Geochemistry and Petrogenesis of the Derbyshire Carboniferous

J. geol. Soc. London, Vol. 141, 1984, pp. 147-159, 11 figs., 4 tables. Printed in Northern Ireland.

Geochemistry and petrogenesis of the Derbyshire Carboniferous

R. Macdonald, K. N. Gass, R. S. Thorpe & I. G. Gass

SUMMARY: Dinantian volcanism in Derbyshire was associated with minor phases of isostatic uplift of a shallow carbonate platform on the northern margin of the St. George's Land E-W land mass. The mainly tholeiitic magmas were generated by varying degrees of partial melting of heterogeneous mantle sources followed by fractional crystallization and final equilibration mainly within the lower crust. Large, high-level magma chambers were not established. Even singlelava sequences, such as the Miller's DaleLava, represent several, isolated magmatic phases. Volcanism was of within-plate type. Magmas generated in isolated mantle pockets rose to the surface through crust attenuated by lithospheric stretching.

The existence of basalticlavas, sills; dykes,breccias, & Tarling1975). In Upper Visean timesthe Der- tuffs and ashes within the Carboniferous Limestone of byshire Block was a roughly circular, level carbonate Derbyshirehas been known for well overa century platform of some 500 km2 that lay either on, or just (Fig. 1). During this time these basaltic rocks have beenoff, the northern edge (Fig. 2) of an E-W land barrier describedpetrographically (e.g. Bemrose 1894, 1907; (St.George's Land). Walkden (1977) hassuggested Tomkeieff 1928), their stratigraphy synthesized (Wal- thatthe area was subjected to numerous (12-15) ters & Ineson 1981), their palaeogeography identified cycles of uplift,during which theplatform rose just (Walkden 1977) andtheir regional setting and signi- abovesea-level, together with subsequent sub- ficance discussed(Francis 1978; Upton 1982). Yet, mergencewhen it was mainly covered by shallow althoughthey are the subject of numerouspublica- seas.Walkden (1977) showedthat during periods of tions, no one. so far as we are aware, has attempted a emergencethe recently deposited limestones were detailedexamination of theirgeochemistry, or pro- erodedand karst surfaces developed. He also made posed magmatic processes to explain their composition;the interesting suggestion that volcanism is temporally this paper attempts tofill the gap. associatedwith periods of uplift: the significance of Asdetailed descriptions of field relations.age, this relation is discussed further below. petrography and regional setting are given elsewhere, This broad palaeogeographic scenario fits, not only onlythose amects relevant to our geochemicaland withour field data,but with the observations of petrogenetic discussionwill be summarized below. others.For instance, the Upper Miller's Dale lavas Withinthe accepted age span (c. 36CL280 Ma) of flowed outover an eroded surface between D, & the Carboniferous Period (Francis & Woodland 1964; DZ thathad long been recognized as such (Cope Fitch et al. 1970), volcanism seems to have been most 1933,1937). Also, Walkden (1977) andWalters & intenseand widespread in theDinantian (345-325 Ineson (1981) identifiedthe widespread tuff and K- Ma),whereas sill emplacementoccurred principally bentonitehorizons as the products of air-fall ash during the Namurian (325-310 Ma). Fitch et al. (1970) deposits on palaeokarst surfaces. An exception to the quoteminimum K-Ar agesfor several Derbyshire uplift-volcanismcorrelation is atCalton Hill, where basalts.For instance, they date the Lower Miller's field evidence suggests to us that a phreatic tuff-ring of Dale Lava (LMDL) at 276 f 17 (282 f 17;this and subaqueousbasaltic agglomerates and ashes was subsequent dates on 1976 decay constants) the Upper formedbefore the effusion of theassociated alkali Miller's Dale Lava (UMDL) at 318 f 9, the Tideswell basalts. Even when the lavas were above sea-level,it is Dale Sill at 287 f 13 and an olivine basalt intrusion at evidentthat the elevation was not great, for the Calton Hill at 295 * 14.Fitch and his colleagues spatiallyvery variable vesicularity of mostlavas considered some of these dates were 'discrepantly low' indicates that at times they flowed over wet sediments due to argonloss. with which Walters & Ineson (1981) and sometimes terminated in shallow water (Cheshire agree. Stratigraphically, the Lower Miller's Dale Lava & Bell 1977; Walkden 1977). Borehole data (Walters is taken as occurring near the top of the Asbian stage & Ineson 1981) showthat most of these volcanic (former D1 zone) in thearea, whereas the Upper productsoccur at depths within Carboniferous sedi- Miller's Dale Lava was erupted at or very close to the ments and that only a minor fraction are exposed at Brigantian-Asbianstage boundary (D21D1 zones); the surface. some 70 m of limestone separate the two lava groups. Poor exposure and lack of continuous outcrop make Throughoutthe Carboniferous Period the British theregional correlation of basalt unitsdifficult. Isles lay in tropical latitudes between CL10"N (Turner Despitethis, however, Walters & Ineson(1981), in

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FIG.1. Distribution of Carboniferous lavas, intrusions and volcanic vents showing locations of analysed samples. Lavas: UMB, Upper Miller’s Dale; LMB, Lower Miller’s Dale; CD, Cave Dale; CRD, Cressbrook Dale; SWB, Shacklow Wood;CBB, Conksbury Bridge; LOB, Lathkill Lodge; LRB, Lower Matlock; WMB, Winstermoor; URB, Upper Matlock; R. Rowsley boreholes. Sills: PFS, Peak Forest; PS, Potluck; WSS, Water Swallows; TDS, Tideswell; BS, Bonsall; IS, Ible. Volcanic vents: SV, Speedwell; CH, Calton Hill; GM, Grangemill. The numbers refer to the stratigraphic sections shown in Fig. 3, and 10 = location of Rowsley borehole.

