U-PB CHRONOLOGY OF THE EDIACARAN GEOLOGY AND ASSOCIATED FOSSILS OF CHARNWOOD FOREST, UK

U-Pb (zircon) geochronology is used here to date horizons containing fossils of the Avalonian-type Ediacara biota from Charnwood Forest (UK). The volcano-sedimentary Charnian Supergroup is dominated by fine- to very coarse-grained volcaniclastics, pyroclastics and subvolcanic intrusions of andesitic to dacitic composition and were affected by late Silurian deformation^[1]. Macrofossils from two distinct assemblages are preserved on finer grained upper bed surfaces of turbidite units or ash-grade tuffaceous horizons. Stratigraphically oldest fossils, including Ivesheadia lobata^[2], occur at a single location in the lowermost Blackbrook Group. Younger fossil forms (e.g. Charnia) occur at five horizons in the overlying Maplewell Group and comprise a more diverse population (Mercian Assemblage^[3]) sharing similarities with the Mistaken Point biota. From the succession containing the Mercian Assemblage, preliminary CA-ID-TIMS data for a subaqueously reworked volcaniclastic lithology with sedimentary clasts yielded 563 ± 1.9 Ma, within error of the acme of the Mistaken Point biota. Further dating using the ET 535 U-Pb tracer confirms and refines this age. Reconnaissance LA-ICP-MS data from a coarse pyroclastic lithology at the base of the Maplewell Group, c. 1200 m stratigraphically below the Mercian Assemblage, indicated c. 570 Ma grains within a general population of c. 615 Ma zircons, and CA-ID-TIMS data for younger grains from the ICP-MS grain mount confirm 565-570 Ma ages, placing a lower age limit on the Maplewell Group. Definitively dating the Blackbrook Group fossil horizon with Ivesheadia lobata (‘Lubcloud Assemblage’³) is more problematic. Petrography and LA-ICP-MS data indicate a mainly monomictic zircon population at c. 615 Ma in volcaniclastic turbidites sampled at and bracketting the fossil horizon; no younger grains were found. CA-ID-TIMS confirms this finding with c. 613 Ma zircon ages from the sampled horizons. The data provide a maximum age for this fossil horizon. Whether these zircon ages date the time of deposition crucially depends upon interpretations of processes occurring between their initial eruption and eventual burial.

[1] Carney et al., 2008, Geol. Mag. 145, 702-713; [2] Boynton and Ford, 1995, Mercian Geologist 13, 165-182; [3] Wilby et al., 2011, Geology 39, 655-658.