Geochemistry of the Chewore Ophiolite and the Kaourera-Arc

lithologies. They may however represent ultra- mafic cumulates. Their position in the stratigra- phy below that of the mafic-cumulates suggests that this might be the case. Thus the lowercrustal portion of the Maunde Ophiolite Group repre- sents a stratified magma chamber, consisting of massive and layered ultramafics and mafic-cumu- lates at the base which grade into massive and layered meta-gabbro representing the main por- tion of the chamber. The major difference between the Maunde Ophiolite Group and that of a typical Phanero- zoic ophiolite is the thickness of each lithostrati- graphical unit, with Phanerozoic examples containing considerably thicker units. According to Brown and Mussett 1981 the thinnest igneous unit within an ophiolite is the pillow lavas which are generally between 300 and 700 m thick. This is in fact as thick as the whole of the type-section of the Maunde Ophiolite Group. This attests to the fact that the Maunde Ophiolite Group has been extensively attenuated during tectono-meta- morphism.

4. Geochemistry of the Chewore Ophiolite and the Kaourera-Arc

4 . 1 . Analytical procedures After the removal of weathered surfaces the samples were crushed in a jaw crusher and pow- dered in a Tungsten Carbide ‘Tema mill’ until the grain size of the powder was less than 200 mm. Major and trace elements were analysed using a Phillipsc PW 145020 X-ray fluorescence spec- trometer with a side-window Rhodium tube at the University of St Andrews. Major elements were analysed on glass discs prepared by fusing the sample with Johnson-Matthey Spectrofluxc 105 in a sample to flux ratio of 5.33:1. Trace elements were analysed using pressed powder pellets, bound with approximately ten drops of polyvinyl alcohol Movial. Calibration of analyses were based on 30 majors and 30 traces international rock standards based on the procedure of Norrish and Chappell 1977. The rare-earth elements of 11 samples were analysed by ICP-AES [Phillipsc simultaneous sequential PV 8060 spectrometer] at Royal Holloway, University of London according to the procedure of Walsh et al. 1981. 4 . 2 . Geochemical alteration Since all the rocks in the Ophiolite Terrane have undergone amphibolite facies metamor- phism, the more mobile elements such as the alkali major elements Na 2 O, CaO and K 2 O and the large lithophile elements Sr, Rb, Ba, Th are likely to have been altered and thus do not repre- sent the original concentrations prior to metamor- phism. It is generally considered Humphris and Thompson, 1977; Brekke et al., 1984; Brouxel et al., 1989 that the high field strength elements HFSE Nb, Ce, Zr, Y, Sc, Cr, Ni and the rare earth elements REE are relatively immobile dur- ing hydrothermal alteration and metamorphism. These elements can be considered with some cau- tion to represent original, pre-metamorphic concentrations. 4 . 3 . The Maunde Ophiolite Group The Chewore Ophiolite 4 . 3 . 1 . Meta-basalts All of the meta-basaltic lithologies both the Mvuu Meta-Volcanic Formation and the Mbizi- Sheeted Dykes Formation have SiO 2 contents less than 54 wt. Fig. 9a and Table 1. In comparison to N-MORB Fig. 10a, the meta- basalts are relatively depleted in the high field strength elements; have variable Cr and Ni con- centrations and reveal negative Nb concentrations in relation to Ce. The REE, when normalised against chondritic values of Nakamura 1974 show a flat pattern Fig. 10a with LaYb ratios between 1.1 and 2.2 Table 1. Such geochemical signatures are evident in modern-day basalts formed within destructive plate margin environ- ments Saunders and Tarney, 1979; Basaltic Vol- canism Study Project, 1981; Pearce, 1982; White and Patchett, 1984; Windley, 1994. Such basalts may have formed