Journal of rhe Geological Society, London, Vol. 144, 1987, pp. 531-542, 10 figs, 2 tables. Printed in Northern Ireland

Tectonic environment of the Devonian Gramscatho basin, south : framework mode and geochemical evidence from turbiditic sandstones

P. A. FLOYD & B. E. LEVERIDGE' Department of Geology, University of Keele, Staffordshire ST5 5BG, UK 1 British Geological Survey, St Just, 30 Pennsylvania Road, Exeter EX4 6BX, UK

Abstra& ThePortscatho Formation, within the allochthonous unit of the Middleand Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded deposited by southerly-derivedturbidity currents into the Gramscatho basin of southCornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantlyacidic composition, similar to averageupper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. Thepresence of an arc to the south of Cornwall during the Devonianimplies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking offlysch-ophiolite nappes.

The DevonianGramscatho Beds in south Cornwall were nappes of flysch and olistostrome, ophiolite and continental defined and described in some detail by Hendricks (1937), basement are related to theclosure of anoceanic basin who referred to the thicksuccession of alternating situated to the southof Cornwall. greywacke sandstones and slates as a flysch facies sequence. Contemporaryattention was, however, focused onthe superincumbentrocks as indicators of regional tectonics, Sandstone discrimination andthey were interpreted wholly, or in part,as tectonic The relationship of sandstone composition to tectonic mBlange beneath thrust nappe (Flett 1933) or the environment, namely thatquartz-rich rocks characterize Lizard-Dodman-Startnappe (Hendriks 1939). Onlylater continentalmargins, quartz-poor rocks are derived from were the character and associations of flysch sequences fully island arcs and intermediate quartz contents are associated described(e.g. Kuenan & Migliorini1950; Crowell 1957) with active continental margins and orogenic belts, had been and their close relationship with tectonically active regimes recognizedby the mid-seventies (e.g. Schwab1975). The detailed (see Aubouin 1965). Following recognition that the concept was developedand aclose correlationbetween Cornish mClangewas sedimentary(Lambert 1965) the sandstoneframework mode proportions and plate setting Gramscatho Beds and the mClange breccias were interpret- was demonstrated, particularlyby Dickinson & Suczek ed as a pre-tectonic flysch/wild-flysch sequence(Dearman (1979) and Ingersoll & Suczek(1979), using standard and et al. 1969; Dearman 1971). An earlyplate model by discriminatory ternaryplots of framework grains. The Mitchell (1974) proposed that the Gramscatho breccias and general validity of thesediagrams has been confirmedby Lizard Complex constituted a subduction association related subsequent work, but so-called error populations have been to closure of a VariscanRheic ocean and subsequent recognizedby Mack (1984). Thesepopulations represent continental collision. sandstonesdeposited in the transitionbetween tectonic It has since beenestablished that a major pari of the regimes, thosefound in settings unrepresented on the Lizard Complex is an ophiolite segment (Strong et al. 1975; diagramsand sediments enriched in quartz as a result of Kirby 1979; Hoyd 1984) of probable Silurian/Devonian age weathering on depositionalreworking. Work on modern (Halliday & Mitchell 1977; Davies 1984) and the presence of deep-seasands has shownsimilara close correlation a RheicOcean south of Cornwallhas been indicated by betweencomposition and environment (Maynard et al. distribution of faunas(Cocks & Fortey 1982) and 1982), although the figured compositional fields may be less palaeomagnetic evidence (Tarling 1979; Scotese et al. 1979). distinct than those proposed for ancient sandstones. Nevertheless,plate models have tended to emphasize the It has been suggested that the bulk chemical composition intracratonicnature of theGramscatho depositional of turbiditic sandstones, as well as finer-grained deep-water environment, be it in a back-arc basin related to northerly argillites, can be used to designate environmental features subduction in S Brittany (Leeder 1982; Floyd 1982, 1984) or (Bhatia 1983,1985). Abundances of rareearth elements an extensional basin generatedin a strike-slip/transfortn (REE)and other stable elements (Ti, Zr, Hf, Y, Sc, Nb, regime (Badham 1982; Sanderson 1984; Barnes & Andrews Ga, Th, U), or their ratios, are particularly promising in this 1986). However, Holder & Leveridge (1986) have proposed contextand have been used to distinguish the tectonic a model for south Cornwall involving southward subduction setting of sedimentary basins in Australia (Bhatia & Taylor during the Devonian.Sedimentation of theGramscatho 1981; Bhatia 1985). The above elements have intermediate flysch and concomitant northward migration and stacking of ionic potential and low ocean residence times (Henderson