theirrecent stratigraphic synthesis using mine and productswere erupted from short-livedcentral vent borehole data as well as surface exposures, recognized volcanoes and that the volcanic rocks extend eastward 30 distinct lava and tuff horizons and identified that underthe Namurian cover. Fig. 3 is basedon there were two main eruptive centres at Matlock and stratigraphic sections by Walters & Ineson (1981) and Miller’s Dale.They also suggested that the volcanic showsthe relationships between the major extrusive

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Shallow Carbonate platforms (B only)

Configuration of S Pennine "sag' as indicated 4 by Westphalian 103ft isopachites (A only) FIG. 2.Palaeogeographical setting of theDerbyshire volcanism. (a) The block-basintopography of theBritish Dinantian, with an indication of the main volcanic areas. The configuration of the late Carboniferous sag, which was superimposed on the earlier structures, is after Leeder's fig. 5 (1982). (b) Detail of the northern edge of St. George'sLand during Dinantian times, showing the relationship of theDerbyshire volcanic centres to shallow carbonate platforms varyingly above or below sea-level.

4 EYAM BOREHOLE 5

FIG. 3. Stratigraphical positions of selected Carboniferous volcanic rocks in Derbyshire, constructed from sections inWalters & Ineson(1981). CHL indicatesthe approximate stratigraphic position of theCalton Hill lava. The sections are numbered as shown on Fig. 1 andare drawn relative to a horizontal line at the Asbian-Brigantian boundary.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/1/147/4887992/gsjgs.141.1.0147.pdf by guest on 28 September 2021 150 R. Macdonald et al. units.One feature deserves special mention: unlike albite,carbonate and epidote. Accessory minerals someprevious workers. we recognize the lavas at include Fe-Ti oxide and apatite set in a microcrystal- Calton Hill Quarry as a sequence separated from the linegroundmass of chlorite,carbonate and chalce- overlying Upper Miller‘s Dale Lava by some 15 m of dony. The lavas are frequently brecciated and veined limestone. by carbonate minerals. TheDerbyshire Carboniferous basalts occur in In contrast, the lavas erupted from the Calton Hill stratigraphic units that are usually between 3 and 30 m vent are distinctive in being petrographically fresh and thick. Normally, exposures are so small or overgrown in containingspinel-lherzolite and harzburgite xeno- that it is notpossible to identify whether such units liths (Tomkeieff 1928; Hamad 1963; Donaldson 1978). representsingle or multiple flows. Even in larger Thelavas contain phenocrysts of olivine (FO~(&~~), outcrops, it has not been possible to identify the single clinopyroxene (augite; WO~~E~~~FS,(,).abundant Fe-Ti or nlultiplenature of the Lower and Upper Miller’s oxidegrains and plagioclase (Anh4)subophitically Dalelavas. Walters & Ineson (1981) showedconclu- enclosed by augite with a higherFeiMg ratio sively from borehole evidence that many of the lava (Wo50En36Fs14), analciteandcalcite (Donaldson unitswere multiple but they followed tradition in 1978, p. 365). In addition to olivine phenocrysts, the referringto them as ‘Lava’. e.g. the Upper Miller’s lavacontains larger xenocrysts of olivine (FoX8)that Dale Lava and the Cressbrook Dale Lava; the terms are clearly derived from the xenoliths, but apparently areused in thesame sense in thispaper. Multiple no xenocrysts of pyroxene or spinel.The xenoliths flows can be seen at Calton Hill quarry where. in the consistmainly (64-85%) of partiallyserpentinized outcropthat has not been covered by rubbish, it is Mg-rich olivine (Fog1_92), 8-2496 orthopyroxene possible to identify five flow units. Here, the massive (WolEnglFsx)with minor (1-2%) clinopyroxene centres of each flow unit are of fresh alkali basalt in (chrome-diopside, Wo,,En,gFs4) and (4% spinel (all which only olivine has been partly altered. In contrast, data from Donaldson 1978). the rock-forming minerals of the rubbly and vesicular The sills are dominantly medium- to coarse-grained topsand bottoms of these flows havebeen totally olivine dolerites. which differ from the lavas in being replaced by secondary minerals. The relation between coarsergrained, having altered olivine phenocrysts, alteration and vesicularity (the more vesicular the rock ophitic intergrowths of clinopyroxene and plagioclase, themore extensive the alteration) suggests that the andan absence or rarity of vesicles andamygdales. mainagents of alterationwere percolating ground Petrographicdescriptions are given by Smith ef al. waters and hydrothermal solutions. (1967, pp. 263-4) for the Ible and Bonsall Sills and by Stevenson & Gaunt(1971, pp. 293-9) forthe Waterswallows, Peak Forest, Potluck, Mount Pleasant Petrography and mineralogy of andTideswell Dale Sills. These sills commonlyhave lavas and intrusions serpentinized olivine phenocrysts set in a sub-ophitic or ophitic intergrowth of generally fresh clinopyroxene Petrographicdescriptions of igneousrocks of (augite)andplagioclase (labradorite). Accessory Derbyshirehave been given by Bemrose(1894, mineralsinclude Fe-Ti oxide and apatite in a micro- 1907),Tomkeieff (1928). Smith er al. (1967),and crystalline matrix that contains plagioclase and secon- Stevenson & Gaunt(1971). This section is basedon dary chlorite. Accessory nepheline has been recorded theseaccounts, on microscopic studies of specimens fromthe Bonsall Sill (Smith et al. 1967. p. 264) and we collected and on electron microprobe analyses of accessory orthopyroxene. altered to chlorite, has been selected samples. reported in thePot Luck Sill (Stevenson & Gaunt Thelavas are usually highly altered,commonly 1971, p. 298). vesicular.fine-grained, olivine-phyric and aphyric To determine the magmatic affinities of the lavas, basaltscontaining amygdales of carbonate,chlorite. electronmicroprobe analyses (Table 1) havebeen chalcedony or albite. The primary igneous mineralogy determined for the cores of clinopyroxenes in repre- is generally completely replaced by secondary miner- sentative fresh samples selected from the range of lava als. Olivine is altered to combinations of serpentine, compositions.These are a Lower Miller’s Dale lava iddingsite. Fe-Ti oxide, chlorite and carbonate. Fresh (sampleD2, 7.7% normative ‘Q’ (‘Q’=normative clinopyroxeneoccurs in severallavas. although it is quartzplus the quartz component of normative generallyreplaced by chlorite.Altered pyroxene is hypersthene)). a LowerMatlock lava (sample D21, commonlyassumed to beclinopyroxene (augite) but 4.9%normative ‘Q‘) anda Calton Hill lava (sample orthopyroxene, altered to carbonate (calcite). chlorite, KG31, 1.1a/r normativenepheline). The Lower Mil- serpentine (bastite) and haematite. has been recorded ler’sDale lava hascoexisting salite (Wo38En46Fsl(, fromthe Upper Matlock Lava (Smith et al. 1967, p. -Wo3gEn42Fsz(~)magnesianpigeonite and 262).Plagioclase varies widelyin thedegree of (Wo8EnhIFsll).The Lower Matlock lava hasaugite preservation: whereas some lavas contain fresh calcic (Wo42En44Fsll)and the Calton Hill lava hassalite labradorite, itis commonly recrystallized and altered to (WolxEn4,,Fsl2).The chemical compositions of the

