ELSEVIER Tectonophysics 311 (1999) 83±111 www.elsevier.com/locate/tecto

New geochemical constraints for the origin of ridge-subduction-related plutonic and volcanic suites from the Chile Triple Junction (Taitao Peninsula and Site 862, LEG ODP141 on the Taitao Ridge)

C. Guivel a,Ł, Y. Lagabrielle a, J. Bourgois b,R.C.Mauryc,S.Fourcaded, H. Martin e, N. Arnaud e a UMR 6538 Domaines OceÂaniques, Universite de Bretagne Occidentale, IUEM, Place Nicolas Copernic, 29280 PlouzaneÂ, France b UMR CNRS, Universite Pierre et Marie Curie, BoõÃte 119, 4 place Jussieu, 75252 Paris Cedex 05, France c UMR 6538, Universite de Bretagne Occidentale, avenue le Gorgeu, B.P. 80, 29285 Brest Cedex, France d UPR 4201, GeÂosciences Rennes, Universite Rennes I, avenue du GeÂneÂral Leclerc, 35042 Rennes Cedex, France e CNRS UMR 6524, Universite Blaise Pascal, Clermont II DeÂpartement des Sciences de la Terre, 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France Received 2 June 1998; accepted 23 April 1999

Abstract

Several features of Neogene and Quaternary magmatism in the region south of the present-day Chile Triple Junction (CTJ) at 46ë120S are directly related to the migration of the triple junction. Due to the obliquity of the ridge orientation with respect to the subduction front, the triple junction migrated from South to North during the last 14 Ma. The Taitao Peninsula Ð the westernmost promontory of the Chile coast Ð and the Taitao Ridge Ð a submarine promontory north of the Taitao Peninsula Ð provide the most complete collection of ridge subduction-related magmatic products in the region. The emplacement of near-trench volcanics, the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the Chile spreading centre and the continental crust. We present new ®eld observations and geochemical data that help to better constrain the problem of the sources and evolution of the Taitao magmas. The new geochemical data were obtained on samples collected from the Taitao Peninsula during a ®eld expedition in 1995, and from samples of the Taitao Ridge during Leg ODP141, Site 862, which have been re-sampled in 1996 by one of us. Selected major- and trace-element compositions of 20 volcanic rocks from the Taitao Ridge are discussed together with 53 analyses from different rock types from the Taitao Peninsula including 24 unpublished analyses. Nd and Sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the Taitao Peninsula together with the oxygen isotope composition of four separated clinopyroxenes. Six main magmatic types are identi®ed: (1) N-type MORB; (2) E-type MORB; (3) LREE-depleted MORB showing some trace-element features typical of arc basalts; (4) moderately Nb-depleted E-MORB; (5) calc-alkaline andesites, dacites and rhyolites; and (6) andesites and dacites with adakitic signature. Chemical similarities exist between some forearc magmas of the Taitao Ridge and the Taitao Peninsula and magmas emplaced at the Chile active spreading ridge. One important result, based on isotope data, is that the lavas emplaced over the continental crust (Taitao Peninsula) did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle-derived

Ł Corresponding author. Present address: Universite de Nantes, Laboratoire de PlaneÂtologie et GeÂodynamique-PeÂtrologie Structurale, 2 rue de la HoussinieÁre, B.P. 92208-44322, Nantes CeÂdex 03, France. E-mail: [email protected]

0040-1951/99/$ ± see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0040-1951(99)00160-2 84 C. Guivel et al. / Tectonophysics 311 (1999) 83±111 magmas. The likely source of these basalts could be the hot convective oceanic mantle of the southern Chile spreading ridge buried at moderate depth (10±30 km).  1999 Elsevier Science B.V. All rights reserved.

Keywords: triple junction; ridge subduction; forearc magmatism; geochemistry

1. Introduction During the last ten years, investigations in the CTJ region have shown that subduction of the Chile The Chile Triple Junction, located at 46ë120S ridge beneath the South American margin was co- (CTJ; Fig. 1) is the site where the Antarctic, eval with the emplacement of magmatic suites and the Nazca and the South America plates meet; it possibly with ophiolite obduction close to the trench is presently a ridge±trench±trench triple junction. axis. Young magmatic products with highly vari-

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2 cm/yr 48˚S active volcanoes plate motion relative to South America plate Fig. 1. The Taitao Peninsula and adjacent areas. Inset: Regional map with location of the plate boundaries separating the Nazca, Antarctic and South America plates. EFZ: Esmeralda Fracture Zone, TMFZ: Tres Montes Fracture Zone. The study area is outlined by a box. The bathymetric map of the three segments of the active Chile spreading ridge is based on gridded bathymetric (500 m contour interval) data from the Simrad EM12 Dual multibeam echo-sounder on the R=V L'Atalante during the Chile Triple Junction cruise. These segments are de®ned as segment 1 (Taitao to Darwin FZ), segment 2 (Darwin to Guamblin FZ) and segment 3 (Guamblin to Guafo (not mapped) FZ). The present-day location of the Chile Triple Junction (46ë120S) is also shown. TP: Taitao Peninsula, TMP: Tres Montes Peninsula, AVZ: Austral Volcanic Zone, SSVZ: Southern South Volcanic Zone. C. Guivel et al. / Tectonophysics 311 (1999) 83±111 85 able chemical characteristics were described in this and the Darwin fracture zones, entered the trench region, especially on the Taitao Peninsula, 50 km ca. 0.3 Ma ago. Previously, two short ridge segments south of the present-day triple junction (Mpodozis et were subducted after 6 Ma and 3 Ma, respectively: al., 1985; Bourgois et al., 1993; Le Moigne et al., one between the Esmeralda and the Tres Montes 1996), and on the Taitao Ridge which protrudes into fracture zones and the second one between the Tres the trench, north of the Taitao Peninsula (Forsythe et Montes and the Taitao fracture zones, west of the al., 1995a). The chemical characteristics of plutonic Taitao Peninsula. and volcanic rocks from these two areas suggest The Chile Triple Junction has migrated rapidly that magmas originated either from mantle sources northwards during subduction of ridge segments for (Kaeding et al., 1990), from slab melting (Bourgois the past 14 Ma. It is now located beneath the north- et al., 1996), or directly from the subducted spread- ern part of the volcanic gap that exists between ing centre interacting with the overlying continental the southernmost volcano of the Southern South crust of the Chile margin (Lagabrielle et al., 1994). Volcanic Zone (SSVZ, Mount Hudson at latitude In the vicinity of the triple junction, the southern 46ë000S) and the northernmost volcano of the Aus- Chile spreading ridge consists of three ®rst-order seg- tral Volcanic Zone (AVZ, Mount Lautaro at latitude ments (Fig. 1), 40±225 km long, separated by fracture 49ë000S) (Fig. 1). Between 42ëS and 46ëS, the SSVZ zones (FZ), namely from north to south: the Guafo, is characterized by basalts and basaltic andesites Guamblin, Darwin and Taitao FZ. Lavas from the ac- with typical calc-alkaline af®nities. South of the vol- tive ridge axis have heterogeneous compositions rang- canic gap, the ®ve active stratovolcanoes of the AVZ ing from typical Mid-Ocean Ridge Basalts (MORB) (Lautaro, Viedma, Aguilera, Burney and Cook) have to basalts and dacites with trace-element character- a spacing of 100 to 170 km. These magmas display istics (e.g., high La=Nb or equivalent ratios) usually adakitic (slab melt) signatures (Stern and Kilian, considered as typical of arc-related volcanics or conti- 1996; Sigmarsson et al., 1998). nental crust. These compositions support the hypoth- The geometry of the slab beneath the Taitao esis of sub-ridge contamination associated with ridge Peninsula is poorly known because of the lack of subduction (Klein and Karsten, 1995). seismicity, but its dip is estimated to be ca. 15ë Therefore, one of the basic problems of the CTJ (Bangs et al., 1992). The continental crust thickness region is to understand how the magmatism related is ca. 30 km (Cande and Leslie, 1986). to a currently subducting spreading ridge may in- teract with the continental crust of the upper plate. In this paper, we present new ®eld observations and 3. Regional geology and previous geochemical geochemical data which are used to better constrain data on young volcanic±plutonic suites in the this problem. The new geochemical data were ob- CTJ area tained on samples collected in the Taitao Peninsula during a ®eld expedition done by two of us (JB and 3.1. The Taitao Peninsula: geological units and YL) in 1995, and from the Taitao Ridge during Leg previous geochemical results ODP141, Site 862 (Fig. 2), the cores of which have been re-sampled in 1996 by one of us (CG). Numerous geological, radiometric and geochemi- cal data have been collected on the magmatic suites and related rocks exposed on the Taitao Peninsula 2. Regional setting and kinematics (Mpodozis et al., 1985; Forsythe et al., 1986; Kaed- ing et al., 1990; Lagabrielle et al., 1994; Le Moigne North of the triple junction, the Nazca plate is cur- et al., 1996; Bourgois et al., 1996). Two ®eld investi- rently subducting beneath the South America plate at gations conducted in 1992 and 1995 (Fig. 3a) allowed arateof9cm=yr, whereas to the South, the Antarc- us to produce an updated geological map (Fig. 3b). tica plate is being subducted at a rate of only 2 Five main rock units have been recognized cm=yr (Cande et al., 1987). Segment 1 of the south- (Fig. 3b): (1) the Bahia Barrientos ophiolite; (2) ern Chile ridge (Fig. 1), located between the Taitao a late Cenozoic volcano±sedimentary sequence of 86 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

