Post-collisional adakite-like magmatism in the Agvanis Massif and implications for the evolution of the Eocene magmatism in the Eastern Pontides (NE Turkey)

In the Anatolia, the Caucasus and northwest Iran, the Eocene epoch is characterized by widespread basic to acidic magmatism, whose temporal and spatial evolutions and origin are poorly understood. In this paper, we provide geochronological and geochemical data on a suite of Early Eocene intrusions from northeast Turkey and discuss their origin within a regional tectonic framework. Post to late-collisional, moderate to small bodies of quartz diorite and leucogranodiorite as well as later dacite porphyries intrude Permo-Triassic low-grade metamorphic rocks in the southern part of the Eastern Pontides very close to the Neo-Tethyan Izmir-Ankara-Erzincan suture. In places, the intrusives display well-developed subsolidus foliations, indicating ongoing penetrative deformation during and shortly after emplacement. Different emplacement depths were inferred based on the contact metamorphic mineral assemblages. Al-in-hornblende barometry and igneous textures (quartz diorite similar to 14-16 km vs dacitic porphyries <= 7 km). Biotite and hornblende Ar-Ar and zircon U-Pb data indicate that the compositionally and texturally different intrusives were emplaced at similar to 51 Ma (Ypresian). Three lines of evidence indicate that the Early Eocene represents a tectonically active time with high exhumation rates (>= 0.6 cm a(-1)): (i) the quartz diorite and dacitic porphyries were emplaced at different crustal depths, (ii) the emplacement ages of these texturally distinct intrusive rocks are geochronologically undistinguishable, and (iii) intrusive rocks locally show a well-developed foliation. The intrusive rocks display high abundances of Al2O3, Sr, Ba. LREEs and low abundances of Y, Sc and HREEs, and are similar to high-silica adakites from supra-subduction zone settings. The geochemical features imply a residual and/or fractionating assemblage rich in hornblende, garnet, pyroxene, and poor in plagioclase. All rock types display similar Sr-Nd isotopic characteristics with initial epsilon(Nd) and Sr-87/Sr-86 values ranging from 1.0 to -1.1 and from 0.70421 to 0.70494, respectively. These three coeval rock groups (quartz diorite. leucogranodiorite and dacite porphyries) were generated from a similar source by variable degrees of partial melting and fractionation. These data together with those from the literature clearly reveal that the adakitic signature is common in Paleocene to Early Eocene igneous rocks in northern Turkey on both sides of the Izmir-Ankara-Erzincan suture, but disappears towards the Middle Eocene. Regional geological constraints indicate that the continental collision between the Pontides and the Anatolide-Tauride block occurred in the Paleocene and that the Middle Eocene represents a period of major extension, characterized by a marine transgression. We therefore suggest that the Early Eocene adakitic magmatism in northern Turkey resulted from the presence of a thickened continental crust following the continental collision along the Neo-Tethyan Izmir-Ankara-Erzincan suture whereas the voluminous Middle Eocene magmatism was related to extensional collapse of a thickened orogen. (C) 2011 Elsevier B.V. All rights reserved.