The scope of this study is to understand better the pre-Early Ordovician history of the west Pontides of northern Turkey by focusing on the best-exposed part of the Bolu Massif, which is located between Bolu and Yedigöller (Seven Lakes). The Palaeozoic rocks of the west Pontides tectonic belt of northern Turkey comprise a transgressive sedimentary sequence known as ‘Palaeozoic of İstanbul.’ In a few areas, the basement of the Palaeozoic sequence is exposed, the largest part of which is the Bolu Massif, which is located in the middle of the west Pontides. The lowermost unit of the Palaeozoic of İstanbul in the Bolu area is the Işığandere Formation, which is made up of fluvial red conglomerates and sandstones of Lower Ordovician age. Three different units are exposed unconformably beneath these continental clastics, forming the Bolu Massif. From the structural base to the top, these are as follows: (1) a high-grade metamorphic unit, known as the Sünnice Group); (2) granitoid intrusions, known as the Bolu Granitoid Complex; and (3) a greenschist meta-volcanic sequence (the Çaşurtepe Formation).

The Sünnice Group is the lowest, structurally speaking. It is a southwest–northeast-trending belt of migmatitic basement, consisting of amphibolites and paragneisses cut by small (< 10 m) metagranitic intrusions. The Sünnice Group is tectonically overlain by the Bolu Granitoid Complex and the Çaşurtepe Formation along the Karadere Fault. In the study area the Bolu Granitoid Complex is represented by two distinct, north-northeast–south-southwest-trending intrusions, the Tüllükiriş and Kapikaya plutons. The granitoids are mainly tonalitic and granodioritic in composition, cut by lam-prophyre and aplite dykes and intruded into the Çaşurtepe Formation. The Çaşurtepe Formation is composed mainly of andesitic and minor rhyolitic lavas, along with a meta-ignimbrite sequence.

The lavas have geochemical characteristics indicative of eruption in a subduction-related tectonic setting. The geochemistry of the intrusions also suggests emplacement in an arc-type setting. Initial Nd isotope data for the Çaşurtepe Formation indicate derivation from a depleted mantle source, whereas those for the granitoids are consistent with greater degrees of crustal contamination.