John A. Gamble

Geochemical and geodynamical constraints on subduction zone magmatism

Abstract The geochemical and geodynamical parameters that may influence the composition of island-arc basalts (IABs) are evaluated. Systematic correlations amongst high-field strength (HFS) elemental ratios (Zr/Nb, Sm/Nb and TiO2/Zr) relative to Nb abundances, indicate that HFS element systematics are not controlled by the presence of residual Nb-bearing phases in the slab. This provides confirmation of models whereby high-field strength (HFS) and HREE elements remain immobile during slab-fluxing processes and are thus derived from the mantle wedge without additional enrichments from the slab. In contrast enrichment of large-ion-lithophile elements (LIL) such as Rb, Cs, Ba, Sr, Pb, U and LREE (i.e., La, Ce) in IABs is consistent with slab involvement, with their relative enrichment, being due to a combination of both their high rock-melt incompatibility and slab-“fluid” mobility. As a consequence, the low abundances of HFS elements such as Nb, Ti, Zr, and Hf in IABs reflect a depleted (relative to MORB source) mantle wedge overlying the subduction slab. Depletion of the arc mantle wedge in HFS elements is attributed to previous melting events in the mantle wedge, and to geodynamic conditions associated with the formation and evolution of coupled island arcs and back-arc basins. These processes ensure a budgetary deficit in the HFS elements relative to those elements derived from the subducted slab (predominantly LILE and LREE). Thus, although in MORBs and OIBs, Nb has a similar incompatibility to U, in subduction zones the main factor controlling its abundance is its highly immobile character, particularly relative to elements like U which are mobile during prograde dehydration reactions in the slab. Based on these observations, a quantitative model has been developed for IAB petrogenesis with the transfer of trace elements from the slab to the mantle wedge being modelled with empirical slab-“fluid” partition coefficients whilst the mantle-wedge to arc-crust transfer is constrained by melt-solid partitioning. The empirically derived slab-“fluid” partition coefficients indicate that the enrichment factors characteristic of slab fluxing processes have a distinctive pattern particularly for the elements Nb, U, Th, and Sr.

Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a review

Abstract Taupo Volcanic Zone (TVZ) volcanic rocks comprise three major compositional series: high-alumina basalt (HAB), andesite, and rhyolite, plus a fourth, minor, dacitic series resulting from mixing of andesite and rhyolite magma. Relatively rare HABs originate as near-primary magma from depleted, chemically and isotopically homogeneous mantle. They erupted exclusively through thin, extensional crust, and have evolved by limited crystal fractionation and crustal assimilation. Andesite volcanoes broadly define the present-day active volcanic front, particularly in the southern and northern sectors of the TVZ. The rocks are generally high-silica, but range in composition from basaltic andesite to dacite. Nine petrologically distinct andesite types are recognised; none are directly related to HAB, the majority originating as AFC derivatives of low-alumina basalt. Rhyolite is volumetrically dominant in the TVZ (> 90% by volume), occurring predominantly in the central part but also offshore to the north as lava, ignimbrite and airfall deposits. Although geochemical and isotopic data, and experimental studies have placed some constraints on petrogenetic models, these remain controversial. Crustal anatexis of meta-greywacke basement can be dismissed as a major source, but basement rocks may be involved through secondary contamination. Least-squares mixing models using geochemistry and a variety of stable and radiogenic isotopes show that TVZ rhyolite could be generated by AFC of a mafic parent. However, these models cannot easily explain the apparent absence of large volumes of intermediate rocks and mafic residues. Melting of pre-existing volcanics or lower crustal granulites is also possible, but the existence of such rocks beneath the TVZ is not proven. Although HAB, andesite and rhyolite have coherent major-element compositions, and their occurrence can be explained in terms of crystal fractionation/crustal contamination/AFC models, all three have distinctive trace-element and isotopic characteristics that cannot be convincingly explained by any single-stage petrogenetic model.

SOUTHERN KERMADEC SUBMARINE CALDERA ARC VOLCANOES (SW PACIFIC) : CALDERA FORMATION BY EFFUSIVE AND PYROCLASTIC ERUPTION

Abstract Three newly identified submarine caldera volcanoes (Brothers, Healy, and Rumble II West), of the southern Kermadec arc (within water depths 80%) and low density (0.3–0.5 g cm−3) erupted pumiceous clasts. These observations are interpreted as evidence of subaqueously quenched, pyroclastic eruption(s), sheathed by ambient water flashing to steam, discharging some 5 km3 of pyroclastic material from Healy volcano. Caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption(s), although it is possible formation as an explosive crater occurred, but we consider the latter unlikely given the volume of erupted material and water depth. Extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas, and syn-eruptive caldera collapse, is possible within water depths of 500–1000 m.

