中国科学院海洋研究所机构知识库

During the transition from compressional to extensional-related magmatism, at Ra's Abdah area of the northern Eastern Desert of Egypt, Ediacaran uraniferous syenogranite dikes with complex morphology sent offshoots into Cryogenian arc granitoids. This study defines new tectonic environment of these dikes and suggests their emplacement timing before the Egyptian late-collision granite intrusions. U-Pb zircon dating for Ra's Abdah dikes and arc granitoids yielded ages 597 +/- 15 and 663.2 +/- 8.4 Ma respectively; that extended the age of late Precambrian post-orogenic dikes in the Egyptian Nubian Shield to 597 +/- 15 Ma. Zircons from dikes show significant enrichment of Th and U; while zircons from arc granitoids display typical igneous zircon patterns with Th and U enrichment but significant negative Nb, La, Pb and Sr anomalies. Average calculated zircon temperature crystallization of Ra's Abdah dikes is 698 degrees C whereas of the granitoids is 713 degrees C. Similar geochemical behavior of some individual trace elements and REE in zircons from dikes and host rocks intimates identical magma source (partial melting of subducted oceanic crust and its overlying magmatic wedge intercalated with older continental crust). High fractional crystallization of the parent magma after the first pulse managed the change from arc granitoids magma with high fO(2) values to syenogranite dikes phase with lower fO(2) following the transformation from a transpressional to an extensional tectonic. Volatile complexing in late-magmatic stage, mineral fractionation and concomitant zircon crystallization would increase all REE in the residual melt with special HREE enrichment and LREE elimination in zircon of dikes. The radioactive mineralization of dikes (Averaging eU = 124.7 ppm; eTh = 263.9 ppm) are syn-genetic followed by supergene enrichment. Suggested model explains the multistage propagation of complex morphology dikes by filling, opening and increasing expansibility of inclined or perpendicular pre-existing fracture framework.