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Rb-Sr Whole-Rock Isochron Ages of Late Precambrian to Cambrian Igneous Rocks from Southern Britain

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Rb-Sr Whole-Rock Isochron Ages of Late Precambrian to Cambrian Igneous Rocks from Southern Britain J. geol. SOC. London, Vol. 137, 1980, pp. 649656, 6 figs., 1 table. Printed in Northern Ireland. Rb-Sr whole-rock isochron ages of late Precambrian to Cambrian igneous rocks from southern Britain P. J. Patchett, N. H. Gale, R. Goodwin & M. J. Hum SUMMARY:Rb-Sr age data are reported for late Precambrian to Cambrianigneous and metamorphic rocks from the border region of England and Wales. A felsic intrusion from Stanner Hill defines a good whole-rock isochron of 702 f 8 Ma. Uriconian pyroclastic rocks from the Wrekin area give a whole-rock age of 558* 16 Ma, while the late- or post-Uriconian Ercall granophyre from the same area gives a good whole-rock isochron of 533 f 13 Ma. A biotite cooling age from the nearby pre-volcanic Rushton Schist is consistent with these dates and suggests that the region cooled after the close of Uriconian magmatism at 536*8 Ma. All these results relate to widespread igneous/metamorphic activityin latest Precambrian to Lower Cambrian time. The Ercall granophyre and Rushton schist are directly overlain by Lower Cambrian sedi- ments and 533* 13 Ma is thus a maximum age for the deposition of these rocks. Rocks underlying themarine Cambrian deposits of rocks comprising the‘Stanner-Hanter complex’, de- southernBritain are exposed ina series of isolated scribed in detail by Holgate & Hallowes (1941). They inliers. Early Rb-Sr mineral and K-Ar dating of these distinguisheda series of intrusiveevents beginning rocks (Moorbath & Shackleton 1966; Lambert & Rex withfine dolerite, followed successively by gabbro, 1966; Fitch et al. 1969)suggested that almostall felsic intrusions and veins, and finally some rare later regions underwent a major thermal event650-600 Ma dolerite dykes. The intrusive rocks are essentially un- ago. Following many discussions of the possible ages metamorphosed,but have been subjected to local of formation, as distinct from ages of cooling, of the shearing and hydrous alteration. rocks (references in Thorpe 1979), Rb-Sr whole-rock Sampling for the present study concentrated mainly andU-Pb zircondating has shown that inseveral on asingle felsic intrusion exposed on the summit cases, thePrecambrian rocks were indeed formed ridge of Stanner Hill, intruded into gabbros and doler- 650-540 Ma ago (Bath 1974; Cribb 1975; Patchett & ites. The southern part of thisbody is fine-grained Jocelyn 1979; Beckinsale & Thorpe 1979). The pres- with phenocrysts of quartz and sodic plagioclase in a ent study is a further contribution to knowledge of the partlygranophyric quartz-alkali feldspar-plagioclase age of the continental crust in southern Britain. groundmass.Northwards the intrusionbecomes coarser-grained, and passes gradually into a medium- Analytical methods grained,sporadically granophyric rock consisting mainly of quartz, microcline and sodic plagioclase, and Sr was extracted from the samples by HF-HNO, dissolu- carryingdrusy cavities partly filled withmuscovite. tion followed by cation exchange separation. A measured Sr The abundance of microcline and the occurrence of blank gave 2.9ng. Micas were analysed by isotope dilution muscovite clearly shows that the felsic intrusion be- using a mixed Rb-Sr spike. All samples were measured on an comesmore alkali-rich towardsthe N, and thisis Oxford-built 30cm radiusmass spectrometer. 3 measure- reflectedin theRb/Sr ratios (Table 1). Atthe ments of the Eimer & AmendSrCO, standard onthis machine at the time of sample analysis yielded a mean of margins of the intrusion to the S, N and E, there is a 0.70829*8, somewhat higher than commonly quoted values. wide zone (10-30 m) of hybridized gabbro containing The resultshave been corrected to a value of 0.70800. poikilitic biotite crystals up to 5 cm, in place of the Uncertainties in the cause of the analytical bias and errors usual pyroxene or amphibole. This hybridization led in the correction are probablyinsignificant in view of the Holgate & Hollowes(1941) to infer thatthe mafic widerange of 87Sr/86Srin the samples.For whole-rocks, rocks were still hot when the felsic bodies and veins Rb/Sr ratios and Rb and Sr concentrations were determined wereemplaced. The samples analysed (Fig. 1) are by X-ray fluorescence as described by Pankhurst & O’Nions mainly from the northern area of this felsic intrusion, (1973). Rb-Srisochron regression calculations were made although asingle sample (P67) from afelsic vein using a programmebased on Williamson (1968); this pro- invading gabbro on the southern partof Stanner Hill is gramme has been tested using data set 3 of Brooks et d. (1972). Errors are quotedat 2u, andthe decay