Age and significance of amphibolites in the bolu massif (İstanbul Zone, NW Turkey)

Abstract

The Bolu Massif represents the Late Neoproterozoic crystalline basement of the Istanbul Zone (NW Turkey), and is made up of two tectonometamorphic units which are separated by a major tectonic line. The lower tectonometamorphic unit comprises amphibolite-facies metamorphic rocks which are dominated by the migmatitic amphibolite and locally trondhjemite. On the other hand, the overlying tectonometamorphic unit comprises greenschist-facies mafic to felsic metavolcanic rocks intruded by a huge granitoid of Late Neoproterozoic age. This study deals with the metamorphic evolution of the underlying amphibolite-facies tectonometamorphic unit. The amphibolite-facies tectonometamorphic unit comprises migmatitic amphibolite (ca. 80% of the exposure area) and concordant to crosscutting trondhjemite sills/dykes/stocks (ca. 20% of the exposure area). Although this amphibolite unit was formerly interpreted to the crustal part of an ophiolite, rare earth element patterns and multi-element variation diagrams involving fluid immobile elements differ from the well-documented ophiolites. The geochemical features of these voluminous amphibolites are more consistent with those in intra-oceanic arcs and magmatic arcs. U-Pb dating of zircons from four trondhjemite samples and from three amphibolite samples consistently yielded ages of 255-260 Ma (Middle to Late Permian). Only zircons from one amphibolite sample defined a discordia with an upper intercept age of 554 ± 34 Ma (2) (Late Neoproterozoic-Cambrian). The geochronological data shows that the amphibolite-facies tectonometamorphic unit does not represent the Late Neoproterozoic basement of the Istanbul Zone. It is related to Late Palaeozoic to Late Mesozoic orogenic events.