S. G. Skolotnev, A. A. Peyve, S. M. Lyapunov, V. A. Simonov, Yu. E. Glazyrin, and V. Yu. Kolobov
N). The
links of basalts of various compositions with particular ocean-floor
geological structural features have been analyzed as well.
Three basaltic varieties have been discriminated. Almost
ubiquitous are high-Mg basalts that are derivatives of N-MORB
tholeiitic melts and that are produced in the axial zone of spreading.
Variety 2 is alkaline basalts widespread on the southwest flank of the
MAR crestal zone in the Sierra Leone region, likely generated through
deep mantle melting under plume impact. Variety 3 is basalts
derivative from T- and P-MORB-like tholeiitic melts and originating
through addition of a deeper mantle material to depleted upper mantle
melts. Magma generation parameters, as calculated from chilled glass
compositions, are different for depleted tholeiites (44-55 km, 1320-1370o
C)
and enriched tholeiites (45-78 km, 1330-1450o C).
Mantle plume impact is shown to affect not only tholeiitic basalt
compositions but also magma generation conditions in the axial
spreading zone, resulting in higher Ti and Na concentrations in the
melts parental to the rift-related basalts occurring near the plume. T- and
P-MORBs are also developed near the areas where mantle plumes
are localized.
The high-Mg basalts are shown to come in several types with
distinctive Ti and Na contents. Nearly every single MAR segment
(bounded by sinistral strike slips and the Bogdanov FZ) is featured by
its own basalt type suggesting that it has formed above an
asthenospheric diapir with its unique magma generation conditions.
These conditions are time variable. The likely causes of the temporal
and spatial instability of mantle upwelling beneath this portion of the
MAR are singular tectonic processes and plume activity.
In the sulfide-bearing rift morphostructures (the so-called "Ore
area'' and the Markov Basin), basalts make up highly evolved suites
generated through olivine and plagioclase fractionation, which is
suggestive of relatively long-lived magma chambers beneath the
sulfide-bearing rift morphostructures. The functioning of these
chambers is a combined effect of a singular geodynamic regime and
plume activity. In these chambers, melts undergo deep differentiation
leading to progressively increasing concentration of the sulfide phase,
eventually to be supplied to the hydrothermal plumbing system.