DATA REPOSITORY 1
2
Analytical methods 3
Mineral Separation 4
Samples (5 to ~50 kg) were jaw-crushed and disk-milled to <420 μm and heavy mineral 5
concentrates prepared using a Gemini table, heavy liquids (methylene iodide) and Frantz LB-6
1 separator. Grains selected for LA-ICPMS were mounted in epoxy blocks and imaged in 7
BSE and CL modes by SEM prior to analysis by Dr. S. Parry and Mr. G. Turner of the British 8
Geological Survey. 9
10
Laser ablation ICP-MS 11
Laser ablation data were obtained on a Nu Instruments multiple collector inductively coupled 12
plasma mass spectrometer (MC-ICP-MS). The NIGL Nu MC-ICP-MS collector block 13
permits simultaneous collection of masses relevant to U-Pb chronology (masses 202 through 14
207, 235, and 238). Further details are given for an almost identical collector configuration 15
by Simonetti et al. (2005). Data collection, reduction and propagation of uncertainties follow 16
Horstwood et al. (2003) and Bauer et al. (2011). Discrete dynode secondary electron 17
multipliers were used to measure
204
Pb+
204
Hg,
206
Pb and
207
Pb, with other isotopes of interest 18
measured on Faraday cups. Targeted zircons were sampled using a New Wave Research 19
UP193-FX 193 nm ArF excimer laser microprobe system. Zircons were ablated for 20 20
seconds using a 25 µm static spot at a laser fluence of ca. 2.5 J cm
-2
. These ablation 21
protocols provided reconnaissance level data with
206
Pb/
238
U ratio uncertainties generally 22
<2%. Instrumental mass fractionation was monitored using a mixed natural Tl-
235
U solution 23
introduced via a Nu Instruments DSN 100 desolvating nebulizer. Fractionation related to 24
laser ablation was corrected in unknowns by analyzing zircon reference materials. During the 25
34
course of this study the following zircon standards were used: 91500 dated at 1062.4 ± 0.4 26
Ma (Wiedenbeck et al., 1995), GJ-1 dated at 600.4 ± 0.6 Ma, (Jackson et al., 2004) and 602.3 27
± 1 Ma (current value from NIGL TIMS data using the EARTHTIME
205
Pb-
233
U-
235
U tracer), 28
and the 337.33 ± 0.38 Ma Plesovice zircon (Sláma et al., 2008). Raw data was reduced using 29
an in-house Excel data reduction worksheet. Given the reconnaissance nature of the LA-ICP-30
MS analytical work, data with <10% discordance were accepted for age calculations where 31
the contaminant was deemed most likely to be common Pb from the abundant melt inclusions 32
in many of the zircons (see DR Fig 2). Zircon data were rejected in instances where mixing 33
was along obvious <3000 Ma – non-zero discordia lines, and grains where sufficient common 34
Pb was present to result in >10% discordance. 35
36
U-Pb (zircon) Chemical Abrasion Isotope Dilution Thermal Ionisation Mass Spectrometry 37
(CA-ID-TIMS) 38
Zircons analysed by TIMS were subjected to “chemical abrasion” (thermal annealing and 39
subsequent leaching pre-treatment; Mattinson, 2005) to effectively eliminate Pb-loss. Zircons 40
were heated in a muffle furnace at 900 ± 20°C for ~60 hours in quartz beakers before being 41
transferred to 3 ml Hex Savillex beakers, which were in turn placed in a Parr vessel, and 42
leached in a ~5:1 mix of 29M HF + 30% HNO
3
for 12 hours at ~180°C. The acid solution 43
was removed, fractions rinsed in ultrapure H
2
O, fluxed on a hotplate at ~80°C for 1 hr in 6 M 44
HCl, ultrasonically cleaned for 1 hr, and then placed back on the hotplate for an additional 30 45
min. The HCl solution was removed and the fractions (single zircon crystals or a single 46
fragment) were selected, photographed (in transmitted light) and again rinsed (in ultrapure 47
acetone) prior to being transferred to 300 µl Teflon PFA microcapsules and spiked with the 48
mixed EARTHTIME
233
U–
235
U–
205
Pb tracer. The single zircons or fragments were dissolved 49
in ~ 120 µl of 29 M HF with a trace amount of 30% HNO
3
at ~220°C for 48 hours, with the 50
35
microcapsules housed within Parr vessels. The zircon digests were subsequently dried to 51
fluorides and then converted to chlorides in 3M HCl at ~180°C overnight. U and Pb were 52
separated using standard HCl-based anion-exchange chromatographic procedures on 0.05 ml 53
PTFE columns manufactured in-house (Corfu and Noble, 1992). Isotope ratios were 54
measured using NIGL’s Thermo-Electron Triton Thermal Ionisation Mass-Spectrometer 55
(TIMS) dedicated to low-blank U-Pb geochronology (Triton 2). Pb and U were loaded 56
together on a single Re filament in a silica-gel/phosphoric acid mixture (Gerstenberger and 57
Haase, 1997). Pb isotopes were measured by peak-hopping on a single SEM detector. U 58
isotope measurements were made in static Faraday mode. Age calculations and uncertainty 59
estimation (including U/Th disequilibrium) were based upon the algorithms of Schmitz and 60
Schoene (2007). All acids were prepared by sub-boiling distillation: HCl and HNO
3
were 61
double-distilled in quartz and HF was double-distilled in Teflon. Ultrapure water with a 62
resistivity of 18 MΩ was prepared with a Milli-Q system. All reagents were blank-checked 63
prior to use. 64
206
Pb/
238
U dates are calculated using the
238
U and
235
U decay constants of Jaffey et al. 65
(1971) and corrected for initial U/Th disequilibrium using an assumed magma Th/U ratio of 66
