'Department of Geology, Royal Holloway and Bedford New College (University of London). Egham, Surrey, TW20 OEX, UK. Received 4 May 1994; revised ...
Journal of
Geochemical variation and magmatic cyclicity within an Ordovician continental-arc volcanic field: the lower Borrowdale Volcanic Group, English Lake District Brett Beddoe-Stephensa7*, Michael G. Pettersonb, David Millward”, Giz F. Marrinerc “British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, UK “British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 SGG, UK ‘Department of Geology, Royal Holloway and Bedford New College (University of London). Egham, Surrey, TW20 OEX, UK
Received 4 May 1994; revisedversion accepted 19 September 1994
Abstract The origin, differentiation and temporal evolution of magmas have been examined from the Ordovician lower Borrowdale Volcanic Group (BVG) - a talc-alkaline, plateau andesite pile of continental arc affinity. The basalt-dacite compositional range can be modelled by POAM-type crystal fractionation of primitive melts, allied with minor crustal assimilation, derived from enriched mantle modified by fluids from the subduction zone. Detailed sampling of sections through 2-3 km of stratigraphy reveals localised and successive highly systematic magmatic evolutionary trends, or cycles. Cycles in which magma becomes less differentiated with time can be viewed as episodes of rapid eruption of a compositionally zoned chamber, possibly due to magma recharge. By contrast, increasing differentiation with time implies reduced recharge and eruption rates whereby fractionation processes dominate. The lack of evidence for compositional change over time represents a balance between recharge, eruption and differentiation processes. Successive cycles within a local sequence can be related in many cases to discrete batches of magma that ascend into sub-volcanic chambers and undergo fractionation. The differences between contemporaneous sequences confirms the current view of the 1owerBVG as a multicentred volcanic field. It was deposited in a subsiding, extensional volcano-tectonic rift zone, consistent with episodes of rapid upward magma flux, with the eruption locally of primitive (high Mg#, Ni and Cr) basaltic lavas.
1. Introduction
Although the compositional variation within a suite of volcanic (or plutonic) rocks can reveal a great deal about the controlling mechanisms of magmatic differentiation, the addition of a time dimension allows a connection to be made between these mechanisms and their physical operation in the magmatic environment. The relative rates of magma supply, differentiation and eruption can be assessed only when the products of a * Corresponding author. 0377-0273/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDIO377-0273(94)00092-l
differentiation cycle can be placed in (relative or absolute) chronological context. Furthermore, temporal control can yield insights into the plumbing systems involved in magma ascent, for example by examining the degree of interaction between consanguineous cycles of differentiationSequences of lavas are particularly useful in this context, and stratigraphic variations in chemistry have been used by various petrologists to gain insights into sub-volcanic magma genesis (e.g., Nixon, 1988; Huijsmans ,and Barton, 1989; Norman and Leeman, 1990; Waiker et al., 1993). On active
82
B. Beddoe-Stephen.7 et al. /Journal
of Volcanology and Geothermal Research 65 (1995) RI-1
volcanoes “petrological monitoring” of eruptive products is an important tool in volcanic hazard prediction (e.g., Newhall, 1979; Luhr and Carmichael, 1990). In this paper, differentiation and magma evolution trends are described from an Ordovician volcanic-arc lavadominated sequence - the lower Borrowdale Volcanic Group (BVG) . Here, excellent exposure and dissection of 2-3 km of stratigraphy, together with detailed 1: 10,000 scale systematic mapping, permit a view into this type of volcanic province that is not usually available in modern analogues. Petterson et al. ( 1992) recently described the lower part of the BVG in terms of a pre-caldera, dominantly effusive sequence. The lower BVG developed as a lowtopography, plateau andesite lava pile but includes subordinate sills, bedded volcaniclastic sandstones and tuffs, and locally important acidic ignimbrites and cogenetic dacitic lavas. The bulk of the 2-3-km-thick pile is composed of basalt to andesite lavas, probably extruded from multiple sources (fissures, vents, etc.) and most plausibly located within an intra-arc, contemporaneously subsiding, extensional tectonic setting. Petterson et al. ( 1992) noted that systematic variations in composition with stratigraphy occur, indicative of magma evolution with time. Geochemical data for the lower BVG (Birker Fell Formation-BGS, 199 1) are here considered in more detail with respect to the origin of the magmas, their differentiation and evolution with time and also the spatial variation in magma evolution and production.
2. Geological setting The BVG is a medium- to high-K talc-alkaline suite of ?Llandeilo to Caradoc age and is considered to be part of a continental margin volcanic arc developed on the south side of the Iapetus Ocean and related to southerly directed subduction and closure of this ancient ocean along the Solway Line (Fig. 1 inset). The geochemical work of Fitton and Hughes ( 1970) and Fitton et al. ( 1982) on the Lake District volcanic rocks was seminal in placing the BVG within its plate tectonic setting. The BVG is predominantly subaerial (Branney, 1988), although there is ample evidence that many volcaniclastic units, in both the lower and upper BVG, were reworked and deposited by non-marine water. However, the BVG is both underlain and overlain by
10
thick marine, mainly turbiditic sedimentary successions (Fig. 1). The BVG is broadly divisible into two parts (Fig. 1) : the lower 2-3-km-thick plateau andesite sequence (Petterson et al., 1992) and an upper succession of thick pyroclastic and epiclastic rocks which have been ascribed to one or more episodes of calderarelated collapse (Branney and Soper, 1988)) and possibly regional intra-arc downfaulting. Similarly, Petterson et al. ( 1992) argued that preservation of a thick, low-topography lava pile indicated its accumulation within a volcano-tectonic graben or depression. Consequently, though the volcanic succession is up to ca. 6 km thick, there is little evidence that large volcanic edifices formed significant positive topographic features. Petterson et al. ( 1992) highlighted distinctive formations or facies associations within a monotonous background of andesitic or basaltic andesitic flows and less abundant intrusive sheets that comprise the plateau andesite sequence of the lower BVG. In the Wasdale area (Fig. 2) the andesite succession is split by a substantial intermediate-acid ignimbrite sequence, called the Craghouse (Tuff) Member. Units of the Craghouse Tuff thicken westwards, and are probably linked with > 1200 m of intra-caldera ignimbrites and other pyroelastic deposits proved by deep drilling beneath cover rocks in the Gosforth area. To the east, probable outflow facies of the Craghouse Tuff thin, and ultimately disappear between Eskdale and Wasdale. A possible time equivalent for this event further south and east may be the Great Whinscale Dacite (Fig. 2) and associated acidic tuffs. Thus although the lower BVG is described as a pre-caldera field with respect to, and underlying, caldera-forming and post-caldera products of the upper BVG, temporally the lower BVG plateau andesite succession is interrupted by a significant caldera-forming event to the west of the exposed BVG. Near the top of the lower BVG succession between Eskdale and Wasdale the Throstle Garth Member is a distinctive unit up to 300 m thick, consisting of heterogeneous, clinkery and scoriaceous basalt interspersed with lenticular tongues and channels of massive aphyric lava (Fig. 2). It is interpreted as a compound au lava field (Petterson et al., 1992). A similar but less well exposed basaltic unit, the Wrighthow Member, is present in the extreme west of the area. Two sections through the sequence, which are dealt with in detail in the following discussion, are close to
B. Beddoe-Stephens et al. /Journal
83
of Volcanology and Geothermal Research 6S (1995) 81-110
Upper Palaeozoic cover
Windermere Group
Upper Borrowdale Volcanic Group Lower
Skiddaw Group
Granite
Fig. 1.Location aad geology of the Lake District showing the bipartite division of the the Solway Line.