531

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1982; Mason & Moore 1982) and as they are readily thickness up to 2 m, being largely classic turbidites (Walker incorporated into sediments they also provide an indication 1979). Bedding,where traceable, is planeparallel, with of the source composition (McLennan et al. 1980; Taylor & scour grove and flute moulds apparent on inverted bases. McLennan 1981, 1985). Internal structures are referrable to the Bouma(1962) model Thepetrographic character of37 GramscathoGroup butthe simple pattern may be disturbed by delayed or sandstonesamples from the allochthonous Portscatho multiple grading, the presence of a coarse traction carpet or Formation has been investigated during this study whereas wavy laminatedbedding whichin wispy siltstone is 44 sampleshave been analysed geochemically. Bulk interspersed with sandstone.Towards the top of the geochemical analysis was determined by XRF spectrometry formation in Gerrans Bay grain-flow sandstones(Stauffer (on an ARL 8420 Quantometer at University of Keele) for 1967) are also common. major and trace elements (method modified after Norrish & Theformation contains fining-up, coarsening-up and Hutton 1969) anda selected subset of20 sampleswere mudstonesubfacies, although there is anoverall upward anslysedfor total rare earthelements (REE) (at Kings coarsening and thickening apparent. It typifies deposition in College, London; Walsh et al. 1981) and Hf, Ta, Th, U and adeep-water fan environment (see Rupke 1977; Walker CS (atUniversities Research Reactor, Risley; Duffield & 1978)with ageneral progradation from outer to mid-fan Gilmore 1979). Modal compositions of samples from the ‘A’ regimes through the formation. The deep-water character is divisions of graded turbidite flows have been determined by furtherindicated by thedark grey mudstonebackground a standard 500 point count of thin-sections. The sandstones sediment,containing sapropelic organic residues through- are largely greywackes, with anaverage of 26% clastic out,the considerable thickness of the succession and the matrix. Apparent matrix proportion is not a constraint on lack of reworking or shallow water depositional structures. thedetermination of frameworkmodal compositions, The PendowerFormation is ahemipelagic sequence because fine-grained lithic grains and even altered feldspars (Holder & Leveridge 1986) of grey, green, brown and black are still recognizable in sandstones with up to 35% matrix. slates with interbeddedautochthonous radiolarian cherts, However, those sandstones with a high proportion of matrix turbiditicpelagic limestones, and coarse lithic greywacke phyllosilicate,white micas andchlorite, do have a strong sandstoneforming sparse, thin, graded beds, lenses and dependent tectonic fabric with recrystallization overprinting small channel fills. The succeeding CarneFormation is a andobscuring grain boundaries, and have been omitted sequence of turbiditesandstone, channel-fill sandstone, from the petrographic study. Also omitted are a few samples slumpfolded and phacoidal sandstone, interbedded with with a secondary carbonate replacement overprint. interlaminatedmudstone, siltstone and sandstone.It is Thepresent investigationattempts to combineboth interpreted (Holder & Leveridge 1986) as an association of petrographic and geochemical data for sandstones from an middle to upper fan distributary channel and interchannel allochthonous unit of the Gramscatho Group as an aid to deposits(Mutti & Ricci-Lucchi 1972; Rupke 1977), slope theidentification of thetectonic environment in south deposits and normal proximal turbidites (Walker 1978). Cornwallduring theDevonian. A representativeselection of samples spanning the full stratigraphic sequence are listed in Tables l (frameworkmodes) and 2(corresponding Sandstone petrography and framework modes chemical analyses). ThePortscatho sandstones are greywackes comprising framework grains set in a microcrystalline matrix, which is commonly quartzo-feldspathic,but may bepredominantly Gramscatho Group phyllosilicate.Framework grains are unsorted and within TheGramscatho Group, as redefined by Holder & sectionsthere is arandom grain-size distribution, with Leveridge (1986), is present in both parautochthonous and maximum grain-size exceptionally in the coarse-sand range allochthonousregions of southCornwall (Fig. 1). On the andgenerally in the medium- to fine-sandrange. Overall NW side of the peninsula the Group is represented solely by frameworkgrain sizes are considerably less thanthe thePorthtowen Formation comprising largely grey and maximum, being in the fine-sand range. The grainsare grey-green slates with thin beds of greywacke sandstone and angular subroundedto anda minor porportion of siltstone.It is in sequencebetween the Meadfoot Group, monocrystalline grains are modified euhedral crystals. which in its northern outcrop is Lower Devonian (House & The origin of the greywacke sandstonematrix is Selwood 1964),and the Mylor SlateFormation whichis generally obscure. Many authors regard it to be a function largely UpperDevonian (Turner et al. 1979). The of burialdiagenesis (e.g. Galloway 1974), but there is no Portscatho, Pendower and Came Formations constitute the directevidence that the differentmatrix types of the Group within the Carrick Nappe to the SE. This succession Portscatho sandstones are the productof selective alteration hashitherto been dated largely on the basis of Eifelian of labilegrains (see Cummins 1962; Buller & McManus conodontswithin limestones of thePendower Formation 1973). Syntectonicrecrystallization of all fine-grained (Veryan Limestone, Sadler 1973), but the reported presence material and the formation of preferred orientation fabrics of Frasnian palynomorphs (Le Gall et al. 1985) has raised in microcrystallinematrix has inevitablyobscured original the possibility that the conodonts may be reworked. textures. The principal formation of the Group is the Portscatho The frameworkmodes of thePortscatho Formation Formation with an estimated structural thickness of 4.5 km sandstones(Table comprise1) approximately equal in Roseland. The samples for this studywere collected from proportions of quartz (Q), feldspar (F) and lithic (L) grains. the Mevagissey Bay coastal section, where some 3.5 kmof Monocrystallinequartz grains (am) are generallyclear the Formation’sexposed (Fig. l).It consistslargely of without vacuole trains, and show gentle undulose extinction. alternating grey andgreen-grey sandstone flags anddark Polycrystalline quartz (Qp) is eitherchert, vein quartzor grey mudstone(), with theformer beds, of varying strainedquartz with sub-grainsbounded by crystal faces.