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Fig. 2. Detailed bathymetric map of the CTJ area (250 m contour interval) showing segment 1 of the active Chile spreading ridge currently being subducted, the Taitao Ridge and the locations of ODP Leg 141 borehole site 862 and the Taitao Peninsula. BBO:Bahia Barrientos Ophiolite, GP: Golfo de Penas. C. Guivel et al. / Tectonophysics 311 (1999) 83±111 87 the Chile Margin Unit (CMU) and the Main Vol- positions including Normal-MORB (N-MORB), En- canic Unit (MVU); (3) the Bimodal Dike Complex riched-MORB (E-MORB), and calc-alkaline lavas. (BDC); (4) the Taitao plutonic intrusions; and (5) the The abundance of pyroclastic material in CMU and Pliocene volcanoes. the evidence for deposition in subaerial to shallow water conditions in the MVU and CMU demon- 3.1.1. The Bahia Barrientos ophiolite strate that these units do not belong to a recently The Bahia Barrientos ophiolite (Bourgois et al., obducted deep-oceanic section created at the Chile 1993) includes mantle peridotites, gabbros and do- spreading ridge (Le Moigne et al., 1996). Accord- leritic dikes. The gabbros are Light Rare Earth Ele- ing to Lagabrielle et al. (1994), the MVU and CMU ment (LREE)-depleted and, according to Le Moigne volcanic rocks were emplaced in or close to their pre- et al. (1996), the peridotites and gabbros show petro- sent-day location by MORB-derived magmas erupt- logical, geochemical and mineralogical characteris- ing through the Chilean continental basement. Thus, tics consistent with an oceanic origin. Pieces of the the lavas likely originated from the buried active ophiolitic complex are also found as xenoliths of var- spreading centre as it was subducted at shallow ious sizes (10 to 100 m) in the later intrusions. Gab- depth. The variety of chemical compositions of the broic and doleritic lenses are found within the Seno magmas have been interpreted as the results of vari- Hoppner pluton. Doleritic xenoliths are found in gran- ous degrees of upper crustal contamination coupled odiorite magmatic breccias exposed along the western with fractional crystallization (AFC) during their coast of the Tres Montes Peninsula. Several plurimet- ascent and possible storage within the Chilean conti- ric fragments of gabbro and dolerite occur within the nental crust (Lagabrielle et al., 1994). acidic dike complex exposed in the central part of the Taitao Peninsula (Fig. 3b). Con®dent geochronologi- 3.1.3. The Bimodal Dike Complex (BDC) cal dating of the ophiolitic rocks is not available yet; Nelson et al. (1993) mentioned that a central K=Ar ages of about 13 and 6 Ma have been obtained sheeted dike unit is exposed within the ophiolitic on a hornblendite vein and a doleritic dike, respec- plutonic suite, in the centre of the peninsula and tively (Bourgois et al., 1992, 1993; Le Moigne, 1994). along the western shore. They also mapped a `cen- Therefore, the timing of obduction and the processes tral' pluton near the centre of the sheeted dike unit. that led to ophiolite emplacement are still under de- This is con®rmed by our own ®eld observations bate. There is no clear evidence for an emplacement and helicopter sampling conducted in 1995 (Guivel linked to recent obduction of the Nazca or the Antarc- et al., 1996). Basaltic dikes are spatially associ- tic oceanic lithosphere due to triple junction tectonics, ated with dacitic to rhyolitic intrusive rocks and but, in turn, there are no arguments for an age clearly volcanic breccias exposed near the central pluton older than that of the triple junction migration. (Fig. 3b). Genetic relationships between the basaltic and acidic dikes are not always clear, but locally, 3.1.2. The volcano±sedimentary units (CMU±MVU) silicic sheeted dikes intrude gabbros and greenschist The Pliocene Chile Margin Unit (CMU) (Fig. 3b), facies dolerites, which are probably part of a former 4±6 km thick, consists of interbedded sedimen- ophiolitic dike complex (Guivel et al., 1996). For tary and volcanic material deposited in a shallow- this reason, the sheeted dike unit is referred hereafter water environment. It unconformably overlies the to as the Bimodal Dike Complex. The basic dikes pre-Jurassic metamorphic basement of the Chile show both diabasic and porphyritic textures. They margin. The Main Volcanic Unit (MVU) consists are ®ne- to medium-grained rocks showing evidence of pillow-lavas and associated sediments that also of greenschist-facies metamorphism (saussuritized accumulated in shallow water environments. It dif- plagioclase and uralitized pyroxene). The lavas con- fers from the CMU by a well developed green- tain secondary chlorite, amphibole and quartz. Their schist metamorphic overprint, a lack of evolved lavas SiO2 contents vary from 50.8 to 53.8 wt%. These such as rhyolites, a greater amount of intermedi- dikes show E-MORB af®nities. Silicic dikes intrud- ate lavas and a lack of pyroclastic deposits. MVU ing the ophiolitic dolerites are amphibole-bearing and CMU lava ¯ows show a large range of com- dacites and rhyolites with SiO2 contents varying 88 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

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43 TAITAO expedition 1992 (a) 24-2 TAITAO expedition 1995 46˚00'S Fig. 3. (a) Location of sampling sites on the Taitao Peninsula. from 65.8 to 76 wt%. Dacites contain dominant above previously eroded basaltic and acidic dikes. subhedral phenocrysts of plagioclase together with They include angular fragments of granite and sub- clinopyroxene phenocrysts replaced by chlorite, am- ordinate coarse-grained ophiolitic dolerites. phiboles and ilmenite; apatite is present as accessory mineral. The groundmass mainly consists of plagio- 3.1.4. The Taitao plutons clase grains. Rhyolites contain quartz and plagio- The Taitao Peninsula includes six plutons labelled clase phenocrysts, and epidotes and chlorite replace P1 to P6 hereafter. The two largest and best studied primary Fe±Mg minerals. Radiometric ages are lack- are the Cabo Raper (P1) and the Seno Hoppner (P2) ing for these rocks. There are no intermediate com- plutons. positions between the two groups of dykes (basaltic The Cabo Raper pluton (P1), located only 17 km and silicic) which display different rare earth ele- east of the trench axis, is a biotite±hornblende-bear- ment (REE) patterns. In addition, polymict volcanic ing granodiorite or tonalite (Fig. 4). New 40Ar=39Ar breccias with glassy rhyolitic matrix are exposed in data obtained recently by one of us (NA; see Arnaud the central part of the dike complex, at the site where et al., 1993, for analytical conditions) on separated Nelson et al. (1993) described the central pluton. biotite from two samples, yielded ages of 5:1 š 0:6 Likely, these breccias were subsequently emplaced and 4:8š0:3 Ma, in contrast with previous data from C. Guivel et al. / Tectonophysics 311 (1999) 83±111 89

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Chaicayan Island and Fiordo San Pedro TAITAO sedimentary sequences (Upper Miocene) Estero Cono PENINSULA Fjordo San Pedro 46˚35'S Tres Montes sedimentary sequence (Middle Eocene) Estero Cono + Pluton P4 + (1) Bahia Barrientos Ophiolite + B 5.9 +- 0.5 + + 6.9 + 1 .. . + F - .. .. Cumulate gabbro + + Cupula + San Pablo 46˚40'S + Gabbro and associated serpentinite lenses + .... . P2 Serpentinite tectonic lenses ...... Seno Hoppner ...... Pluton ...... (2) Volcanic sedimentary units + P1 Pan de Chile Margin Unit (CMU) + Azucar Cabo + 46˚45'S Raper + + Bahia Main Volcanic Unit (MVU) Pluton + + + Barrientos + + + P3 + + Pluton B 5.1 +- 0.6 + + + + + (3) Bimodal dike complex (BDC) F 4.5 - 0.2 + + + + + + + + (4) Upper Miocene-Pliocene plutons + Peninsula Cabo Raper + Tres + Montes Lauquen 46˚50'S (5) Pliocene volcanics + + Pluton B 4.8 +- 0.3 Rhyolite breccia + + P5 F 4.2 - 0.1 +

Cabo Helena Caldera TRES MONTES PENINSULA Main volcanic dome 46˚55'S Major shear zone

+ 40 39 B 5.0 - 1.0 Ar/ Ar date B : biotite F : K-feldspar (b) 46˚00'S Fig. 3 (continued). (b) Geological map of the Taitao Peninsula based on ®eld observations obtained during two ®eld expeditions conducted in 1992 and 1995 and new 40Ar=39Ar dates obtained by NA.

Mpodozis et al. (1985) which gave younger K=Ar (NA) are 5:9 š 0:5Maand6:9 š 1 Ma, respectively, ages (3:6 š 0:6and3:3 š 0:3 Ma on biotite). The and are consistent with previous K=Ar ages from Cabo Raper and Tres Montes plutons have chemical Mpodozis et al. (1985) .5:5 š 0:4and5:2 š 0:3Ma characteristics similar to those of the adakitic or on biotite). trondhjemite±tonalite±dacite suites (Bourgois et al., The remaining plutons are smaller or have been 1996) which are believed to derive from partial poorly explored. The Bahia Barrientos pluton (P3) melting of metabasalts under amphibolite±eclogite has been described in Mpodozis et al. (1985) and transition conditions (Defant and Drummond, 1990; the Estero Cono pluton (P4) in Nelson et al. (1993). Drummond and Defant, 1990; Kay et al., 1993; The Tres Montes pluton (P5) has been sampled dur- Martin, 1993). ing our ®eld expedition in 1995. It shows tonalitic The Seno Hoppner pluton (P2) is a ®ne- to me- to granodioritic compositions (Fig. 4). Along the dium-grained biotite and hornblende-bearing granite seashore, north of Cabo Helena (Fig. 3b), this plu- with SiO2 contents ranging from 68 to 75 wt%. Its ton consists of magmatic breccias, showing evidence composition is granitic to trondjhemitic (Fig. 4) and of hydraulic fracturing, and locally includes clasts unlike the Cabo Raper pluton, it displays character- of greenschist facies dolerites. Granite fragments istics of typical calc-alkaline series. New 40Ar=39Ar (P6) observed within the rhyolite breccias associated ages obtained from biotite and feldspar by one of us with the bimodal dike complex as well as the cen- 90 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