Devoke Water (SD 16 97) in the south of the area under study, and at Haycock (NY 14 11) to the north (Table 1; Fig. 2). 2.1. Devoke Water A complete sequence through the 2600-m-thick Birker Fell Formation (BGS, 1991) is present from
Borrowdale
’
10 Kilometres
I
I
Volcanic Group. inset shows the relation to
south of Devoke Water to Stainton Pike (SD 15 94; Figs. 2 and 3). The lowest part comprises a thick sequence of weakly bedded basic lapilli tuffs, the Devoke Water Member, up to about 620 m thick (Petterson et al., 1992). These are overlain by up to 480 m of clinkery, autobrecciated plagioclase + pyroxenephyric andesites and basaltic andesites. At least 25 sheets are present, most of which are interpreted as aa
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
84
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All samples are from basaltic andesite, andesite and dacite lava flows or sills, except where indicated. Units are numbered from base to top of stratigraphic sections. Full sample locations obtainable from the authors. ‘National Grid Reference.
23
:; BIRKBY17
0
Devoke Water section (DVK) ; SD 148 962 to SD 155 942”; 37 samples; Great Whinscale Dacite (GWD) = DVK-30 Greyfriar section (GRY); NY 258 010 to NY 260 003; 15 samples; Cockley Beck (tuff) Member (CKT) = GRY-2 Throstle Garth area (TGB); NY 22 05; 16 samples of Throstle Garth (basalt) Member (TGH) Stony Tarn section (SIT); NY 200 029 to NY 211 049; 15 samples; Craghouse Tuff Member (CHT) = SIT-6 Haycock section (HAY); NY 135 120 to NY 166 106; 30 samples; Craghouse (tuff) Member (CHT) =HAY-19 to HAY-22 Seatallan Dacite (STA) = HAY-l 8 Santon Bridge area (SNB) ; NY 113 024 to NY 107 037; 11 samples; Craghouse (tuff) Member (CHT) = SNB-3 to SNB8. Wrighthow basalt Member (WGT) =SNB-10 to SNB-12
FELL 15 MEMBER
T
Dacite, an unusually widespread, large-volume lava flow ( Kanaris-Sotiriou et al., 199 1) . 2.2. Haycock EVOKE WATER MEMBER
FAULT Skiddaw -UP
Fig. 3. Stratigraphy of the Devoke Water section showing analysed (numbered) units and defined members.
lava flows. Typically, most flows are less than 50 m thick and many are separated by thin interbeds of bedded volcaniclastic sandstone and tuff. The upper part of the sequence is dominated by generally thicker block flows of plagioclase-phyric andesite, with interbedded volcaniclastic sandstone and, locally, coarse debrisflow breccias (Fig. 3). Stratigraphically important marker units include the crystal-rich and nodular, welded rhyolitic Little Stand Tuff, which contains common alkali-feldspar phenocrysts, an unusual occurrence within the BVG, and the aphyric Great Whinscale
The 2.2~km-thick lower BVG stratigraphy exposed in the Haycock area (Fig. 2) is shown in Fig. 4. Unfortunately, the complete succession is not exposed due to intrusion of the Ennerdale granite at the base and erosion of the top; however, the thickness of the sequence, which is comparable to that of the lower BVG elsewhere in the area, suggests that it represents a high proportion of the original succession. As in the Devoke Water area it is dominated by basalt to andesite lava flows, often with thin interbeds of volcaniclastic sandstone. A 200-m-thick unit of bedded, reworked volcaniclastic sandstones with primary tuff and lapilli tuff occurs toward the base of the succession, and has been designated the Eagle Crag Member (Petterson et al., 1992). This section also intersects a distinctive, thick, porphyritic dacite lava (Seatallan Dacite), which is local to this area of the lower BVG, and an overlying ignimbrite unit which has been correlated with the Craghouse Tuff. The Craghouse Tuff consists of crystal-rich, massive to eutaxitic lapilli tuffs comprising several ignimbrite flow units. The Craghouse Tuff here
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
86 EXPLANATION
?
27
Volcamc sandstone and Ml
26
Basalt-andmrte
mapping control permits sampling unit by unit, thus enabling detailed temporal variations in magma chemistry to be examined through some 2-3 km of volcanic stratigraphy (Figs. 3 and 4). In addition, several of the more distinctive lithological packages have been sampled in order to obtain geochemical data from the complete spectrum of lower BVG rock types across the area. Table 1 summarises the samples obtained and acronyms used in the following discussion and Fig. 2 shows localities. During collection of samples, care was taken to collect a large representative sample from the centres of the individual units, avoiding zones of brecciation and/or vesiculation that typify the margins of most basalt to andesite sheets. The samples were analysed by XRF, using fused beads for major oxides and pressed powder pellets for trace elements, at Royal Holloway and Bedford New College, University of London, and the British Geological Survey, Keyworth. In the plots used the major oxides were normalised to 100%.
-0
4. Petrography
-
500 m&es
ENNERDALE GRANITE Fig. 4. Stratigraphy of the Haycock section showing analysed (numbered) units and defined members.
