Contrary to general belief, not all large igneous provinces (LIPs) are characterised by rocks of basaltic composition. Silicicdominated LIPs, such as the Whitsunday Volcanic Province of NE Australia, are being increasingly recognised in the rock record. These silicic LIPs are consistent in being: (1) volumetrically dominated by ignimbrite; (2) active over prolonged periods (40-50 m.y.), based on available age data; and (3) spatially and temporally associated with plate break-up. This silicicdominated LIP, related to the break-up of eastern continental Gondwana, is also significant for being the source of Ͼ 1:4 × 10 6 km 3 of coeval volcanogenic sediment preserved in adjacent sedimentary basins of eastern Australia. The Whitsunday Volcanic Province is volumetrically dominated by medium-to high-grade, dacitic to rhyolitic lithic ignimbrites. Individual ignimbrite units are commonly between 10 and 100 m thick, and the ignimbrite-dominated sequences exceed 1 km in thickness. Coarse lithic lag breccias containing clasts up to 6 m diameter are associated with the ignimbrites in proximal sections. Pyroclastic surge and fallout deposits, subordinate basaltic to rhyolitic lavas, phreatomagmatic deposits, and locally significant thicknesses of coarse-grained volcanogenic conglomerate and sandstone are interbedded with the ignimbrites. The volcanic sequences are intruded by gabbro/dolerite to rhyolite dykes (up to 50 m in width), sills and comagmatic granite. Dyke orientations are primarily from NW to NNE.The volcanic sequences are characterised by the interstratification of proximal/near-vent lithofacies such as rhyolite domes and lavas, and basaltic agglomerate, with medial to distal facies of ignimbrite. The burial of these near-vent lithofacies by ignimbrites, coupled with the paucity of mass wastage products such as debris-flow deposits indicates a low-relief depositional environment. Furthermore, the volcanic succession records a temporal change in: (1) eruptive styles; (2) the nature of source vents; and (3) erupted compositions. An early explosive dacitic pyroclastic phase was succeeded by a later mixed pyroclasticeffusive phase producing an essentially bimodal suite of lavas and rhyolitic ignimbrite. volcanic lithofacies, the volcanic sequences are interpreted to record the evolution of a multiple vent, low-relief volcanic region, dominated by several large caldera centres. ᭧
Abstract:Emerald from the deposits at Poona shows micrometre-scale chemical, optical, and cathodoluminescence zonation. This zonation, combined with fluid inclusion and isotope studies, indicates early emerald precipitation from a single-phase saline fluid of approximately 12 weight percent NaCl equivalent, over the temperature range of 335-525 • C and pressures ranging from 70 to 400 MPa. The large range in pressure and temperature likely reflects some post entrapment changes and re-equilibration of oxygen isotopes. Secondary emerald-hosted fluid inclusions indicate subsequent emerald precipitation from higher salinity fluids. Likewise, the δ 18 O-δD of channel fluids extracted from Poona emerald is consistent with multiple origins yielding both igneous and metamorphic signatures. The combined multiple generations of emerald precipitation, different fluid compositions, and the presence of both metamorphic and igneous fluids trapped in emerald, likely indicate a protracted history of emerald precipitation at Poona conforming to both an igneous and a metamorphic origin at various times during regional lower amphibolite to greenschist facies metamorphism over the period~2710-2660 Ma.
The Neoproterozoic Aries kimberlite was emplaced in the central Kimberley Basin, Western Australia, as a N-NNE-trending series of three diatremes infilled by lithic-rich kimberlite breccias. The breccias are intruded by hypabyssal macrocrystic phlogopite kimberlite dykes that exhibit differentiation to a minor, high-Na-Si, olivine-phlogopite-richterite kimberlite, and late-stage macrocrystic serpentine-diopside ultramafic dykes. Mineralogical and geochemical evidence suggests that the high-Na-Si, olivinephlogopite-richterite kimberlite was derived from the macrocrystic phlogopite kimberlite as a residual liquid following extended phlogopite crystallization and the assimilation of country rock sandstone, and that the macrocrystic serpentine-diopside ultramafic dykes formed as mafic cumulates from a macrocrystic phlogopite kimberlite. Chemical zonation of phlogopite-biotite phenocrysts indicates a complex magmatic history for the Aries kimberlite, with the early inheritance of a range of high-Ti phlogopite-biotite xenocrysts from metasomatized mantle lithologies, followed by the crystallization of a population of high-Cr phlogopite phenocrysts within the spinel facies lithospheric mantle. A further one to two phlogopite-biotite overgrowth rims of distinct composition formed on the phlogopite phenocrysts at higher levels during ascent to the surface. Ultraviolet laser 40 Ar/ 39 Ar dating of mica grain rims yielded a kimberlite eruption age of 815. 4 ± 4. 3 Ma (95% confidence). 40 Ar/ 39 Ar laser profiling of one high-Ti phlogopite-biotite macrocryst revealed a radiogenic 40 Ar diffusive loss profile, from which a kimberlite magma ascent duration from the spinel facies lithospheric mantle was estimated (assuming an average kimberlite magma temperature of 1000 C), yielding a value of $0. 23-2. 32 days for the north extension lobe of the Aries kimberlite.
The Mad Gap Yards ultramafic lamprophyre (UML) dykes in the East Kimberley region of northern Western Australia form part of a widespread Neoproterozoic (~842-800 Ma) alkaline mafic-ultramafic magmatic province in the north, east and central regions of the Kimberley Craton of Western Australia. The NE-trending Mad Gap Yards dykes lie at the southeastern margin of the Kimberley Basin adjacent to the Greenvale Fault and intrude the Paleoproterozoic Elgee Siltstone. The dykes are classified as alnöite, and contain abundant macrocrystic olivine in a groundmass of phlogopite, perovskite, spinels, diopside, apatite, andradite-hydroandradite, serpentine, calcite, pseudomorphs after melilite and rare gittinsite. Mantle-derived olivine macrocrysts have compositions in the range Mg#91-92, similar to moderately refractory peridotite from other parts of the Kimberley Craton, whereas magmatic olivine phenocrysts have Mg#88-90. Olivine and chromian spinel were the earliest phenocrysts; they record equilibration temperatures of ~1030-920˚C under moderately reducing conditions with fO2 values below the fayalite-magnetite-quartz (FMQ) oxygen buffer (Δ FMQ = mostly -0.8 to -1.7 log units). Magnetite rims and groundmass grains crystallised at ~850-740˚C under more oxidising conditions with Δ FMQ ~+0.6 to -0.75 log units. Perovskite is well preserved in parts of the dykes and indicates crystallisation inside this fO2 range. The perovskite yielded a SHRIMP 206Pb/238U age of 842±8 Ma. The Mad Gap Yards dykes carry rare partially-altered spinel peridotite xenoliths containing olivine (Mg#86.3-90), Cr-diopside, enstatite and Al-Cr spinel, and well as
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