within a variety of destructive plate margin settings, i.e. within the arc, fore-arc or marginal basin. The association of these meta- basalts with a suite of lithologies which resemble S .P . Johnson , G .J .H . Oli 6 er Precambrian Research 103 2000 125 – 146 137 Table 1 Data table presenting representative analyses for Maunde Ophiolite and Kaourera Island Arc lithologies a Kaourera Group Maunde Ophiolite Group Meta-basalts Talc- Rhyolite Dacite Massive Andesite Oceanic within-plate meta-basalts Island-arc meta-basalts High- Meta- strain cumulate bearing layered meta- meta- ultramafic gabbro gabbro SJ 203.1 SJ 206.1 SJ 208.2 SJ 105.3 SJ 213 Hb SJ 215 g SJ 213 Ha SJ 236 Sample SJ 286 SJ 287.2 SJ 67 SJ 95 SJ 144.2 SJ 224 A SJ 220 A SJ 222 B1 Sample 48.60 47.48 47.55 41.10 46.34 45.24 SiO 2 46.77 48.51 49.73 48.09 49.11 49.48 50.65 60.85 66.14 78.89 SiO 2 0.78 0.74 0.80 0.22 0.51 0.72 0.70 0.70 TiO 2 0.68 0.66 2.98 2.54 1.94 1.80 3.00 0.65 TiO 2 14.66 17.00 17.93 2.69 16.68 13.48 11.80 16.75 Al 2 O 3 15.20 16.54 12.52 12.63 13.08 12.60 15.92 8.94 Al 2 O 3 11.33 10.93 11.77 14.29 6.68 14.03 9.84 11.13 Fe 2 O 3 10.78 11.10 18.28 18.01 15.48 11.90 1.96 3.73 Fe 2 O 3 0.22 0.24 0.16 0.13 0.13 0.23 0.23 0.19 0.21 0.18 0.25 0.25 0.22 0.22 0.05 0.05 MnO MnO 10.50 8.47 7.72 30.57 10.53 11.11 15.17 8.55 MgO 8.64 8.20 4.97 4.37 5.28 1.93 0.23 1.01 MgO CaO 9.35 8.65 9.59 2.09 17.60 12.41 12.86 10.87 9.70 11.05 9.30 9.16 9.84 3.48 3.05 2.16 CaO Na 2 O 2.99 2.51 3.07 0.01 0.58 1.38 1.27 2.58 3.39 2.75 1.35 1.33 1.95 3.22 8.35 0.11 Na 2 O 0.32 2.66 0.61 0.00 0.20 0.64 0.34 0.43 K 2 O 0.23 0.27 0.46 0.54 0.54 2.22 0.24 2.44 K 2 O P 2 O 5 0.09 0.13 0.10 0.00 0.03 0.03 0.04 0.10 0.05 0.07 0.40 0.32 0.19 0.52 0.46 0.24 P 2 O 5 2.00 1.90 1.30 8.60 1.10 1.10 1.40 0.90 2.10 LOI 1.80 1.20 1.90 0.70 1.30 1 2 LOI 100.84 100.71 100.60 99.70 100.38 100.37 100.42 100.71 Total 100.28 100.71 100.79 100.50 99.94 100.02 100.40 100.22 Total 1 1 1 2 1 1 Nb 2 2 15 13 8 21 24 25 Nb 33 46 42 4 15 8 13 36 31 32 166 161 118 347 413 463 Zr Zr 16 17 18 2 3 11 5 17 Y 15 16 48 51 34 63 84 67 Y Sc 42 30 31 13 55 56 51 40 38 36 40 40 42 20 13 7 Sc Cr 700 184 77 3239 1045 644 1410 174 199 177 127 161 134 5 11 4 Cr 270 211 151 1739 365 224 542 204 Ni 237 206 39 40 56 6 8 7 Ni La 2.2 4.4 3.4 3 1 2.7 2.2 2.5 20.1 33.3 13.5 52 4 52 La 5.0 9.9 8.6 5 7 3 4 5.9 Ce 5.9 5.2 43.3 45.3 29.6 101 24 118 Ce 0.8 1.4 1.5 NA NA NA NA 0.7 Pr 1.0 0.8 6.2 8.9 4.1 NA NA NA Pr 4.0 7.0 6.8 NA NA NA NA 4.4 Nd 4.4 4.0 26.9 38.4 17.9 NA NA NA Nd 1.3 1.9 1.9 NA NA NA NA 1.4 1.1 1.3 Sm 6.9 8.9 4.9 NA NA NA Sm 0.6 0.9 0.9 NA NA NA NA 0.7 Eu 0.4 0.7 2.4 2.9 1.8 NA NA NA Eu Gd 1.8 2.4 2.6 NA NA NA NA 2.0 2.1 1.9 7.8 9.2 5.7 NA NA NA Gd Dy 2.6 3.0 3.0 NA NA NA NA 2.8 2.6 2.7 8.2 8.7 6.3 NA NA NA Dy 0.6 0.6 0.7 NA NA NA NA 0.6 Ho 0.6 0.6 1.7 1.8 1.3 NA NA NA Ho Er 2.0 2.2 2.3 NA NA NA NA 2.0 1.8 2.0 5.5 5.6 4.1 NA NA NA Er 1.9 2.0 2.0 NA NA NA NA 1.8 Yb 1.9 1.8 5.6 4.8 3.5 NA NA NA Yb 0.3 0.3 0.3 NA NA NA NA 0.3 0.3 0.3 Lu 0.7 0.7 0.5 NA NA NA Lu 1.15 2.20 1.70 – – – – 1.50 1.15 1.38 LaYb 3.59 6.93 3.85 – – LaYb a NA indicates that no analysis is available. Fig. 9. Major element variation plots for the Maunde Ophiolite and Kaourera Island Arc Groups. For representative analyses, see Table 1. Key to abbreviations: A, andesite; B, basalt; BA, basaltic andesite; BTA, basaltic trachyandesite; D, dacite; P, phonolite; PB, picro basalt; PT, phonotephrite; R, rhyolite; TA, trachyandesite; TB, trachy-basalt; TD, trachydacite; TP, tephriphonolite. a Total alkali vs. silica TAS diagram for meta-basalts within the Maunde Ophiolite Group pillow lavas, massive sheet flows and sheeted dykes are undifferentiated. b TAS diagram for all lithologies within the Kaourera Island Arc Group. c K 2 O vs. SiO 2 plot for the