An

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Granite Trondhjemite Ab Or

Cabo Raper pluton (P1) Tres Montes pluton (P5)

Seno Hoppner pluton (P2) Central pluton (P6)

Fig. 4. Classi®cation of the Taitao Peninsula plutonic rocks according to their normative anorthite±albite±orthoclase composition after O'Connor (1965). tral pluton of Nelson et al. (1993), also belong to 3.2. The Taitao Ridge the plutonic suite. They plot in the ®eld of trond- hjemites within the normative An±Ab±Or triangle The Taitao Ridge is located immediately south of of O'Connor (1965) (Fig. 4). They show strong al- the Chile Triple Junction at the boundary between teration and do not contain Fe±Mg minerals except two segments of the active margin that exhibit strong epidote and chlorite which replace amphibole. differences in their tectonic regime: a linear trench to the North, suggesting active subduction-erosion, 3.1.5. The Pliocene volcanic edi®ces and a wider forearc domain to the South with devel- Volcanic and hypovolcanic edi®ces (volcanoes, opment of an accretionary prism (Behrmann et al., stocks, calderas) occur in the eastern part of the 1994; Bourgois et al., 1997). The Taitao Ridge has studied area in a region lacking extensive deforma- been de®ned as an anomalous bathymetric ridge in tion. Radiometric ages published by Mpodozis et the trench region (Fig. 2). According to Behrmann al. (1985) on two edi®ces (Pan de Azucar and the et al. (1994), the volcanic pile of the Taitao Ridge Fjordo San Pedro Caldera) range from 3:8 š 0:8Ma was probably formed at a ridge-transform intersec- to 5:1 š 1:3 Ma. From limited data (Le Moigne, tion and represents a possible candidate to become 1994), the geochemical composition of the Pan de an ophiolite body emplaced into the South American Azucar and Fjordo San Pedro volcanoes is adakitic forearc. However, a recent bathymetry and geophys- and appears to be very similar to that of the Cabo ical survey of the triple junction region in 1997 by Raper pluton especially with respect to low heavy R=V L'Atalante has revealed that the basement of REE (HREE) contents. the Taitao Ridge is not only volcanic but also con- C. Guivel et al. / Tectonophysics 311 (1999) 83±111 91 sists of tectonic units made up of continent-derived element contents and LREE-enrichments. Their Nb sediments (Bourgois et al., 1997). depletion and low Ce=Pb and K=Pb ratios are con- At ODP Site 862, the Taitao Ridge crest is com- sistent with a subducted-sediment component in their posed of a 20-m-thick sedimentary layer overlying mantle source. The available data suggest that there ma®c and silicic volcanic rocks (Fig. 5). The vol- is a continuum from N-type MORB to arc basalts canic rocks have been drilled down to 42.9 m at among the samples recovered from the Taitao Ridge. hole 862B and 102.1 m at hole 862C. The sediments Rhyolites and dacites are presumably melts of and sedimentary rocks are composed of silty clay to metama®c sources. Trace-element patterns and Pb clayey silt and ®ne silty sand. Microfossils recov- isotopic compositions indicate that their sources ered from the sediments indicate a Late Pliocene age were chemically similar to those of the Group 2 consistent with 40Ar=39Ar ages of 1:54 š 0:08 and calc-alkaline basalts. The higher 87Sr=86Sr ratios of 2:2 š 0:4 Ma obtained on hornblende separated from the rhyolites might result from incorporation of sea- the rhyolitic samples (Forsythe et al., 1995b). water-derived ¯uids in these sources (Forsythe et al., The cores contain sedimentary rocks in depo- 1995a). sitional contact with the glassy rims of pillowed basaltic ¯ows. Volcanogenic material within the cores is found as isolated clasts or glass shards 4. A summary of geochemical data from the within the sediments, which is consistent with a Late southern Chile spreading ridge Pliocene magmatic activity (Forsythe et al., 1995a). The underlying volcanic units are bimodal and Samples have been collected from four ridge seg- consist of an alternation of plagioclase±clinopyrox- ments (segment 4 to segment 1, from N to S) of the ene±olivine phyric basalts and plagioclase±hornblen- southern Chile ridge between the Chiloe F. Z. and the de phyric dacites=rhyolites (Fig. 5) among which Chile Margin Triple Junction including the segment Forsythe et al. (1995a) have distinguished three main currently being subducted at the Chile Trench (seg- groups: two groups of basalts (Group 1 and Group 2) ment 1) (Klein and Karsten, 1995). Their analysis and one group of dacites=rhyolites. Group 1 basalts reveals that the sub-ridge mantle appears to be highly differsfromGroup2byhavinglowerSiO2,Na2O heterogeneous at the scale of each of these segments * and K2O with higher FeO , MgO and CaO, and lower as evidenced by the occurrence of N-MORB and Sr and Ba contents. The REE patterns of Group 1 two types of E-MORB. Segment 4 lavas have many basalts are characterized by strong LREE depletion, trace-element similarities with E-MORB and ocean whereas Group 2 basalts exhibit LREE-enriched pat- island basalt (OIB). Segments 1 and 3 show trace- terns. The Sr±Nd isotopic composition of one Group element patterns rather similar to those of convergent 1basaltissimilartothoseofcommonMORBbut margin magmas (Klein and Karsten, 1995). its 207Pb=204Pb ratio is higher. Group 2 basalts have The most enriched samples from segments 1 and 3 distinctly different Sr±Nd compositions (87Sr=86Sr D are similar to arc lavas regarding their high abun- 0.7033, žNd DC8:0). Group 2 basalts, and rhyolites, dances in the most incompatible trace elements, their have similar Pb isotopic ratios, which are higher in enrichment in Pb (relative to Ce and Sr) and their de- 207Pb=204Pb and 208Pb=204Pb than the Northern Hemi- pletion in Nb and Ta (relative to U and K), although sphere Reference Line or the Nazca Plate Basalts. all segment 1 samples are LREE-depleted. In con- Distinct sources are required to account for the dif- trast, enriched samples from segment 4 display little ferences in major, trace elements and isotopic com- or no depletion in Nb and Ta and exhibit enrichments positions of Group 1 and Group 2 basalts. Group 1 that are often regarded as typical of OIB. basalts most resemble N-type MORB in their major, The corresponding trace-element variations have trace elements and isotopic geochemistry, except for been modeled by bulk mixing of a depleted MORB their Rb enrichment, high Pb contents and isotopic ra- source mantle with a mantle contaminated by various tios which are consistent with small amounts of sub- amounts of oceanic sediments and altered oceanic ducted sediments in their source. Group 2 basalts are crust or melts=¯uids derived therefrom (Klein and calc-alkaline with higher K2O, incompatible trace- Karsten, 1995). To explain the presence of the sub- 92 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

862B 862C 1228.6 mbsl IA

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B3X01-5 b C5R01-2

c Unit II B4X01-1 g C6R01-3 B4X01-4 d h B4X01-7 e C6R01-8 B4X02-10 f i g C6R01-9 j C7R01-3 k C7R01-7 l C8R01-2 m C8R01-11

