Geology and Petrology of Rabaul Caldera, Papua New Guinea

R. F. HEMING* Department of Geology and Geophysics, University of California, Berkeley, California 94720

ABSTRACT parison with experimental data for high- off the north coast of New Guinea and ter- alumina basalt also suggests a shallower minates at Rabaul. Only the New Britain Rabaul caldera is unusual in that it was origin. A progressive increase in Ti02, section has the classical features of an is- formed by two episodes of construction and A1203, and alkalis along the New land arc: a deep submarine trench to the collapse on an older basalt volcano. One Britain—New Guinea arc toward Rabaul at south, an inclined seismic zone dipping collapse occurred around 3,500 yr B.P. and its eastern end cannot be explained. Key northward, and a string of volcanoes on the the latest around 1,400 yr B.P. Both were words: igneous petrology, volcanology. side of the island opposite the trench. West accompanied by the eruption of daciiic of New Britain, the trench disappears, and pumiceous ash flows. Following initial col- INTRODUCTION in the vicinity of Long Island (Fig. 1), the lapse, volcanism was confined to a large Rabaul caldera, situated at the northeast- simple Benioff zone is replaced by a north- andesite volcano in the southern part of the ern tip of the island of New Britain, is the and a south-dipping zone (Denham, 1969). caldera, but a renewal of basaltic volcanism easternmost volcano of the New West of here, the volcanic arc lies well occurred at a point on the eastern ring frac- Guinea—New Britain arc (Fig. 1). Rabaul is offshore of the mainland, and the under- ture. The later collapse of the southern cone a current center of active volcanism, and lying seismic zone is located beneath New left the caldera as a large ellipse, 14 by 9 the region is often shaken by large earth- Guinea. Earthquakes of intermediate depth km, breached on the southeast side, though quakes. Vulcan (on the western side of the are frequent in the New Britain portion of more recent volcanism has built small cones caldera) and Tavurvur (on the eastern side) the arc but do not occur in the arc west of on the caldera floor. erupted simultaneously in 1878 and 1937, the island. No trench, however, is as- The rocks of the caldera cannot un- and Tavurvur was active again in 1941 and sociated with the western portion of the arc equivocally be labeled as calc-alkalic because 1942. Other cones have been built within (Fig. 1). the contemporaneous basalts are high- the caldera, the most notable being Sulphur In New Britain, the volcanic chain has alumina types and the suite shows mild iron Creek, which was reported by early mis- two pronounced breaks, one west of enrichment. This mild iron enrichment is sionaries and explorers to have erupted in Rabaul of over 100 km and another of the not found in the lavas of some parasitic vol- 1858. same length west of Talasea. The first canoes, suggesting that it is an artifact of The first description of the geology of the break, between Rabaul and Mount the magma chamber beneath the caldera. caldera was given by Fisher (1939) in his Ulawun, contains a number of north- Differences in chemistry and mineralogy report on the eruption of Vulcan in 1937, west-trending faults and a graben (Macnab, persist between lava erupted on the western which made local authorities aware of the 1970). The second break has a more un- side of the caldera and that on the east; vulnerability of Rabaul, and a volcanologi- usual character. From Mount Ulawun the chemical variation is explained by crystal cal observatory was built on the north rim volcanic chain continues along the north fractionation at low pressures. Lack of a of the caldera. coast of New Britain then abruptly passes clearly defined Benioff zone and location north along the Talasea peninsula before close to a transform fault suggest that the STRUCTURE OF THE turning west again to the Witu Islands (Fig. origin of the magma was not controlled by NEW BRITAIN ARC 1). No active volcanoes are found between partial melting near a Benioff zone. Corn- The major regional structures are de- Talasea and Langila, at the western tip of picted in Figure 1. The New Guinea—New the island, and the arc apparently is offset * Present address: Department of Geology, University Britain volcanic arc begins west of Manam about 100 km between Talasea and Mount of Auckland, Private Bag, Auckland, New Zealand.

Geological Society of America Bulletin, v. 85, p. 1253-1264, 10 figs., August 1974

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Figure 1. Structure of the New Guinea region. Bathymétrie contours are in meters with depths below 6,000 m depicted by a horizontal line pattern, and below 8,000 m, by dots. Bathymetry from Australian Department of National Development map (1970); seismic data from Denham (196i>); plate boundaries and relative motions of crustal plates are those suggested by LePichon (1968) and Johnson and Molnar (1972).

Langila. Dacite and rhyolite are common in GEOLOGIC SETTING OF Fisher, 1944). Similar rocks are exposed on the volcanoes of the arc east of Talasea, but THE CALDERA southern New Ireland (Fig. 1), where again west of that point, andesite is the most silice- active northwest-trending faults control the ous rock. The geology of the area surrounding structure (French, 1966). Rabaul caldera lies close to a postulated Rabaul caldera is typical of young, Xenoliths provide few clues as to the na- junction between three crustal plates and circum-Pacific volcanic arcs. West of ture of the basement ber.eath Rabaul. Some only a few kilometers west of a transform Rabaul, the Baining Mountains (Fig. 1) diorite blocks are found, and a gabbro from fault separating the Pacific and Bismarck consist of Tertiary flows, volcanic sedi- near Kabakada (Fig. 4) resembles leu- Sea plates. The simple model of trench, in- ments, and limestones; all are intruded by cogabbro from the Baining Mountains clined seismic zone, and volcanic arc is not rocks of varied composition including (described by Macnab, 1970). West of apparent near the Rabaul caldera. Shallow leucogabbro, adanellite, and granite, Rabaul, the volcanic rocks interfinger with seismic events of large magnitude are con- which Macnab (1970) considers to be unconsolidated sediments that may concentrated along St. George's Channel, while equivalents of the calc-alkalic volcanic tinue some distance beneath the Rabaul earthquakes of intermediate foci scatter suite. The range is cut by northwest- volcanics. Miocene limestone exposed in north and west of Rabaul but are concen- trending faults, the largest of which, the the Rembarr Range (Fig. 4) most probably trated mostly along large faults. Baining fault, is still active (Macnab, 1970; forms an isolated fault block. Uplifted Pleis-

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tocene reef limestone is found around Watom Island and at Tawui Point. Although there is no direct indication of the composition of the basement beneath the caldera, circumstantial evidence suggests that it may be an extension of that found in the Baining Range, with the possible addition of a veneer of unconsolidated sediments.