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-Greenstone

Major Thrusts

Q. 1. Geological map of south Cornwall (after Holder & Leveridge 1986) showing the allochthonous unit of the Gramscatho Group between the Camck and Dodman thrusts. The Portscatho Formation (Pto) comprises the main unit within the allochthon. Turbiditic sandstone samples for this study were collected from the stratigraphic sequence at Mevagissey Bay.

The feldsparcontent includes plagioclase (P) and potassic showing a marked and progressive decrease, and metamor- feldspar (K), the former being largely albite/oligoclase and phic grains showing a complementary increase. the dominant phase. Lithic grains are of three categories: volcanic (Lv), sedimentary (Ls) and metamorphic (Lm)-the last derivingfrom both igneous and sedimentaryparents. Provenance and tectonic discrimination Among the volcanic grains arethose composed of a few On theQ-F-L diagram (Fig.2a) the Porscatho data plot near intergrown crystals of quartz and feldspar (somewith mica), the centre largely within the magmatic arc provenance field, microcrystalline quartzofeldspathicrocks often showing butpoints straddle the boundaryand overlap into the felsic recrystallization,some of which containmicrophe- recycled orogenprovenance field of Dickinson & Suczek nocrystic feldspars, and aminor proportion composed of (1979). Within the arc field the data are nearer theQ-F join microcrystalline chlorite and disseminated ore. Sedimentary than the L pole. This represents proportions of monocrys- grains are mudstone, siltstone and fine sandstone, including tallinegrains derived from plutonics in a dissected arc quartzite. Quartz and mica schists are predominant in the rather than volcanics of an undissected arc that furnish an metamorphic group, but volcanic rocks showing low-grade abundance of polycrystalline grains. The presence of pre-incorporation metamorphic fabrics are also present. non-volcanogenic metamorphic grains also indicates dissec- Proportions of the main categories of framework modes tion intothe continental core of thearc massif. Thearc of the whole grain population show little deviation from the provenance field generallyhas less quartzthan the other mean Q,,F,,~, through the sampled Portscatho sequence. majorsettings, but thetrend away from L, representing Similarly,with polycrystalline quartz assigned to the lithic increased dissection of thearc convergeswith that grains, there is no marked variation trend from the mean representing decreasing maturity away from Q towards F in Qm2,F31Lf42.Partial grain populations do however indicate the continental block provenance field. By assigning Qp to changes through the succession. Considering polycrystalline the lithics modein the Qm-F-Lt diagram (Fig. 2b) grains (Qp20Lvm52Lsm28)quartz fluctuates about the mean ambiguities regarding provenance field are largely removed. but theproportion of volcanic and meta-volcanicgrains As all thequartz present is not monocrystalline quartz (Lvm)shows a significant decrease above 1500m whichis derivedfrom thearc roots, a majority of the Portscatho largely balanced by anincrease in theporportion of plots move away from the convergence area into the central sedimentary and metasedimentarygrains (Lsm). These zone of thearc provenance field. This possibly reflects changesrepresent variations in proportions of the labile continued volcanism in the dissecting arc source. lithic grains(mean Lv,~L~,,L~~~)withvolcanic grains Further discrimination of source is afforded by the

