Etude geochimique des elements en traces dans les series de roches volcaniques du rift d'Asal; identification et analyse des processus d'accretion

Abstract

Study of samples of the main volcanic units of the Afar rift, from margins to late axial series, have yielded the following conclusions: 1) the magmatic liquids are strongly differentiated by a fractional crystallization process in shallow chambers, the evolution of which is limited by injection of new primary liquids as shown in the latest axial series; 2) primary liquids are generated by melting of a homogeneous mantle source, a process whereby successive partial melting in a closed system is stopped by percolation of the generated liquid. The degrees of partial melting are closely related to spreading rates; 3) the magmatic processes involve a discontinuous dynamic evolution of spreading displayed by a pulsatory magmato-tectonic activity which is controlled by the speed of decompression of mantle material.

Crustal structure and magmato‐tectonic processes in an active rift (Asal‐Ghoubbet, Afar, East Africa): 2. Insights from the 23‐year recording of seismicity since the last rifting event

“…The weight of the Fieale topographic edifice may also impede lava ascent [ Pinel and Jaupart , 2000; Muller et al , 2001]. The “light”, viscous liquids and crystal mush rich in plagioclases [see Joron et al , 1980; Bizouard et al , 1980] that may arise first from the original melts may be those most arrested by the capping plug. By contrast, the denser, less viscous basaltic liquids may more easily escape laterally, away from the reservoir and overlaying plug [ Walker , 1999, and references therein].…”

“…A large volcano, Fieale, occupies the eastern part of the subaerial inner floor, where a long‐lived magma chamber is suspected [ Allard et al , 1979; Demange et al , 1980; Joron et al , 1980, Van Ngoc et al , 1981]. That volcano has been active from ∼300 to 80 ka [ Manighetti et al , 1998].…”

“…The long‐term, ∼300‐ka‐long evolution of the “modern” rift is marked by alternating phases of dominant magmatic activity, with large volumes of basaltic lavas filling the inner floor and burying most previous faults, and phases of major faulting [ Stein et al , 1991; Manighetti , 1993; De Chabalier and Avouac , 1994; Manighetti et al , 1998]. In the last ∼300 ka, volcanism evolved from fissural to central in the SE subaerial part of the rift where the Fieale‐Shark Bay magma chamber is suspected [ Allard et al , 1979; Demange et al , 1980; Joron et al , 1980, Van Ngoc et al , 1981]. In the last ∼100 ka, faulting has been dismantling the whole lava pile including the Fieale edifice [ De Chabalier and Avouac , 1994], only interrupted by short phases of fissural volcanism.…”

Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti

“…The locations of these samples are shown in the auxiliary material, except those of the Hayyabley Basalts which are given by Daoud et al [2010]. In addition to this sample set, we have shown, as envelopes in Figures 3a–3d, the fields of previously published analyses of (1) the Gulf Basalts (Initial Gulf Basalts) [ Richard , 1979; Joron et al , 1980a; Vidal et al , 1991; Deniel et al , 1994], (2) the Asal Basalts [ Richard , 1979; Joron et al , 1980b; Vidal et al , 1991; Deniel et al , 1994], and (3) offshore basalts dredged along the gulf axis [ Barrat et al , 1990, 1993].…”

“…In the TAS plot of Figure 3a, they lie on both sides of, but close to, the line separating the fields of subalkalic and alkalic basalts [ Irvine and Baragar , 1971], with the exception of Hayyabley Basalts which are clearly subalkalic. The Gulf, Goumarre and Somali Basalts can therefore be considered as transitional basalts with a dominant tholeiitic tendency [ Richard , 1979; Bizouard and Richard , 1980; Joron et al , 1980a, 1980b; Vidal et al , 1991; Deniel et al , 1994]. The various groups may be quite readily distinguished using the TAS plot (Figure 3a), which has been used, together with other major and trace element data (Figures 3b–3d) to draw the boundaries between them in the new geological sketch map (Figure 2a).…”