Post-Subduction Alkaline Volcanism Along the Antarctic Peninsula

Journal of rhe Geological Society, London, Vol. 145, 1988, pp. 985-998, 14 figs, 2 tables. Printed in Northern Ireland

Post-subduction alkaline volcanism along the Antarctic Peninsula

M. J. HOLE British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK

Abstract: Following the cessation of subduction along the Antarctic Peninsula, there was a change in the character of the magmatism from calc-alkaline to alkaline. A Miocene-Pliocene suite of basanites, tephrites and alkali and olivine basalts from Alexander Island, south-west Antarctic Peninsula, was erupted between 18 and 30 Ma after the cessation of subduction. These basalts exhibit incompatible trace element characteristics similar to those reported for continental alkali basalts and OIB. Major andtrace element variations are consistent with an origin by low butvariable degrees of partial melting of agarnet lherzolite source withresidual garnet f clinopyroxene, followed by limited fractional crystallization of olivine and clinopyroxene. Sr- and Nd-isotope compositions cover alimited range of 87Sr/ssSr 0.70275-0.70343 and 143Nd/144Nd0.51298-0.51286. These ratios are consistentwith the derivation of the basalts from a depleted source region broadly.similar to that of MORB and non-Dupal OIB. Correlated LILE/HFSE-isotope trends demonstrate that the olivine basalts are enriched in the LILEand 87Sr butdepleted in 143Nd relativeto other Alexander Island samples.The LILE- enrichment is attributed to the inheritance of a subduction component from mantle material which previously constituted the mantle wedge during Mesozoic subduction along the Antarctic Peninsula.

The traceelement and isotopiccharacteristics of subduction- alkali basalts were generated and to establish the related magmatic rocks havebeen the subject of much relationship between ridge crest-trench collisions and attention in- the literature. However, relatively few studies alkaline magmatism. have beencarried outon the character of magmatism following the cessation of subductionalong continental destructive plate margins (e.g. Rogers et al. 1985; Saunders Geological setting of the post-subduction basalts of et al. 1987). Alexander Island Following morethan 200 Ma of subduction-related The geology of Alexander Island is dominated by a magmatism along theAntarctic Peninsula,a sequence of Mesozoic accretionary prism complex comprising athick, ridge crest-trench collisions took place resulting in the variably deformedmetasedimentary sequence with local almost complete cessation of subduction by 4Ma (Barker occurrences of thrust-soled slices of pillow lava and 1982). Following ridge crest-trench collision there was a associated cherts (Burn 1984). Theinterpretation of the change in the character of the magmatism from calc-alkaline complex as an accretionary prism carries the implication that to alkaline. The alkaline volcanic rocks occur in scattered it is underlain by Mesozoic oceanic crust. A major Tertiary exposures along the length of the Antarctic Peninsula batholith (46 f 3 Ma, Pankhurst 1982) intrudes the accre- (Smellie et al. 1988). In the north (James Ross Island, Seal tionary prism complex in the nolth and there is a narrow, Nunataksand ArgoPoint), Plio-Pleistocene alkali and but laterally extensive belt of Tertiary (40-60 Ma) olivine basalts were erupted to the east of the Mesozoic- calc-alkaline andesitesand dacites which are thought to Tertiary magmatic arc (Nelson 1975; Saunders 1982) represent coeval volcanic cover (Burn 1981). West of the whereas in the south at AlexanderIsland (Fig. l), Antarctic Peninsula the oceaniccrust is segmented by at post-subduction alkalic basalts overlie a Mesozoic accretion- least 10 discretefracture zones. One of thesezones, the ary prism complex (Bell 1973; Care 1980; Burn & Thomson Heezen Fracture Zone lies offshore of the central portion of 1981). Alexander Island. In the south ridge crest-trench collision The sequence of plate motions which led to the change occurred between 50 and 39 Ma,whereas subduction of from acontinental destructive plate margin to a passive oceanic crust continued beneath northern Alexander Island plate margin are relatively well constrained (Barker 1982). until 25 Ma with decoupling from the stationary southern Collision times vary from c. 50 Ma opposite southern segment along the line of the Heezen fracture zone (Barker Alexander Island to less than 4 Ma opposite Anvers Island, 1982). Post-subduction volcanism commenced ataround The SouthShetland Islands trench represents the last 7 Ma in northernAlexander Island and continued forat remaining site of active subduction in the region. With its least 5 Ma in that area, whereas no post-subduction volcanic well constrained history of platemotions, the Antarctic rocks older than 2.5 Ma have yet been found in southern Peninsula is an ideal region to examine the geochemical Alexander Island. characteristics of post-subduction magmatism at a continen- Figure 2 illustrates the relationship between the cessation tal destructive platemargin. The presentstudy considers of subductionand calc-alkaline magmatism and the post-subduction alkalic basalts in AlexanderIslands, initiation of post-subduction alkaline volcanism. Based on south-west Antarctic Peninsula. Acomprehensive set of the ridge crest-trench collision times of Barker (1982) and trace elementand Sr- andNd-isotope data is used to the published K-Ar ages for Alexander Island basalts (Table examine the character of the mantle source from which the l), the shortest period between cessation of subduction and

985

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 986 M. J HOLE

~ I 5% 70 I 1 Ma iLLL3-L Collision time Fig. 2. Illustration of the timing ridge crest-trench collisions and the onset of alkaline magmatism. Block shaded areas represent periods of alkaline volcanism and the open rectangle represents the period of Tertiary calc-alkaline magmatism. Note that southof the Heezen fracture zone (HFZ), ridge crest-trench collision was Fe. 1. Simplified geological map of Alexander Island showing the oblique. Pecked lines represent the landward trace of oceanic distribution of the post-subduction basalts. Inset: location of fracture zones. After Barker (1982), with additional geochronologi- Alexander Island off the west coastof the Antarctic Peninsula. cal data from Smellie et al. (1988). TFZ, Tula fracture zone; ThFZ, Location of post-subduction basalts: 1, Mount Pinafore, Elgar Tharp fracture zone. Uplands; 2, Hornpipe Heights; 3, Rothschild Island; 4, Mussorgsky Peaks, Beethoven Peninsula; 5, Gluck Peak, Beethoven Peninsula. + , Tertiary granitoids; V, Tertiary calc-alkaline volcanics; stipple, basalt. Spine1 lherzolitenodules are common within the Jurassic to Cretaceous fore-arc sediments(Fossil Bluff Formation); flows and as clasts within the hyaloclastites. The Rothschild , Mesozoic accretionary prism metasediments (LeMay Group). Island exposures are likely to representremnants of a similar sequence to that described for Mount Pinafore (Care 1980). theonset of alkaline volcanism is 18 Ma fortephrite sequences in northern Alexander Island. Olivine basalts in southernAlexander Island post-datethe cessation of The agglutinate sequences subduction in that area by more than 30 Ma. A thin sequence (maximum ?20 m) of subaerial lapillistones, tuffs and incipiently to completely welded agglutinates crops Petrography and mode of occurrence of the basalts out atHornpipe Heights(Fig. 1). The wholesequence The petrography and field characteristics of the basalts are unconforrnably overlies an irregular surface of LeMay group described elsewhere (Smellie 1987; Hole et al. in press) and metasediments. The basal lapillistone bed is laterally only details pertinent to the presentstudy will be given here. discontinuous and infills and drapes over irregularities in the Three distinct modes of occurrence are recognized: underlying metasediments. Much of the exposure consists of (1) columnar jointed flows with associated hyaloclastites, thin massive flows (<0.75 m) interbedded with thicker (up (2) agglutinate sequences, to 6 m) agglutinates with welded lenses. The agglutinates are (3) pillow lava, pillow breccia and palagonitized vol- almost entirelymade up of brick red, aerodynamically caniclastic sequences. moulded, incipiently welded bombs of olivine-phyric basalt. Fragments of spine1 lherzolite inclusions areabundant Columnar jointed flowsand associated hyaloclastites throughout the sequence. Amphibole megacrysts are also a common feature of the finer grained agglutinate beds. The Columnarjointed flows and assoicated hyaloclastites are lavas are olivine-phyric with a fine-grained groundmass of geographically restricted to the area around Mount Pinafore plagioclase laths, greenish clinopyroxene andopaque and Rothschild Island (Fig. 1). The flows are up to 60 m in oxides. Rare,scattered, partially resorbedpurple clino- thickness, are composed of dark, fine-grained or glassy pyroxene crystals within this sequence are likely to be olivine-phyric basalt and unconformably overlie accretionary xenocrystal in origin (Smellie 1987; Hole et al. in press). prism metasediments or Tertiary calc-alkalineandesites. The local presence of basal diamictites with glacially striated calc-alkaline volcanic rocks led Burn & Thomson (1981) to Pillow lava and pillowbreccia sequences suggest thatthese flows were eruptedduring a Miocene Thick sequences of weakly palagonitized volcaniclastic rocks intraglacial period. The hyaloclastites are predominantly of and associated pillow lava and pillow breccia crop out on palagonitized lapillistones with rare clasts of pillow-form five isolated nunataks on Beethoven Peninsula, south-west

