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Geochemical characteristics of mafic lavas from the Neotethyan ophiolites in western Turkey: implications for heterogeneous source contribution during variable stages of ocean crust generation

E. ALDANMAZ^*,, M. K. YALINIZ, A. GÜCTEKIN^* and M. C. GÖNCÜOLU

^* Department of Geology, University of Kocaeli, Izmit 41040, Turkey
Department of Civil Engineering, Celal Bayar University, Manisa, Turkey
Department of Geology, Middle East Technical University, Ankara 0653, Turkey

View larger version (51K): [in this window] [in a new window]   Figure 1. (a) Regional tectonic map showing the major geological features of the Aegean section of the eastern Mediterranean. (b) Simplified geological map showing the distribution of main lithological units within the Tethyan suture zone in western Turkey (based on the data compiled from Erdoan, 1990; Okay, Tansel & Tüysüz, 2001; Yalnz & Göncüolu, unpub. TÜBTAK rep. no. 199Y100, 2005).

View larger version (19K): [in this window] [in a new window]   Figure 2. (a) Zr/117–Nb/16–Th diagram showing the basaltic lavas with respect to the fields for within-plate basalt (WPB), volcanic-arc basalt (VAB), normal (N) MORB and enriched (E) MORB (after Wood, 1980). (b). Zr/Y v. Zr discriminant diagram for the basaltic lavas within the Tethyan suture zone in western Turkey. The fields of within-plate basalt (WPB), mid-ocean ridge basalt (MORB) and island arc basalt (IAB) are from Pearce & Norry (1979).

View larger version (32K): [in this window] [in a new window]   Figure 3. N-MORB normalized multi-element patterns for the (a) MORB-, (b) OIB- and (c) SSZ-type basaltic lavas from the Tethyan suture zone in western Turkey. Average E-MORB and OIB compositions are shown by grey and dark heavy lines, respectively. E-MORB, OIB compositions and N-MORB normalizing values are from Sun & McDonough (1989).

View larger version (25K): [in this window] [in a new window]   Figure 4. Chondrite-normalized REE element patterns for the (a) MORB-, (b) OIB- and (c) SSZ-type basaltic lavas from the Tethyan suture zone in western Turkey. Chondrite normalizing values are from Boynton (1984). Average N-MORB and OIB compositions are shown by dark and grey heavy lines respectively. N-MORB and OIB compositions are from Sun & McDonough (1989).

View larger version (41K): [in this window] [in a new window]   Figure 5. Plot of Th/Yb v. Ta/Yb for the MORB-, OIB- and SSZ-type basaltic lavas from the Tethyan suture zone in western Turkey. Also plotted for comparison are some typical oceanic basaltic and mantle compositions including the depleted MORB mantle, primitive mantle (PM), N-MORB and E-MORB. Partial melting trends predicted using the depleted MORB mantle and PM compositions as the source are shown as solid lines with the calculated degrees of melting on each curve (shown as thick marks). Contours parallel to the mantle array denote the percentage of subduction-derived element Th in the mantle source, assuming that Ta is subduction-immobile (e.g. Pearce et al. 2005).

View larger version (26K): [in this window] [in a new window]   Figure 6. Variation of Ta/Nd with Th/Nb for the MORB-, OIB-and SSZ-type basaltic lavas from the Tethyan suture zone in western Turkey. The predicted mixing trend between MORB and OIB compositions defines the vertical trend comprising the compositional variations of the MORB- and OIB-type lavas from western Turkey, while the model prediction constituting the subduction influx into the depleted MORB mantle can account for the compositions of the lavas with SSZ geochemical signatures. End-member components shown in the figure are defined by 17 % melting of a depleted mantle source (A), 4 % melting of an enriched mantle source (B) and 20 % melting of a subduction-enriched mantle source (C).

View larger version (24K): [in this window] [in a new window]   Figure 7. Plot of La/Yb v. Zr/Nb showing melt curves (or lines) obtained using the non-modal batch melting model (see Aldanmaz et al. 2000 and Aldanmaz, 2002 for details). The parameters and common geochemical reservoirs used in the diagram are as described in Aldanmaz et al.(2000). The enriched mantle (EM) is assumed to have incompatible element concentrations similar to that proposed in Aldanmaz et al.(2006). The mantle array is defined using melt-residual compositional trends from depleted MORB mantle (DMM) and primitive mantle (PM; similar in composition to CI chondrite) compositions. Mantle depletion and enrichment trends are defined by melt extraction from the mantle (towards the residues) and melt addition to the mantle (or influx of material from outside the mantle prior to solid-state mixing and homogenization) respectively. The melting trends from depleted MORB mantle and enriched mantle compositions are shown by solid curves (or lines) while the dashed curves (or lines) represent the melting trends from primitive mantle. Thick marks on each curve (or line) correspond to degrees of partial melting for a given mantle source. MORB field defines the compositional range of basalts from the East Pacific Rise (Niu et al. 2001).

View larger version (30K): [in this window] [in a new window]   Figure 8. Plot La v. La/Sm showing melt curves (or lines) obtained using the parameters described in the caption of Figure 7. The straight line array represents a mixing line between melts from enriched (e.g. enriched mantle, EM) and depleted (e.g. depleted MORB mantle) components. The numbers in italic denote the mixing proportion of the depleted component in the final melt produced. The inset diagram compares the variations of highly versus moderately incompatible elements (La v. Nd) for the MORB- and OIB-type lavas from the ophiolitic suites of western Turkey with model partial melting curves (or lines) for both depleted and enriched mantle components obtained assuming the non-modal batch melting model. Continuous partial melting of any single source predicts curved trends for moderately versus highly incompatible elements. In contrast, highly and moderately incompatible trace elements from the MORB and OIB lavas from western Turkey generally exhibit individual quasi-linear trends suggesting a mixing between melts derived from sources with different compositions. Descriptions of geochemical reservoirs are as given in Figure 7.

View larger version (40K): [in this window] [in a new window]   Figure 9. Hf–Nd co-variations (after Pearce et al. 1999) in the basaltic lavas from the Tethyan suture zone ophiolites in western Turkey. Hf and Nd define extent of positive and negative displacements of samples from the mantle array (shown as shaded area) defined by average MORB–OIB compositions (Pearce et al. 1999). values indicate displacements from the mantle array and positive value corresponds to high element ratios with respect to the mantle array. Equal Hf anomalies (Hf) are represented by straight lines parallel to the MORB–OIB array, with positive Nd values corresponding to negative Hf anomalies. Positive Nd values record the proportion of subducted Nd in mantle sources. Hf describes the negative Hf anomaly as a Hf depletion (that is, Hf < 1), whereas Nd describes the anomaly as a Nd enrichment. Both Hf and Nd can be expressed in terms of the element compositions of the mantle and subduction zone end-members. Mixing trends between depleted MORB mantle (DMM) and subducted pelagic sediment (see Pearce et al. 1999 for detail) are also shown for a mass fraction of subduction component in the mantle between 0.05 and 0.2 and a range of values of rNd and rHf at rYb equal to 0 (dashed lines) or 2 (solid lines). Representative ratios (r) for Hf and Nd between the subduction component and the mantle are shown as inset.