GEOCHEMICAL INSIGHTS INTO MANTLE FLOW BENEATH THE ANATOLIAN PLATE

Anatolia is a natural laboratory for studying and identifying complex geodynamic processes between continental lithosphere, convecting asthenosphere and subducting slabs. New topographic, tomographic and geochemical studies enable an integrated picture of both regional and small-scale geodynamic interactions in space and time. The complex tectonics of the Anatolian microplate are generally well constrained. The African and Arabian plates currently push Anatolia against the relatively stable Eurasian plate, resulting in WSW-directed ‘escape tectonism’ over the past 12 Ma that is accommodated by three bounding fault systems: 1) the Aegean-Cyprean arc, 2) the dextral North Anatolian Fault Zone, and 3) the sinistral East Anatolian Fault Zone. There is strong geophysical evidence for slab rollback in Western Anatolia and slab detachment in Eastern Anatolia (e.g. Le Pichon & Angelier 1981; Bozkurt 2001; Keskin 2003; Şengör et al. 2003); beneath Central Anatolia new tomographic evidence suggests a tear in the slab (Biryol et al. 2009). To gain better insight into the interaction between downgoing slabs and the upwelling mantle in these three environments, we use observed differences in the geochemical signatures of primitive mafic lavas to place constraints on the timing and pathways of mantle flow associated with slab rollback, slab detachment and tear(s) in the downgoing African slab. Late Miocene-Quaternary mafic volcanism throughout Anatolia preserves the signatures of multiple distinct source regions for basalt genesis. The relative abundances of key incompatible trace elements coupled with new radiogenic isotopic Sr, Nd, Pb and Hf data indicate variable mixing between asthenospheric mantle and other source domains within the lithosphere, including subduction-modified mantle. Specifically, Sr-Nd isotopic values measured on Late Miocene-Quaternary primitive basalts from Western, Central and Eastern Anatolia converge on a common asthenospheric source component that is compositionally similar to the shallow mantle underlying mid-ocean ridges worldwide. The asthenospheric component is not observed in Mid Miocene and earlier volcanism indicating a significant temporal change in available sources. Local involvement of the other source domains produces a unique composition for each volcano and each region of Anatolia. Systematical geographical source variations are clearly observed in Pliocene-Quaternary basalts from the Central Anatolian Fault Zone. Karapinar, the southernmost volcano, displays the most enriched trace element contents (Ba/Nb = 25-50, Th/La = 0.2-0.04; Alici Şen et al., 2004). Sivas, the northernmost volcano, has the most primitive mafic lavas (Ba/Nb = 9-20, Th/La = 0.1-0.25; this study); their trace element compositions require a substantial asthenospheric component. Within Central Anatolia, negative Nb-Ta anomalies, characteristic of subduction-related lavas, decrease from south to north accompanied by decreasing Ba/Nb and Th/La ratios. Isotopic variations within Central Anatolia show similar patterns. Sr-Nd isotopic values are more radiogenic in Karapinar basalts (¹⁴³Nd/¹⁴⁴Nd = 0.51260-0.51265; Alici Şen et al., 2004) than in contemporaneous Sivas lavas (¹⁴³Nd/¹⁴⁴Nd = 0.51257-0.51280; this study). Interestingly, both Karapinar and Sivas have enriched Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb>18.5, ²⁰⁸Pb/²⁰⁷Pb>38.5, ²⁰⁷Pb/²⁰⁴Pb>15.6) that suggest contribution from a recycled oceanic sediment source component, presumably related to subduction of the African plate. Apparent mixing trends between enriched and depleted source compositions observed in all Anatolian mafic lavas indicate a dynamic interaction between the continental lithosphere, the convecting upper mantle and the subducting slab(s). In particular, the geochemical signature seen in the Sivas basalts requires a contribution from upwelling asthenospheric mantle placing important constraints on the nature of mantle flow around subducting slabs in Anatolia.