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 Alexander Island (Fig. 1). At Mussorgsky Peaks 200 m of (16.0-16.7%). CaO abundances are relatively consistent for interbedded palagonitized lapillistones and tuffs are overlain the basanites, the samplesfrom Rothschild Island having by approximately 100 m of pillow basalt and pillow breccia. higher absolute CaO abundances (c. 10%) than the samples Individual pillows are isolated and are separated by up to from Hornpipe Heights (Fig. 3). The olivine basalts exhibit 3 m of poorly sorted, weakly palagonitized pillow breccia. a slight increase in CaO with increasing F/(F + M). The This sequence is similar to the subglacial 'tindar' deposits of tephritesfrom KG.3616 show a negative correlation with SW Iceland (Jones 1970). All the samples collected from F/(F + M) ratiowhereas the alkali basalts exhibit consis- this area arefresh, fine grained olivine-phyric or tently high CaO abundances (c. 10%). Ni andCr olivine-vitrophyric basalts, with rare plagioclase-phyric abundances are variable for all groups of basalts and exhibit basalts occurring at Gluck Peak, south-eastBeethoven alinear negative correlation with F/(F+ M) ratios. The Peninsula. Anumber of dykes cross-cut the volcaniclastic basanites cover a similar range in abundancesCr rocks andappear to be feedersfor the overlying pillow (167-495 ppm) to that for the olivine basalts (113-316 ppm) lavas. andtephrites (177-347 ppm)whereas the alkali basalts contain less than 120 ppm Cr.The olivine basalts and tephrites have noticeably lower Ni abundances (70- Geochemistry of the post-subduction basalts 140 ppm)than the basanites (120-340 pprn), the alkali Major elements were analysed on fused glass discs by standard XRF basalts having the lowest Ni contents (<50ppm) of any of methods at the University of Keele. Trace elements were analysed the post-subduction basalts. Total alkalis rangefrom a on pressed powderpellets by standard XRF techniquesat the maximum of 7.2% for the basanites to 5.2% for the alkali University of Nottingham. To ensure compatibility of data, 18 rock basalts. The olivine basalts, alkali basalts and tephrites form sampleswere analysed fortrace elements at both the Keele and a slightly more sodic series (Na,O/K,O 2.1-2.4) than the Nottingham laboratories. In allcases therewas good agreement basanites (Na,O/K,O 1.6-1.8). between the two laboratories. The rare earth elements (REE) and Absoluteabundances of incompatibletrace elements Th, Hf and Ta were analysed by instrumental neutron activation generally increase in theorder olivine basalt < alkali analysis at Royal Holloway and Bedford New College, London. Nd- basalt < tephrite < basanite. For example, Nb abundances and Sr-isotope compositions were determined on a multi-collector VG354 mass spectrometer at the British Geological Survey Isotope increase from 22 ppm in the olivine basalts to a maximum of Geology Unit. Representative whole-rock geochemical analyses and 84 ppm in the basanites. However, Y (23-28 ppm) and Yb details of standarddata for isotopedeterminations are given in (1S-2.2 ppm) abundances are relatively constant across the Table 1. The full set of analytical data will bestored in the completerange of compositions. Binary trace element UK-IGBA data file from which it can be retrieved via the National variation diagrams (Fig. 4) reveal a number of trace element Geochemical Data Bank of the British Geological Survey or from trends for the Alexander Island samples. On Zr vs Y and the World Data Center-A. Nb vs Y plots all the Alexander Island samples form a subhorizontal array. Zr vs Nb and Sr vs Nb plots delineate The post-subduction basalts cover a broad compositional two positive linear correlations; the alkali and olivine basalts spectrum from slightly silica saturated (<5% normative hy) and tephrites have higher Zr/Nb and Sr/Nb ratios (6-7 and olivine basalts to Si-undersaturated (up to 16% normative 17-20, respectively) than the basanites (Zr/Nb 2-4, Sr/Nb ne) basanitoids. According tothe classification scheme of 11-12). Large ion lithophileelement (LILE) vs Nb plots De La Roche et al. (1980), the northern Alexander Island exhibit three positive linearcorrelations. The Beethoven samples representa tephrite-basanite association whereas Peninsula olivine basalts have consistently higher Ba/Nb, the southern Alexander Island samples fall within the fields Rb/Nb,K/Nb and Th/Ta ratios thanthe alkali basalts, for alkali and olivine basalts (Smellie 1987). Lava sequences tephrites and basanites. All the samples analysed are from individual outcrops are compositionally restricted and LREE-enriched with La/Yb, ratios increasing from 7 to 9 can be divided intofour distinct groups based onthe for the olivine basalts up to 13 for the basanites (Fig. 5). predominant rock type at eachexposure. The lavas from Analysis of Sr- and Nd-isotopic compositions have been Elgar Uplands are predominantly tephrites(up to 8% carried outon representativea set of samplesfrom normative ne) with local occurrences of basanite, whereas AlexanderIsland. The post-subduction basalts exhibit the Hornpipe Heightsand Rothschild Island occurrences restricted ranges of both 87Sr/R6Srratios (0.70275-0.70342) (11-16% normative ne) are dominated by basanites with and '43Nd/'"Nd ratios (0.51286-0.51296) (Fig. 6, Table 1). minor occurrences of tephrites. Hy normative olivine basalts The alkali basalts and basanites have almost identical are restricted to Mussorgsky Peaks, whereas the remainder Sr- and Nd-isotope compositions (87Sr/R6Sr 0.70300, of the Beethoven Peninsula outcropsare slightly ne I4'Nd/'"Nd 0.51296) whereas the tephrites have unusually normative (up to 4%) alkali basalts. low X7Sr/86Srratios (0.70275) but similar '43Nd/'"Nd ratios The basanites are characterized byhigh MgOcontents to the basanites (0.51294). The olivine basalts contain more (8.5-11.2%) whereas MgO contents vary from 6.5 to 8.5% radiogenic Sr and less radiogenic Nd (H7Sr/H6Sr0.70342, in the tephrites and olivine basalts. The alkali basalts do not 143Nd/'"Nd 0.51286) than the other post-subduction basalts. exceed 5.5% MgO. Al,03 shows a progressive increase with increasing (Fe0 + Fe,O,)/(FeO + Fe,O, + MgO) ratio [F/(F + M)] for each group of basalts (Fig. 3). The basanites Basalt type and tectonic setting increase from 13% Al2O3at F/(F + M) = 0.46 to 15% Al,O, Onthe Ti-Zrplot of Pearce (1982) (not shown), the at F/(F+ M) = 0.56 whereas the olivine basalts increase majority of the Alexander Island samples plot above the line from 15%AlZO3 at F/(F+ M) = 0.56 to 16% A1,0, at dividing basic and evolved compositions, and have Ti and Zr F/(F + M) = 0.61. The alkali basalts from Beethoven contents characteristic of within plate basalts (WPB). Only Peninsula and the tephrites from locality KG.3616 exhibit the tephrites from Mount Pinafore (locality KG.3616) plot the highest F/(F + M) ratios (c. 0.65) and A1,03 contents in the area for evolved rocks. In Ti/Y-Nb/Y space (Fig. 7)