marine sedimentary strata basaltic volcanics

hydrothermal ocher zone (with sulfides) rhyolitic and dacitic volcanics Fig. 5. General unit stratigraphy for holes 862B and 862C on the Taitao Ridge and location of the samples used in this study (modi®ed from Behrmann et al., 1992). ducted component contaminant beneath the ridge 5. New geochemical data on young volcanic suites axis, Klein and Karsten (1995) propose two differ- from Taitao Ridge and Taitao Peninsula ent models: (a) slab fragments reintroduced by the shallow asthenospheric ¯ow and entrained in the up- 5.1. Sampling and analytical methods welling and melting regime beneath the ridge due to slab breaking, or (b) the presence of a slab window 5.1.1. Major- and trace-element data providing a major connection between the metasom- New major- and trace-element whole-rock analy- atized sub-arc mantle and the adjacent sub-oceanic ses of 24 magmatic rocks from the Taitao Peninsula mantle. (®eld expedition Taitao 1995) and 20 volcanic rocks C. Guivel et al. / Tectonophysics 311 (1999) 83±111 93 from the Taitao Ridge (Site 862, Leg ODP 141) have for isotopic analysis using a VG SIRA10 triple col- been performed for this study. They are presented here lector instrument. Results are expressed as ½ δ18O together with 29 analyses from the Taitao Peninsula units relative to the V-SMOW standard reference. previously published by Le Moigne (1994). Table 3 shows selected major-element parameters of Whole-rock analyses (major, trace and rare earth the studied whole rocks and the corresponding mea- elements) were obtained by inductively coupled sured 87Sr=86Sr, 143Nd=144Nd ratios and δ18O-values plasma-atomic emission spectrometry (except Rb on whole rock and clinopyroxene. wich was obtained by atomic absorption spec- troscopy). Calibrations were made using our own 5.2. Rock classi®cation and geochemical standards and controlled with international reference characteristics samples: PM-S; WS-E; BE-N; AC-E; JB-2 and SY-4 (Universite de Bretagne Occidentale). Relative stan- 5.2.1. Major- and trace-element geochemistry dard deviation for major elements is 2% except for Taitao Ridge and Peninsula lavas display in the MnO and P2O5. That for trace elements is 5%. alkali±silica diagram (Fig. 6a) a wide compositional Analysed samples include 2 greenschist facies range from basalts to intermediate and acidic lavas dolerites from the Bahia Barrientos ophiolite, 12 do- with SiO2 contents ranging from 47 to 72.5%. This lerites and 6 dacites and rhyolites from the Bimodal diagram also shows the wide range of variation of Dike Complex, and 2 fresh basalts and 2 rhyolites alkalis (up to 8%) in the basaltic lavas of the Taitao from the CMU. The 20 analysed rocks of the Taitao Peninsula, re¯ecting their high degree of alteration Ridge drill hole include 8 samples from ODP hole or metamorphism. In an AFM triangle (Fig. 6b) most 862B and 12 from hole ODP 862C. Selected geo- of the lavas emplaced at the Chile Triple Junction chemical analyses are listed in Tables 1 and 2, with area plot within the calc-alkaline trend rather than detection limits for the trace elements. Site locations within the tholeiitic one. are shown in Fig. 3a. In the Harker diagrams shown in Fig. 7a, most major elements are negatively correlated with SiO2 * 5.1.2. Strontium, neodymium and oxygen isotopic (MgO, Fe2O3, CaO and TiO2) except for Na2O data on Taitao Peninsula samples and K2O. Cr and Ni decrease from near primitive New analyses were conducted on both whole contents to very low concentrations in the most rocks and separated clinopyroxenes, to avoid the evolved lavas (Fig. 7b). But differences at equivalent effects of alteration on the isotopic data. Clinopyrox- SiO2 contents are observed among the basalts. The enes were separated by rock crushing followed by Taitao Ridge basalts have higher Ni contents than heavy liquid separation and ®nally handpicking. basalts from the Taitao Peninsula consistent with the Nd and Sr compositions have been obtained from occurrence of olivine crystals in the former. one gabbro of the Bahia Barrientos ophiolite (TA65), The rock classi®cation used in this study is not one ophitic dolerite, one dacite and one rhyolite of based on major-element variations as the rocks have the Bimodal Dike Complex (TA55, TA103, TA91, suffered various degrees of alteration especially on respectively), one dacite from CMU (T18a), and the Taitao Peninsula. The magma types shown in Ta- one basaltic andesite from MVU (T16d). The Nd ble 4 are distinguished using some key trace-element and Sr compositions have been obtained from both ratios [(La=Nb)N,(La=Yb)N and (Sr=Y)] and multi- whole rock and separated clinopyroxenes for sam- element patterns of incompatible trace elements nor- ples TA55, TA65, T18a and T16d. As the magmatic malized to N-MORB (Sun and McDonough, 1989). rocks are young, initial radiogenic isotopic ratios When considering the available whole-rock anal- have not signi®cantly evolved since emplacement yses on both the volcanic and plutonic rocks found and age corrections are unnecessary. on the Taitao Ridge and on the Taitao Peninsula, six Oxygen isotope compositions have been obtained main magma types can be recognized. They include on separated clinopyroxenes. Oxygen was extracted four types of basalts and two types of intermedi- from the clinopyroxenes using the BrF5 method ate to evolved lavas and their plutonic equivalents: (Clayton and Mayeda, 1963) then converted to CO2 (Type 1) N-type MORB; (Type 2) E-type MORB; 94 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

Table 1 Selected whole-rock analyses of Taitao Peninsula Bimodal Dike Complex magmatic rocks

Sample: Detection limit (ppm) TA89 TA99 TA104 TA107 TA109 TA90 TA105 TA106 TA108 Sampling site a: 24-01 24-03 24-03 24-04 24-04 24-01 24-03 24-04 24-04 Unit: BDC BDC BDC BDC BDC BDC BDC BDC BDC Nature b: diabase diabase diabase diabase diabase dacite rhyolite trachydacite trachydacite

SiO2 52.20 51.35 53.50 52.75 53.80 66.70 72.50 65.80 69.60 TiO2 1.70 1.61 1.92 1.87 2.30 0.50 0.28 0.65 0.52 Al2O3 15.55 15.25 15.25 15.24 14.95 16.10 13.83 15.86 15.00 * Fe2O3 10.25 9.58 10.28 11.27 11.10 3.85 2.04 3.91 3.03 MnO 0.19 0.14 0.08 0.19 0.11 0.03 0.02 0.05 0.05 MgO 5.80 7.10 5.12 5.42 5.34 2.35 1.12 2.18 1.36 CaO 8.62 10.10 7.80 8.25 7.30 4.30 0.54 1.53 0.87 Na2O 3.23 3.39 5.01 3.93 3.20 4.10 3.64 4.98 5.01 K2O 0.12 0.33 0.13 0.17 0.25 0.55 4.28 2.79 3.19 P2O5 0.24 0.21 0.25 0.23 0.30 0.10 0.05 0.14 0.10 L.O.I. 2.02 1.21 1.01 0.94 1.80 1.46 1.38 2.01 1.29 Sum 99.92 100.27 100.35 100.26 100.45 100.04 99.68 99.90 100.02 Mg# 57.11 63.55 53.96 53.09 53.09 58.95 56.37 56.74 51.36 Rb 1 1 5.7 1.6 2.2 6 25 124 80 99 Sr 1 200 209 188 161 163 222 98 146 118 Ba 3 74 109 82 85 53 185 635 500 440 Sc 0.2 37 39 30.5 32 31.5 9 4.2 8 6.5 V 3 265 274 285 300 345 59 30 80 52 Cr 2 125 123 13 12 17 37 12 9 3 Co 2 3334293329114 9 6 Ni 2 3252232518278 7 4 Y 0.5 34.5 31 38 37.5 42 14 19 23 19.5 Zr 2 46253131561154170180 Nb 1 8.5 11.7 11 11.1 14.6 4.9 5.5 11.2 10 La 1 11.6 10.4 11.6 11.1 14.1 15 21.5 20.2 20.6 Ce 2 27.5 23 28 25 32.5 31 41 41 41 Nd 2 18.5 15 18.8 16.8 21.7 15 19 19 18 Eu 0.2 1.56 1.36 1.73 1.61 1.88 0.79 0.64 0.89 0.84 Dy 0.4 6.3 5.1 6 6.8 7.3 2.6 2.9 3.7 3.1 Er 1 3.6 3 3.6 3.6 4.2 1.5 1.7 2.3 1.9 Yb 0.2 2.91 2.76 3.48 3.35 3.61 1 1.56 2.21 1.8 Th 0.1 1.25 0.95 1.5 1.35 1.65 3.75 8.8 8 7.45

(La=Nb)N 1.42 0.92 1.09 1.04 1 3.18 4.06 1.87 2.14 (La=Yb)N 2.86 2.7 2.39 2.38 2.8 10.76 9.89 6.56 8.21 a Sampling sites are shown in Fig. 3. b * Major elements in wt%, Fe2O3 as total iron. Trace elements in ppm. ICP-AES analyses: J. Cotten, Brest (France). Analytical methods discussed in the text. Mg# D 100.Mg=Mg C Fe2C/. (La=Nb)N: chondrite-normalization from Sun and McDonough (1989).

(Type 3) LREE-depleted N-MORB with trace el- the Taitao Peninsula and the southern Chile ridge is ement characteristics of arc basalts; (Type 4) Nb- presented in Table 4. depleted E-MORB; (Type 5) calc-alkaline andesites, dacites and rhyolites; and (Type 6) andesites and 5.2.2. Basalts dacites with adakitic af®nities. The spatial distri- The distinctions among the basalt types are bution of these types throughout the Taitao Ridge, based on their chondrite-normalized (La=Nb)N and C. Guivel et al. / Tectonophysics 311 (1999) 83±111 95

Table 2 Selected whole-rock analyses of Taitao Ridge lavas, Leg ODP141, Site 862

Sample: Detection limit a B1W01-1 B2XCC-1 B3X01-5 B4X01-4 B4X01-7 C1W01-3 C3R01-2 C5R01-2 C6R01-9 Interval (cm) 1-5 33-38 28-35 28-41 60-66 41-46 9-13 10-18 61-69 Unit: Taitao Taitao Taitao Taitao Taitao Taitao Taitao Taitao Taitao Ridge Nature: rhyolite basalt basalt basalt basalt dacite basalt dacite basalt

SiO2 70.00 49.50 49.50 51.20 50.50 68.90 51.90 64.90 52.00 TiO2 0.32 1.07 1.06 1.23 1.18 0.33 1.09 0.75 0.97 Al2O3 14.35 17.05 17.15 16.48 16.20 14.35 17.70 16.00 16.50 * Fe2O3 2.38 8.95 8.70 8.22 8.12 2.76 6.92 3.89 7.63 MnO 0.04 0.15 0.13 0.12 0.12 0.04 0.11 0.06 0.12 MgO 0.65 8.65 8.34 7.43 7.72 0.62 6.33 1.63 8.60 CaO 2.09 11.40 11.40 9.65 9.35 2.00 10.24 3.92 9.60 Na2O 4.48 2.89 2.93 3.40 3.53 4.47 3.44 3.94 3.19 K2O 2.88 0.09 0.13 0.48 0.45 2.70 0.58 2.09 0.39 P2O5 0.08 0.09 0.09 0.14 0.14 0.09 0.12 0.13 0.11 L.O.I. 2.85 -0.13 0.72 1.55 2.25 3.41 1.09 2.20 0.46 Sum 100.12 99.71 100.15 99.90 99.56 99.67 99.52 99.51 99.57 Mg# 39.12 69.46 69.28 68.02 69.11 34.58 68.28 49.65 72.62 Rb 1 95 1.6 1.6 9.5 13.3 94 13 71 14 Sr 1 115 88 88 145 140 115 144 172 120 Ba 3 450 10 7 64 76 450 83 392 79 Sc 0.2 3.2 31 31 28.5 26.5 3 28 8.2 26 V 3 25 210 210 195 190 25 195 81 182 Cr 2 6 345 340 260 216 5 330 9 365 Co 2 4 38.5 38.5 34 38.5 6 34 10.5 37 Ni 2 4 167 170 176 205 11 162 11 205 Y 0.5 16.5 24.7 25.5 29 26 17 24.5 20 23.5 Zr 2 172 60 60 93 101 193 96 155 89 Nb 1 6.6 1 0.9 2.7 2.7 6.6 3.3 6.6 2.7 La 1 19.8 1.75 1.85 5.8 5.6 19.6 5.7 17.2 5.2 Ce 2 38 5.8 6.5 15.4 14 37 14.5 34 12.5 Nd 2 15.3 7.3 7 11.3 11.4 15.3 10.4 16 9.7 Eu 0.2 0.65 1.01 1 1.14 1.12 0.65 1.07 0.9 0.98 Dy 0.4 2.6 4.3 4.3 4.7 4.5 2.7 4.3 3.3 4 Er 1 1.85 2.7 2.6 2.7 2.7 1.9 2.6 2.1 2.5 Yb 0.2 1.71 2.37 2.44 2.76 2.53 1.73 2.35 1.94 2.28 Gd 0.3 3.2 3.6 3.6 4.3 4.1 3.1 3.8 3.6 3.6 Th 0.1 10.7 0.1 0.2 1.5 1.45 10.9 1.6 7.1 1.5