STRUCTURE OF THE CALDERA

The Rabaul caldera is elliptical, measur- ing 14 km from north to south and 9 km from east to west, with a breach 5 km wide in the southeast wall (Fig. 2). Two well-preserved composite cones rise above the eastern wall (Fig. 4): The Mother (Kombiu) and South Daughter (Turan- guna). The northern wall is formed by the deeply dissected cone of North Daughter (Tovanumber). Straddling the eastern wall is the breached volcano Palangaigia and within it the low cone of Rabalanakaia (Figs. 2, 4). Within the caldera lie the active volcanoes of Tavurvur and Vulcan, the fissure of Sulphur Creek and its associated explosion craters, and the two oddly shaped Beehives that are the remnants of another ash cone. The outer slopes of the caldera decrease gradually to the south and west, though in detail the terrain is highly dissected. South- southwest of Rabaul is Mount Varzin, the remnants of a satellite volcano; 11 km to the northwest is another satellite, Watom Island (Fig. 4). The bathymetry of the caldera floor is shown in Figure 2. The northern part is es- sentially a long, narrow trough that plunges southward into the basin of Karavia Bay, which reaches a depth of 295 m. Remnants of the caldera wall are seen at the seaward entrance to the caldera, and east of them are two deep valleys that extend into the deep trench beneath St. George's Channel. The dominant feature in the area is the elliptical fault of the caldera ring fra.cture (Fig. 4), which is partially masked by extensive pumice ash deposits. Where it is well exposed, between South Daughter and The Mother and east of North Daughter, it is arcuate and nearly vertical. Figure 3 is a copy of seismic reflection records made on the R.V. Mahi in Karavia Bay, with the steep escarpment of the caldera ring fault clearly shown. Displacement took pls.ee on Figure 2. Topography and bathymetry of the Rabaul caldera. Contours in meters. Bathymétrie contours are at a number of concentric arcuate faults. 20-fm intervals (—36 m). Also shown is the track of R. V. Mahi along which lies the profile of Figure 3. Around Rabaul these additional faults are hidden, but some evidence for them may be caldera. These features are presumed to be One scarp forms the east rim of the caldera found. Within 5 km or so of the western surface expressions of faults parallel to the while the other trends parallel to the ring and southern walls of the caldera, drainage major fracture. fault and cuts the western slopes of Palan- patterns are parallel to the escarpment Two faults within the caldera are not gaigia (Fig. 4), a basaltic cone that straddles (Figs. 2, 4) rather than radially away from mantled by ash and, because they form the main caldera ring fracture. it. Two valleys that originate outside the steep scarps with only a small accumulation The amount of displacement on the cal- caldera wall run at an acute angle to it, but of talus at their base, are thought to result dera faults is not known. Around Karavia they eventually cut across to drain into the from a more recent episode of movement. Bay where the wall is over 400 m high, the

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deeply weathered. Xenoliths in these beds include diorite, andesite, and basalt. South- erly dips of 15° to 20° are consistent with a source within the caldera, and the pyroclastic flows may have been erupted during the first caldera collapse. Minor outcrops, mainly of pyroclastic rocks, lie to the west and north of the caldera and may be the remnants of small explosive vents. At Kabakada (Fig. 4), a road cut has exposed a succession of dark basaltic tuffs interbedded with palaeosols and Figure 3. Profile in the southern part of the caldera (see Fig. 2). Sparker record (125 to 250 Hz) made on board R. thin pumice bands and succeeded upward V. Mahi and reproduced here with the permission of the Hawaii Institute of Geophysics. Note the inclination of the caldera ring fault on the right of the photograph and the apparent rotation of the sediments west of the fault, llie by dacitic ash flows and bedded pumiceous two domes west (left) of the basin are caused by a course change while crossing a ridge but the cone is a real feature. ash. Fragments of gabbro, diorite, and basalt occur in the ash beds, and some throw must exceed 600 m if the depth of caldera unit, the Karavia Bay unit, the cal- blocks have impact pits beneath them sug- the Kara via Bay basin is taken into account. dera unit, and the postcaldera unit. gesting the presence of a nearby explosive Stepped topography of the caldera wall in vent. this area suggests that the throw is taken up PRECALDERA UNIT Lithologies similar to those just described on several faults. Younger faults show The precaldera unit consists of lava flows are also found near the northwest rim of the smaller displacements, with 100 m on the and pyroclastic rocks erupted from the caldera. Again the lower part of the section eastern caldera wall and a few tens of me- Rabaul volcano and its parasitic cones, as is composed predominantly of basaltic ters at Palangaigia, but these are minimum well as pyroclastic flows erupted during the pyroclastic rocks. A pyroclastic flow de- displacements as recent alluvium and ash first caldera collapse. Early volcanism pro- posit in the caldera wall west of North abuts the wall in both places. duced basaltic lava flows and scoria but Daughter is of similar composition to that Faults radial to the caldera have not been later eruptions were predominantly of at Kabakada, but additionally it contains detected but their presence may be inferred. andesite and dacite pyroclastic rocks. basalt fragments. East of North Daughter In addition to the caldera rir.g fracture, the Basalt and basaltic andesite crop out are two dacite flows, with tubular cross sec- other dominant feature is the extensive along the eastern and northern caldera tions and pronounced concentric jointing breach in the southeast wall, the age and walls and in the parasitic cones of North analogous to undrained lava tubes de- nature of'which is unknown. The breach is and South Daughter, The Mother, Mount scribed by Lutton (1969). in line with the wide depression in the Varzin, and Watom Island (Fig. 4). In the northwest rim of the caldera wall (Fig. 2); eastern caldera wall, basalt and basaltic KARAVIA BAY UNIT together they may indicate a large graben andesite form massive flows up to 20 m The Karavia Bay unit consists of andesite that trends northwest, more or less parallel thick that are interbedded with fine gray lava flows and pyroclastic rocks erupted to major faults in the Baining Mountains and brown tuffs, usually only a few cen- from a volcano that was built in the south- and southern New Ireland. There is, how- timeters thick, which dip eastward at a shal- ern part of the caldera following the first ever, little evidence other than topographic low angle. caldera collapse. The base af the unit is not for such a graben. All of the most recent North and South Daughter, The Mother, exposed but it is everywhere overlain by volcanoes, apart from Rabalanakaia, lie and Mount Varzin consist of basalt and pyroclastic rocks of the su:ceeding caldera within, or close to, the postulated graben. basaltic andesite flows and scoria interbeds. unit. Lineaments outside the caldera trend North Daughter also contains beds of Andesite lava flows crop out in the west- either northwest or approximately east. scoriaceous volcanic breccia with blocks of ern caldera wall (south of Vulcan) with a The northwest-trending group can be fol- basalt as much as 2 m across. North distinctive flow banding caused by alterna- lowed across the young ash mantle, and the Daughter and Mount Varzin are deeply tion of vitric and vesicular bands. The ve- most easterly traces form a high scarp over- eroded volcanoes, while in contrast, South sicular layers are often wea :hered a dull red looking the Keravat River valley (Fig. 4). Daughter and The Mother are well- color, and large xenoliths of andesite are The east-trending group cuts the limestones preserved cones, the latter even retaining a especially numerous near the base of the of the Rembarr Range, and one other small recognizable summit crater. South Daugh- flow. fault of similar trend cuts a series of ash ter may have been active since the first cal- Outcrops of pyroclastic: rocks appear layers and a soil horizon north of Mount dera collapse as a lobe of olivine basalt and mainly along the south shore of Karavia Varzin but does not affect the overlying scoria apparently mantles the caldera wall. Bay as a sequence of dark-brown tuffs and ash. Hence, this latter group may be older Watom Island is a low cone composed of ash beds, palaeosols, and volcanic breccias, than the northwest-trending faults. interbedded basalt flows and thick, the latter becoming more important up- scoriaceous volcanic breccias. The summit ward and eventually dominating the sec- STRATIGRAPHY OF THE of the volcano contains a crater 1.5 km tion. Xenoliths of basalt and basaltic ande- RABAUL VOLCANIC SERIES across, and a low basaltic cone within the site similar to those in the eastern caldera crater has obliterated the eastern rim. wall are found throughout the section. Dips The geology of the caldera is shown in The major outcrops of pyroclastic rocks in the ash beds are near 40° south- Figure 4. As the rocks are largely mantled occur around Mount Varzin where erosion southwest. Further small outcrops of pyro- by pumiceous ash, much of the data was has stripped much of the young pumice and clastic rocks occur in deeply incised valleys obtained from steep valley walls and road ash mantle to expose an older series of in- west and southwest of Vulcan. cuts as shown in Figure 5. durated pyroclastic rocks including The Rabaul volcanic series is divided into pumiceous tuff, volcanic breccia, and CALDERA UNIT four informal units on the basis of lithology pumice flow deposits. No palaeosols are This unit consists of pyroclastic beds and apparent time relationships: the pre- found in this sequence, but some tuffs are with compositions varying from basalt to

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LEGEND

Figure 4. Geological map of Rabaul caldera. Numbered points refer to sections in Figure 5.