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 2 c! U M

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 A NT ARC TIC PENINSULA ALKALINE VOLCANISMANTARCTICALKALINEPENINSULA 989

- 200 -20 /- / / l- /77

- 100

I I I I

-'O.O CaO - 9.0 Q '4 '-. - 8.0 \- - '\ .3 \* I l \$ I

Fig. 4. Binary trace element plots for Alexander Island samples. Lines of constant inter-element ratio are shown on each plot. Symbols as for Fig. 3.

consensus being that it is impossible to distinguish between the two on both trace element and isotopic grounds. As the Fig. 3. Ni, CaO and A1,0, vs (Fe0 + Fe,O,)/(FeO + Fe,O, + Alexander Island post-subduction basalts were erupted onto MgO) [F/(F + M)] ratios for the Alexander Island basalts. accretionary prism rnetasediments, they must be considered F/(F M) ratios calculated from total Fe Fe,O,as XRF analyses. + to represent continental alkali basalts rather than OIB. As W, Olivine basalts; A, alkali basalts; tephrites; a, basanites. +, mentionedabove, the accretionary prism is likely to be The higher CaO basanites are from Rothschild Island, the remainder are from Hornpipe Heights. underlain by Mesozoic oceanic crust, and the provenance of the prism metasediments is likely to have been the Mesozoic magmatic arc. The crust through which the post-subduction all the samples are unequivocally identified asWPB with basalts were erupted can thus be considered to be similar Ti/Y and Nb/Y ratios to ocean island basalts (OIB) geologically immature. Thiscontrasts with the geological from Gough and Bouvet islands. setting of most continental alkali basalts which were erupted On primordial mantle normalized multi-element plots all through ancient continental crust and mature subcontinental the basalts exhibit smooth convex upwards patterns which lithosphere.Thus the Alexander Island post-subduction peak close to Nb and Ta and converge at Y (Fig. 8). This basalts have a greater affinity to OIB than continental alkali type of pattern is also exhibited by continental alkali basalts basalts in terms of tectonic setting. from Africa, the western USA, and Greenland (Thompson et al. 1983, 1984) and some Pacific and Atlantic OIB (e.g. St Partial melting and fractional crystallization trends Helena, Mangaia, Rurutu, Bouvet Island; Verwoerd et al. The convex-upward shape of the primordialmantle 1974; Le Roex & Erlank 1982; Palacz & Saunders 1986). normalized diagrams, their convergence at Y values and the The similarity between OIB and continental alkali basalts observed fractionation of the LREE relative to the HREE has been discussed in some detail by a number of workers suggests that Y and the HREE were compatible during the (Thompson et al. 1983; Fitton & Dunlop 1985), the general generation of the post-subduction basalts. Thiscontrasts

Table 2. Average LILEINb and ThlTa ratios and l-sigma standard deviations for the Alexander Island basanite-tephrite sequence. Average N-type MORB values from Saunders & Tarney (1985).

BASANITETEPHRITE ALKALIBASALT OLIVINE BASALT MORB

Th/Ta 1.20 f 0.20 1.19f0.14 1.32 2.281.17 f 0.21 Rb/Nb 0.37 f 0.14 0.30 f 0.06 0.43 0.77 f 0.07 0.40 Ba/Nb 4.0 f 0.14 3.00 f 0.30 4.30 6.904.80 f 0.23 K/Nb 280 f 22 337 f 31 420 510 f 11 332

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 990 M. J. HOLE

100

z0 U 0 5 \* 0 a

KG.3624.6

KG.3619.1 0 KG.3624.10 A

KG.3606.9 A KG.3620.7 m KG.36 16.5 A KG.3605.1 KG.3627.8 + KG.3609.1 o KG.3612.5 II I II I II II I It I 1 La CO Nd Sm Eu Tb Yb Lu La Ce Nd Sm Eu Tb Yb Lu Fii. 5. Chondrite-normalized rare earth element plotsof (a) olivine basalts, alkali basalts and tephrites; KG.3627.8 alkali basalt, KG.3616.5 and KG.3609.1 tephrites; the remainder are olivine basalts;(b) basanites. KG.3619.3 from Rothschild Island, others Hornpipe Heights. Analysed by INAA (see Table 1).

1000

Ti/Y

14a 144 Nd/ Nd 500

0.5 133

0.5131

0.5 129 Society Imlands / -..

Gough Tristan I 1 I 1 0.5125 c 1 2 34 NWY 0.70300.7040 0.7050 0.7060 Fig. 7. Ti/Y vs Nb/Y for the Alexander Island basalts, OIB (Gough and Bouvet islands) and E-type MORB (mid-Atlantic ridge 87 88 45" north). The field for within-plate basalts (WPB), outlined by Sr/ Sr dashed line, is taken from Pearce (1982). Data for alkali basalts Fe. 6. 143Nd/144Ndvs ''Sr/%r for Alexander Island samples and from mid-Atlantic ridge45" north are included to demonstrate that alkali basalts from Marie Byrd Land (MBL; Futa& LeMasurier the Alexander Island basalts are not enriched MORB. Data 1983). For comparison, fields for MORB and someOIB are also sources; Gough Island, Le Roex(1985): Bouvet Island, Venvoerd shown (data from White 1985; Palacz & Saunders 1986). Symbolsas et al. (1974): MAR 45" north, Tarney et al. (1978). Symbols as for for Fig. 3. Fig. 3.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 A NT AR CTIC PENINSULA ALKALINE VOLCANISMANTARCTICALKALINEPENINSULA 991

1O( 1 oc

W W -l 2 c c 2 2 4 I 2 \ \ Y Y V V P P

la 10

1111111111111111

Rb Ba Th K Ta Nb La Ce Sr Nd P H1 Zr Sm TI Y Rb B. Th K Ta Nb La Ce Sr Nd P Hi Zr Sm TI Y Fig. 8. Primordial mantle-normalized multi-element plots for(a) Alexander Island basanites (open symbols) and olivine and alkali basalts (filled symbols); (b) continental alkali basalts and OIB: 0,San Quintin post-subduction alkali basalt, Baja California from Saunderset al. (1987): B,St Helena OIB and +, Tanzania continental alkali basalts from Thompson et al. (1984): (-) Bouvet Island from Le Roex & Erlank (1982): (- - - -) N-type MORB from Saunders& Tarney (1985). Normalizing values shown on the abscissae are from Wood et al. (1979).

with the incompatibility of most of the trace elements and However, limited fractional crystallization of the suggests that all the basalts could havebeen derived by observed phenocryst phases is evidentfrom the observed partial melting of asource leaving an HREE-, Y-rich decrease inNi with increasing F/(F+ M) ratios for each residuum. As Nb abundances show an progressive increase individual group of basalts. Furthermore, correlations with increasing degree of silica undersaturation (Table l), between compatible and incompatibleelements, (e.g. Ni Nb vs Y correlationsshould allow examination of partial melting and fractional crystallization trends. Figure 9 shows that Nb/Y ratios increase with increasing Nb abundances, the olivine basalts exhibit the lowest Nb/Y ratios (0.95-1.2) 3’ whereas the basanites have the highest Nb/Y ratios (up to 3.5) and highest Nb contents.Calculated fractional *a crystallization vectors for 50% crystallization of the 2. : observed phenocryst phases, in this case olivine plus minor *** plagioclase, from a theoretical starting composition are also shown in Fig. 9. Due to the similarity in K, values for Y Nb/Y and Nb in olivine and plagioclase (and also clinopyroxene; Pearce & Norry 1979), fractional crystallization vectors for these phases are subhorizontal. Therefore, fractional crystallization of an olivine + plagioclase + clinopyroxene assemblage should result in progressive increases in Nb content but relatively constant Nb/Y ratios.This is illustrated using data frompost-subduction alkalic basalts c from the Salahi Unit of Oman (Alabaster er al. 1982) and OIB from Bouvet Island (Venvoerd er al. 1974; Le Roex & Erlank 1982). Inboth thesecases, incompatible trace 20 40 60 00 100 element variations have been attributedto high-level fractional crystallization of an olivine + plagioclase + Nb PPm clinopyroxene assemblage, resulting in sub-horizontal trends Fig. 9. Plot of Nb/Y vs Nb. Fractional crystallization vectors for in Fig. 9, which are in good agreement with the calculated 50% crystallization of olivine (01) plagioclase (pl) and clinopyroxene fractional crystallization trajectories. The much steeper (cpx) calculated using the K, data of Pearce & Norry (1979) and trend defined by all the post-subduction basalts from simple Rayleigh fractionation (CI/Co = p,-’)) are also shown. Alexander Island implies that fractional crystallization Data for WPB fractional crystallization series from Oman arefrom involving the observed phenocryst phases must have been Alabaster er al. (1982) and Alabaster (pers. comm.) andfor Bouvet limited during their genesis. Island from LeRoex & Erlank (1982). Symbols as for Fig. 3.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 992 M. J HOLE