is probably associated with a major caldera-related ignimbrite sequence largely hidden beneath upper Palaeozoic cover to the west of the area shown in Fig. 1 (Petterson et al., 1992), possibly as a localised or outflow facies. Overlying the Craghouse Tuff in the Haycock succession is a prominent scoriaceous basaltic lava flow with a reworked top surface. 3. Sampling and analysis The geochemical sampling strategy was focused on well-exposed sections through the lower BVG where
Phenocryst mineralogy of the BVG was summarised by Fitton et al. ( 1982)) and the principal petrographic characteristics of the lower BVG rocks were described by Petterson et al. (1992). In addition, Allen et al. ( 1987) have described in detail lower BVG composite lava flows from the Wrynose area. In discussing magmatic composition it is important to recognise that the BVG has undergone significant secondary mineralogical alteration. In the map area (Fig. 2), a broad area of contact metamorphism can be recognised, related to the intrusion of the Eskdale and Ennerdale granites. This is manifested by the development of actinolitehornblende, predominantly as a replacement of pyroxene, and less extensively by fine-grained biotite. The BVG has also been subject to a regional low-grade metamorphism, possibly burial (Oliver et al., 1984)) which reached prehnite-pumpellyite facies and involved the formation of albite, chlorite and epidote as secondary phases. Sericite is also extensively developed, predominantly as a pseudomorph after feldspar. To the east of the map area (Fig. 2)) carbonate- rather than epidote-bearing assemblages become more common (N.J. Fortey, pers. commun., 1994) and sericitecarbonate alteration may in part relate to areas of strong
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-l 10
cleavage that affect the BVG within the SE-facing limb of the Westmorland Monocline. Within the area sampled, however, tectonic strain (cleavage, etc.) is low and, despite mineralogical alteration, textural features of the lavas and pyroclastic rocks are usually well preserved (e.g., porphyritic and groundmass texture, welding foliation, etc) . The basalt to dacite lavas/sheets are commonly porphyritic, although aphyric lavas characterise the Throstle Garth (basalt) Member. Olivine and orthopyroxene phenocrysts are never fresh but the relict crystal habit of some chlorite pseudomorphs indicates that both were present, with olivine restricted to the most mafic members. Clinopyroxene (diopside-augite) is a conspicuous phenocryst in the more mafic rocks locally up to 1 cm in size. It is sporadically fresh but mainly pseudomorphed by chlorite, or in part by actinolite-hornblende near the contacts of the granitic plutons. Plagioclase phenocrysts are variably altered to sericite, epidote and more rarely to carbonate. Magmatic compositions are preserved locally but albitisation is common. Fe-Ti oxide microphenocrysts occur in minor amounts and accessory apatite is common, particularly in the more acidic rocks. Garnet phenocrysts are a feature of the BVG (Fitton, 1972; Beddoe-Stephens and Mason, 199 1) . They are particularly conspicuous as euhedra in dacitic-rhyolitic rocks of the upper BVG, but also occur locally, usually as partly resorbed crystals, within andesites and some tuff units of the lower BVG. Within the lower BVG especially, and the BVG as a whole there is only rare evidence for the presence of primary hornblende phenocrysts which often are a characteristic of continental arc volcanic rocks. Altered biotite microphenocrysts occur in some dacitic rocks. Because of the altered state of the BVG, elements recognised to be mobile under conditions of low-grade hydrothermal alteration, such as Rb, Ba, Sr, Na and K (cf. Allen et al., 1987), can only be utilised with care in discussing magma petrogenesis. The discussion relies on the more immobile trace elements and major oxides which, given the lack of significant carbonate alteration or gross textural alteration, retain magmatic signatures. 5. Compositional
variation within the lower BVG
Harker diagrams (Figs. 5 and 6) show that the lower BVG ranges in composition from basalt ( < 52% SiO,)
87
to dacite (63-69% Si02), Although there will be some sampling bias, particularly in the 52-54% SiO,! range due to a disproportionate number of Throstle Garth basalts analysed (Table l), the most common compositions are basaltic andesite and basic andesite-this is probably close to being representative due to the systematic sampling strategy employed. Most of the dacitic compositions correspond to ignimbritic pyroelastic rocks of the Craghouse Tuff member of the western part of the outcrop, but also several lava flows, including the distinctive Great Whinscale dacite (Kanaris-Sotiriou et al., 1991). The remaining analyses pertain to basalt-andesite flows and sheets (Table 1). The variation diagrams present a characteristic calcalkaline trend of decreasing Fe203*, MgO, Ti02, CaO and V against increasing SiOz (Figs. 5 and 6). A1,03 shows a level to decreasing pattern with differentiation (Fig. 5B). Na,O and KzO (Fig. 5F) show considerable scatter due to secondary mobility but total alkalis retain a reasonable positive correlation with SiO*, suggesting that gross enrichment or depletion of alkalis has not occurred. An exception may be some units within the Haycock succession which are anomalously high in K (Fig. 5F), possibly related to K loss from the Ennerdale granite. The immobile HFS incompatible elements (Zr, Nb, Th, Y and REE) (Table 2; Fig. 6) show well defined positive correlated trends with SiOz. By contrast Rb, Ba and Sr show considerably more scattered trends (not shown). Both Cr and Ni exhibit wide variation at the basic end of the spectrum, but overall sharply declining (exponential type) trends towards silicic compositions (Fig. 6A and B) . Given the wide spread of samples the covariation between immobile elements is good, throughout the suite. For example, the Cc/Y ratio shows a restricted, very slightly increasing value from mafic to acidic compositions (Fig. 6F) - an analogous La/Y plot was used by Fitton ( 1972) to rule out significant garnet control on the compositional spectrum of the BVG. The markedly concave Ni, Cr and MgO trends (Figs. 5D, 6A and B) suggest that the evolution of the suite was dominated by crystalliquid fractionation of basaltic magmas. Gross mixing between rhyolitic crustal melts and mantle-derived basaltic magmas, as has been invoked for the generation of some andesites (e.g., Eicheberger, 1975; Grove et al., 1982), does not appear to be important. Local sequences within the lower BVG display slightly different, albeit sub-parallel, trends. In partic-
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-l 10
88 I
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Fig. 5. Harker variation diagrams for maior oxides with ornamentation reflecting local sequences (Table 1 and Fig. 2). (A) TiO,. (B) A&O,. (6)Fe203*. (D) MgO. (E)CaO. (F) I&O.
ular the Haycock (HAY) sequence plots at lower MgO, Ni, Cr and higher A1203 for a given SiOp than the other sequences, whereas the Devoke Water rocks are slightly richer overall in V (Figs. 5 and 6). The data suggest that subtly different magma compositions were supplied in different parts of the region but that similar differentiation processes generated the variation in composition of volcanic units within the localised areas. 5.1. Origin of the compositional
variation
Least squares mixing calculations were made to assess the role of crystal fractionation. Rather than using discrete rock compositions, which might be subject to localised alteration and consequent element
mobility, a trend was constructed using a smoothing procedure described by Cleveland ( 1979, 1981) and compositions at 52, 54, 56, 58, 60, 62 and 64% SiO, defined-these are given in Table 2. In calculating these, the Haycock samples were excluded and will be dealt with separately, because they define significantly different trends for Ni, Cr and MgO (see Figs. 5 and 6). Mineral compositions are not generally known, the exceptions being clinopyroxene (Fitton et al., 1982) and plagioclase. Therefore, Mg/Fe ratios in olivine, clino- and orthopyroxene were calculated using Fe/Mg distribution coefficients (cf. Grove and Bryan, 1983; Reagan et al., 1987). Reasonable estimates of Ca, Mn, Ti and Al in pyroxene can be derived from data in Fitton et al. ( 1982) and published data on similar rock types (Gill, 1981). Similarly the Ca/Na content of plagio-
B. Bedoe-Stephens
et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
89
200 * 0 45.00
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Fig. 6. Harker variation diagrams for trace elements. (A)
/
l
._~.
__
Cr. (B) Ni. (C) V. (D) Zr. (E) Ce. (F) Cc/Y.Ornamentation as Fig.5.
