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Evolution of the Neotethyan branches in the Eastern Mediterranean: Petrology and ages of oceanic basalts

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Evolution of the Neotethyan branches in the Eastern
Mediterranean: Petrology and ages of oceanic basalts
M.Cemal Göncüoglu1, U. Kagan Tekin2, Kaan Sayit1, Yavuz Bedi3 and Seda Uzuncimen-Keceli2
1) Middle East Technical University, Department of Geological Engineering, Ankara. mcgoncu@metu.edu.tr; 2)
Hacettepe University, Department of Geological Engineering, Ankara. 3) General Directorate of Mineral
Research and Exploration (MTA), Department of Geological Research, Ankara.
Introduction
The Anatolian Peninsula in the Eastern Mediterranean includes remnants of the Proto-, Paleo-, Neo-
and Para-Tethys oceans. From these, the Neotethys with its various branches has been studied
relatively well. Disregarding the embayments of the major oceanic strands, three main oceanic
branches are distinguished by the presence of ophiolitic suture belts, separating terranes of
continental crust origin (Fig. 1). From S to N these are the Southern branch of Neotethys, the Izmir-
Ankara-Erzincan branch and the Intra-Pontide branch. In the last twenty years we studied
systematically the petrology of oceanic volcanism and the radiolarian ages of oceanic sediments,
mainly cherts, in primary depositional contact with them. To have a wider range of tectono-
magmatic settings and formation ages, we studied not only the epi-ophiolitic cover of major
ophiolitic massifs but also blocks within subduction-accretion prisms/mélanges.
The Southern Branch of Neotethys is obviously the main oceanic strand between N Africa-
Arabia and the Apulian-Tauride microplate. Oceanic assemblages of this branch are distributed as
allochthonous bodies from the Aegean arc in the W Mediterranean towards E and W Antalya to
Cyprus- SE Anatolia- Zagros and Oman in the east (Fig. 1). Whether the ophiolitic bodies and the
remnants of the subduction accretion prism along this belt were formed in a single oceanic basin or
in relation with a complex multi-armed oceanic system is still a matter of debate. The paleontological
data from the sediments in association with oceanic basalts suggest a long period of oceanic crust
generation from the Late Triassic (mid Carnian) to Late Cretaceous (Maastrichtian) (Fig. 2). The
petrological evaluation of the basalts indicates that the Late Triassic volcanism is OIB and E-MORB
type, so that the initial ocean floor spreading should predate the Late Triassic. Yet, the no data is
obtained on the ages of N-MORB. Hence the duration of the ridge-spreading is highly speculative.
The supra-subduction-type (SSZ) oceanic crust generation in this branch commenced during the
Cenomanian. With a time-gap in Turonian the SSZ-type magmatism lasted until mid Maastrichtian
(Fig. 2), suggesting that the Sothern Branch of Neotethys closed by intra-oceanic subduction. No
oceanic magmatism younger than Late Cretaceous has been recorded yet from the mélanges of the
Sothern Neotethys.
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Figure 1- Distribution of the Neotethyan ophiolites and mélange complexes in Turkey
The Izmir-Ankara-Erzincan Branch separated during the interval from the Mesozoic the
Sakarya Composite Terrane and the Tauride-Anatolide Block (Fig. 1). Considering the radiolarian
record from the oceanic sediments within the subduction accretion prism, it has been the site of
almost continuous oceanic deposition. The oldest oceanic deposition in this branch is Ladinian. The
time-gap recognized in Early Jurassic (Fig. 2) is crucial and may have important implications for the
geodynamic interpretation. Geochemical data from the oldest basalts indicate that already during
the Norian the oceanic branch was wide enough to be the side of OIB generation. Plume-related OIB
formation is proven from Late Jurassic to late Early Cretaceous. The SSZ-type basalts found yet are of
Early Late Cretaceous in age. However, recent radiometric ages obtained from metamorphic soles
indicate that the intra-oceanic subduction has started as early as the Middle Jurassic (Fig. 2). On the
other hand, the presence of MORB basalts of Early Cretaceous age is indicative for ongoing spreading
in another segment of the IAEO.
The Intra-Pontide Oceanic Branch is the least known one among the Neotethyan oceans. Its
existence has been even refused by some authors. It may stand for the eastern extension of the
Vardar Ocean, where successive intra-oceanic subduction-accretion events were realized during the
Mesozoic. The suture of it is largely followed by the N Anatolian Transform Fault (Fig. 1), so that it is
cryptic. The ophiolitic mélanges stretch from the Aegean coast to northern Central Anatolia, where it
probably joins the suture of the Izmir-Ankara Ocean. Radiolarian data yet obtained from mélange
blocks is limited (Fig. 2). The oldest ages are Middle-Late Triassic. After a gap in Jurassic, oceanic
sediments were deposited during the Lower and middle Cretaceous. Geochemically, the Late Jurassic
mélange basalts, the HP/LT as well as the LP/LT metabasic rocks display dominantly supra-
subduction-type characteristics. The presence of late Middle Jurassic SSZ basalts and latest Jurassic
MORB in the Intra-Pontide belt is indicative for coeval intra-oceanic subduction and MOR-spreading
within the Intra-Pontide Ocean.
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Figure 2- Geochemical features and radiolarian ages of basaltic lavas from the Neotethyan oceanic branches.
Conclusion
To conclude, three main branches of Neotethys have been recognized in the E
Mediterranean by the distribution of continental terranes and sutures separating them. A combined
study of the petrology of oceanic basalts and the radiolarian ages of associated sediments has
resulted in a more detailed picture of the geological evolution of these oceanic branches. Available
data suggest that in all of these branches oceanic crust generation started prior to Middle Triassic by
the involvement of a mantle plume. One of their common characteristics is the absence of relicts of
the Jurassic oceanic crust. Another common feature in their tectono-magmatic evolution is that they
all started to close along N-directed intra-oceanic subduction zones creating SSZ-type oceanic basins.
The youngest oceanic volcanism in all these oceanic basins is middle to late Late Cretaceous.
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... Inner Tauride ocean, is inferred to exist between Central Anatolian Crystalline complex and TaurideAnatolide block. The northern branch is considered to be a marginal basin called Vardar or İzmir-Ankara-Erzincan (İAE) ocean (Stampfli, 2000;Handy et al., 2010;Göncüoğlu et al., 2015, and references therein), and the southern branch, part of which still exists in the Aegean part of the Eastern Mediterranean Sea, is referred to simply as the NeoTethys ocean (Stampfli, 2000), and that the eastern Mediterranean Sea is considered to be part of the NeoTethys (Fig. 1). ...
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