Records of early Gondwana assembly on the southwestern Baltica margin: Insights from the Holy Cross Mts., Poland

The western periphery of Baltica has been viewed as a passive continental margin formed during the fragmentation of Rodinia and the opening of the Iapetus and Tornquist Oceans. This view is supported by the Volyn Large Igneous Province (VLIP) of Ediacaran age in Eastern Europe, which may be associated with the opening of the Tornquist Ocean. However, the sedimentary succession overlying the VLIP in western Ukraine contains latest Ediacaran to early Cambrian detrital zircon with mixed ƐHf(t) values that can be interpreted to reflect deposition in a convergent margin setting. To further investigate this issue, we conducted research in the Holy Cross Mts. (HCM), Poland, where tightly folded and slightly metamorphosed middle Cambrian sandstone and slates are unconformably overlain by Lower Ordovician sedimentary rocks. We applied ⁴⁰Ar/³⁹Ar single grain fusion geochronology on white mica defining cleavage in lower Cambrian rocks and analysed detrital zircons to constrain their age and ƐHf(t) signature. Nine samples of shale and sandstone collected from the HCM showed consistency between stratigraphic age and the calculated maximum depositional age (MDA), ranging between c. 502-538 Ma. All samples have similar Proterozoic and Archean detrital zircon populations with major age peaks at c. 1200, 1500, 1800, and 2100 Ma, suggesting affinity with Baltica-associated sources. New U-Pb detrital zircon ages suggest that the HCM remained a coherent unit throughout the Cambrian and Early Ordovician. Importantly, ƐHf(t) signatures from Ediacaran-Cambrian detrital zircon of the HCM display a wide spread of values from -18 to +12. We interpret these results to reflect a continental magmatic arc setting, where there is significant mixing between mantle derived magmas and evolved crustal material. ⁴⁰Ar/³⁹Ar geochronology on white mica from one sample yielded two age populations. We interpret a group of ages between 537-640 Ma as detrital, while a younger group of ages yielded a weighted mean age of 510 ± 3 Ma (MSWD = 1.5). Interestingly, this younger age is corroborated by the presence of low-U zircon rims on detrital zircon from the same sample, which have an age of c. 510 Ma. We interpret this c. 510 Ma age population in both muscovite and zircon rims to record docking of a peri-Gondwana terrane, collision with a Baltica-derived terrane or the subduction of a seamount or oceanic plateau and shallowing of the down-going plate triggering deformation. With the present results, none of them can be ruled out. Mixed ƐHf(t) signatures and geochronologic evidence for deformation support the presence of an active margin at the periphery of southwestern Baltica during the Ediacaran and Cambrian. Furthermore, we suggest that this new ⁴⁰Ar/³⁹Ar geochronologic data may provide a new age constraint for early Gondwanan assembly.

This work was funded by the National Science Centre (Poland) project no. 2019/33/B/ST10/01728 to Majka.