MEIMECHITES - HIGHLY MAGNESIAN LITHOSPHERE-CONTAMINATED ALKALINE MAGMAS FROM DEEP SUBCONTINENTAL MANTLE

Meimechites are highly magnesian alkaline lavas from the Meimecha-Kotu j region of northern Siberia. They contain abundant large phenocrysts of olivine and smaller grains of chromite in a matrix of smaller olivi ne grains, titanian clinopyroxene, ilmenite, altered glass, and in mos t but not all cases, biotite. A small proportion of the larger olivine grains have a pronounced parallel parting or show irregular and patch y extinction or kink bands: these are interpreted as xenocrysts. Abund ances of MgO range from 8 to 40 wt.%, SiO2 is low (40-43 wt.%) and FeO (to 16 wt.%) and K2O are high (1-3 wt.%). Trace elements concentratio ns are generally high and strongly fractionated, ranging from primitiv e mantle values for the heavy rare earth elements to 100 times greater for incompatible elements such as Rb, Nh and La. Isotopic composition s indicate a depleted source: Nd-143/Nd-144 ranges from 0.51262 to 0.5 1282 (initial epsilon(Nd) values +2.8 to +5.9): inital Sr-87/Sr-86 val ues are between 0.70299 and 0.70338. Compositions of olivine phenocrys ts and xenocrysts vary from Fo(84) to Fog(93) and compositions of smal ler matrix grains from Fo(81) to Fo(92). These values indicate that th ese rocks formed from highly magnesian liquids: maximum MgO contents o f these liquids are calculated to have been at least 25 wt.%, and perh aps as high as 29 wt.%. The high levels of incompatible trace elements and the strongly fractionated patterns are explained by very low degr ee-melting (similar to 1%) of a source with primitive mantle abundance s, or low-degree melting (similar to 7%) of an enriched source. The ve ry high MgO contents in such low-degree melts indicates that the site of melting was very deep, as much as 200 km, and either in the lowermo st continental lithosphere or in the underlying asthenosphere. The mel ting probably was linked with the arrival of the mantle plume that was the source of Siberian basaltic flood volcanism. The olivine xenocrys ts, however, were most likely picked up by the magma during its passag e through the lithosphere. The composition of the meimechites is there fore a consequence of melting under unusual conditions, with contribut ions from several mantle sources.