Kaan Sayıt 1,2 , Cemal Göncüoglu 1 , Tanya Furman 2

E-MORB- and OIB-type Metabasalts from The Karakaya Complex: Trace Element Evidence for Melting Across The Spinel-Garnet TransitionE-MORB- and OIB-type Metabasalts from The Karakaya Complex: Trace Element Evidence for Melting Across The Spinel-Garnet TransitionKaan Sayıt1,2, Cemal Göncüoglu1, Tanya Furman2

1 Middle East Technical University, Department of Geological Engineering, 06531, Ankara, TURKEY, 2 The Pennysylvania State University, Department of Geosciences, University Park, PA16802, USA

ABSTRACTABSTRACTThe variably metamorphosed and deformed pre-Liassic Karakaya Complex that covers much of Northern Anatolia comprises abundant metabasalts with subordinate gabbroic and ultramafic rocks. In Central and Western Anatolia, these mafic and ultramafic rocks with enriched geochemical signatures form an extensive melange unit within this ancient subduction/accretion complex and holds a key to the understanding of Palaeotethys events. However, the tectonic setting of these mafic and ultramafic rocks is controversial, as they are interpreted as relics of a continental rift (e.g. Bingöl et al. 1973), an oceanic plateau (Okay 2000), oceanic island(s) and/or seamount(s) (Pickett and Robertson 1996, 2004; Yaliniz and Goncuoglu 2002; Sayit and Goncuoglu 2007) or an oceanic plateau associated with seamounts (Genc 2004).

Metaigneous rocks from Central and Western Anatolia are dominantly alkaline with minor tholeiitic units as characterized by incompatible element abundance patterns. They are variably enriched in LREE and incompatible elements relative to N-MORB, with “hump-backed” patterns similar to E-MORB and OIB-type magmas; they lack negative Nb-Ta anomalies. The fractionation of LREE compared to HREE ([La/Yb]N = 1.9-23.0) suggests that garnet was a residual phase during genesis of these rocks. Little or no crustal involvement is suggested by high Ti/Yb (ave = 7318) and Nb/U (ave = 50) values, as well as the absence of negative Nb-Ta anomalies. They plot within the MORB array on Th/Yb vs Nb/Yb, and their low La/Ta (ave = 11.8), La/Nb (ave = 0.7) values indicate derivation from asthenospheric sources with no crustal involvement.

Geochemical modelling based on REE ratios reveals that the studied samples could have been generated by involvement of both spinel- and garnet-facies mantle sources. The Karakaya metaigneous rocks can be modelled by mixing of 5% spinel-facies partial melts with 0.5-5% partial melts derived from garnet-bearing mantle source. We suggest that the OIB-type rocks represent oceanic island(s) and/or seamount(s) fed by magmas derived dominantly from garnet-facies mantle with minor contribution from spinel lherzolite, while the E-MORB-type magmas formed by partial melts coming mainly from a spinel lherzolite source. These oceanic islands and/or seamounts were accreted into an forearc-accretionary prism during latest Triassic where they chaotically mixed with continent-derived material and created huge metabasaltic melange units within the Karakaya Complex.

AGU Fall Meeting 2008 Paper Number: V43B-2163AGU Fall Meeting 2008 Paper Number: V43B-2163

EVIDENCE FOR MINIMAL CRUSTAL CONTAMINATIONEVIDENCE FOR MINIMAL CRUSTAL CONTAMINATION

Figure 3. Low La/Nb, La/Ti, Ce/Nb and high Ti/Yb ratios suggest that crustal contamination is of negligible importance (A, B, C, D). There is no positive relationship on the plots of Ce/Y-La/Nb and Zr/Nb-La/Ti that we would expect in the case of crustal contamination. The positive trend of Ce/Y against Ti/Yb also supports this idea. The samples are of low Ce/Nb and Th/Nb ratios with no trends towards to the composition of continental crust, thus confirming the minimal or no crustal contamination (bulk continental crust composition from Taylor and McLennan 1995). Some low values observed in La/Nb may have resulted from low-grade metamorphism, leading to remobilization of La.

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Figure 4. A) High Nb concentrations result in low Ce/Nb values, suggesting limited influence of subduction events, B) Coupled behavior of Zr/Nb and Ce/Y suggest mixing of enriched and depleted sources, C) HFSE abundances suggest involvement of distinct source regions and/or melting behavior (dividing line from Fitton et al. 1997), D) Th-Nb-Yb values that plot along the MORB array indicate within-plate enrichment and minimal crustal contamination. High values in the OIB-type samples suggest lower degrees of partial melting than observed for the E-MORB suite (N-NORB, E-MORB and OIB compositions from Sun and McDonough 1989).

SOURCE PROCESSES AND FEATURESSOURCE PROCESSES AND FEATURES

Figure 5. We used several REE-ratio plots (including also Lu/Hf) for the geochemical modeling in order to examine what type of mantle sources may have been involved for the generation of the Karakaya samples and what degrees of partial melting can produce the observed REE ratios. For this purpose, we applied non-modal batch melting on a garnet-lherzolite “primitive mantle (PM)” source with a composition of 0.598 ol: 0.211 opx: 0.076 cpx: 0.115 grt, and a spinel-lherzolite “depleted mantle (DM)” source with 0.612 cpx: 0.203 opx: 0.135 cpx: 0.025 spi composition. All three plots give consistent results, suggesting the studied samples can be generated by mixing of 5% spi-facies melt with variable amount grt-lherzolite (0.5%-5%). Note that the OIB-type samples suggest more contribution from the garnet-facies mantle, whereas the E-MORB group largely requires spinel-bearing source (PM values and grt-lherzolite source mode from McKenzie and O’Nions 1991; DM values and spi-lherzolite source mode from Haase et al. 1997).

MELTING MODELS FOR THE KARAKAYA COMPLEXMELTING MODELS FOR THE KARAKAYA COMPLEX

Figure 1. Distribution of the Karakaya Complex in Turkey (after Okay and Göncüoglu 2004. Sample locations are indicated by the letters A, B and C together with compositional type).

AAE-MORB & OIBE-MORB & OIB

BBCC

only OIBonly OIB

E-MORB & OIBE-MORB & OIB

INTRODUCTIONINTRODUCTION

Figure 2. We define two sample groups on the basis of immobile trace element abundances. The OIB-type samples are highly enriched in HFSE (Nb and TiO2) relative to the E-MORB group (A and B). These samples have distinctive trace element patterns, including REE. variable enrichment of incompatible elements relative to N-MORB, D) fractionation of HREE relative to LREE indicating that residual garnet may have been involved in the source. The high LREE/HREE of OIB-type samples suggests garnet is involved in their petrogenesis, whereas the less fractionated patterns of E-MORB group mostly require spinel rather than garnet in the source (normalization values: N-MORB from Pearce 1983; C1-Chondrite from Sun and McDonough 1989)

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Documents

Kaan Sayıt 1,2 , Cemal Göncüoglu 1 , Tanya Furman 2