(La=Nb)N 3.11 1.82 2.13 2.23 2.15 3.08 1.79 2.71 2 (La=Yb)N 8.31 0.53 0.54 1.51 1.59 8.13 1.74 6.36 1.64 a * Major elements in wt%, Fe2O3 as total iron. Trace elements in ppm. ICP-AES analyses: J. Cotten, Brest (France). Analytical methods discussed in the text. # 2C Mg D 100.Mg=Mg C Fe /. (La=Nb)N: chondrite-normalization from Sun and McDonough (1989).

(La=Yb)N ratios (Fig. 8). Magmas of Type 1 and 3 5.2.3. Type 1 and Type 3 basalts are N-type MORBs with respect to their LREE de- Type 1 and Type 3 basalts can be distinguished pletion [(La=Yb)N < 1; Fig. 8]. Type 2 and 4 are one from another using the spidergram pattern E-type MORBs which display enriched light rare of the incompatible trace elements normalized to earth elements patterns (Fig. 9) with (La=Yb)N ratios N-MORB. Compared to Type 1, Type 3 N-MORB ranging from 1.3 to 3.4 (Fig. 8). show enrichments in large ion lithophile elements 96 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

Table 3 Isotopic and chemical data for Taitao Peninsula magmatic rocks

87 86 143 144 18 Sample SiO2 LOI K2O Sr= Sr Nd= Nd žNd δ O (wt%) (wt%) (wt%) (½ vs. V-SMOW) CMU T18a WR 64.25 1.5 1.28 0.704390 š 11 0.512793 š 5 C3.02 Cpx 0.704039 š 15 0.512839 š 5 C3.92 C6.06 š 0.06 magma C6.36 MVU T16d WR 53 3.38 1.57 0.705309 š 11 0.512862 š 5 C4.37 Cpx 0.704394 š 14 0.512889 š 8 C4.89 C5.9 š 0.07 magma C6.2 BBO TA55 WR 48.6 3.1 0.42 0.704399 š 14 0.513107 š 7 C9.15 Cpx 0.704303 š 17 0.513017 š 10 C7.39 C5.92 magma C6.2 BBO TA65 WR 51 0.5 0.01 0.702658 š 12 0.513113 š 13 C9.26 Cpx 0.702650 š 10 0.513151 š 34 C10.01 C6.29 magma C6.6 BDS TA91 WR 76 1.26 0.16 0.704148 š 11 0.512801 š 5 C3.17 BDS TA103 WR 66.5 2.02 2.22 0.704995 š 13 0.512686 š 7 C0.93

WR: whole rocks; Cpx: separated clinopyroxenes; and magma: δ18O-magmatic values obtained from δ18O-clinopyroxenes C0.03½ (Sheppard and Harris, 1985).

(LILE) such as Rb, Ba and K and slight depletion show (La=Nb)N ratios lower than 1.2, whereas Type in high ®eld strength elements (HFSE; Fig. 9). Type 4 basalts display (La=Nb)N ratios ranging from 1.2 1 basalts are found in the Taitao Peninsula, mainly to 2.35 (Fig. 8). These ratios emphasize the HFSE in the Bahia Barrientos ophiolite. These basalts dis- depletion of Type 4 magmas relative to LILE and play LILE variations as illustrated by their K=Ti LREE, which is a characteristic feature of arc mag- ratios, ranging from 0.03 in a dolerite from the Bahia mas. Type 2 is unknown in the Taitao Ridge but Barrientos ophiolite to 0.6. However, due to the we found some E-MORB in the Taitao Peninsula LILE mobility during alteration and metamorphism, in CMU and MVU. Some of the greenschist facies these features may be secondary. Type 3 magmas are dolerites of the BDC may also be classi®ed as Type N-MORB with selective trace element enrichments 2. In Fig. 9, they display REE-enriched but Rb- usually interpreted as tracing a continental and=or and K-depleted patterns possibly due to alteration subduction-related component. They are found in the or metamorphism. In the AFM plot (Fig. 6b) they Taitao Ridge (Group 1 basalts of Forsythe et al., de®ne a tholeiitic trend with Fe and Ti enrichment 1995a). Uncommon N-type MORB found on the in the early stages of crystallization. They also have Taitao Peninsula display similar patterns (Fig. 9), the highest Nb and Yb contents (Fig. 7b) of our set, together with rather variable Sr and P contents. and, compared to other magma types, rather constant trace element ratios, e.g. (La=Nb)N (Fig. 8). Type 4 5.2.4. Type 2 and Type 4 basalts magmas are enriched mid-ocean-ridge basalts with Distinction between Type 2 and 4 is based on trace element features usually considered as typical the ratios between LILE and HFSE, especially the of subduction-related magmas. This type is typical of behaviour of Nb compared to La. Type 2 basalts most basalts drilled on the Taitao Ridge and is equiv- C. Guivel et al. / Tectonophysics 311 (1999) 83±111 97

9

Rhyolite 8

7

6 (wt. %) (wt. O 2 5

O + K Basaltic 2 andesite

Na Basalt 4 Andesite

Dacite 3

2 40 50 60 70 80 (a) SiO2 (wt. %)

TAITAO TAITAO SCR P. R. TYPE 1

TYPE 2 TYPE 3 TYPE 4 FeO*

TYPE 5 TYPE 6

Tholeiitic

Calc-alkaline

(b) Alk MgO

Fig. 6. (a) TAS diagram (Le Maitre, 1989) Na2O C K2O (wt%) vs. SiO2 (wt%) from the Taitao Peninsula, Taitao Ridge and Southern Chile Ridge (SCR). Our new data for the Taitao Peninsula and the Taitao Ridge are compared with Taitao Peninsula data published by Le Moigne (1994). Southern Chile ridge data are from Klein and Karsten (1995). Symbols as in Table 4. (b) AFM diagram showing the * * boundary between the calc-alkaline ®eld and the tholeiitic ®eld after Irvine and Baragar (1971); FeO calculated assuming Fe2O3 D 0.9 FeO*. 98 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

3 15

TAITAO TAITAO SCR TiO (wt %) P. R. CaO (wt %) 2 TYPE 1 TYPE 2 TYPE 3 2 TYPE 4 10

TYPE 5 TYPE 6

1 5

10 8 MgO (wt %) Na2O(wt %) 7 8

6 6 5

4 4

2 3

6 11 Fe2O3* (wt %) K2O (wt %) 5

9 4

7 3

5 2

3 1

20 0.4

Al2O3 (wt %) P2 O5 (wt %) 18 0.3

16 0.2 14

0.1 12 (a) 10 0.0 40 50 60 70 80 40 50 60 70 80 Si O2 (wt %) SiO2 (wt %) Fig. 7. Harker diagrams showing (a) major, and (b) trace element variations of magmatic rocks from the Taitao Peninsula, Taitao Ridge and Southern Chile ridge (shaded ®eld). Data sources as in Fig. 6. We convert FeO* of the Southern Chile ridge basalts (Klein and * * Karsten, 1995) assuming Fe2O3 D 0.9 FeO . C. Guivel et al. / Tectonophysics 311 (1999) 83±111 99

300 20

TAITAO TAITAO SCR Ni p.p.m. P. R. Nb p.p.m. TYPE 1 TYPE 2 15 200 TYPE 3 TYPE 4

TYPE 5 10 TYPE 6

100 5

500 30

Cr p.p.m. La p.p.m. 400

20 300

200 10

100

200 400 Rb p.p.m. Sr p.p.m.

150 300

100 200

50 100

1000 4 Ba p.p.m. Yb p.p.m. 800 3

600 2 400

1 200 (b) 0 0 40 50 60 70 80 40 50 60 70 80 SiO (wt %) SiO (wt %) 2 2 Fig. 7 (continued). 100 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

Table 4 Magma types and their geographic distribution a

a This distribution is based on our new data from the Taitao Peninsula and the Taitao Ridge, on the Taitao Peninsula data published by Le Moigne (1994) and on the Southern Chile ridge data published by Klein and Karsten (1995). Symbols as in Figs. 6±8 and 11. alent to the Group 2 of Forsythe et al. (1995a). On display LILE and LREE enrichments (Fig. 9) and the Taitao Peninsula, they occur in the CMU, MVU clear negative Nb anomalies relative to neighbouring and BDC. In the N-MORB-normalized plot, they incompatible elements.

