LOW-TITANIUM ALDAN LAMPROITES (SIBERIA) - MELT INCLUSIONS IN MINERALS

The geology and composition of the Aldan lamproites from the Little Mu run, Ryabinovyi, and Yakokut Massifs, from the Upper-Yakokut Trough an d Molbo River have been characterized. According to the mineral crysta llization temperatures established in a study of melt inclusions, the Aldan rocks are similar to common lamproites and K-basaltoids. Crystal lization of pyroxene and apatite in them occurred at 1,280-1,170 degre es C and 1,150-1,030 degrees C, respectively, while olivine crystalliz ed at temperatures greater than 1,200 degrees C. In contrast, accordin g to their major element composition (Mg, Al, Ti), (K2O+Na2O)/Al2O3, S iO2/Al2O3, and 100FeO/(FeO+MgO+TiO2) ratios, the Aldan lamproite rocks are intermediate between common lamproites and K-basaltoids. Because of variable alkali-to-alumina ratios derivative melts had either agpai tic or miaskitic characteristics. Decreasing concentrations of Mg at n early constant Ti favored a significant increase in 100FeO/(FeO+MgO+Ti O2) to 40-60 mol.% in differentiated melts. The primary fluid componen t of the melts was nitrogen: at the time of olivine crystallization, t his is estimated as 60 mol.% (with 39% CO2) and during pyroxene crysta llization, as 100 mol.%. The volatiles F, Cl, and SO3 were also signif icant components or the fluids. It is concluded that the Aldan lamproi tes cannot be direct derivatives formed by crystallization or common l amproitic or K-basaltoid melts. Most probably, they either result from mixing of these magmas or reflect a higher level of melting as compar ed with common lamproites. Low-pressure crystallization could lead to depletion of Ti and Zr in the melts, while mixing of melts could expla in the similarities between the Aldan lamproites and derivatives of K- basaltoid magma.