A FIFTY YEAR PERSPECTIVE OF MAGMATIC EVOLUTION ON RUAPEHU VOLCANO, NEW ZEALAND : VERIFICATION OF OPEN SYSTEM BEHAVIOUR IN AN ARC VOLCANO

Geochemical and petrological data for samples from well documented eruptions that occurred at Ruapehu volcano over the period 1945–1996 can be used to illustrate the complexity of short term geochemical variation in an arc-type volcano. Collectively, data from Ruapehu Volcano show trends with time of increasing SiO2 abundance and rising 87Sr/86Sr ratios, consistent with broad control by assimilation and crystal fractionation processes (AFC). However, the magmas emplaced during the past fifty years show geochemical variability that spans most of the range shown by lavas erupted over the entire history of the volcano. Magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes associated with magma recharge events within the volcano. These complex trends are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short-term effects that are imposed during recharge on the overall AFC trend. We show that the temporal geochemical and petrographic variations among erupted magmas are modulated by processes of mixing and mingling between fresh magma from below and stagnant melt and entrained crystals from earlier events remaining in the volcanic edifice, probably in dikes and sills. These processes are probably replicated over longer time periods (hundreds to thousands of years) as melts arrested at different levels in the near surface conduit system are progressively displaced by new magma batches. Arc type volcanoes such as Ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or less continuous activity. This style of activity is difficult to predict through the usual (seismicity, ground deformation, lake water geochemistry) volcano monitoring techniques, and petrology and geochemistry may provide the basis for an alternative strategy.

40Ar/39Ar geochronology of magmatic activity, magma flux and hazards at Ruapehu volcano, Taupo Volcanic Zone, New Zealand

We have determined precise eruption ages for andesites from Ruapehu volcano in the Tongariro Volcanic Centre of the Taupo Volcanic Zone (TVZ) using 40Ar/39Ar furnace step-heating of separated groundmass concentrates. The plateau ages indicate several eruptive pulses near 200, 134, 45, 22 and 300-m section of lavas in Whangaehu gorge as well as some lavas in Ohinepango and Waihianoa catchments on eastern Ruapehu, and this suite of lavas belongs to the Waihianoa Formation. This pulse of activity is not represented on nearby Tongariro volcano, indicating that the two volcanoes have independent magmatic systems. A younger group of lavas yields dates between 50 and 20 ka and includes lava flows from the Turoa skifield and in the Ohinepango and Mangatoetoenui catchments and is consistent with two pulses of magmatism around the time of the last glacial maximum, relating it broadly to the Mangawhero Formation. Syn- and post-last glacial activity lavas, with ages <15 ka are assigned to the Whakapapa Formation, and include the voluminous flows of the Rangataua Member on southern Ruapehu. Magma flux, integrated over 1000-yr periods, averages 0.6 km3 ka−1 assuming a volcano lifespan of 250 ka. Fluxes for the Te Herenga, Waihianoa and Mangawhero Formations are consistent at 0.93, 0.9 and 0.88 km3 ka−1, respectively. These fluxes are broadly comparable with those measured at other modern andesite arc volcanoes (e.g. Ngauruhoe, 0.88; Merapi, 1.2 and Karymsky 1.2 km3 ka−1). The relatively low flux (0.17 km3 ka−1) calculated for the Whakapapa Formation may derive from underestimates of erupted volume arising from an increase in phreatomagmatic explosive eruptions in postglacial times. However, using volume estimates for the 1995–1996 eruptions and a recurrence interval of 25 yr has yielded an integrated 1000-yr flux of 0.8 km3 ka−1 in remarkable agreement to estimates for the prehistoric eruptions. Overall, Ruapehu shows consistency in magma flux, but at time scales of the order of one hundred to some thousands of years, field evidence suggests that short bursts of activity may produce fluxes up to twenty times greater. This is significant from the perspective of future activity and hazard prediction.

