Pre-Early Ordovician Cadomian arc-type granitoids, the Bolu Massif, West Pontides, northern Turkey: geochemical evidence

Abstract

The West Pontides tectonic belt of northern Turkey comprises a Lower Ordovician–Lower Carboniferous transgressive sequence. A stratigraphic basement to this Paleozoic sequence is exposed in the Bolu area. The tectono-stratigraphy of the basement closely resemble that of the Cadomian belt of western Europe. Three rock units forming the basement imply development of an Andean-type active continental margin during the pre-Early Ordovician period. High-grade metamorphics (the Sunnice Group), granitoids (the Bolu Granitoid Complex) and evolved felsic meta-volcanic rocks (the Casurtepe Formation) are exposed unconformably beneath the Lower Ordovician fluvial clastics, between the Bolu-Yedigoller area, to the north of Bolu. The Bolu Granitoid Complex comprises a group of intrusive rocks of variable composition and size, generated through multiple episodes of magmatism, and is represented by two separate intrusive bodies within the study area, the Tullukiris Pluton in the west and the Kapikaya Pluton in the east. Both plutons are mainly tonalite and granodiorite in composition. More felsic and mafic compositional varieties also occur. Major and trace element chemical characteristics of the granitoids, as well as biotite chemistry, indicate that these are volcanic arc-type granitoids and are products of an immature arc developed during early stages of a subduction. Furthermore, textural and chemical characteristics of the plutons show that these are subvolcanic intrusions, emplaced at shallow depths, and are calc-alkaline in composition. The granitoidic plutons intrude the Casurtepe Formation. The Casurtepe Formation is represented by arc-type volcanics and volcaniclastics. Both the Casurtepe Formation and the granitoids represent subduction-zone magmatism constructed on a continental crust, represented by the Sunnice Group. The history is very similar to Cadomian active margins as exposed in western Europe (i.e., the North Armorican and Bohemia massifs) and therefore the basement to the Paleozoic of the West Pontides is considered to be a preserved remnant of the Cadomian belt.