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'L A / n h

rlfted continental margin (back arcbasins1 Lv Ls

Fig. 2. Portscatho sandstones plotted on various triangular framework mode diagrams that discriminate between different provenances and depositional settings (diagrams from Dickinson& Suczek 1979; Ingersoll & Suczek 1979).

Qp-Lvm-Lsm triangulardiagram (Fig. 2c)showing the Graham et al. (1976) that lithic populationsremain polycrystalline sub-populations of the framework, The fields proportionatelyconstant in linked sediment dispersal for active sources are well separated and continental block systems throughnon-marine into marine environments. sources withlow contents of lithic clasts are absent. They define combined provenance and depositional setting Portscathosandstones plot largelywithin thearc orogen fields on the meansand standard deviations of some 465 source field but away from the Lvm pole. The field extends data sets. Fore-arc areas are characterized by predominant from the Lvm pole towards the Lsm pole, the Lsm content volcanic and subordinate metamorphic lithic grains derived reflecting the metamorphic basement input of a continental fromaneroded continental margin arc.Continental margin arc. back-arc areas, in contrast, are typified by a volcanic and Although the continentalmagmatic arc provenance of variablesedimentary lithic componentfrom the rifted thePortscatho sandstones is indicated by thismethod, it continental margin. The Portscatho data plot in and around doesnot permit differentiation of thesetting of the the fore-arc field (Fig. 2d). depositional basin betweenback-arc, fore-arc or trench environment.The Lm-Lv-Ls triangularplot of lithic sub-populationsintroduced by Ingersoll & Suczek(1979) Geochemical features does however provide a means of such discrimination. The The majority of the sandstones (Table 2) have a SiO, range basis of the diagram is their confirmation of the concept of of 67-74 wt.%, moderate Fe203*(total Fe asFe,O,) + MgO

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70 Ni ppm

I5t Hf ppm t .. ..: .I 50t ..

100 300 500 15t Nb ppm Zr

.". A 2.5 interelementFig.sandstone 3. Portscatho .. relationshipsshowing coherencegood for most element pairs, except Zr-Nb. Average upper 100 3 00 500 6 0 10 12 and lowercontinental crust compositions from Zr PPm Th PPm Taylor & McLennan (1985).