I 2 3 4 SiOz 54.20 52.52 52.12 49.50 TiOz 0.41 0.95 1.18 1.65 A1103 0.51 2.28 2.51 5.54 Crz03 0.00 0.58 0.43 0.57 Fe0 19.29 9.67 8.72 6.85 MnO 0.51 0.27 0.25 0.14 MgO 15.73 21.59 14.85 13.18 CaO 3.99 18.14 19.96 22.20 0.07 0.33 0.31 NaZO 0.33 0.07 0.55 100.57 100.20 100.33 100.18Total100.33 100.20 100.57 atomic %

61.2 46.0 43.6 40.0 43.6 Mg 46.0 61.2 0 I I 1 Fe 30.7 15.9 14.4 11.6 1 2 3 Ca 8.1 38.1 42.1 48.4 TiOp (%)

1. Pigeonite Lower Miller’s Dale Lava (D2) 2. Salite Lower Miller’s Dale Lava (D2) b 3. Augite Lower Matlock Lava (D21) 4. Salite Calton Hill Lava (KG31)

clinopyroxenesare compared in terms of SiOz and AlZ03(Fig. 4a) and tetrahedral AI (AI,) and TiOz s102 (Fig.4b; cf. LeBas 1962, figs 2 & 5 respectively). These diagrams confirm that the clinopyroxenes from each rock sample are distinctive, indicating a range of magmatic compositions. The clinopyroxenes from the Lower Miller’s .Daleand Lower Matlock rocks are relativelySi02-rich and Ti0,- and Alz03-poor and plot within the fields of ‘non-alkaline’ (specifically, in thiscase, tholeiitic) pyroxenes. The Lower Miller’s Dalepyroxene has more ‘tholeiitic’ characteristics, consistent with chemical features to be described later. The Calton Hill clinopyroxenes have higher contents of A1203 (>4%) and TiO, (>1.5%) and plot within the field of normalalkaline rocks, again consistent with the silica-undersaturated composition of the host FIG. 4. (a) Plot of AI, against TiOz and (b) Plot basalt. of SiOz againstAl2O3 for clinopyroxenes from lavas. In spite of thealteration suffered by theDerby- Open circles,UMB: closed circles.LMB; and open triangles. CH. Abbreviations a5 in Fig. 1. shirelavas, the overall petrological characteristics The broken lines separate three groups of clino- thereforeindicate that they form aseries ranging pyroxene composition: A, non-alkaline (tholeiitic. from‘tholeiitic’ types containing minor olivine, high alumina and calc-alkaline) lavas: B. alkalinc relictorthopyroxene and clinopyroxene, to alkaline lavas and C. peralkaline lavas (from Le Bas 1962). varietiescontaining more abundant olivine together with analcime.The existence of thisrange isfully confirmed by analyses of clinopyroxenes,since these Geochemistry rangefrom pigeonite and SiOz-rich, A1203- and Ti02-poor clinopyroxene in the most ‘tholeiitic’ repre- 60 rockswere analysed, mainly by standardX-ray sentatives (e.g. Lower Miller’s Dale) to the salites in fluorescence methods using U.S.G.S. standard rocks, the more alkaline lavas of Calton Hill. The sills also for major elements and Ba. Cr, Nb, Ni, Rb, Sr, Y and range between relatively ‘tholeiitic’ (e.g. the Pot Luck Zr. Six werealso analysed by instrumentalneutron Sill) andrelatively ‘alkaline’ (e.g. the Bonsall Sill) activation analysis for Hf, REE. Ta and Th, follhwing dolerites,but the overall compositional range of the theprocedure of Potts et al. (1981). Thespecimens sills is smallerthan that represented by thelavas. represent the various stratigraphic lava sequencesas

