Source components and magmatic processes in the genesis of Miocene to Quaternary lavas in western Turkey: constraints from HSE distribution and Hf–Pb–Os isotopes

Abstract

crystallization trends indicate distinctly lower bulk partition coefficients for IPGE in more evolved lavas, possibly reflecting a change in the fractionating assemblages. Pd and Re in the primitive melts display negative correlations with MgO, demonstrating moderately incompatible behavior of these elements during fractionation, while the significantly scattered variation in Pt against MgO may indicate the effects of micronuggets of a Pt-rich alloy. Osrich alkaline primary lavas (>50 ppt Os) exhibit a limited range of 187 Os/ 188 Os (0.1361-0.1404), with some xenolith-bearing lavas displaying depletions in 187 Os/ 188 Os (0.1131-0.1232), suggesting slight compositional modification of primitive melts through contamination with highly depleted, Os-rich mantle lithosphere. More radiogenic Os isotope ratios ( 187 Os/ 188 Os > 0.1954) in the evolved lavas reflect contamination of the magmas by high 187 Os/ 188 Os crustal material during shallow differentiation. The OIB-type lavas show limited variations in Hf and Pb isotopes with Hf ratios correlate negatively with 208 Pb*/ 206 Pb*, suggesting the effects of similar mantle processes on the evolution of time-integrated Th/U and Lu/Hf. These lavas have distinctly higher 176 Hf/ 177 Hf and lower 208 Pb*/ 206 Pb* than the Early-Middle Miocene lavas of the region, which are interpreted as melts of enriched mantle with an overprint by sediment-derived subduction component. The source region for the OIB-type alkaline melts is interpreted to be a sub-lithospheric reservoir enriched in Hf and Pb isotopes with respect to depleted MORB mantle. Combined evaluation of Hf, Pb, and Os isotopes suggests that the relative enrichment in this domain is related to mixing of ancient oceanic crust with the ambient mantle through long-term plate recycling processes.Abstract Hf-Pb-Os isotope compositions and highly siderophile element (HSE) abundance variations are used to evaluate the mantle source characteristics and possible effects of differentiation processes in lavas from western Turkey, where the eruption of Late Miocene to Quaternary OIB-type intraplate mafic alkaline lavas followed pre-Middle Miocene convergent margin-type volcanism. Concentrations of Os, Ir, and Ru (IPGE) in the OIB-type intraplate lavas decrease with fractionation for primitive melts (MgO > 10 wt%), suggesting that these elements reside predominantly in olivine and associated HSE retaining trace phases and behave compatibly during olivine-dominated fractionation. Fractional Communicated by

Some remarks on the nature of mantle metasomatism beneath western Anatolian–Aegean region: Contrasting isotopic signatures recorded in the Miocene lavas from the Söke Basin

“…In this context, the partial subduction of a subduction–accretion complex containing material with a wide variety of isotopic compositions may represent a viable explanation for the observed compositional variations. This is consistent with the hypothesis of Aldanmaz et al (), who used Hf isotopic evidence to suggest that some of the lava compositions within the Early to Middle Miocene suite require the involvement of zircon‐deficient pelagic sediments with relatively low 207 Pb/ 204 Pb values, while the compositions of other lavas could likely be explained by the involvement of terrigeneous sediments of an upper continental crustal origin with remarkably high 207 Pb/ 204 Pb values.…”

“…Yet, the abundances of K, Rb, and Th in the lavas remain almost constant over a range of SiO 2 , indicating that the influence of crustal contamination during magmatic differentiation was limited. Moreover, looking more broadly, the geochemical data from the lavas of the Söke Basin, as well as those from nearly the entire Early to Middle Miocene suite in the northern part of west Anatolia, do not in general show any clear covariance of 87 Sr/ 86 Sr with the elements that reside preferentially in the crust or with differentiation indices such as magnesium or silica contents, and even the least differentiated samples within this suite possess radiogenic 87 Sr/ 86 Sr signatures (> 0.7060) (Aldanmaz et al, ; Ersoy & Palmer, ). This may further indicate that crustal contamination was not the main process that created the observed isotopic enrichments and that the mantle source itself also carried the radiogenic signatures of crust‐derived material.…”

Shallow melting of MORB‐like mantle under hot continental lithosphere, Central Anatolia

“…Symbols as in Figure 4, with addition of average ocean island basalt (red star) [ Sun and McDonough , ]. Fields show Kula basalt compositions [ Chakrabarti et al ., ; Aldanmaz et al ., ], most circum‐Mediterranean anorogenic basalts (CiMACI) [ Lustrino and Wilson , ], MORB, and OIB; note that MORB field is corrected from that shown in Davidson et al . [].…”

“…Basalt compositions are from same studies as investigated by Çoban [] as well as results of Aldanmaz et al . [] and Ku¨rkçu¨oğlu et al . [], except for locations in Central Anatolia that are the subject of the present study.…”

“…A mantle T P of ∼1420°C appears to characterize deeply derived intraplate‐like mantle ascending under Anatolia (Figure ; Table ). Under Western Anatolia, a slab tear separates the Aegean and Cyprean slabs (Figure ) [ Biryol et al ., ], which enables toroidal flow of asthenospheric mantle into the former (Western Anatolia) and present (Aegean) mantle wedge [ Aldanmaz et al ., ; Klaver et al ., ], resulting in the deep equilibration pressures and intraplate‐like geochemical signatures of Western Anatolia basalts (Figure ). With respect to Central to Eastern Anatolia, the westward transition in melt extraction conditions and T P is proximal to the projected location of the Cyprean slab edge (within 200 km of it geographically; Figure ) [ Biryol et al ., ].…”