constant included. ha7Rb=1.42~ 10-” a-1 is used. The analyses fromthe felsicintrusion (Table 1) define a good whole-rock isochron of 702 f 8 Ma (Fig. 2), with initial 87Sr/86Sr = 0.7053*3 and MSWD = StaMer Hin 1.15. The vein sampleP67 was omittedfrom the Stanner Hill (Figs 1 and 6) is the northernmost of 3 calculation. The microcline-rich,muscovite-bearing isolated,mainly fault-bound outcrops of intrusive samples P60-62, P91 andP97, collected fromthe 0016-7649/80/0900-0649$02.00. @ 1980 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/5/649/4886846/gsjgs.137.5.0649.pdf by guest on 27 September 2021 650 P. J. Parchett et al. which exposes a succession of pyroclastics with sub- sidiary lavas. In the lowest part of the quarry, several Stanner Hill distinctive units 0.75-2.0 m thick consist of devitrified glassfragments, some with characteristic shard / faultsagainst Silurian shapes,quartz and feldspar crystalfragments, and d other,indeterminate devitrified glassy material. The a felsic intrusions unitsshow variable development of sizesorting of gabbro + doler ite glass and crystal fragments into 1-100 mm beds. The R horizons are interpreted aspyroclastic rocks related to A hill summits ignimbriteeruptions, but subjected to variablea amount of sorting by water at the time of deposition. Of the two units sampled (Table l), unit 1 shows an irregular, contorted bandingwith virtually no size sort- ing of fragments. Possible devitrified pumice fragments were identified in one of these samples. Unit 2, on the other hand, is well sorted into bandsof different grain size, butnevertheless shows many devitrified glass shards. The two units are separatedby c. 2 m of crystal tuff. They have systematically different Rb/Sr ratios, TABLE1: Rb-Sr isotopic data Rbppm Sr ppm "Rb/'% s7Sr/86Sr Sample *5% *5% *2% (2~) *2~, FIG. 1. Sample locality map for Stanner Hill. The Stanner Hill intrusion boundaries of the felsic intrusion78 in the area be- P59 172 1.303 0.71830* 12 tween the twosummits have been remapped, P60 207 50 12.032 0.82595*20 resulting in a further extension of the intrusion to- P6 1 174 48 10.539 0.81144+10 wards the N as compared to Holgate188 & Hallowes P62 45 12.242 0.82831 * 18 (1941). The road junction is at 69SO 262582. P63 101 1.974 0.72535f 16 P67 78 116 1.930 0.72309*20 P91 135 43 9.135 0.79687*-4 northernmost outcrops (Fig. l),are primarily responsi- P94 132 126 3.023 0.73492*6 blefor defining theage; the remaining 249 3 samples, P97 42 17.6460.88180*64 however, appear to be completely consistent with this Uriconian jelsic tuffs isochron. Unit 1 P40A 128 155 2.371 0.72313* 16 P40 B 127 P40B 253 1.440 0.71557* 18 Wrekin area P4 1 116 122 2.772 0.72640* 20 P42A 71 161 1.264 0.71483*24 In the Wrekin area (Figs 3 and 6) the following rock P42B 91 148 1.767 0.71889*10 groups underlie the Lower Cambrian Wrekin quart- Unit 2 zite: 64 P44A 243 0.750 0.71037*6 1. The Rushton Schist, consisting 32of psammitic and P44B 329 0.268 0.70657*6 peliticlayers regionally metamorphosed in theam- P45 21 277 0.207 0.70620 f6 phibolitefacies to garnet,biotite 38and muscovite- P47 325 0.328 0.70695 *4 Ercall granophyre bearing schists. P27A 104 P27A 41 7.358 0.76076*8 2. The Uriconian volcanic suite, consisting of essen- P27V 111 37 8.753 0.77304*26 tially unmetamorphosed calc-alkaline lavas156 and pyro- P29D 19 23.865 0.88631*24 clastic rocksof a basalt-andesite-dacite-rhyoliteassocia- P29F 101 24 12.261 0.79963*8 tion (Greig et al. 1968; Thorpe 1974).107 P29J 25 12.690 0.80208* 10 3. Dolerite and granophyre intruding the114 volcanic P72A 35 9.391 0.77712*24 rocks,and assumed to bepart of 97the same broad P72B 35 8.006 0.76722 f 24 magmatic event. PZ8 131 27 13.967 0.81153*6 All these rocksare described by Pocock et al. (1 938) Rushton schist P32whole rock 153 63 6.9320.78754* 10 and Greig et al. (1968). P32 muscovite 202 115 5.1330.77363*42 The Uriconian volcanic rocks were sampled at the 239biotite P32 17 42.4621.05888*40 workingLeaton quarry at Wrockwardine (Fig. 3), Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/5/649/4886846/gsjgs.137.5.0649.pdf by guest on 27 September 2021 Rb-Sr ages of late Precambrian to Cambrian igneous rocks, southern Britain 65 1 STANNER HILL felsic intrusion / /*P91 age 702 2 8 Ma Sro = 0.7053 t 3 MSWD = 1.15 1 I l I 5 10 15 20 87Rb/86Sr FIG.2. Rb-Sr isotopic data for the samples from Stanner Hill. P67, not from the felsic intrusion, is excluded from the regression calculation. and also considerable internal variation, due to vari- evidence that the alkali contents (hence Rb, Sr) have able content of feldspar crystalfragments. The two not been disturbed after crystallization. The samples unitsanalysed separately give indistinguishable but (Table 1) definea statistically perfectisochron of ratherimprecise age information, so they are com- 533f 13 Ma (Fig.
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