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TABLE2. Representative analyses of Carboniferous lavas and sills in Derbyshire

L. Miller's U. Miller's Culton Hill Lava U. Matlock U. Matlock Conkshwylble Tideswell Dale Lava Dale LavaDale Lava Lava Bridge Lava Sill Dale Sill D2 KGSSU KG31KG34KG33 D17 D20 SKI43 D23 KGSIH

~ ~ SiOz 51.5 47.6 46.0 47.8 44.8 48.7 48.9 47.4 49.9 51.2 TiOl 1.70 1.92 2.28 2.22 2.42 1.92 1.93 2.25 1.77 1.98 A1203 15.26 13.82 13.27 14.58 13.53 16.12 12.74 14.46 16.01 15.36 FezOi 3.89 2.53 2.83 3.22 5 .23 3.55 5.27 4.65 4.34 4.14 Fe0 6.86 8.73 9.12 7.14 6.93 4.85 3.61 7.34 5.57 6.24 MnO 0.16 0.16 0.17 0.09 0.37 0.06 0.09 0.12 0.09 0.12 MgO 6.47 9.69 10.96 9.88 9.82 4.84 4.79 8.65 7.53 6.47 CaO 8.92 7.10 9.37 9.14 10.14 9.04 8.63 8.36 8.83 8.59 NaZO 2.93 2.80 2.55 2.21 2.02 2.74 2.04 2.29 2.71 2.82 KZ0 0. 62 0.78 1.12 0.80 0.92 1.44 1.92 0.94 0.54 0.73 P205 0.17 0.25 0.49 0.31 0.44 0.30 0.33 0.34 0.22 0.27 H,O' 0.92 3.60 2.46 1.83 2.74 2.62 3.34 2.91 1.81 1.46 CO2 - 0.11 0.05 0.10 0.06 3.33 5.62 0.20 - - Total 99.40 99.09 100.67 99.32 99.42 99.51 99.21 99.91 99.32 99.38 Ba 140 209 574 239 387 288 455 249 129 214 Cr 265 - - - - 222 357 - 398 - Nb 11 24 42 29 42 28 33 33 16 18 Ni 89 - _- - 87 107 - 276 - Rb 13 18 22 16 16 20 29 12 10 14 Sr 274 238 522 337 495 442 32 1 533 266 29 1 Y 25 23 23 22 21 22 22 22 22 26 Zr 99 117 178 140 180 129 133 151 100 121

follows: Lower Miller's Dale Lava 8, Cave Dale Lava Representative analyses aregiven in Table 2 and the 1. Upper Miller's DaleLava 9, Calton Hill Lava 7, INAA data in Table 3. The complete body of data and Cressbrook Mill Lava 1, ShacklowWood Lava 1, details of analytical procedures are available from the CressbrookDale Lava 1, LowerMatlock Lava 4, authors on request.These data are also available in UpperMatlock Lava 4, ConksburyBridge Lava 2, machine-readableform from the recently established with a further two specimens from an unnamed lavain UK-IGBA data base housed in the National Geoche- a borehole near Rowsley. One sample came from each mica1 Data Bank of the Institute of Geological Scien- of the Peak Forest, Waterswallows, Pot Luck, Bonsall ces, London. andIble Sills and 15 fromasystematic collection through the Tideswell Dale Sill. Problems of interpretation of chemicaldata TABLE3. Additionaltrace element data for selected lavas Allthe analysed samples show varying degrees of Lower Miller's Upper Miller's Calton Hill alteration,from those largely retaining original Dalt, Lava Dale Lava LULYl igneousmineralogy to others inwhich the original mineralsare completely replaced. Secondary hydra- KG14 KG25KG40KG55L KG34 KG33 tion is ubiquitous, resulting in variable replacement of La 7.7 9.3 8.3 18.8 18.8 29.9 theferromagnesian minerals by serpentine-chlorite Ce 16.3 17.8 19.2 34.5 39.8 59.6 assemblagesand alteration of the plagioclase to Nd 11.0 11.9 13.1 21.1 23.8 32.8 sericite f clay minerals.Many samples show partial Srn 3.1 3.1 3 .? 5.1 5.3 6.6 replacementand sometimes veining by carbonate f Eu 1.22 1.17 1.42 1.63 1.87 2.3 silica minerals,and albitization of plagioclase is a Gd 3.0 4.0 4.4 4.9 5.3 7.4 commonfeature. An indication of thetypes of Tb 0.56 0.62 0.74 0.78 0.90 1.01 alteration in theanalysed rocks is givenwith the Tm 0.22 0.25 0.32 0.32 0.39 0.32 Yb 1.25 1.54 1.77 1.70 1.96 1.89 chemical data. Lu 0.18 0.23 0.25 0.25 0.27 0.25 During secondary alteration, mobilityof Si, Ca, Mg. Na, K, Ba. Rh and Sr is likely. Such mobility may be Hi' 2.47 2.82 2.40 3.06 3.33 4.40 evaluated by plottingdata for areputedly stable Ta 0.64 0.77 0.57 1.42 1.68 2.82 Th 1.24 1.56 1.15 2.11 2.35 3.94 incompatibletrace element, such as Zr, against anotherstable element and an element of suspected

