Penelope J. Lancaster

Interrogating the provenance of large river systems: multi-proxy in situ analyses in the Millstone Grit, Yorkshire

Establishing the source(s) of sedimentary material is critical to many geological applications, but is complicated by the ability of some minerals to be recycled. To test the relative utility of current proxies for determining a unique provenance, new samples have been collected from the Namurian Millstone Grit Group of Yorkshire, England. Two K-feldspar 206Pb/204Pb isotope populations between 12.5 and 15.5 and c. 18.4 are consistent with Archaean–Proterozoic basement and Caledonian granites, respectively. Zircon U–Pb age populations at c. 2700, 2000 – 1000 and 430 Ma reflect a mixture of Archaean basement, overlying Proterozoic sediments and intrusive Caledonian granites, and εHf values in zircons of all ages indicate crystallization from reworked crust. Garnet major element compositions are relatively rich in Fe and low in Ca, indicative of derivation from a granulitic or charnockitic source. Rutile Cr/Nb ratios indicate that source rocks were dominantly metapelitic, and Zr-in-rutile thermometry records two populations representing lower (c. 650°C) and higher (c. 800°C) metamorphic grade material. Combining these results with published monazite and muscovite data suggests overall derivation from the Greenland Caledonides, with additional contributions from NE Scotland and western Norway, highlighting the power of multi-proxy provenance work, especially in tectonically and geologically complicated regions. Supplementary material: Sample details, full analytical methods, data tables and references for compilation figures in the text are available at https://doi.org/10.6084/m9.figshare.c.3515457.

U–Pb zircon geochronology and geodynamic significance of ‘Newer Granite’ plutons in Shetland, northernmost Scottish Caledonides

U–Pb zircon ages obtained from the late- to post-tectonic ‘Newer Granite’ suite in Shetland, northernmost Scottish Caledonides, indicate a significantly more protracted intrusion history than was inferred previously from K–Ar data. Emplacement of the Brae Complex (c. 465 Ma), Graven Complex (c. 440 Ma) and the Muckle Roe Granophyre (c. 438 Ma) followed regional deformation and metamorphism of metasedimentary successions during the Grampian orogenic event, and is attributed to NW-directed subduction beneath Laurentia. The almost complete absence of plutons of this age along-strike in mainland Scotland suggests a change in subduction angle and/or the distance between the subduction zone and the Laurentian margin. Intrusion of the Ronas Hill Granite (c. 427 Ma) was approximately coeval with displacement on the Moine Thrust in mainland Scotland, and therefore probably occurred during Baltica–Laurentia collision. A gap of c. 35 myr followed before emplacement of the Mangaster Voe Intrusion and Eastern Granophyre (c. 390 Ma), and a further gap of c. 20 myr before emplacement of the Sandsting Complex (c. 370 Ma). Both periods of magmatism are attributed to pulses of localized lithospheric melting in the vicinity of the Walls Boundary Fault during Devonian sinistral relative displacements between Laurentia and Baltica. Supplementary material: Petrological details, full analytical methods and data tables are available at https://doi.org/10.6084/m9.figshare.c.3655706

The Eoarchaean foundation of the North Atlantic Craton

Abstract The Archaean North Atlantic Craton underpins much of North America, Greenland and northern Europe, and incorporates the Earths oldest extant continental crust. This paper reviews the current understanding of the regions crustal evolution, and considers our ability to investigate interrelationships between different fragments of the North Atlantic Craton. Detrital zircons from Mesoproterozoic to Cambrian basal sediments in NW Scotland have been re-examined in light of new data from the Archaean Tarbet supracrustal unit and the Palaeoproterozoic Rubha Ruadh granite. Hf model ages are recorded from 4160 to 1410 Ma, peaking at c. 3350 Ma, and are associated with U–Pb crystallization ages from 3670 to 1070 Ma, peaking at c. 2700 and 1700 Ma. The Rubha Ruadh granite is consistent with partial melting of Northern Region basement without contamination by juvenile magmas or supracrustal material, while the Tarbet Supracrustals record a minimum model age of c. 3200 Ma. Each of these units records Hf model ages that imply remelting of Eoarchaean (4000–3600 Ma) crust. Similar distributions of crystallization and model ages have been identified around the North Atlantic Craton, suggesting that Eoarchaean crust was once extensive in the region and constitutes the foundation of both Scotland and the North Atlantic Craton. Supplementary material: All new zircon U–Pb-Hf-O data from this study are available at www.geolsoc.org.uk/SUP18776.