Reconstructing craton‐scale tectonic events via in situ Rb‐Sr geochronology of poly‐phased vein mineralization

Fault‐ and fracture‐hosted multi‐stage mineral assemblages that formed by fracture reactivation and fluid migration, constitute archives of the tectonic evolution of Precambrian cratons. Complex intergrowth patterns of these mineral records often hinder absolute dating of mineralization events for geological models. We apply LA‐ICP‐MS/MS in situ Rb‐Sr dating of single crystal growth zones in sub‐mm‐wide vein mineralization assemblages including illite, K‐feldspar, albite, calcite, mica, zeolites, fluorite and/or epidote at three Palaeoproterozoic crystalline bedrock sites over 300 km apart in the Fennoscandian Shield. The dating campaign reveals multiple age clusters between ca. 1757 ± 15 and 355 ± 12 Ma correlating with fluid flow and fracture reactivation events initiated by far‐field orogens and their foreland basin evolution. This new approach for reconstructing geological histories of Precambrian cratons connects micro‐scale age determinations of different mineral growth zones in fractures with regional‐scale crustal dynamic responses to tectonic events.


| INTRODUC TI ON
Minerals precipitated in veins, shear zones and extensional fractures can be used to track diverse processes including brittle and ductile deformation (van der Pluijm et al., 2001), ore genesis (Oze et al., 2017), fluid migration and mixing (Uysal et al., 2011) and ancient microbial activity (Drake et al., 2015). Once a discontinuity in the bedrock has formed, repeated reactivation may result in a record of multiple authigenic mineral generations precipitated along fault planes (e.g., slickenfibres) and walls of re-opened fractures, forming poly-phased veins (Bons et al., 2012;Sibson et al., 1975).
Dating of fault gouge and veins has shown that faults record reactivation and mineral neo-crystallization in various tectonic regimes, including rifting (Scheiber et al., 2019) and shearing of conjugate faults (Goodfellow et al., 2017). Crust within Precambrian cratons usually experienced complex histories of tectonically induced episodic fracture reactivation. These tectonic effects can act in the far-field over entire cratons and include compression, shearing or extension of crustal blocks, for example, due to foreland basin development and crustal thickening, or mountain range build-up and collapse (e.g., Andersen, 1998). Vein mineralization dating is well suited to decipher the complex tectonic evolution of cratons.
However, no attempts have yet been made to (a) use micro-scale geochronology to decipher multi-stage reactivation of ductile shear zones and of sub-mm-sized veinlets of discrete fracture networks within Precambrian cratons and (b) interpolate such age populations between sites in a craton for detailed reconstructions of tectonicrelated effects on crustal blocks. Here, we apply in situ Rb-Sr geochronology of fine-grained authigenic minerals (listed in Table 1) with the aim to decipher and provide age constraints to the cryptic multi-stage reactivation of semi-ductile to brittle shear zones and veins within three geographically distinct sites, 170-350 km apart, across the Fennoscandian Shield. The absolute geochronological determinations are supported by detailed petrographic documentation. This robust methodology allows us to identify the temporal and spatial relationships between periods of fracture reactivation and terrane-spanning tectonic events, and thus record key processes in the geological development of Precambrian cratons.

| Geological and geochronological background
Sub-mm-wide veins and fracture coatings were sampled from drill cores covering the upper kilometre of the crystalline crust from three sites in Sweden: Laxemar, Forsmark and Siljan ( Figure 1). The sampled mineral assemblages are representative of the vein and fracture generations occurring at these sites. The Laxemar area comprises ca.

Ga granitoid rocks belonging to the Transscandinavian Igneous
Belt (TIB; Wahlgren et al., 2008). 40 Ar/ 39 Ar cooling ages of rockforming micas suggest that the crystalline rocks at Laxemar have been in the brittle regime since ~1,620 Ma (Söderlund et al., 2008).
The basement at Siljan is composed of 1.79 Ga and 1.71-1.68 Ga granitoids (Ahl et al., 1999) and is heavily fractured due to a meteorite impact dated to 380.9 ± 4.6 Ma using 40 Ar/ 39 Ar geochronology on impact breccia (Jourdan & Reimold, 2012). Previously recognized fracture reactivation and fluid flow events in this area include impact-induced hydrothermal quartz breccia systems (Hode et al., 2003) and authigenic carbonate mineral growth on at least four occasions between 80 and 22 Ma by ambient microbial activity .

| RE SULTS AND D ISCUSS I ON
A total of 27 Rb-Sr isochrons from 19 samples (Table 1) were derived for a variety of mineralization assemblages and structures (Dataset S1) and group into several age populations. Petrographic and geochronological characteristics of the episodic mineralization on individual veinand/or grain-scale, and their relation to tectonic episodes affecting the Fennoscandian Shield on the craton-scale, are presented below.

