Stratigraphy and tectonic evolution of the Kazdagi Massif (NW Anatolia) based on field studies and radiometric ages

摘要

The Kazdagi Massif was previously considered as the metamorphic basement of the Sakarya Zone, a microcontinental fragment in NW Anatolia. Our new field mapping, geochemical investigations, and radiometric dating lead to a re-evaluation of previous suggested models of the massif. The Kazdagi metamorphic succession is subdivided into two major units separated by a pronounced unconformity. The lower unit (the Tozlu metaophiolite) is a typical oceanic crust assemblage consisting of ultramafic rocks and cumulate gabbros. It is unconformably overlain by a thick platform sequence of the upper group (the Sankiz unit). The basement ophiolites and overlying platform strata were subjected to a single stage of high-temperature metamorphism under progressive compression during the Alpine orogeny, accompanied by migmatitic metagranite emplacement. Radiometric age data obtained from the Kazdagi metamorphic succession reveal a wide range of ages. Metagranites of the Kazdagi metamorphic succession define a U-Pb discordia upper intercept age of ca. 230 Ma and a lower intercept age of 24.8 ± 4.6 Ma. This younger age agrees with ~(207)Pb/~(206)Pb single-zircon evaporation ages of 28.2 ±4.1 to 26 ± 5.6 Ma. Moreover, a lower intercept age of 28 ± 10 Ma from a leucocratic metagranite supports the Alpine ages of the massif within error limits. Reconnaissance detrital zircon ages constrain a wide range of possible transport and deposition ages of the metasediments in the Sarikiz unit from ca. 120 to 420 Ma. Following high-tempera ture metamorphism and metagranite emplacement, the Kazdagi sequence was internally imbricated by Alpine compression, and the lowermost Tozlu ophiolite thrust southward onto the Sankiz unit. Field mapping, internal stratigraphy, and new radiometric age data show that the Sarikiz unit is the metamorphic equivalent of the Mesozoic platform succession of the Sakarya Zone. The underlying metaophiolites are remnants of the Palaeo tethys Ocean, which closed during the early Alpine orogeny. After strong deformation attending nappe emplacement, the unmetamorphosed Miocene Evciler and Kavlaklar granites intruded the tectonic packages of the Kazdagi Massif. During Pleistocene time, the Kazdagi Massif was elevated by EW trending high-angle normal faults dipping to Edremit Gulf, and attained its present structural and topographic position. Tectonic imbrication, erosion and younger E-W-trending faulting were the main cause of the exhumation of the massif.