Abstract
Magmatism between 1.3 and 0.9 Ga at the southwestern margin of Fennoscandia, comprising mainly granitic batholiths and subordinate bimodal volcanic rocks, provides a nearly continuous magmatic record of the Fennoscandian tectonic evolution. Here, we present new and published zircon Hf, K-feldspar Pb and whole-rock Sr isotopic data from the granitic rocks. The εHf isotopic evolution since 1300 Ma starts out as relatively juvenile, with a flat superchondritic trend at 1300–1130 Ma followed by a steeper trend towards lower, but still superchondritic values at 1070–1010 Ma. During the 1000–920 Ma period, the trend flattens out at near-chondritic values. The variations between flat and steep εHf trends correspond to previously documented extensional and compressional periods, respectively. Although the change to a steeper εHf trend at ca. 1100 Ma may indicate the emergence of a new isotopic reservoir (i.e. a colliding continent), there is no corresponding change in the K-feldspar Pb or whole-rock Sr isotopic composition. We argue that the trends are better explained by varying proportions of isotopically evolved crust and juvenile mantle in the magma source regions, similar to Nd and Hf isotopic pull-downs and pull-ups observed in many accretionary orogenic systems. We therefore conclude that continuous accretionary processes without involvement of exotic sources is the best explanation for the isotopic evolution before and during the Sveconorwegian orogeny, and that the orogeny involved generation of significant volumes of new crust to the SW margin of the Fennoscandia.
Highlights
Two fundamental processes of active continental margins are trench advance and trench retreat, which are recorded in both the metamorphic (Collins, 2002) and magmatic record (DeCelles et al, 2009)
The samples used in this study are from the Telemark and Bamble lithotectonic units and comprise 59 previously dated samples analysed for Hf, 34 samples for whole-rock Sr and 20 samples for Pb in K-feldspar
Most of the available Lu\\Hf data for the Sveconorwegian granitoids in the Telemark lithotectonic unit show a trend starting at 1.5 Ga that follows through the Sveconorwegian orogen (Fig. 7 in Roberts and Slagstad, 2015); whereas only a few inherited zircon grains indicate a Palaeoproterozoic source (Andersen et al, 2002)
Summary
Two fundamental processes of active continental margins are trench advance and trench retreat, which are recorded in both the metamorphic (Collins, 2002) and magmatic record (DeCelles et al, 2009). The input of juvenile components into arc magmas leads to isotopic “pull-ups”, while isotopic “pull-downs” indicate reworking of pre-existing crust (Boekhout et al, 2015; DeCelles et al, 2009; Kemp et al, 2009; Kohanpour et al, 2019). When the tectonic setting transitions from an active margin to continent collision, the magmatism changes from arc-related to collisionrelated. This implies a change from a mixed crust-mantle source to a dominantly (or exclusively) crustal source. In this case, even if the exotic and indigenous crustal source have similar isotopic compositions the result is a strong isotopic pull-down (Collins et al, 2011). In cases where the tectonic setting is unclear, such as in ancient orogens, the isotopic record may provide a means to identify the underlying tectonic setting and evolution (e.g. Spencer et al, 2019)
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