Late Triassic subduction-related ultramafic-mafic magmatism in the Amasya region (eastern Pontides, N. Turkey): Implications for the ophiolite conundrum in Eastern Mediterranean

The eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic-early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we report the geology, petrology, geochemistry, Nd-Sr-Pb isotopes and U-Pb zircon ages from a ultramafic-mafic complex in Aksalur which forms part of a series of Alaskan-type ultramafic-mafic bodies in the Amasya region located at the south-western corner of the eastern Pontides orogenic belt. These ultramafic-mafic intrusions occur as minor (30-300 m in diameter), scattered, roughly circular bodies within the low-grade metamorphic rocks of the Tokat metamorphic massif. Petrographic studies characterise the rock types as gabbro and wehrlite, with the latter showing typical cumulate texture and mainly comprising serpentinized olivine, clinopyroxene, hornblende, mica and opaque minerals. The gabbroic rocks are characterized by relatively high initial ratios of Nd-143/Nd-144 (0.512618-0.512633) and low initial ratios of Sr-87/Sr-86 (0.70420-0.70435). Their initial Pb isotope ratios range from 17.306 to 17.771. The gabbros and cumulate wehrlites display geochemical distribution patterns analogous to the Alaskan-type intrusions that were derived from a subduction-related, high-alumina, hydrous basaltic parental magma in a convergent plate setting. LA-ICPMS analyses of zircons separated from the gabbros show high Th/U ratios (1.35-5.26) typical of magmatic origin. The U-Pb isotopic data define a weighted mean Pb-206/U-238 age of 203.3 +/- 4.9 Ma. Detailed evaluation of the magma tectonics suggests that the Alaskan-type Aksalur ultramafic-mafic intrusions were accumulated in magma chamber(s) located at relatively shallow crustal levels. Two distinct magmatic pulses are identified: the first phase produced the gabbroic intrusions and the second phase was characterized by numerous small-scale werhlitic cumulate intrusions. In contrast to the previous models which assigned a mid-oceanic ridge-derived ophiolite affinity for ultramafic-mafic complexes in the eastern Pontides orogenic belt, our study reveals that these intrusions were derived from subduction-related high-alumina hydrous basaltic magmas transferred into the overlying relatively thin continental crust along deep-seated fractures. (C) 2011 Elsevier Ltd. All rights reserved.