The petrology, phase relations and tectonic setting of basalts from the taupo volcanic zone, New Zealand and the Kermadec Island arc - havre trough, SW Pacific

Volcanism in the Taupo Volcanic Zone (TVZ) and the Kermadec arc-Havre Trough (KAHT) is related to westward subduction of the Pacific Plate beneath the Indo-Australian Plate. The tectonic setting of the TVZ is continental whereas in KAHT it is oceanic and in these two settings the relative volumes of basalt differ markedly. In TVZ, basalts form a minor proportion (< 1%) of a dominant rhyolite (97%)-andesite association while in KAHT, basalts and basaltic andesites are the major rock types. Neither the convergence rate between the Pacific and Indo-Australian Plates nor the extension rates in the back-arc region or the dip of the Pacific Plate Wadati-Benioff zone differ appreciably between the oceanic and continental segments. The distance between the volcanic front and the axis of the back-arc basin decreases from the Kermadec arc to TVZ and the distance between trench and volcanic front increases from around 200 km in the Kermadec arc to 280 km in TVZ. These factors may prove significant in determining the extent to which arc and backarc volcanism in subduction settings are coupled. All basalts from the Kermadec arc are porphyritic (up to 60% phenocrysts) with assemblages generally dominated by plagioclase but with olivine, clinopyroxene and orthopyroxene. A single dredge sample from the Havre Trough back arc contains olivine and plagioclase microphenocrysts in glassy pillow rind and is mildly alkaline (< 1% normative nepheline) contrasting with the tholeiitic nature of the other basalts. Basalts from the TVZ contain phenocryst assemblages of olivine + plagioclase ± clinopyroxene; orthopyroxene phenocrysts occur only in the most evolved basalts and basaltic andesites from both TVZ and the Kermadec Arc. Sparsely porphyritic primitive compositions (Mg/(Mg+Fe2) > 70) are high in Al2O3 (>16.5%), and project in the olivine volume of the basalt tetrahedron. They contain olivine (Fo87) phenocrysts and plagioclase (> An60) microphenocrysts. These magmas have ratios of CaO/Al2O3, A12O3/TiO2 and CaO/TiO2 in the range of MORB and MORB picrites and can evolve to the low-pressure MORB cotectic by crystallisation of olivine±plagiociase. Such rocks may be the parents of other magmas whose evolutionary pathways are complicated by interaction of crystal fractionation, crystal accumulation and mixing processes and the filtering action of crust of variable density and thickness. The interplay of these processes likely accounts for the scatter of data about the cotectic. More evolved rocks from both TVZ and KAHT contain clinopyroxene and orthopyroxene phenocrysts and their compositions merge with basaltic andesites and andesites. Stepwise least-squares modelling using phenocryst assemblages in proportions observed in the rocks suggest that crystal fractionation and accumulation processes can account for much of the diversity observed in the major-element compositions of all lavas. We conclude that the parental basaltic magmas for volcanism in the TVZ and KAHT segments are similar thereby implying grossly similar source mineralogy. We attribute the diversity to secondary processes influencing liquids as they ascended through complex plumbing systems in the sub arc mantle and cross.

Arc and back-arc geochemistry in the southern Kermadec arc-Ngatoro Basin and offshore Taupo Volcanic Zone, SW Pacific

Abstract Back-arc basin basalts from the Ngatoro Basin (the southern end of the Havre Trough) are similar geochemically to, yet subtly distinct from, basalts of the Havre Trough to the north. Whole rock and glass chemistry are consistent with derivation from a fertile mantle source with subsequent evolution by fractionation of olivine (+ Cr-spinel) + plagioclase, and then clinopyroxene. Basalts from the vicinity of Rumble IV seamount at the southern end of the Kermadec island arc, and the eastern Ngatoro rift escarpment, are strongly porphyritic relative to the back-arc basin basalts and show trace element (high LIL abundances and highly depleted HFS abundances) and isotopic signatures of subduction zone basalts. At its southern end, the Ngatoro Basin penetrates the continental crust of New Zealand creating a major, 3000 m deep bathymetric re-entrant in the slope-break; the slope-break marks the transition from oceanic to continental crust. Basalts from the floor of the Ngatoro Basin re-entrant are isotopically distinct from the basalts of the oceanic sector in that they have higher Sr and correspondingly lower Nd isotope ratios and are comparable to basalts of the Taupo Volcanic Zone (TVZ) to the south. In contrast to the basalt-dominated oceanic sector, basalts from the offshore TVZ, a 100 km long area extending roughly NNE from White Island to the submarine Whakatane arc volcano at the edge of the continental slope-break, occur in association with andesites, dacites and rhyolites. These basalts are generally strongly porphyritic (olivine + plagioclase + clinopyroxene) and show trace element abundances typical of suprasubduction zone rocks. However, offshore TVZ basalts show subtle distinctions from onshore TVZ basalts to the south; the former have more radiogenic Sr isotopes. Furthermore, their high field strength element and transition element systematics appear to overlap with those of basalts from the Kermadec arc to the north. The authors attribute these lateral (along arc) and transverse (across arc) variations to source heterogeneity and variable fertility in the sources of the arc and back-arc basin magmas. Sources of the arc-front magmas are more refractory and also more susceptible to contamination by slab-derived fluids than sources for back-arc basin magmatism, reflecting the dynamic nature of flow from the back-arc into the mantle wedge beneath the volcanic front.

