Ion Probe U-Pb Dating of the Central Sakarya Basement: A peri-Gondwana Terrane Intruded by Late Lower Carboniferous Subduction/Collision-related Granitic Rocks

Abstract

Ion probe dating is used to determine the relative ages of amphibolite-facies meta-clastic sedimentary rocks and crosscutting granitoid rocks within an important 'basement' outcrop in northwestern Turkey. U-Pb ages of 89 detrital zircon grains separated from sillimanite-garnet micaschist from the Central Sakarya basement terrane range from 551 Ma (Ediacaran) to 2738 Ma (Neoarchean). Eighty five percent of the ages are 90-110% concordant. Zircon populations cluster at ~550-750 Ma (28 grains), ~950-1050 Ma (27 grains) and ~2000 Ma (5 grains), with smaller groupings at ~800 Ma and ~1850 Ma. The first, prominent, population (late Neoproterozoic) reflects derivation from a source area related to a Cadomian-Avalonian magmatic arc, or the East African orogen. An alternative Baltica-related origin is unlikely because Baltica was magmatically inactive during much of this period. The early Neoproterozoic ages (0.9-1.0 Ga) deviate significantly from the known age spectra of Cadomian terranes and are instead consistent with derivation from northeast Africa. The detrital zircon age spectrum of the Sakarya basement is similar to that of Cambrian-Ordovician sandstones along the northern periphery of the Arabian-Nubian Shield (Elat sandstones). A sample of crosscutting pink alkali feldspar-rich granitoid yielded an age of 324.3±1.5 Ma, whilst a grey, well-foliated biotite granitoid was dated at 327.2±1.9 Ma. A granitoid body with biotite and amphibole yielded an age of 319.5±1.1 Ma. The granitoid magmatism could thus have persisted for ~8 Ma during late Early Carboniferous time, possibly related to subduction or collision of a Central Sakarya terrane with the Eurasian margin. The Central Sakarya terrane is likely to have rifted during the Early Palaeozoic; i.e. relatively early compared to other Eastern Mediterranean, inferred 'Minoan terranes' and then accreted to the Eurasian margin, probably during Late Palaeozoic time. The differences in detrital zircon populations suggest that the Central Sakarya terrane was not part of the source area of Lower Carboniferous clastic sediments of the now-adjacent İstanbul terrane, consistent with these two tectonic units being far apart during Late Palaeozoic-Early Mesozoic time.