contents of between about 6 and 9 wt. %, and low oxidation by Maynard et al. (1982) and also in terms of their relative ratios (generally <0.3). A1,03/Si0, ratios are variable and La-Th-Sc distributions(not illustrated) are typical post- reflect grain-size variation;coarser-grained quartz-rich Archean fine-grained sediments (Taylor & McLennan 1985). turbiditicunits commonlyhaving the lowest ratios. Large-ion-lithophile (LIL) elements, such as K, Rb, CS, Sr, La, Th and U, showa considerable range of abundances, Chemical stratigraphy although specific element pairs are constant throughout the Throughout a stratigraphic sequence chemical variation in stratigraphicsequence, e.g. K/Rb = 200, La/Th = 3.5, the form of abundance trends or specific chemical breaks Th/U ~3.3. Similarly, theREE and transitiontrace may provideevidence of a changein source composition elements(Ni, Cr, V, Sc)exhibit constantinter-element that is also reflected in clast populations. However, absolute ratios,e.g. La,/Yb, = 10, Cr/Ni=2.The high field elementabundances can be influencedby grain-size strength elements, Ti, Zr, Y, Nb, Hf, with ionic potentials variations or, more specifically, in turbiditic sandstones, the between 3 and 7, do not always showconsistent proportion of quartz to other clasts. The AI,O,/SiO, ratio inter-relationships (except Zr/Hf) as their distributions are provides a crudemeasure of the proportion of quartz to partlygoverned by differenthost stable heavy mineral feldspars + phyllosilicates againstwhich elementaltrends phases. Some of the above relationships are shown in Fig. 3 and relative abundances can be monitored before they are and illustrate the chemical coherence and uniformity of the ascribed to differential source compositions. Portscatho sandstone sequence as a whole for these element Chemicalvariation with stratigraphic height inthe groupings. PortscathoFormation is shown in Fig. 5 and exhibitsa Compared to sandstone means(Levinson 1974; Taylor & chemical break between loo0 and 1400 m from the base of McLennan 1985) their base metal contents are on average the measured section. It is illustrated by decreases in Zr, enrichedabout X3 for Pb and X4 for Zn, whereas Cu is Zr/Y and possibly TiO, together with increases in Cr, Rb abnormally low at 5-13 ppm. and Al2O3/SiO2 from the basal to the upper sequence. A Chemical classification of the sandstonesindicate that t-test for Rb, C-, Zr and Al2O3/SiO2 inthe data set showed they are virtually all Fe-rich quartz-intermediate greywackes thatthe designated basal andupper portions of the with K,O/Na,O ratios between 0.4 and 1.0 (Fig. 4). They stratigraphy differ significantly at the 95% confidence level, are not dissimilar to many Palaeozoic greywackes analysed whereas TiO, is not significantly different (means are

Fe203*+ MgO

Fig. 4. Major element chemical classificationof the Portscatho sandstones asFe-rich, quartz- QUARTZ-POOR intermediate greywackes (diagrams from Blatt et 01. 1972; Crook 1974). Palaeozoic greywacke Na2 0 0.1 1.0 10 Na20 field from Maynard et al. (1982).

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Ti02 Cr Rb Zr ZrlY Al203lSiO2 LVlLLmlL strat. wt% strat. ppm ppm PPm O/O O/O 0.59 4,O , t0 l 12,O 2% 400 7 15 0.250.13 l?O -r

+ +I( 1, 1, '+ : :;ll l+I

+:t t+, + I l .l +l .? : .. k

Fig. 5. ChemostratigraphyPortscatho of the \+ '001 ... .. i+ sandstones from the MevagisseyBay se- \ ."I : *\ quence (3.5 km sampledthickness). For 500 '' " ...... 1 +t 0. t e. .. .. $1 + comparison are shown the stratigraphic I distribution of the Lv and Lm components of .2.'. :it the frameworkthe modes, bothplotteda as 0.7wt% 0Oppm 8C percentagelithics,total of the L. Ti02 Cr Ftb

similar,although the coefficient of variation is markedly the basal sequence are governedby the presence of residual largerfor theupper portion). Reasonable correlations zircon as might be expected in the more Si0,-rich and/or (r 20.6) are also seenbetween Rb andCr for the basal coarserfractions of the sandstones; chemicallyindicated portion of thesequence as well as thetop. The relative here by the typically lower A1203/Si02 ratios.However, distributions of the elements are also different in that the only poor negative correlations are obtainedbetween Zr elementconcentrations aremore uniform in the basal and A1203/Si0, in both portions of the stratigraphy (e.g. sequence, whereas in the upper portion variably increasing r = -0.3 for basalportion). The high andvariable Cr or decreasing trends are common. The exception to this is content of theupper sequence (especially for values Zr, whichis highly variablein the basalsequence. These 3100 ppm Cr) is probably a reflection of changing source featuresare illustrated by comparing the coefficient of composition and the incoming of detrital material of more variation (as a measure of relative dispersion) which is lower pelitic or intermediate/basic character. This is supported to in the basal portion for Rb and Cr (e.g. Cr basal = lO%, Cr somedegree by the increase upthe sequence of upper=22%), whereasit is higherfor Zr (e.g. Zr metamorphic lithic fragments at the expense of acid volcanic basal = 25%, Zr upper = 15%). and plutonic clasts (Fig. 5). Figure 6 shows the division of the sandstones into basal (below 1400 m) and upper stratigraphic units on the basis of Cr and Zr distributions. Although useful for correlation or Chemical discrimination of provenance discrimination purposes it is unlikely that this variation can We will now consider if the bulk geochemical features of the beinterpreted entirely in terms of sourcevariation. For sandstonescan berelated to andsupport the framework example, the high Zr contents (and also associated Hf) of mode interpretation. Previous chemical studies have related greywacke compositions to different tectonic environments (e.g. Bhatia & Taylor 1981; Bhatia 1983), but have lacked +top portlon of sequence supportive framework data on the same sample set. Trace (vanable A12 03 /Si02 l elements considered useful in this context include the REE, . basal portlon of sequence Ti, Zr,Hf, Y, Th and U, andanumber of chemical (low E unlform AI203 151021 diagrams can be used to determine the bulk composition of

