Geochemical characteristics of accreted material beneath the Pozanti-Karsanti ophiolite, Turkey: Intra-oceanic detachment, assembly and obduction

Abstract

The mid- to late Cretaceous sub-ophiolitic dynamothermal metamorphic sole and the underlying unmetamorphosed Aladag melange, defined here as the Aladag accretionary complex, were accreted to the base of the Pozanti-Karsanti ophiolite in southern Turkey during intra-oceanic subduction, transport and final obduction of the ophiolite onto the Menderes-Taurus block. Geological, geochemical and structural investigations of the dynamothermal metamorphic sole and melange indicate that these units represent a progression of high to low temperature accreted appendages to the base of the Pozanti-Karsanti ophiolite formed within the Neo-Tethyan ocean in the mid to late Cretaceous. Intra-oceanic detachment of the Pozanti-Karsanti ophiolite appears to have begun in the vicinity of ocean islands generated at an off-axis plume or at a plume-centered spreading ridge within Neo-Tethys during the early late Cretaceous. During the early stages of the intra-oceanic subduction, the dynamothermal 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 southerly vergent upper plate. The highest temperature accretion to the base of the overthrust ophiolite occurred at ∼90–94 Ma, and involved dominantly OIB-like alkali basalts metamorphosed under lower to upper amphibolite facies conditions. Subsequently, because of decreasing 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. Formation of the greenschist facies rocks was succeeded by igneous activity in which unmetamorphosed mafic to intermediate dyke swarms intruded the ophiolite and dynamothermal metamorphic sole, apparently due to a ridge subduction event. Later, low-temperature accretion occurred during intra-oceanic subduction and final emplacement of the ophiolite forearc basement onto the Menderes-Taurus block. This is recorded by development of the Aladag melange complex accreted to the base of the higher-temperature metamorphic sole. The melange is composed of three major thrust-fault-bounded tectonic packages, each of which has a different block and matrix lithology, geochemical signature and deformation style. The geochemical analyses of the pelitic protoliths in the metamorphic 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-arc volcanic sources, whereas the sediments accreted in the relatively later stages were derived from mixed sources 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 emplacement and obduction, the Pozanti-Karsanti ophiolite was thrust onto the northward-facing Devonian-early Cretaceous Menderes-Taurus carbonate margin. These high- to low-temperature accreted packages and continental margin sediments are strongly folded and disrupted along southward-directed thrust surfaces and now represent an allochthonous to para-autochthonous sequence of thrust sheets resting on the eastern Tauride autochthon.