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The evolution of magma during ascent through the crust in the region of the Okinawa Trough is recorded by its mineralogy. In this study, we utilized the characteristics of plagioclase minerals in basaltic magma to deduce magma generation processes, by examining (via back‐scattered electron images) the compositional features of plagioclase phenocrysts in basalts dredged from the middle and southern portions of the Okinawa Trough, which provided a detailed account of the evolution and crystallization of the magma. The crystallization and evolutionary histories of magmas from the middle Okinawa Trough are distinct from those in the southern Okinawa Trough. In the middle Okinawa Trough, we identified seven textural varieties of plagioclase: homogeneous‐core, fine oscillatory‐zoned, core‐sieved, mantle‐sieved, patchy‐core, glomerocrystic and microlitic. Anorthite (An) contents in the cores of plagioclase crystals are 80–87%, while concentrations of Fe, Mg and Sr vary more strongly, especially in patchy‐core plagioclase. The homogeneous‐core, fine oscillatory‐zoned and glomerocrystic plagioclase crystallized in deep‐seated chambers. In contrast, the core‐sieved, mantle‐sieved and patchy‐core textures developed when magmas ascended slowly and were injected into shallow chambers, during which plagioclase was resorbed and dissolved due to decompression. In the southern Okinawa Trough, homogeneous‐core plagioclase crystallized when magma ascended rapidly through deep conduits and was injected into a shallow chamber; under these conditions, only homogeneous‐core and core‐sieved plagioclase phenocrysts crystallized in the shallow chamber. The core of homogeneous‐core plagioclase (An86–An93) was partially resorbed and contaminated by crustal material. Degassing and undercooling promoted the rapid crystallization of microlitic plagioclase, especially in the outermost rims, during and after the extraction of magma from shallow chambers, in both the middle and southern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.
The evolution of magma during ascent through the crust in the region of the Okinawa Trough is recorded by its mineralogy. In this study, we utilized the characteristics of plagioclase minerals in basaltic magma to deduce magma generation processes, by examining (via back‐scattered electron images) the compositional features of plagioclase phenocrysts in basalts dredged from the middle and southern portions of the Okinawa Trough, which provided a detailed account of the evolution and crystallization of the magma. The crystallization and evolutionary histories of magmas from the middle Okinawa Trough are distinct from those in the southern Okinawa Trough. In the middle Okinawa Trough, we identified seven textural varieties of plagioclase: homogeneous‐core, fine oscillatory‐zoned, core‐sieved, mantle‐sieved, patchy‐core, glomerocrystic and microlitic. Anorthite (An) contents in the cores of plagioclase crystals are 80–87%, while concentrations of Fe, Mg and Sr vary more strongly, especially in patchy‐core plagioclase. The homogeneous‐core, fine oscillatory‐zoned and glomerocrystic plagioclase crystallized in deep‐seated chambers. In contrast, the core‐sieved, mantle‐sieved and patchy‐core textures developed when magmas ascended slowly and were injected into shallow chambers, during which plagioclase was resorbed and dissolved due to decompression. In the southern Okinawa Trough, homogeneous‐core plagioclase crystallized when magma ascended rapidly through deep conduits and was injected into a shallow chamber; under these conditions, only homogeneous‐core and core‐sieved plagioclase phenocrysts crystallized in the shallow chamber. The core of homogeneous‐core plagioclase (An86–An93) was partially resorbed and contaminated by crustal material. Degassing and undercooling promoted the rapid crystallization of microlitic plagioclase, especially in the outermost rims, during and after the extraction of magma from shallow chambers, in both the middle and southern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.
Pumice is the most widely distributed magmatic rock in the Okinawa Trough. Numerous scholars have conducted extensive research on the mineralogy, petrochemistry and whole‐rock isotopic characteristics of pumice. However, even with extensive research, some information about pumice is still unknown, including the initial provenance of magma, influences from subducting plates and the possibility of crustal material (including sediments) mixing in during pumice formation. As pumice is loose and porous, seawater re‐modification and injection of sediments may occur during the condensation and rock‐forming process after magma erupts from the seafloor, or in the seawater immersion process after pumice formation. In order to get a better understanding of the isotopic characteristics of magmatism phase, phenocrysts were first separated from the pumice and divided into both light and heavy minerals to analyse their Sr–Nd–Pb isotopic composition. This information was combined with existing isotopic data of volcanic rocks from the Okinawa Trough and its adjacent areas to assess the magma source composition, influence from subduction components and mantle properties in the magmatic source. The research results indicated light minerals had an average 87Sr/86Sr ratio of 0.70454, while heavy minerals averaged 0.70546. The magma, formed pumice, was a product of the crystal fractionation evolution of basic magma, with the additional injection of crustal material. The magma source in the central trough is a mixture of depleted MORB mantle (DMM) and enriched mantle (EMI‐like and EMII‐like) end members, where the EMI‐like end member is the residual subcontinental lithosphere or lower crustal material and the EMII‐like end member is subducted oceanic sediments. The Pb isotopic composition indicated that the magma source was affected by subduction components (sediments) and that pumice magma was mixed with more sediment than basalt magma material in the same area. The mantle of the magma source in the Okinawa Trough showed signatures of the Indian Ocean‐type mantle. Copyright © 2016 John Wiley & Sons, Ltd.
As an active margin of the Pacific Plate, the ocean‐continent transition zone records major tectonic events from the Late Mesozoic to the present‐day. The development of the transition zone between the Western Pacific Ocean and the East Asian Continental Margin occurred in two stages. In the first stage, the formation of the Songliao and Bohai Bay basins was initiated earlier in the Mesozoic and then the active tectonic events that moved southeastwards to develop the East China Sea and Japan Sea basins in the Cenozoic. We use the methods of tectonics and geophysical analysis for the tectonic reconstruction of the transition zone in the most significant periods of tectonic activity in this area. We propose that the deep processes have a direct influence on the Meso‐Cenozoic surface geomorphologic system in the transition zone. This paper tries to reveal the mechanism of continental margin dynamics by comparing the stratigraphy, sedimentary character and magmatism for each of the four basins, as well as the gravity anomalies, the results of mantle tomography, and thickness of the crust and lithosphere. Our results show that the Songliao and Bohai Bay basins and the East China Sea and Japan Sea basins have similarities in their geomorphological characteristics and sequential tectonic evolution. The Meso‐Cenozoic morphotectonic processes are dominated by the subduction retreat of the (Palaeo‐)Pacific Plate and the eastward tectonic jump. Copyright © 2016 John Wiley & Sons, Ltd.
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