15:30 〜 15:45
[HRE13-05] Tectonic and volcanic controls on the deposition of the Kuroko Zn-Cu-Pb deposits of NE Japan
キーワード：Kuroko, Tohoku, Japan Sea, Massive sulfide
Northeast Japan hosts important volcanicanogenic massive sulfide deposits that formed during Mid-Miocene crustal extension occurring in the final stages of the opening of the Japan Sea. This paper examines the available geological and geochemical data to identify links between the type of volcanism and tectonic factors that favoured ore-deposition at the regional and local scale.
Volcanic activity accompanied all stages of the formation of the Japanese archipelago, from rifting of the east Asian continent margin in the Eocene, to back-arc spreading in the Miocene, to arc front volcanism from the late Miocene onwards. A compilation of new and previously published whole-rock chemical analyses of volcanic rocks from NE Japan indicates overall decreasing La/Yb and Sr/Y from the Eocene rift stage to the Mid-Miocene, likely reflecting a thinning crust and shallowing magma production and fractionation during progressive extension. This stage is followed by a new increase of these ratios as the tectonics reverted to compressive and the volcanism switched to arc-front type. A decrease of Nb/Yb until the Mid-Miocene suggests progressively less “enriched” (likely less contaminated) magmas during crustal extension and thinning, and is followed by an increase during the arc stage from the Late Miocene. Ratios of fluid-soluble to insoluble elements, such as Pb/La, Ba/La and Th/Nb, increase and become more variable with time, suggesting an increased influence of slab-derived fluids during magma production, even during the back-arc extension stage.
At the local scale, the textures and compositions of minerals and melt inclusions of Mid-Miocene felsic rocks hosting the Kuroko of the Hokuroku district demonstrate stalling and protracted crystallisation of magmas at very low pressure (mostly ca. 0.5 kbar, estimated based on the normative melt composition at the haplogranitic cotectic) under water-saturated conditions. The plagioclase-melt equilibrium indicates estimated H2O contents of 1.0 to 3.0 wt%, avg 2.0 wt%. Considering the pressure dependence of water solubility in granitic melts, these H2O estimates provide pressure values (0.5 ±0.2 kbar) compatible with the estimates based on normative compositions.
Despite the fact that protracted submarine volcanism could have provided continuous favourable conditions for hydrothermal circulation and ore deposition, the Kuroko of the Hokuroku district appears to have been deposited over a short time span at the end of rift extension and basin deepening. Most ore bodies are hosted in palaeotopographic lows interpreted as submarine calderas, and bathyal conditions have been inferred, although quantifying water depth has proven challenging. A model is proposed to explore the effect of sea water deepening on the depth of water-saturation of shallowly emplaced magma, based on recent studies demonstrating a first-order control of volatile saturation on the depth of magma stalling in the crust. It is proposed that the increasing hydrostatic pressure would have caused a progressive shallowing of the fluid-saturation depth in the crust, encouraging successive magma batches to rise further towards the ocean floor, and causing higher heat flow in the overlying crust. In this model, it is likely that a threshold of heat-flow in the shallow crust was exceeded, triggering vigorous hydrothermal circulation and ore-deposition. The system was then rapidly shut down when tectonics reverted from compression to extension, leading to basin shallowing, eventually leading to sub-aerial conditions.
Volcanic activity accompanied all stages of the formation of the Japanese archipelago, from rifting of the east Asian continent margin in the Eocene, to back-arc spreading in the Miocene, to arc front volcanism from the late Miocene onwards. A compilation of new and previously published whole-rock chemical analyses of volcanic rocks from NE Japan indicates overall decreasing La/Yb and Sr/Y from the Eocene rift stage to the Mid-Miocene, likely reflecting a thinning crust and shallowing magma production and fractionation during progressive extension. This stage is followed by a new increase of these ratios as the tectonics reverted to compressive and the volcanism switched to arc-front type. A decrease of Nb/Yb until the Mid-Miocene suggests progressively less “enriched” (likely less contaminated) magmas during crustal extension and thinning, and is followed by an increase during the arc stage from the Late Miocene. Ratios of fluid-soluble to insoluble elements, such as Pb/La, Ba/La and Th/Nb, increase and become more variable with time, suggesting an increased influence of slab-derived fluids during magma production, even during the back-arc extension stage.
At the local scale, the textures and compositions of minerals and melt inclusions of Mid-Miocene felsic rocks hosting the Kuroko of the Hokuroku district demonstrate stalling and protracted crystallisation of magmas at very low pressure (mostly ca. 0.5 kbar, estimated based on the normative melt composition at the haplogranitic cotectic) under water-saturated conditions. The plagioclase-melt equilibrium indicates estimated H2O contents of 1.0 to 3.0 wt%, avg 2.0 wt%. Considering the pressure dependence of water solubility in granitic melts, these H2O estimates provide pressure values (0.5 ±0.2 kbar) compatible with the estimates based on normative compositions.
Despite the fact that protracted submarine volcanism could have provided continuous favourable conditions for hydrothermal circulation and ore deposition, the Kuroko of the Hokuroku district appears to have been deposited over a short time span at the end of rift extension and basin deepening. Most ore bodies are hosted in palaeotopographic lows interpreted as submarine calderas, and bathyal conditions have been inferred, although quantifying water depth has proven challenging. A model is proposed to explore the effect of sea water deepening on the depth of water-saturation of shallowly emplaced magma, based on recent studies demonstrating a first-order control of volatile saturation on the depth of magma stalling in the crust. It is proposed that the increasing hydrostatic pressure would have caused a progressive shallowing of the fluid-saturation depth in the crust, encouraging successive magma batches to rise further towards the ocean floor, and causing higher heat flow in the overlying crust. In this model, it is likely that a threshold of heat-flow in the shallow crust was exceeded, triggering vigorous hydrothermal circulation and ore-deposition. The system was then rapidly shut down when tectonics reverted from compression to extension, leading to basin shallowing, eventually leading to sub-aerial conditions.