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The evolution of the Triassic-Cretaceous Cuyo Basin, located in the Andean Precordillera (NW Argentina), was mainly controlled by extensional-transtensional tectonics along NW-trending structures inherited from Paleozoic sutures. Stratigraphic and isotopic studies are herein presented in order to constrain the sedimentary infilling and source areas, as well as the age of Triassic strata related to the active faulted margin of the Cacheuta subbasin. The whole stratigraphic package of the Cuyo Basin is interpreted as a second-order depositional sequence, comprising lowstand (alluvial fan deposits), transgressive (fluvial-deltaic and lacustrine deposits), and highstand (oxidized lacustrine and high-sinuosity fluvial deposits) systems tracts. This stacking pattern was mainly controlled by the interplay between sedimentary influx and accommodation space, with important volcaniclastic contribution, especially during the early stages of basin development. Sm-Nd depleted-mantle model ages ([T.sub.DM]) for both basement and volcano-sedimentary samples range from 867 to 1345 Ma. All samples have negative [[epsilon]Nd] (t) values (-0.33 to -5.02). Basal deposits present older [T.sub.DM] model ages compared to intermediate-positioned deposits, and upper deposits present a wider range of model ages. This time-integrated factor correlates with paleo-current data, which indicate changes in the source areas through time. The Nd isotopic signature also suggests that sources of sediments for the Cacheuta sub-basin during the Triassic were restricted to units of Mesoproterozoic (Grenvillian) age, probably related to the Cuyania terrane. The U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon age of 243 [+ or -] 5 Ma, obtained in juvenile magmatic zircons from a lithoclast-free ignimbrite interlayered within the basal alluvial fan facies of the Cacheuta sub-basin, places the lowstand deposits in the Olenekian and establishes a potential chronostratigraphic horizon for future correlations. Keywords: Nd isotopes, provenance, chronostratigraphic horizon, Triassic, South America.

Eastern Australia hosts a long track of Cenozoic age-progressive volcanoes, mostly alkaline in composition. Of these, Warrumbungle and Comboyne are coeval and occur at the same latitude (31°S), but they are ∼300 km apart, on either side of the Great Dividing Range. The lavas from both volcanoes often contain complex crystal assemblages, including plagioclase, olivine and clinopyroxene, which permit a comparative study of pre-eruptive magma histories in a large, complex, continental setting. Here we combine mineral and whole-rock geochemistry with 40Ar/39Ar geochronology to temporally constrain the processes operating in the magma plumbing systems. 40Ar/39Ar geochronology indicates that volcanic activity took place for ∼3 Myr, in two separate stages. The first stage (18–17·5 Ma) is evident only at the larger Warrumbungle volcano. In Stage 2 (∼17–15·5 Ma) the two volcanoes were active contemporaneously. The dominantly porphyritic and relatively evolved (MgO from 7·25 to 0·39 wt %) nature of the lavas suggests that the magmas stalled and differentiated in the crust prior to eruption. At the Warrumbungle volcano, Stage 1 magmas fractionated olivine and minor clinopyroxene and subsequently differentiated during ascent. The crystal cargo in Stage 2 magmas at the Warrumbungle volcano became increasingly more complex with time and the samples have been divided into two subgroups, according to age and petrological variation. Stage 2.1 magmas sampled olivine, clinopyroxene and plagioclase mushes at Moho depths of ∼41 km. Disequilibrium textures in plagioclase and clinopyroxene macrocrysts indicate differences in composition between the mush and the ascending magmas. Stage 2.2 magmas, by contrast, carried a combination of antecrysts and phenocrysts. Clinopyroxene antecrysts show strong disequilibrium textures and are reversely zoned. In plagioclase, anorthite contents increase close to the rim of the crystals, to levels (An60–55) similar to those found at the core of primitive, normally zoned, euhedral antecrysts (An53–50). At the Comboyne volcano mineral phases have a similar complexity to those of Stage 2.2 at the Warrumbungle volcano, with disequilibrium textures and reversely zoned antecrysts providing evidence of magma mixing, only lacking the primitive, normally zoned, euhedral plagioclase crystals. The complex crystal assemblage evident in Stage 2.2 lavas at the Warrumbungle volcano and throughout Stage 2 at the Comboyne volcano indicates a coeval rejuvenation of evolving crystal–melt mushes with the intrusion of more primitive, hotter, and crystal rich or -poor magmas shortly before eruption. Forward modelling using Rhyolite-MELTS replicates the composition of melts and fractionated minerals along a polybaric fractional crystallization path at depths from 24 to 7 km at the Warrumbungle volcano and from 15 to 7 km at Comboyne, supported by barometry estimates on clinopyroxene crystals. This study has identified that the two temporally associated, but spatially discrete, continental alkaline volcanoes were fed by parallel plumbing systems, which become more complex throughout the life of the volcanoes. Multiple mush zones, in which magmas stagnated and fractionated, were periodically replenished with more primitive magmas, triggering eruptions intermittently over a protracted period of ∼3 Myr. [ABSTRACT FROM AUTHOR]

The Macquarie University Deformation-DIA (MQ D-DIA) multi-anvil apparatus at the Australian Synchrotron provides a new experimental facility that enables simultaneous high-pressure and high-temperature in situ synchrotron experimentation in Australia. The MQ D-DIA can be easily deployed at any of a number of beamlines at the Australian Synchrotron, and we describe its installation at the x-ray absorption spectroscopy beamline, which enables in situ x-ray absorption near-edge spectroscopy and energy-scanning x-ray diffraction. A simple, reliable, and x-ray transparent high-pressure cell assembly has been developed for the D-DIA for which load/pressure and heater power/temperature relationships have been calibrated using in situ x-ray diffraction and "offline" mineral equilibration experiments. Additionally, we have mapped temperature distribution within the assembly using a new quantitative electron microprobe mapping technique developed for fine-grained polyphase samples. We are now investigating the speciation of geologically important trace elements in silicate melts (e.g., Zr, U, and Th) measured in situ under high pressure and temperature conditions corresponding to the Earth's mantle. Pressure-dependent changes in speciation influence partitioning behavior, and therefore the distribution in the Earth, of many trace elements. However, previous ex situ investigations are hampered by uncertainty as to whether high-pressure speciation can be faithfully recorded in samples recovered to ambient conditions. We present preliminary results showing an increase in the coordination number of Zr dissolved as a trace component of a sodium-rich silicate melt with pressure. These results also indicate that silicate melt composition exerts a strong influence on Zr speciation.

Subduction zone or arc magmas are known to display a characteristic depletion of High Field Strength Elements (HFSE) relative to other similarly incompatible elements, which can be attributed to the presence of the accessory mineral rutile (TiO2) in the residual slab. Here we show that the partitioning behavior of vanadium between rutile and silicate melt varies from incompatible (∼0.1) to compatible (∼18) as a function of oxygen fugacity. We also confirm that the HFSE are compatible in rutile, with D(Ta)> D(Nb)>> (D(Hf)>/∼ D(Zr), but that the level of compatibility is strongly dependent on melt composition, with partition coefficients increasing about one order of magnitude with increasing melt polymerization (or decreasing basicity). Our partitioning results also indicate that residual rutile may fractionate U from Th due to the contrasting (over 2 orders of magnitude) partitioning between these two elements. We confirm that, in addition to the HFSE, Cr, Cu, Zn and W are compatible in rutile at all oxygen fugacity conditions.