U/Pb and Pb/Pb zircon ages for arc-related intrusions of the Bolu Massif (W Pontides, NW Turkey): evidence for Late Precambrian (Cadomian) age

Abstract

We present new U/Pb and Pb/Pb radiometric age data from two tectono-stratigraphic units of the regionally extensive Bolu Massif, in the W Pontides (Istanbul Fragment), N Turkey. A structurally lower unit (Sunnice Group) is cut by small meta-granitic intrusions, whereas the structurally higher unit comprises meta-volcanic rocks (Casurtepe Fm) cut by meta-granitic plutons (Tullukiris and Kapikaya plutons). U/Pb single-crystal dating of zircons from the Kapikaya Pluton yielded a concordant cluster, with a mean U-238/Pb-206 age of 565.3 +/- 1.9 Ma. Zircons from the Tullukiris Pluton (affected by Pb loss) gave a Pb-207/Pb-206 age of 576 +/- 6 Ma age (Late Precambrian). Small meta-granitic intrusions cutting the Sunnice Group yielded a less precise Pb-207/Pb-206 age of 262 +/- 19 Ma (Early Permian). The older ages from the Bolu Massif confirm the existence of latest Precambrian arc magmatism related to subduction of a Cadomian ocean. We infer that the Bolu Massif represents a fragment of a Cadomian active margin. Cadomian orogenic units were dispersed as exotic terranes throughout the Variscan and Tethyan orogens, and the Bolu Massif probably reached its present position prior to latest Palaeozoic time. Our dating results also confirm that NW Turkey was affected by Hercynian magmatism related to subduction of Palaeotethys, as inferred for other areas of the Pontides.