HEMING, FIGURE 4 Geological Society of America Bulletin, v. 85, no. 8 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/8/1253/3418429/i0016-7606-85-8-1253.pdf by guest on 02 October 2021 GEOLOGY AND PETROLOGY OF RABAUL CALDERA, PAPUA NEW GUINEA 1257

TYPICAL COLUMN 1 6 7 10 r—r-n O- • ash flow with •o' . M o.': • '0- a pumice blocks • .' a o • • IO "i . a 1 • . ! volcanic breccia 1 " . /N, 1 O' O- ' •-'C? o: • !• .Qi 0. :• ô ash flow o-

M.Ò'Ò'x? ? •'•'ai graded.pumiceous \ 1 • • 1 •CP \ 1 • . . I b' ash(blockto fine m ssässä ash) o: • • • C •. o soil

¿vAooi Vv •. • 0- d 9 o ash flow il OÓ VERTICAL e bedded ash,(lapilli lì :o.à>;o;p.c to fine ash) SCALE il • f o'o. Ni ' o. '

o •• oo. ooc 0<3i

LsSÄ 3 a* - Figure 5. Stratigraphic sections through the pyroclastic sequences around Rabaul caldera. Numbers refer to selected locations on Figure 4. 4 METERS es »v T-

rhyolite. The upper beds of the unit, which 4) as well as in the foothills of the Baining Lake (Williams, 1942) and the Valley of contain dacitic pyroclastic flows and bed- Mountains, 30 km and more from the cal- Ten Thousand Smokes (Curtis, 1965). The ded ash, were deposited during the second dera rim. finer ash of the interfluves was deposited by caldera collapse. The base of the unit is The lowermost bed (b) consists of graded the ash-hurricane component of the ash diachronous; on the western side of the dacitic pumice lapilli containing numerous flow (Anderson and Flett, 1912). caldera, the lowest bed is a buff-white pumice blocks and forming repetitive units The volcanic breccia (a') is found be- rhyolitic ash (Fig. 5, sec. 1), but elsewhere grading upward from coarse to fine. At a tween the two ash-flow deposits. It is a various members of the unit rest uncon- point north of Vulcan (Fig. 5, sec. 6), it is coarse bed composed of fragments of formably on Karavia Bay and precaldera over 15 m thick. basalt, andesite, obsidian, mudstone, and units. Resting unconformably on (b) are two diatomite in a matrix of dark coarse ash The lower part of the caldera unit is dacitic pumiceous ash beds (Fig. 5, a" and a) and pumice. The stratification is crude, but dominated by rhyolitic ash-flow deposits. and an intervening volcanic breccia (a'). in places the breccia shows signs of rework- However, in some places (Fig. 5, sees. 1, 2, The two ash beds are unsorted and un- ing by water with development of cross- 3), an intervening bed of basaltic lap. lli (e) graded and are typical of ash-flow deposits; bedding and shallow channels. Many of the 30 cm thick appears and is divided they are best preserved in the deep valleys lava fragments are extremely altered and approximately in half by a thin oxidized around the caldera, locally reaching thick- stained bright red and green, similar to layer, thus probably representing two nesses of 30 m, and contain blocks of weathered blocks found in fumarolic areas. closely spaced eruptions. The rhyolite ash pumice, obsidian, and basalt that are often The source of the mudstone, diatomite, and beds are unsorted and in places display a found concentrated into horizontal lenses rotten varicolored lava fragments may have crude columnar jointing. approximately 1.5 m thick and 5 m wide. been a dormant crater lake which was A thick palaeosol (c) overlying the rhyo- These features are similar to those de- evacuated by strong explosions prior to the lite ash (d) contains carbon that has yielded scribed by Kuno (1941) in ash-flow de- eruption of the second ash flow (a). an age of 1,200 yr B.P.; because of its wide posits at Komagatake. On the interfluves The two ash flows (a, a") are unwelded, distribution, the palaeosol provides a useful the ash-flow deposit is only a few meters but at one locality on the south shore of horizon, as well as representing a lengthy thick with pumice lumps 6 or 7 cm across. Karavia Bay there is an intervening bed of period of relatively inactive volcanism. It is also finer grained and does not contain welded tuff. This tuff overlies the volcanic The overlying beds (Fig. 5, b, a", a', a) are the coarser lenses typical of the deposit breccia (a1) and is over 6 m thick, with a all composed of pumiceous dacitic ash and within the valleys. In the few places where moderately welded base containing lumps record the sequence of eruptions during the the bed can be traced continuously from of flattened pumice; toward the top, the tuff second collapse. These beds mantle most of ridge top to valley bottom, the change in is less dense and more friable. These fea- the caldera wall and rest unconformably on grain size is gradual. In nature and distribu- tures, as well as the andesitic composition, the Karavia Bay and caldera units. They tion, these deposits are similar to pyroclas- are reminiscent of the sillars described by crop out over most of the mapped arsa (Fig. tic flows from other calderas, such as Crater Fenner (1948) from southern Peru.

