Richard C. Price

A-type granites revisited: Assessment of a residual-source model

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 granites, 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.

Primitive basalts and andesites from the Mt. Shasta region, N. California: products of varying melt fraction and water content

Quaternary volcanism in the Mt. Shasta region has produced primitive magmas [Mg/(Mg+Fe*)>0.7, MgO>8 wt% and Ni>150 ppm] ranging in composition from high-alumina basalt to andesite and these record variable extents ofmelting in their mantle source. Trace and major element chemical variations, petrologic evidence and the results of phase equilibrium studies are consistent with variations in H2O content in the mantle source as the primary control on the differences in extent of melting. High-SiO2, high-MgO (SiO2=52% and MgO=11 wt%) basaltic andesites resemble hydrous melts (H2O=3 to 5 wt%) in equilibrium with a depleted harzburgite residue. These magmas represent depletion of the mantle source by 20 to 30 wt% melting. High-SiO2, high-MgO (SiO2=58% and MgO=9 wt%) andesites are produced by higher degrees of melting and contain evidence for higher H2O contents (H2O=6 wt%). High-alumina basalts (SiO2=48.5% and Al2O3=17 wt%) represent nearly anhydrous low degree partial melts (from 6 to 10% depletion) of a mantle source that has been only slightly enriched by a fluid component derived from the subducted slab. The temperatures and pressures of last equilibration with upper mantle are 1200°C and 1300°C for the basaltic andesite and basaltic magmas, respectively. A model is developed that satisfies the petrologic temperature constraints and involves magma generation whereby a heterogeneous distribution of H2O in the mantle results in the production of a spectrum of mantle melts ranging from wet (calc-alkaline) to dry (tholeiitic).

The effects of weathering on rare-earth element, Y and Ba abundances in Tertiary basalts from southeastern Australia

Tholeiitic basalts showing anomalously high Ba, Y and rare-earth element (REE) concentrations occur within the Tertiary Newer Volcanic Province of Victoria and these geochemical characteristics are related to incipient alteration accompanying weathering. REE and Y released during weathering of basalts are retained either in secondary phosphate or clay minerals and, in rocks showing very high Ba concentrations, the Ba is present as BaSO4. Leaching experiments suggest that glassy or fine-grained rocks are more likely to release REE, Y and Ba while more extensively altered rocks are likely to hold these elements, probably because they substitute in, or are adsorbed on, clay minerals or because secondary BaSO4 and REE phosphates are resistant to acid leaching. Samples taken sequentially across a weathering profile in a tholeiitic basalt flow, from fresh to intensively altered rock, illustrate that, in the very earliest stages of alteration, extreme variations in the abundances of Ba, Y and REE can occur. This is reflected in the broad patterns of trace-element behaviour manifested by an extensive analysed sample suite from the Victorian Newer Volcanic Province where there is no correlation between anomalous enrichments in Ba, Y, or REE and the level of incipient alteration. Ba anomalies are not necessarily accompanied by “abnormal” Y and REE behaviour and this is probably because the mobility of each trace element is a function of: (a) the ionic radius and charge of the ionic species it forms in solution; (b) local groundwater parameters (such as Eh, pH, groundwater flux, and concentrations in the groundwater of species such as SO42−); and (c) the secondary mineral assemblages developed in a particular situation. Sr isotopic compositions are not strongly affected by incipient weathering and do not show significant changes until alteration has progressed to the point where plagioclase begins to break down. The study illustrates that relatively slight alteration at the earliest stages of weathering can produce drastic changes in the concentrations of REE, Y and Ba and petrographic assessment of the freshness of samples is not precise enough to eliminate, prior to analysis, cases where these effects may be manifested. Where weathering has caused changes in abundance patterns these may be recognised by careful comparisons of appropriate elemental ratios (e.g., Ba/Rb) throughout a particular analysed sample suite.

STRONTIUM ISOTOPIC AND TRACE ELEMENT HETEROGENEITY IN THE PLAINS BASALTS OF THE NEWER VOLCANIC PROVINCE, VICTORIA, AUSTRALIA

Abstract The plains sub-province of the Newer Volcanics of Victoria Australia is a lava plain, ranging in age from 4.5 Ma to Projected onto an east-west profile through the sub-province, the strontium isotopic data reveal a north-south boundary (the Mortlake Discontinuity) separating eastern and western sectors with different mean strontium isotopic ratios. The basalts of the eastern sector show a higher mean 87Sr/86Sr ratio (0.7047) and the ratios are more variable (0.7037–0.7058) than is the case in the western sector (mean is 0.7042 and all but one sample in the range 0.7037–0.7046). The Mortlake discontinuity coincides with a major tectonic boundary separating the two principal Palaeozoic mobile belts of southeastern Australia. It is proposed that the tectonic boundary extends into the subcontinental lithosphere and that the isotopic and geochemical differences observed in the basalts of eastern vs. the western sectors arise because of differences in the geochemistry of heterogeneous lithospheric mantle on either side of this boundary. Strontium isotopic analyses in conjunction with other geochemical information, geomorphological and petrographic observations, and available geochronological information have also been used to define isotopic domains in the sub-province ranging in size from a few up to several hundred square kilometres. The isotopic domains define individual flows or groups of flows representing individual magma batches and it is postulated that the domains in part reflect small scale geochemical heterogeneity in the lithospheric mantle.

