Sources of rare-metal-bearing A-type granites from Jabel Sayed complex, Northern Arabian Shield, Saudi Arabia

Abstract

Jabel Sayed, in the northern part of the Arabian Shield, is an alkaline granite complex. It consists of syenogranites and aegirine–riebeckite alkali feldspar granites intruded into an older granodiorite–tonalite suite. Elemental geochemistry attests that the syenogranite and the alkali feldspar granite represent a genetically related post-collisional A2-type granite intrusion (e.g., high K2O+Na2O, FeO∗/MgO, Ga/Al, and Y/Nb, and low CaO, Ba, and Sr). U–Pb zircon dating and Nd isotopes of the alkali feldspar granites indicate an age of 593Ma, εNd(t) values of +4.9 to +5.3 (mean=+5.1), and depleted mantle model ages (tDM) between 0.78 and 0.88Ga. These isotope results are in agreement with a juvenile magma source of Neoproterozoic age like that responsible for forming most other Arabian–Nubian Shield crust. The alkali feldspar granites have substantial modal content of albite and fluorite, wide variation of HFSE and REE and exhibit tetrad effects in REE patterns that indicate interaction with hydrothermal F-rich fluids. The fluids caused secondary metasomatic alteration superimposed on the granitic rocks. The result of the fluids–granitic rock interaction is a metasomatic pegmatite rim, which carries economic levels of HFS elements and REE, and a red granite variety (altered syenogranite) characterized by large variation in trace element abundances. Geochemical features in the granitic rocks such as strong linear positive correlation of Ta versus Nb and Zr versus Hf indicate magmatic origin of the fluids and emphasize that the behavior and enrichment of Ta and Nb are largely controlled by magmatic processes. Based on the geochemical and Nd isotopes compositions, we propose an origin for the Jabel Sayed granites which involved fractional crystallization of a parental magma derived by the partial melting of a crustal protolith accreted between 780Ma and 880Ma. This crustal protolith has juvenile geochemical and isotopic composition and was extracted from a subduction modified mantle source.