Detrital zircon geochronology of Lower Paleozoic sedimentary rocks from COSC-2 borehole

The Caledonian Orogeny in the Scandinavian Caledonides (COSC) project aims to investigate the orogenic processes involving Caledonian allochthons together with the underlying sedimentary cover and Proterozoic igneous basement. The basement comprises Transscandinavian Igneous Belt (TIB) rocks with Hallandian and Central Scandinavian Dolerite Group intrusions and is overlain by a regolith (sub-Cambrian peneplain?). A Lower Cambrian(?) sedimentary succession of conglomerate, carbonate and shale covers this immature soil, followed by coarse-grained gravity flows fining upwards and showing a transition into the Alum Shale Formation. The undisturbed middle part of the formation separates the lower sedimentary cover from its overlying turbiditic part and the Lower Ordovician(?) turbidite sequence fining up to the top of the COSC-2 core.

First results of detrital zircon geochronology from the Cambrian succession show that the basal section of the autochthonous cover is characterized by mainly late Paleoproterozoic (c. 45% of all grains) – early Mesoproterozoic (c. 52%) detrital grains with age signatures of c. 1.77 Ga, 1.66 Ga and 1.44 Ga and a subordinate 1.25 Ga age peak. The middle part of the succession is dominated by late Paleoproterozoic detritus (c. 62% of all grains) with minor Mesoproterozoic (c. 21%) and Archean (c. 11%) input. The main age signatures are c. 1.80 Ga and 1.90 Ga with subordinate age peaks at c. 2.72 Ga, 2.00 Ga, 1.16 Ga. The upper part of Lower Cambrian(?) succession is characterized by Archean to Cambrian detritus. Archean grains constitute 12% of grains with dominant age signature at c. 2.67 Ga. Paleoproterozoic grains (25%) are grouped in 2.15-1.65 Ga interval with peaks at c. 2.12 Ga, 1.80 Ga, 1.76 Ga and 1.67 Ga. The Mesoproterozoic population (41%) is characterized by major age peaks at c. 1.55 Ga and 1.20 Ga. Neoproterozoic – Cambrian group (17%) contains major populations at c. 0.60 Ga and 0.53 Ga and a significant peak at c. 0.72 Ga. The maximum depositional age calculated via the maximum likelihood age algorithm yielded 530.5±4 Ma for the upper part of the Lower Cambrian succession. Two samples from the Ordovician succession show Mesoproterozoic – Neoproterozoic sources (c. 75% of grains), with more than 38% of grains yielding late Mesoproterozoic – early Neoproterozoic (1.2-0.9 Ga) ages. The dominant population of c. 1.06-1.02 Ga is accompanied by c. 1.50-1.47 Ga, 1.15 Ga and 0.99-0.97 Ga age peaks.

The autochthonous Lower to Lower Middle Cambrian passive margin succession in the lower part is dominated by local detritus provided solely from the Eastern Segment of Sveconorwegian Orogen (including the basement investigated by the COSC-2). The provenance shifts up the profile towards TIB-1 and Svecofennian Orogen sources, with the youngest part of the succession characterized by an input of Timanian Orogen detritus, including the uplifted Karelian protocraton. The Ordovician succession is characterized by Meso-Neoproterozoic age populations most likely sourced from the Sveconorwegian Orogen with a minor cratonic contribution. The youngest detritus is early Neoproterozoic, suggesting a passive margin setting with no early Caledonian input present.

This work was funded by the National Science Centre (Poland) projects no. 2019/33/B/ST10/01728 and 2018/29/B/ST10/02315.