This stage coincides with the development of the present geometry of spreading centres in the North Fiji (Auzende et aI., 1995) and Lau Basins (Taylor et aI., 1 996) (see Figure 4.2D).
Volcanic centres <3 m.y old are shown in Figure 4.4.4. In contrast to the arc-related magmatism of the previous stages (outlined above), volcanism in Fiji during the late rifting stage, with the exception of Kadavu, which will be discussed separately, was predominantly of alkali-basalt affinity. Alkali basalts erupted within oceanic plates (ocean island basalts (OIB)) have high concentrations ofHFSE, with Ti02 > 1 .5
IOOkm
KADAVU GROUP
1800 178°W
• Late Pliocene-Recent Fijian ocean island basalt (FOIB) centres
_ Late Pliocene-Recent Kadavu Group calc-alkaline, adakitic andesites and dacites
• Late Pliocene(?)-Recent Ngaloa Group high-Nb basalts
Figure 4.4.4 Distribution of late rifting stage and Kadavu Group volcanic centres -3-0 Ma.
YL - Yalewa; SE - Seatura; VR - Vatu-i-Ra; SS - Savusavu; KR - Koro; NR - Narai; TA - Taveuni; MO - Moala; VB - Vanua Balavu; KF - Katafaga; MG - Mago; KB - Kabara. See text for detail.
Table 4.4.4. Geochemical comparison ofPlio-Pliestocene basalts from the Fiji area. From Gill and Whelan (1989b) and references therein.
Late Rifting FOIB Samoa OIB NFB basalt Early Rifting
I II shoshonitic Tholeittic ne norm% * >4 0-3 0-1 0 0-5 Ti02 wt%* 2.1 1 .5-1 .8 2.5-4 2 0.5-0.7 0.6-0.8 Nb (ppm)* 50 24 30-75 25 2-3 2-3 ZrlNb* 4.0 6.5 4-6 6.5 >20 >20 Y/Nb* 0.6 1 .0 0.5-0.9 1 5-1 0 > 1 0 KlNb* 225 275 200 265 >8000 > 1 500 LalNb 0.7 0.7 0.7-1 .1 0.7 >3 >2 TiN 60 60 70-1 00 43 <15 1 5-20 TIISc 440 400 1 00 1 00 Rb/ee 0.4-0.6 0.2-0.4 0.27 0.4 >2 1 .0 Sr/Nd 20 22 8-1 3 1 1 >75 >40 Balla 1 3-1 6 1 0-1 2 6-9 1 2 30-90 30-50 E Nd 5 4 <8 4 8 9 87Srjl6Sr 0.7036-38 0.7033-61 0.7046-74 0.7036-38 0.7037-43 0.7033-41
* Criteria which distinguish between types I and II
wt% and NblLa > 1 at MgO > 8 wt% (Gill and Whelan, 1 989b). Twelve subaerial Fijian ocean island basalt (pom) volcanoes have been recognised (Figure 4.4.4), with the majority surrounding the "hook" in the northern Lau Ridge/Koro Sea. Three fonned large shield volcanoes (Seatura, Koro and Taveuni), and the rest are small cinder cones and plugs (Gill and Whelan, 1 989b). The oldest known FOIB are 3.3 Ma from Seatura Volcano, Vanua Levu (Hindle and Colley, 1981 ;Whe1an et aI., 1985), with the most recent activity occurring on southern Taveuni at 2050 ± 1 50
yrs BP (Rodda and Kroenke, 1 984).
Gill and Whelan (1989b) subdivide FOIB into two types on the basis of chemical composition, using the criteria in Table 4.4.4. Both types are widespread, with several volcanoes (eg. Taveuni and Koro and Moala, see below) evolving from type-II to type-I with time. Type-I FOIB typically contain only olivine phenocrysts in magnesian compositions and are joined by plagioclase microphenocrysts in more evolved compositions. Augite is generally absent in type-I FOIB. Type-II FOIB contain abundant plagioclase phenocrysts with lesser olivine and rare clinopyroxene. Type-I FOIB are more silica-undersaturated than Type-II (> 4% nonnative ne, Table 2.), have higher concentrations of both LILE and HFSE (eg. Ba
and Nb) at similar MgO contents and are generally more mafic with MgO > 9 wt% (Gill and Whelan, 1 989b). Sr and Nd isotope compositions are generally similar in 'both Fom types, with 87Sr/86Sr = 0.7036-38 in type-I basanitic FOIB, versus 0.7033-41 in type-II basalts and hawaiites. However, one type-II basalt (Yalewa Kalou, see figure 4.4.4) has 87Sr/86Sr= 0.7061 (Gill, 1976, 1 984: Gill and Whelan, 1 989b). Additionally, Gill (1 984) noted that both type-II samples analysed for Pb have higher 207PbP04Pb and 208PbP04Pb. ENd = +5.1 (type-I FOm) and +4.3 (type-II FOm) in the two samples for which Nd isotopes were analysed by Gill and Whelan (1989b). These ENd values of both FOm-types are lower and the 207PbP04Pb and 208pbP04Pb ratios of type-II FOIB are higher than in all earlier, subduction-related Fijian rocks (Gill and Whelan, 1 989b)
Gill and Whelan (1989b), did not include the island of Moala as an FOIB centre. However, fieldwork conducted by the author during 2001 identified the occurrence of widespread Fom lavas (type-I and type-II), that conformably overlie absarokite
flows and breccias. Moala Fom contain occasional peridotite micro-xenoliths in the north, while large (up to 20cm) and abundant peridotite xenoliths occurr in a flow at the southwestern point (previously reported by Coulsen, 1976). Peridotite xenoliths in FOm have also been reported from Koro (Coulsen, 1976) and from Mago Island (Lau Group) by Cole et al. (1985).