the turbiditic sandstones and also infer their provenance. As +*+ + seenin Fig. 7 the Portscathosandstones have a uniform K/Rb ratio of 200 that lies close to a typical differentiated magmatic suite or 'main trend' with a ratio of 230(Shaw 1968). Thisfeature, together with the high K andRb contents suggest that the sandstones could be the erosional I I I I I products of seriesa ofmagmatic rocks ranging from 200 300 400 500 Zr PPm predominantly acidic to intermediate compositions. The presence of both acidicvolcanic and (to a lesser extent) Fig. 6. Chemical discrimination of Portscatho sandstones into basal plutonicfragments in the sandstonessupports the overall and upper portions on Cr and Zr abundances. chemical composition. Note that the variation displayed (see

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10 - 10 - acid intermediate K- compositions / Wt%

compositions arkosic sands 1.0 - 1.0 - 7. (a) Distribution of K and metagreywackes Fig. Rb in Portscatho sandstones rela- lower continental lower / crust A / tive to a K/Rb ratio of 230 (=Main / / Trend of ShawUpper 1968). and lower continental crustcomposi- tions from Taylor & McLennan (1985). (b) Distribution of K and Rb in various unmetamorphosed arkosic sands (van de Kamp et al. (a) b 1976), low-grade metagreywackes (Condie et al. 1970; Caby et al. ,I ,I ,Igrade higher I and 1977) metavol- 1 100 1 100 canic tuffs (van de Kamp 1968)-all Rb ppm Rb ppm derived from magmatic precursors.

also Fig. 3) could begenerated by mixingtypical upper sedimentscould have been derived from such asource continentalcrustal materials (granodioritic/granitic)with originally. The normalized REE pattern is similar to average small portions of moreintermediate/basic compositions. post-Archeanargillaceous sediments and also the North However,the K-Rb coherence could result from their American shale composite, NASC (Nance & Taylor 1976; redistribution by hydrothermal fluids duringregional Fleet 1984), thatare considered to reflect theaverage metamorphism/metasomatism accompanying theVariscan composition of the continental source region. Arc and active orogenyand emplacement. Even thoughthe continental margin volcanic rocks show variableLIL sedimentsare mildly metamorphosedthis is probably contents and ratios (Jakes& White 1972; Thorpe et al. 1976; unlikely in the light of the constancy of element ratios that Saunders & Tamey 1982), as do the sediments derived from are unaffected by element mobility (e.g. see Fig. 3). Also, them (van de Kamp et al. 1976; Bhatia & Taylor 1981). As by comparison (Fig.7) other lithologically similar, shown in Fig. 9 sediments derived from acid-dominated arcs low-grade metasedimentaryrocks derived from acid and have low and uniform La/Thratios and Hf contents of intermediate igneous precursors have uniform K/Rb ratios about 3-7 ppm. With the progressive dissection of the arc, around 230 and fall in about the same areaof the diagram as andthe erosion of its plutonicroots and continental the Portscatho sandstones. basement of ancient(meta)sedimentary rocks, the Hf Supporting chemical data for the predominantly acidic contentincreases via therelease ofzircon-its main host nature of thesandstones is exhibited by thechondrite phase. A plot of La/Th against Hf (Fig. 9) provides a useful normalizedpatterns for the REE (Fig. 8). These show a bulk rock discrimination between different arc compositions stronglyenriched light REE pattern (La,/Yb,= lO), a andsources, and can berelated to the framework mode small negative Eu anomaly (Eu/Eu* - 0.7)and flat or analysis given above. uniform heavy REE. All these features can be matched with Stabletrace elements rapidly incorporated into sedi- activecontinental margin rhyolites (Taylor et al. 1968; ments can provide a means of discriminating both source Ewart et al. 1968) ratherthan andesites and suggest the composition and, to some extent by inference, the tectonic

100 -

calc-alkall rhyolltes envelope

talc-alkallandesltes envelope Fig. 8. (a) Chrondritenormalized REE plot for Portscatho sandstones. (b) Typical REE distributions for continental margin rhyolites and andesites (from Tayloret al. 1968; Thome et al. 19761, Dost-Archean .. (b) I..