trend delineated by the Nb-Y correlation and variations in I Ni B thedegree of LREE-enrichment.The low, but relatively e I consistent Y and HREE abundances across the whole range of compositions suggests that the degree of partial melting must have been insufficient to completely consume garnet in all cases. It is difficult to placeabsolute values on the degrees of partial melting required to produce the different basalt types. Pearce & Norry (1979) modelled simple equilibrium partial melting trends for thegeneration of basaltic melts, and estimated that a steep trend on plots of 5tS incompatible elements vs Y andthe HREE would be l produced by between 15% and30% melting of a garnet \a lherzolite. Clague & Frey (1982) suggested thatHawaiian b I basanites and alkali basalts were generated by >5.5-11% I partial melting of garnet lherzolite. More extreme estimates l 10 t of degrees of partial melting were modelled by Fitton & l Dunlop (1985) who suggested that alkali basalts of the l CameroonLine could havebeen generated by <0.34% l partial melting of MORB-likea source. Using similar 15 !l- calculations, Smellie (1987) suggested limiting values of 0.6-0.9% melting of a MORB-like source for thegeneration I 15 $ of the Alexander Island basanites. The specific degrees of melting required to produce the Alexander Island basanite 20 94 40 60 80 Nb . to olivine basalt sequence are not fundamentalto the present study. It is sufficient to state that the trace element Fig. 10. Plot of Ni vs Nb for the Alexander Island post-su uction basalts. The trendsfor olivine fractional crystallization were data presentedhere are in generalterms consistent with calculated using the Rayleigh fractionation equation with an derivation of the all Alexander Island basalts from a garnet assumed DNi of 10 and DNbof 0.01 (Pearce & Norry 1979). lherzolite source with residual garnet. However, it is Numbers on trendA are for percentage crystallization from starting important to state that relative degrees of partialmelting composition KG.3624.11 for which Ni = 150 ppm and Nb = 23 ppm. decrease in the order olivine basalt, alkali basalt, tephrite, The basanite trend (B) was calculated in the same way using a basanite. starting composition with Ni = 250 ppm and Nb= 62 ppm. Note that there is little variation in Nb abundancesfor more than 20% olivine Incompatible trace element fractionation crystallization. Symbols as for Fig. 3.

andNb; Fig. 10) exhibit steep linearcorrelations. In the High field strength element fractionation olivine basalts for example, Ni abundances fall from 170 to For the Alexander Island samples Nb/Y ratios cah be used 65 ppm with onlya slight concomitantincrease in Nb as an index of degree of partial melting. Ti/Y and Nb/Y contents (23-28 ppm). These observations are most easily ratios (Fig. 7) bothincrease with decreasingdegrees of explained by limited crystallization of olivine for which K, partial melting, resulting in a good positive correlation for values for Ni are much greater than unity whereas K,, Nb 01 all the Alexander Island post-subduction basalts. However, c. 0.01. Assuming a moderate distribution coefficient for Ni whereas Nb/Y ratios show a more than threefold increase (10) and that olivine was the sole crystallizing phase during from olivine basalt to basanite (l-3S),Ti/Y ratiosonly the differentiation of the olivine basalts, than less than 15% increase 1.5 times (500-800) resulting in decreasing Ti/Nb fractional crystallization would be capable of producing the ratios with decreasingdegrees of partial melting (olivine observed Ni-Nb trends (Fig. lO), without having any major basalts 350-475, basanite 200-300). Zr/Y-Nb/Y correla- effect on incompatible trace element abundances. Further- tions (Fig. 11) show that the basanites plot at significantly more, the consistent CaO/Al,O, ratios for the basanites and higher Nb/Y ratios at a given Zr/Y ratio than the olivine olivine basalts suggests that a Ca- and Al-free phase must basalts, alkali basalts and tephrites.This variation is havebeen responsible for theirmajor and trace element reflected in Zr/Nb ratios: the olivine basalts and alkali characteristics. In terms of major element compositions the basalts have Zr/Nb ratios (6-7.5) which overlap with those tephrites from KG.3616 are the most fractionated samples in for the tephrites (5-6.5) whereas the basaniteshave this area; decreasing CaObut increasingA1203 with significantly lower Zr/Nb ratios (3-4). It is again noticeable, increasing F/(F + M)ratio suggest that clinopyroxene that like Ti/Nb ratios, Zr/Nb ratios are lowest in the most crystallization may havebeen important during their undersaturatedsamples which were generated by the genesis. Also, the high CaOcontent of the alkali basalts smallest degrees of partial melting. Similar trends are [F/(F + M) = 0.651 reflects their plagioclase phyric nature. delineated by P/Nb, Sr/Nb and Hf/Ta ratios, the basanites However, in terms of trace elementcompositions, all the always having lower high field strength element (HFSE)/Nb AlexanderIsland samples must have evolved by limited and HFSE/Ta ratios than the basalts and tephrites. fractional crystallization of olivine (?<15%) or in the case of the tephrites olivine plus clinopyroxene. LIL-element fractionation Generation of the Alexander Island basalts by variable, The behaviour of theLILE relative tothe HFSE is but low degrees of partialmelting of a garnet lherzolite illustrated in Figs 12 & 13. Like Nb/Y ratios, Ta/Yb ratios source is the most likely explanation for the petrogenetic increase with decreasingdegrees of partial melting.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 ANTARCTIC PENINSULA ALKALINE VOLCANISM 993

30 Rb/Y

1 .o

0.8

0.6 10 0.4 /

Ba/Y i

10 1 2 345 Nb/Y a.11. Plot of Zr/Y vs Nb/Y for the Alexander Island post-subduction basalts. Gough Island, Bouvet Island and MAR45" north are also shown. Symbols as for Fig.3. 5 Furthermore, due to the similarly low D values expected for Th and Ta duringpartial melting of a garnet Iherzolite, Th/Ta ratiosshould remain fairly constantregardless of 1 2 34 degree of partial melting. This behaviour of Th, Ta and Yb Nb/Y is illustrated in Fig 12 using the Th/Yb vs Ya/Yb plot of Fig. U. Plots of Rb/Y vs Nb/Y and Ba/Y versus Nb/Y. These are analogous to Fig. 12 with within-plate enrichment and subduction Pearce (1982), in which the basanites, tephrites and alkali zone enrichment trendsfollowing the same trajectories as in Fig. 12. basalts form alinear array with a slight scatteraround a Symbols as for Fig. 3. slope defining a Th/Ta ratio of unity. This array is consistent with the partial melting or 'within-plate enrichment' trend ratios increasing with increasing 'within-plate enrichment' or described by Pearce (1982), with both Th/Yb and Ta/Yb decreasing degrees of partial melting of a garnet lherzolite. However, the olivine basalts fall outside this array and are displaced vertically away from the 'within-plate enrichment' trend. Such a shift is generally considered to be a diagnostic "L4 of subduction-related magmatism. Plots of LILE/Y vs Nb/Y (Fig. 13) are analogous to the Th/Yb vs Ta/Yb plot. Again 3 the tephrites, alkali basalts and basanites form a linear array consistent with variabledegrees of partial melting of a 2 garnet lhenolite whereas the olivine basalts exhibit consistently higher LILE/Nb and LILE/Ta ratios than any other AlexanderIsland basalts. Inter-elementvariations Th/Yb such as these could have been produced in two main ways: (1) variations in the degree of trace element compatibility 1 with varying degrees of partial melting and/or (2) melting of a mantle source that was heterogeneous with respect to both LILE and HFSE abundances.

