[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-P22] Decoding evolution of cooling open solid-dominant system from rock records retained over a wide temperature range

*Kazuhito Ozawa¹, Eman Saad², Hideto Yoshida³ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Geology Department, Faculty of Science, Assiut University, 3.Department of Earth and Planetary Science, University of Tokyo)

Keywords: open rock system, open magma system, syenite, Wadi Dib ring complex

Living Earth materials experience a long-lived solid-dominant state in coexistence with fluid/melt due to the multi-component nature before ending in a dormant solid state near the Earth’s surface. Such active solid-dominant state, irrespective of under metamorphic or igneous condition, is susceptible to various open processes before complete solidification by cooling with removal of melt or fluid to have microstructure and chemistry of constituent minerals completely frozen before they were brought to near the Earth’s surface accessible to mankind on diverse scales from geological units to rock samples. Rock samples (< ~ 0.1 m in size) represent a consequence of integration of various complex open processes and internal reaction driven by changes in pressure, temperature, and chemical environment. In order to understand evolution of such rock system over a wide temperature range, we need to resolve the integrated entity by deconvolution of intricate records of microstructure and chemical heterogeneity in minerals as well as whole-rock chemical and isotopic compositions. An ideal deconvolution is possible if older microstructure and chemical heterogeneity were not completely erased by more recent reaction and deformation processes, which is contrasting to the whole-rock geochemical features with complete time integration. For clarification of entire history of open reacting rock systems, rock records convolved in time and space must be identified and properly placed in the chronological order. Useful criteria for this purpose are: (1) temperature-dependent chemical reactions involving minerals and melts/fluids stops at specific closure temperatures and (2) microstructures tend to be downscaled with temperature. Because of this and the continuous temperature decrease of rock systems in general, products of reaction and deformation sequentially frozen with decreasing temperature. We applied this concept to a frozen open mushy magma system, syenites of the Wadi Dib ring complex (WDRC), Eastern Desert, Egypt. The WDRC is one of the alkaline ring complexes formed intermittently over the Phanerozoic time in the Nubian shield in northeast Africa. The ring complex has 586.7Ma age and consists of volcanic, plutonic, and dike units (Saad et al., 2023). The plutonic unit has complete ring structure and was intruded from the margin (outer ring) to the center (granitic core) through the intermediate zone (inner rings). We examined syenites from the outer ring, which is shown to be a frozen mushy boundary layer (Saad et al, 2023). The syenites experienced various events such as crystallization, feldspar accumulation, and migration of fractionated melt, which implies the rocks represent a frozen open rock system involving influx and separation of interstitial melt inducing various reactions accompanying heat loss to the country rock. The syenites have various microstructures, such as intergrowth, exsolution, reaction rim, and deformation as well as complicated zoning in feldspars. We exploited strongly temperature-dependent chemical and microstructural behaviors of Ca-Na-K ternary feldspar to resolve various reactions and deformation took place over a wide temperature range by clarifying formation order of chemical zoning and microstructures. We were successful in revealing open processes operated in the wall mushy boundary layer of a magma body formed in shallow crustal depths, from which an important role of wall boundary layer played in magma fractionation in a crustal magma chamber is argued for.