| Popis: |
The Ririba rift represents the southern termination of the Main Ethiopian Rift and formed from the southward propagation of this latter during, or shortly after, the emplacement of subalkaline basalts that produced a widespread basaltic lava basement, at ~3.7 Ma.The activity of the Ririba rift was short-lived and ceased between 2.8 and 2.3 Ma, when deformation migrated westward into an oblique, throughgoing rift zone directly connecting the Ethiopian and Kenyan rifts. Rifting was followed by the eruption of limited volumes of Late Pleistocene-Holocene alkaline basalts, associated to several, monogenetic volcanic centres, forming the Dilo-Dukana and Mega volcanic fields.We provide new petrological, geochemical and isotopic data on the still poorly studied magmatic products emplaced by the volcanic activity of the Ririba rift with the aim of investigating the source of both the older Pliocene lava basement and the Late Pleistocene-Holocene alkaline basalts as well as their pathways to the surface.Major and trace elements, besides discriminating the Pliocene lavas from the other younger alkaline products, reveal that the Dilo-Dukana and Mega samples always overlap in composition. On the whole they display variable major and trace element contents compared to a limited variation in silica (43-46wt.%) describing slightly defined trends. Regular and evident trends are observed comparing some incompatible trace elements (e.g., Rb, Ba, Zr, Nb) suggesting a prominent role of fractional crystallization for their differentiation.Isotopes reveal that the products of the two volcanic fields have small but significantly different behaviour: the Dilo-Dukana products are isotopically homogeneous and clustered around 87Sr/86Sr values of 0.70303 and 143Nd/144Nd values of 0.51292, whereas the Mega lavas and pyroclastics display a small but wider variability partially overlapping the Dilo-Dukana samples, with 87Sr/86Sr ranging from 0.70300 to 0.70334 and 143Nd/144Nd from 0.51293 to 0.51290. Conversely, isotopes data corroborates the evidence that the younger Dilo-Dukana and Mega products are well distinct from the Pliocene basaltic lava basement (as well as with respect to all the older magmatic rocks of the area) and are characterised by a more prominent mantle signature.Moreover, the Sr vs Nd isotopes variation among the younger Holocenic lavas of Mega describe a negative well-defined trend allowing to make inferences about the possible role of crustal contamination during the Ririba rift magmatic activity.All these evidences are consistent with the interpretation that the two young volcanic fields of Dilo-Dukana and Mega are fed by deep structures directly transferring mantle melts up to the surface, as also suggested by the large abundance of mantle xenoliths in the different products. As a consequence, this strongly corroborates the interpretation that the two volcanic fields are not related to the major faults of the Ririba rift, but are associated to different, deep, NE-SW-trending inherited structures which cut the roughly N-S boundary faults of the rift. |
