On the cratonization of the Arabian-Nubian Shield:Constraints from gneissic granitoids in south Eastern Desert,Egypt

• 作者: Ali A.Khudeir Jean-Louis Paquette Kirsten Nicholson3,(A)ke Johansson Tyrone O.Rooney Sadiq Hamid Mohamed A.El-Fadly Loretta Corcoran Shawn J.Malone Mohamed Ali Abu El-Rus
• 来源期刊: 地学前缘（英文版）
• 年，卷（期）: 2021,（4）
• 所属期刊栏目: Research Paper
• 页码范围: 215-243
• 页数: 29页
• 语种: 英文

摘要

The Shaitian granite complex (SGC) spans more than 80 Ma of crustal growth in the Arabian-Nubian Shield in southeast Egypt.It is a voluminous composite intrusion (60 km2) comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite,granodiorite and monzogranite.The host tonalite,in turn,en-closes several,fine-grained amphibolite enclaves.U-Pb zircon dating indicates a wide range of crystallization ages within the SGC (800 ± 18 Ma for tonalites;754 ± 3.9 Ma for trondhjemite;738 ± 3.8 Ma for granodiorite;and 717 ± 3.2 Ma for monzogranite),suggesting crystallization of independent magma pulses.The high positive εNdi (+6-+8) indicate that the melting sources were dominated by juvenile material without any significant input from older crust.Application of zircon saturation geothermometry indicates increasing temperatures dur-ing the generation of melts from 745 ± 31 ℃ for tonalite to 810 ± 25 ℃ for trondhjemite;840 ± 10 ℃ for grant-diorite;and 868 ± 10 ℃ for monzogranite.The pressure of partial melting is loosely constrained to be below the stability of residual garnet (＜10 kbar) as inferred from the almost flat HREE pattem((Gd/Lu)N =0.9-1.1),but ＞3 kbar for the stability of residual amphibole as inferred from the significantly lower NBN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases.The inverse relation between the gen-eration temperatures and the ages estimates of the granitoid lithologies argue against a significant role of frac-tional crystallization.The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting.It lacks distinctive features for melt derived from a subducted slab (e.g.high Sr/Y and high (La/Yb)N ratios),and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantle wedge.Comparison with melts produced dur-ing melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite,trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fu-sion of a tonalite protolith.Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios be-tween monzogranite and tonalite samples,it is reasonable to suggest that the renewed basaltic underplating may have caused partial melting oftonalite and the emplacement of monzogranite melt within the SGC.The emplace-ment of potassic granite (monzogranite) melts subsequent to the emplacement of Na-rich granites (tonalite-trondhjemite-granodiorite) most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield.Eventually,after complete consolidation,the whole SGC was subjected to regional deformation,most probably during accretion to the Saharan Metacraton(arc-continent collisions) in the late Cryogenian-Ediacaran times (650-542 Ma).