DETRITAL RUTILE AGES FROM THE CARBONIFEROUS FLYSCH OF THE ISTANBUL ZONE, TURKEY: NEW INSIGHTS FOR PROVENANCE STUDIES

Rutile and zircon are among the most stable heavy minerals in sedimentary environments (erosion, transport and burial). While detital zircon is extensively analysed to gain information of the sediment provenance, the study of detrital rutile is still in its infancy. Nevertheless, it would be highly desirable to collect U/Pb ages of detrital rutile, as rutile records cooling ages of metamorphic terranes, which is another layer of information in addition to U/Pb ages of zircon, which generally record magmatic episodes of its source rocks. We have recently developed a method that allows precise and accurate U/Pb determinations of single rutile grains, employing a LA-ICP-MS, a common Pb correction based on ²⁰⁸Pb and the usage of primary rutile standards (Zack et al., CMP, in revision).

As part of a thorough provenance study targeting the Carboniferous flysch of the Istanbul Zone, Turkey (see Okay N et al., this conference), we started the first pilot study on dating detrital rutiles. The age spectrum of the analysed detrital zircons shows three main age groups (335-390 Ma; 520-640 Ma and 1700-2750 Ma), consistent with an Amorican Terrane source. This is significant, as older sediments in the Istanbul Zone all have a clear Avalonian signature (e.g. a significant zircon population with Grenvillian ages of ca 1000 Ma). The preferred model to explain these results involves docking of the Istanbul Zone as a distant arm of the Avalonian Terrane, with the Amorican Terrane at the time of deposition as the source of the Carboniferous flysch.

In contrast to the results from detrital zircons, detrital rutile U/Pb ages from the Carboniferous flysch of the Istanbul Zone are prominently from only one age population, giving a pooled concordia age of 360 ±7 Ma. With one exception (1 of 30 detrital rutile is ca 550 Ma old), rutiles do not record any event older than the youngest age group of the zircons, e.g. the zircon age group of 1700 to 2750 Ma is not represented. If we follow the interpretation that rutile U/Pb ages represent cooling ages of their source area, the implication is that the source area of the Istanbul Zone flysch has a uniform cooling history.

In the complex Variscan collage, results from detrital rutiles of the Istanbul Zone imply that zircon ages in the age range of 1700 to 2750 Ma, often considered to be of West African Craton origin, by no means have to come directly from a distant source. In contrast, they have rather been involved in several metamorphic cycles, with a Variscan metamorphic cycle (360 Ma) in the case of source rocks from the Istanbul Zone flysch. We suggest to systematically search for age spectra of detrital rutile with several age groups, as they seem to be a tell-tale signature of continental scale river systems, therefore greatly helping to improve plate tectonic reconstructions.