clase can be estimated by assuming an equilibrium distribution between melt and plagioclase. Sodium is a potentially mobile element; however, the trend-fitting procedure employed produced consistent Na,O and K20 values, though there is considerable individual scatter of these oxides due to alteration (cf. Fig. 5F). It seems unlikely that the BVG as a whole is enriched or depleted in these elements so average abundances are probably close to original magmatic values, although Na,O values are rather low compared with analogous suites. To account for the contribution of an Fe-Ti oxide phase pure Fe0 and TiO, were included in the calculation, then combined to give a composite FeTi oxide composition. Finally, an ideal apatite composition was included. The best results of the least-squares calculations for each 2% increment of SiOz are shown in Table 2. Residuals are generally very small and show that the basic end of the compositional spectrum can be derived by ol+cpx+plag+
oxide + apatite fractionation, whereas the more intermediate-acidic compositions are best explained by plag + opx + cpx + oxide + apatite control. The change in assemblage at 57-58% SiO, is marked by a significant change in slope on the calculated trend for MgOSiOz (cf. Fig. 5D). There is no need to invoke hornblende or garnet as a fractionating phase. The calculated assemblages from Table 2 and published partition coefficients (Nagasawa, 1970; Pearce and Norry, 1979; Gill, 198 1) can be used to predict the behaviour of various immobile trace elements during Rayleigh fractional crystallisation, as shown in Table 3. Most significant from this modelling is that calculated values for HFS incompatible elements (Zr, Nb, Th, Ce, Y) all show significantly greater enrichment with fractionation than observed. It is unlikely that this can be attributed to uncertainty in the values of the partition coefficients since in order to reproduce the observed trends one requires D values significantly
90
B. Bea’doe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-l IO
Table 2 Smoothed lower BVG compositions
(excluding
Haycock analyses)
and least squares mixing calculations
using calculated mineral compositions
AV52
AV54
AV56
AV58
AV60
AV62
AV64
SiOz TiO,
52.00
40, Fe& MnO MgO CaO Na,O K,O P?O5
15.58 9.53 0.28 10.00 8.40 2.00 I .60 0.22
54.00 1.04 16.02 9.12 0.28 7.80 7.80 2.20 1.90 0.22
56.00 I.06 16.50 8.61 0.25 5.70 7.20 2.40 2.20
58.00 1.06 17.00 8.00 0.20 4.18 6.35 2.60 2.50 0.22
60.00 0.94 17.15 7.14 0.20 3.22 5.40 2.70 2.90 0.22
62.00 0.82 17.30 6.40 0.20 2.40 4.40 2.70 3.40 0.22
64.00 0.68 17.44 5.50 0.18 1.72 3.40 3.20 4.00 0.21
Mg# Ca#
61.5 69.9
62.9 66.2
56.1 62.4
so.9 57.4
47.2 52.5
42.6 41.4
38.2 37.0
wt.%
I .oo
Least-squares mixing F (melt fraction) F (cumulative)
0.786 0.786
Fractionating assemblage (%) Olv
0.22
0.823 0.647
0.841 0.544
0.826 0.449
0.843 0.379
0.871 0.330
17.7 22.3 51.9 7.6 0.5 0.0011
20.2 16.8 55.1 7.4 0.5 0.0008
24.8 13.5 52.9 8.1 0.6 0.2328
51.9 71.2 74.8 72.6
41.3 61.4 71.2 68.9
CPX OPx Plag Oxide Apatite Residuals sR*
30.7 20.4
31.6 20.9
22.2 28.5
42.5 5.9 0.5 0.0027
42.1 5.0 0.5 0.0027
43.4
Equilibrium mineral compositions %An 13.6 %Fo 87.4 Mg# cpx 89.3 Mg# opx 88.1
70.2 85.0 87.1 85.8
66.5 81.4 84.0 82.4
61.8 71.5 80.5 78.7
57.0 74.9 78.1 76.1
cpx-liq 0.25
opx-liq 0.28
K,(CalNa)
pl-liq 1.20
G(FelMg)
ol-liq 0.30
higher than any published. For example, using the maximum values for D, given by Gill ( 1981) reduces calculated Zr from 325 to 285 ppm at 64% SiOz, still 35 ppm in excess of that observed (14% too high). Other HFSE behave similarly. More complex in situ (Nielsen, 1990) or RTF (O’Hara, 1977) differentiation processes do not affect the rate of increase of Zr with SiO*. A more plausible mechanism is combined assimilation-crystal fractionation - De Paolo, has been some continental arc such as Andean CVZ,
5.5 0.5 0.0069
and crustal contamination (e.g., 6”O 87Sr/86Sr al., 1984; James, 1984). element ratios (e.g., with fractionation also reflect AFC processes, even distinction between and assimilant al., 1988). High ible element ratios (e.g., Fig. in lower BVG suite not vary significantly with fractionation, but the abundances elements in assimilant are small and/or are greatly different from those in fractionating
B. Beaiioe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
91
Table 3 Smoothed trace-element
compositions
modelled by Rayleigh fractionation
AV52
AV54
Calc
AV56
cak
AV58
cak
AV60
talc
AV62
WC
AV64
Calc
265 900 120 24
135 450 140 26
81.2 409.5 149.4 27.6
66 240 166 28
30.8 226.4 178.4 30.8
28 115 191 30
15.7 116.1 208.2 33.4
20 78 208 32
10.4 46.7 246.9 36.8
15 60 227 33
7.6 21.4 287.4 40.0
11 35 250 34
6.0 10.7 324.5 42.4
Nb 10.7 Ce 44 Th 6 V 210 melt (F)
11.7 48 6.6 200 0.786
13.2 53.3 7.5 179.4
12.4 52 7.2 188 0.823
15.6 62.3 9.1 164.6
12.6 58 8.1 169 0.841
18.1 71.1 10.7 147.2
13.3 64 9.3 139 0.826
21.0 82.8 12.7 113.5
14.2 72 10.6 107 0.843
D
CPX
OPx
Plag
Oxide
Ap
Ni Cr Zr Y
Ni Cr Zr Y Nb Ce Th V
01
15 1.1 0.01 0.01 0.01
3.5 10 0.25 1.5 0.3
8 7 0.08 0.45 0.35
0.01 0.01 0.03 0.04 0.025
10 32 0.4 0.35 1
0 0 0 15 0
0.4 0.05 1.1
0.02 0.15 1.5
0.1 0.005 0.01
0 0.5 24
15 1 0
0.009 0.01 0.03
Partition coefficients
derived from Nagasawa
(1970).
24.0 94.5 14.9 91.6
15.0 80 11.8 74 0.871
26.8 104.9 16.9 75.2
Pearce and Nony ( 1979). Gill (1981) and Reagan et al. (1987).
magma this affect may not be seen. Unfortunately, LILE such as K, Rb and Ba which are likely to be enriched in the assimilant are rendered useless by their mobility during secondary alteration. The influence of AFC in the lower BVG suite can be illustrated by Fig. 7 in which calculated Zr-SiOz trends for r (assimilation rate/crystal fraction rate) = 0, 0.2 and 0.4 are plotted together with the average lower BVG trend from Tables 2 and 3. In Fig. 7A the assimilant is average Skiddaw Group sedimentary rock (54% SiOz, 189 ppm Zr - O’Brien et al., 1985) which models bulk assimilation of upper crustal material. Compositionally average Skiddaw Group rock is not dissimilar to the BVG in many major and trace elements, although it is significantly more peraluminous. Consequently its assimilation does not greatly affect the slope of the SiO*-Zr trend. A feature of the BVG is the trend from diopside-normative to corundum-normative composition with differentiation (Fitton et al., 1982) - the crossover being at 5860% SiOz. This is a feature shared with other talc-alkaline suites (cf. Cawthorn and Brown, 1976), and the common occurrence of garnet in some upper BVG dacitic rocks, and
sporadic occurrence in some lower BVG andesites (Fitton, 1972), reflects this peraluminosity. Fitton et al. ( 1982) have suggested that this is due to assimilation of Skiddaw Group pelitic rocks. Thirwall ( 1989) also indicates involvement of Skiddaw Group or similar sedimentary rocks in BVG magma genesis based on Pb isotope data. However, AFC type calculations indicate that bulk assimilation of average Skiddaw Group composition cannot be much greater than r = 0.05 otherwise increase of bulk A1,03 will significantly exceed that shown in Fig. 5B. The trend to corundum-normative magmas, therefore, may reflect very minor bulk contamination by aluminous sediment, but can also be enhanced by clinopyroxene-dominated fractional crystallisation, especially under conditions of moderate pressure (and water acivity) due to the expansion of the diopside stability field relative to that of plagioclase (e.g., Baker and Eggler, 1987). In Fig. 7B an AFC model is shown based on assimilation of a scavenged crustal partial melt. The composition of such a contaminant can only be hypothetical, but a plausible analogue is the composition of the Ordovician Eskdale granite (Fig. 2). This
t?. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
92
kbar) fractionation of this assemblage to be the most common and viable explanation for the derivation of many andesitic suites. Although the modelling described above is crude, it suggests that high-level crystal fractionation processes involving the observed phenocrysts, allied with minor assimilation of partially melted wall rock, can largely account for the broad geochemical variation of the lower BVG from basaltic (mantle-derived) parental melts through to dacite. This does not, however, rule out a degree of mixing between magmas at different stages of evolution, as clearly exemplified by the presence of composite lava flows within the sequence (Allen et al., 1987)) or imply a single parental magma, as will be shown. 01 45
sl
55
a
70
66
75
sloz ---o-WI I
-
,so-o.z ----o-
r-0.4
-
BVG M
I
Fig. 7. Average lower BVG trend compared with calculated AFC Zr-SiOz trends for assimilation rate/crystal fractionation rate (r) = 0,0.2, 0.4. Partition coefficient for Zr as in Table 3. For Si02 an average bulk partition coefficient of 0.82 was used, derived from data in Table 2. Symhols at values of F (fraction magma remaining) as in Table 2. (A) Bulk assimilation of upper crust sedimentary rock (average Skiddaw Group composition). (B) Assimilation of silicic crustal partial melt (average Eskdale granite composition). See text for discussion.