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All that remains of Palangaigia volcano timated that 22 x 108 m3 of material was along the western side of Palangaigia, on (Fig. 4) is a horseshoe-shaped rim with the erupted, most of it in the first 17 hr. The the eastern side of the caldera, and in the breach on the southern side partly filled by Beehives, north of Vulcan, are remnants of western caldera wall south of Vulcan. The the younger cone of Rabalanakaia (Fig. 2). a cone of similar structure to Vulcan. breach in the caldera wall may have been Palangaigia was developed on the eastern A striking feature of postcaldera vol- formed at this time. caldera ring fault and postdates the first col- canism is the lithologic contrast between 5. Renewed volcanism took place on the lapse. Outcrops of basalt and basaltic andesites erupted on the eastern side and floor of the caldera with the building of scoria from Palangaigia are overlain by dacitic pumice and ash on the west. submarine volcanoes in Karavia Bay and both bedded ash (b) and volcanic breccia the cones of Rabalanakaia, The Beehives, (a'). The basaltic lapilli (e) may well have FORMATION OF THE CALDERA and Matupit Island. Active volcanism con- been a product of a Palangaigia eruption. The following stages in the history of the tinues at the present time at Vulcan, Tavur- caldera are proposed: vur, and Sulphur Creek. Figure 6 is a POSTCALDERA UNIT 1. Construction of the ancestral volcanic schematic illustration of the development of superstructure carrying the parasitic cones the Rabaul caldera. This unit comprises a number of small of North Daughter, The Mother, and South volcanoes built in the caldera since the sec- Daughter. Mount Varzin and Watom Is- AGE AND VOLUME OF THE ond collapse. Rabalanakaia, Sulphur land were active during this stage. PRIMITIVE VOLCANO' Creek, and Tavurvur (Figs. 2, 4) lie along 2. Eruption of dacite and rhyolite pyro- the eastern side of the caldera, while Vulcan clastic flows and air-fall ash penecontem- Pyroclastic rocks around Mount Varzin, lies on the western side. Additional to these poraneous with formation of the caldera by west of North Daughter and at Kabakada, well-preserved cones are The Beehives and collapse on the major ring fault and parallel a::e thought to represent ash flows erupted Matupit Island, which are remnants of ash arcuate faults. when the ancestral volcano collapsed. No cones, and the submarine volcanoes in 3. Construction of the Karavia Bay vol- carbonized wood has been found in any of Karavia Bay. canic crater and Palangaigia volcano. The these flows; however, the age of this col- Distinct differences exist: between the lower part of the caldera unit was deposited lapse may be inferred from a palaeosol ex- eruptive products of volcanoes near the during this period, including the basalt posed in the Baining Mountains 40 km eastern caldera wall and those on the west- lapilli (e) and the rhyolite ash (d); some south of Rabaul. At the base of the section ern side of the caldera. Rabalanakaia, Sul- pumice and ash beds of the precaldera unit is a palaeosol overlain by fine white clay 1 phur Creek, and Tavurvur are composite may also date from this period. m thick. Above the clay is another volcanoes that have erupted andesite. In 4. After a period oi quiescence of possi- palaeosol covered by up to 2 m of fine, contrast, Vulcan and The Beehives consist bly 200 to 300 yr, during which palaeosol c w.aterlaid pumice. This upper bed was only of dacitic pumiceous ash. Matupit Is- was formed, the Karavia Bay center found- probably deposited during the final collapse land, which is also built of dacite pumice ered after a series of violent eruptions had of the caldera. The radiocarbon age of the and ash, spoils the symmetry of this divi- laid down the white bedded pumiceous ash upper palaeosol is 1,450 ± 60 yr B.P. sion as it lies toward the eastern side of the b and the two pumice flows a and a". The (Macnab, 1970), which is in good agree- caldera (Fig. 4). volcanic breccia (a') suggests that two cen- ment with ages yielded by charred logs Rabalanakaia, lying eccentrically within ters were involved in the eruptions. Further within the chaotic pumiceous ash around the skeleton of Palangaigia, retains a well- collapse took place on arcuate fractures the caldera (Table 1). The white clay may preserved crater surrounded by an ash be an altered product of the ash fall of the rampart. The latest lava flow was a glassy earlier collapse. The carbon layer beneath it andesite that covered the lowland to the has an age of 3,500 ± 65 yr B.P. (Macnab, southwest as far as the shoreline north of 1970). Tavurvur. Few clues remain about the form and size Sulphur Creek is a fissure, now filled by of the primitive volcano. Ii: seems to have the sea, with two small craters at the land- been composed predominantly of basaltic ward end. The two craters were active in flows with pyroclastic rocks being relatively 1850 when they erupted blocks of andesite. scarce near the base but becoming more Tavurvur may be of the same age as voluminous upward. An approximate cal- Rabalanakaia but has had more historic ac- culation of the volume of the primitive vol- tivity. Eruptions in 1878, 1937, and 1941 cano is based on a concept of two coalesc- were explosive with no recorded lava flows. ing cones, a southern one with a diameter Early lava flows from Tavurvur were por- of about 9 km and a northern one with an phyritic basalts, but later eruptions have approximate diameter of 6 km. Compari- mainly been explosive, ejecting andesite son with well-preserved composite cones bombs and ash. such as The Mother gives ar. angle of slope Vulcan is a cone of dacitic pumice and of about 20° above the 200-m contour. ash produced during one eruption in 1937, Usir.g these figures, a height of 1,600 m was though a previous eruption had formed an obtained for the southern corie and 1,100 m island in 1878; charts prior to that date for the northern one; these dimensions yield 3 show a submarine bank according to Fisher a total volume of 71 km . Fisher (1939), as- (1939) who gave details of the 1937 erup- suming the case of a single cone 3,280 m 3 tion. It began at 4:15 p.m. on May 29th high, obtained a volume of 104 km . These Figure 6. Hypothetical stages in the development of 3 and was preceded by a number of earth- the caldera: (a) ancestral cones, i'b) the first collapse and figures compare with a volume of 62 km quakes and severe ground tilting; by 9 a.m. formation of the caldera, (c) growth of the volcano in for Crater Lake in Oregon (Williams, 1942) 3 on May 30th the cone had reached a height Karavia Bay, and (d) subsequent collapse of the Karavia and 75 km for Lake Dakataua (Lowder Bay volcano and growth of postcaldera cones. All sec- and Carmichael, 1970), a caldera 10 by 14 of 180 m. The eruption was virtually com- tions are north-south; the volcano shown on the ex- pleted by the evening of June 2d. Fisher es- treme left is North Daughter (see text). km in size.

TABLE 1. C" DATES FROM RABAUL CALDERA least 650 km'2, and soon after the eruption, it probably mantled an area at least twice Sample Age in years Date Stratigraphie Location number (B.P.) (A.D.) position that size. Thus a conservative estimate, tak- ing into account the volume that entered 7030* 1,505 ± 90 445 a (Fig. 5) In road cut near the sea and that contained beneath the 8032* 1,390 ± 90 560 a" (Fig. 5) 3 section 7 (Fig. 4) 0.5-m isopach, yields about 24 km . 8033* 1,430 ± 90 520 a (Fig. 5) 1001Bf 1,280 ± 81 670 a (Fig. 5) Base of ash flow in PETROGRAPHY valley between sections 1 and 2 The lavas of Rabaul caldera and the ad- P1344+ 1,450 ± 60 500 Upper carbonaceous jacent volcanoes of Watom Island and layer Mount Varzin vary in composition from P134S+ 3,500 ± 65 1,550 B.C. Lower carbonaceous Collected near layer Arumbum, Baining basalt to rhyolite, but the full range is found Mountains (Macnab, 1970) only in rocks around the caldera; nothing more silicic than basaltic andesite is found * Dated at GEOCHRON Laboratories, Cambridge, Massachusetts, U.S.A. t Dated by N.2.D.S.I.R., Lower Hutt, New Zealand. on either Watom Island or Mount Varzin. Estimates of the relative volumes of the various rock types are unreliable because of the INTRACALDERA STAGE Carmichael, 1973) at 830° to 1035°C, large volume lost during formation of the which should have been more than ade- caldera; however, the following volumes After the foundering of the primitive quate to permit welding. were calculated as rough approximations: cones, volcanism was centered in the srea basalt and basaltic andesite, 53 percent; of Karavia Bay and Palangaigia. The latter AGE AND VOLUME OF THE andesite, 30 percent; dacite, 15 percent; volcano can be dated, relative to other FINAL COLLAPSE and rhyolite, about 2 percent. Modal events in the caldera, with a fair amount of Pieces of carbonized wood were found in analyses of representative rocks are listed in certainty. It straddles the ring fault and the pumice flows, and dates from them are Table 2. Reported mineral analyses were rests unconformably on flows of The listed in Table 1. The spread in ages is 225 obtained using an electron microprobe. Mother. The western flanks of Palangaigia yr, but the greatest care was taken with Fuller descriptions and analyses of the are cut by another fault that formed during samples 8032 and 8033, which yielded the andesite, dacite, and rhyolite are provided the second collapse. Also, the basaltic lapilli most consistent results of 1,430 B.P. for the by Heming and Carmichael (1973). horizon (e) found in the base of the caldera upper pumice flow a and 1,390 B.P. for a". unit is very similar in composition to that None of these dates agree with the age of Basalt erupted from Palangaigia. 1,200 yr B.P. obtained on the palaeosol c, Porphyritic basalt is most common in the In the case of the Karavia Bay unit, the but water and interference by organisms precaldera unit where many flows are ves- evidence is rather more circumstantial. The would make that a minimum age only. All icular. Phenocrysts are of plagioclase (An95 andesite lava flows and pyroclastic rocks of the ages, however, are accurate to only to An70), olivine (Fo85 to Fo70), diopsidic have dips that suggest Karavia Bay was a ±90 yr so they overlap. augite, and less commonly, titanomagnetite center. The lava pile forms a high area near The Karavia Bay volcano had a diameter and orthopyroxene. Plagioclase pheno- the caldera rim which divides the annular at sea level of at least 7 km and a height crysts are usually twinned and zoned; many drainage and covers the arcuate faults of conservatively estimated at 1,600 m. In- have sieved cores and more rarely contain the first collapse. In addition, the sequence cluding the part beneath sea level, the cone small crystals of pyroxene, olivine, and of pumice beds b to a, which is thought to had an estimated total volume of 26 km3. titanomagnetite. Coarse-grained aggregates have been erupted from the Karavia Bay This value is the collapse volume of the of pyroxene, olivine, plagioclase, and center, reaches its greatest thickness in this Karavia Bay center; a further small volume titanomagnetite, with similar compositions area. was lost by collapse in other parts of the to the phenocrysts, are common. caldera, as well as at Palangaigia. The groundmass is commonly finely crys- FINAL COLLAPSE The volume of dacite pumice is consider- talline and contains labradorite, clino- Violent eruptions, followed by collapse able; a pumice layer 0.5 m thick is exposed pyroxene with a range of compositions of the Karavia Bay center, gave rise to the in the foothills of the Baining Mountains including pigeonite and subcalcic augite, caldera as we see it today. Early eruptions where it is associated with a bed of ac- orthopyroxene and ubiquitous titano- from this center may have formed the rhyo- cretionary lapilli 2 to 3 cm across. The area magnetite. Small interstitial patches of lite ash flows (d and f) of the caldera unit, now covered by the dacite pumice ash is at brown glass are found in some rocks. but the final eruptions from the center were

