxene and olivine (Fa_12-17) of the second generation more ferrous and more titan spinel composition.

Our data confirm complex multistage nature of the meimechite’s magmas that is proves by composition and the mineralogy of the effusive varieties. These proof speak that the deep magmatic cameras exist. In such camera ultrabasic liquids with the crystals are contained. Macro compositions and the mineralogy of phenocrysts indicate the possibility of the prolonged stages of the equilibrium crystallization of olivine and spinel in the regime of very high pressures and temperatures.

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