Kaourera Island Arc Group samples. Notice that there is a poorly defined calc-alkaline series. Since K 2 O is readily mobile during metamorphism, this poorly defined calc-alkaline series is probably an artifact resulting from the alteration of once, low-K lithologies. that of an ophiolite i.e. oceanic-type crust, indi- cate that they formed within an extensional spreading tectonic regime, i.e. a back-arc mar- ginal basin and not that of an arcfore-arc. This is not altogether surprising since the majority of Phanerozoic ophiolitic fragments are interpreted to be relict marginal basins Saunders et al., 1979; Wilson, 1989; Taylor et al., 1992; Smith, 1993; Windley, 1994. Within Fig. 10a and d the fields for basalts from modern-day marginal basins data includes analyses from the Bransfield Strait, Guyamas Basin, Mariana Basin and Scotia Sea Brekke et al., 1984; Gribble et al., 1998; Hawkins et al., 1990 has been superimposed and a good correlation is evident. In comparison with the Kaourera Island Arc Group, tholeiitic island-arc meta-basalts, these marginal basin meta-basalts display a similar geo- chemical signature compare Fig. 10a with b. Many authors including Gribble et al. 1998 and Brouxel et al. 1989 indicate that marginal basin basalts which display strong subduction-zone geo- chemical signatures, result from a combination of adiabatic mantle decompression coupled with fluid-induced melting processes. Such geochemical signatures are evident in modern-day, immature back-arcs where spreading has yet to dominate over rifting Gribble et al., 1998. As the mar- ginal basin matures from the rifting to spreading stage, adiabatic decompression melting of the mantle becomes predominant and the input from the subducting slab decreases. Trace element con- centrations of the spreading centre marginal basin basalts therefore become distinct from those produced within the associated arc. The Maunde Ophiolite Group meta-basalts might therefore represent a young, immature marginal basin. 4 . 3 . 2 . Ultramafics, meta-gabbros and meta-cumulates All Ngwena Ultramafic Formation lithologies contain less than 44 wt. SiO 2 for representitive analyses, see Table 1 and are thus classified as ultramafic Le Maitre, 1989. The highly altered nature of this formation is illustrated in Fig. 11 which is a plot of the MgO wt. versus MgO CaO wt. after Meisel et al. 1997. The figure shows the field for all known, unaltered mantle xenoliths and massive peridotites which fall on a line intersecting the position of primitive upper Fig. 10. MORB-normalised spider plots after Pearce, 1983 and chondrite-normalised rare earth elements REE plots normalisa- tion after Nakamura, 1974 for Maunde Ophiolite and Kaourera Island Arc Group meta-basalts. For representative analyses, see Table 1. a, d Pillow, massive and sheeted-dyke meta-basalts from the Maunde Ophiolite Group. Superimposed field includes data from present-day marginal basins including Bransfield Strait, Guyamas Basin, Mariana Basin and Scotia Sea Brekke et al., 1984; Gribble et al., 1998; Hawkins et al., 1990. b, e High field strength elements HFSE-depleted, island-arc-type meta-basalts from the Kaourera Island Arc Group. Superimposed field includes data from present-day island-arcs including Mariana Arc, Northern New Hebrides and Palau-Kyushu Range Barsdell et al., 1982; Brekke et al., 1984; Gribble et al., 1998. c, f HFSE-enriched, oceanic within-plate-type meta-basalts from the Kaourera Island Arc Group. Superimposed lightly-shaded field include data from present-day oceanic within-plate hot spots and ridge-centred hot spots including Hawaii, Kergeuelen and Iceland Basaltic Volcanism Study Project, 1981; Schilling et al., 1983; West et al., 1992; Mahoney et al., 1995. The stippled field is that for