Crustal evolution and metamorphism in east-central Eritrea, south-east Arabian-Nubian Shield

Abstract

The crust of the Arabian-Nubian Shield (ANS) was formed in oceanic settings in the Mozambique Ocean during the Neoproterozoic (c. 0.9–0.6 Ga). Convergence started to coalesce island arcs, closed the ocean, and continental collision followed (<c. 0.7 Ga). The investigated rocks of east-central Eritrea occupy a southeasterly position in the ANS. Mafic and felsic rocks from both the greenschist facies Bizen and amphibolite facies Ghedem sub-domains are calc-alkaline to tholeiitic with typical arc geochemical characteristics. The Nd and Sr isotopic composition of five felsic, mafic and metasedimentary gneisses from the Ghedem sub-domain are all highly juvenile ( ε Nd at 800 Ma: +4.9 to +7.2, and 87Sr/ 86Sr: 0.7027–7054). SIMS geochronology of two amphibolite-facies orthogneiss samples yielded ages of magmatically zoned zircon at 795 ± 14 and 818 ± 9 Ma. A few cores, indicating the presence of relatively older crust, yielded ages of maximum c. 890 Ma, and no metamorphic rims were observed. TIMS geochronology on monazite from an orthogneiss and a kyanite schist from the Ghedem sub-domain yielded 593 ± 5 and 587 ± 2.5 Ma, respectively, while rutile from the latter gave 565 ± 7 Ma. Metamorphic conditions in garnet-bearing kyanite schists from the Ghedem and Barka sub-domains in eastern and western Eritrea, respectively, were estimated using garnet–biotite–plagioclase–kyanite–quartz thermobarometry. Peak metamorphic conditions for the Ghedem sample was c. 700 °C and 8 kbar and for the Barka sample c. 600 °C and 9 kbar. The geochemistry and Nd–Sr isotopes of the east-central Eritrean rocks confirm a juvenile oceanic arc origin for the crust in this region, allowing only very small contributions from older crust. The c. 800 Ma igneous ages are typical for juvenile Eritrean and ANS crust. The peak amphibolite facies metamorphic conditions were reached during continental collision between east and west Gondwana, at c. 590 Ma in this area, while the c. 565 Ma rutile age represents cooling and exhumation through c. 400 °C and 3–4 kbar after collision.