ePoster
Talk Description
We provide combined Lu-Hf and U-Pb isotope data on 14 samples of enderbites and charnockites that constitute a granulite complex in the Dniester-Bouh Domain of the Ukrainian Shield. These rocks have traditionally been regarded as early Archean (c. 3.7-3.6 Ga), while early multigrain zircon dating indicated that many of them crystallised at ca. 2.8 and 2.0 Ga. It was believed that these younger ages reflected remelting and remobilisation of the early Archean crust. According to our data, the oldest 3786 ± 32 Ma zircon population defines the primary igneous event in enderbite sample C10-U4. Hafnium TDMmafic and TDMfelsic model ages for these zircons are 4.2 and 4.0 Ga, respectively, whereas εHf(3790) ≈ 1 and whole-rock εNd(3790) = 2. This was followed by an igneous/metamorphic event (possibly involving migmatisation) at 3569 ± 22 Ma that closely resembled the initial one with respect to Hf and Nd isotopic systematics. The next main event took place between ca. 3100 and 2800 Ma, and consisted of several phases separated by extended periods of inactivity. Hafnium isotope composition of zircons attained DM values (εHf = 5 to 6), whereas some remobilisation of the Eoarchean crust is apparent in negative εHf values that are recorded from several samples. The extended duration of this event and significant input of juvenile material imply large-scale crust-forming processes. Felsic igneous rocks of this age also occur in the Middle Dnieper and Azov domains of the Ukrainian Shield, signifying the importance of this time interval in its geological evolution. The last major event that affected the early Archean rock assemblage in the Dniester-Bouh Domain took place in the Paleoproterozoic, between ca. 2.15 and 1.90 Ga. Hafnium isotope systematics of zircons indicate that significant juvenile input accompanied recrystallisation of the rocks formed during the previous events. Several rocks contain zircons with εHf greater than +5. This event resulted in granulite facies metamorphism with emplacement of numerous granitic intrusions, and broadly correlates with the collision of the Sarmatian and Volgo-Uralian segments of Baltica at 2.1-2.05 Ga, with subsequent subduction at the northern margin of Sarmatia. However, timing of the metamorphic and igneous events in the Dniester-Bouh Domain does not match this collision in detail. Moreover, igneous and metamorphic activity was not synchronous in different domains of the Ukrainian Shield and were probably caused by different tectonic events. We interpret that the activity in the Dniester-Bouh Domain was caused by an unknown tectonic event that was happening at the southern margin of Sarmatia during assembly of the Columbia/Nuna supercontinent (collision with Amazonia or West Africa?). Our U-Pb and Lu-Hf isotope data from granulite-facies rocks of the Dniester-Bouh Domain of the Ukrainian Shield therefore establish three major juvenile crust-forming events, at 3.8-3.6, 3.1-2.8, and 2.15-1.90 Ga respectively, that may extend across the whole of the shield.