On the track of a Scottish impact structure: a detrital zircon and apatite provenance study of the Stac Fada Member and wider Stoer Group, NW Scotland

Abstract

The Stac Fada Member of the Stoer Group, within the Torridonian succession of NW Scotland, is a melt-rich, impact-related deposit that has not been conclusively correlated with any known impact structure. However, a gravity low approximately 50 km east of the preserved Stac Fada Member outcrops has recently been proposed as the associated impact site. We investigate the location of the impact structure through a provenance study of detrital zircon and apatite in five samples from the Stoer Group. Our zircon U–Pb data are dominated by Archaean grains (> 2.5 Ga), consistent with earlier interpretations that the detritus was largely derived from local Lewisian Gneiss Complex, whereas the apatite data (the first for the Stoer Group) display a single major peak at c. 1.7 Ga, consistent with regional Laxfordian metamorphism. The almost complete absence of Archaean-aged apatite is best explained by later heating of the > 2.5 Ga Lewisian basement (the likely source region) above the closure temperature of the apatite U–Pb system (c. 375–450°C). The U–Pb age distributions for zircon and apatite show no significant variation with stratigraphic height. This may be interpreted as evidence that there was no major change in provenance during the course of deposition of the Stoer Group or, if there was any significant change, the different source regions were characterized by similar apatite and zircon U–Pb age populations. Consequently, the new data do not provide independent constraints on the location of the structure associated with the Stac Fada Member impact event.

Provenance of altered carbon phases and impact history of the Stac Fada Member, NW Scotland

“…From disordered carbon, we detect a graphitic precursor and a rare kerogenous potential biotic precursor (Figure 6); as such, one would expect two types of impact diamonds derived from high pressure alteration of either. However, given the low abundance of shocked phases discovered, in agreement with earlier studies (Kenny et al, 2019;Osinski et al, 2020), impact diamonds formed after kerogen within the Stac Fada Member must be exceedingly rare.…”

“…This is mixed with highly altered and partially devitrified vesicular glass which makes up ~20%–30% by volume and is interpreted to be an impact melt (Amor et al., 2008). Previous work has identified shocked quartz (Amor et al., 2008; Osinski et al., 2020), a high‐pressure zircon polymorph reidite (Reddy et al., 2015), and shock features in zircon (Kenny et al., 2019), directly linking the deposition of the Stac Fada Member to a hypervelocity impact event. However, evidence for shock within the member is notably sparse compared to other terrestrial hypervelocity impact deposits (Kenny et al., 2019; Osinski et al., 2020).…”

“…Previous work has identified shocked quartz (Amor et al., 2008; Osinski et al., 2020), a high‐pressure zircon polymorph reidite (Reddy et al., 2015), and shock features in zircon (Kenny et al., 2019), directly linking the deposition of the Stac Fada Member to a hypervelocity impact event. However, evidence for shock within the member is notably sparse compared to other terrestrial hypervelocity impact deposits (Kenny et al., 2019; Osinski et al., 2020).…”

The Mesoproterozoic Stac Fada Member, NW Scotland: an impact origin confirmed but refined

“…Thus, our results corroborate the findings of Amor et al (2008) and Reddy et al (2015) and further confirm that the Stac Fada Member contains material derived from an impact event. However, the amount of shocked material in the Stac Fada Member is extremely low compared to other impact ejecta deposits (Table 1) (see also the recent detrital zircon and apatite study by Kenny et al (2019)), which we discuss further below.…”

Evaluating sediment recycling through combining inherited petrogenic and acquired sedimentary features of multiple detrital minerals

“…An increase in compositional maturity is a hallmark of sedimentary recycling and reflects the breakdown and removal of labile detrital components during multiple cycles of chemical weathering and diagenesis (Cox & Lowe, 1995; Garzanti, 2017). Important progress towards accounting for the compositional changes imparted by sedimentary recycling in provenance studies has been facilitated by advances in in‐situ micro‐analytical techniques over the past two decades, which have allowed provenance fingerprinting of a variety of labile detrital minerals, such as feldspar (e.g., Barham et al, 2020; Flowerdew et al, 2019; Johnson et al, 2018; Mulder et al, 2019; Tyrrell et al, 2006, 2009), monazite (e.g., Hietpas et al, 2010; Moecher et al, 2019), mica (e.g., Mulder et al, 2017; Sun, Kuiper, Tian, Li, Gemignani, et al, 2020; Sun, Kuiper, Tian, Li, Zhang, et al, 2020) and apatite (e.g., Kenny et al, 2019; Nauton‐Fourteu et al, 2021; O'Sullivan et al, 2020). By employing a multi‐proxy provenance approach, first‐cycle detrital zircon can be identified as grain populations that are accompanied by a labile mineral population derived from the same source, whereas detrital zircon populations with no corresponding labile phase may have been recycled (Tyrrell et al, 2009) (Figure 1).…”