Succession, Magma Origin And Tectonic Setting Of Silurian Volcaniclastic Rocks In The North Daba Mountains

The Taohekou Formation (TK) is a volcanic-sedimentary terrane formed in the early Silurian in the Daba Mt, China. Based on integrated 1/10,000 geological mapping, detailed observations along seven transections across the TK, and geochemical analyses for the whole rock and pyroxene phenocrystals, the volcanic-sedimentary succession, associations of the lithofacies, magmatic origin, and tectonic setting of alkaline basalts within the TK are systematically discussed in the present thesis. The results show that the TK is composed primarily of volcaniclastic rocks, minor amounts of interbedded alkaline basalts and submarine sedimentary rocks, with an upward volcaniclast-decreasing megasequence consisting alkaline basalts, tuff breccia, structureless or imbricate tuffaceous conglomerate, tuffaceous sandstone, fossil-bearing limestone and/or mudstone. The succession of the TK contains twenty-two lithofacies that are ascribed to five associations of lithofacies, including coherent facies, primary volcaniclastic facies, syneruptive volcaniclastic facies, volcanogenic sedimentary facies and sedimentary rocks facies. Where, coarse or fine porphyritic basalts are commonly located at the lower of the succession, while amygdaloidal or pillow basalts are always at the upper. Geological mapping indicates that volcanism is most likely controlled by the faults, and volcanic activities in the form of the fissure eruptions are much stronger in the west portion of the study area than in the east. The association of lithofacies indicates that the volcanic eruption is mainly the Strombolian-Hawaiian type, possibly the Sertseyan type at the beginning.Pyroxene phenocrystals within basalts are mainly sahlite and diopside, with minor fassaite. These pyroxene phenocrystals are enriched in LREE and show a strong differentiation between LREE and HREE. The major, rare earth and trace elements of these pyroxene phenocrystals hosted in different basalt show highly geochemical affinity with the same magmatic source, suggesting that these pyroxene phenocrystals are generated by the crystallization and fractionation of the same magma chamber. The geochemical results indicate that alkaline basalts were derived from the combined HIMU with the EMI and EMII end-members, and overprinted by metasomatization of the mantle melt enriched in the incompatible elements, similar to that of oceanic island basalt (OIB).By the calculation of clinopyroxene equilibrium to the melt, this study gets three groups of crystallization temperature-pressure of the pyroxene phenocrystals,T1=1246-1324 C, P1=1.13-2.05 GPa, T2=1200-1211℃, P2=0.44-0.62 GPa, P3=1159-1183℃, P3=0.14-0.28 GPa, which indicates that magma generated volcaniclastic rocks and alkaline basalts might have been originated from the mantle, and stagnated at the depths of 40-68 km,15-20 km and 5-9 km during its migration. Pyroxene shows a tendency of Mg decrease, Ca and Fe increase, while alkaline basalts show the increasing tendency of Si, alkali and volatile contents, accompany with magma crystallization and fractionation at different depths. In conclusion, this study suggests that alkaline basalt and volcaniclastic rocks in the TK are formed in a passive margin on the north margin of the Yangtze block in the early Silurian.