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Ulf B. Andersson, Swedish Museum of Natural History (Sweden)
Mengist Teklay, University of Asmara (Eritrea)
Martin J. Whitehouse, Swedish Museum of Natural History (Sweden)
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The Neoproterozoic basement of Eritrea, which is part of the juvenile (c. 900-550 Ma) Arabian-Nubian Shield (ANS), dominantly consists of low-grade metavolcano-sedimentary terrains of the highlands and high-grade gneiss terrains of the lowlands. The amphibolite facies rocks of the lowlands have been considered as an older basement to the greenschist facies rocks of the highlands; a concept that has been refuted by recent studies from central and eastern Eritrea. In order to collect more information on character and timing of crust formation, a suite of low- and high-grade rocks from the Felcat area in northernmost Eritrea was studied. Five samples from the Felcat area, northernmost Eritrea, and one sample from central Eritrea were analyzed for zircon SIMS geochronology. A low-grade felsic metavolcanic rock yielded an intrusion age of 877±10 Ma, while two high-grade metagranitoids yielded magmatic ages of 838±17 Ma and 814±15 Ma, respectively. In the former, metamorphic zircon overgrowths formed in the age range 670-585 Ma. This rock also carries inherited cores up to 990 Ma old. A veined metasediment from the same area mostly contains grains with ages in the range 900-800 Ma, except for two grains that indicate ages in the range 1.5-1.4 Ga. Low Th/U (<0.1) overgrowths in this rock yield an age of 672±15 Ma. A late tectonic granite that cuts the >800 Ma metagranitoids yielded an intrusive age of 603±5 Ma, and a post-tectonic granite from central Eritrea yielded an age of 588±6 Ma. The tholeiitic to calc-alkaline mafic rocks show trace element characteristics similar to modern volcanic arc rocks, with variable enrichments superimposed on MORB. A few samples show enrichments reminiscent of OIB compositions. The felsic rocks are of volcanic arc type. 147Sm/144Nd values are in the range 0.118-0.232 for the mafic rocks and 0.094-0.107 for the felsic rocks. Initial εNd ratios for both mafic and felsic rocks are strongly depleted, in the range +4.5 to +10, and initial Sr ratios 0.7020-0.7039.
The geochronology shows that the earliest, crust-forming Pan-African rocks from northernmost Eritrea formed in the age range 900-800 Ma, with a minimal crustal inheritance. The present data, together with previous data, show slightly higher ages for the highland rocks (875-840 Ma) compared to lowland rocks (840-790 Ma). Zircon overgrowths indicate metamorphism from 670 to 585 Ma, overlapping with the intrusion of the c. 605 Ma late-tectonic granite, and the c. 590 Ma post-tectonic granite from central Eritrea.
The mafic samples represent melts derived from MORB-like depleted mantle sources spatially variably overprinted by subduction-generated H2O-rich fluids, and presumably partly also plume-derived melts. The strongly depleted isotopic character of both mafic and felsic rocks shows that the felsic rocks were derived exclusively within the juvenile Pan-African crust, and that the enrichment of the MORB-type sources occurred shortly before melt extraction.
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