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/1/147/4887992/gsjgs.141.1.0147.pdf by guest on 28 September 2021 The Derbyshire Carboniferous basalts 153 section is concentrated on those elements, Zr, Nb, Y, a Ti and P, considered to be stable during alteration of M 4c basalticrocks (e.g. Pearce & Cann 1973; Floyd & Winchester 1975; Pearce & Norry 1979) and on those rocksdeemed to be 'fresh'. In the following discus- n 3c sion,the term 'fresh' is appliedto 29 specimens (14 fromlavas, 15 from sills) wherethe pyroxene and c/n 2 plagioclaseare almost wholly fresh,although the 20 olivine may be partly or totally serpentinized.

Magmatic affinities 10 A Zr-Nbplot (Fig. 5) permitsthree important 50C observations: (i)The sills andsome of theextrusive rocks have comparablecompositions and, despite the age n 400 difference, may be discussed together. 5n Y (ii)The sills andextrusive rocks show asignificant range of incompatible trace element abundances 300 (morethan a factor of fourfor some stable elements). (iii) Individual volcanic sequences, such as the Upper 200 Miller's Dale and Calton Hill Lavas, may cover up to about half the compositional spectrum. The 60 geochemistrythus provides additional evidence thatat least the main lava sequences consist of 0 severalchemically distinct flows. TheTideswell 40 Dale Sill shows internal chemical differentiation. Such inter- and intra-sequence variation, especially . in incompatible trace element abundances, may result n . of 30 from several factors: varying degrees partial melting 0 of thesource rocks, heterogeneity of thesource, n5 0 Yn differing degrees of fractional crystallization (perhaps A A a 20 A involving different mineral assemblages) of a parental, or series of parental, magma(s) and crustal contamina- a tion of mantle-derived melts. It is difficult to assess the 0 10 degreeto which theDerbyshire rocks have been affected by crustal contamination, but positive correlation between the degree of silica-undersaturation and 0 such lithophile elements as K, Ba and Rb suggests that 100 160 180 suchcontamination, if itoccurred, must have been slight. We first use the geochemical and mineralogical data FIG. 5. Plot of Nb, Sr andRb against Zr for to establish the magmatic affinities of the lavas. Here, analysedlavas and sills. Symbolsused areas four lines of evidence are particularly relevant: follows: Closed circles, LMB; open circles. UMB; (1) C.I.P.W. norms have been calculated for the fresh opentriangles, CH; closed squares,LRB; open samplesonly, on a volatile-free basis andwith squares. URB; open diamonds, other lavas; Tides- Fe203arbitrarily set at 1.5 wt%. The range is from wellsill = +, other sills = X. Abbreviations of lavas as in Fig. 1. q- tone-normative, but the majority are 01 + hy-normative. (2) Thestable incompatible elements Nb, Y and Zr mobility. Graphs of Zr against Nb, Sr and Rb (Fig. 5) areparticularly useful in assessing magmatic indicatethat Nb was stable and that Rb was affinity in that they are assumed to be unaffected secondarilymobilized in the Derbyshire rocks, as by alteration processes. Zr/Nb ratio decreases and indicated by the varying degrees of scatter of the data. ZrlYand NblY ratios increase with increasing Althoughthe scatter for Sr suggests some mobiliza- alkalinity of basalts(e.g. Pearce & Cann 1973; tion,it was not sufficiently intenseto obscure the Floyd & Winchester 1975; Pearce & Norry 1979). nositive correlation with Zr. Further discussion in this In Fig. 6. theZriY ratio is dotted againstthe