Source heterogeneities versus partial melting trends Trace element fractionation atlow degrees of partial melting 1 2 3 45 Clague & Frey (1982) showed that during small degrees of Ta/Yb partial melting of garnet lherzolite incomplete consumption Fig. U. Plot of Th/Yb vs Ta/Yb. Vectors for within plate of trace element-rich minor phases such as rutile, ilmenite enrichment (W)and subduction zone enrichment(SZ) are also andperovskite and major phases such as garnet, shown. Adapted from Pearce(1982). Symbols asfor Fig. 3. clinopyroxene, hornblendeand phlogopite may result in

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 994 M. J. HOLE

fractionation of trace element pairs which would normally coincidence with the 'within-plate enrichment' trend of behave incompatibly during higher degrees of partial Pearce (1982) suggests thatTh and Ta were similarly melting. For theAlexander Island samples,Nb and Ta incompatible during generation of all the above rock types. appear to have behaved incompatibly during partial melting Rb/Y-Nb/Y and Ba/Y-Nb/Y covariations imply that Rb as Nb abundances correlate positively both with Nb/Y ratios and Ba also behaved incompatibly duringpartial melting. and degree of Si-undersaturation. It follows that the lower This contrasts with highly undersaturated lavas from other Ti/Nb,Zr/Nb, Hf/Ta, Sr/Nb and P/Nb ratiosin the locations which show major relative fractionation of LILE basanites relative tothe olivine basalts, alkali basalts and dueto residual phlogopite or hornblendeduring partial tephrites may be the result of the greater compatibility of melting (e.g. Hawaii (Clague & Frey 1982) and Pacific OIB Ti, Zr, Hf, Sr and P over Nb and Ta during the low degrees (Palacz & Saunders 1986)). Interms of thedegree of of partial melting envisaged forthe generation of the LREE-enrichment and Nb/Y ratios, the alkali basalts and basanites. The compatibility of Ti over Nbmay be the result divine basalts are likely to have beenderived by similar of greater volumes of residual garnet and/or clinopyroxene degrees of partial melting from a mantle source region with during the generation of the basanitesas K, Ti, cpx and similar HFSE abundances. This elevation of LILE/Nb and gt > K, Nb, cpx and gt (Pearce & Norry 1979). Indeed, the LILE/Ta ratios for the olivine basalts relative to the other existence of partially resorbed clinopyroxene xenocrysts in Alexander Island samplesis thus likely to be a result of their the basanites, but not in any other of the Alexander Island generation from a LILE-enriched source region rather than samples, attests to the importanceof clinopyroxene at some being an artifact of partial melting. time duringtheir genesis (cf. Smellie 1987). Therole of residual clinopyroxene in the genesis of the basanites may also be reflected in Na,O/K,O ratio variations. The bulk of Isotopic constraints on mantle source compositions the Na,O in a garnet lherzolite mineral assemblage would be The low "Sr/?3r and high 143Nd/144Ndratios of the held in clinopyroxene,whereas K,O would behaveas an Alexander Island basanites, tephritesand alkali basalts incompatible trace element during garnet lherzolite melting. demonstrate that they were all derived from a mantle source Consequently,complete consumption of clinopyroxene region with time-integrated a low Rb/Srratio but duringpartial melting of a garnet lherzoliteshould yield time-integrated high Sm/Nd ratio, broadly similar to that of melts with higher Na,0/K20 ratios than melts produced MORB and some OIB. This implies that the source region leaving residualclinopyroxene. Themore sodic nature of for thesebasalts underwent long term depletionin the olivine basalts, alkali basalts and tephrites relative to the incompatible trace elements. Similar arguments have been basanites could thus be a result of residual clinopyroxene in applied by Futa & Le Masurier (1983) to the alkali basalts of the source of the basanites. Furthermore, the similarity of Mane Byrd Land, and a number of workers have noted the Cr abundances in the olivine basalts and basanites may also necessity for a depleted mantle component as a source for reflect greater volumes of residualclinoproxene in the continental alkali basalts andsome OIB (Norry & Fitton basanites relative to the other post-subduction basalts since 1983; Palacz & Saunders 1986; Zindler & Hart 1986). The distribution coefficiences for Cr in clinopyroxene are high. implication madehere was thatthe high absolute trace Clague & Frey (1982) suggested that decreasing Zr/Nb element abundances and LREE-enrichment exhibited by the ratios with smaller degrees of partial melting for Hawaiian Antarctic alkali basalts,and also OIB, may be relatively volcanic rocks could also have been the result of residual recentmantle phenomena resulting from low degrees of clinopyroxene and garnet at low F. However,the overall partial melting or localized metasomatic mantle enrichment range in Zr/Nb ratios reported by them was only 3.4-4.5, (Futa & Le Masurier 1983; Norry & Fitton 1983). The which contrasts greatly with the variation from 3.0-7.5 for higher x7Sr/s6Srbut lower '43Nd/'"Nd ratios of the olivine theAlexander Islandsamples. Furthermore,the apparent basalts relative tothe other Alexander Island basalts compatibility of Zr relative toNb inHawaiian samples demonstrates that they contain a component which evolved occurred only in highly Si-undersaturated nephelinites and with higher Rb/Sr but lower Sm/Nd ratios than the alkali melilitites (15-25% normative ne) and not in samples with basalts, tephrites and basanites. This implies that the olivine similar normative ne contentsto the Alexander Island basalts may have beenderived from sourcea which basanites (11-16% normative ne). Thesame arguments underwent long-term incompatibletrace element enrich- apply to the observed Sr/Nb, P/Nb, etc., ratiovariations: by ment relative tothe other Alexander Island basalts. comparison with Hawaii, HFSEelement fractionation is Alternatively,this relative elevation in R7Sr/R6Srand much greater for the Alexander Island basanites relative to lowering of '43Nd/'"Nd ratios may be the result of the the tephrites, olivine and alkali basalts than the total range incorporation of a crustal component containing radiogenic observed for Hawaii. Sr but unradiogenic Nd during the genesis of the olivine Whilst the possibility remains thatthe observed basalts. variations in HFSEcontents of theAlexander Island post-subductionbasalts may partlyresult from residual mantle phases at low degrees of partial melting, the greater Correlated trace element isotope trends magnitude of thesevariations by comparison with Hawaii It is generally accepted that isotope ratios are unlikely to be may suggest thatthe Alexander Islandpost-subduction fractionatedduring partial melting and simple fractional basalts werederived from a heterogeneous mantle source crystallization. Consequently any correlation between trace region. elementandisotope ratios would imply thatthe interelement variations represent source region characteris- Heterogeneity in LIL-elements tics. In Fig. 14, Th/Ta and Rb/Nb ratios are plotted versus The coherentvariation of Th/Yb with Ta/Ybfor the Nd- and Sr-isotopic compositions. Two clusters of data are basanites, alkali basalts andtephrites, and their apparent evident. The alkali basalts, tephrites and basanites all plot at

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 A N TAR CTIC PENINSULA ALKALINE VOLCANISMALKALINEANTARCTICPENINSULA 995

being due to residual mantle phases at low degrees of partial -0.7035 melting. However, the olivine basalts have significantly higher Th/Ra, Rb/Nb, Ba/Nb and K/Nb ratios than any other Alexander Island post-subduction basalts and average N-type MORB. It is clear that the alkali basalts, tephrites and basanites exhibit both trace element and isotopic evidence suggestive of theirderivation from an incompatible trace element -0.7025 depleted source region broadly similar to thatof MORB and Rb/Nb some OIB. Similarly, theapparent isotopically enriched .2 .4 .6 nature of the olivine basalt source region is reflected in incompatible traceelement ratios;they exhibit Th/Ta, Ba/Nb and Rb/Nb and K/Nb ratios which are all greater than the other Alexander Island basalts and N-type MORB. Comparison of interelement ratios shows that this relative degree of LILE-enrichment for the olivine basalts follows -0.5 129 the sequence Rb > Th = Ba > K. $4 Origin of the LILE-enriched component in the -0.5 128 olivine basalts

