Characteristics Of The Intermediate-silicic Volcanic Rocks And Their Magmatic Process Records In The Middle Okinawa Trough

Back arc basins are important places for seafloor volcanic activities and hydrothermal activities.Clarifying the characteristics and evolution process of the magma systems is significant for understanding the geological evolution and heatmatter circulation of hydrothermal system in the back arc basin.The Okinawa Trough(OT)is a nascent back arc basin that has developed in the eastern margin of the Asian continent.It is characterized by high heat flow values,widespread hydrothermal activities and volcanic activities,and abundant intermediate to acidic volcanic rocks.Therefore,the OT is ideal for studying the magma systems in the back arc basin.This thesis analyzed the whole-rock geochemistry(major and trace elements,Sr-Nd-Pb isotopes),mineral structure,and in situ mineral geochemistry of the intermediate to acidic volcanic rocks from the Iheya North Knoll(T3 and T6)and Iheya Ridge(T5-2 and C11)in the middle part of the OT(MOT);discussed the petrogenesis of the volcanic rocks;and probed into the characteristics and magmatic processes(including magma evolution and mixing)of the magma chambers.The major findings include:(1)The rhyolites from the Iheya North Knoll(T6)belong to high K calc-alkaline volcanic rocks,which indicates a cryptic K-rich source in the OT.The high K rhyolites have more depleted Sr-Nd isotopic compositions but much more enriched 207 Pb and 208 Pb isotopic compositions than the volcanic rocks from the MOT axial zone,reflecting a DUPAL-like signature.The isotopic anomaly cannot be explained by the incorporation of subducting sediments or crustal contamination but instead imply the existence of a DUPAL-like mantle source beneath the OT.The decrease in the DUPAL anomaly from the high K rhyolites to rocks from the MOT axial zone is consistent with the injection of a source of N-MORB mantle into preexisting DUPAL-like mantle during back-arc extension.(2)The crystal-poor pumice collected from two sampling sites(T3 and T6)on the Iheya North Knoll falls along a line of descent of comagmatic evolution.The pumice-forming melt may have been extracted from a deep source before it ascended nearly adiabatically and experienced physical and chemical differentiation prior to eruption.The extracted melt first experienced crystallization-driven differentiation.Then,the melt ascended rapidly,during which the melt was physically differentiated;the more crystal-poor and volatile-rich melt reached a higher lever in the magma chamber as decompression-driven crystallization simultaneously continued the chemical evolution of the melt.The melt may have been erupted before quartz saturation was reached;thus,no quartz is found in the pumice.(3)Andesite T5-2 from the Iheya Ridge contains two distinct types of disequilibrium mineral assemblages,which crystallize from basaltic melt and rhyolitic melt,respectively.A 4:6 mixture of basalt and rhyolite could produce a melt with similar compositions to andesite T5-2.Rhyolite C11 from the Iheya Ridge also contains a disequilibrium mineral assemblage,which is from a less evolved melt and joined in rhyolite C11 via magma mixing.In addition,the trace elements and Sr-Nd isotopic compositions of type 2 rhyolites from the MOT also exhibit magma mixing signature,implying that magma mixing is very common in the MOT.The above suggests that the magmatisms in the MOT are very complex.Beneath the Iheya Ridge,the basaltic melt,andesitic melt as well as rhyolitic melt were stored at a depth of 9 km to 3 km,with the temperature being from 1100°C for mafic melt to 870°C for rhyolitic melt;a basaltic melt could intrude into a rhyolitic melt and produce an andesitic hybrid melt;a rhyolitic melt could mix with or assimilate another rhyolitic melt with different degree of evolution.Beneath the Iheya North Knoll,volatile-rich melt(about 850°C)could ascend rapidly from the depth of 7–10 km to a shallower depth(about 2 km).It is such complex magma systems that maintain the high heat flow values and widespread seafloor hydrothermal activities in the MOT.