Statement of significance
We present a new methodological approach, distinguishing discrete vein generations in complex inter-grown networks and linking them to tectonic events up to a billion years apart. Coupled textural and structural analysis and in situ dating of growth zones in single grains and veins enhance the understanding of micro-and meso-scale responses to fluid migration and fracture reactivation episodes across a craton. Our recorded age clusters represent an extensive contribution to constraining the deformation history of the Fennoscandian Shield, demonstrating the advancing applicability of the methodology for tectonic reconstruction.

| Mineral precipitation age clusters
The oldest age determined in the study is from K-feldspar-albite fabric in a Laxemar mylonite (1757 ± 15 Ma). The oldest ages at Siljan (1,680 ± 43 and 1,670 ± 100 Ma) are from K-feldspar ± calcite ± albite (nuclei in zoned vein crystals).
Mesoproterozoic ages between 1,495 ± 55~1,429 ± 18 Ma are most frequently observed (n = 10), and occur in all areas. These ages are mainly from K-feldspar-calcite assemblages in veins, such as in Forsmark (Figure 2), but also from K-feldspar ± calcite ± albite ± har-

| Linkage of fracture activation/reactivation to tectonic events
40 Ar/ 39 Ar closure ages of TIB hornblende indicate rapid cooling down to ~500℃ shortly after the 1.8 Ga TIB rock emplacement at Laxemar (Söderlund et al. 2008). This is in agreement with our 1757 ± 15 Ma-aged K-feldspar-albite fabric in a major semi-ductile NE-trending shear zone (Figure 3f bly influenced by up to 6-7-km-thick foreland basin development (Guenthner et al., 2017).

The Caledonian Orogeny affected NW parts of Fennoscandian
Shield in the Late Cambrian to Early Devonian (Roberts, 2003), with a main "Scandian" stage of collision and subsequent orogenic collapse between 430 and 380 Ma (Corfu et al., 2014). Hence, we relate the 447 ± 37~444 ± 12 Ma old adularia-calcite ± illite vein assemblages at Laxemar and Siljan (Figures 3 and 4) to far-field effects related to a pre-Scandian tectonic event in the Caledonides. Our studied fracture sets show no straightforward orientation trend (Figure 3), but previous investigations found a dominance of WNW-ESE-directed fractures carrying the same mineral assemblage (Drake et al., 2009), in agreement with direction of the Caledonian bulk crustal shortening (Roberts, 2003).
During post-orogenic collapse of the Caledonides after ca.
408 Ma (Andersen, 1998), a foreland basin had started to develop on the Fennoscandian shield to the ESE-SSE of the orogenic belt (Cederbom, 2001). This development probably featured local extensional stress directed perpendicular to the orogen, in response to a migrating forebulge (Alm et al., 2005). The most frequently detected Palaeozoic vein mineral ages cluster between 402 ± 23~387 ± 10 Ma at Laxemar and Forsmark (Figures 3 and   4). These mainly occur in steep NNE-striking fractures, including fractures with Mesoproterozoic mineral precursors (Figure 2, Dataset S1). Consequently, we correlate this event of fracture activation/reactivation to WNW-ESE-directed extension following orogenic collapse and foreland basin initiation.
Thermochronological constraints exist for heating of the wall rock minerals at Laxemar to 150℃ during the peak thickness of the Caledonian foreland basin (Guenthner et al., 2017). We propose that the 358 ± 8~355 ± 12 Ma adularia ± illite ± calcite formation in reactivated veins at Laxemar and Forsmark occurred during foreland basin development featuring enhanced fluid circulation induced by the increased heat and tectonics associated with crustal

| Poly-phase precipitation processes and gaps of mineralization
The recognized multi-stage discrete mineral growth in eight of 19 samples supports that fluid migration reutilized pre-existing crustal structures. This ratio of reactivated samples to new structures is a minimum due to the potential to detect more reactivation episodes with lower error margins and extended sampling along and across fracturing planes.

CO N FLI C T O F I NTE R E S T
The authors declare that there is no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.