A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications

Abstract

An isotope dilution thermal ionization mass spectrometry U–Pb zircon age of 411.5 ± 1.3 Ma obtained from an andesitic lava occurring within the Lower Devonian Rhynie Outlier (Aberdeenshire, NE Scotland) effectively dates the Rhynie Chert Konservat-Lagerstätte. Biostratigraphical constraints on the Rhynie Chert-bearing succession indicate that this age lies within the interval early (but not earliest) Pragian–(?)earliest Emsian. Accordingly, the Pragian–Emsian boundary must post-date or closely approximate to 411.5 ± 1.3 Ma, while the Lochkovian–Pragian boundary must predate 411.5 ± 1.3 Ma. Integration of this new high-precision age with an improved temporal framework for late Caledonian intrusive activity in NE Scotland suggests that the Rhynie hot-spring system (the ‘parental' hydrothermal system to the Rhynie cherts) was unrelated to any ‘Newer Granite' intrusion. Rhynie was instead powered by a basaltic andesite magma whose generation and ascent were directly linked to the transcurrent fault movements responsible for the formation of the Rhynie basin. Supplementary material: Details of analytical techniques (ID-TIMS U–Pb geochronology) and photomicrographs of zircon and titanite grains recovered from the Milton of Noth Andesite are available at http://www.geolsoc.org.uk/SUP18463 .

Lava–sediment interactions in an Old Red Sandstone basin, NE Scotland

“…Taken collectively, these ages can be considered to be early Devonian (Lokchavian-Pragian) and thus are consistent with the palynological data (Richardson et al 1984;Lavendar & Wellman 2002). A precise zircon u-Pb age of 411.5 ± 1.3 Ma for an andesite from the Rhynie outlier (Parry et al 2011) suggests that the volcanic and hydrothermal activity at the Rhynie Outlier was coeval with that of the Montrose Volcanic Formation. Additionally, although there complexities surrounding the interpretation of the zircon data, the Glencoe Igneous Complex may also have been emplaced at this time (emplacement of the complex has been dated at 406 ± 6 Ma (Fraser et al 2004) and the emplacement of a fault intrusion at 419.6 ± 5.4 Ma (Neilson et al 2009)).…”

Mantled fungal reproductive units in land plant tissue from the Lower Devonian Rhynie chert

“…The fossil-bearing cherts occur in the Windyfield Shales Member, i.e. within the lower part of the Dryden Flags Formation (Parry et al 2011). This Lagerstätte is made up of fossiliferous beds containing lacustrine shales and cherts that have been interpreted as a series of ephemeral freshwater pools which existed within a hot spring environment (Rice et al 2002, Rice & Ashcroft 2004.…”

“…The cherts have been dated as Pragian-?earliest Emsian based on spore assemblages (Wellman 2006. High-precision age constraints on the Rhynie Lagerstätte indicate absolute ages of 407.1 ± 2.2 Ma (Mark et al 2011) and 411.5 ± 1.3 Ma (Parry et al 2011).…”

Reply to Discussion on ‘A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications’

“…Thirdly, there is neither physical nor geochronological evidence (Parry 2004, unpubl. data;Parry et al 2011) for any other Devonian igneous activity of similar age to or younger than the Rhynie Outlier volcanism in the local area. On the basis of the collective evidence, which points to both a spatial and temporal association, we still consider it perfectly reasonable to infer a genetic link between the Rhynie…”

“…In the course of their discussion, Mark et al partially reinterpret the U-Pb dataset of Parry et al (2011) Ma is of no value whatsoever in terms of statistically distinguishing between an age of 407.6 ± 2.2 Ma and one of 411.5 ± 1.3 Ma. Furthermore, it is unclear why only those zircons apparently carrying a Proterozoic inherited component would be affected by the proposed hydrothermal Pb-loss.…”