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A Permian (~265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes. Cumulates, which include dunite, olivine clinopyroxenite, olivine gabbro, and hornblende gabbro-norite, are related products of parent-magma fractionation. Both primary (magmatic) and secondary platinum-group minerals occur within dunite at one locality. Using the composition of cumulus minerals, mafic dykes and melt inclusions, we have determined that the parent magmas of the complex were hydrous, low-K island-arc tholeiites of ankaramitic affinities. Progressive magmatic differentiation of this parent magma generated fractionated melt of high-alumina basalt composition which is now preserved only as dykes which cut the Complex. Field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate-rock formation. Recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface. Similar processes are inferred to account for volcanic-rock compositions in other parts of this arc terrane and in modern island-arc systems.

A residual-source model for the origin of A-type granites is assessed by examining the likely mineral content and geochemistry of the residue remaining from generation of an I-type granite. Although this model may explain some characteristics of A-type granite, available data suggest that a residual source is unlikely to generate a partial melt with the appropriate major element characteristics. An alternative model for the origin of some A-type granites involves partial melting of crustal igneous rocks of tonalitic to granodioritic composition. Modeling the partial melting of these source rocks suggests that partial melts with water contents appropriate for A-type granites may be generated by 15% to 40% melting. This model can predict many other characteristics of A-type granites.

The Alcurra Dolerite in the Musgrave Province, central Australia, is part of the c. 1075 Ma Warakurna large igneous province (LIP), which is interpreted to have been emplaced contemporaneously with intracontinental rifting. The Alcurra Dolerite in the eastern Musgrave Province comprises two types of continental tholeiite. Dominant Group 1 dolerites are mainly magnesian tholeiites, whereas subordinate Group 2 dolerites are normal tholeiites. These two groups have distinct chondrite-normalized rare earth element (REE) patterns, primordial mantle-normalized trace element patterns, and incompatible element ratios (e.g. Th/Yb, Nb/Yb, La/Yb and Zr/Y). Both groups are low-Ti and low-Zr, but Group 1 dolerites have higher Ti/Zr (>60) and Ti/Y (>310) values than those of Group 2. Group 1 mainly comprises olivine-microgabbros, and is volumetrically dominant, whereas Group 2 is proportionally minor and includes noritic-microgabbros. Dolerites from both groups plot in discrete fields in 143Nd/144Nd versus 147Sm/144Nd space, along a mixing trend between chondritic or depleted mantle and the c. 1·59 Ga Musgravian basement. Group 1 dolerites have low Rb, Th and K2O contents that preclude their contamination by Musgravian continental crust. They also have incompatible element ratios that overlap with mid-ocean ridge basalt or primitive mantle. Group 2 dolerites have more negative ɛNd values and are also the more incompatible element enriched group; they may have, to some extent, been directly contaminated by Musgravian basement. Low concentrations of light REE (LREE) in Group 1 dolerites indicate a moderate to high percentage of partial melting of a relatively depleted source. Mixing this partial melt with a small proportion of melt derived by a moderate percentage of partial melting of a more incompatible element enriched mantle source could have resulted in the low (La/Sm)N ratios and low absolute La contents seen in these dolerites. Group 1 is divided into two subgroups (1A and 1B) on the basis of LREE abundances and Nb/Yb ratios. These two subgroups may reflect different percentages of partial melting of the depleted source. Petrogenetic interpretations for the Alcurra Dolerite of the eastern Musgrave Province must address published geophysical evidence for asthenospheric mantle upwelling and underplating in the Musgrave Province, and for contemporaneous low- and high-Ti Alcurra Dolerites indicative of a plume to the west where they are associated with Giles Suite layered intrusions. The large proportion of probable asthenosphere-derived melt in the petrogenesis of the Group 1 dolerites from the eastern Musgrave Province indicates either substantial heat transfer, or significant decompression in the mantle. The Warakurna LIP could result from a complex interaction between a plume and mantle upwelling. [ABSTRACT FROM AUTHOR]

