RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES4001, doi:10.2205/2005ES000181, 2005

Introduction

[2]  The third crustal layer in slow-spreading ridges, such as the Mid-Atlantic Ridge (MAR) and the Southwest Indian Ridge, is known to consist of a broad spectrum of plutonic rocks, which include, along with primitive magnesian troctolites and gabbro, also ferrogabbro (Fe-Ti-oxide gabbro) and closely associated with it norites, gabbronorites, hornblende gabbronorites, and gabbronorite-diorites, quartz diorite, as well as veins and small bodies of plagiogranites (trondhjemites) [Dick et al., 2000; Ozawa et al., 1991; Pearce, 2002; Silantyev, 1998].

[3]  These rocks are still studied relatively poorly because they are commonly regarded as late derivatives of tholeiitic basalts in mid-oceanic ridges (MORB) [Dick et al., 1992; Silantyev et al., 1998]. However, Simonov et al. [1999] were the first to note that the parental melts of the Fe-Ti-oxide gabbroids could not be derivatives of either N-MORB or E-MORB, because they are oversaturated with TiO2. These rocks are sometimes regarded even as evidence of the existence of anomalous mantle sources [Cannat et al., 1992]. The nature of the trondhjemites is also uncertain: according to Dick et al. [1991], these rocks were generated by the fractional crystallization or anatexis of the host amphibolized gabbroids, whereas Silantyev [1998] believes that they had had a separate deep source with anomalous geochemical characteristics typical of source like E-MORB.

[4]  Our petrological and geochemical data obtained on these rocks from the Sierra Leone MAR segment indicate that these rocks display some unusual features which have not been adequately fully highlighted before. First of all, these rocks are characterized by low contents of incompatible elements, including LREE, which suggests that the parental melts of these rocks were derived from sources that had already been remelted, and, thus, these rocks could hardly be MORB derivatives. Moreover, these melts exhibit some extraordinary compositional features: on the one hand, they were saturated or even oversaturated with silica and had high water contents, as is typical of magmas of the suprasubduction calc-alkaline series, and, on the other hand, were rich in Fe, Ti, and Nb, as is characteristic of within-plate (plume-related) magmas.

[5]  All of these features seem to suggest that slow-spreading ridges contain, along with MORB (which are typical of such environments) and their derivatives, also an unusual type of magmatic melts, which are enriched in silica and do not fall into any of the currently adopted systematics for magmatic. This publication is devoted to the petrological-geochemical characterization of these rocks and their genesis.


RJES

Citation: Sharkov, E. V., N. S. Bortnikov, T. F. Zinger, and A. V. Chistyakov (2005), Silicic Fe-Ti-oxide series of slow-spreading ridges: petrology, geochemistry, and genesis with reference to the Sierra Leone segment of the Mid-Atlantic Ridge axial zone at 6° N, Russ. J. Earth Sci., 7, ES4001, doi:10.2205/2005ES000181.

Copyright 2005 by the Russian Journal of Earth Sciences

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