Late Cretaceous arc and back-arc formation within the Southern Neotethys: whole-rock, trace element and Sr-Nd-Pb isotopic data from basaltic rocks of the Yüksekova Complex (Malatya- Elazığ, SE Turkey)
Keywords:mélange basalts, petrology, arc-back arc, Southern Neotethys
The remnants of the Southern Neotethys are represented by ophiolitic bodies and subduction/accretion complexes along the Southeast Anatolian-Zagros suture belt in the Eastern Mediterranean. Around Malatya and Elazığ areas (SE Turkey), they are found within imbricated slices of a mélange complex, known as the Yüksekova Complex. The studied basaltic rocks are common members of this mélange complex, and show distinctive features of sources with tholeiitic to tholeiitic-transitional character. Petrography, whole-rock trace element and isotopic data reveal two different compositional groups. Group I is transitional between island arc tholeiites and normal mid-ocean ridge basalt, and Group II includes back-arc (BABB), enriched mid-ocean ridge (E-MORB), and ocean island basalt type (OIB) compositions. According to the known Late Cretaceous stratigraphic ages, the studied basaltic rocks have (87Sr/86Sr)i = 0.703666- 0.706394, (143Nd/144Nd)i = 0.512734-0.512927, 208Pb/204Pb = 38.3216-39.3400, 207Pb/204Pb = 15.5018-15.6262 and 206Pb/204Pb = 18.5655-19.3209. In addition, εNdi values vary between +3.99 and +8.01 with two-stage TDM values ranging from 0.25 to 0.59 Ga. In the Sr versus Nd isotope correlation diagram, most samples plot along the mantle array line. In the correlation diagrams of 206Pb/204Pb versus 208Pb/206Pb and 207Pb/206Pb, the rock samples also plot nearly on the Northern Hemisphere Reference Line, similar to Atlantic and Pacific Ocean ridge basalts. As a whole, the isotopic data reveal that the studied rocks were derived from a depleted mantle source. When plotted in the conventional tectonic discrimination diagrams based on immobile trace elements, the samples can be related to an arc environment (Group I basalts), and back arc basin (Group II basalts) developed by generation and maturation of the rifted arc in later stages. This interpretation is supported by ages obtained from pillow lava-radiolarian chert associations indicating that the arc stage was realized during the late Cenomanian-early Turonian, followed by Coniacian-early Maastrichtian spreading in a back arc basin within the closing southern branch of Neotethys.