4, typical for magmatic systems. A value of
238
U/
235
U
zircon
= 137.818 ± 0.045 (Hiess et al., 67
2012) was used in the data reduction calculations. Compared to calculations using the old 68
‘consensus’ value (
238
U/
235
U = 137.88) this has the effect of reducing
207
Pb/
206
Pb dates by ca. 69
0.98 Myr at the age range of interest (ca. 560 to 620 Ma) and reduces the
206
Pb/
238
U dates by 70
<5 kyr. For U–Pb dates of this age, the
206
Pb/
238
U dates are the most precise and robust. In 71
contrast, the
207
Pb-based dates (
207
Pb/
235
U and
206
Pb/
207
Pb) are considerably less precise and 72
hence are only used to assess concordance of the U–Pb (zircon) systematics. 73
74
75
36
Detailed geochronology sample descriptions 76
Blackbrook Group 77
Three Blackbrook Group samples were examined in this study. Sample JNC 916 was 78
collected at Morley Quarry (BNG SK 4766 1787) from a several meters-thick succession of 79
volcaniclastic sandstones, siltstones and mudstones, just above the exposed base of the Ives 80
Head Formation. At Morley Quarry, individual graded units (Bouma A-E divisions) typically 81
commence in structureless, very coarse-grained volcaniclastic sandstone in which are 82
embedded sporadic angular fragments of laminated volcaniclastic siltstone ripped up from the 83
underlying beds. They show an upward transition into medium-grained sandstone, which in 84
turn develops a diffuse parallel-stratification before passing up to parallel-laminated siltstone 85
and mudstone. An outstanding petrographical feature of JNC 916 is the general uniformity of 86
the angular to subrounded dacitic volcanic grains, which enclose small quartz and plagioclase 87
phenocrysts; their groundmasses are extremely fine-grained and microcrystalline although 88
some show a slightly coarser, microgranular texture. Plagioclase and quartz also occur as 89
discrete, fragmented euhedra between the lithic grains. This graded bed is comparable to the 90
‘secondary monomagmatic volcaniclastic turbidites’ of Schneider et al. (2001) which show 91
mild reworking and clast heterogeneity. 92
JNC 836 was sampled (BNG SK 4772 1700) from the middle part of a 2.5 m thick 93
volcaniclastic turbidite (see Fig. 3a of Carney, 1999). The position of this turbidite is critical 94
in terms of palaeontology, since its uppermost bedding plane contains impressions of 95
Ivesheadia, Blackbrookia and Shepshedia (Boynton and Ford, 1995; Liu et al., 2011). Sand-96
size, angular to subrounded volcanic grains predominate in this sample. These grains are 97
remarkably homogeneous with ~85 per cent having uniformly microcrystalline groundmasses 98
and the remainder exhibiting varying degrees of patchy coarsening to microgranular or faintly 99
spherulitic textures truncated at grain edges (DR Fig. 1a). Many grains contain quartz 100
microphenocrysts indicating a dacitic composition for the parental magmas; one grain was 101
37
also seen to contain a euhedral, acicular zircon crystal. Sharply angular to locally subhedral 102
quartz and plagioclase phenocryst fragments are particularly common within the matrix to the 103
lithic grains. These petrographic characteristics strongly resemble those of JNC 916 and thus 104
JNC 836 is interpreted to have had a similar origin. 105
JNC 917 is from the South Quarry Breccia Member located about 600 m 106
stratigraphically above the other two samples. The sample was obtained from the South 107
Quarry type locality (BNG SK 4637 1712). Exposed faces in the quarry consist of a few 108
meters of stratified to massive coarse-grained volcaniclastic sandstone, passing upwards into 109
a breccia with large contorted rafts of laminated mudstone embedded in a volcaniclastic 110
sandstone matrix. In thin section, the analysed sample contains about 50-60% plagioclase and 111
quartz, present as phenocrysts in dacitic lithic volcanic grains, or as fragmented to partially-112
fragmented crystals concentrated within the matrix between the grains. Lithic grains show a 113
range of crystallinities from exceedingly fine-grained, virtually aphanitic, to more coarsely 114
crystalline varieties with microgranular textures. Patchy recrystallization is commonly seen 115
within the confines of a single grain (DR Fig. 1b). Some lithic grains contain very large 116
embayed quartz euhedra surrounded by a thin ‘skin’ consisting of the microcrystalline matrix. 117
In other outcrops, a degree of heterogeneity is shown by the lithic volcanic grains, and some 118
examples possess a perlitic texture (Carney, 1994). The sedimentary features of the South 119
Quarry Member are consistent with a history of secondary reworking involving submarine 120
slumping of incompletely consolidated volcaniclastic strata. 121
Maplewell Group 122
The oldest sample in the Maplewell Group to yield dateble zircons is JNC 918 from the 123
Benscliffe Breccia, a highly distinctive unit at the base of the Beacon Hill Formation (Fig. 1). 124
Sample JNC 918 was collected from the Benscliffe Breccia Member at the ‘Pillar Rock’ type 125
locality in Benscliffe Wood (BNG SK 5146 1246). These exposures show 3+ meters of 126
massive breccia in which lapilli- to small block-size fragments of andesite are set in a poorly 127
38