Gill and Whelan (1989b) explained the occurrence of "post-subduction" FOIB as
reflecting the introduction of sub-Pacific mantle from Samoa beneath Fiji and the northern Lau and North Fiji Basins since the breakup of the arc. The occurrence of basalts with type-II FOIB affinity dredged from the NFB (Sinton et aI., 1 988, (see also Nohara et al., 1994» , the "Dupal-like" isotopic compositions of northern Lau Basin tholeiites ego Niua Fo'ou (Ewart and Hawkesworth, 1987) and the Yalewa Kalou type-II FOm in northwestern-most Fiji (Figure 4.4.4) with 87Sr/86Sr
=
0.7061 , lend support to their hypothesis.Magmatism in southern Fiji, in the Kadavu Island Group (Figure 4.4.4), contrasts markedly with other volcanism of the same age elsewhere in Fiji, which was dominated by the eruptions ofFOIB. Pleistocene to Recent (2.9-0.48 Ma, Whelan et al., 1 985) volcanism on Kadavu consisted of predominantly medium to high-K "adakitic" andesites and dacites (Verbeeten et aI., 1995;Verbeeten, 1 996). These rocks become progressively younger towards the west and typically contain phenocrysts of plagioclase, clinopyroxene, hornblende and magnetite, frequently joined by biotite and orthopyroxene in more evolved dacitic compositions. Andesites and dacites of the Ono, Eastern, Central and Western Kadavu Groups of Verbeeten (1996) contain low abundances of Y (15.6-19.6 ppm), high Sr (553-
1667), high SrN (79.2-88.9) and strongly fractionated REE patterns
(LaNbN=
12- 25) akin to known adakitic compositions derived from the melting of garnet amphibolite or eclogite (eg. Defant and Drummond, 1990). Additionally, Kadavuadakites display Sr, Nd and Pb isotopic compositions that fall within the range of Pacific MORB (Gill, 1984), consistent with derivation of these magmas by partial melting of subducted MORB crust (Verbeeten, 1996). However, the occurrence of high-Mg adakites as satellites to the main felsic volcanoes (Tetro eva, in prep.), indicates that the genesis of adakites is complex, and likely involves interaction
Temporally and spatially associated with the adakites is an unusual suite ofhigh-Nb basalts. and basaltic-andesites, known as the Ngaloa Group (Verbeeten, 1 996) (Figure 4.4.4). Two main outcrops of Ngaloa Group rocks occur in Kadavu. One comprises Ngaloa Island (0.36 ± 0.05 Ma, Whelan et aI., 1985) immediately south of Vunesia Station. The other is exposed on Kadavu's northeastern coast between the villages of Levuka and Yaro. Ngaloa Group rocks typically contain microphenocrysts of olivine, clinopyroxene and abundant magnetite. Chemically, they are very distinctive, with high Na20 (2.7-5.5 wt%), Ti02 ( 1 .4-1 .7 wt%), Sr (2055-2957 ppm), relatively high Nb (8-1 6 ppm) and unusually low FeO* (5.8-6.8 wt%). Interestingly, Ngaloa Group rocks also have "adakitic" trace element characteristics (eg. SrN=93-205, LaNbN=1 9-29). However, their relatively high MgO (5. 1-7.9 wt%) and Ni (up to 200 ppm) contents, in addition to primitive phenocryst compositions (eg. olivine F089-91) preclude an origin solely from the partial melting of subducted crust (Verbeeten, 1996). Evidence for mixing/mingling between contemporaneous high-Nb basalt (Ngaloa Group) and andesite-dacite adakite magmas (at least in Western Kadavu), comes from the observation of relatively abundant mafic enclaves, with distinctive Ngaloa Group high-Nb basalt composition, hosted by evolved andesite, dacite and their hypabyssal equivalents (authors observations, 2003 ; Tetroeva, in prep.).