has a restricted high-Si02 composition, is mildly peraluminous and has other chemical and petrographic features which suggest a derivation from a crustal source. It has an average Zr content of 79 ppm (O’Brien et al., 1985) and its assimilation clearly suppresses the rate of increase in Zr against SiOz (Fig. 7B). The same pattern is produced for the other HFSE which all have low abundances in the Eskdale granite (Nb = 12, Th = 9, Ce = 3 1, Y = 2 1 ppm) as might be expected for a crustal partial melt in which accessory minerals, such as zircon, are residual phases. However, the degree of assimilation required to model the observed BVG trend is small (r < 0.2)) and consequently does not materially alter the major oxide chemistry which is dominated by crystal fractionation effects. The controlling mineral assemblage defined above for the lower BVG has been given the acronym POAM (plag-ol/opx-augite-magnetite) by Gill ( 198 1) , who considered water-undersaturated, crustal-level ( < 10
6. Character of the magma source Cr-Y and Ni-Zr logarithmic plots (cf. Gill, 1981; Pearce, 1982) are useful in separating the influences of mantle melting from differentiation processes (Fig. 8A and B). On both plots the lower BVG data define steep linear trends, with the most mafic BVG samples falling close to mantle primary partial melt compositions in terms of Ni and Cr content. The tight correlation between Ni and Cr (Fig. 9) for these most basic rocks indicates that accumulation of olivine or clinopyroxene was not a causal factor. These rocks occur within the Birkby Fell (Devoke Water), Throstle Garth and Wrighthow basalt members (Fig. 2) and example compositions are given in Table 4. High Mg# [ lOOMg/ (Mg + Fe) ] in some basalts also confirm that these are close to primary magmacompositions and as such these should reflect closely the geochemical signature of the sub-BVG mantle source zone. On a MORB-normalised spidergram (Pearce, 1983), BVG basalts are typified by enriched Th and LREE together with a negative Nb spike (Fig. 10). The elements Rb, K and Ba are also strongly enriched, but rather variably, almost certainly due in part to secondary mobility. However, elevated abundances of these elements together with the more immobile Th are typical of a subduction zone component (Pearce, 1983; Pearce and Parkinson, 1993) introduced by aqueous fluids derived from a subducting slab. The lesser enrichments of Nb, Zr and possibly P over a baseline defined by Y and Ti (i.e., compared with MORB or oceanic island-arc basalts) are indicative of either a
93
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
1000
100 t
Calculated cry~tol
NI
x tracilonatlon path
i-
Cr
100
‘0 i 10
, ---
,
----------t-B
10
lCC+l
100 Zr
1
L--
-_1c~--___-,
10
100 Y
Fig. 8. (A) Log Ni-Zr diagram showing generalised partial melting curve based on non-modal melting of olv-diop-opx f gt peridotite with 2400 ppm Ni and 11ppm Zr. (B) Log 0-Y diagram showing partial melting curves for garnet-bearing and garnet-absent peridotite with 3200 ppm Cr and 5 ppm Y. Average crystal fractionation trends from Table 3. Ornamentation as Fig. 5.
within-plate asthenospheric (OIB), sub-continental lithosphere or lower crustal component. Hildreth and
1OOW
1
1
L-----x? 1
’
““‘1
10
’
100
Moorbath ( 1988) have argued with regard to a segment of the Chilean Andes that an OIB source or sub-continental lithosphere mantle are unlikely to be significant contributors to the geochemical signature of erupted magmas. Rather, they consider that subduction-modified, MORB-like asthenospheric mantle and lower crustal material dominate the geochemical and isotopic characteristics of derived magmas (MASH hypothe-
‘““‘1 1OW
NI 0.1
Fig. 9. Log Ni versus Cr diagram. Arrows indicate the effect of selective olivine or clinopyroxene accumulation. Chnamentation as Fig. 5.
4111(11/1111/ Sr
K
Rb Ba
Th
Nb La Ce
P
Zr
Ti
Y
Cr
Fig. 10. MORB-normal&d element patterns for lower BVG basalts (normalising values after Pearce, 1983).
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
94 Table 4 Representative
lower BVG basalt analyses
I Section:
DVK
2 DVK
3 SNB
4 SNB
5 HAY
6 HAY
7 TGB
8 TGB
9 TGB
Unit no.:
17
30
11
10
7
8
3
4
12
10
II
TGB 16
STT 1
wt.% 48.61 1.10 14.30 11.31 0.33 10.40 10.24 1.23 2.21 0.17
Total LO1
100.02 1.89
54.21 0.95 14.21 9.99 0.35 9.12 5.96 2.62 2.12 0.16 -100.35 2.13
pm Ni Cr V Zr Y Nb La Ce Rb Ba Sr Th
144 415 276 70 21 6 11 29 117 161 248 3
296 857 204 129 25 I 15 35 IO 622 173 5
Si02
TiOz Al203
FezOl MnO MgO CaO Na,O %O p205
Mg#
64.6
65.8
51.92 1.07 15.31 11.05 0.22 9.00 7.80 2.29 0.47 0.27
51.55 0.79 11.71 11.45 0.32 11.99 8.08 1.89 1.26 0.28
99.39 2.85
99.32 2.80
-
146 470 200 114 21 9 17 39 19 132 270 5 61.7
150 624 187 62 13 5 10 24 30 249 256 2 61.5
51.20
50.76 1.30 16.90 10.40 0.42 7.30 7.49 2.27 2.95 0.17 99.97 2.82
51 155 211 104 22 8 12 24 99 373 184 4
1.30 17.09 10.47 0.43 7.38 7.56 2.26 2.96 0.17
52.93 1.09 16.40 9.46 0.27 8.62 6.32 3.82 1.23 0.21
53.48 0.92 14.11 8.51 0.23 9.93 9.08 1.71 1.78 0.21
52.79 0.80 15.93 9.25 0.35 7.88 9.31 2.41 1.16 0.19
53.50
100.0 2.25
100.35 2.57
99.96 2.32
100.07 1.49
99.60 1.27
100.81 3.65
-
46 126 244 126 28 10 17 33 26 250 208 3
340 1083 203 130 26 11 22 44 30 155 252 7
58.3
58.1
Analyses 1.2, 7-10 Royal Holloway aad Bedford New College; analyses 36, “Total Fe.