of dacite pumice, which mantled most of TABLE 2. MODAL ANALYSES* the outer flanks of the caldera and its inner walls. Two cones were involved. The crater Sample Plagioclase Olivine Augite Hypersthene Oxides Groundmass Glass number of the second cone had been dormant for a considerable period of time and was filled 8014 24.1 3.9 1.2 0.0 1.6 69.2 with mudstone and diatomite; it was the 7084 40 1 6.4 9.0 0.6 0.8 43.0 eruption of this material which formed the 8082 45 4 2.1 7.2 0.1 1.7 41.2 distinctive volcanic breccia (a'). The two 8080 32 3 1.3 4.4 0.4 0.3 61.3 ash flows involved were similar in composi- 6985 34 0 0.7 3.0 0.1 0.7 61.5 7066 31 8 0.6 7.2 1.8 1.3 58.2 tion, both being dacites but neither showing 8015 13 7 0.1 4.2 0.4 1.1 80.4 any signs of welding, a feature they hold in 8087 14 3 0.2 1.8 0.3 0.6 82.7 common with many other ash flows in 8016 9 5 0.4 1.6 0.4 1.0 87.0 island-arc volcanoes. The temperature of 8042 6 7 0.0 0.7 0.2 0.1 92.32

the ash flows has been estimated using the Note: The mode of 7007 is comparable to that of 6985, and of 8047 and 8058 to that of 8042. Fe-Ti oxide geothermometer (Heming and * Volume percent.

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Basaltic Andesite bleb-shaped grains. Ilmenite, when present, CHEMISTRY AND PETROLOGY is enclosed by glass. Basaltic andesite is usually porphyritic, The mineralogy of rhyolite is remarkably Chemically, the lava flows have many though aphyric varieties occur. The mineral similar to that of dacite, the only difference similarities to those from other content is similar to that of the basalt, ex- being the greater paucity of phenocrysts in circum-Pacific volcanic arcs (Table 3). They cept for the appearance of titanomagnetite rhyolite (2.4 percent, Table 2, Heming and are quartz normative and have relatively as a phenocryst phase. Plagioclase Carmichael, 1973). Despite the difference low Ti02 and high A120;, in comparison to (bytownite-labradorite) predominates in bulk composition, the composition of the oceanic volcanic rocks. Compared with the along with diopsidic augite and small various phenocryst minerals is little differ- volcanic rocks of Talasea (Lowder and phenocrysts of olivine (FoS0 to Fo60). The ent in rhyolite than iri dacite. Carmichael, 1970), the Rabaul series is olivine is commonly rimmed with or- higher in Ti02, A1203, and total alkalies thopyroxene that also occurs as pheno- XENOLITHS find slightly higher in content of total iron, crysts, which range in composition from Xenoliths are common in the pyroclastic but lower in magnesium. One notable fea- En75 to En70. The groundmass is crystalline rocks around the caldera, most being of ture of the analyses is the lack of a rock or partly glassy and contains titanomag- older lava flows from the caldera. The vol- with a Si02 composition of between 55.5 netite, diopsidic augite, augite, subcalcic canic breccia (a') commonly contains frag- and 60 percent, which is only emphasized augite, and pigeonite. Common textural ments of mudstone, diatomite, coral, and not bridged by further analyses. A simi- features are phenocryst clots and sieve tex- basalt, and andesite. Xenoliths of plutonic lar gap was noticed by Lowder and Car- ture in either cores or marginal zones of rocks are rare. Diorite occurs in the caldera michael (1970) at Talasea, by Brothers plagioclase phenocrysts. unit pyroclastic rocks north of the caldera (1970) in the Kermadec Group, and seems and in the ash beds around an explosion to be a common feature of circum-Pacific Andesite vent near Kabakada. The diorite contains volcanic suites. Two major types of andesite may be dis- intergrown crystals cf pyroxene altered to Trace element abundances are listed in tinguished by texture. One type contains amphibole along with ilmenite, titan- Table 4. Ba, Zr, and 2',n increase from clots of phenocrysts of similar composition omagnetite, and interstitial quartz and basalt to dacite while Si, V, and Cu de- to those in basaltic andesite. These have a feldspar, both plagioclase and a potassic crease. In contrast, Rb shows only a slight glassy, or at most, finely crystalline variety. Gabbro had been found only near enrichment in dacite, while Ni, although groundmass in which are set phenocrysts of the Kabakada vent. It has twinned, un- d splaying a slight decrease over-all, is diopsidic augite, orthopyroxene (En78 to zoned bytownite and titanomagnetite set in markedly enriched in some rocks, notably En65), Fe-Ti oxide, and plagioclase that is poikilitic pyroxene. sample numbers 8080 and 8082. Sr reaches mainly sodic labradorite with rare cores of bytownite or even anorthite. This type of

andesite is found in lava flows of the North TABLE 3. CHEMICAL ANALYSES AND CIPW NORMS Daughter and in the northern caldera wall, OF LAVA FROM THE RABAUL CALDERA* in the cones of Rabalanakaia, Sulphur Sample number Creek, and the younger flows of Tavurvur. 8014 7084 8082 8080 7007 6985 7066 8015 8087 8016 8D42 8047 8058 The other type, found in the Karavia Bay (1) (2) (3! (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) unit, lacks the phenocryst clots and the S10i 48.19 50.11 50.E8 51.00 52.44 54.71 55. >7 60.27 61.56 62. 26 64.54 64.66 64.95 groundmass is always glassy, but otherwise T10j 0.81 0.95 o.sa 0.96 0.99 1.07 0. 36 0.90 0.96 0..8 8 C.91 0.86 0.84 the phenocrysts are of similar composition AhO, 18.99 18.89 18.CI 18.50 17.45 16.70 17..4 0 16.10 15.40 15..4 3 15.52 15.41 15.25

to the first type. Fe20j 8.31 3.67 4.66 4.10 3.08 2.99 3.• ISO 2.15 1.70 1..7 6 1.93 1,83 2.04 The welded tuff in Karavia Bay consists FeO 1.31 5.80 5.33 4.79 6.80 6.77 4..1 7 4.36 4.80 4..2 6 3.27 3.15 3.00 of phenocrysts of plagioclase (labradorite MnO 0.16 0.18 0.25 0.28 0.19 0.35 0. "5 0.28 0.16 0. 16 0.18 0.16 0.24 to andesine), diopsidic augite, hypersthene, MgO 5.64 4.64 4.83 4.40 4.42 3.79 3. (10 2.30 2.42 2..0 4 1.51 1.51 1.42 CaO 12.81 10.86 10.09 10.06 9.58 8.20 8. ::2 5.45 5.26 4. 89 3.71 3.70 3.64

