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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]

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]