A late Precambrian (∼ 710 Ma) high volcanicity rift in the southern Eastern Desert of Egypt

Abstract

ZusammenfassungDie spfitprfikambrischen Shadli-Metavulkanite in der sfid6stlichen Eastern Desert von )~gypten sind eine schwach metamorphe bimodale Basalt-Rhyodazit-Abfolge, die bisher als Ausdruck eines Inselbogen-Vulkanismus gedeutet wurde. Zwei Basalt-Typen k6nnen aufgrund ihrer unterschiedlichen chemischen Zusammensetzung unterschieden werden: der stratigraphisch untere Typist ein N-MORB fihnlicher Ferrobasalt, w/ihrend der tiberlagernde Typ Charakteristika eines leicht fraktionierten E-MORB aufweist. AbstractThe late Precambrian Shadli Metavolcanics of SE Egypt constitute a slightly metamorphosed bimodal sequence that has been previously interpreted as manifesting volcanic activity at an island arc. We report the first Rb-Sr geochronologic, trace element (including REE), and Nd isotopic data for these rocks. Two types of basalt are recognized, the stratigraphically lower suite having compositions like N-MORB ferrobasalt while the overlying basalt is similar to slightly fractionated E-MORB. The two basalt types were derived from melting of a strongly depleted source, most likely within the upper 60-75 km of the upper mantle. The origin of the felsic melts is problematic, and these could either have fractionated from a mafic melt or resulted from melting of juvenile crust. The mafic and felsic lavas yield a Rb-Sr isochron age of 712 ___ 24 Ma that probably represents the time of volcanic eruption. The trace element characteristics of both mafic and felsic members of the Shadli Metavolcanics show few of the hallmarks of subduction-related melts, and we reject the hypothesis that these formed at an island arc. Instead, the field and geochemical data are most consistent with the hypothesis that these rocks originated in a magmatic rift, where the eruption of large volumes of lava accompanied large-scale lithos-*) Authors' addresses: R. J. STERN, The University of Texas at Dallas,

The Neoproterozoic Kolet Um Kharit bimodal metavolcanic rocks, south Eastern Desert, Egypt: a case of enrichment from plume interaction?

“…El Ramly et al (1982) studied the KUKh metavolcanic rocks and found it difficult to evaluate their tectonic setting but accepted an island-arc setting, in spite of the recognition of relatively high TiO 2 contents. Based on a series of geological and geochemical arguments Stern et al (1991) rejected this hypothesis for metavolcanic rocks from Um Samiuki area (Fig. 1) and suggested a continental rift origin for these rocks.…”

“…1 and Ba/La (4.29-25.9 vs. 7). Stern et al (1991) distinguished ''Basalt I'' from more primitive lower TiO 2 ''Upper basalt'' or ''Basalt II'', which is not recognized in KUKh area. Also, the KUKh felsics are significantly more enriched in potassium than are the Um Samiuki felsics .…”

“…The associated felsic pyroclastics are found in the northern side of KUKh. Stern et al (1991) reported Rb/Sr whole rock age of 712±24 Ma for correlative rocks at Um Samiuki. KUKh metavolcanic rocks have identical paleomagnetic pole position to those of El Shadli, implying similar ages (H. Lofty, personal communication, 2004).…”

“…According to Irvine and Baragar (1971) the Al 2 O 3 content of the tholeiitic series should be lower than 16%, which is the case of the investigated rocks. The diagram of Peccerillo and Taylor (1976) classifies the KUKh mafic metavolcanic rocks and Basalt I of Stern et al (1991) as a low-K tholeiitic magma series and the felsic rocks as high-K magma series (Fig. 3b).…”

“…They constitute the largest metavolcanic exposure (80·25 km and attain a maximum thickness of 10 km (Shukri and Mansour 1980;Stern et al 1991). The SMB has been traditionally regarded as a bimodal island-arc metavolcanic rocks (e.g.…”