Fe-Ti oxide, and hornblende. Partially C O 3.03 3.37 3. 4.37 4.39 4. 59 4.89 4.70 flattened pumice lumps are concentrated Na20 2.20 2.76 3.11 3.01 near the base of the flow, while toward the K;0 0.50 0.94 0.95 1.39 0.97 1.21 1. 53 2.22 2.44 2..5 7 2.81 2.79 2.89 0.13 0.2S 0.26 0.19 0.20 0. ?3 0.28 0.37 0..3 4 0.27 0.32 0.30 top are rounded clasts of glass with P,0S 0.19 Hj0+ 0.63 0.79 0.98 0.98 0.54 1.07 0..19 0.49 0.47 0.61 0 14 0.90 0.55 phlogopite grains clustered around their HiO" 0.10 0.20 0.23 0.54 0.21 0.05 _0.J:3 _ 0.09 0.09 _0._31 _ 0 01 0.05 0.04 margins. The groundmass appears glassy, Total 99.78 99.98 100.57 100.27 99.89 100.48 99..8 7 99.26 100.02 99. 92 99. 69 100.04 100.04 but there are numerous feldsoar microlites 2.79 1.41 2.35 2.13 3.85 6.49 7. 88 11.25 11.92 12. 55 15.41 16.70 16.19 and much of the glass is partly recrystal- Q Or 2.95 5.55 5.6' 8.21 5.73 7.15 9..0 4 13.12 14.42 15..1 9 16.61 16.49 17.08 lized. Ab 18.62 23.35 26.32 25.47 25.64 28.52 28. 94 36.98 37.15 38.84 41.38 39.77 41.29 An 40.46 36.38 32.38 32.86 31.15 26.87 27. £1 17.76 15.11 13. 91 12.10 12.71 11.77 Dacite and Rhyolite D1 17.31 13.20 12.86 12.35 12.43 10.33 9. 37 6.15 7.09 6..7 3 3.74 2.92 4.05 Dacite and rhyolite without exception Hy 6.02 11.53 10.61 9.36 13.56 13.18 8. 35 7.96 8.63 6. 96 5.16 5.47 4.43 are glassy lava flows or purrtice with few Mt 2.40 5.32 6.7E 5.94 4.47 4.34 5.51 3.12 2.46 2. 55 2.80 2.65 2.96 phenocrysts. Plagioclase is the predominant Ilm 1.54 1.80 1.88 1.82 1.88 2.03 1.6! 1.71 1.82 1. 67 1.73 1.63 1.60 phenocryst and is usually fo-ind as clear, Hm 6.66 0.00 3.00 0.00 0.00 0.00 0. 00 0.00 0.00 0. 00 0.00 0.00 0.00 Ap 0.31 3.62 0.45 0.47 poorly zoned, and rarely twinned calcic an- 0.45 0.62 _0. i± 0.66 0.88 0.81 JL§i 0.76 0.71 Total 99.06 99.00 99.38 98.77 99.15 99.37 98. 8ti 98.70 99.48 99. 20 99.56 99.11 99.47 desine. Orthopyroxene and clinopyroxene, Note: Sample locations—6014, porphyritic basalt, western scopes of South Daughter; 7084, porphyritic basalt, of similar composition to those in andesites, southern slopes of Mount Varzin; 3032, porphyritic basalt, southern flanks of North Daughter; 8780, basalt, south form large euhedral phenocrysts. Titan- slopes of North Daughter; 7007, basalt, eastern caldera wall; 6955, basaltic andesite, eastern c,\ldera walli 7066, basaltic andesite, east coast of Ms tort Island; 8015, andesite, ¡< rominent lava flow on southwest slopes of Rabalana- omagnetite is common, while ilmenite, apa- kaiai 8087, porphyritic glassy andesite. Sulphur creek explosion craters; 8016, glassy andesite bomb, Tavurvur; 8042, glassy dacite core of pumice bomb in chaotic pumiceous asl lying on southern slopes of South Daughter; 8047, tite, and rarely pyrrhotite are accessory. glassy dacite from coarse pumiceous ash on coast northwest of Ncrth Daughter; 8058, glassy daciv.e bomb, Vulcan 1937 Within titanomagnetite, apatite occurs as eruption. euhedral cyrstals and pyrrhotite as small * Analyses by R. F. Heming and J. Hampel.

TABLE 4. TRACE ELEMENTS IN RABAUL LAVA Carmichael, 1970). Rb values are low and BY X-RAY FLUORESCENCE* Rb:Sr ratios are slightly higher than for Sample number both Talasea and Bagana, an active volcano 8014 7084 8082 8080 7007 6985 7066 8015 8087 8016 8042 8047 8058 on Bougainville southeast of Rabaul.