Hf isotopes in zircon reveal contrasting sources and crystallization histories for alkaline to peralkaline granites of Temora, southeastern Australia

Peralkaline granites exhibit the hallmark features of A-type igneous rocks, but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins. We present the first in situ Hf isotope data from zircons of peralkaline granites, aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous/weakly peraluminous granites. This study used rocks of the Devonian Narraburra Complex in southeastern Australia, and found that correlations between Hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites. The latter have a depleted mantle-like ancestry, whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile, probably alkaline magmas. Juxtaposition of crust- and mantle-derived magmas reflects the high heat flow and lithosphere-scale faults associated with continental extension, and explains the diversity of A-type granites.

Geochemistry and evolution of Iherzolite-bearing phonolitic lavas from Nigeria, Australia, East Germany and New Zealand

Abstract The major and trace element chemistry of phonolites containing spinel Iherzolite xenoliths from Bokkos (Nigeria), Phonolite Hill (northeastern Australia) and Heldburg (East Germany) is consistent with an origin by fractional crystallization of basanitic magmas at upper mantle pressures (10–15 kbar). At Bokkos, spatially associated lavas ranging from hawaiitic nepheline mugearite to nepheline benmoreite can be modeled very well by fractional crystallization of kaersutitic amphibole + olivine + Fe-Ti-spinel + apatite, a crystal extract consistent with experimentally-determined near-liquidus phase relationships for mugearitic liquids. Further fractional crystallization of aluminous clinopyroxene + mica + apatite will yield the phonolites. A similar model relating the unusual Iherzolite-bearing mafic nepheline benmoreite from Pigroot (New Zealand) to basanitic lavas of the East Otago province is not supported by major and trace element data. The Pigroot lava is possibly the product of melting of a mantle source region previously enriched in Sr and light rare earth elements, with subsequent minor fractional crystallization of olivine + kaersutite. Dynamic flow crystallization processes operating within conduit systems from mantle pressures are capable of yielding large volumes of evolved phonolitic liquids from primary basanitic liquids, if magma flow rates are appropriate. This mechanism may provide an explanation for the volumetric bias towards salic differentiates in some alkalic provinces.

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.

Basalt basement from the Kerguelen Plateau and the trail of a Dupal plume

The first samples of volcanic basement recovered from the Kerguelen Plateau are Lower Cretaceous transitional tholeiites. Isotope and incompatible element abundance ratios for these rocks are similar to ocean island basalts from the southern hemisphere Dupal anomaly region, and geochemical, geological and geophysical data are consistent with volcanic activity associated with a mantle plume. A reconstruction of plate motions suggests that the Kerguelen Plateau formed above a mantle plume in the interval 118-95 Ma, during the opening of the Indian Ocean between India and Australia-Antarctica. This plume was the source of other plateaus and ridges of the eastern Indian Ocean and possibly the Bunbury Basalt of southwestern Australia, and is now beneath Heard Island.

The petrology of a large intra-oceanic silicic eruption: the Sandy Bay Tephra, Kermadec Arc, Southwest Pacific

Abstract The pumiceous pyroclastic deposits known as the Sandy Bay Tuff on Macauley Island in the Kermadec Group represent a medium-scale silicic eruption in an oceanic subduction setting. The Sandy Bay eruption occurred about 6310 yr BP, forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic Macauley Volcano. The Sandy Bay magma contained

Basalts of the North Fiji Basin: the generation of back arc basin magmas by mixing of depleted and enriched mantle sources

Active spreading ridges in the North Fiji Basin range from well-developed stable ridges where largescale mantle upwelling is in progress to proto-ridges where spreading is incipient. South of 17°S, where the central ridge of the North Fiji Basin has a bathymetric profile normally expected of a fast-spreading, steadystate mid-ocean ridge, basalts are evolved N-type MORBs. North of 17°S, where the central ridge is propagating northward into old North Fiji Basin crust and spreading is in the initial stages, two types of basalt have been recovered: N-type MORBs from this northern arm of the central ridge are believed to be samples of older North Fiji Basin crust; basalts with transitional alkalic chemistry (up to 0.5% Ne in the Norm) and characterized by strong relative enrichments in Rb, Ba, K, Nb, La, Ce, Sr, P, Zr, and Ti are believed to be associated with incipient rifting. Among the latter group are compositions that are intermediate between transitional alkalic types and MORBs and these are geochemically similar to the back-arc basin (BABB) magma type defined by Sinton and Fryer (1987) from a study of Mariana back arc basin basalts. Dredges along the South Pandora Ridge, a transform zone characterized by short spreading segments, are dominated by basalts that are enriched in large-ion lithophile and high field strength minor and trace elements and compositions range from types resembling ocean island tholeiites to transitional alkalic varieties. Basalts from Rotuma are regarded as alkalic end-members of the South Pandora Ridge magmatic spectrum. In areas of the North Fiji Basin where relatively fast spreading must be accompanied by largescale asthenospheric upwelling, depleted (N-type) MORBs dominate, whereas in areas of slow mantle upwelling, or where some other tectonic effect (e.g. a transform fault) causes a transient thermal disturbance within the lithosphere or upper asthenosphere, enriched (alkalic) magmas either dominate or make a significant and noticeable contribution to the overall chemical characteristics of basalts being erupted. The MORBs have a depleted asthenospheric source, and the alkalic component is believed to derive from an enriched lithospheric or shallow asthenospheric source. The BABB magma type may simply be part of the spectrum of mixed magmas that can occur in the transitional tectonic settings represented by the early development of most back-arc basins.