A.Ya. Medvedev

Compositional variations of spreading basalts of the King’s trough (Central Atlantic) and possible reasons for them

194 The King’s trough is a remarkable tectonic struc ture in the eastern Central Atlantic (Fig. 1) [1]. It is represented by a chain of deep troughs (5000 m and more) and surrounding ridges with a total length of 450 km and a width up to 80 km. The King’s trough is located 700 km to the northeast from the Azor Islands and elongated in the east southeasterly direction from the eastern slope of the Mid Atlantic Ridge (MAR) to the Azores–Biscay height. The origin of this mysteri ous structure has long been subjected to discussions, and the two stage formation of trough is the most grounded model at present [2].

Two types of alkaline volcanics in the southwestern Iberian margin: The causes of their diversity

The diverse geodynamic conditions of the parental magma’s melting are responsible for the compositional diversity of the alkaline volcanics near the southwestern margin of Iberia. The petrological-geochemical data show that the volcanics of the Gorringe Bank originated within the continental plate. The parental melilitite melts depleted in silica and anomalously enriched with trace elements could have been generated only in deep settings with a low degree of metasomatically enriched mantle matter melting. The volcanic melilitite-nephelinite-phonolite series is widespread in alkaline provinces of the Eurasian, African, and other continental plates. The Ampere, Josephine, and other seamounts and islands of the region are largely composed of volcanic rocks belonging to the picrobasalt-hawaiite-mugearite association. Their parental magmas were generated within the oceanic plate at shallower depths under a higher degree of moderately enriched oceanic lithospheric mantle melting. Both series of volcanics were formed under the influence of mantle plumes.