Kerguelen only Mahoney et al., 1995. Fig. 11. Whole rock MgO vs. MgOCaO wt. variation diagram after Meisel et al. 1997. The Ngwena Utramafic Formation plot away from that of unaltered mantle and primitive upper mantle PUM values indicating alteration due to metasomatism and serpentinisation. For representitive analyses, see Table 1. which range from picro-basalt to rhyolite Fig. 9b and Table 1. These rocks also are variable in total alkalis Fig. 9c; however, due to amphibolite facies tectono-metamorphism and the mobile na- ture of the alkali elements it is likely that this does not reflect the original pre-metamorphic concen- trations. It is impossible therefore to determine if this rock suite displays both a low-K and calc-al- kaline trend as tentatively indicated in Fig. 9c. The trace element composition of the meta-basalts re- veal the presence of two distinct geochemical sig- natures Fig. 10b and c. The first has HFSE-concentrations lower than that of N- MORB depleted and display a negative Nb anomaly compared with Ce, while the second have HFSE-concentrations greater than that of N- MORB enriched and no significant Nb anomaly. The HFSE depleted meta-basalts are spatially associated only with the with the silica-variable suite of lithologies Fig. 3. When normalised against chondritic values of Nakamura 1974, the REE display a relatively flat pattern Fig. 10e with LaYb ratios of between 0.7 and 1.5 Table 1 indicating slight, HREE enrichment. Such geo- chemical signatures are characteristic of basalts produced within destructive plate margin settings Saunders and Tarney, 1979; Basaltic Volcanism Study Project, 1981; Pearce, 1982; White and Patchett, 1984; Wilson, 1989. Again such basalts may have formed within a variety of destructive plate margin settings, i.e. within the arc, fore-arc or marginal basin. The association of these meta- basalts with the silica-variable, volcanic suite sug- gests that they may have formed within the main portion of an island-arc. In Fig. 10b and e, the field for modern-day, ensimatic island-arc, low-K, tholeiitic basalts has been superimposed data in- cludes analyses from the Mariana Arc, Northern New Hebrides and the Paulau-Kyushu Range Barsdell et al., 1982; Brekke et al., 1984; Gribble et al., 1998, and a close match is evident. The lack of plutonic material within the Kaour- era Island Arc Group indicates that during either arc accretion, or later deformation and metamor- phism, only the upper crustal section of the Kaourera Arc have been preservedexposed. The HFSE-enriched meta-basalts are located toward the southern margin of the Kaourera Is- mantle PUM, McDonough and Sun, 1995. The Ngwena Ultramafic Formation lithologies plot outside this field towards lower MgO and MgO CaO wt. values indicating that all lithologies have undergone metasomatic alteration and re- moval of major elements due to serpentinisation. Both the meta-gabbroic Twiza Meta-Gabbro Formation and meta-cumulate Ingwe Meta- Mafic Cumulate Formation rocks have SiO 2 con- tents less than 51 Table 1 and range from picro-basalt to basalt. When normalised against N-MORB, both groups display similar HFSE con- centrations Fig. 8 indicating that the two share a similar parental source. This enforces the field interpretation that the cumulate formation is derived by the gravity settling of the original meta-gabbroic magma. 4 . 