Geochemistry, petrogenesis and radioactive mineralization of two coeval Neoproterozoic post-collisional calc-alkaline and alkaline granitoid suites from Sinai, Arabian Nubian Shield

摘要

The Younger Granites of Yahmid-Um Adawi area, located in the southeastern part of Sinai Peninsula, comprise two coeval Late Neoproterozoic post-collisional alkaline (hypersolvous alkali-feldspar granites; 608-580 Ma) and calc-alkaline (transsolvous monzo- and syenogranites; 635-590 Ma) suites. The calc-alkaline suite granitoids are magnesian and peraluminous to metaluminous, whereas the alkaline ones are magnesian to ferroan alkaline to slightly metaluminous. Both granitoid suites exhibit many of the typical geochemical features of A-type granites such as enrichment in Nb ( 20 ppm), Zr ( 250 ppm), Zn ( 100 ppm) and Ce ( 100 ppm) and high 10000*Ga/Al2O3 ratios ( 2.6) and Zr + Nb + Y + Ce ( 350 ppm). Accessory mineral saturation thermometers demonstrated former crystallization of apatite at high temperatures prior to zircon and monazite separation from the magma for both granitoid suites. The mild zircon saturation temperatures of the studied Younger Granites (around 800 degrees C) imply low-temperature crustal fusion and incomplete melting of the largely refractory zircon. The two Younger Granite suites were semi-synchronously evolved during the post-collisional stage of the Arabian-Nubian Shield subsequent to the collision between the juvenile shield crust and the older pre-Neoproterozoic continental blocks of west Gondwana. Their parental magmas has been generated by melting of crustal source rocks with minor involvement from mantle, which might participated chiefly as a source of heat necessary for fusion of the crustal precursor. Extensive in-situ gamma-ray spectrometry revealed anomalously high radioactivity of some Younger Granite exposures along Wadi Urn Adawi (eU; 388-746 ppm and eTh; 1857-2527 ppm) and pegmatitic pockets pertaining to the calc-alkaline suite (equivalent U and Th; 212-252 ppm and 750-1757 ppm, respectively). The radioactivity of the syngenetic pegmatites arises from the primary radioactive minerals uranothorite and thorite together with the U- and/or Th-bearing minerals zircon, columbite, samarskite and monazite. The anomalously high radioactivity of some Younger Granite exposures in Wadi Urn Adawi stem from their appreciable enclosure of the epigenetic uranium minerals metatorbenite and uranophane.