Was Baltica part of Rodinia?

Late Ediacaran opening of the Iapetus Ocean is typically considered to reflect separation of Baltica and Laurentia during final breakup of the Rodinia supercontinent, with subsequent closure during the Caledonian Orogeny. However, evidence of the pre‐opening juxtaposition of Baltica and Laurentia is limited to purportedly similar apparent polar wander paths and correlation of Rodinia‐forming orogenic events. We show that a range of existing data do not unequivocally support correlation of these orogens, and that geologic and palaeomagnetic data instead favour separation of Baltica and Laurentia as early as 1.1–1.2 Ga. Furthermore, new detrital zircon U–Pb age and Ar–Ar thermochronological data from Norway point towards an active western Baltican margin throughout most of the Neoproterozoic and early Palaeozoic. These findings are inconsistent with the majority of palaeogeographic reconstructions that place Baltica near the core of the Rodinia supercontinent.

, presented a series of papers documenting a similar Palaeo-and Mesoproterozoic evolution of the SW Baltican and SE Laurentian margins, culminating in late Mesoproterozoic orogeny interpreted to reflect collision with Amazonia. More recent work on the Sveconorwegian Orogen has, however, shown that this orogeny was characterized by a lack of crustal thickening and near-continuous heating by mantle-derived magma, refertilizing the crust on time scales of 150-250 Myr (Bingen et al., 2021;. These tectonic features are very different from the Grenvillian Orogeny, characterized by crustal thickening and radiogenic self-heating (Jamieson et al., 2007;Rivers, 2015), and inconsistent with late Mesoproterozoic continent-continent collision at the SW Baltican margin (Slagstad et al., 2020). The Neoproterozoic evolution of western Baltica is poorly constrained but was dominated by widespread deposition of clastic sediments and intermittent, rift-related magmatism (Siedlecka et al., 2004;Nystuen et al., 2008; see also Figure 1).
Palaeomagnetic reconstructions of Rodinia are inherently poorly constrained given later metamorphic overprinting (Meert & Torsvik, 2003;Torsvik, 2003). Historically, the late Mesoproterozoicearly Neoproterozoic proximity of Baltica and Laurentia was accepted based on a rather vague similarity of the Grenvillian and Sveconorwegian segments of the respective apparent polar wander paths (APWPs) (e.g. McWilliams & Dunlop, 1978;Piper, 1980;Piper, 2009). More recent analyses of available palaeomagnetic data, however, indicate that direct comparison of the APWPs for Baltica and Laurentia is problematic at best because the relevant polar tracks are only partly coeval and, with the exception of a ca.
30 Myr period, represented by the Laurentian Keweenawan track, are characterized by relatively poor data resolution with gaps in the palaeomagnetic record that in some cases exceed 100 Myr (Evans et al., 2021;Kulakov et al., 2022). Whilst detailed analysis of the relevant APWPs is beyond the scope of this paper a detailed review of APWPs for Baltica and Laurentia are given by Kulakov et al. (2022).
Thus, an ocean must have existed at that time, separating Baltica and Laurentia.
Given sparse and highly equivocal palaeomagnetic constraints on the Baltica-Laurentia relationship through the Neoproterozoic, widely different orientations of the two continents have been proposed, even with Baltica inverted in some reconstructions (Hartz & Torsvik, 2002;McCausland et al., 2007). Figure 2b shows that Laurentia appears to have resided at low southern latitudes for a significant time interval, at least between ca. 830 and 720 Ma (Eyster et al., 2020;Maloof et al., 2006). In contrast, Baltica occupied polar latitudes at 848 ± 27 Ma (Walderhaug et al., 1999). The precise latitudinal position of Baltica between ca. 850 and 615 Ma is difficult to assess due to a lack of well-dated, high-quality palaeomagnetic data, however, the palaeomagnetic pole from the Katav formation (Pavlov & Gallet, 2009) reconstructs Baltica at low latitudes at ca. 800 Ma. However, the age of the Katav formation as well as the age of magnetic remanence is ill-defined and can fall anywhere between ca. 860 and 700 Ma (Ovchinnikova et al., 1998;Ovchinnikova et al., 2000;Pavlov & Gallet, 2009).
Thus, although an orientation like modern-day Baltica is by far the most favoured reconstruction, this interpretation stems largely from the poorly established correlation of the Grenville and Sveconorwegian orogens and equally poorly constrained APWPs.
Thus, there is no unique geologic or palaeomagnetic support for such an interpretation. Here, we present new geochronologic data that do not require the assumption of Baltica-Laurentia proximity and, instead, appear incompatible with such a configuration.

| Detrital zircon geochronology
New detrital zircon data presented here (see Data Supplements S1-S3), along with earlier work in SW Norway (Sláma & Pedersen, 2015), show that late Cambrian through Middle Ordovician (par)autochthonous metasedimentary units deposited on Baltica ( Figure 2) are dominated by Palaeo-through Mesoproterozoic detrital zircon grains (Figure 3a). Detrital zircon of this age is ubiquitous in metasedimentary sequences around the North Atlantic region and typically interpreted to reflect erosion from the Grenville-Sveconorwegian orogen (e.g. Kirkland et al., 2007;Krabbendam et al., 2022). In addition, Cambrian to Middle Ordovician sedimentary successions in SW Norway contain sparse 850 to 700 Ma and abundant 700 to 500 Ma zircon grains (Sláma & Pedersen, 2015; this study). εHf t values for the Neoproterozoic grains range widely, from −27 to +13 (Sláma & Pedersen, 2015), indicating both juvenile and evolved crustal sources. Sedimentary sequences of similar

