The Astrolabe Islands are a small group of broadly north-south aligned volcanic islands that lie within a large lagoon, surrounded by the Great Astrolabe Reef, forming the northeastern part of the Kadavu Island Group (Figures. 4.4.3 and 5.1 .2A). The islands, from north to south are; Solo lighthouse (small rocky islet), Vanuakula, Dravuni, Yanuyanu-i-Sau, Yanuyanu-i-Loma, Namara, Nggasimbale, Yaukuvelevu, Yaukuve1ailai and Buliya.
Buliya is the largest of the Astrolabe Islands and forms a distinct conical shape with a diameter of approximately 2km and a maximum elevation of � 140m; other islands
generally have low topography. The central parts of the Astrolabe islands are heavily vegetated with outcrop largely limited to the coastal exposures.
P.J. Woodrow conducted fieldwork on Kadavu and the Astrolabe Islands from 1 974 to 1 976, from which the 1 :50,000 geological maps were drawn. Woodrow (1980) classified the rocks of the Astrolabe Islands as basaltic to biotite-andesite lavas and pyroclastic breccias, cut by a number of dykes. Other geological work on the Astrolabe Islands includes Carman, (1986, BSc hons thesis) and Verbeeten (1996, PhD-thesis). In addition, the author visited and collected rock several specimens from Dravuni, Yanuyanu-i-Sau, Yanuyanu-i-Loma, Namara, Yaukuve1evu and Yaukuvelailai islands in August, 2003.
..
Al:Ea-okie -vok:;a1K:: rrecria ardaa-fbVlS
CJ
Stoshonle lavaflowCJ
Banckle lava fowFigure S.1.2A. Simplified geology of the Astrolabe Islands, after Woodrow (1980). Black dots denote sample localities. Samples with AV-prefix were collected by A.c. Verbeeten (1993), other samples (eg. YKSI, YK3 and YK6) were collected by the author in August, 2003.
Figure 5.1.2. B, biotite-clinopyroxenite inclusion in Dravuni banakite; C, boatman Epelli stands next to absarokite dyke, crosscutting absarokitic breccia, Yaukuvelevu Is; D, author next to jointed absarokite dyke,
Namara Is; E, porphyritic absarokite dyke (Ab) and micro-absarokite dyke (mAb) at the northern end of Yaukuvelevu Is; F, close up of micro-absarokite dyke, crosscutting absarokite breccia, Yaukuvelevu Is.
The generalised geology of the Astrolabe Islands is shown in Figure 5. 1 .2A and follows the shoshonitic nomenclature of Iddings (1895). The most evolved rocks of the Astrolabe Islands are banakites, that occur primarily on the southern half of both Dravuni and N amara islands, with another outcrop forming the rock platform at Solo Lighthouse
01
ebeeten, 1996 sample A VI82b). Banakite lavas on Dravuni and Namara are light-grey, and moderately porphyritic with phenocrysts of plagioclase, clinopyroxene, biotite and rare hornblende. Banakite flows on both islands are crosscut by shoshonite dykes.A feature of the Namara and Dravuni banakites is the presence of moderately abundant inclusions of phlogopite-clinopyroxenite (Figure 5.1 .2B) that range in size from 2-15cm, are dark-green and are predominantly angular with sharp contacts against the host banakite lava. Inclusions are holocrystalline and consist of coarse euhedral clinopyroxene crystals poikiolitically enclosing biotite, magnetite and apatite. Large biotite blades (up to 2cm long) and smaller grains of orthoclase, plagioclase and apatite are interstitial to clinopyroxene (Figures 8.5.1C and D). Olivine is rare and is generally altered to iddingsite.
Shoshonite flows (sensu stricto) constitute the island ofBuliya (Fig. 5.1 .2A), but are relatively rare on the northern islands, occurring as dykes within banakite on Dravuni and Namara islands and as minor west-dipping flows on Yanuyanu-i-Sau, Yanuyanu-i-Loma, Yaukuvelailai and the central part of Namara Island. Shoshoriites are dark-grey to green-grey and are weakly to moderately porphyritic rocks with phenocrysts of plagioclase, clinopyroxene and microphenocrysts of magnetite, with olivine generally rare or resorbed/altered. Glomerocrysts of clinopyroxene and magnetite are common (Carman, 1986). Shoshonite flows on Yanuyanu-i-Sau, Yanuyanu-i-Loma and Yaukuvelailai, are crosscut by absarokite dykes.
Absarokite forms the most abundant rock type on the northern Astrolabe Islands, occurring as volcanic breccias and aa-flows on Vanuakula, northern Dravuni, Yaukuvelevu and Yaukuvelailai islands, and as dykes on all of Astrolabe Islands with the exception of Buliya and the Solo Lighthouse islet. Absarokite breccia flows
large (up to 80cm) angular-blocky clasts of highly porphyritic absarokite welded together by smaller more rounded, vesicular clasts that are notably less porphyritic than ·the large blocky-clasts. Individual breccia flows are separated by I -2m thick, red-brown aa-flows. Breccia clasts are highly porphyritic containing between �30- 60% total phenocrysts of euhedral clinopyroxene (generally 3-6mm in size), subhedral-euhedral olivine grains (1-3mm) and microphenocrysts of magnetite and occasionally plagioclase set in a glassy-black to waxy-grey microcrystalline groundmass.
