[S-GC33] 固体地球化学・惑星化学

[SGC33-P09] Temporal increase of assimilation in a small magma body: mechanism revealed in an alkaline ring complex in Egypt

*Eman Saad¹、Kazuhito Ozawa²、Takeshi Kuritani³、Ali Khudeir¹ (1.Geology Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt、2.Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan、3.Department of Earth and Planetary Science, Hokkaido University, Hokkaido, Japan.)

キーワード：alkaline, magma, fractional crystallization, assimilation

Assimilation of crustal materials has long been regarded as one of the important processes that produce chemical diversity of magmas from more uniform mantle derived magmas (Bowen, 1928; Taylor, 1980). Because of low temperature of the crustal materials, particularly near the Earth’s surface, such as in a subvolcanic environment, heat generated by crystallization of the magma is required to melt the cold solid crustal materials to be incorporated into the molten magma (Bowen, 1928; Kelemen, 1986). Therefore, coupling of assimilation and fractional crystallization (AFC) is a key factor and several geochemical mass balance models were proposed (e.g., Depaolo, 1981; O’Hara and Mathews, 1981). In this regard, “energy-constrained assimilation–fractional crystallization” (EC-AFC) model (Spera and Bohrson, 2001; Bohrson and Spera, 2001) is important in that energy balance is built in the model. Spera and Bohrson (2001) modeled EC-AFC process by describing thermal and material interaction among several magma subsystems but did not consider thermal and material structures of the magma - crust system and their evolution. We examined an alkaline ring complex in northeast Africa to clarify how crystallization and assimilation was coupled in a three-dimensional crustal magma body. The studied complex is ~600Ma Wadi Dib ring complex (WDRC) located in the Eastern Desert of Egypt (Saad et al., 2023). The WDRC has ~2 km diameter and consists of multiple circular rings of the oldest volcanic unit and the middle-stage plutonic unit, which are cut by the youngest dike unit. The complex represents subvolcanic magma plumbing system (Saad et al., 2023). The plutonic unit consists of an outer ring (syenite), inner rings 1 (quartz-bearing syenite) and 2 (quartz syenite), and a granitic core (syenogranite). The plutonic unit is progressively more fractionated inwards from the outer ring to the granitic core through the inner rings. Exotic blocks of Neoproterozoic granitoids with ~50cm in size are observed only in the outer margin of the outer ring. The sample initial values of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd of quartz-bearing syenite, quartz syenite, and syenogranite show deviations from those of isochron initial ratios, which is defined by less fractionated rocks, such as trachybasalts, trachytes, and syenites. The deviations are explained only by open processes involving influx of exotic materials with low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd, which implies more depleted geochemical characteristics of the exotic materials than the WDRC. We adopted an AFC model to reproduce Nd isotope ratios and trace and major element compositions of the three rock types from respective parental melts derived from the trachybasalt by fractionation closed to material input. The optimized Nd isotope ratios and major and trace element compositions are comparable to those of the host Neoproterozoic granitoids. The obtained Ma/Mc, mass assimilated to mass crystallized, is ~0.15 for quartz-bearing syenite from the inner ring 1 and is 1 – 2 for quartz syenite from the inner ring 2 and the syenogranite from the granitic core. The Ma/Mc thus increases from zero to >1 not only with time but also from the complex margin to the center of the magma body existed beneath the WDRC. The extensive assimilation is thus restricted in the center far apart from the country rocks, which occur exclusively as blocks in the outer ring. These indicate that AFC became effective only during the later stage with supply of exotic melts derived from the bottom boundary layer by melting the country rocks sunk from the carapace of the magma body by stoping in the earlier stage. The estimated Ma/Mc >1 for the quartz syenite and syenogranite implies that the heat generated by crystallization of these rocks was not enough to melt the host rocks, which requires heating of exotic materials to near or even above the solidus before incorporation in the magma body. This and extensive assimilation in the late stage suggest that exotic materials originally consisting of the roof were heated up not only before sinking to the bottom but also during the residence in the bottom boundary layer by accumulation of heat generated by continuous crystallization of the magma body beneath the WDRC.