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I I 1 I I 1

1 I I 1 I I 2 4 6 8 10 12 I ZrINb .t FIG. 8. Plot of YiNb against ZriNb. Symbols, as in l I I l I Fig. 5, indicate the normative characteristics calcu- 4 6 a 0 2 10 lated for fresh rocks only. Fields are for q + hy and Ne Q+Q in Hy 01 + hy normativerocks. The generalized ranges (fulllines) were then used todetermine the FIG.6. Plot, for fresh samples only, of ZriY against magmaticaffinities of alteredsamples; those in normatwe nepheline (Ne) or normative quartz (Q) areas of overlap assigned on overall characteristics plusquartz in normativehypersthene (Hy). Sym- of their lava sequence. bds as in Fig. 5. variationin the LREE in comparisonwith the degree of silica-undersaturation(expressed as heavy REE (HREE). The CeN/YbN ratioranges normativenepheline or normative quartz plus from 2.9-8.0, with the Lower and Upper Miller’s quartzin normative hypersthene, termed ‘Q’). Dale showing lower REE contents and significant- The good negative correlation of ZriY with ‘Q’ is ly variedCeN/YbN (2.9-5.2) incomparison with takento confirm that the range in magmatic theCalton Hill lavas (CeNiYbN = 5.2and 8.0). affinitiessuggested by thenorms is real.Similar Notingthe higher CeNiYbN ratios of alkaline in conclusions are reached by using ZriNb and YiNb comparison with tholeiitic lavas (e.g. Kay & Gast ratios instead of ZrN. 1973) andthe relative stability of theREE to (3) Thereare significant, negative correlations be- hydrothermalalteration, these data confirm that tweenTi, P, Zr,Hf, Nb, Ta, Th, the LREE thelavas form a spectrumbetween relatively abundancesand ‘Q’ in all samplesand between tholeiitic and alkaline types. those elements and Ba and Rb in fresh samples. A plot of ZriNbagainst YiNb (Fig. 8) allows the The range from ne- to q-normative compositions determination of the affinities of alteredspecimens must therefore be related to magmatic variations sinceboth ratios correlate with degree of silica- rather than to hydrothermal alteration. undersaturation. The range of normative characteris- (4) Six lavas covering the range of lava compositions tics of eachsequence is shown in Table 4, and have been analysed for REE (Fig. 7). All samples arelight-REE (LREE) enriched with chondrite- TABLE4: Magmatic afjnities of Derbyshire lavas and normalizedCe(CeN) = 19-70, andYbN = 6-8. sills. TheREE patterns are characterized by large Unit characteristics Normative ConksburyBridge Lava 01 + hy : somenear CPSU* 0 KG14 CPSU*near someMatlockUpperLava KG25 iLMB CPSU*near someMatlockLowerLava UnnamedLava 9 + hY Shacklow Wood Lava q + hY CressbrookdaleLava 01 + hy : close toCPSU UpperMiller’s Dale Lava 01 + hy, q + hy Cressbrook Mill Lava q + hy or 01 + hy Calton Hill Lava ol + ne, 01 + hy LowerMiller’s Dale Lava q + hy Cave Dale Lava 9LavaCaveDale + hY Ible, Tideswell Dale and WaterswallowsSills9 + hY 1, , IIII III Bonsall, Pot Luck and Peak La Ce Nd SrnEuGd Tb Trn Yb Lu Forest SillsForest 01 + hy FIG. 7. Chondrite-normalized REE plot for lavas. Abbreviations of lavas as in Fig. 1. *cPsU = critical plane of Silica-undersaturation.

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4 2 U ea V X W v vi ._c L V C 0 L V .m > m 1

1 '. \, .' d

I I I I Ill 1 I 1 II I I II I I Ba Rb Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm FIG.9. Chondrite-normalized (except for Rb, K and P). incompatible element patterns for: Calton Hill lava (KG 33); UpperMiller's Dale lava (KG 40); olivinetholeiite (BHVO-l), Kilauea.Hawaii (Thompson 1982, fig. 15); median composition at 7.5% MgO, late Carboniferous quartz dolerite suite, Scotland (MVQD; Macdonald et al. 1981, table 7). Normalizing factors from Thompson (1982, p. 105).

indicatesthat the bulk of theDerbyshire volcanism basaltsstraddle CPSUthe (critical plane of was tholeiitic, in the sense of containing normative hy. silica-undersaturation) in thenatural basalt system, This is confirmed by trace element abundances (Fig. which is a thermal divide only at low pressures. 9); the Derbyshire rocks are compositionally similar to Itmay be noted that the Derbyshire basalts, such typically tholeiitic suites such as the late Carbo- consideredeither in toto or asindividual sequences niferousquartz dolerites of theMidland Valley and such as the Calton Hill lavas, do not show decreasing Whin Sill(s). MgO, Ni orCr concentrations along the down- temperature direction of the 9 kbar cotectic. They do Petrogenesis notrepresent asimple fractionation series at this pressure.Chemical variation within therocks is A widely accepted criterion (e.g. Frey et al. 1978) is relatedeither to higher-pressure fractionation events that only basalts with Mg number >66-67 are likely to or to partial melting conditions. beprimary basalts. assuming mantlea olivine of The REE data may be used to commenton relevant composition Fo >87. On this basis, only certain of the models. As notedearlier, the REE patternsare Calton Hill lavas would be possible primary magmas. characterized by largevariation in the LREE in Following the interpretation of Thompson (1982) for comparison with the HREE. Such variation is incom- BritishTertiary Volcanic Province lavas, it would patiblewith low-pressure fractional crystallization of seem that the Derbyshire rocks plotting within field A olivineand plagioclase, which generates lavas with (Fig. 10) have compositions related to cotectic equilib- sub-parallel REE patterns with different abundances. ria at about 9 kbar, whereas those in field B reached Extensivefractional crystallization of clinopyroxene their final compositions at lower pressures, within the might cause LREE enrichment but this seems unlikely, uppercrust. Undoubtedly, this iswhy some of the asclinopyroxene has DF&, = 10 andthe lavaswith

