Subsolidus re-equilibration and diffusion during melting process of the granitic system

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(1)Cooling process of the Okueyama granitic body deduced from the extents of subsolidus reationsThe Okueyama granititc body situated in Miyazaki Prefecture represents a single magmatic chamber solidified to a zonal plutonic body. We examined the extents of development of subsolidus reaction textures such as myrmekite and perthite from the roof boundary towards the interior of the body. (1)The mean width of myrmekite increases systematically from 10 μm at the roof contact to 100 μm at 950 m below the contact. (2)The width of albite lamella in perthite increases similarly from 10 μm to 230 μm. (3)The volume fraction of albite lamella increases fiom 0.013 to 0.278. (4)The spacing between the albite lamella increases from 35 μm to 138 μm. These lines of evidence show that the Okueyma body has been cooled effectively from the roof. We could not detect the systematic change of the extent of subsolidus reactions from the wall contact.We also estimated the duration during which the subsolidus reactions had been progressing, by employing plagioclase -hornblende geothermometer. The durations are estiomated to be 15-25 years at the roof contact and 150-300 yeras at 950 m below the contact.(2)Experiments on the diffusion -controlled melt growth in granitic systemsWe carried out melting experiments under non-equilibrium conditions to understnad the partial melting process of the crust. We got basic data on the melting kinetics by the experiments. The systems we examined are sanidine -albite -quartz -H2O, orthoclase -quartz -albite -H2O, and their partial systems. Main results are : (1)We found concentration gradients in the m,elt layers, which suggest diffusion -controlled melt growth. (2)We can estimate diffusion coefficients of the rate-controlling species based on the parabolic growth rate law.