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 and trace 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