sis) . However, in the Andean segment studied by Hildreth and Moorbath (1988) primary basalts are nowhere present and it is more plausible for the lower BVG, where basalts with near primary characteristics were erupted, that the pattern of increasing Y-Zr-Nb enrichment is attributable to the sub-crustal mantle wedge. The enriched levels of LREE relative to Nb-P (Fig. 10) suggest that subduction-modified enriched lithospheric mantle most likely acted as a source or interacted with ascending magmas, as has been suggested to occur in some volcanic provinces (e.g., Davidson et al., 1988; Davis and Hawkesworth, 1993). The comparatively high SiOz content of the high Mg# basalts (see Table 4) reflects the melting of mantle modified by the influx of subduction zone hydrous flu-
51.09 1.oo 15.37 8.97 0.27 10.56 9.24 2.57 0.74 0.22
70.0
11 British Geological
239 872 205 146 30 12 19 46 72 160 222 8 64.3
298 943 180 121 25 10 19 42 43 591 236 6 69.8
88 554 210 107 23 9 24 50 36 300 327 8 62.8
1.19 17.71 7.48 0.29 6.68 8.29 3.00 1.22 0.26
51 189 168 155 25 13 23 50 41 251 226 7 63.9
Survey. Totals exclusive of LOI.
ids and mobile alkalis, as suggested by Gill ( 198 1) . A clue to the nature of the source mantle may be indicated by the melting curves shown in Fig. 8B for garnet-present and garnet-absent lherzolitic assemblages. The Y levels in the BVG rocks are too high to be derived from the garnet lherzolite model based on commonly accepted Y levels within the mantle ( 500 m) and the overlying Wrighthow (basalt) Member. The Craghouse ignimbrite is zoned from 65% SiO, at its base to 57% Si02 toward its top (cf. Table 5)) typical of an ash-flow eruption from a zoned magma chamber (Smith, 1979). The overlying Wrighthow basalts have morphological similarities to the Throstle Garth Member, though they more commonly carry clinopyroxene phenocrysts. They also show relatively primitive geochemical signatures (Analyses SNB 11 and 12-Table 4)) suggestive of rapid uninterrupted rise to the surface.
8. Discussion 8.1. Magmatic cycles
Systematic geochemical variations with time characterise the lower BVG, and can be categorised into essentially three types: ( 1) A trend to more basic compositions with time (type AB) The lower group of lavas at Devoke Water
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothemd
c
a--
5
7--
=
6--
txxds
0:
Research 65 (1995) 81-110
CKI@lGUSe
'.
Zrnbf19
5 .-
5--
,*
4 .-
.
2 .-
,
17
=:
50.00
55.00
_,'
l
4 --
:,
3 --
'I ,'
4
3 --
.
:*
2 --
0
17
t 60.00
65.00
:
0.00
70.00
.
2.00
.-_ '.. I-: : 4.00
so2
(0 15 -
i
13 .12 --
,!o /'
I1 .-
't
15 '.t '\ *
2
8 -7-6--
14 --
_,'
13 -12 --
a, 1'
.
P c
11 .-
\
: \ ', '. 0' x \
i, \
i
.
f
.
__,_
loo
150
. '.__ ', .
l
. '\\ '~
2 .: 2m
:
:
4
250
300
350
Ii
17 1
‘.
.
4 -3 --
1. =
;.. 5 --
.
.,, ,:
:
_:/
i
;
2 --
50
.
,'
5 --
14
0)
.e
4 -3 --
1
8.00 10.00 12.00
\S'
9 .-
5 =
/
6.00 WJ
iA
14 --
10 --
103
100
10
< lea
Cr
Fig. 17. Chemical variation through the Stony Tam sequence. (A) Si02. (B) MgO. (C) Zr. (D) Cr.
display this trend and, as shown above, in fact comprise three successive subcycles which record the eruption of decreasingly differentiated magmas with time. At Haycock two cycles of this type occur, one at the base and one at the top of the sequence. (2) A trend to more acidic compositions with time (type BA) . The middle part of the Haycock sequence shows this characteristic as does the Stony Tarn section excluding the Throstle Garth basalts. (3) No overall trend with time (type C).The upper group of andesitic lavas at Devoke Water and the Throstle Garth basalt pile both show a range of compositions but no clear evolution of magma chemistry with time. It was shown earlier that differentiation of the lower BVG suite is dominated by crystal fractionation of mantle-derived magmas. Magmatic variation with time, as exemplified by the trends described, should
therefore reflect magma dynamics operating during ascent through, and ponding in, the crust. Type AB magmatic cycles have an analogue in the eruption of zoned pyroclastic deposits, such as the Craghouse tuffs, whereby the deposit presents a mirror image of the stratified compositional zoning in the precursor magma chamber. In the case of the zoned ashflow deposit, the timescale of eruption of the magma is orders of magnitude less than the timescale of the differentiation process. Within the shield-like lava sequences on Santorini, Huijsmans and Barton ( 1989) have documented several successive andesite-to-basalt magma cycles of this type. They point out that preservation of this type of cycle in a series of lava flows requires relatively high eruption rates due to the instability of the high thermal and compositional gradients in the chamber prior to eruption. They attribute compositional zoning to crystal-liquid fractionation as well
104
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
3x33
/
5500
to.00
65.00
7oJxJ
Si02
. . l . . . . . .
.
