[S-CG58] New Developments in fluid-rock Interactions: From Surface to Deep Subduction Zone

[SCG58-P14] Cockade-breccia and focused fluid flow in upper crustal hydrothermal circumstances

*Geri Agroli¹, Manzshir Bayarbold^1,2, OTGONBAYAR DANDAR^1,2, Masaoki Uno, Atsushi Okamoto, Noriyoshi Tsuchiya (1.Graduated School of Environmental Studies, Tohoku University, 2.Geoscience Center, Mongolian University of Science and Technology, Mongolia)

Keywords:Khungui,Mongolia, focused fluid flow, Cockade-breccia

Focusing fluid flow occurs where highly permeable structures access a pressurized fluid reservoir. The subsequent hydrothermal fluid could alter the permeability and dimension of the host rock by episodic fracturing and fragmentation resulting in distinctive rock texture like breccia. Breccia is believed to be a proxy of abrupt changes in fluid and stress conditions in the earth's interior. Here we present novel observations of “cockade-breccia” in the Khungui area, Zavkhan Terrane, Western Mongolia. The term cockade is referred to as breccia where the concentric matrix encompasses the clast (sensu stricto). Khungui area itself is mainly composed of metamorphic rock sequences like eclogite, gneiss, and amphibolite with lenses of dacite/andesite porphyry dikes intruding on those rocks. The narrow area of dacite porphyry surrounded by eclogite facies shows intense fracturing follow by brecciation. The breccia has a monomict clast of dacite cemented by greenish-quartz and calcite matrix. 3D observations using micro-tomography show the suspension of the clast and barely touching each other. The matrix appears to have multiple generations under cathodoluminescence. Initial euhedral quartz (qz-1) of CL-bright to CL-gray zoning growth as a concentric band upon the clast. The second quartz matrix (qz-2) fills the cavity and shows CL-dark intensity. While calcite is likely the last product of hydrothermal activity in this area and fills the fracture. Other intriguing features observe in this cockade-breccia are the intense dissolution and replacement of the clast by qz nodule, which is unusual for this type of breccia. Thus, it needs an alternative mechanism to explain the formation. From all those, we can see the multiple matrix components of cockade-breccia are a manifestation of different fluid conditions that some have suggested about the contribution of seismic events and draining the pressurized reservoir by focused flow mode. And though the observation of breccia in Khungu hopefully improves our understanding of subsurface fluid conditions which are related to the mineralization, alternating seismic and coseismic events (EQ swarm) as well as future energy production like geothermal.