3 N-MORB ARC IMPRINT

2 N (La/Nb)

1 TAITAO TAITAO SCR E-MORB P. R. TYPE 1

TYPE 2 TYPE 3 TYPE 4 0 01 2 3 4

(La/Yb)N

Fig. 8. (La=Yb)N vs. (La=Nb)N showing the differences between three types of basalts. Chondrite-normalization values are from Sun and McDonough (1989). C. Guivel et al. / Tectonophysics 311 (1999) 83±111 101 CMU-MVU CMU-MVU nt patterns of the RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb

1

1

10 10 MORB-N Rock/ Rock/ MORB-N Rock/ 0.1 0.1 100 100 Taitao Yb Yb Yb Peninsula Er Er Er BDC Dy Dy Dy BDC Y Y Y BBO Ti Ti Ti Eu Eu Eu P P P Nd Nd Nd BDC-CMU-MVU Sr Sr Sr Ce Ce Ce La La La Nb Nb Nb K K K Ba Ba Ba Rb Rb Rb RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb

1 1

1 1

10 10 Rock/ MORB-N Rock/ MORB-N Rock/ 10 MORB-N Rock/ 10 Rock/ MORB-N Rock/ 0.1 0.1 0.1 0.1 100 100 100 100 Yb Er Dy Y Ti Eu P Nd Sr Ce La Taitao Ridge Taitao Nb K Ba Rb RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb

1 1

10 10 Rock/ MORB-N Rock/ 0.1 MORB-N Rock/ 0.1 100 100 Yb Yb Yb Er Er Er Dy Dy Dy Y Y Y Ti Ti Ti Eu Eu Eu P P P Nd Nd Nd Sr Sr Sr Chile Ridge Ce Ce Ce Southern La La La Nb Nb Nb K K K Ba Ba Ba Rb Rb RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb Rb

1 1 1

1 10 MORB-N Rock/ 10 10 Rock/ MORB-N Rock/ MORB-N Rock/ 10 MORB-N Rock/ 0.1 0.1 0.1 0.1 100 100 100 100 Fig. 9. Multi-element plots ofbasalts the from Taitao the Ridge Southern and Chile Taitao ridge Peninsula (Klein basaltic and lavas Karsten, normalized 1995) to are the shown N-MORB for of comparison Sun (bold-face and curves). McDonough (1989). Trace-eleme TYPE 1 TYPE 2 TYPE 3 TYPE 4 102 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

5.2.5. Evolved lavas: Type 5 and Type 6 noes are typically mantle derived and calc-alkaline All of the intermediate and evolved lavas (SiO2 > (Lopez-Escobar et al., 1993). 53%) show chemical characteristics of arc magmas, Type 5 magmas (andesites, dacites and rhyo- i.e. depletion in HFSE relative to LILE and LREE lites) occur in the MVU, CMU and BDC of the (Fig. 10). They have low TiO2 contents (<1%) Taitao Peninsula and also include all the evolved and high LILE=HFSE and HREE=HFSE ratios com- lavas drilled on the Taitao Ridge. Their chemical pared with other mantle-derived magmas. Their features are typically calc-alkaline. Finally, Type N-MORB-normalized trace-element plots display 6 magmas, which display adakitic characteristics, Nb and Ti negative anomalies with respect to adja- are represented by the Pliocene volcanic edi®ces of cent elements (Fig. 10). Fig. 11 is a Sr=Yvs.Yplot Chaicayan Islands, Pan de Azucar and Fjordo San for all of the intermediate and evolved lavas showing Pedro, as well as the Cabo Raper pluton. the expected ®elds for slab melts (solid envelop) and mantle-derived magmas (dashed envelop). It allows 5.2.6. Isotope geochemistry us to discriminate between Type 5 and Type 6 mag- The 87Sr=86Sr values vary from 0.702650 in one mas. The distinction between Type 5 (calc-alkaline gabbro of Type 1 from the Bahia Barrientos to or mantle-derived magmas) and Type 6 (adakites or 0.704394 in Type 5 magmas and the žNd from slab melts) is based on the higher contents of Sr and C10.01 in Type 1 to C3.92 in one dacite from lower Y and lower heavy REE contents in Type 6 Type 5. The δ18O magma values deduced from lavas (Fig. 10), characteristic of adakitic composi- δ18O-values measured on clinopyroxenes vary from tions (Drummond and Defant, 1990). Lavas from the C6.2 to C6.6½ which is slightly higher than typical Austral Volcanic Zone and from the Southern South mantle values (C5:7 š 0:3½) and than those mea- Volcanic Zone have been plotted for comparison in sured on the Chile ridge basalts (C5.6 to C6.0½, Fig. 11. Austral Volcanic Zone lavas display adakitic Sherman et al., 1997b). compositions (Stern and Kilian, 1996; Sigmarsson et A positive correlation between whole-rock 87Sr= al., 1998). The Southern South Volcanic Zone volca- 86Sr isotopic ratios and loss on ignition (LOI) is ob-

1000 1000 1000 BDC CMU- MVU 100 100 100

10 10 10

TYPE 5 MORB-N Rock/ Rock/ MORB-N Rock/ Rock/ MORB-N Rock/ 1 1 1

0.1 0.1 0.1 RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb

SiO2 (wt %) : 65-72

1000 Chaicayan 100 Islands

10 TYPE 6 Rock/ MORB-N Rock/ 1

0.1 RbBa K NbLaCeSr Nd P Eu Ti Y Dy Er Yb Taitao Ridge Taitao Peninsula

Fig. 10. N-MORB-normalized trace-element plots of the Taitao Ridge and Taitao Peninsula andesite, dacite and rhyolite lavas. C. Guivel et al. / Tectonophysics 311 (1999) 83±111 103

200

TAITAO TAITAO SSVZ AVZ P. R. TYPE 5

150 TYPE 6 Slab melts

100 Sr/Y

50 Mantle derived magmas

0 0 10203040 Y Fig. 11. Sr=Y vs. Y diagram showing ®elds expected for slab melts (solid curve) and mantle-derived magmas (dashed curve) (Maury et al., 1996). Dacites from the Chaicayan Islands plot within the slab melt ®eld while all the andesites, dacites and rhyolites from the Taitao Peninsula and Taitao Ridge plot within the mantle-derived ®eld. The Austral Volcanic Zone (AVZ) lavas classi®ed as adakites (Stern and Kilian, 1996) and the Southern Volcanic Zone (SVZ) lavas that include `normal' dacites (Lopez-Escobar et al., 1993) are also shown for comparison. served in Fig. 12a, whereas 143Nd=144Nd ratios and dacitic and rhyolitic ¯ows occur on both structures. LOI appear uncorrelated (not shown). This may be The ma®c±felsic sequence recovered from the Taitao attributed to interaction of the studied rocks with sea- Ridge recalls the volcano±sedimentary sequences water, which results in an increase of their 87Sr=86Sr found in the Taitao Peninsula. In both cases, they ratios whereas their 143Nd=144Nd ratios remain un- may represent a volcano±sedimentary blanket of a changed. This is well illustrated by sample T16d for forearc region in the vicinity of the Chile Triple which the 87Sr=86Sr ratio of the whole rock is clearly Junction. However, Taitao Ridge and Taitao Penin- higher than that of the separated clinopyroxenes at sula lavas display distinct alteration=metamorphic equivalent 143Nd=144Nd ratios (Table 3). If, in a con- features. The degree of alteration of basalts from servative way, we only consider the behaviour of Site 862 on the Taitao Ridge is low, with smec- δ18O, 87Sr=86Sr and 143Nd=144Nd ratios measured on tites as the main secondary minerals (Kurnosov et handpicked clinopyroxenes, no correlation appears al., 1995). In contrast, the Taitao Peninsula volcanic between oxygen isotopic ratios, which remain ap- units have experienced moderate to strong develop- proximately constant, and strontium and neodynium ment of greenschist facies minerals. Available ages ratios which vary signi®cantly (e.g. Fig. 12b). of these magmatic units are rather similar: in the Taitao Peninsula, Cabo Raper and Seno Hoppner 5.2.7. Comparison between the Taitao Peninsula and plutons are dated between 6 and 3.5 Ma (Mpodozis Taitao Ridge lavas et al., 1985) and the volcano±sedimentary units be- The similarities between the Taitao Ridge and tween 4.6 and 1.5 Ma (Mpodozis et al., 1985; Bour- some of the Taitao Peninsula units (BDC, CMU and gois et al., 1992). On the Taitao Ridge, sediments MVU) reside in the nature of magmatic products yield Late Pliocene ages and a radiogenic date avail- erupted in these two areas, and their association with able on a rhyolitic ¯ow is 1.5 Ma (Forsythe et al., sediments. Spatially associated basaltic as well as 1995b). 104 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

0.706 5.2.8. Comparison with the southern Chile (a) spreading ridge basalts The four kinds of basalts discussed also occur 0.705 among the southern Chile ridge lavas. Type 1 mag-

Sr mas have been sampled along all four segments of the 86 0.704 southern Chile ridge (Klein and Karsten, 1995; Kar- Sr/

87 sten et al., 1996; Sherman et al., 1997a). Sherman et R= 0.793 = 0.703 al. (1997a) characterized these basalts by their K Ti ratios lower than 0.15. Type 2 magmas are only found Clinopyroxene analysis Whole rock analysis on segment 4 and have trace-element af®nities with 0.702 01234ocean island basalts (OIB) (Klein and Karsten, 1995). LOI Type 3 magmas are only represented by sample D20-1 10 from segment 1 (Klein and Karsten, 1995) which is characterized by its K=Ti ratio higher than 0.15. In 9 addition, Type 4 magmas have been sampled on seg- ment 3 of the southern Chile ridge (Klein and Karsten, 8 1995). Finally, Sherman et al. (1997a) mentioned O