1.o 2.0 Th/Ta Accretionary prism metmedimem One possible source of the LILE-enrichedcomponent is Fig. 14. Th/Ta vs 143Nd/1"Nd and 87Sr/ssSrvs Rb/Nb. Symbols as from the accretionary prism complex metasediments for Fig. 3. 2-sigma error bars are shown for Nd-isotope analyses. introduced in crustal magma chambers prior to the eruption Errors on Sr-isotope analysesare encompassed by the symbols. of the olivine basalts. Thisseems likely for anumber of geological reasons. Firstly, the lack of ultramafic nodules within the olivine basalts contrasts with theirubiquitous significantly lower Th/Ta, Rb/Nb and f17Sr/R6Srratios but occurrence within the tephritesand basanites, suggesting higher 143Nd/1"Nd ratios thanthe olivine basalts.This that the olivine basalts paused in crustal magma chambers linked elemental and isotopic covariation provides unequiv- prior to their eruption, and would thus have had adequate ocal evidence thatthe observed Th/Taand Rb/Nb ratio opportunity to interact with the continental crust. Secondly, variations in the Alexander Island basalts are not an artifact the occurrence of partially recrystallized arkosic fragments of degree of partial melting. Since Ba/Nb and K/Nb ratios within the volcaniclastic rocks associated with the olivine also show a positive correlation with Th/Taratios, it is basalts testifies tothe existence of fusible crust beneath evident that all the observed LILE/Nb and LILE/Ta ratio BeethovenPeninsula. Significantly however, the alkali variations similarly reflect real source heterogeneities. basalts are geographically closely associated with the olivine The basanites, tephrites and alkali basalts have Sr- and basalts, are also nodule free and contain large fragments of Nd-isotopic compositions and Th/Ta ratios similar to both partially fused arkosic material yet do not exhibit the LILE- some Pacific OIBand north Atlantic MORB. Palacz & and 87Sr-enrichment characteristic of the olivine basalts. Saunders (1986) noted that basalts from Mangaia and Nevertheless, the localised nature of this enrichment may Rurutu showed isotopic evidence of derivationfrom an suggest that it is dueto crustal contamination.However, incompatible element depleted source region even though introduction of arkosic, feldspar-rich crustalmaterial into the basalts themselves are enriched in terms of absolute the olivine basalts by AFC (assimiliation with fractional trace element abundances. However, they also pointed out crystallization) might be expected to fractionate Sr relative that certainincompatible element ratios (e.g. Rb/Nb, to the HFSE. Sr/Nb ratios are comparable for the olivine K/Nb, Th/Ta) for Mangaia and Rurutu are similar to those basalts, alkali basalts and tephrites. This lack of correlation for MORB and may also reflect the depleted nature of their between Sr/Nb and 87Sr/ssSr ratios argues against AFC and mantlesource region. Similarly, although the Alexander crustal contamination as a mechanism for the introduction Island alkali basalts, tephrites and basanites all exhibit of the enriched component into the olivine basalts. Indeed, absoluteenrichment in incompatibleelements, they have constancy of Sr/Nb ratios across the broad compositional Rb/Nb,Th/Ta, Ba/Nb and K/Nb ratios similar to those range from olivine basalt totephrite would arguethat reported for N-type MORB (Table 2). It is noticeable that isotopic exchange between radiogenic and unradiogenic Sr thetephrites, which exhibit the most depletedSr-isotope could have taken place during the genesis of the olivine characteristics (87Sr/"6Sr 0.70275) also exhibit the lowest basalts rather than bulk introduction of high 87Sr/x6Sr,low LILE/HFSE ratios for the Alexander Island samples. This 143Nd/1"Nd ratio crustal material. implies that the tephrites were derived from a source which experienced greaterdegrees of long-lived incompatible elementdepletion than the basanites. The higher Zr/Nb, Sr/Nb,etc., ratios noted for thetephrites relative tothe LILE-enriched mantle basanites may also reflect the depleted nature of the mantle There is a wealth of evidence to suggest that the high Rb, source of the tephrites relative to the basanites, rather than Ba,Th, Kand LREE abundances and high LILE/HFSE

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 996 M. J. HOLE

ratioscharacteristic of calc-alkaline subduction-related The talc-alkaline to alkaline transition and ridge basalts from intra-oceanic island arcs, are the result of the crest-trench collision incorporation of subductingslab-derived LILE-enriched Rogers et al. (1985) described alkali basalts from San components into the overlying mantle wedge (Hawkesworth Quintin,north west Baja California, which were erupted & Powell 1980; Saunders et al. 1980; Hole et al. 1984; about 12.5 Ma after ridge crest-trench collision in that area. Thirlwall & Graham 1984). LILE/HFSE ratios may also be The SanQuintin samples exhibit similar traceelement increased during subduction-related magmatism by the characteristics to Alexander Island post-subduction basalts stabilization of HFSE-bearing minor phases in the hydrous (Fig. 8) which were erupted around 18-30Ma after ridge conditionsassociated with slab dehydration.However, as crest-trench collision. This may be significant in terms of noted by Thompson et al. (1983), the nature of such minor the temporalrelationship between ridge crest-trench phases is conjectural. collision and post-subduction magmatism. It is clear that The bulk of theRb,?Th and K budget of there is change from calc-alkaline to morealkaline subduction-related magmas is likely to bederived from magmatism within less than 18Ma of ridge crest-trench fluids released during dehydration of subducting, hydrother- collision. However, the existence of the currentlyactive mally altered MORB. Furthermore, the subducting slab is Tres Virgines calc-alkaline strato-volcano at Baja California also likely to carry a significant volume of pelagic and/or (Rogers et al. 1985) demonstratesthat subduction-related terrigenoussediment intothe mantle. The melting and volcanism may persist after the cessation of subduction. In incorporation of such sedimentsinto mantlederived southernAlexander Island,the minorsubduction zone magmas may furtherenrich subduction-relatedbasalts in component identified in the olivine basalts eruptedmore Th, Rb,Ba, K, LREE, P and X7Sr and causerelative than 30 Ma after ridge crest-trench collision, also implies decreases in 143Nd/144Nd ratios. that the subcontinental mantle may retain a subduction zone Subduction processes are therefore an effective way of signaturemore than 40Maafter the cessation of decoupling Rb, Ba, Th, K and the LREE from all other subduction-related magmatism. To what extentplate incompatible trace elements. It is likely that the long period configurations prior to cessation of subduction and the exact (a200Ma) of subductionprior to theonset of alkaline mechanism of ridge crest-trench collision influence the magmatism onAlexander Islandresulted in LILE- character of post-subduction magmatism is atpresent enrichment of the subcontinental mantle. Such mantle unknown. Indeed, the apparent absence of bajaites (Rogers enrichment eventsappear tobe recorded during the et al. 1985) within theAntarctic Peninsula,but their generation of extensionalback-arc basalts (Weaver et al. occurrence in other locations where ridge crest-trench 1979; Saunders & Tarney 1985). Addition of asmall, but collision and ridge subduction have occurred may be related geochemically significant subduction componentinto the to differences in plate configurations prior to andduring olivine basaltscould have produced thedisproportionate ridge crest-trench collision in various arcs. From the present enrichment in Rb,Ba, Th, K and 87Sr relative tothe study it would seem that by farthe most volumetrically basanites andtephrites. The observeddecreases in important magmas produced following ridge crest-trench 143Nd/1"Nd ratios would require recycling of low collision are alkalic basalts which are geochemically '43Nd/'"Nd ratio sedimentary material into the subduction indistinguishable from OIB or continental alkali basalts. zoneas thedehydration products of alteredMORB are unlikely tocontribute significant amounts of unradiogenic Nd to the mantle wedge. Depth of generation in relation to tectonic setting The striking isotopic and trace element similarities between the Alexander Island post-subduction basalts and continen- Long-lived mantle enrichment tal alkali basalts and OIB have already been noted. There A third possible explanation fortheLILE-enriched are at present a variety of models for the generation of OIB character of the olivine basalts is that it reflects a long-lived andcontinental alkali basalts. Fitton & Dunlop (1985) enrichedmantle source region. Apparent subduction argued that alkali basalts from the oceanic and continental enrichment of some OIB (Dupal OIB) is considered by a sectors of the Cameroon line, western Africa, were derived number of workers to be a long-lived mantle phenomenon froma common MORB-like source within the convecting (Dupre & Allegre 1983; Hart 1984), and may have resulted upper mantle. Alternative hypotheses for the generation of from recycling of pelagic and/or terrigenousmaterial into OIB, require long-term recycling of subducted oceanic crust the mantle more than 1 Ga ago(Cohen & O'Nions 1982; into the asthenospheric mantle. Ringwood (1982) suggested Weaver et al. 1986). The olivine basaltsexhibit broadly that recycled subductedoceanic crust may reside at c. similar trace elementcharacteristics toDupal OIB, but 650 km depth within the mantle and its subsequent melting Dupal islands such as Gough and Tristan da Cunha exhibit after time periods longer than 1 Ga, produces diapirs which much greaterdegrees of Sr- and Nd-isotopicenrichment are ultimately the source of OIB. Hofmann & White (1982) (87Sr/ssSr up to 0.7055, '43Nd/'44Nd <0.5126). This does not proposed a similar model, but suggested that the subducted preclude a long-lived enriched source region for the olivine slab resides at the core-mantle boundary prior to melting basalts, butthe intimate temporal and geographical and the generation of OIB-typediapirs. In short,the association between theAlexander Islandbasalts andthe general consensus at present seems to be for a deep (? up to relatively recent cessation of a long period of subduction is 650 km) origin for continental alkali basalts and OIB. perhaps geologically compelling evidence thatthe LILE- The subducted slab may act as a physical barrier to the enrichment is related to the Mesozoic subduction history of uprise of deep-seated OIB-type magmas during subduction the AntarcticPeninsula ratherthan a long-lived mantle and until the time of ridge crest-trench collision. In order to phenomenon. erupt OIB-type alkali basalts after ridge crest-trench