The Pounamu Ultramafic Belt is an elongate zone of serpentinites and associated rocks within the Alpine Schist of north Westland, New Zealand. It includes metamorphosed pyroxenite-peridotite or boninite, gabbro and a breccia dominated by clasts of basalt and gabbro, overlain by pelagic sediments, a mid-Cretaceous tuff (108-105 Ma), and quartzofeldspathic turbidites with Cretaceous (< 105 Ma) detrital zircon minimum age populations. Rare earth element geochemistry indicates metabasites are similar to fractionated normal mid-ocean ridge basalt (N-MORBs), with minor enriched mid-ocean ridge basalt (E-MORB). However, because high field strength element concentrations of these metabasites are depleted relative to N-MORB, they have affinities to fore-arc magmas. The Pounamu ophiolite is likely, therefore, to have formed in a supra-subduction zone setting. Formation of oceanic crust and sedimentation during the Cretaceous is most likely in the re-entrant plate boundary zone between northwest and southeast Zealandia. Subsequent (c. 70 Ma) metamorphism occurred during accretion, subduction and interfolding of this exotic Pounamu Terrane with Rakaia Terrane-derived Otago Schist beneath western southeast Zealandia. [ABSTRACT FROM AUTHOR]

This study uses the geochemistry and petrology of xenoliths to constrain the evolutionary pathways of host magmas at two adjacent andesitic volcanoes in New Zealand’s North Island. Egmont (Mt. Taranaki) is located on the west coast of the North Island and Ruapehu lies 140 km to the east at the southern end of the Taupo Volcanic Zone, the principal locus of subduction-related magmatism in New Zealand. Xenoliths are common in the eruptives of both volcanoes but the xenoliths suites are petrographically and geochemically different. Ruapehu xenoliths are predominantly pyroxene–plagioclase granulites derived from Mesozoic meta-greywacke basement and the underlying oceanic crust. The xenolith population of Egmont Volcano is more complex. It includes sedimentary, metamorphic and plutonic rocks from the underlying basement but is dominated by coarse grained, mafic and ultramafic igneous rocks. Gabbroic xenoliths (Group 1) are composed of plagioclase, clinopyroxene and amphibole whereas ultramafic xenoliths are dominated by amphibole (Group 2) or pyroxene (Group 3) or, in very rare cases, olivine (Group 4). In Group 1 xenoliths plagioclase and clinopyroxene and in some cases amphibole show cumulate textures. Amphibole also occurs as intercumulate poikilitic crystals or as blebs or laminae replacing pyroxene. Some Group 2 xenoliths have cumulate textures but near monomineralic amphibole xenoliths are coarse grained with bladed or comb textures. Pyroxene in Group 3 xenoliths has a polygonal granoblastic texture that is commonly overprinted by veining and amphibole replacement. Group 1 and most Group 2 xenoliths have major, trace element and Sr, Nd and Pb isotope compositions indicating affinity with the host volcanic rocks. Geochemical variation can be modelled by assimilation fractional crystallisation (AFC) and fractional crystallisation (FC) of basaltic parents assuming an assimilant with the composition of average crystalline basement and Group 1 xenoliths have compositions approximating the solid material extracted during these processes. Some Group 2 xenoliths have relatively unevolved Sr and Nd isotopic compositions and they are interpreted to have derived as crystal cumulates from a more primitive parental basalt or through metasomatic alteration of other xenolith types by isotopically less evolved fluids or melts. Some Group 3 xenoliths could have originally been pyroxene cumulates but the granoblastic textures of others are more consistent with an origin as restites generated during anatexis of amphibolite. Group 4 xenoliths have textures similar to those of mantle-derived peridotite xenoliths found in intraplate basalts. The geochemical variation is consistent with a system fed by mantle-derived magmas that underplated and intruded the lower crust. At this level AFC and FC and crustal anatexis generated cumulates and pyroxene restite represented by the mafic and ultramafic xenoliths. The magmas segregating at these deep levels moved higher into the crust where a complex dispersed magma storage and plumbing system formed. Here magmas evolved further through AFC and FC with the formation of cumulates and crystal mushes that are represented by some Group 1 and Group 2 xenoliths. Xenoliths were further modified by interaction with host magmas or by alteration at the side walls of magma storages and conduits. [ABSTRACT FROM AUTHOR]

