Origin of submarine volcanism at the eastern margin of the central atlantic: Investigation of the alkaline volcanic rocks of the carter seamount (Grimaldi Seamounts) |
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| Authors: | S G Skolotnev V V Petrova A A Peyve |
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| Institution: | 1.Geological Institute,Russian Academy of Sciences,Moscow,Russia |
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| Abstract: | This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of
the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58
Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea
Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites,
ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization
of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from
the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered
by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very
low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high
Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of
their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies
on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and
the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics
was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material
(HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere,
whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt
assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated
into the oceanic lithospheric mantle during rifting and the breakup of Pangea. |
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