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 A N TA R C T IC PENINSULA ALKALINE VOLCANISMANTARCTICALKALINEPENINSULA 997

collision, a mechanism must beinvoked whereby the The post-subduction basalts of Alexander Island subducted slab is rendered permeable to rising magmas or is represent an excellent example of the transition from removed from beneath the accretionary prism complex. The calc-alkaline to alkaline magmatism following the cessation former case has been invoked by Alabaster et al. (1982) to of subduction along a continental destructive plate margin. explain the production of alkalic basalts (Salahi Unit) during The intimatetemporal and spatial association of calc- obduction of the Oman ophiolite onto the Arabian passive alkaline and alkaline volcanic rocks in the geological record, continental margin. They suggested thatthe magmatic may allow detailed analysis of the timing and longevity of precursors to Salahi unit magmas penetrated the subducted ancient subduction activity. slab via fracturezones and inheriteda minor subduction componentfrom the overlying LILE-enrichedmantle Thesamples for this studywere collected during the 1985/86 wedge. In the second case, the removal of the slab to allow Austral summer. T. Brewer, G. Lees, P. Floyd and G. F. Marriner uprise of deepseated magmas may befacilitated if the provided analytical facilities for major, trace and rare earth element subducted slab was decoupled fromthe adjacent oceanic analysis. Isotope analyses were carried out under the watchful eye plate and continued descending into the mantle after ridge of I. L. Millar at the BritishGeological Survey’s isotope geology crest-trench collision. In this case the 18 Ma timelapse unit. T. Alabaster, J. L.Smellie and A. D. Saundersprovided between collision and alkali basalt magmatism may much constructive criticism of the text and ideas at various stagesof represent the minimum period for the removal of the slab their evolution. from beneath the accretionary prism complex. The Heezen fracture zone, west of Alexander Island, is known to have References been subductedfor more than 50Ma prior to ridge ALABASIER,T., PEARCE,J. A. & MALPAS,J. 1982. The volcanic stratigraphy crest-trench collision (Barker 1982) and it could have of the Oman Ophiolite. Contributions to Mineralogy and Petrology, 81, provided an ideal pathway for the uprise of deep-seated 168-83. magmas. However, there is no obvious correlation between BARKER,P. F. 1982. Cenozoic subduction history of the Pacific margin of the the landward trace of the fracture zone and the distribution AntarcticPeninsula: ridge crest-trench interactions. Journal of the Geological Sociery, London, 139,787-801. of alkali basalts and either of the above hypotheses seem BELL, C. M.1973. The geology of BeethovenPeninsula, south-western feasible. Alexander Island. British Antarctic Survey Bulletin, 32, 75-83. BURN,R. W. 1981. Early Tertiary calc-alkaline volcanism on Alexander Island. British Antarctic Survey Bulletin, 53, 175-93. Conclusions - 1984. The geology of the LeMay Group, AlexanderIsland. British Antarctic Survey Scientific Report, 109, 29-34. Following the cessation of subduction along the west coast - & THOMSON,M. R. A. 1981.Late Cenozoic tillites associated with of AlexanderIsland, south west AntarcticPeninsula, a intraglacialvolcanic rocks, Lesser Antarctica. In: HAMBREY,M. J. & series of alkalic basalts ranging in composition from olivine HARLAND,W. B. (compilers) Earth’s pre-Pleistocene glacial record. International geological correlation program, project 38: pre-Pleistocene basalt to basanite were erupted. Post-subduction magmatism tillites. Cambridge University Press, 199-203. commenced 18-30Ma after the cessation of subduction and CARE,B. W. 1980. The geology of Rothschild Island, north-west Alexander continued until c. 2.5 Ma. Island. British Antarctic Survey Bulletin, 50, 87-112. The basalts have trace element characteristics similar to CLAGUE,D. A. & FEY, F. A. 1982.Petrology and trace element geochemistry of the Honoluluvolcanics Oahu: Implications for the OIB and continental alkali basalts. All the post-subduction oceanic mantle below Hawaii. Journal of Petrology, 23, 447-504. basalts could have been derived by variable but low degrees COHEN,R. S. & O’NIONS,R. K. 1982. Identification of recycled continental of partial melting of agarnet lherzolite source leaving material for Sr. Nd and Pb isotope investigations. Earth and Planetary residual garnet f clinopyroxene, followed by limited (?< Science Letters, 61, 73-84. 15%) fractional crystallization mainly of olivine. The mantle DE LA ROCHE.H., LETERRIER,J., GRANDCLAUDE,P. & MARCHAL, M. 1980. A classification of volcanic and plutonic rocks using Rl-R2 diagram and source region from which theywere generated may have majorelement analysesits relationshipwith currentnomenclature. been heterogeneous with respect toHFSE abundances, Chemical Geology, 29, 183-210. although heterogeneities can only be recognized in the most DUPE, B. & ALLEGRE,C. J. 1983. Pb-Sr isf)tope variation in Indian Ocean undersaturated basanitoids. This leads to the possibility that basalts and mixing phenomenon. Nature, 303, 142-6. FITKIN,J. G. & DUNLOP, H. M. 1985.The Cameroon Line, West Africa, and traceelement fractionation at low F may in part have its bearing on the origin of continental and oceanic alkali-basalts. Earth contributed to the observed HFSE variations. and Planetary Science Letters, 72,23-38. Sr- and Nd-isotope compositions demonstratethat the FUTA, K. & LEMASURIER, W. E. 1983.Nd- and Sr-isotopic studies on majority of the post-subduction basalts were derived from a Cenozoicmafic lavas from West Antarctica: anothersource for continental alkali-basalts. Contributions to Mineralogy and Petrology, 83, source which evolved with low timeintegrated Rb/Sr but 38-44. high time-integrated Sm/Ndratios, broadly similar to the HART,S. R. 1984. A large-scale isotope anomaly in the Southern Hemisphere source of MORB and some non-Dupal OIB. Incompatible mantle. Nature, 309, 753-7. traceelement ratios (Th/Ta,Rb/Ta, Ba/Ta) for the HAWKESWORTH,C. J. & POWELL,M. 1980. Magmagenesis in the Lesser Antilles island arc. Earth and Planetary Science Letters, 51, 297-308. majority of the Alexander Island samples are similar to HOFMANN,A. W. & WHITE, W. M. 1982. Mantle plumes from ancient ocean those reported forMORB and may also reflect the crust. Earth and Planetary Science Letters, 57, 421-36. incompatible trace element depleted nature of their source HOLE,M. J., SAUNDERS,A. D., MARRINER,G. F. & TARNEY,J. 1984. region. Subductionofpelagic sediments: implications for theorigin of The olivine basalts of Mussorgsky Peaks exhibit both Ce-anomalousbasalts from the MarianaIslands. Journal of the Geolgoical Sociery, London, 141, 453-72. traceelement and isotopic signatures of minora but -, SMELLIE,J. L. & MARRINER,G. F. in press. Geochemistry, and tectonic geochemically significant subductionzone component in setting of Cenozoicalkaline basaltsfrom Alexander Island, Antarctic their source. This subduction zone component was probably Peninsula. In: THOMSON,M. R. A., CRAME,J. A. & THOMSON,J. W. (eds). Geological evolution of Antarctica. CambridgeUniversity Press, inherited from portiona of mantle which originally Cambridge. constituted the mantle wedge during the previous 200 Ma of JONES,J. G. 1970. Intraglacial volcanoes of the Laugarvatn region, southwest subduction along the west coast of Alexander Island. Iceland 11. Journal of Geology, 78, 127-40.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021 998 M. J HOLE

