研究業績

The Wadi Hawashiya, North Eastern Desert of Egypt, is a mountainous granitic terrain exposing two Ediacaran granitoid suites, namely the Hawashiya Older Granites (HOG) and the Hawashiya Younger Granites (HYG). The HOG comprise normal to high-K, calc-alkaline, metaluminous granitoids (mainly quartz diorites), while the HYG are strongly high-K, calc-alkaline, slighty peraluminous syeno- and monzogranites. Zr is strikingly high (av. 335 ppm) in the HOG and is notably depleted (av. 108 ppm) in the more evolved HYG. The HYG have very low Nb contents almost matching those of the HOG (Nb ~ 6 ppm), which is typical of I-type granites generated at arc-type settings. Although the REE patterns of the HOG and HYG are comparably LREE-enriched with moderately steep slopes, the patterns of the HYG have significant deep negative Eu anomalies of variable magnitudes, whereas those of the HOG usually exhibit very shallow negative and occasional positive Eu anomalies. The assessed apatite crystallization temperatures for both granitoid suites are higher than those estimated from the zircon and monazite thermometers indicating that apatite crystallized before zircon and monazite from the melt. HOG evolved by assimilation-fractional crystallization processes from mantle-derived magmas, which subsequently interacted with crustal sources during ascent and residence in higher crustal levels. Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments. The HYG were emplaced (586?601 Ma) after the collision between the juvenile crust of the Arabian-Nubian Shield (ANS) and the older pre-Neoproterozoic continental blocks of Saharan Metacraton. The emplacement of HOG (622-611 Ma) had occured earlier (ca. 10 Ma) in a normal mature continental arc. The geochemical signatures and age data suggest that the terminal collision between East and West Gondwanaland in the northern tip of the ANS occurred before ~620 Ma, most likely at ~630 Ma, and transitioned to an extensional setting at ~600 Ma.