[S-CG48] 岩石・鉱物・資源

[SCG48-02] Estimation of melting conditions and potential temperature for highly alkaline fractionated trachybasalts: Wadi Dib ring complex in the Nubian Shield, Eastern Desert of 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.)

Decompression melting is one of the main mechanisms of magma generation, and the pressure and temperature (P-T) of melting have been estimated from less differentiated magmas by adding olivine until the melt composition is in equilibrium with the mantle (e.g., Herzberg and Asimow, 2015). Potential temperatures may also be estimated from the melting conditions by assuming adiabatic melting and a mantle solidus. The melting P-T conditions and potential temperature of highly alkaline magmatisms (such as trachybasalt, basanite, and nephelinite) is important for better understanding temporal and spatial changes of lithosphere-asthenosphere interaction beneath continental regions. However, we cannot apply such simple olivine addition approach to the alkaline magmatism because (1) highly alkaline nature of magmatism suggests very low degree of partial melting (< a few %), (2) high sensitivity of the source mantle composition to P-T estimates due to (1), and (3) possible extensive fractional crystallization involving fractionation of clinopyroxene and plagioclase even if assimilation of the crustal material is correctly treated. In order to properly estimate melting conditions for fractionated alkaline magmas, it is imperative to resolve these issues. We have developed a novel approach in estimation of melting conditions and potential temperature of highly alkaline fractionated magmas by coupling major and trace elements and isotope ratios for multiple rocks representing frozen magmas underwent various melting and crystallization history. This approach allows estimation of major and trace element compositions of primary magma from significantly fractionated magmas, for which pure olivine addition cannot be applied. It also allows estimation of potential temperature without assuming adiabatic decompression by considering the effect of heat loss during decompressional melting. We applied this approach to a ring complexes distributed in the Arabian-Nubian Shield, where numerous episodes of intra-plate alkaline magmatism took place extending from the Late Proterozoic to Tertiary. We focus on the ~600Ma Wadi Dib ring complex in the Eastern Desert. Geochemical and mineralogical data of the Wadi Dib ring complex provide information to constrain environment of magma generation in the upper mantle. The mineralogy and petrology data show that the WDRC formed at a low oxygen fugacity < -1 log unit lower than the fayalite-magnetite-silica buffer, and that the parental magma, trachybasalt having ~4wt% MgO, was deficient in water ~0.2wt%. Radiogenic isotope ratios and trace element data show that the Wadi Dib source mantle derived by partial melting of the bulk silicate earth with degree of melting as low as ~1.4%, which is more enriched than the enriched MORB mantle. Primary magmas were estimated by adding olivine, clinopyroxene, and plagioclase in equilibrium with the melt in terms of Mg-Fe exchange, from which melting pressure of 1.9 - 2.6GPa (63-85km in depth) and melting temperature of 1390-1480°C were estimated. The depth of magma generation is consistent with heavy rare earth element ratios, such as Dy/Yb. The potential temperature is estimated to be ~1480°C. The potential temperature is higher than the present-day mid ocean ridge 1350-1400°C (Putirka, 2009). We infer that the asthenospheric mantle beneath continent at ~600Ma was in the thermal state hotter than that of beneath the present-day mid ocean ridges, which is attributable to secular cooling of the upper mantle or effective insulation of continental lithosphere.