LE ROEX,A. P. 1985. Geochemistry, mineralogy and magmatic evolution of SMELLIE,J. L. 1987. Geochemistry and tectonic setting of alkaline volcanic thebasaltic and trachytic lavas from Gough Island,South Atlantic. rocks in the Antarctic Peninsula.Journal of Volcanology and Geothermal Journal of Petrology, 26, 149-86. Research, 32, 269-85. - & ERLANK,A. J. 1982. Quantitativeevaluation of fractional -, PANKHURST,R. J., HOLE, M. J. & THOMSON,J. W.1988. Age, crystallisationin Bouvet Island lavas. Journal of Volcanology and distribution and eruptive conditions of Late Cenozoic alkaline volcanism Geothermal Research. W, 309-38. in the Antarctic Peninsula and Ellsworth Land: review. British Antarctic NELSON,P. H. H. 1975. The James Ross Island Volcanic Group of north-east Survey Bulletin. 80. Graham Land. Brihh Antarctic Survey Scientific Report, 54. TARNEY,J., SAUNDERS, A. D.,WEAVER, S. D., DONELLAN,N. C. B. & NORRY, M. J. & FITTON,J. G. 1983. Compositionaldifferences between HENDRY,G. L. 1978. Minor-element geochemistry of basalts from leg 49, continentaland oceanic basic lavas and their significance. In: North Atlantic Ocean. Initial Reportr of the Deep Sea Drilling Project, HAWKESWORTH,C.J. & NORRYM. 3. (eds) Continental Baralts and 49. US Government Printing Office, Washington, D.C., 657-91. Mantle Xenolih. Shiva Publishing Ltd, Nantwich, 5-19. THIRLWALL,M.F. & GRAHAM,A. M.1984. Evolution of high-Ca. high& PALACZ,Z. A. & SAUNDERS,A. D. 1986. Coupled trace element and isotopic C-series basalts from Grenada, Lesser Antilles: the effectsof intracrustal enrichment in the Cook-Austral-Samoa islands, southwest Pacific. Earth contamination. Journal of the Geological Society, London,141, 427-447. and Planetary Science Letters, 79,270-80. THOMPSON,R. N., MORRISON,M. A., DICKIN,A. P. & HENDRY,G. L. 1983. PANKHURST,R. J. 1982. Rb-Sr geochronology of Graham Land.Journal of the Continental flood basalts: Arachnids rule O.K. In: HAWKESWORTH,J. C. Geological Society, London, l39.701-11. & NORRY,M. J. (eds) Continental Baralts and Mantle Xenoliths. Shiva PEARCE, J. A. 1982. Trace element characteristics of lavas from destructive Publishing Ltd. Nantwich, 158-85. plate boundaries. In: THORPE,R. S. (ed.) Orogenic Andesites. J. Wiley __, , HENDRY,G. L. & PARRY,S. J. 1984.An assessment of the & Sons, 525-48. relative roles of crustand mantle in magmagenesis: an elemental -& NORRY,M. J. 1979. Petrogenetic implications of Ti, Zr Y and Nb in approach. Philosophical Tramactions of the Royal Society of London, volcanic rocks. Contributions to Mineralogy and Petrology, 69, 33-47. Serial A, 310, 549-90. RINGWCOD,A. E. 1982.Phase transformations and differentiation in VERWOERD,R. J., ERLANK,A. J. & KABLE,E. J. D. 1974. The geology and subducted lithosphere: implications for mantledynamics, basalt geochemistry of Bouvet Island. In: GONZALEZ-FERRAN,0. (ed.) Andean ptrogenesis and crustal evolution. Journal of Geology, W, 611-43. and Antarctic Volcanological Problems. IAVCEI Symposium, Santiago, ROGERS,G., SAUNDERS,A. D., TERRELL, D. J., VERMA,S. P. & MARRINER, Chile, 203-37. G. F. 1985. Geochemistry of Holocene volcanic rocks associated with WEAVER,B. L., WOOO, D. A., TARNEY,I. & JORON, J.-L. 1986. Role of ridge subduction in Baja California, Mexico. Nature, 315, 389-92. subducted sediment in the genesis of ocean island basalts: Geochemical SAUNDERS,A. D. 1982. The petrology and geochemistry of alkali-basalt from evidence from south Atlantic Ocean islands. Geology, 14, 275-8. Jason Peninsula, Oscar I1 Coast, Graham Land. British Antarctic Survey WEAVER,S. D., SAUNDERS,A. D., TARNEY, J.& PANKHURST,R. J. 1979. A Bulletin, 55, 1-9. geochemicalstudy on magmatismassociated with the initial stages of -& TARNEY,J. 1985. Geochemical characteristicsof basaltic volcanism in back-arc spreading: the Quaternary volcanics of Bransfield Strait from back-arc basins. In: KOKELAAR,B. P. & HOWELLS,M. F. Marginal basin South Shetland Islands. Contributions to Mineralogy and Petrology, 68, geology, Special Publication of the GeologicalSociety, London, 16, 689-92. 59-76. WHITE,W. M. 1985. Sources of oceanic basalts: Radiogenic isotope. evidence. -, ROGERS,G., MARRINER,G. F., TERRELL, D. J. & VERMA,S. P. 1987. Geology, 13, 115-8. Geochemistry of Cenozoic volcanic rocks from Baja California, Mexico: WOOD,D. A., JORON,J.-L., TREUIL,M., NORRY, M.J. & TARNEY, 1979.J. implicationsfor the genesis of post-subduction magmas. Journal of Elemental and Sr isotope variations in basic lavas from Iceland and the Volcanology and Geothermal Research, 32,223-46. surrounding ocean floor. Contributions to Mineralogy and Petrology, 70, -, WEAVER,S. D. & TARNEY, J.1980. Tranverse geochemical variation 319-39. acrosstheAntarctic Peninsula: implications forthe genesis of ZINDLER, A.& HART,S. 1986. Chemical geodynamin. AnnualReviews in calc-alkaline magmas. Earth and Planetary Science Letters, 46, 344-60. Earth and Planetary Sciences, 14, 493-571.

Received 30 October 1987; revised typescript accepted 14 June 1988.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/6/985/4889675/gsjgs.145.6.0985.pdf by guest on 27 September 2021