珪長質マグマの発生と上昇, マグマ溜り  [in Japanese] Generation, ascent, and magma chamber of felsic magma  [in Japanese]

珪長質マグマの発生の条件を, 高温マントルと玄武岩質マグマの2つの熱源モデルで考察した。次に, 珪長質マグマの溜まりうる場所は2箇所あることを理論的に検討した。珪長質マグマは高粘性のため下部地殻ではダイアビルで上昇を始める可能性が強く, 地殻熱流量が大きい場合は中部地殻まで上昇できる。粘性に支配されたマグマ溜りの深さを修正されたストークス球のモデルで計算した。上部地殻をクラックで輸送されたマグマは, 浅所に付加体やリフトの堆積盆のような厚い堆積岩が存在すれば, ここで母岩との密度差がなくなり最終的にトラップされ, 密度に支配されたマグマ溜りを形成する。これらの結果は地球物理的観測や地質学的観察と矛盾しない。また, 地殻の熱構造に支配されるマグマ溜りの深さと水平規模はカルデラの構造の違いをもたらす可能性がある。珪長質マグマを伴う火成活動史を温度場や応力場等で分類し議論した。

If felsic magma is produced in the partially melted crust, there are two cases to heat the crust: one is the heat supply formed the conduction from the local high-temperature mantle; the other is that formed the accumulation of basaltic magma. In the former case, it is difficult to heat the crust above the melting temperature for D_L (the distance between the high-temperature mantle and the Moho) > 18-19 km. In the latter case, the degree of magma accumulation in the lower crust depends on the stress field. The tensile stress field has the advantage of the accumulation of basaltic magma over the hydrostatic stress field. Moreover, the tensile stress decreases D_L mechanically so that it can accelerate crustal melting. The depth of a magma chamber of felsic magma is governed by the crustal viscosity and the density difference between magma and the crust. A diapir in this paper contains small cracks (<2m thick) filled with felsic magma which are not easy to grow and propagate because of its high viscosity. Fracturing a diapir prevents its ascent. By the modified Stokes's Law, for example, the possible final depths at surface heat flows of 120, 90, and 60 mWm<-2> are estimated to be around 8-10 km, 14-16 km, and 22-24 km, respectively (the viscosity-controlled magma chamber). The sedimentary rocks with porosity have a small density at the shallow crust (0-10 km in depth), and can provide the level of neutral buoyancy (LNB) for felsic magma (the density-controlled magma chamber). In the accretionary prism (orogenic region) and in the rift with the sedimentary basin (an orogenic region), both the viscosity-contolled magma chamber and the density-controlled magma chamber can develop. In the continental crust without a thick cover of sedimentary rocks, only the viscosity-controlled magma chamber can develop. Geophysical and geological observations support above the oretical results. The depth of a magma chamber as well as its horizontal size, that is, the ratio of the depth to the horizontal size is one of the parameters controlling the caldera structure. Finally, under the small differential stress (Δσ), the variations of polygenetic volcanoes with calderas in the cold crust, and in the hot crust are discussed; under the large Δσ, monogenetic volcanoes in the cold crust, and calderas in the hot crust are also discussed.