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/1/147/4887992/gsjgs.141.1.0147.pdf by guest on 28 September 2021 156 R. Macdonald et al. bearing assemblages such as those involvedin eclogitefractionation (e.g. O’Hara 1968). However, as em ha sized by Shimizu & Arculus (1975). the high DGtg- in garnetmeans that different degrees of such high pressure fractional crystallization would be unlikely to generate almost constant HREE concentrations. Also, the high values of DE and D:: wouldgenerate an inversecorrelation between CeNiYbN andCr as a result of eclogitefractionation. Such a correlation is notobserved. However, the presence of garnet through varying degreesof partial melting is consistent with constant HREE abundances (Shimizu & Arculus V V 1975) and this model is preferred for the Derbyshire M W, XI H” lavas.This is consistentwith the conclusion of FIG. 10. Compositions of freshDerbyshire basalts Donaldson (1978) that the source of theCalton Hill plotted in terms of normative di, 01. hy, ne and q. xenolithswas ‘probably close tothe spineligarnet Symbols as in Fig. 5. The cotectic curves at 1 atm. transition which would have been crossed by either a and 9 khar(anhydrous) represent the equilih- smallpressure increase or a small temperaturede- ria olivine + plagioclase + clinopyroxene + natural crease’ (Donaldson 1978, p. 371). basaltic liquid (Thompson 1982. fig. 2). Arrows It appears, therefore, that the Derbyshire rocks had markdirection of falling temperature. Field A is a complicated genesis, viz, variable degrees of partial slightly enlarged over that which encloses Tertiary melting of thesource rocks generated arange of Plateau-type mafic rocksfrom Skye and Mull. B silica-undersaturation in theparental melts and vari- enclosesrepresentative Non-Porphyritic Central magmatype maficrocks fromthroughout the ableamounts of subsequentmoderate-pressure frac- BritishTertiary Volcanic Province (from data tional crystallization produced the rangesin Cr, Ni and plotted in Thompson (1982, fig. 2)). MgO contents. Thisis consistent with the observations of Donaldson(1978, p. 372). who noted the lack of evidence for differentiationof the Calton Hill magmas, the highest CeN/YbN do not show any depletionof Cr atleast at depths <45 km. It follows that in those in comparison with lavas with lower CeN/YbN. Further, sequences which showa substantial range of silica- theapparent rarity of clinopyroxenephenocrysts undersaturation,such as the Upper Miller’s Dale andtextural evidence for near-simultaneous crystal- Lava, the eruptive rocks represent equilibration over a lization of plagioclaseand clinopyroxene argues range of pressures. Unfortunately, the scarcity of data againstextensive fractionation of clinopyroxene. We forfresh rocks makes it impossibleto analyze the therefore conclude that the REE data do not reflect Derbyshirevolcanics in terms of parent-daughter low-pressurefractional crystallization of mineral relationships. phases present within the basalts. It is possible to use the incompatible trace element Thecharacteristic large variation of LREE in data to comment on the heterogeneity of the mantle relation to HREE has been observed in other basaltic sources of theDerbyshire volcanic rocks. Particular volcanic provinces and has been analysed by Shimizu use is made of Zr and Nb, which are insensitive to the & Arculus (1975) for a suite of basanitoids and alkali degree of partialmelting or fractional crystallization basaltsfrom Grenada, which havesimilar REE because of low partitioncoefficients for likelysolid characteristicsto the Derbyshire basalts. The small phases (Pearce & Norry 1979). variation in HREE in relation to LREE indicates the ZriNbratios (Fig. 8) varyoverall from 3.6 to 9. presence of amineral withhigh HREE partition Even in the rocks of one sequence there is consider- coefficients such as garnet. Assuming that the mantle ablevariation, e.g. 4.6 to 8.9 in the Upper Miller’s sourcemay have 2-3 timeschondritic REEabund- DaleLava. These ranges are quite outside those ances (e.g. Kay & Gast 1973; Sun & Hanson 1975) the possiblefrom varying degrees of partialmelting or formation of lavas with HREE (e.g. YbN) concentra- differing fractionation histories, if the mineral-liquid tions of 6-8 X chondritic implies formation by partial distribution coefficients compiled by Pearce & Norry melting or fractional crystallization involving garnet or (1979) for mantle rocks and basalt phenocryst Dhases someother mineral(s) with high HREE partition areat all representative.This indicates that the coefficients. Derbyshire volcanics were derived from source mantle Intheir analysis, Shimizu & Arculus (1975)ex- rocks which were heterogeneous as regards Zr and Nb cludedlow-pressure fractional crystallization (cf. distribution. The significance is that each lava or sill. above) and investigated high-pressure fractional crys- even within a single stratigraphic sequence, represents tallizationand partial melting models. The high- a discrete melting event. pressurefractionation might have involved garnet- It may also be noted that within seqdences where a

