Petrogenesis of contrasting magmatic suites in the Abu Kharif area, Northern Eastern Desert, Egypt: implications for Pan-African crustal evolution and tungsten mineralization

The Abu Kharif area in the Northern Eastern Desert consists of contrasting granitic magma suites: a Cryogenian granodiorite suite (850–635 Ma), an Ediacaran monzogranite suite (635–541 Ma) and a Cambrian alkali riebeckite granite suite (541–485 Ma). Tungsten mineralization occurs within W-bearing quartz veins and a disseminated type confined to the monzogranite. Whole-rock geochemical data classify the granodiorite as a late-orogenic I-type with calc-alkaline affinity, while the monzogranite and alkali riebeckite granite represent respectively a post-orogenic highly fractionated I-type with calc-alkaline affinity and an anorogenic A1-subtype with alkaline affinity. Geochemical modelling indicates that the three intrusions represent separate magmatic pulses where the granodiorite was generated by ∼75 % batch partial melting of an amphibolitic source followed by fractional crystallization of hornblende, biotite, apatite and titanite. The monzogranite was formed by 62 % batch partial melting of the normal ‘non-metasomatized’ Pan-African crust of calc-alkaline granite composition followed by fractional crystallization of plagioclase, biotite, K-feldspar, magnetite, ilmenite, with minor apatite and titanite. The alkali riebeckite granite was generated by 65 % batch partial melting of metasomatized Pan-African granite source followed by fractional crystallization of plagioclase, K-feldspar, amphibole and biotite with minor magnetite, apatite and titanite. In general, the parent magmas of the three intrusions were originally enriched in W, but with different concentrations. This W-enrichment would be caused by magmatic-related hydrothermal volatile-rich fluids and concentrated within the monzogranite.

S- to I- to A-type magmatic cycles in granitic terranes are not globally recurring progressions. The cases of the Cape Granite Suite of Southern Africa and central Victoria in southeastern Australia

Recurring progression from S- to I- to A-type granites has been proposed for a subset of granitic rocks in eastern Australia. The wider applicability and the validity of this idea is explored using the Cape Granite Suite (CGS) of South Africa and the granitic and silicic volcanic rocks of central Victoria, in southeastern Australia. Within the CGS there is presently little justification for the notion that there is a clear temporal progression from early S-type, through I-type to late A-type magmatism. The I- and S-type rocks are certainly spatially separated. However, apart from a single slightly older pluton (the Hoedjiespunt Granite) there is no indication that the S- and I-type granites are temporally distinct. One dated A-type granitic sample and a syenite have poorly constrained dates that overlap with those of the youngest S-type granites. In central Victoria, the granitic magma types display neither a spatial separation nor a temporal progression from one type to another. All magma varieties are present together and were emplaced within a far narrower time window than in the CGS. Thus, a progression may or may not exist in a particular region, and the occurrence of such a progression does not hold true even in a part of southeastern Australia, which afforded the type example. Thus, the idea that, globally, there should be a progression from S- to I- to A-type magmatism is unjustified. The critical factor in determining the temporal relationship between granitic magmas of different types is probably the compositional structure of the deep crust in a particular region, a reflection of how the individual orogen was assembled. In turn, this must reflect significant differences in the tectonic settings.

Widespread Permian granite magmatism in Lower Austroalpine units: significance for Permian rifting in the Eastern Alps

The Grobgneis complex, located in the eastern Austroalpine unit of the Eastern Alps, exposes large volumes of pre-Alpine porphyric metagranites, sometimes associated with small gabbroic bodies. To better understand tectonic setting of the metagranites, we carried out detailed geochronological and geochemical investigations on the major part of the porphyric metagranites. LA–ICP–MS zircon U–Pb dating of three metagranites sampled from the Grobgneis complex provides the first reliable evidence for large volumes of Permian plutonism within the pre-Alpine basement of the Lower Austroalpine units. Concordant zircons from three samples yield ages at 272.2 ± 1.2 Ma, 268.6 ± 2.3 Ma and 267.6 ± 2.9 Ma interpreted to date the emplacement of the granite suite. In combination with published ages for other Permian Alpine magmatic bodies, the new U–Pb ages provide evidence of a temporally restricted period of plutonism (“Grobgneis”) in the Raabalpen basement Complex during the Middle Permian. Comparing the investigated basement with that of the West Carpathian basement, we argue that widespread Permian granite magmatism occurred in the Lower Austroalpine units. They belong to the high-K calc-alkaline to shoshonitic S-type series on the base of geochemical data. Zircon Hf isotopic compositions of the Grobgneis metagranites show εHf(t) values of − 4.37 to − 0.6, with TDM2 model ages of 1.31–1.55 Ga, indicating that their protoliths were derived by the recycling of older continental crust. We suggest that the Permian granitic and gabbroic rocks are considered as rifted-related rocks in the Lower Austroalpine units and are contemporaneous with cover sediments.