Origin and emplacement of the Late Cretaceous Baer–Bassit ophiolite and its metamorphic sole in NW Syria

The Baer–Bassit ophiolite, of inferred Late Cretaceous age, was emplaced from the south Tethys ocean onto the leading edge of the Arabian continental margin in latest Cretaceous (Maastrichtian) time. Dismembered sequences in different thrust sheets can be correlated to produce a complete ophiolite sequence, with a metamorphic sole at the base, overlain, in turn, by upper mantle tectonite, rare cumulates, massive and layered gabbros, localized high-level plagiogranites, sheeted dykes, basic extrusives and minor Fe–Mn sediments (umbers). The restored ophiolite sequence is similar to that of the more intact Troodos and Hatay ophiolites, but dissimilar to Oman. The Baer–Bassit extrusives are magnesian and strongly depleted, comparable to primitive island arc tholeiites and some boninitic lavas, which favours a subduction-related origin. A well-developed metamorphic sole is divisible into high-grade (amphibolite facies) assemblages in the north and lower-grade (greenschist facies) assemblages in the centre of the region, but no complete inverted metamorphic gradient is preserved in any one local area. The protoliths of both the high-grade (amphibolites) and lower-grade rocks (greenschists) are interpreted as alkali basalts and pelagic sediments, including chert, similar to the volcanic rocks in an underlying unmetamorphosed melange. The metamorphic fabrics progressively evolved from ductile to brittle during tectonic transport towards the southeast. Most of the metamorphic fabrics evolved during intense shearing, but some of the alkaline metabasites apparently were metamorphosed under relatively static strain conditions. The apparent absence of preserved inverted metamorphic gradients in Baer–Bassit may reflect a complex deformation history during emplacement. Alternatively, differing P–T conditions may be recorded at different depths at the base of the over-riding mantle wedge. During emplacement, the front of the ophiolite was tectonically imbricated and overthrust by the main ultramafic slab (Bassit massif). The thickest massif (Baer) is underlain by a high-grade metamorphic sole and was internally shortened but without thrust duplication of the entire ophiolite sequence. Following covering by Late Maastrichtian–Palaeogene marine calcareous sediments, the area was subjected to mid-Tertiary regional folding. This was followed by Neogene dominantly left-lateral, strike-slip deformation along the African–Eurasian plate boundary, extending from south of Cyprus to the Dead Sea transform fault. As a result, the originally emplaced thrust sheets were dissected into three main composite units (Baer, Bassit and the Southeastern units), separated by strongly faulted and sheared ophiolitic blocks and unmetamorphosed volcanic–sedimentary melange.