POSTCOLLISIONAL MANTLE DYNAMICS OF AN OROGENIC LITHOSPHERE: LAMPROITIC MAFIC ROCKS FROM SW. ANATOLIA, TURKEY

Several mantle-derived ultrapotassic occurrences of lamproitic affinity are exposed in southwestern Anatolia, mostly intruding the Menderes massif. Their origin was poorly understood, including the accurate timing of emplacement and their relation with the more voluminous shoshonitic volcanism that occur in the region. To elucidate these issues, the rocks from all lamproite occurrences have been dated using the Ar-Ar method on phlogopite, leucite, and glass, and are characterized using their mineral and whole rock major and trace element geochemistry, as well as their Pb, Sr, Nd and Hf isotopic compositions.

From north to south, the volcanism shows increasingly younger ages ranging between 20 and 4 Ma which are coeval with more voluminous shoshonitic, K-calk-alkaline and ultrapotassic volcanoes in the Simav-Selendi, Usak, Kirka, Koroglu, Afyon and Isparta-Golcuk areas. Sr, Nd, Pb and Hf initial isotopic compositions, together with major, trace and rare earth element analyses in whole rocks and Cpx phenocrysts, show that the southward decrease of the ages goes along with changes in geochemical composition, in particular with a decrease of ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb, Zr/Nb, Th/Nb, and an increase of ¹⁴³Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb and Ce/Pb, i.e., systematic changes from crust-like to convecting mantle-like signatures. In contrast, K₂O abundance remains relatively invariable around 7%.

The SW Anatolian volcanism suggests that magma genesis is controlled by postcollisional extensional events initiated after major lithospheric thickening. It can be explained in terms of the P-T relationships of mineral stabilities, mantle solidi and geothermal gradients. The geochemical constraints suggest that the SW Anatolian mantle dynamics underwent complex multistage processes:

i) The first episode leeds to ultra-depletion of the mantle and subsequent metasomatic enrichment, which enabled the coupling of ultradepleted harzburgite with crust-derived sediments. We think this happened during the final stages of southern Neotethyan ocean closure and accretion of forearc oceanic lithosphere as shallowly subducted material to already assembled Anatolia.

(ii) The second episode is related with postcollisional tectonics, most probably with the collapse of the orogenic belt and extension originating horst-graben structures. Our data suggest polybaric partial melting of the phlogopite-bearing harzburgitic lithospheric mantle. Geochemical resemblance of the lamproites with more voluminous coeval magmas implies that the shoshonitic and ultrapotassic volcanic rocks are derived from mantle that had been affected by similar processes. The spatial and temporal variations in the isotopic signatures reflects the increasing contribution of the convecting mantle component in the younger volcanic rocks.

Our data are incorporated into a geodynamical model that explains the overall distribution of volcanism in SW Anatolia, using lamproites as a proxy for mantle dynamics. During the Tertiary extensional tectonics, lithospheric mantle underwent intense astenospherization due to reaction to southward delamination and increase of intensity of lithosphere-asthenosphere interaction. These mantle processes are tightly associated with the major Menderes uplift episodes. Our petrogenetic model has implications for the melt generation and interaction between sub-lithospheric mantle and the SCLM.