major difference between the two types of lavas in a level of incompatible
trace element differ by a factor of >5x for La, Nb, Zr and Ta; this explains
differences in the level of partial melting or the magma were generated from
the source with different trace element composition. Chondrite-normalized rare earth element diagrams
of both lavas of the area shows LREE (light rare earth element) enrichment over
HREE (heavy rare earth element); the enrichment history of LREE over HREE is
more pronounced on the melanephelinite lavas.
high enrichment of LREE over the heavy one of the melanephelinite lava crosses
the basalt lavas Chondrite-normalized diagram and together with non-correlation
trend on a variation diagram between them; reflecting their derivation from
various mantle source region.
Hirna characterized by high La/Sm ratio (for the basalt = ~ 2.7; for the Melanephelinite
= ~ 7), Sm/Y ratio (for the basalt = ~ 3.4; for the Melanephelinite= ~ 6.4) relatively
fractionating HREE pattern (Tbn/Ybn = 2.03 to 2.7) and
together with Lan/Ybn ratiovalue(5 to 28) which most likely suggest
mantle source containing garnet rather thanspinel. The lavas of Hirna exhibit enrichment of Ba
and depletion of Rb and K relative to other elements with a similar degree of
incompatibility in their primitive mantle-normalized variation diagram (fig.
5). Such depletion and enrichment are thought to be related to
amphibole/phlogopite in the mantle source (e.g. Furman and Grham, 1999; Ayalew
et al., 2006; Jung et al., 2005; 2012; Mayer et al., 2014; Rooney et al.,
Ba are compatible with phlogopite while Sr, Rb and Ba are moderately compatible
with amphibole. Melts in equilibrium with phlogopite are expected to have
significantly higher Rb/Sr and lower Ba/Rb values whereas melt in equilibrium
with an amphibole-bearing source are expected to have a higher Baand Ba/Rb
value. Furman et al.(1999) presented
lava from many volcanic areasthat have low K2O/Na2O
(<0.75) and low Rb/Sr values (<0.06) and interpreted these result as consistent with the melting of amphibole bearing source, Hence the Hirna lavas characterized by higher Ba content, low K2O/Na2O (0.23 to 0.47) and Rb/Sr value (< 0.06), this trace element ratio distribution consistent with the melting of an amphibole bearing-source region. Amphibole is not stable mineral at a temperature of the convicting upper mantle or the thermal mantle plume in the deep mantle; it is stable at pressure-temperature conditions of the lithospheric mantle (up to 3Gpa and 1050 to 1150oC, Class and Goldstein, 1997; Mayer et al., 2014). Therefore identification of amphibole mineral in the source region of Hirna lavas is a very strong evidence for the generation of their primary magma from the lithospheric mantle. In all case we suggest that, the continental lithospheric mantle (CLM) source for Hirna lavas is garnet lehrzolite consist of amphiboles mineral. Pilet et al. (2008) explain that, metasomatic veins in the lithosphere can produce melts with elemental compositions matching those of extreme alkaline lavas (nephelinites). The veins consist of a group of minerals that are stable in the lithosphere (amphiboles) but not in the seismic low-velocity zone (LVZ). The results provide an evidence for a lithospheric origin of alkaline magmas which are enrichedin highly incompatible trace elements such as Ba, Rb, Th, U, Nb, and light rare earth elements. Carbonatitemetasomatized mantle show an increase in Zr/Hfratio value ranging between ~ 45 and 100 (Duputy et al., 1992; Rudnick et al., 1993). The melanephelinitelava of the area haveZr/Hfratio value ranges between 44 to 45;which is very high compared with the primitive mantle value (Zr/Hf ratio ~36: Sun and McDonough, 1989). The elevated Zr/Hf values (and overall high incompatible trace element enrichment) of the melanephelinite lava of the area suggested that, small-volume carbonatite metasomatic fluid have enriched the source region. In general it is concluded that, the melanephelinite rock suites are more likely generated from amphibole-bearing garnet lherzolite continental lithospheric mantle (CLM) containing veins of small-volume carbonatite metasomatic fluid.Whereas, the basalts (silica saturated) lavas of the area comes from un-metasomatized continental lithospheric mantle containing amphibole-bearing garnetlherzolite. This is more likely the plausible explanation for the existed trace element variation between the two distinctive lavas of the area.