GEOCHEMICAL CHARACTERISTICS OF ACCRETED MATERIAL BENEATH THE POZANTI-KARSANTI OPHIOLITE, TURKEY - INTRAOCEANIC DETACHMENT, ASSEMBLY AND OBDUCTION

The mid- to late Cretaceous sub-ophiolitic dynamothermal metamorphic s ole and the underlying unmetamorphosed Aladag melange, defined here as the Aladag accretionary complex, were accreted to the base of the Poz anti-Karsanti ophiolite in southern Turkey during intra-oceanic subduc tion, transport and final obduction of the ophiolite onto the Menderes -Taurus block. Geological, geochemical and structural investigations o f the dynamothermal metamorphic sole and melange indicate that these u nits represent a progression of high to low temperature accreted appen dages to the base of the Pozanti-Karsanti ophiolite formed within the Neo-Tethyan ocean in the mid to late Cretaceous. Intra-oceanic detachm ent of the Pozanti-Karsanti ophiolite appears to have begun in the vic inity of ocean islands generated at an off-axis plume or at a plume-ce ntered spreading ridge within Neo-Tethys during the early late Cretace ous, During the early stages of the intra-oceanic subduction, the dyna mothermal metamorphic sole formed through accretion of portions of the downgoing oceanic slab to the base of the Pozanti-Karsanti ophiolite. This ophiolite formed the forearc basement and leading edge of the so utherly vergent upper plate. The highest temperature accretion to the base of the overthrust ophiolite occurred at similar to 90-94 Ma, and involved dominantly GIB-like alkali basalts metamorphosed under lower to upper amphibolite facies conditions. Subsequently, because of decre asing temperature and pressure and continued subduction of the oceanic crust unaffected by intra-plate volcanism, tholeiitic MORBs and their open ocean pelagic cover were metamorphosed and accreted to the base of the amphibolite facies rocks under greenschist facies conditions. F ormation of the greenschist facies rocks was succeeded by igneous acti vity in which unmetamorphosed mafic to intermediate dyke swarms intrud ed the ophiolite and dynamothermal metamorphic sole, apparently due to a ridge subduction event. Later, low-temperature accretion occurred d uring intra-oceanic subduction and final emplacement of the ophiolite forearc basement onto the Menderes-Taurus block. This is recorded by d evelopment of the Aladag melange complex accreted to the base of the h igher-temperature metamorphic sole, The melange is composed of three m ajor thrust-fault-bounded tectonic packages, each of which has a diffe rent block and matrix lithology, geochemical signature and deformation style. The geochemical analyses of the pelitic protoliths in the meta morphic sole and pelitic matrix in the Aladag melange complex suggest that the sediments accreted to the base of the overriding plate in the early stages were primarily derived from open-ocean pelagic sediments and juvenile oceanic island-are Volcanic sources, whereas the sedimen ts accreted in the relatively later stages were derived from mixed sou rces that included mafic/oceanic and felsic/continental sources. These were probably orogenic in nature and presumably formed as the forearc encroached on the Menderes-Taurus stable margin. During its final emp lacement and obduction, the Pozanti-Karsanti ophiolite was thrust onto the northward-facing Devonian-early Cretaceous Menderes-Taurus carbon ate margin. These high- to low-temperature accreted packages and conti nental margin sediments are strongly folded and disrupted along southw ard-directed thrust surfaces and now represent an allochthonous to par a-autochthonous sequence of thrust sheets resting on the eastern Tauri de autochthon.