. . i 1
Fig. 18.Chemicalvariation throughthe Greyfriar sequence. (A) SiOz. (B) MgO. (C) Zr. (D) Cr. as mixing of evolved magma with primitive magma. At Arena1 volcano, Reagan et al. ( 1987) also record a small (56-54s Si02) type AB magma cycle as “stage 1” of recent activity resulting from the relatively rapid discharge of a zoned magma chamber due to the influx of, or recharge by, new magma. The generation of compositional zoning in calc-alkaline magma chambers is not now believed to be due to crystal settling, but to inhomogeneous crystallisation along the walls and roof zone (side wall crystal fractionation - SWCF) , with migration of buoyant liquids to form a density stratified chamber (McBirney, 1980; McBirney et al., 1985). Rapid discharge of magma is most likely as a result of new magma being emplaced from below. Huppert and Sparks (1980) show from theoretical considerations that input of hot magma triggers eruption of already present evolved magma. Baker et al. ( 1991) ascribe a type AB trend (basaltic andesite
to high-alumina basalt) within the Giant Crater lava field at Medicine Lake volcano to rapid effusion of a chemically stratified chamber due to input of primitive magma, although they model development of the zoned chamber as a combination of fractionation, wall-rock melting and mixing. Similarly, at Volcan Colima, Mexico, Luhr and Carmichael ( 1990) describe eruptive cycles that terminate with more basic products as being due to eruption rate outstripping fractionation, and probably due to chamber recharge by a new batch of magma. Acid-to-basic magmatic cycles within the Salmon Creek volcanics, Idaho, are also ascribed by Norman and Leeman (1990) to high rates of chamber recharge by mafic magma relative to crystallisation. At Devoke Water it has been demonstrated that three such cycles are related to the differentiation of separate magma batches. Thus the eruption of the sub-group 1 sequence can be viewed as being due to the emplace-
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (199.5) 81-1 IO
ment of sub-group 2 magmas. The lack of interaction or mixing between the sub-groups (cf. Fig. 12A) is evidence that eruption of the lavas was rapid and efficient (cf. Blake and Ivey, 1986). Type BA magma cycles are common within orogenie volcanic provinces (Gill, 198 1) . Paracutin volcano (Wilcox, 1954) is one of the best known examples, others being Jorullo volcano (Luhr and Carmichael, 1985), also from the Mexican monogenetic cone province, and Bagana, Papua New Guinea (Bultitude et al., 1978). Stage 3 of recent activity at Arena1 volcano also shows a small trend of this type, and Reagan et al. ( 1987) conclude that this is due to declining eruption and recharge rates such that side-wall crystal fractionation becomes a noticeable effect driving the magma to a more evolved composition with time. On Santorini Huijsmans and Barton ( 1989) record a trend to more acidic compositions at the top of the lava pile prior to caldera collapse and eruption of acidic pyroelastics. They attribute this to declining eruption rates which allow the development of a silicic magma chamber. A trend to more silicic composition with time in the Salmon Creek volcanics is related to cooling (and hence crystal fractionation) in the magma chamber and likewise due to reduced recharge (Norman and Leeman, 1990). Izalco volcano in Central America shows a small trend from more to less mafic basaltic andesite, which Woodruff et al. ( 1979) show is due to the crystal fractionation of a single parental magma. Long-term evolution of polygenetic stratocones is commonly toward more silicic compositions (Rose et al., 1977; Gill, 1981), and probably reflects declining eruption rates due to the dampening effect of increased lithostatic load in promoting longer residence time for magmas and hence increased differentiation. Thus this category of magma cycle can be related to periodic tapping of a fractionating chamber. Recharge and eruption rates are sufficiently low to allow differentiation processes to dominate the composition of the magma. By contrast, McMillan and Dungan ( 1986) attribute this type of cycle within a package of lavas within the Taos volcanic field of New Mexico to progressive mixing of a dacitic magma with a basaltic magma, though they have to invoke forceful injection of basalt in order to overcome viscosity and density contrasts that inhibit mixing. The middle (basic to acid) cycle within the Haycock sequence shows clear geochemical evidence of mixing (Fig. 15B). This may he a simple progressive
105
two-component mixing, or a series of fractionationmixing-eruption episodes. In either case, the timescale of eruption is likely to be longer compared to the type AB case. A type C magmatic cycle is evident within stage 2 of recent activity at Arena1 volcano (Reagan et al., 1987). This shows broadly constant SiOZ with time, although subtle changes in trace elements indicate that successive compositions were formed by progressive mixing of two magmas. Measured eruption rates are intermediate between those of stages 1 and 3 at Arena1 described above. In the dominantly basaltic Servilleta lavas of the Taos Plateau field, there is no systematic temporal change in composition and Dungan et al. ( 1986) attribute the bulk of geochemical variation to open-system mixing between basaltic and dacitic/ andesitic compositions, related to episodic replenishment by mafic magma. Stratigraphic section variation in lava geochemistry at Iztaccihutal volcano, Mexico, shows a type C trend-a dominant dacitic composition with occasional more basic spikes, but no overall trend with time. Nixon (1988) has shown that this quasi steady state is due to a long lived chamber being periodically replenished by basic magma, convectively and thoroughly mixed and undergoing dynamic crystallisation. Constant post-caldera dacite compositions were erupted over the last 2200 yr on Santorini and Barton and Huijsmans (1986) also accredit this to magma chamber recharge maintaining the temperature of the magma over this period. Within the Devoke Water upper group there is evidence for at least two magma batches (cf. Fig. 11) , one slightly more silicic occurring later in the sequence and exhibiting geochemical evidence for mixing. There is, however, no clear evolutionary trend with time and it is suggested that rates of differentiation (crystal fractionation) are roughly compensated by recharge, mixing and eruption. 8.2. Models for the lower BVG On the basis of the discussion above and the data presented, dynamic models for the Devoke Water and Haycock sequences can be developed and are illustrated in Figs. 19 and 20. Devoke Water
An initial batch of mantle-derived magma is emplaced into a sub-volcanic chamber where crystal fractionation, most likely due to side-wall crystallisa-
106
B. Beddoe-Stephens et al. /Journal of Volcanology and Geothermal Research 65 (1995) 81-110
f rapid srupUon of BATCH 1
Fig. 19. Schematic model for the Devoke Water sequence. (A) Development of a zoned chamber, probably by side-wag crystal fractionation (SWCF) and density stratification of liquids. (B) Rapid emption of chamber due to influx of new magma batch to form lava pile. Cycles (A) and (B ) then repeat two more times during development of the lower group lava pile. (C) Generation of new (upper group) magma chamber fed by possibly two discrete magmas ( UGMl and UGM2). one of which may be related to the lower group. Steady eruption rate broadly balances recharge, mixing and fractionation. Eruption from lower group chamber ceases.
tion and rise of buoyant differentiated liquids, leads to a compositionally stratified magma (Fig. 19A). This magma is expelled due to influx of a new magma batch, and forms the sub-group 1 sequence of lavas (Fig. 19B). This two-stage process is repeated for subgroups 2 and 3. The lack of interaction (i.e., mixing) between the three magma batches forming the lower group sequence indicates that the episodic emptying of the chamber was rapid and efficient. Composite lava flows elsewhere in the lower BVG succession (Allen et al., 1987) record evidence for localised mixing but where eruption occurred before homogenisation was attained. During eruption of the final lower group lavas, the initiation of upper group extrusion indicates the
development, or connection to the surface, of a separate magma chamber/plumbing system (Fig. 19C). At least one batch of magma contributing to the upper group lavas appears to be independent of the lower group (UGM2 - Fig. 19C) but, following cessation of lower group effusion, magma from the lower group chamber may have been channelled into the upper group system, resulting in mixing. The relatively constant composition over time of the upper group lavas indicates a discharge rate that balances crystal fractionation differentiation on the one hand with mixing and replenishment on the other. The one exception to this is the Great Whinscale Dacite, which may have fed to the surface via an entirely separate route.