More than 1 00 dykes have been recognised on the Astrolabe Islands (Carman, 1 986), the vast majority of which are of absarokite composition. Absarokite dykes display a wide textural range and variation in phenocryst abundance relative proportions. The majority of dykes consist of highly porphyritic absarokite, essentially indistinguishable from the large absarokite clasts that compose the volcanic breccias (Figures 5.1 .. 2C, D and E). These dykes tend to have approximately equal to sub equal proportions of clinopyroxene and olivine, generally with cpx
�
01. However, on Yaukuvelevu and Yaukuvelailai several other types of dyke have also been noted. These include glassy to weakly porphyritic varieties where clinopyroxene is the dominant phenocryst phase, with subordinate plagioclase and minor olivine (eg sample YKS-1), highly porphyritic picro absarokites (sample YK3), containing predominantly olivine (2-8mm), with subordinate bright-green diopside « 4mm). In addition, two relatively thin dykes (20cm and 60cm thick) of weakly porphyritic, light-grey "micro-absarokite" (eg. sample YK-6) occur on northernmost Yaukuvelevu (Figure 5.1 .2F). These dykes display a prominent laminated fabric and are moderately vesicular when broken. Micro-absarokites are composed of elongate "hopper"-olivine and abundant prismatic clinopyroxene, set in a glassy-dark-grey groundmass (see Figure 9. 1 . 1B). Dykes on Yanuyanu-i-Sau, Yanuyanu-i-Loma, Namara and Nggasimbale islands in the west of the lagoon intrude shoshonite flows, are generally less porphyritic than the dykes observed on the eastern islands (eg. Yaukuvelevu and Yaukuvelailai) and tenci
'to be less mafic (transitional) plagioclase-absarokites.Dykes in the Astrolabe Islands are generally vertical to sub-vertical with the majority displaying dips in excess of 70° (Carman, 1986). Dykes on the eastern
islands Yaukuvelevu and Yaukuvelailai tend to be either vertical or dip steeply to the east, whereas the dykes that occur on the western islands ego Yanuyanu-i-Sau, Yanuyanu-i-Loma, Natnara and Nggasimbale, tend to dip steeply west (�800) and form erosion-resistant, topographic "spines" to these islands. Dykes range in thickness from 40cm (eg. Yaukuvelevu micro-picrite dyke, Figure 5.1 .2F), up to 1 5m, although most are I -3m thick. Dykes on Vanuakula trend predominantly NE SW (Carman, 1 986), whereas on Namara, dykes show contemporaneous crosscutting relationships with both NNE-SSW and NNW -SSE trends. Dykes on
Yaukuvelevu and Yaukuvelailai display a very consistent NW -SE orientation. Carman (1986) noted that the average dyke orientation throughout the island group is NW-SE.
Limited bathymetric data within the lagoon suggests that there is minimal sea floor topography (Carman, 1 986); therefore dyke and bedding orientations provide the only means available to interpret the location of the main eruptive centres in the Astrolabe region. Dyke trends and bedding orientations from the western (Yanuyanu-i-Sau, Yanuyanu-i-Loma, Namara and Nggasimbale) and eastern (Yaukuvelevu and Yaukuvelailai) indicate a radial trend about an eruptive centre situated in the middle of the present day lagoon (Carman, 1986). Dyke orientations on Vanuakula, with W -to SW -dipping lava flows and breccias, suggest a separate absarokite-dominated centre to the northeast of Vanuakula. Solo Islet (�l lkm NNE of Vanuakula) , is enclosed by a small circular reef and is accordingly interpreted to be a relic of an individual volcanic cone (Carman, 1 986). Buliya's relatively coherent shape with markedly conical topography and radiating bedding orientations indicates that the island has suffered significantly less erosion than the other islands further north. It is therefore interpreted as the youngest of the Astrolabe eruptive centres (Carman, 1986).
The evolutionary model for the geological development of the Astrolabe Islands proposed by Carman (1986), excluding the arguably separate Solo and Bulyia centres, shows notable similarities to that proposed by Setterfield et al. (1991) for Tavua Volcano. The development of the central Astrolabe Volcanic Centre consisted of an initial shield-building phase, involving the extrusion of voluminous
submarine shield volcano. Subsequent absarokite eruptions eventually produced a subaerial expression to the volcano and alternating volcanic breccias and aa-flows were deposited and intruded by coeval subvolcanic absarokite dykes and sills. The initial volcanic phase culminated in the eruption of more evolved shoshonitic flows and the emplacement of hypabyssal equivalents as dykes. The second eruptive phase relates to a significant change in magma composition, involving the subaerial eruption of voluminous banakite flows. Banakite extrusion was likely associated with contemporaneous shoshonitic magmatism, as evidenced by shoshonite dykes intruding banakite on southern Dravuni and Namara islands. A caldera probably formed during the late stages of banakite eruption and was situated near the centre of the present day lagoon. Magma removal resulted in fault-accommodated subsidence and "downsagging" similar to the model proposed by Setterfield et al. (1991) for Tavua Volcano. Following caldera-collapse, subsequent development of the central Astrolabe volcano was dominated by continued subsidence and erosional processes. The Astrolabe Islands can therefore be regarded as relics of the pre existing caldera-rim stratigraphy.
Rock specimens used in this study were collected from the Astrolabe Islands by A.C. Verbeeten in 1992 (A V-prefix). Additional samples were collected by the author in August 2003. Sample localities are shown in Fig. 5. 1 .2A and analyses of Astrolabe Group rocks are presented in Table 5.2C.