11:45 〜 12:00
[SCG52-11] １次元熱伝導モデル解析が示す足摺岬花崗岩体貫入と前弧海盆三崎層群堆積の時間的関係
キーワード：沈み込み帯、花崗岩、1次元熱伝導モデル、前弧海盆
During the Middle Miocene, a number of granite intrusions occurred in the SW of Japan. Aside from their intrusive nature, the mechanisms by which the granites were emplaced have not been determined (e.g. buoyancy-driven or passive). Intrusive and diapiric magmatic bodies cause contact metamorphism creating unique heating and thermal regimes. Here we present a study of the middle-Miocene Ashizuri Granite, SW Japan its metamorphic aureole, and measurements and models of the thermal alteration it caused in surrounding host rocks.
The Ashizuri Granite intrudes the Tertiary Shimanto accretionary complex, with the accretionary complex forming a basement for overlying fore-arc sediments (the Misaki Group). Here the basin-fill shows an upward coarsening and shallowing sequence which from the base to top progress; 1) muddy beds (> 90 % mudstone with alternating sandstone beds) that are seafloor deposits, 2) to sandy beds (> 70 % sandstone) that are shelf sheet sand deposits and finishes with 3) a thick accumulation of sandstone (subtidal channel deposits and supratidal deposits). Measurements of vitrinite reflectance and an assumed duration of heating equating to 1 Ma indicate a maximum heating temperature of 140 °C for the top sedimentary units furthest from the granite, while the oldest and deepest units indicate a maximum heating temperature of 300 °C. There is a notably rapid change in the maximum heating temperature from the middle to uppermost fore-arc basin sediments of 274 to 164 °C. This rapid change in the heating temperature coincides with changes in thermal diffusivity of 1.64 to 1.2 mm2s-1 (measured by the Hot Disk method) and porosity of 1.8 to 17.0 %. This big difference in physical properties divides the section into two regions; the side closest to the granite with high thermal diffusivities of ~1.7 m2s-1 and the side further from the boundary with a low diffusivity of ~1.2 m2s-1.
To date, for 1D modeling purposes, the region has been divided into two sections. Within the model, it is assumed that the heating caused by the intruding granite was nearly instantaneous with cases evaluated where the initial temperature for granite-side of the system was 600 or 800 °C (lower or higher than the solidus temperature of the granite at 650-700 °C), while for the upper parts of the forearc-basin sediments it was 90 or 140 °C (temperatures representing the unheated Boso Peninsula and the lowest vitrinite reflectance values measured in this study, respectively). As the result, for the granite side, modeled and measured temperatures matched (255.1 compared to 256.0 °C). On the other hand, for the upper fore-arc basin, to achieve a match, it was found necessary to the lower initial temperature of the sediments to 20 °C. Such a geological scenario is only possible if the Ashizuri granite intruded soon after or near contemporaneously with the deposition of fore-arc basin sediments.
The Ashizuri Granite intrudes the Tertiary Shimanto accretionary complex, with the accretionary complex forming a basement for overlying fore-arc sediments (the Misaki Group). Here the basin-fill shows an upward coarsening and shallowing sequence which from the base to top progress; 1) muddy beds (> 90 % mudstone with alternating sandstone beds) that are seafloor deposits, 2) to sandy beds (> 70 % sandstone) that are shelf sheet sand deposits and finishes with 3) a thick accumulation of sandstone (subtidal channel deposits and supratidal deposits). Measurements of vitrinite reflectance and an assumed duration of heating equating to 1 Ma indicate a maximum heating temperature of 140 °C for the top sedimentary units furthest from the granite, while the oldest and deepest units indicate a maximum heating temperature of 300 °C. There is a notably rapid change in the maximum heating temperature from the middle to uppermost fore-arc basin sediments of 274 to 164 °C. This rapid change in the heating temperature coincides with changes in thermal diffusivity of 1.64 to 1.2 mm2s-1 (measured by the Hot Disk method) and porosity of 1.8 to 17.0 %. This big difference in physical properties divides the section into two regions; the side closest to the granite with high thermal diffusivities of ~1.7 m2s-1 and the side further from the boundary with a low diffusivity of ~1.2 m2s-1.
To date, for 1D modeling purposes, the region has been divided into two sections. Within the model, it is assumed that the heating caused by the intruding granite was nearly instantaneous with cases evaluated where the initial temperature for granite-side of the system was 600 or 800 °C (lower or higher than the solidus temperature of the granite at 650-700 °C), while for the upper parts of the forearc-basin sediments it was 90 or 140 °C (temperatures representing the unheated Boso Peninsula and the lowest vitrinite reflectance values measured in this study, respectively). As the result, for the granite side, modeled and measured temperatures matched (255.1 compared to 256.0 °C). On the other hand, for the upper fore-arc basin, to achieve a match, it was found necessary to the lower initial temperature of the sediments to 20 °C. Such a geological scenario is only possible if the Ashizuri granite intruded soon after or near contemporaneously with the deposition of fore-arc basin sediments.