18 that some samples (station 12, segment 1) are dacitic ∂ 7 (SiO2 D 65%) and that they show calc-alkaline af®n- ity. They could represent equivalents of our Type 5. 6 (b) 5.2.9. Isotopic composition of the Chile Triple 5 0.702 0.703 0.704 0.705 0.706 Junction area magmas 87Sr/ 86Sr A similar range of Sr, Nd isotopic variations is ob- 0.5132 served for the Taitao Peninsula (our data on clinopy- roxenes plus those on whole rocks from Kaeding et 0.5130 al., 1990), the Taitao Ridge (data on whole rocks from Forsythe et al., 1995a) and the southern Chile 0.5128 Nd ridge (data on whole rocks from Klein and Karsten,

144 1995). The observed patterns range from the ®eld of Nd/ 0.5126 typical MORB to Nd and Sr ratios close to 0.5127 143 CONT. and 0.705, respectively, and have been considered by 0.5124 CRUST former authors as indicative of continental crust or (c) subduction-related imprint (Fig. 12c). 0.5122 0.700 0.705 0.710 0.715 0.720 87Sr/ 86Sr 6. Discussion TAITAO PENINSULA SOUTHERN CHILE RIDGE TAITAO RIDGE Continental Rocks 6.1. Magma emplacement in the forearc domain Fig. 12. Sr±Nd±O isotopic plots. (a) Correlation of Sr isotopic during active ridge subduction compositions with losses on ignition of the Taitao Peninsula lavas. Linear regression through all of the data yields the equa- There is an ongoing discussion on the origin of 87 86 tion Sr= Sr D 6.3 C 0.8 LOI with a poor coef®cient of magmas emplaced in the near-forearc in the sit- correlation (0.71). (b) Combined Sr and O isotopic plot (for clinopyroxenes only). (c) Compilation of the available Sr and uation of active spreading ridge subduction. This Nd isotopic data for the Taitao Peninsula from this study and debate mainly concerns peripaci®c geologic history, Kaeding et al. (1990), the Taitao Ridge (Forsythe et al., 1995a) punctuated by collisions between spreading ridges and the southern Chile ridge lavas (Klein and Karsten, 1995). and trenches. In central Hokkaido, the Hidaka mag- Continental rock data are from Kaeding et al. (1990). matic suite include plutons of N-MORB af®nity as C. Guivel et al. / Tectonophysics 311 (1999) 83±111 105 well as granitic plutons which have been emplaced away from the trench axis, has been already dis- within the accretionary prism during the subduction cussed by Kaeding et al. (1990) and Lagabrielle et of the Kula±Paci®c ridge in Late Paleocene to Early al. (1994) who pointed out that where basalts were Eocene time. The diversity of the magma is inter- generated is not far enough inland to expect a sub- preted as interaction of mantle-derived N-MORB continental mantle wedge as a source. Assuming a magma of the spreading ridge and the accretionary slab dip of 15ë (Bangs et al., 1992), the Chile conti- prism through AFC (assimilation and fractional crys- nental lithosphere should not be thicker than 15 km tallization) processes. Partial melting of forearc ac- at a distance of ca. 40 km from the trench (Fig. 13) creted material is supposed to have generated the and consequently there is no subcontinental mantle granitic magma (Maeda and Kagami, 1996). Mid- wedge overlying the subducting slab. In these condi- dle Tertiary bimodal volcanism in the Santa Maria tions, the only possible source for the Taitao basalts Province of west-central California, including basalts appears to be the hot convective oceanic mantle and basaltic andesites with rhyolites and dacites, has of the subducted Chile spreading ridge. The diver- been interpreted as the result of interactions between sity of basaltic magma types erupted in the Chile subducting segments of the East Paci®c Rise and the Triple Junction, both on the Taitao Ridge and on the North American continental crust (Cole and Basu, Taitao Peninsula, should thus re¯ect (a) the small- 1992). In most examples, MORB magmas (often scale chemical diversity of the sub-oceanic mantle, referred as in situ basalts, Kinimani et al., 1994) already evidenced by Klein and Karsten (1995), and are supposed to derive directly from the subducted (b) additional heterogeneities resulting from interac- mid-ocean ridge with variable interaction with the tions between the ascending basaltic magmas and the continental crust (Johnson and O'Neil, 1984; Hib- continental crust. bard and Karig, 1990; Sharma et al., 1991; Harris et Former studies of Taitao Peninsula lavas led to al., 1996; Osozawa and Yoshida, 1997). the recognition of crustal contamination as an im- In the case of the Taitao region, the recent age and portant process likely responsible for Sr±Nd isotopic the present-day location of the intrusions, 17 to 30 variations (Kaeding et al., 1990) and random varia- km away from the trench axis, are not easy to explain tions of La=Nb ratios in basalts (Lagabrielle et al., according to current models of magma genesis and 1994). However, these studies were published prior emplacement in typical subduction zones. Age con- to the discovery of the important chemical variabil- straints indicate that the studied volcano±plutonic ity of basalts from the Chile ridge axis (Klein and suites have been emplaced in the Triple Junction Karsten, 1995; Karsten et al., 1996; Sherman et al., region during its migration along the Chile margin. 1997a). Our data on clinopyroxenes from gabbroic Kinematic reconstructions show that the triple junc- samples (TA65 and TA55) fall within the range of tion migrated northward in front of the Golfo de Sr and Nd isotopic compositions found at the Chile Penas between 6 and 3 Ma and was located at the ridge axis (Fig. 14) but corresponding δ18Omagma latitude of the Peninsula around 3 Ma ago. The triple values (C6.2 and C6.6½) are slightly higher than junction migrated along the trench toward the pre- those measured on the Chile ridge basalts (C5.6 to sent-day location of the Taitao Ridge between 3 and C6.0½, Sherman et al., 1997b). 2 Ma. This kinematic evolution is in good agreement The geochemical characteristics of the basaltic with the age pattern of the magmatic rocks from lavas erupted in the Chile Triple Junction region south to north: 6±2 Ma on the Taitao Peninsula and may be the result of processes linked to the present- 2±1.5 Ma on the Taitao Ridge. day ridge±trench con®guration by direct emission of the sub-oceanic mantle magmas contaminated by 6.2. Constraints on magma genesis in the Taitao recent re¯ux of subduction-related material into the area mantle source. Klein and Karsten (1995) proposed that the sub-oceanic Chile ridge mantle was con- 6.2.1. Origin of basaltic magmas taminated either by slab fragments reinjected into The problem of the emplacement of basaltic mag- the asthenospheric convection or, alternatively, by mas in the Taitao Peninsula, only some kilometres passage through a slab window of the metasoma- 106 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

Zone of normal Near trench arc volcanism magmatism 0 50100 150 200 250

0 . . .++++ +++ + + + + + + ++ + continental ...... 15˚ . . . . . +++++ + + + + + ++ +

. . . crust

. . .oceanic++ lithosphere+ + + + + + + + +

VVV VVVV V 30

.

VV VV . . V

... .

. ... mantle wedge .. V ...... 60 km . . ..

W E

0 +++ + + +++++ + + + + + + ...... altered oceanic crust ...... +++++ + possible limited AFC ...... +++ + + + + + ...... +++ ...... heterogeneous 5 km sub-oceanic mantle

0 5 10 15 km Fig. 13. Tectonic setting of near-trench magmatism and origin of volcanic products emplaced in relation with the subduction of an active spreading ridge. Note that the sketch only concerns the very distal toe of the subduction zone. tized sub-continental mantle wedge. In all the areas The high diversity of the chemical characteristics investigated, a model involving direct emission of of the lavas (trace-element patterns, isotopic data) magmas from the buried active spreading centre as precludes a simple closed-system fractionation of a it was subducted at shallow depth is still acceptable single primitive magma source to generate the sili- (Lagabrielle et al., 1994), as similar chemical diver- cic lavas from the Taitao Peninsula or the Taitao sity is found along the Chile ridge and the South Ridge. The decrease of Yb with SiO2 contents in American margin. Fig. 7b precludes fractional crystallization as the main process. The correlations between Sr, Nd iso- 6.2.2. Origin of the intermediate=evolved topic ratios and SiO2 contents (Kaeding et al., 1990), calc-alkaline magmas and between La=Nb ratios and SiO2 (Lagabrielle The main difference between the magmatic rocks et al., 1994) led these authors to propose assimi- from the Taitao Peninsula and the Taitao Ridge and lation of continental crust or assimilation coupled those from the Chile ridge resides in the occurrence with fractional crystallization (AFC) as a mecha- of abundant intermediate or evolved rocks in the for- nism accounting for the origin and abundance of the mer areas. They include basaltic andesites, andesites, intermediate=evolved lavas. For instance, Kaeding et dacites and rhyolites from CMU, MVU, BDC (Kaed- al. (1990) concluded that their isotopic data were ing et al., 1990; Lagabrielle et al., 1994; Guivel et consistent with assimilation of 5±10% of pre-Juras- al., 1996; Le Moigne et al., 1996) and dacites and sic metasedimentary rocks by tholeiitic magmas. rhyolites from the Taitao Ridge (Forsythe et al., A very different explanation was proposed by 1995a). Forsythe et al. (1995a) to account for the geochemi- C. Guivel et al. / Tectonophysics 311 (1999) 83±111 107

ODP sed. And CC 9 9 ODP sed.

