Fluid alteration of Ordovician limestones in the Baltoscandian Basin during Caledonian collision revealed through trace-element mapping and U-Pb dating by LA-ICP-MS

Combined trace-element mapping and U-Pb geochronology of calcite in situ (in thin sections) by LA-ICP-MS provides opportunities to assign absolute ages to calcite crystallization and recrystallization with petrographic and geochemical context. The ability to date multiple, texturally distinct generations of primary, diagenetic, and metamorphic carbonate phases enables deciphering of complex depositional and post-depositional histories sedimentary successions have experienced. We have applied this approach to two samples of Ordovician bioclastic limestones from the Viki drill core (western Estonia), representing the eastern part of the Baltoscandian Basin. The depositional ages of the samples are constrained by biostratigraphic correlation to ca. 460 and 445 Ma (Hints et al., 2014). Several lines of evidence¾such as very low organic-matter maturation and properties of clay minerals¾indicate that this sequence did not experience temperatures above 100 °C, and likely not above 50°C, since deposition (Kirsimäe et al., 2020). Optical petrography and backscatter-electron (“BSE”) imaging reveal low-porosity, “BSE-bright” calcite spar cement in pore spaces between “BSE-dark” micro-porous calcite bioclasts. Trace-element mapping of several areas (several mm2 each) in each thin section by LA-quadrupole-ICP-MS reveals variably elevated Mn/Sr, U concentration, and U/Pb in the calcite spar cement. The trace-element maps were subsequently used to guide the placement of laser spots for U-Pb dating by LA-multicollector-ICP-MS. Primary bioclastic calcite in both samples has low U/Pb (238U/206Pb < 7) and, thus, does not yield precise Concordia-intercept dates. One sample, however, yields an intercept date of ca. 450 Ma, consistent with the deposition age, albeit with a large uncertainty. Calcite spar cement has higher U/Pb (238U/206Pb up to ~15.7) and including all analyses, yields intercept dates of ca. 420 Ma in each sample. Additionally, several of the domains with the highest U/Pb from each sample yield slightly younger dates of ca. 400­-380 Ma. The timing of calcite (re)crystallization and cementation identified here overlaps with the Scandian (collisional) phase of the Caledonian orogeny. We interpret this to be a result of fluid flow in response to the collision at a relatively far-inboard position (i.e., far from the orogenic front).

References:

Hints, O., Martma, T., Männik, P., Nõlvak, J., Põldvere, A., Shen, Y., Viira, V. 2014. New data on Ordovician stable isotope record and conodont biostratigraphy from the Viki reference drill core, Saaremaa Island, western Estonia. GFF 136, 100–104.

Kirsimäe, K., Somelar, P., Jõeleht, A. 2020. Illitization of the lower Cambrian (Terreneuvian) Blue Clay in the northern Baltic Palaeobasin. Estonian Journal of Earth Sciences 69, 200–213.