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stratigraphy can be established, e.g. Calton Hill, there Excludingthe far-fetched concept of membrane is no tendency for composition to vary systematically tectonics (Turcotte & Oxburgh 1973) and the unlikely withtime, nor can any simple relationship between relationship to a fossil subduction zone in this area(cf. compositionaland geographical location be estab- Elston1982), two principal mechanisms have been lished,e.g. in the Upper Miller’s DaleLava. These invokedto explain within-plate Cenozoic volcanism. lines of evidence are consistent with the eruption of The first (Gass et al. 1978; Pollack et al. 1981) theDerbyshire extrusive rocks from relatively short- proposesthat a slow-moving andior thermally sus- livedsmall-sized vents. This in turnsuggests that ceptiblelithospheric plate is partiallyfused over a high-levelmagma chambers of substantial size or mantle thermal high. Melts produced in thisway are longevity were never developed during Carboniferous alkalic and commonly use faults as lines of easy egress volcanism in Derbyshire. Rather, the melts may have tothe surface. However, they are usually also passed through the crust in a system of small conduits, associatedwith areas of markedtopographic uplift, dykesand sills whichwere largely unconnected reflecting the production by partial melting of zones of between the vents erupting lavas within (and between) anomalousmantle within the thicker continental each lava sequence (cf. Thompson et al. 1972 for Skye lithosphere. In the case of the Derbyshire magmatism, volcanism and Moore et al. 1976 for the San Francisco itis relevantto note that there is no evidencefor volcanic field, Arizona). major uplift before,during or after the volcanic activity. Discussion Thesecond mechanism is relatedtotensional stressesproduced by theirregular impaction of two Duringthe period of Carboniferousvolcanism the continentalplates. Upton (1982) hassuggested, with BritishIsles laywell withinthe Laurasian continent particularreference tothe Midland Valley and whosenorthern, western and eastern margins were NorthumberlandBasins, that stresses produced by many hundreds of kilometres away whilst its southern continentalimpingement could have produced ten- boundary lay justto the S of theBritish Isles. sional stresses that reactivated linesof weakness in the Although during the late Devonian-early Carbonifer- underlyingbasement, thereby controlling the site of ousthe ocean separating Laurasia from morea volcanicactivity and initiating trough formation. In southerly continent was closing (Dewey 1982, fig. 38), Derbyshire, however, the volcanic activity does not lie Upton(1982, p. 256) maintainedthat this volcanism alonga Caledonide trend. Indeed, Wilkinson (1967) was‘thoroughly intraplate in character’and ‘not has noted that intrusive bodies and volcanic vents tend influenceddirectly by anysubduction processes’. to coincide with WNW-ESE anticlinal axes. The lack Upton’s conclusions are confirmed by the geochemical of evidencethat the Derbyshire area was a zone of results of this study (Fig. 11). major lithospheric weakness during the Carboniferous (Leeder 1982) apparentlyrequires atleast some modifications to Upton’s (1982) proposals. 1 Noting the seismic evidence for crustal thinning of 1c some 6 km beneath the central Carboniferous basin, 5 Leeder (1982) hasattempted to apply McKenzie’s ZrIY (1978) model of lithospheric stretching to Carbonifer- ous basin formation in continentala lithosphere 1 rendered inhomogeneous by the presence of granitic plutons in theupper crust (cf. aspects of Francis’s (1978) model). The driving force for the stretching was takento be plate motions and stresses caused by ZrJY subductiontheto S of Britain.Leeder (1982) envisaged that the lithosphere N of St. George’s Land 5 wassubjected to stretching during the Dinantian, causing regional tension and rift formation in areas of less buoyant (i.e. granite-free) crust. basalts The statusof the Derbyshire area during this stageis 0 uncertain; the northern margin of St. George’s Land I MORE was not a fault, though it may have been the flexured 100 Zr (ppm) margin of ahalf-graben (Leeder 1982, fig. 7B). It is perhaps no coincidence that the Derbyshire volcanism FIG. 11. ZriY-Zrplot (Pearce & Norry 1979) indicatingthe within-plate character of theDer- wasvolumetrically much smaller than that in the byshirebasalts. Symbols as in Fig. 5. Ininset, Midland Valley of Scotland and the Northumberland WPB, within platebasalts; MORB, mid-ocean trough, where reactivated lines of Caledonianweak- ridge basalts; and IAB, island arc basalts. ness allowed melts easier access to the surface.

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Leeder (1982) further suggested that there was an the mantle was sufficiently fertile for minor quantities importantchange in tectonic behaviour in northern of magma to be generated in isolated pockets and that Britainduring Namurian times. The fault-bounded some of these came to the surface along fractures in rifting of theDinantian was replaced by a general the attenuated crust and in later Carboniferous times crustalsagging, centred in N Lancashire,caused by along the flanks of the S Pennine sag. asthenospheric cooling. Derbyshire was on the south- Walkden’s (1977) correlation of minorphases of ernlimb of thissag and the renewed Namurian isostaticuplift with volcanism is interesting, in that volcanism (expressed as sills) might be related to uplift Gasse (1975) andFaure (1976) indicatedsimilar of a rejuvenated St. George’s Land, for which there is temporalcorrelations for the 0-30 Ma Cenozoic goodsedimentary evidence. It may be noted that, volcanics of the Afar depression. It appears that the whereasa different style of magmaticemplacement energy in thesevolcanogenic lithothermal systems accompaniedthis transition from rift to sag. the produces either uplift or volcanism. When volcanism change is not reflected in the chemistry of the volcanic occurs, uplift stops.The 12-15 cycles of uplift rocks. Magmatic processes during both episodes were identifiedby Walkden (1977) arenot all associated apparently identical. with volcanic products but, as only minor quantities of We conclude, therefore, that the Derbyshire Carbo- basaltwere erupted and the amount of uplift was niferousvolcanism. unlike the Midland Valley of limited in bothvertical and surface extent, it seems Scotlandand Northumberland Trough occurrences, likely thatthe mantle processes that triggered these wasnot associated with any major lithospheric effectswere also minor and restricted as is also fracturing.The lack of associatedisostatic uplift indicated by thegeochemically-based magmatic precludes the production of extensive partial melting mechanisms we describe. in theunderlying upper mantle. The S Pennine crustal sag of Leeder (1982), associated as it is with underlyingcrustal attenuation, could well havere- ACKNOWLEDGMENTS.We are grateful to Dr P. J. Potts, Dr sultedfrom lithospheric stretching as invoked by 0.Williams Thorpe. A. Tindle, J. Bowman, K. Waterhouse McKenzie (1978). If these inferences are corrcct, then and L. Griffithsfor help with samplepreparation and it seems most probable that no major zone of partial analysis, to J. Taylor & H. Boxall for drawing the diagrams meltingunderlay Carboniferous Derbyshire but that and to P. A. Budd for typing the manuscript.

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Received 3 February 1983; revised typescript received 8 June 1Y83

R. MACDONALD& K. N. GASS, Department of EnvironmentalSciences, University of Lancaster, Bailrigg. Lancabter LA1 IYQ. R. S. THORPE,& I. G.GAS. Department of Earth Sciences, TheOpen University, Walton Hall. Milton Keynes, MK7 6AA.

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