Revealing the early geological history of the Istanbul Zone (Far-East Avalonia) through zircon U-Pb-Hf isotopic data

Abstract

The Istanbul Zone, located in the far-eastern part of Avalonia, is built over a composite late Neoproterozoic basement consisting of an oceanic arc and a continental domain. This basement is unconformably overlain by uppermost Cambrian to Middle Ordovician clastic sedimentary rocks. New U-Pb-Hf isotopic analyses of zircons extracted from amphibolite-facies continental metamorphic rocks and the unconformably overlying clastic rocks on the oceanic arc elucidate the earliest geological history and refine the timeline for the collision between the oceanic arc and the continent. The continental metamorphic rocks are subdivided into two distinct amphibolite-facies domains: (i) an upper amphibolite-facies domain with peak metamorphism at 568 ± 10 Ma (2σ), and (ii) a lower amphibolite-facies domain with peak metamorphism at 591 ± 6 Ma (2σ). The zircon grains in the amphibolite-facies metaclastic rocks display mainly early Neoproterozoic to Paleoproterozoic ages (0.9–2.1 Ga) with predominantly positive initial εHf values, apart from late Neoproterozoic metamorphic overgrowths. Their protoliths were likely deposited in an intracontinental rift setting during the Tonian. In contrast, the uppermost Cambrian to lowermost Ordovician sandstones reveal detrital zircon age distributions with major peaks at 601–608 Ma and minor peaks at 701–710 Ma and 1500–1501 Ma. The late Neoproterozoic zircons predominantly demonstrate negative initial εHf values (∼78 %), indicating significant involvement of crustal material in the mantle-derived magmas. Thus, the provenance of the detritus for the uppermost Cambrian to lowermost Ordovician sandstones on the oceanic arc is mainly a neighboring late Neoproterozoic continental magmatic arc with negligible input from the Meso- and Paleoproterozoic basement. A comparative analysis of the detrital zircon data with previously published data from other far-eastern Avalonian terranes reveals notable similarities. These include (i) the occurrence of metaclastic rocks with Tonian maximum deposition ages, (ii) major zircon age peaks at 589–608 Ma and minor peaks at 701–738 Ma in upper Ediacaran and uppermost Cambrian–Ordovician clastic rocks, and (iii) the occurrence of 575–615 Ma and 695–715 Ma igneous rocks that correspond to the detrital zircon age peaks in the uppermost Cambrian and Middle Ordovician clastic rocks of the Istanbul Zone, although such rocks have not yet been documented. Thus, the far-eastern Avalonian terranes were part of a unified Neoproterozoic crustal domain and were likely never separated by a large ocean. Integration of these data with published detrital zircon U–Pb ages from upper Neoproterozoic to Cambrian clastic successions in Baltica supports a provenance linkage between far-eastern Avalonian terranes and Baltica, indicating that the Istanbul Zone was accreted to Baltica during the late Neoproterozoic to early Cambrian. Subsequent post-collisional uplift and erosion likely removed the upper crustal portion of the arc by the latest Cambrian.