It I I I I I III''"' llllllllllilll~l fine-grained sediments and NASC La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (Nance & Taylor 1976).

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’ La/Th 15 La l Th (a) ,, ,S \yJ tholelitit ocean island source

lowercontinental crust 1C A

Fig. 9. (a) Source and composi- tional discrimination of turbiditic sandstones in terms of La/Th ratio and Hf abundance (data 5 a from Bhatia & Taylor 1981; ._ ‘.- passive 2;. a ----- _____, .. McLennan er al. 1984; Taylor & ...... - - - - . Increasing old sedime$.,,’? ? McLennan 1985; Floyd 1986). component source a‘up;er continentalcrust (b) Discrimination of Portscatho I I I l I l sandstones indicates derivation 5 10 15 5 10 15 from an acidic arc source. Hf PPm Hf PPm

monocrystalline grainsfrom the plutoniccore, but lithic clasts from the metamorphic basement. The decrease of Lv l oceanlc and concomitant increase of Lm in the upper part of the PortscathoFormation suggests either a change of source area or simply deeper dissection through arc magmatic rocks to metamorphicbasement. The higher Crcontent in the upperpart of the sequence can be tentatively correlated with the influx of meta-argillaceous materialrather than basic igneous clasts which are very limited throughout the succession. Onefurther significant chemical featureconcerns the actlvecontlnental margin basemetal content of the sandstonesrelative to Cornish basic volcanic rocks. Both the Gramscatho greywackes and -4-2 0 2 4 6 the geographically adjacent Mylor Slate Formation argillites FUNCTION- I have higher than average concentrations of Pb and Zn, but Fig. 10. Plot of Portscatho sandstones within a major element lowCu (this paper;Edwards 1976), whereasmetabasalts discrimination function diagram (redrawn from Bhatia 1983) for show theopposite relationship withhigh Cucontents turbiditic sandstones relative to tectonic environment. averaging c. 85ppm (Floyd1968). This suggests thatthe basicvolcanic and local (non-mineralized)sedimentary environment. Bhatia (1983) used major element discrimina- rocks could havebeen the original source rocks for the tion function analysis to determine the tectonic situation of associated Cu and Pb + Zn mineralization respectively, via derived sandstones. Figure 10 (redrawn from Bhatia 1983) granite-induced hydrothemal leaching. shows thatthe Portscatho sandstones plot astride the On chemical grounds the sandstonesstudied hereare continental island arcand active continental margin field almost identical to some flysch sequences within the Tasman boundary.The main difference betweenthese two geosyncline, Australia, that have beenderived from environments is essentially the thickness of the continental continental arcs (Bhatia & Taylor 1981). In particular, they crust involved,and thus the discriminationachieved compare with the Hill End trough greywackes, composed of generallysupports thetrace element inference and felsic volcanicrock clasts plusa complement of older framework mode analysis. sedimentary fragments, that were deposited in an interarc basin initially formed by crustalextension (Cas & Jones 1979). The Gramscatho framework mode compositions also Conclusions and discussion compare closely with sandstones from the fore-arc basins of Frameworkmodes and bulkgeochemical data of the the NW Pacific (Galloway1974). Dickinson (1982) has sandstones show remarkable consistency throughsome confirmed that sandstonesfrom circum-Pacific fore-arc 3.5 km of the allochthonousPortscatho Formation. The basins have asimilar compositioneverywhere reflecting petrographic and geochemical data indicate that this unit of varying degrees of dissection of magmatic arcterranes. the Gramscatho flysch had a continental margin magmatic Moderndeep-sea sands retrieved from continental margin arcprovenance and that accumulation was in a fore-arc strike-slip environments also havevariable, but pre- basin. In general terms the composition of the arc can be dominantlyarc provenance compositions (Maynard et al. consideredas predominantly acidic together with minor 1982). Thisappears to be dueto strike-slip development contributions from an intermediate/basic source or, on the along thetrend of arc systems inresponse to oblique other hand, the derived sediments chemically represent the subduction which exposesarc rocks for erosion after admixture of typical upper continental crust with material of interruption of arc magmatism (see Dickinson & Valloni moreintermediate/basic character. The arc was probably 1980). Thus, both geochemical andframework mode data deeplydissected, supplying not only volcanic lithics and indicate the presence of an arc at the southern margin of the