4 . The Kaourera Island-Arc Group Kaourera Arc The Kaourera Island-Arc Group comprises an SiO 2 -variable suite of fine grained extrusive rocks land-Arc Group where meta-basalt lithologies predominate over the more silica-rich lithologies, and in the west of the region where they are tectonically interleaved with Maunde Ophiolite Group, marginal basin meta-basalts Fig. 3. When normalised against chondritic values of Nakamura 1974, the REE display LREE en- richment with LaYb ratios of between 3.5 and 6.9 Table 1, similar to those of oceanic within- plate basalts. In Fig. 10c and f the field of mod- ern-day oceanic withinplate and ridge-centred hot spot basalts has been superimposed data in- cludes analyses from Hawaii, Kerguelen Plateau and Iceland Basaltic Volcanism Study Project, 1981; Schilling et al., 1983; West et al., 1992; Mahoney et al., 1995, and a good correlation is evident. Oceanic within-plate volcanic suites asso- ciated with mid-plate hot spots such as Hawaii or ridge-centred hot spots such as Ascension Island and Iceland are characterised by alkalic fraction- ation trends and the production of trachybasalts Wilson, 1989. The lack of such lithologies within the Kaourera Island Arc Group, especially those associated with the oceanic within-plate meta-basalts, suggests that these basalts were not formed within these environments. Also superim- posed on Fig. 10f is the field for the seamount large igneous province of Kerguelen, which also displays oceanic within-plate geochemical trace element signatures and which is dominated early in its history by simple, tholeiitic volcanics Ma- honey et al., 1995; Kerr et al., 1996. A good correlation is evident. Fig. 12 is an YNb versus ZrNb discrimination plot, which distinguishes between P-type, T-type and N-type MORB. The fields for oceanic mid-plate Hawaii, ridge-cen- tred Ascension Wilson, 1989 and seamount large igneous provinces Kerguelen Mahoney et al., 1995 basalts are plotted. Both the Kaourera within-plate basalts and Kerguelen basalts plot as T-MORB while the mid-plate Hawaii and ridge- centred Ascension basalts plot clearly as P- MORB. The Kaourera Island Arc HFSE- depleted meta-basalts plot as N-MORB. The sim- ilarity in trace element geochemistry of the Kaourera oceanic within-plate basalts with those of Kerguelen and the similar HFSE and REE trace element concentrations suggest that these meta-basalts might represent the remnants of a seamount-type, within-plate feature. The spatial volume of these meta-basalts compared to the Ophiolite Terrane is relatively small and thus not what one would expect from an accreted mid- plate seamount; however, this again might be due to attenuation and interleaving during tectono- metamorphism. It is possible that the geochemistry of these oceanic within-plate-type meta-basalts may be the result of contamination of the Kaourera arc-type meta-basalts from the subduction and dehydra- tion of an oceanic seamountHawaiian-type plateau under the are. Turner and Hawksworth 1998 have illustrated that marginal basin and arc-type basalts within the northern sector of the Lau Back Arc Basin display oceanic within-plate characteristics. This is interpreted to be the result of contamination of these lavas, from the interac- tion of mantle plume material and the subduction of oceanic seamounts below the marginal basin Fig. 12. Trace element variation diagram of YNb vs. ZrNb after Wilson, 1989, discriminating between N- normal, T- transitional and P plume-type MORB. Kaourera Island Arc Group oceanic within-plate meta-basalts plot clearly as T-MORB while the Kaourera Island Arc Group, island-arc- type meta-basalts plot predominantly as N-MORB. Also shown are the fields for Ascension Island, Kerguelen and Hawaii Basaltic Volcanism Study Project, 1981; Schilling et al., 1983; West et al., 1992; Mahoney et al., 1995. and island-arc. Such basalts are spatially re- stricted to the northern sector of the arc where these three components interact. If the Kaourera oceanic within-plate-type metabasalts result from a similar process, this might account for the re- stricted volume and sporadicinterleaved nature of these meta-basalts.

5. Discussion