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

Conclusions

[83]  1. The material dredged from the MAR axial zone (within the Sierra Leone MAR segment at 6o N) contains both plutonic rocks and basalts, including fresh varieties with volcanic glass crusts. The plutonic rocks belong to two associations: (i) primitive high-Mg troctolites, olivine gabbro, and gabbro, whose geochemistry is close to that of MORB, and (ii) diverse rocks with orthopyroxene, variable amounts (from 0.5 vol % to 30 vol %) of magmatic brown hornblende of the kaersutite series, ilmenite, and magnetite. The composition of these rocks ranges from ultramafic cumulates (harzburgites and lherzolites) through pyroxenites, norites, and gabbronorites to mineralized hornblende Fe-Ti-oxide gabbrodiorites and gabbrodiorites and, then, to diorites and plagiogranites (trondhjemites). Subvolcanic analogues of this association are hornblende Fe-Ti-oxide dolerites, and its volcanic analogues are basalts containing Fe-Ti oxides.

[84]  2. The rocks of the second association, which account for no less than one-third of the dredged material, were recognized as a silicic Fe-Ti-oxide magmatic series. All rocks of this series (except only the granites and diorites) are depleted in REE, Zr, Th, Ba, Hf, K, Ni, and Cu relative to the primitive mantle and MORB and enriched in Pb, U, Ta, Nb, Zn, Cs, and Rb. This uncommon fractionation of trace elements makes this series different from the rocks of both the spreading (i.e., derivatives of MORB) and the plume (i.e., derivatives of OIB) association. The series is similar to the latter rocks only in having high contents of Ti, Ta, and Nb in the Fe-Ti-oxide gabbroids themselves, whereas most incompatible elements are deficient.

[85]  3. The parental melts of the silicic Fe-Ti-oxide association were depleted in incompatible elements, i.e., were derived from a source that had already undergone melting, and, correspondingly, these melts could not be derivatives of MORB. At the same time, they are enriched in elements usually mobilized by fluids, particularly aqueous ones, at high temperatures. This suggests that the melts were derived from water-bearing rocks. The petrological-geochemical characteristics of this series are intermediates between those of island-arc and within-plate rocks.

[86]  4. Data particularly important for understanding the genesis of the melts that produced the Fe-Ti-oxide association pertain to the plagiogranites (trondhjemites), the latest acid derivatives of this series. These rocks are characterized by high concentrations of incompatible elements, negative Eu and Sr anomalies, and are slightly enriched in LREE. According to the Nd-Sr isotopic data, the petrogenesis of these late differentiation products was participated by seawater.

[87]  5. The genesis of the parental magmas of this association is thought to have been related to the melting of the hydrated oceanic lithosphere during the emplacement of a new asthenospheric plume (protuberance on the surface of an asthenospheric lens beneath the ridge). The newly formed mantle melt passed through the upper, colder part of the plume, accumulated on its surface, and produced a large chamber, which started to ascend in compliance with the zone melting mechanism, i.e., by melting the roof rocks of the chamber and crystallizing near the bottom. The melt was thereby continuously enriched in components not only from the melting rocks of the chamber roof but also from the partly melted rocks at the peripheries of the melting zone, as well as in fluid material from the strongly heated rocks in the distant periphery. This seems to cause the unusual characteristics of the mantle-crustal melts, for example, their enrichment in SiO2, Fe, Ti, Pb, Cu, Zn, etc.

[88]  6. Because of the large-scale assimilation of the hydrated material of the upper mantle and the lower oceanic crust and as a result of crystallization differentiation in the chamber, the composition of this melt continuously evolved, as also did the compositions of the settling crystalline phases. Hence, the ascending chamber left behind a trail in the form of a cumulate series ranging from ultramafic to mafic compositions, whose inner structure resembles that of a layered intrusion.

[89]  7. A consequence of this mechanism of the origin of the parental melts is the occurrence of inclusions of foreign minerals in the cumulus phases of the rocks, which seem to be incompletely melted fragments of the roof rocks that served as "seeds'' during the crystallization of minerals in the magmatic chamber. Geochemical consequences of this mechanism are the aforementioned unusual distributions of trace elements and REE, which can explain the genesis of the HIMU isotopic source without invoking subduction processes.

[90]  8. The hydrous melts of the silicic Fe-Ti-oxide association were characterized by limited vertical mobility because of a drastic decrease in the water solubility in melts at pressures of 1 kbar. Because of this, volcanic rocks of this association are very rare. At the same time, the solidification of the chambers of such melts at depths of 3-4 km should have been associated with the release of significant amounts of mineralized fluids enriched in Cu, Pb, Zn, and other ore components. This process is believed to be responsible for the origin of sulfide ore mineralization in the form of black smokers and massive hydrothermal-metasomatic ores, analogous to those found in the Markov depression.


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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