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Gramscatho basin duringthe Devonian. It is unlikely, CONDIE, K. C., ~CKE,J. E. & REIMER,T. 0. 1970.Petrology and therefore,that the basin is anintracratonic strike-slip geochemistry of early Precambrian greywackes from the Fig Tree Group, feature as suggested by Barnes & Andrews (1986). Such a South Africa. Bulletin of the Geological Society of America, 81, 2759-76. CROOK, K. A. W. 1974.Lithogenesis and geotectonics: the significanceof setting would be expected to produce sandstones, derived compositional variation infiysch arenites (greywackes). Society of from the adjacent uplifted basement, with high Q, F and EconomicPalaeontologists and Mineralogists, Special Publication, 19, low L modes (Dickinson & Vallori 1980), high SO2, Zr, Hf 304-10. and K20/Na20 ratiosand low Crand Sc (Taylor & CROWELL,J. C. 1957. Origin of pebbly mudstones. Bulletin of the Geological Society of America, 68,993-1010. McLennan 1985Fnone of which are exhibited by the CUMMINs, W. A. 1962. The greywacke problem. Liverpool & Manchester Portscatho turbidites. Geological Journal, 3, 51-72. Anarc source for the sediments and a fore-arc depo- DAVIES, G. R. 1984. Isotopic evolution of the Lizard Complex. Journal of the sitionalenvironment is consistent with thepresence of a Geological Society, London, 141, 3-14. subductionzone tothe south of Cornwall(cf. Holder & DEARMAN,W. R. 1971.A general view of the structure of Cornubia. Proceedings of the Ussher Society, 2, 220-36. Leveridge 1986). Furtherpossible support for this comes -, LE~RIDGE,B. E. & TURNER,R. G. 1969. Structural sequences and the from the chemistry of the volcanicrocks in the melange ages of slates and phyllites from southwest England. Proceedings of the zone of south Cornwall (Barnes et al. 1979) which has been Geological Society of London, 1654, 41-5. considered to be stratigraphically higher than the Portscatho DICKINSON,W. R. 1982.Compositions of sandstones in&cum-Pacific subduction complexesand fore-arc basins. Bulletinof the American Formation (Holder & Leveridge 1986; Barnes & Andrews Association of Petroleum Geologists, 66, 121-37. 1986). Ratherthan the enriched MORB features as - & SUCZEK,C. A. 1979. Plate tectonics and sandstone compositions. previously thought (Barnes 1984; Floyd 1984), many of the Bulletin ofthe AmericanAssociation of Petroleum Geologists, 63, 2164-82. metabasalticrocks within the melange(e.g. Nare Head; -& VALLONI,R. 1980. Plate settings and provenance of sands in modern Meneage)have higher than usual LIL/HFSratios (e.g. Ocean basins. Geology, 8, 82-5. La/Nb = 1.5-2.5) andchondrite normalized negative Nb DUFFIELD, J. & GILMORE,G. R. 1979.An optimum method for the and Ta anomalies, more indicative of a subduction-related determination of rare earth elements by neutron activationanalysis. environment than a spreading ridge. Journal of Radioanalytical Chemistry, 48, 135-45. EDWARDS,R. P. 1976. Aspects of trace metal and ore distribution in Cornwall. Transactions ofthe Institution of Mining & Metallurgy (Applied Earth Science), 85, 1383-90. P. A.Floyd acknowledges the University of KeeleResearch EWART,A., TAYLOR,S. R. & CAPP,A. C. 1968. Trace and minor element Committee for financialsupport for field work andchemical geochemistry of the rhyolitic volcanic rocks, central North Island, New for Zealand. Contributions to Mineralogy and Petrology, 18, 76-104. analyses.D. Emley and M. Aikin are thanked technical FLEET,A. J. 1984. Aqueous and sedimcntary geochemistry of the rare earth assistance in producing the XRF data. The paper is published by elements. In: HENDERSON,P. (ed.) RareEarth Element Geochemistv. permission of the Director of the British Geological Survey, Natural Elsevier, Amsterdam, 343-73. Environment Research Council. Fan, J. S. 1933. The geology of the Meneage. 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Received 20 May 1986; revised typescript accepted 19 September 1986.

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