5:15 PM - 7:15 PM
[SCG53-P05] Characteristics of orthoclase megacrysts in the Yakushima Pluton and estimation of their formation environment
Keywords:granite, orthoclase megacryst
Orthoclase megacrysts are observed in granitic rocks worldwide, and their origin has been a subject of long-standing debate. However, the growth process, formation environment and selective growth of orthoclase to be megacrysts size are still in debate. Yakushima Island, located south of Kyushu Island, hosts a granitic pluton containing orthoclase megacrysts that are strongly euhedral, with grain sizes ranging from 1 to 14 cm. Kawachi and Sato (1978) have suggested that megacryst formation began in the early stages of magma crystallization. However, the processes leading to the development of these megacrysts and the factors contributing to their growth require further investigation. This study aims to understand the characteristics of the orthoclase megacrysts in the Yakushima Granite and to estimate their formation environment.
The field investigations revealed that the megacrysts exhibit preferred orientation within granitic dikes, suggesting the rotation of the megacrystals in the magma with a relatively low crystal fraction. Thin-section observations showed that the megacrysts contain quartz, plagioclase, and biotite crystals as inclusions. Notably, plagioclase inclusions exhibit a preferred orientation, with their long axes parallel to the megacryst’s euhedral surface. This alignment indicates that the plagioclase was rotated in a melt-rich environment during the entrainment into the orthoclase megacryst. Compositional analyses revealed that the orthoclase megacrysts show a "reverse zoning", with the potassium content increasing from core to rim. Though, the potassium content of alkali feldspar typically decreases during crystallization. The potassium content in alkali feldspar can increase as crystallization progresses under low H2O conditions (Nekvasil, 1992). This implies that orthoclase megacrysts in Yakushima Granite was formed under low H2O conditions. Additionally, potassium contents in the megacryst rims and groundmass orthoclase crystals are similar or slightly higher in the groundmass, suggesting that the groundmass feldspar crystallized after the megacrysts had nearly completed their growth.
Based on these findings, a crystallization model for the Yakushima Granite is proposed. First, quartz, plagioclase, and biotite crystals nucleated, followed by the delayed nucleation of orthoclase in a melt-rich environment. The orthoclase then grew rapidly, incorporating surrounding crystals and reaching megacryst sizes. In the final stages of crystallization, groundmass minerals formed, completing the orthoclase porphyritic texture observed today.
The field investigations revealed that the megacrysts exhibit preferred orientation within granitic dikes, suggesting the rotation of the megacrystals in the magma with a relatively low crystal fraction. Thin-section observations showed that the megacrysts contain quartz, plagioclase, and biotite crystals as inclusions. Notably, plagioclase inclusions exhibit a preferred orientation, with their long axes parallel to the megacryst’s euhedral surface. This alignment indicates that the plagioclase was rotated in a melt-rich environment during the entrainment into the orthoclase megacryst. Compositional analyses revealed that the orthoclase megacrysts show a "reverse zoning", with the potassium content increasing from core to rim. Though, the potassium content of alkali feldspar typically decreases during crystallization. The potassium content in alkali feldspar can increase as crystallization progresses under low H2O conditions (Nekvasil, 1992). This implies that orthoclase megacrysts in Yakushima Granite was formed under low H2O conditions. Additionally, potassium contents in the megacryst rims and groundmass orthoclase crystals are similar or slightly higher in the groundmass, suggesting that the groundmass feldspar crystallized after the megacrysts had nearly completed their growth.
Based on these findings, a crystallization model for the Yakushima Granite is proposed. First, quartz, plagioclase, and biotite crystals nucleated, followed by the delayed nucleation of orthoclase in a melt-rich environment. The orthoclase then grew rapidly, incorporating surrounding crystals and reaching megacryst sizes. In the final stages of crystallization, groundmass minerals formed, completing the orthoclase porphyritic texture observed today.