Fig. 20. Schematic model for the Haycock sequence. (A) Initial development of zoned magma chamber as in Fig. 19A. (B) Influx of new magma initially rapidly empties chamber of zoned magma& followed by declining emption rate allowing crystal fractionation and mixing processes to drive empted magmas back toward andesitic compositions. (C) Hiatus in eruption during which a strongly zoned chamber becomes established. Eruption of dacite and acidic ignimbrites due to buildup of volatiles and/or influx of hot primitive basic magma. (D) Quiescent eruption of lower parts of zoned chamber or progressively mixed magma.%
B. Beddoe-Stephens et al. /Journal
of Volcanology and Geothermal Research 65 (1995) 81-110
Haycock
As at Devoke Water prior to the initiation of lava effusion, magma emplaced into a sub-volcanic chamber underwent differentiation by crystal fractionation processes and became compositionally stratified (Fig. 20A). Input of new magma similarly rapidly expelled at least part of the zoned magma as the series of lavas recording a zonation from dacite to basalt in the lower part of the sequence. Subsequent reversal of this eruptive compositional trend reflects waning discharge rates allowing magma differentiation processes to dominate (Fig. 20B). This is manifested by the jump to a more evolved composition during the hiatus in extrusion represented by the Eagle Crag Member (Fig. 4). These lava compositions do not lie on a simple fractionation trend, but a trend indicative of mixing. Most plausibly this can be viewed as a series of fractionation-mixingeruption events, whereby local (side-wall) crystallisation generates more evolved liquids that mix back into the main body of magma, analogous to the in situ fractionation model of Nielsen ( 1990). The eruption of the Seatallan Dacite records the entry of a new discrete magma batch into the scenario. This possibly represents the refilling of the same magma chamber in which the underlying lavas evolved, and which was responsible for their extrusion. The development of the highly evolved Seatallan Dacite suggests a hiatus during which fractionation could proceed. That the Seatallan Dacite was immediately succeeded by the local representatives of the Craghouse Tuff suggests that they are cogenetic and probably derived from a zoned chamber in which differentiation was advanced and volatiles built up (Fig. 20C). The top of the Haycock sequence records a return to effusive volcanism with a series of lavas trending from andesite to basaltic andesite (Fig. 20D). These appear to lie on the same chemical lineage as the Seatallan Dacite and Craghouse ignimbrites and can be interpreted as tapping the deeper levels of the same compositionally zoned chamber, possibly in response to recharge by, and possibly mixing with, a new batch of mantle-derived magma. Although the details of the two sequences described above are different, they contain common features. Most significantly for lower BVG magmatism is that these lava-dominated sequences record the uprise and differentiation of discrete pulses or batches of magma. Type AB cycles suggest episodes of relatively rapid eruption,
107
8.3. Relation to morphology and sub-volcanic plumbing of the volcanicfield Petterson et al. ( 1992) deduced from field characteristics that the lower BVG developed as a multi-centred, plateau-like lava field. It was stressed that a single, polygenetic stratocone was an inappropriate model and analogies were drawn with the monogenetic volcano provinces of Central America and the western USA. It was also pointed out that the bias within the lower BVG to basaltic andesite together with not uncommon fairly primitive basalts was a characteristic of plateau fields which develop within extensional or transtensional environments and which promote ready flux of magma from mantle source to surface (e.g., Bacon, 1990). These conclusions are enhanced by the detailed examination of magma chemistry described here. As described above, polygenetic stratocones commonly show a long-term trend to more silicic compositions (Gill, 1981). There is no evidence from the stratigraphic sections analysed from the lower BVG of this type of pattern: even though an individual section through the flanks of a stratocone would be unlikely to contain a complete eruptive sequence, it should still show evidence of the broad trend. Furthermore, a series of sampled sections through a hypothetical stratocone should show a broad correlation and cogenetic relations between magmas from each section. In the lower BVG there is clearly no chemical correlation between the Haycock and Devoke Water sequences, which are about 15 km apart. In addition, although the units within each section form coherent cogenetic suites, the two sections exhibit separate differentiation trends and thus two discrete plumbing systems must have been operating. Sequences at Greyfiiar and Stony Tarn are also quite different, indicating, on a very rough basis, that individual plumbing systems or magma chambers had a spatial frequency of greater than one every 4 km. The Wrighthow and Throstle Garth basalt members also indicate the presence of local&d volcanic centres erupting discrete magmas. This multi-centrism can also be extended to the mantle source. In addition to showing different magma cyclicity, Figs. 5 and 6 show that the Haycock basalts are depleted in MgO, Ni and Cr for a given Si02 content compared with those at Devoke Water, and the two sequences lie on separate crystal fractionation trends. Thus they cannot be related by, say, polybaric olivine
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( * clinopyroxene) fractionation of a common parental melt. This difference may be due to the degree of melting locally (cf. Fig. 8)) variable H20, or heterogeneous mantle depletion/enrichment. In the monogenetic volcanic field of the Cascade Range, Prueher and McBirney (1988), Bacon (1990) and Barnes (1992) similarly record the production of discrete batches of partial melt with different source characteristics feeding different volcanic centres. At Taal volcano Miklius et al. ( 199 1) interpret contemporaneous but different magmatic lineages as evidence for discrete magma batches processed independently through separately evolving plumbing systems. Similarly, at Tatara-San Pedro volcano, the production of different parental magmas feeding isolated sub-volcanic reservoirs is inferred by the lack of chemical interaction evident between different lava types, despite their temporal and spatial overlap (Ferguson et al., 1992). Whatever the process in detail, it is evident that the lower BVG field developed over a broad zone of melt production with numerous channelways feeding a series of fissures and/or vents. Contemporaneous volcano-tectonic activity (subsidence and faulting) during the aggradation of the volcanic field is likely to have been a controlling factor in the establishment of the sub-volcanic plumbing system. Localised episodes of crustal tension, manifested as block faulting (Petterson et al., 1992), would facilitate recharge of magmachambers and rapid eruption of type AB lava sequences whereas concomitant compressive volcano-tectonism would reduce magma flux, promote magmatic differentiation and lead to type BA trends.
(4) Individual stratigraphic sections reveal highly systematic magmatic evolution trends (or cycles). These record the flux from mantle to crustal chambers of discrete batches of melt which differentiated by crystal fractionation coupled with some internal remixing. Acid to basic eruptive sequences represent the rapid evacuation of compositionally zoned magma chambers, probably due to recharge by subsequent pulses of magma. Basic to acid eruptive sequences indicate reduced eruption rates whereby ongoing crystal-liquid differentiation dominates the composition of each subsequent unit. Sequences that show no overall trend with time suggest recharge, mixing and crystal fractionation broadly balancing eruption rates. The development of these magmatic cycles probably reflects the control of changing stress patterns on magmatic plumbing within an actively subsiding volcanic field. (5) Despite magmatic coherence within individual sections or localities there is no correlation of magmatic cycles between adjacent areas. This indicates that a number of crustal chambers or plumbing systems were operating contemporaneously and independently, consistent with the view of the lower BVG as a multicentred, plateau-like volcanic field. (6) Magmatic compositions for the different areas lie on subparallel but significantly different differentiation trends suggesting that the production of partial melt and the degree of differentiation suffered prior to eruption varied across the area. This further indicates melt production over a wide area and its ascent via a number of independent channelways into the crust.
Acknowledgements
9. Conclusions ( 1) The lower BVG ranges from basalt to dacite.
Lavas dominate the succession, basaltic andesite to basic andesite (53-60s SiOZ) being most common. Acid andesite to dacite compositions include localised ignimbritic pyroclastic rocks. (2) The overall compositional variation can be ascribed to POAM-type crystal fractionation processes allied with minor assimilation of crustal partial melts. (3) Parental magmas were derived by partial melting of enriched (possibly lithospheric) mantle, further enriched in Th and LIL incompatible elements by subduction-derived aqueous fluids.
Thanks are due to Prof. Ray Macdonald and Dr. Paul Henney for their helpful comments on various drafts of the manuscript. Dr. Eric Johnson provided samples and stratigraphical details for the Greyfriar section. Dr. Godfrey Fitton and an anonymous reviewer are also thanked for their comments. This paper is published with the permission of the Director, British Geological Survey (NERC). It forms part of the Lake District regional survey, a multidisciplinary programme of mapping and related work.
B. Bedoe-Stephens
et al. /Journal of Volcanology and Geothermal Research 6.5 (1995) 81-110
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