THE ORIGIN AND STRUCTURE OF THE LOWER CRUST OF OCEANS AND BACK-ARC SEAS: EVIDENCE FROM THE MARKOV DEEP (MID-ATLANTIC RIDGE) AND THE VOIKAR OPHIOLITE ASSOCIATION (POLAR URALS)

The Markov Deep (the axial part of the slow-spreading Mid-Atlantic Ridge, 6°N, Sierra Leone oceanic core complex) and the Paleozoic Voikar ophiolite association (Polar Urals) formed in the back-arc sea conditions. In both cases, the lower crust of a close structure was formed on the basements composed ofdepleted peridotites of the ancient lithospheric mantle. The available data show that the composition of the lower crust of the oceans and back-arc seas is dominated by layeredmafic-ultramafic intrusions originating from the MORB melts, and suggest a similar asthenospheric source of magmas. Sills and dykes formed from other magma sources represent the second structural element of the lower oceanic crust: in case of the ocean, mainly ferrogabbroids originating from specific melts with the OIB involvement, and, in case of the back-sea sea, gabbro-norites of the supra-subduction calc-alkaline series. In both cases, the upper crust originates frombasaltic flows that occurred later and are associated with new episodes in the tectonic development. According to [Sharkov,2012], the development of slow-spreading ridges takes place in discrete impulses and non-simultaneously along their entire length. Furthermore, oceanic core complexes (OCC) in their axial parts are the ridge segments, where spreading is resumed. At the OCC stage, newly formed basalt melts move upwards from the magma generation zone into fractures (dykes) through the lithospheric mantle, and the thickness of the lower crust is built up by sills. As spreading develops in this area, the crust becomes thicker from below due to underplating in form of large layered intrusions. The newly formed restites, in their turn, cause an increase in the lithospheric mantle thickness from below. Apparently, the lower crust formed in the back-arc seas according to a similar scenario, although complicated by the processes taking place in the subduction zone.