Nd 10 20 20 10 15 15 30 20 30 20 25 25 25 The Rabaul suite does not fit easily into Pr <5 5 <5 <5 <5 10 <5 <5 5 5 10 5 5 any of the classification schemes that are Ce 43 43 37 37 46 46 51 30 30 55 45 35 55 commonly applied to island-arc suites. In La <5 <5 5 <5 <5 5 10 10 10 5 10 15 5 comparison with schemes proposed for Ba 107 278 235 230 186 238 299 390 385 450 475 450 470 Japanese rocks (Kuno, 1959, 1966), the Nb 7 7 7 <5 6 <5 <5 5 <5 <5 10 10 10 Rabaul suite has characteristics of both the Zr 35 42 56 70 51 64 77 118 124 118 165 153 171 calc-alkalic and alkali lava suites. In Figure Y 13 35 13 13 13 13 39 23 23 35 23 29 23 7, where Si02 is plotted against total alkali 538 Sr 608 650 678 451 444 554 371 342 353 336 341 330 content, rocks from Rabaul straddle the Rb 13 21 28 28 19 19 26 41 41 47 47 35 47 boundary between high-alumina and alkali Ga 15 15 19 20 16 18 14 16 16 16 16 16 16 basalt types. In the inset diagram, another Zn 73 87 77 73 89 94 71 86 102 90 94 90 98 Cu 121 106 189 126 152 138 129 53 78 29 49 45 24 of Kuno's criteria is used. Here, total alkalis Ni 34 24 174 29 27 13 27 16 94 8 77 69 8 are plotted against a differentiation index Cr 25 15 15 10 19 5 14 10 10 10 10 <5 5 (Kuno, 1959), and the Rabaul suite lies in a V 294 353 310 262 313 291 275 122 131 113 66 70 61 portion of the diagram where calc-alkalic Cl 100 240 210 80 470 680 350 1 ,490 1,600 1 ,560 1 ,760 1 ,750 2 ,100 and alkali-basalt fields overlap. Figures 8 S 20 40 490 70 30 20 100 500 270 180 260 and 9, however, show that the suite dis- K/Rb 319 371 282 411 424 529 488 449 494 609 454 493 510 plays a mild iron enrichment, a commonly K/Ba 39 28 34 50 43 42 43 47 53 47 45 51 51 acknowledged feature of tholeiitic rocks. V/Nî 8.6 14.7 1.78 9.03 11.6 22.4 10.1 7.6 1.4 14 0.86 1.01 7.6 Mineralogic evidence also supports a tho- Rb/Sr 0.024 0.034 0.043 0.041 0.012 0.043 0.047 0.110 0.120 0.099 0.140 0.138 0.142 leiitic affinity by the presence of ground- * Concentrations in ppm. Analyses by p. Jack and P. - F. Heming. mass pigeonite and by the reaction relation between hypersthene and olivine displayed in the orthopyroxene rinds on olivine a peak in sample number 8082, which also K/Rb ratios are higher in the Rabaul phenocrysts. contains a much higher amount of plagio- rocks than generally found in island-arc en- clase than any of the other basalts. Highest vironments, with andesite having values values of V are found in basalt and basaltic close to 500 and occasionally as high as CHEMICAL VARIATION andesite, and it is relatively depleted in the 600, compared to an average value of 430 WITHIN THE NEW andesite and dacite, a trend that is also in most andesite (Taylor and others, 1969), BRITAIN-NEW GUINEA ARC reflected in the V content of titanomagnet- 300 to 500 in Japanese andesite (Taylor Comparison of chemical data for the ite. The relative abundance of Cu varies and White, 1966), and as low as 200 to 300 Rabaul suite with those for other volcanoes rather similarly to that of Ni. In andesite in New Zealand andesite (Ewart and Stipp, within the New Britain—New Guinea arc and dacite, this can be linked with the ap- 1968). The Rabaul values are, however, has revealed some distinct variations. These pearance of pyrrhotite, but no sulfide phase much lower than the notably high ratios variations also seem to be more or less has been noted in basalt. found in Talasea andesite (Lowder and progressive, generally displaying an increase in the amount of a component eastward along the arc. Briefly, the trends from west to east are (1) an increase in total alkalies (Fig. 7), (2) an increase in TiO-2 (Fig. 10), (3) a progressive increase in

Si02 Figure 8. AMF diagram for the Rabaul lavas and other volcanic series. Th-Th represents Thingmuli Figure 7. Total alkalies versus silica. Rabaul analyses, large dots (this paper); small dots, unpub. analyses and (Carmichael, 1964); S-S, Skaergaard (Wager, 1960); from Heming and Carmichael (1973). Other analyses from volcanoes in the New Britain-New Guinea arc: Talasea, and C-C, average Cascades trend (Carmichael, 1964). triangles (Lowder and Carmichael, 197(1); Manam, crosses; Ulawun and Langila, squares. The fields of alkali All available analyses for Rabaul lavas are plotted as basalts, high alumina basalts, and tholeiitic basalts are from Kuno (1966). Inset shows Kuno's (1966) differentiation solid dots. Circled dots represent analyses from North index versus total alkalies (abscissa) on which are plotted the Rabaul and Talasea suites (R and T). Daughter and Watom Island volcanoes.

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In Figures 8 and 9, there is a more or less smooth progression from basalt to rhyolite with cessation of iron enrichment occur- ring in basaltic andesite. Cessation of iron enrichment is usually correlated with the enhanced crystallization of magnetite. T itanomagnetite is more abundant in basaltic andesite than in the basalt (Table 2), but not spectacularly so. Titanomagnetite, however, increases in the groundmass and orthopyroxene is more common. Possibly, it is the combination of the more voluminous crystallization of titanomagnetite along with the appearance of orthopyroxene that terminates the iron enrichment in the Rabaul series.

PETROGENESIS Before discussing the origin of the Rabaul suite, one particular aspect of the pattern of eruption at Rabaul is worth amplifying. Certain spatial contrasts in magma composition have persisted through the last few thousand years of the volcano's lifetime. Figure 9. Absolute iron enrichment in the Rabaul series. Numbered analyses, this paper; other analyses from This contrast appeared during the construc- Heming and Carmichael (1973). Circled dots are analyses from North Daughter and Watom Island. Th is trend for tion of the Karavia Bay volcano, which was Thingmuli (Carmichael, 1964); Ta for Talasea (Lowder and Carmichael, 1970). B, Ba, A, D, R refer to average rock erupting andesite lava while Palangaigia compositions from the Cascades (Carmichael, 1964). erupted basalt similar in composition to the

K20:Na20, and (4) a small rise in A1203. Watom Island and North Daughter are un- early basalt of the primitive Rabaul vol- Despite a certain amount of overlap, which affected by it. This suggests that lack of iron cano. More recently, the explosive dacite is to be expected, the over-al] pattern seems enrichment of these rocks is some artifact of pumice eruptions on the wesi.ern side of the clear, and Rabaul, at the eastern end of this the magma chamber beneath these vol- caldera are in contrast to eruption of glassy arc, lies at the culmination of these trends. canoes. andesite from cones on the eastern side. This chemical variation in the rock analyses seems, from the limited data available, to be reflected in the compositions of the miner- Rabaul als from the various volcanoes. Titano- magnetite from Manam Island, at the west- U lawun ern end of the arc, is extremely low in Ti02 Talasea (<4 percent) as are the rocks themselves. Ka rkar The higher amount of alumina in the Rabaul rocks is in turn reflected in the high -1.00 Manam alumina content of clinopyroxene from that volcano. Such progressive changes in chemistry have been noted Dreviously (Kuno, 1959, 1966; Dickinson, *1968) but always across an island arc, not along it. CM -0.60 VARIATION DIAGRAMS O i- When plotted on an AMF diagram (Fig. 8), the Rabaul analyses display a mild but distinct iron enrichment. Not all of the 5 analyses lie on this smooth curve, as three points fall away from an iron-enrichment -0.40 trend. Modal analyses (Table 2) do not re- veal any major differences between these samples (numbers 7066, 8080, and 8082) and other samples from the area, except for larger amounts of plagioclase in number 8082 and of augite in numbers 8082 and 30 50 70 90 7066. Titanomagnetite, which if present in l_ large amounts would readily explain the Diff. Index lack of any iron enrichment, shows no Figure 10. Variation in Ti02 alorg the New Britain—New Guinea arc. Ti02 (wt percent) versus differentiation significant variation. Whatever the cause of index. Dashed lines are indications of the spread of data for a particular volcano. Rabaul (R) and Manam (M) show the iron enrichment, the lava flows from the two extremes of this variation.

