17:15 〜 18:30
[SMP25-P05] Episodic fluid explosion in shallow and middle crust revealed by hydrothermal brecciation
キーワード：Fluid explosion, Breccia, Erdenet, Sør Rondane Mountains, Yuze
High energy released in the middle to shallow crust in the form of episodic fluid explosion. This phenomenon is triggered by several factors primarily an accumulation of fluid pressure from the dehydrated magmatic-hydrothermal system. The episodic fluid explosion is exemplified by intensive crustal failure through fracturing and brecciation. Further investigation of breccia elucidates the origin and characteristic of fluid and brecciation mechanism. Here we examined samples from 3 different locations to demonstrate how common fluid explosions occur in the middle to the shallow crust.
The Sør Rondane Mountains, in East Antarctica is part of the Pan-African collision zone between East and West Gondwana during Precambrian (650-500 Ma). Northeastern terrane of this mountain occupied by metamorphic sequence (granulite) and some locality of syn-metamorphic intrusion of old granite/diorite followed by younger granite which later makes up the breccia clast on this area and represents explosive activity in the middle crust. On the other hand, Breccia on porphyry deposit of Erdenet in Mongolia which occur in Mesozoic (245-235 Ma) and Cenozoic (9-7 Ma) volcanic breccia in Yuze, Japan represent brecciation processes in the shallow crust. The breccia in Sør Rondane Mountains (SRM) comprise of granulite clast cemented by carbonate rock (refer as carbonate breccia dyke) and subsequent granitic dyke which capture carbonate, granite, and gneiss as the clast. Estimated P-T conditions of this brecciation were P=200Mpa and T=580-700°C which indicating middle crustal condition. Erdenet Porphyry (EP) comprises of 2 distinct breccia type of polymict-breccia which consist of granodiorite porphyry (with potassic alteration) cemented by biotite (phlogopite) located in the bottom part of the drilling-core and monomict-breccia which consist of quartz-porphyry (with sericite alteration) cemented by calcite, quartz, and gypsum occupied the middle and shallower part. We estimated this breccia occurs as multiple fluid processes during the evolution of ore processed. Phlogopite capture fluid temperature at ~700-750°C with pressure <200Mpa and subsequent brecciation occur in lower temperature mark by low titanium content in quartz-matrix as well as the presence of gypsum which perhaps derived from hydration process of anhydrite indicate the ingress meteoric fluid in the system. Yuze breccia (YB) exhibit polymict breccia where volcanic and plutonic clasts cemented by aphanitic rock texture. Particle/Clast size distribution (PSD) shows that SRM clast primarily has boulder size ( >5m) with an aspect ratio (Rs/Rl) of 0.4-0.5 and YB have cobble-pebble size ~50-100 mm with an aspect ratio of 0.5-0.6. As opposed to EP which represents hand specimen size breccia has a fine clast <5mm primarily with an aspect ratio of ~0.6 for all breccia types. Cumulative distribution for all breccia shows that fractal distribution with fractal dimension less or equal to two. And clast arrangement exhibit typical fluid assisted brecciation texture such as jigsaw and mosaic without significant rotation and subsequent wearing processes.
This suggests that brecciation due to overpressurized fluid commonly occurs from the middle to shallow part of the earth's crust. This overpressurized fluid follows by an episodic explosion which abruptly changes the crustal condition and enhanced permeability hence, the fluid can percolate and bring a high amount of energy or precipitate precious metal up to the shallower part of the crust.
The Sør Rondane Mountains, in East Antarctica is part of the Pan-African collision zone between East and West Gondwana during Precambrian (650-500 Ma). Northeastern terrane of this mountain occupied by metamorphic sequence (granulite) and some locality of syn-metamorphic intrusion of old granite/diorite followed by younger granite which later makes up the breccia clast on this area and represents explosive activity in the middle crust. On the other hand, Breccia on porphyry deposit of Erdenet in Mongolia which occur in Mesozoic (245-235 Ma) and Cenozoic (9-7 Ma) volcanic breccia in Yuze, Japan represent brecciation processes in the shallow crust. The breccia in Sør Rondane Mountains (SRM) comprise of granulite clast cemented by carbonate rock (refer as carbonate breccia dyke) and subsequent granitic dyke which capture carbonate, granite, and gneiss as the clast. Estimated P-T conditions of this brecciation were P=200Mpa and T=580-700°C which indicating middle crustal condition. Erdenet Porphyry (EP) comprises of 2 distinct breccia type of polymict-breccia which consist of granodiorite porphyry (with potassic alteration) cemented by biotite (phlogopite) located in the bottom part of the drilling-core and monomict-breccia which consist of quartz-porphyry (with sericite alteration) cemented by calcite, quartz, and gypsum occupied the middle and shallower part. We estimated this breccia occurs as multiple fluid processes during the evolution of ore processed. Phlogopite capture fluid temperature at ~700-750°C with pressure <200Mpa and subsequent brecciation occur in lower temperature mark by low titanium content in quartz-matrix as well as the presence of gypsum which perhaps derived from hydration process of anhydrite indicate the ingress meteoric fluid in the system. Yuze breccia (YB) exhibit polymict breccia where volcanic and plutonic clasts cemented by aphanitic rock texture. Particle/Clast size distribution (PSD) shows that SRM clast primarily has boulder size ( >5m) with an aspect ratio (Rs/Rl) of 0.4-0.5 and YB have cobble-pebble size ~50-100 mm with an aspect ratio of 0.5-0.6. As opposed to EP which represents hand specimen size breccia has a fine clast <5mm primarily with an aspect ratio of ~0.6 for all breccia types. Cumulative distribution for all breccia shows that fractal distribution with fractal dimension less or equal to two. And clast arrangement exhibit typical fluid assisted brecciation texture such as jigsaw and mosaic without significant rotation and subsequent wearing processes.
This suggests that brecciation due to overpressurized fluid commonly occurs from the middle to shallow part of the earth's crust. This overpressurized fluid follows by an episodic explosion which abruptly changes the crustal condition and enhanced permeability hence, the fluid can percolate and bring a high amount of energy or precipitate precious metal up to the shallower part of the crust.