Significance Statement
The manuscript presents arguments against the widely held hypothesis that Baltica formed part of the core of the Rodinia supercontinent, and that the Iapetus Ocean opened at ca. 600 Ma during separation of Baltica and Laurentia. The manuscript points out obvious weaknesses in the sparse data used to argue for such a configuration and opening history, and reviews recently published and presents new data that support the presence of an active Baltican margin where Laurentia is located in most Neoproterozoic reconstructions. The alternative views presented are innovative and will almost certainly be provocative. We do, however, believe they are well  (Cawood et al., 2012). Earlier work discussing these Neoproterozoic detrital zircon data have argued for their derivation from the Ediacaran Timanian Orogen at the E and NE margin of Baltica (Andresen et al., 2014;Sláma & Pedersen, 2015;Zhang et al., 2015). While the Timanides record calc-alkaline magmatism as old as ca. 700 Ma, it appears that the orogenic evolution took place outboard of Baltica, with oceanic subduction away from the Baltican passive margin, driving arc magmatism at the active margin of a hypothesized Arctida microcontinent (Kuznetsov et al., 2007). According to these authors, accretion of the active margin of Arctida onto Baltica was marked by intrusion of a suite of 560 Ma syn-collisional granites. Whilst the Timanian Orogen is purported to have continued north of the Varanger Peninsula into N Norway (Figure 2), evidence of such a westward arm is lacking. We also note that the type area of Timanian Orogeny and the Varanger Peninsula are located ca. 2,200 and 1,500 km, respectively, from the study area in SW Norway. Hence, we stress that comparatively few 'Timanian'-age grains are found in  Sláma & Pedersen, 2015), Rendalen formation (B2005, Bingen et al., 2005) and data presented in this study. Extension-related magmatism includes the 850 Ma Hunnedalen mafic dikes (Walderhaug et al., 1999), the 686 Ma Vinoren ailikite dike, the 616 Ma Egersund dikes (Bingen et al., 1998) and the 583 Ma Fen carbonatite (Meert et al., 1998) A southerly, Avalonian source has also been proposed for these Neoproterozoic detrital zircon grains (Andresen, 2021); however, considering that Avalonia and Baltica collided in the latest Ordovician to earliest Silurian (Domeier, 2016) and the general lack of grains younger than early Middle Ordovician, we consider this interpretation unlikely.
Hence, a more likely derivation of these Neoproterozoic detrital zircon grains is from a source region west of the present western Baltican margin. Available detrital zircon data from (par)autochtonous sedimentary rocks in Norway are, therefore, indicative of a long-lived, active margin west of present-day Baltica (Figure 4), which rules out most pre-Iapetus reconstructions of Baltica and Laurentia in which the western Baltican margin is placed adjacent to the eastern Laurentian margin.  (Figure 4c). Evidence of longlived extension is restricted to S Norway, which is characterized by comparatively thin and weak lithosphere (Figure 1). Divergent settings generally produce little magma that crystallizes zircon (Cawood et al., 2012), and it is, therefore, unlikely that these rift events reflect continental rifting, as generally assumed, but rather extensional basins formed behind, and filled by erosion of, an active continental margin. The geographic distribution of Neoproterozoic detrital zircon and thermal events constrained by the Ar-Ar data suggest the presence of an active margin outboard of the present western margin of Baltica.

| WHEN D ID BALTI C A AND L AURENTIA S EPAR ATE?
The timing of Baltica-Laurentia separation and Iapetus opening remains unknown, but we speculate that widespread rifting around 1.2 Ga, recorded on both continents (Bingen et al., 2002 and references therein), marks this event, consistent with a very similar tectonic evolution up until this point and a rather more dissimilar evolution thereafter (Karlstrom et al., 2001;Slagstad et al., 2019;Spencer et al., 2019). Available palaeomagnetic data indicate latitudinal separation of at least 20° between the two continents as early as ca. 1090 Ma (Figure 4a, adopting the classic right-way-up position of Baltica; Kulakov et al., 2022), consistent with contrasting styles of Grenvillian-Sveconorwegian orogeny (Slagstad et al., 2019). We F I G U R E 3 (a) Detrital zircon probability density plot from (par) autochthonous metasedimentary units in SW Norway (Sláma & Pedersen, 2015; this study). Data for the Rendalen formation from Bingen et al. (2005). (b) Detrital zircon age data from lower allochthonous and parautochthonous rocks in N Norway (Andresen et al., 2014;Zhang et al., 2015). (c) Ar-Ar biotite ages from this study, plotted with detrital zircon probability density plots from SW and N Norway and ages of extension-related magmatism in SW Norway (see Figure 1  Slagstad , following a period of accretionary tectonics along both margins (Barnes et al., 2019;Gasser et al., 2021;Majka et al., 2014;Zagorevski et al., 2006), and argue that the commonly held interpretation of Baltica being located adjacent to Laurentia in the supercontinent Rodinia and during most of the Neoproterozoic is founded on incomplete information and is inconsistent with presently available geologic and palaeomagnetic data.

| CON CLUS IONS
The available geologic and palaeomagnetic data are best explained by separation of Baltica and Laurentia well before assembly of Rodinia.
In contrast, the available age and thermal history information suggest that the western Baltican margin was active throughout much of the Neoproterozoic and located some unconstrained distance

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that supports the findings of this study are available in the