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Balykin P.A.,RAS Institute of Geology and Mineralogy | Polyakov G.V.,RAS Institute of Geology and Mineralogy | Izokh A.E.,RAS Institute of Geology and Mineralogy | Hoa T.T.,the Vietnamese Academy of science and Technologies | And 3 more authors.
Russian Geology and Geophysics | Year: 2010

The Jinping-Song Da rift structure in the Emeishan Large Igneous Province is composed of Permian high- and low-Ti volcanic and volcanoplutonic ultramafic-mafic associations of different compositions and genesis. High-Ti picrites, picrobasalts, basalts, and dolerites are enriched in LREE and depleted in HREE and show low Al2O3/TiO2 ratios (~4), commensurate εNd(T) values (+0.5 to +1.1), and low (Th/Nb)PM ratios similar to those of OIB-enriched mantle source. The established geochemical characteristics evidence that the parental melts of these rocks were generated from garnet lherzolite at the depths of garnet stability (~200 to 400 km). Later, high-Mg low-Ti volcanics (komatiites, komatiitic basalts, and basalts) and associating small peridotite-gabbro massifs and komatiite-basalt dikes were produced as a result of ~20% partial melting of depleted water-poor (≤0.03 wt.% H2O) peridotite substratum from the hottest upper part of mantle plume at relatively shallow depths (100-120 km). The LREE-depleted komatiites and komatiitic basalts are characterized by low (Ce/Yb)CH values, 187Re/188Os = 0.05-1.2, 87Sr/86Sr = 0.704-0.706, positive εNd(T) values (+3 to +8), γOs = -0.5 to +0.9, and strong negative anomalies of Ba, K, and Sr on the spidergrams. The scarcer LREE-enriched komatiites, komatiitic basalts, and basalts vary greatly in chemical composition and values of εNd(t) (+6.4 to -10.2), 87Sr/86Sr (0.706-0.712), and γOs (+14.8 to +56), which is due to the different degrees of crustal contamination of parental magmas. The Rb-Sr isotopic age of basaltic komatiite is 257 ± 24 Ma. The Re-Os age determined by analysis of 12 komatiite samples is 270 ± 21 Ma. These data agree with the age of flood basalts of the Emeishan Large Igneous Province. The komatiite-basalt complex of the Song Da rift is still the only Phanerozoic PGE-Cu-Ni-complex of this composition. The geochemistry of accompanying Cu-Ni-PGE-ores confirms their relationship with komatiite-basaltic magmatism. © 2010.

Izokh A.E.,RAS Institute of Geology and Mineralogy | Smirnov S.Z.,RAS Institute of Geology and Mineralogy | Egorova V.V.,RAS Institute of Geology and Mineralogy | Anh T.T.,the Vietnamese Academy of science and Technologies | And 3 more authors.
Russian Geology and Geophysics | Year: 2010

Study of the chemical composition of clinopyroxene and garnet megacrysts from the Dak Nong sapphire deposit and model calculations have shown that megacrysts originated from the crystallization of alkali basaltoid magma in a deep-seated intermediate chamber at 14-15 kbar, which is close to the Moho depth (50 km) in this part of southeastern Asia. The chamber was a source of heat and CO2 fluids for the generation of crustal syenitic melts producing sapphires and zircons. The formation conditions of sapphires and zircons are significantly different. The presence of jadeite inclusions in placer zircons points to high pressures during their crystallization, which is confirmed by the ubiquitous decrepitation of CO2-rich melt inclusions. Sapphires crystallized from iron-rich syenitic melt in the shallower Earth's crust horizons with the participation of CO2 and carbonate-H2O-CO2 fluids. The subsequent eruptions of alkali basalts favored the transportation of garnet and pyroxene megacrysts as well as sapphire and zircon xenocrysts to the surface. It is shown that sapphire deposits can be produced only during multistage basaltic volcanism with deep-seated intermediate chambers in the regions with thick continental crust. The widespread megacryst mineral assemblage (clinopyroxene, garnet, sanidine, ilmenite) and the presence of placer zircon megacrysts can be used as indicators for sapphire prospecting. © 2010.

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