Mantle xenoliths in recent volcanism from Syrian rift

In direct continuity with the Dead Sea fault, the Syrian rift, which links the rigid Arabian plate to the mobile ophiolite belt of Cyprus-southern Tur key, plays a very important role in the regional geodynamic structure. Its exact position, as well as the related fracture system, has been documented from the analysis of a complete aerial photo coverage of the whole Syrian territory. The rift corresponds to a transform fault, with lateral displace ments decreasing from more than 100 km, to the south, to less than 30 km to the north. Several major episodes of volcanic activity have occurred since early Mesoz oic times, with eruptive centers located in three major domains, southern ( S), center (M) and northern (N), respectively. The recent (Neogene-Quaterna ry) volcanism is compared to the Cretaceous one (bhannes-Tayasir episode). Erupted lavas are in general very basic (picrobasalts, basanites), with rat her primitive magmas, except for one occurrence at El Kafr (southern Syria) which corresponds to a silica-undersaturated, strong differentiated phonol ite, Major and partial (Rb, Ba, Nb, Sr, Y, Lu) trace-element data show over all similarities between recent and Cretaceous volcanisms, with however a m ore distinct alkaline trend and stronger variations of LILE-elements for re cent lavas. Few volcanoes contain a number of ultrabasic xenoliths, notably lherzolites , harzburgites and pyroxenites. Rare garnet-bearing varieties have also bee n observed in M and S-domains, including few grenatites, Xenolith texture i s protogranular or granular, with a variable (mostly limited) degree of loc al melting by the enclosing basalt. Olivines are Mg-rich (mg* = 0.93-0.83), as are ortho- and clinopyroxene. The last mineral, which may show spectacu lar Opx and spinel exsolution lamellae, is relatively abundant, as shown by the frequent occurrence of pyroxenite (Cpx-Opx) and Cpx-rich lherzolite (w ehrlite). Because of the possible occurrence of Cretaceous rocks with kimbe rlitie affinities [Nabi Mata, Sharkov et al., 1993], garnet has been studie d in detail. On the garnet triangle, most analyses plot well away from high -pressure mantle rocks, notably kimberlites, but close to infracrustal garn et-bearing rocks (granulites). Very few analytical points (3 out of a total of 20) could correspond to garnet peridotite. All other rocks are equilibr ated in the field of spinel peridotite or infracrustal granulites. Pure CO2-bearing fluid inclusions have been found in olivine and pyroxenes from xenoliths and in phenocrysts from enclosing basalts. Highest density f luids (up to 1.15 g/cm(v)), are observed in pyroxenites, especially from M- domain. They occur in primary, tubular inclusions adjacent to or even conta ining minute spinel grains, oriented along the exsolution lamellae of the c linopyroxene-host, P-T conditions of mineral equilibration in the xenoliths have been estimated from the pyroxene thermometer [Bertrand and Mercier, 1 986; Brey and Kohler, 1990] and maximum density of fluid trapped in primary inclusions. They correspond to about 1 100-1 300 degreesC for the temperat ure, 10-13 kb for the pressure. These P-T conditions do not show any signif icant variation between different regional occurrences, but well between va rious petrographical types, the maximum conditions being recorded in pyroxe nites. These results suggest that some clinopyroxene at least has been form ed by mantle metasomatism caused by ephemeral carbonate magmas, in a mantle plume located under the Arabic plate.