Magma Plumbing System And Subsidence Mechanism Of The Bonghwa Cauldron In The Yeongju Batholith, Korea

The Yeongju Batholith, located in the Yeongnam Massif, is a plutonic complex composed of intrusive phases such as the Buseok, Chunyang, and Jangsu plutons. The Buseok and Chunyang plutons are divided as differentiated phases of hornblende biotite tonalite, biotite granodiorite, and biotite granite, and into the into Chunyang granite stock and the fine-grained granite dikes depending on the intrusive forms, respectively. The SHRIMP zircon U-Pb ages of the plutons show a decreasing age trend from tonalite (186.6 +/- 0.73 Ma) and granodiorite (185.8 +/- 0.69 Ma), through biotie granite (185.8 similar to 177 Ma), to Chunyang granite (174.8 +/- 1.4 Ma), and ring granite dikes (172.2 +/- 1.4 Ma). The Bonghwa cauldron, 6.0x12.8 km in diameter, is bounded by the ring faults and ring dikes preserved as fine-grained granite dike. Early and late differentiated phases, tonalite/granodiorite and biotite granite, respectively, occur along the inside and outside of the boundary, and might be emplaced in the lower/middle and uppper parts of the chamber. Based on the geological map pattern, ring dike and fault kinematics indicate that the biotite granite moved down relative to the surrounding metamorphic rocks, tonalite and granodiorite, suggesting its subsidence into an underlying magma chamber. The structural and intrusive relationships allow us to evaluate the subsidence by the models (1) retreat of magma in the magma chamber and (2) sinking of the solidified granite in the roof into its magma chamber, and thus suggests the cauldron subsidence regardless of volcano-plutonic system in the upper crust level. The cauldron shows the evolutionary sequence of (1) differentiation and emplacements of the Buseok pluton, (2) development of subvertical ring faults in the middle crust level, and (3) cauldron subsidence and injection of ring dikes.