Mineralogy, geochronology, and geochemistry of the calc-alkaline Um Takha white granite pluton, South Sinai, Egypt

The Um Takha white granite pluton (UTP) is exposed in west central Sinai (Egypt), in the northernmost segment of the Neoproterozoic Arabian-Nubian Shield (ANS). It is a member of the youngest suite of calc-alkaline granites in south Sinai. The UTP intrudes metamorphosed country rocks and diorites and is in turn intruded by the Serbal alkaline granite pluton. The UTP comprises a single phase of leucocratic monzogranite that contains alkali feldspar, plagioclase, quartz, biotite, and rare muscovite. U-Pb ion probe zircon dating for two samples of the white granite yields ages of 614.9 & PLUSMN; 6.3 and 581.5 & PLUSMN; 10.9 Ma, but the younger sample reveals abundant evidence that its zircons were affected by late corrosive fluids (e.g., discordance and patchy cathodoluminescence zoning); the 614.9 & PLUSMN; 6.3 Ma age is preferred. Geochemically, the UTP displays a limited compositional range (SiO2 = 72.9-75.1 wt%) of mostly high-K calc-alkaline rocks of metaluminous to weakly peraluminous character, typical of the calc-alkaline granitoids of the early post-collisional stage of the northern ANS. The variation diagrams and parallel rare-earth element (REE) patterns for the white granite samples (n = 22) suggest that they represent one cogenetic suite. The UTP was likely generated by partial melting of a Na-rich amphibole-bearing tonalite source from the juvenile crust. This is consistent with the Hf isotope ratios of zircons from the UTP, which consistently yield positive epsilon Hf(t) values. The proposed trigger for the formation of the UTP is lithospheric delamination; its emplacement was associated with extensive denudation of the pre-630 Ma orogenic edifice as well as the 630-600 Ma post-collisional products, heat transfer into the lower crust, and post-collisional magmatism. Although the UTP is a cogenetic body, the traditional liquid-line-of-descent fractional crystallization approach fails to explain the large dynamic range of trace element contents over a limited range in major elements. We show that an alternative variable-proportion cumulate-liquid sampling interpretation of the wholerock compositions, in which the most evolved sample represents a liquid composition and the least evolved sample is a low melt-fraction crystal mush, offers a physically reasonable scenario for the observed suite.