Seafloor geology and petrology in the oceanic to continental transition zone of the Kermadec‐Havre‐Taupo Volcanic Zone arc system, New Zealand

Abstract To the north of New Zealand, the oceanic Tonga‐Kermadec island arc system strikes roughly NNE‐SSW and intersects the continental shelf of New Zealand in the vicinity of the Whakatane arc volcano, a submarine edifice which rises around 1000 m from the surrounding seafloor. A major bathymetric slope‐break here coincides with the proposed oceanic‐continental transition zone. Northwest of the Whakatane volcano, dredging on Colville Knolls recovered samples of deformed Mesozoic(?) metasedimentary rocks, whose petrography and geochemistry resemble Waipapa Terrane rocks of onshore New Zealand, thereby confirming the extent and “continental” status of the offshore region. The transition zone from continental crust to oceanic crust is delineated by a marked slope‐break at the 2000 m isobath, oriented roughly parallel to the Bay of Plenty coastline. The linearity of this boundary is broken by a well‐defined reentrant structure, the Ngatoro Basin, which here is recognised as a southern extension of the ocea...

Discovery of hydrothermal sulfide mineralization from southern Kermadec arc volcanoes (SW Pacific)

Abstract We report the discovery of hydrothermal sulfide mineralization within the summit caldera of two southern Kermadec frontal arc volcanoes (Brothers and Rumble II West) of the ∼1200 km long, Kermadec–Havre arc–back-arc system. The Brothers and Rumble II (West) calderas, both with re-surgent domes and comprising mostly dacite and basalt–andesite host rocks, respectively, rise to water depths of ≤1500 m. These calderas comprise mostly effusive lavas and volcaniclastic deposits, including talus breccias along their inner caldera walls. These two southern Kermadec hydrothermal sites have similar geological settings and mineralogy to other arc front vent sites. Two principle ore assemblages are identified, comprising: (i) chalcopyrite–pyrite–barite, and (ii) sphalerite–marcasite–barite ± pyrite. The most prevalent ore texture consists of massive chalcopyrite + pyrite ± sphalerite, commonly overprinting barite. Early marcasite cores within large, euhedral pyrites, and trails of pyrite inclusions within chalcopyrite are indicative of recrystallization. The ore mineralogy is consistent with postulated venting temperatures of ≥300°C. Sulfide geochemistry is consistent with the ore mineralogy with concentrations of Cu, Fe, and Zn up to 15.3, 19.1, and 18.8 wt%, respectively. The sulfide geochemistry is distinct, however, from comparable western Pacific vent sites, and may reflect source heterogeneity due to proximity to continental New Zealand and sediment recycling along the southern Kermadec trench–arc system. The recovery of two partial caridean vent shrimps suggests that present-day hydrothermal venting is occurring within the Brothers caldera.

The age, geology, and geochemistry of the Tapuaenuku Igneous Complex, Marlborough, New Zealand

Abstract The Tapuaenuku Igneous Complex (TIC) is a mid‐Cretaceous layered intrusion and dike swarm cropping out at the axis of the Inland Kaikoura Ranges, South Island, New Zealand. The TIC is part of an extensive, but poorly preserved, igneous province that formed during or after cessation of subduction along the margin of Gondwana c. 100 Ma ago. The TIC is a complex, multiphase intrusion that is subdivided into eight units (in order of relative emplacement): (1) radial dike swarm—basanite, trachybasalt, and shoshonite/latite dikes; (2) Layered Series (LS)—basin‐shaped mafic cumulate rocks; (3) Staircase Intrusives— noncumulate gabbro and monzogabbro enclosed by a discontinuous sheet of cumulate gabbro; (4) Lower Hodder Gabbro and minor gabbroic intrusives—small stocks of noncumulate gabbro, monzogabbro, and monzonite; (5) Red Hills Breccia Pipe—heterolithic breccia emplaced in a pipe or diatreme; (6) Hodder Intrusives—a monzonite and sodalite syenite laccolith intruding the LS; (7) monzonite, orthoclase...