This andesite differs from that in the various amounts from one lava it should be clinopyroxene in the first case and 0.3 g of Karavia Bay unit in the large volume of possible to derive the next lava along a liq- orthopyroxene in the dacite. mafic clots found within it. Differences like uid line of descent. This hypothesis was The worst fit to a fractionation model oc- these may be due to a magma chamber roof tested with a computer program used by curs when deriving andesite 8015 from which is much higher in the west, allowing Lowder and Carmichael (1970). Knowing basaltic andesite 6982, and rhyolite 6830 the concentration of a less dense, gas-rich the composition of the phenocryst minerals from dacite 8042. In both cases large com- dacite magma. Volcanoes on the eastern from microprobe analyses, we can compute position gaps occur between the two rocks side would tap much lower levels of this the amounts of phenocryst phases to be which may or may not be an artifact of hypothetical, differentiated magma cham- subtracted from a parent magma in order to sampling. As the basalt and basaltic ande- ber, presumably entraining lumps of cumu- give a chosen daughter. The results of this site of Watom Island and North Daughter late crystals during the explosive spasms. calculation are given in Table 5. In order lie on a different trend to other rocks from Many mechanisms have been proposed that one can judge the closeness of the cal- Rabaul, a separate test was made of the for the generation of an orogenic suite such culated to the observed values, the sum of fractional crystallization model in their as that found at Rabaul; they have been squares of the residuals for all the oxides case. The result requires that basaltic ande- summarized previously by Lowder and are also listed. What values these should site 7066 be accumulative in orthopyrox- Carmichael (1970) and Green and Ring- reach before a fractionation hypothesis may ene, and as these rocks seem to be partly ac- wood (1968). Hypotheses involving partial be discounted is a matter of personal cumulative, the result may seem reasonable, melting of subcrustal material (Green and judgement. but the amount (12.0 g) appears unlikely. Ringwood, 1968) are difficult to evaluate The proposed path involves removal of Over-all, the computed proportions of on the evidence available, but some such reasonable amounts of phenocryst phases, basalt:basaltic andesite:andesite:dacite: scheme is generally favored by many pe- and the maximum difference between ob- rhyolite are 10:3:2:1.8:1.2. These values trologists for the source of island-arc suites served and calculated weight percentages of reflect the hierarchy of abundances ob- like Rabaul. Partial melting of overlying the ten oxides used is less than 0.3 in all but served at Rabaul but not their proportions. sialic material (Holmes, 1932; Turner and one. As titanomagnetite is present in 8014, However, the fractionation model pro- Verhoogen, 1960) is not in accord with the it was included in the initial fractionation posed above would seem to be a plausible observed relative abundance of rock types step, but nevertheless, only very small one that is in accord with chemical and at Rabaul. Sr87:Sr86 ratios for the Rabaul amounts are required to be removed from mineralogical data for the series. rocks (Peterman and Heming, 1974) are parent liquids until one attempts to derive low, with a mean of 0.7038, and are not andesite 8015 from basaltic andesite 6982. PARTIAL MELTING HYPOTHESIS compatible with any scheme involving par- Here it is necessary to remove 4.7 g of tial melting of older crustal material. titanomagnetite, a much larger amount Current hypotheses on the generation of than previously, in order to drive the liquid island-arc magmas are closely allied to the FRACTIONATION SCHEMES around the inflection in the iron enrichment hypothesis of sea-floor spreading (Isacks Variation diagrams for the Rabaul series curve (Figs. 8, 9). It is also possible to re- and others, 1968; LePichon, 1968). It is show that the rocks lie on relatively smooth move small amounts of titanomagnetite thought that partial melting at the top of trends of increasing silica and alkalies, and from the basalts without compromising an the underthrusting lithosphere slab gener- each composition may be used to locate a iron enrichment trend. In the derivation of ates a magmatic liquid, which through dif- point along a liquid line of descent. The dacite 8042 and andesite 8087, it is re- ferentiation yields the observed variety of only exception to these smooth trends are quired to add a small amount of pyroxene. rock types. lava flows from North Daughter and The mineralogical evidence does not pre- At Rabaul it is not possible to identify an Watom Island volcanoes. clude this, for many rocks do contain inclined seismic plane with any certainty. If fractional crystallization in relatively phenocryst clots of very similar composi- Much of the seismic activity is of relatively shallow magma chambers has controlled tion to the phenocrysts of a more basic and shallow focus and seems to be concentrated chemical variation in this series, then by presumably parental lava. Anyway, along the numerous northwesterly trending subtracting combinations of phenocrysts in amounts involved are small with 1.0 g of faults in the area.

TABLE 5. RESULTS OF THE CRYSTAL FRACTIONATION PROGRAM

Plagioclasii CIinopyroxens Olivine Orthopyroxene Magnetite Difference Oxides Sum of squares of residuals

Basalt 7007 -25.8 Ab, from basalt 8014 Ans, -14.9 EmeUOi.1>FS 20 -2.7 Fost -2.1 Uspj» -0.27 Na20 0.0923 Basaltic andesite 6982 from 7007 -13.8 An,, Ab22 - 6.0 EnssWo»;ÏFS2 2 -1.8 FOSÎ -1.4 Usp,i -0.09 NaiO 0.0149 Andesite 8015 -16.8 An , En,eWOi,:¡FSÎ 2 from 6982 e Ab,o -11.3 -0.9 FOs; -4.7 Usp26 +0.301 K2O 0.1523 Andesite 8087 from 8015 - 6.2 AnS00r iAb,e + 1.0 EnaitWOi, SFSÎO -0.9 Enss -0.4 USP28 -0.127 MnO 0.0260 Andesite 8016 - 2.3 An 0r lAbj, En,* Wo from 8087 6s : - 1.5 IH,FS 2 2 -1.07 En5s -0.6 USP2S -0.06 Ti02 0.0091 Dacite 8042 from 8016 - 5.0 An6i,0r iAb,s - 4.9 EnjsWo»:,FS2 1 +0.3 Ens» -1.4 USP2, +0.10 TÌO2 0.0301 Rhyolite 8030 from 8042 -30.4 AnvaOl'îAbso - 0.2 EniiHo», .FS,, -5.6 Enss -4.3 Usp,s -1.5 K2O 4.0705 Basaltic andesite 7066 -12.8 EnasWOi, : from basalt 8082 AnB2 Abis - 8.9 ,FS2, -9.6 Fo„ +12.2 Enss -2.9 Uspsi -0.16 Na:0 0.1254 Note: Figures beneath mineral names refer to the number of grams of that phase which has to be subtracted from each 100 g of starting composition in order to obtain the composition of the derived lava. Difference = maximum difference between observed and calculated composition of the derived lava in weight percent of oxide indicated in adjacent column.

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Petrol- I thank my many colleagues in that organi- New Ireland: Australia Bur. Mineral. Re- cgy, v. 1, p. 364-398. zation, especially R. W. Johnson and N. H. sources Rec. No. 179, 21 p. Fisher. Support for this research was af- Green, T. H., and Ringwood, A. E., 1966, Origin Willians, H., 1942, The geology of Crater Lake forded by National Science Foundations of the calc-alkaline igneous rock suite: National Park, Oregon, with reconnais- Grant GA-32445X (Carmichael). Ian Earth and Planetary Sci. Letters, v. 1, p. sance of the Cascade Range southward to 307-316. Mount Shasta: Carnegie Inst. Washington Carmichael and Howel Williams critically Pub. 540, 162 p. read the manuscript, offered many sugges- 1968, Genesis of the calc-alkaline igneous tions, and also provided a continuous rock suite: Contr. Mineralogy and Petrol- ogy, v. 18, p. 105-162. source of stimulation during my research. I Heming, R. F., and Carmichael, I.S.E., 1973, MANUSCRIPT RECEIVED BY THE SOCIETY MAY owe a special debt to the late G.A.M. High-temperature pumice flows from the 1j., 1973 Taylor, G. C., who first stimulated my in- Rabaul caldera, Papua New Guinea: Contr. REVISED MANUSCRIPT RECEIVED DECEMBER terest in volcanoes. Mineralogy and Petrology, v. 38, p. 1-20. 26,1973

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