8 And CC 8 O

7 O 7 18 ∂ 18

∂ 10 10 10 5 10 5 5 1 5 1 6 6 N-MORB 1 (Chile ridge) N-MORB (Chile ridge)

5 5 0.702 0.703 0.704 0.705 0.706 0.707 0.5122 0.5124 0.5126 0.5128 0.5130 0.5132

87 86 143 144 Sr/ Sr Nd/ Nd a

ODP sed. ODP sed. And CC 9 9

8 8 O O

18 7

18 7 ∂ ∂ And CC 10 10 10 5 5 1 5 1 6 6 1 1 DM (Chile ridge) DM (Chile ridge)

5 5 0.702 0.703 0.704 0.705 0.706 0.707 0.5122 0.5124 0.5126 0.5128 0.5130 0.5132 87 86 143 144 Sr/ Sr Nd/ Nd b

gabbro basaltic andesite dacite

Fig. 14. O±Sr±Nd isotopic variations for the Taitao Peninsula magmas compared to calculated mixing models for: (a) crustal contamination of ma®c magma (N-MORB type) by various amounts of Andean continental crust (And CC) or ODP Leg 141 sediments (ODP sed.; bold curve); and (b) source contamination of a depleted mantle (DM) by Andean continental crust (And CC) or ODP Leg 141 sediments (ODP sed.; bold curve). Numbers on the curves indicate the mass fraction of crustal or sediment component. See Table 5 for parameters. 108 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

Table 5 Chemical and isotopic parameters used in the calculation of mixing models

Component Sr Nd 87Sr=86Sr 143Nd=144Nd δ18O (ppm) (ppm) (½ vs. V-SMOW) DM 8.8 0.857 0.702674 0.513162 C6.0 N-MORB 88 8.57 0.702674 0.513162 C6.0 Andean continental basement 220 23.6 0.715 0.5123 C12 ODP sediments 305 21 0.708 0.5124 C10

DM: depleted mantle [sample D34-1 from segment 2 of the Southern Chile ridge (Klein and Karsten, 1995)]. N-MORB: normal oceanic crust [sample D34-1 from segment 2 of the Southern Chile ridge (Klein and Karsten, 1995)]. And. CC: Andean continental basement (see Stern and Kilian (1996) and references therein). ODP sed.: ODP Leg141 sediments [see Stern and Kilian (1996) and references therein]. cal features of the Taitao Ridge dacites and rhyolites. the Chile ridge (D34-1 from Klein and Karsten, Indeed, these authors regarded the Taitao Ridge as 1995), and Andean continental crust or ODP Leg built on an oceanic substratum and, therefore, did 141 detrital sediments (see Table 5 for calculation not consider continental crust contamination. Two parameters), and rhyolites were analysed for Sr, Nd and Pb isotopes (b) A theoretical depleted sub-oceanic mantle (Forsythe et al., 1995a). The sample displaying the (DM) isotopically similar to N-MORB D34-1 but strongest continental (or subduction-related) imprint having Sr and Nd concentrations ten times lower, has Nd and Pb isotopic ratios similar to those of and Andean continental crust or ODP Leg 141 detri- calc-alkaline basalts from the Taitao Ridge but a tal sediments. slightly higher 87Sr=86Sr ratio. On the basis of trace Mixing curves in Fig. 14a and b approximate elements patterns, these authors proposed that the crustal contamination of ma®c magmas and mantle rhyolites were formed by 10% melting of hydrother- source contamination processes, respectively (Tay- mally altered gabbroic equivalents of the calc-alka- lor, 1980; James, 1981; Davidson, 1985; Davidson line basalts (Type 4), leaving a residue of hornblende and Harmon, 1989; Fourcade et al., 1994). Although and plagioclase at 800ëC and 1.5 GPa. Thermal we have taken for the MORB=DM pole a δ18O anomaly due to the subduction of a spreading ridge of C6.0½, corresponding to the upper bracket of segment may lead to such a high temperature peak the data on the Chile ridge basalts, none of the (DeLong et al., 1979). The latter pressure, however, mixing curves ®ts closely with our analytical data. seems inconsistent with the estimated depth of the It seems thus impossible to decipher between the Chile ridge ocean crust below the Taitao Ridge (less two models (source contamination of DM by less than 10 km). than 5% sediments or assimilation of less than 10% In order to discuss the origin of the geochemical continental crust or derived sediments by N-MORB signatures of the Taitao Peninsula magmatic rocks, magmas). Of course, the predominantly mantle-like we have plotted their 87Sr=86Sr, 143Nd=144Nd, and signature of the studied clinopyroxenes precludes the δ18O ratios deduced from measurements on primary hypothesis that their host rocks derived either from clinopyroxene phenocrysts in Fig. 14 together with anatectic melting of continental crust=sediments or literature data on the Chile ridge basalts. In addi- from contamination=AFC processes involving large tion to the two N-MORB gabbros discussed here- amounts of a continental component. The fact that above, data on clinopyroxenes have been obtained the dacite and the basaltic andesite clinopyroxenes on a basaltic andesite (T16d) showing a typical trace are slightly less radiogenic in Nd than the gabbros element calc-alkaline imprint (Type 4) and on a could give some support to the crustal contamination dacite (T18a) also typically calc-alkaline. The mix- hypothesis but corresponding Sr isotopic ratios are ing curves in Fig. 14 correspond to bulk mixing nearly identical (Fig. 14a). The hypothesis of dacite± between: rhyolite production through partial melting of hy- (a) One of the least Sr radiogenic N-MORB from drothermally altered oceanic crust can also account C. Guivel et al. / Tectonophysics 311 (1999) 83±111 109 for the available isotopic data. Indeed, hydrothermal Bourgois et al. (1996) suggested a high rate of alteration of the upper part of the hole 504B basaltic subduction-erosion below Taitao. Such a process sequence resulted in an overall increase of δ18Oval- necessarily results in the dissemination of frag- ues by C1toC2 units (Alt et al., 1996). Such a ments of continental crust within the Chilean mantle process may explain the δ18O differences between wedge. If a slab window exists beneath Taitao, as Taitao Peninsula lavas and Chile ridge basalts. suggested by recent seismic data (Russo and Mur- die, 1997), it may provide a way of access to the sub-ridge mantle for these continental materials. 7. Conclusions

The Taitao Peninsula and the adjacent Taitao Acknowledgements Ridge are characterized by a remarkably variable Pliocene±Quaternary magmatic activity which em- This work was supported by the ECOS program placed over a Benioff zone less than 10 km (Taitao C93U05 and the `Dynamique et Bilan de la Terre' Ridge) to 20 km (Taitao Peninsula) deep: (1) N-type program of the CNRS-INSU. The data collected in MORB; (2) E-type MORB; (3) LREE-depleted the Taitao Peninsula have been obtained during the N-MORB showing trace element features typical of Taitao 95 expedition by Jacques Bourgois and Yves arc basalts; (4) moderately Nb-depleted E-MORB; Lagabrielle. The helicopter used during the Taitao 95 (5) calc-alkaline andesites, dacites and rhyolites; (6) expedition was funded through the ECOS program andesites and dacites with adakitic signature (Bour- by our Chilean partner, PI: Professor Jose Frutos. gois et al., 1996). The bathymetric data shown on Figs. 1 and 2 are The chemical variability of the studied basalts is from the CTJ cruise of the R=V L'Atalante (Bourgois similar to that of the Chile spreading ridge basalts et al., 1997). ICP-AES analyses were performed in regarding trace elements and Sr±Nd isotopes. This Brest by Jo Cotten, and Sr±Nd isotope analyses were observation suggests that their most likely origin is performed in Clermont-Ferrand by FrancËoise Vidal. the actively subducting Chile ridge. There is no need for important assimilation or melting of continental material during the ascent of ridge-generated MORB References through the Chile crust (Lagabrielle et al., 1994). 18 The slightly higher δ O of Taitao Peninsula mag- Alt, J.C., Teagle, D.A., Bach, W., Halliday, A.N., Erzinger, mas (with respect to Chile spreading ridge basalts) J., 1996. Stable and strontium isotopic pro®les through hy- deduced from the analysis of their primary clinopy- drothermally altered upper oceanic crust, hole 504B. Proc. roxenes could result from (a) minor amounts of man- Ocean Drilling Program, Sci. Results 148, 417±434. Arnaud, N.O., Brunel, M., Cantagrell, J.M., Tapponnier, P., 1993. tle source contamination by continental components, High cooling and denudation rates at Kongur-Shan (Xinkiang, (b) minor assimilation of Chilean continental crust China), revealed by 40Ar=39Ar thermochronology on alkali by basaltic to dacitic magmas, and, for the acidic feldspars. Tectonics 12 (6), 1335±1346. rocks, (c) remelting of previously hydrothermally Bangs, N., Cande, S.C., Lewis, S.D., Miller, J., 1992. Structural altered oceanic crust (Forsythe et al., 1995a). The framework of the Chile margin at the Chile ridge collision origin of the continental imprint of the Chile ridge zone. Proc. Ocean Drilling Program, Initial Rep. 141, 11±21. Behrmann, J.H., Lewis, S.D., Musgrave, R.J., ODP Leg 141 basalts (and consequently of their Taitao Peninsula Scienti®c Party, 1992. Site 862. Proc. Ocean Drilling Program, and Taitao Ridge equivalents) remains highly conjec- Initial Rep., 141: 301±340. tural. Klein and Karsten (1995) already envisioned: Behrmann, J.H., Leslie, S.D., Cande, S.C., ODP Leg 141 Sci- (1) The reintroduction of slab fragments beneath enti®c Party, 1994. Tectonics and geology of spreading ridge the ridge due to slab breaking and entrainment by subduction at the Chile Triple Junction: a synthesis of results shallow asthenospheric ¯ow, or from Leg 141 of the Ocean Drilling Program. Geol. Rundsch., 83: 832±853. (2) The presence of a slab window providing a Bourgois, J., Lagabrielle, Y., Maury, R.C., Le Moigne, J., Vi- major connection between the metasomatized sub-arc dal, P., Cantagrel, J.M., Urbina, O., 1992. Geology of the mantle and the adjacent sub-oceanic mantle. Taitao Peninsula (Chile margin triple junction area, 46±47ëS): 110 C. Guivel et al. / Tectonophysics 311 (1999) 83±111

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