Permian-Jurassic ultramafic to felsic intrusive complexes at Bluff Peninsula and in the southern Longwood Range along the Southland coast represent a series of intra-oceanic magmatic arcs with ages spanning a time interval of 110 m.y. New SHRIMP U-Pb zircon data for a quartz diorite from the Flat Hill complex, Bluff Peninsula, yield an age of 259 ± 4 Ma, consistent with other geochronological and paleontological evidence confirming a Late Permian age. The new data are consistent with an age of c. 260 Ma for the intrusive rocks of the Brook Street Terrane. SHRIMP U-Pb zircon ages for the southern Longwood Range confirm that intrusions become progressively younger from east to west across the complex. A gabbro at Oraka Point (eastern end of coastal section) has an age of 245 ± 4 Ma and shows virtually no evidence of zircon inheritance. The age is significantly different from that of the Brook Street Terrane intrusives. Zircon ages from the western parts of the section are younger and more varied (203-227 Ma), indicating more complex magmatic histories. A leucogabbro dike from Pahia Point gives the youngest emplacement age of 142 Ma, which is similar to published U-Pb zircon ages for the Anglem Complex and Paterson Group on Stewart Island. [ABSTRACT FROM AUTHOR]

Mt. Shasta andesite and dacite lavas contain high MgO (3.5-5 wt.%), very low FeO*/MgO (1-1.5) and 60-66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50-60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10-15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10-15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption. [ABSTRACT FROM AUTHOR]

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 <15 ka and, based on our and previous field mapping confirm the lavas of the Te Herenga Formation as the oldest exposed part of the volcanic edifice. The pulse at ∼134 ka includes an entire >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. [Copyright &y& Elsevier]

Highlights: [•] Two volcanic Lineages (Mason Spur and Riviera Ridge) and their entrained mantle xenoliths are characterized at Mount Morning. [•] A source with 4 component mixing of depleted mantle, enriched, HIMU-like and Nb-enriched (carbonatite) components is shown. [•] Refertlization may have occurred c. 530-490 Ma ago during Gondwanaland subduction &amalgamation at Mount Morning. [•] Source of chemical & isotope variation across many SW Pacific Cenozoic eruptive centres resides (in part) in the lithosphere. [•] SW Pacific xenoliths & lavas record a Nb-enriched lithospheric mantle reservoir that helps explain the Nb paradox. [Copyright &y& Elsevier]

Regular large-scale edifice collapse and regrowth is a common pattern during the long lifespans of andesitic stratovolcanoes worldwide. The >130 k.y. history of Mount Taranaki, New Zealand, is punctuated by at least 14 catastrophic collapses, producing debris avalanche deposits of 1 to >7.5 km³. The largest of these sudden events removed as much as one-third of the present-day equivalent cone. The resulting deposits show similar sedimentary and geomorphic features, suggesting similar proto-edifice characteristics, failure trigger mechanisms, and runout path conditions. Each collapse was followed by sustained renewed volcanism and cone regrowth, although there are no matching stepwise geochemical changes in the magma erupted; instead a stable, slowly evolving magmatic system has prevailed. Last Glacial climatic variations are also uncorrelated with the timing or magnitudes of edifice collapse. We demonstrate here that, if the magmatic composition erupted from stratovolcanoes is constant and basement geology conditions are stable, large-scale edifice collapse and the generation of catastrophic debris avalanches will be governed by the magma supply rate. Using a mass balance approach, a volume-frequency model can be applied to forecasting both the probable timing and volume of future edifi ce failure of such stratovolcanoes. In the Mount Taranaki case, the maximum potential size of a present collapse is estimated to be 7.9 km³, while the maximum interval before the next collapse is <16.2 k.y. The current annual collapse probability is ~0.00018, with the most likely collapse being a small one (<2 km³). [ABSTRACT FROM AUTHOR]