Mapping and Analyzing the Volcano-Petrology and Tectono-Seismicity Characteristics Along the Syrian Rift – NW the Arabian Plate - Advances in Data, Methods, Models and Their Applications in Geoscience - page 145

3.2 Composition of the lithospheric mantle
A number of volcanoes along the rift contain a number of ultrasiques xenoliths, notably
lherzolites, harzburgites and pyroxenites(Bilal and Touret 2001).Major rock -forming
minerals are Olivine (Ol), Ortho- and Clinopyroxene (Opx and Cpx),with as common
accessories spinel and amphibole. Microstructure varies from coarse-grained, coarse-
grained-tabular to rare porphyroclastic (Ismail et al.2008).Most mantle peridotites are very
well preserved with however a small variable possibility of local melting by the enclosing
basalt around intergrain boundaries (Fig.5, A, B, C).
Most abundant rock types are harzburgites (Ol+Opx),which from their mineral composition
and geochemistry can be divided into three groups (Ismail et al 2008): Group I, issued from
a residual, depleted mantle, Groups II and III which correspond to a refertilized mantle,
caused by the percolation of undifferentiated basaltic melt or ephemeral carbonate magmas
through the residual lithosphere. Both groups correspond to a different degree of melting of
the mantle peridotite (large for Group II, small for Group III).They are characterized by
undispread mantle metasomatism with a carbonatite signature (Frezzotti et al 2002, Gregoir
et al 2000), as notably indicated by the composition of clinopyroxene in some pyroxenites
(Bilal and Sheleh 2004).
Rare garnet-bearing varieties have also been observed in the middle and south domains
(Mheilbeh,Tel Thenoun) including few grenatites. These correspond most probably to lower
crustal granulites, even if the occurrence of some high-pressure basaltic derivates cannot be
excluded (Bilal and Touret 2001). The possible occurrence of xenoliths corresponding to
lower crustal granulites is further indicated by the occurrence of sapphirine in some garnet
and/or spinel-bearing websterites (Opx and Cpx-bearing pyroxenites, Fig. 5 D ,E) (Bilal
2009b, Bilal et al.2011). These basalts result from a complex polybaric melting process, first
starting in the garnet peridotite stability field, then proceeding within the field of spinel
peridotite( Bilal and Touret 2001,Bilal et al.2011).
3.3 Fluid inclusions
A great of pure CO2-bearing fluid inclusions have been found in olivine and pyroxenes
from xenoliths, and in phenocrysts from enclosing basalts (Bilal and Touret 2001).This type
of inclusions occur in virtually all mantle xenoliths in basalts worldwide(Roedder 1984),but
in the present case some features confirm the occurrence of mantle metasomatism seen
above in group II and III.The CO2 density in inclusions is very variable ,most commonly
around or lower the critical point(about 0,4 g/Cm3).Fluid pressure at trapping conditions
,for a reference temperature of about 1000C,correspond to a depth of about 5 Km, namely
the last magma chamber prior to eruption .But some primary inclusions contain fluid of
much higher density recording deeper episode of the rock evolution. Highest fluid densities
(up to 1, 15 g/cm 2 ) are found in pyroxenites, notably in clinopyroxene. Fig 5 (F , G, H, I)
show primary inclusions, of tubular shape, aligned along orthopyroxene or plagioclase
exsolution lamellae within the clinopyroxene host .It is belived that these fluids are formed
by a reaction illustrating the mantle metasomatism carbonatite connection:
Olivin+Carbonate (from the Carbonatite)→Clinopyroxene (with plagioclase and
orthopyroxene in solid solution)+Co2.
P-T conditions of mineral equilibration in the xenoliths can are deduced from the pyroxene
mineral assemblage (pyroxene thermometry) for the temperature (Wells 1977, Bertrand and
Mercier 1986,Brey and Kohler 1990, Kohler and Brey 1990),and from the maximum fluid