Les enclaves du volcanisme recent du rift syrien

Abstract

Abstract 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 Turkey, 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 displacements 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 Mesozoic times, with eruptive centers located in three major domains, southern (S), center (M) and northern (N), respectively. The recent (Neogene-Quaternary) volcanism is compared to the Cretaceous one (Bhannes-Tayasir episode). Erupted lavas are in general very basic (picrobasalts, basanites), with rather primitive magmas, except for one occurrence at El Kafr (southern Syria) which corresponds to a silica-undersaturated, strongly differentiated phonolite. Major and partial (Rb, Ba, Nb, Sr, Y, Lu) trace-element data show overall similarities between recent and Cretaceous volcanisms, with however a more distinct alkaline trend and stronger variations of LILE-elements for recent lavas. Few volcanoes contain a number of ultrabasic xenoliths, notably lherzolites, harzburgites and pyroxenites. Rare garnet-bearing varieties have also been observed in M and S-domains, including few grenatites. Xenolith texture is protogranular or granular, with a variable (mostly limited) degree of local melting by the enclosing basalt. Olivines are Mg-rich (mg (super *) = 0.93-0.83), as are ortho-and clinopyroxene. This last mineral, which may show spectacular Opx and spinel exsolution lamellae, is relatively abundant, as shown by the frequent occurrence of pyroxenite (Cpx-Opx) and Cpx-rich lherzolite (wehrlite). Because of the possible occurrence of Cretaceous rocks with kimberlitic affinities [Nabi Mata, Sharkov et al., 1993], garnet has been studied in detail. On the garnet triangle, most analyses plot well away from high-pressure mantle rocks, notably kimberlites, but close to infracrustal garnet-bearing rocks (granulites). Very few analytical points (3 out of a total of 20) could correspond to garnet peridotite. All other rocks are equilibrated in the field of spinel peridotite or infracrustal granulites. Pure CO 2 -bearing fluid inclusions have been found in olivine and pyroxenes from xenoliths and in phenocrysts from enclosing basalts. Highest density fluids (up to 1.15 g/cm 3 ), are observed in pyroxenites, especially from M-domain. They occur in primary, tubular inclusions adjacent to or even containing minute spinel grains, oriented along the exsolution lamellae of the clinopyroxene-host. P-T conditions of mineral equilibration in the xenoliths have been estimated from the pyroxene thermometer [Bertrand and Mercier, 1986; Brey and Kohler, 1990] and maximum density of fluid trapped in primary inclusions. They correspond to about 1 100-1 300 degrees C for the temperature, 10-13 kb for the pressure. These P-T conditions do not show any significant variation between different regional occurrences, but well between various petrographical types, the maximum conditions being recorded in pyroxenites. These results suggest that some clinopyroxene at least has been formed by mantle metasomatism caused by ephemeral carbonate magmas, in a mantle plume located under the Arabic plate.