GEOCHEMISTRY AND PETROGENESIS OF PERMIAN ULTRAMAFIC-MAFIC COMPLEXES OF THE JINPING-SONG DA RIFT ( SOUTHEASTERN ASIA )
P.A. Balykina, G.V. Polyakova, A.E. Izokha, Tran Trong Hoab, Ngo Thi Phuongb, Tran Quoc Hungb, and T.E. Petrovaa
aV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, prosp. Akad. Koptyuga 3, Novosibirsk, 630090, Russia
bGeological Institute of the Vietnamese Academy of Sciences and Technologies, Hanoi, Vietnam
Keywords: Mantle plum; parental melts; Permian high-Ti picrites, picrobasalts, and dolerites; low-Ti komatiites, komatiitic basalts, and basalts; peridotite-gabbro massifs; komatiite-basalt dikes; Cu-Ni-PGE deposits and ore occurrences
Pages: 611-624
Abstract
The Jinping-Song Da rift structure joining 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...+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...+8), γOs = -0.5...+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...-10.2), 87Sr/86Sr (0.706-0.712), and γOs (+14.8...+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.
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