Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts [Review]

摘要

The Ethiopian continental flood basalt (CFB) province (similar to30 Ma, > 3 x 10(5) km(3)) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 x 10(4) km(3), which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite +/- Ti-magnetitc, richterite, and eckermanite) suggests temperatures in the range of 740 to 900degreesC. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high Nd-143/Nd-144 and low Sr-87/Sr-86 ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle. Rb-Sr isochrons on whole rocks and mineral separates (30.1 +/- 0.4 Ma for Wegel Tena and 30.5 +/- 0.4 Ma for Jima ignimbrites) show that most of the silicic volcanism occurred within < 2 Ma during the Oligocene. Ignimbritic eruptions resumed in the Miocene during two episodes dated at 15.4 +/- 0.2 Ma and 8.0 +/- 0.2 Ma for the Debre Birhan area. The Rb-Sr isochron ages of ignimbrites (both Oligocene and Miocene rhyolites) are indistinguishable within uncertainties from the Ar-40/Ar-39 ages of the underlying flood basalts. The Oligocene ignimbrites and the underlying trap basalts are synchronous with a shift in the oxygen composition of foraminifera recorded in Indian and Atlantic Ocean cores. The temporal coincidence of Ethiopian Oligocene volcanism, which released immense volumes of S (> 1.4 x 10(15) mol) and Cl (6.4 x 10(15) mol) into the atmosphere over a short time span, with the global cooling event at 30.3 Ma suggests that this volcanism might have accelerated the climate change that was already underway. Copyright (C) 2002 Elsevier Science Ltd. [References: 114]