Ross E. Pogson

Passive-margin prolonged volcanism, East Australian Plate: outbursts, progressions, plate controls and suggested causes.

Prolonged intraplate volcanism along the 4000 km-long East Australian margin for ca 100 Ma raises many genetic questions. Studies of the age-progressive pulses embedded in general basaltic activity have spawned a host of models. Zircon U–Pb dating of inland Queensland central volcanoes gives a stronger database to consider the structure and origin of Australian age-progressive volcanic chains. This assists appraisal of this volcanism in relation to plate motion and plate margin tectonic models. Inland Queensland central volcanoes progressed south-southeast from 34 to 31 Ma (∼5.4 cm/yr) until a surge in activity led to irregular southerly progression 31 to 28 Ma. A new inland southeastern Queensland central volcano line (25 to 22 Ma), from Bunya Mountains to North Main Range, followed 3 Ma behind the adjacent coastal progression. The Australian and Tasman Sea age-progressive chains are compared against recent plate motion modelling (Indian Ocean hotspots). The chain lines differ from general vector traces owing to west-facing swells and cessations in activity. Tectonic processes on the eastern plate margin may regulate these irregularities. These include subduction, rapid roll-back and progressive detachment of the Loyalty slab (43 to 15 Ma). West-flowing Pacific-type asthenosphere, related to perturbed mantle convection, may explain the west-facing volcanic surges. Such westward Pacific flow for over 28 Ma is known at the Australian–Antarctic Discordance, southeast of the present Australian plume sites under Bass Strait–West Tasman Sea. Most basaltic activity along eastern Australia marks asthenospheric melt injections into Tasman rift zone mantle and not lithospheric plate speed. The young (post-10 Ma) fields (Queensland, Victoria–South Australia) reflect new plate couplings, which altered mantle convection and stress regimes. These areas receive asthenospheric inputs from deep thermal zones off northeast Queensland and under Bass Strait.

Carboniferous clay deposits from Jenolan Caves, New South Wales: implications for timing of speleogenesis and regional geology

K – Ar dating of illite-bearing clays from eight locations in Jenolan Caves yielded ages from 394 Ma (Early Devonian) to 258 Ma (Late Permian) (18 dates of individual size fractions). There were two distinct clusters among the dates. Seven dates ranged from 342 to 335 Ma (Carboniferous, Visean). Three dates ranged from 394 to 389 Ma (Early Devonian). Fission track dating of 11 zircons from one sample yielded pooled ages of 345.9 Ma (nine grains) and 207.2 Ma (two grains). XRD peak width measurements and SEM studies indicated that the clays are well crystallised and showed no signs of transport. This suggests that the clays formed and matured in place in the caves. Sedimentary strata of Visean to Namurian age are not found in the surroundings of the caves. XRD peak width measurements, SEM studies, and additional K – Ar illite dating rule out illite-bearing materials in the immediate catchment of the caves as a source for the Carboniferous clays. The most likely origin for the Carboniferous clays is from volcaniclastics, associated with the emplacement of Carboniferous granites, entering the caves. The volcaniclastics reacted with thermal waters, which had excavated the caves, altering feldspars and glasses to kaolinite and illite. The Early Devonian clays are interpreted as volcaniclastic palaeokarst deposits related to an unconformity at the base of the overlying Lower Devonian volcanics. Whatever their origin, the Carboniferous clays are hundreds of millions of years older than absolute dates of cave deposits reported in recent reviews and appear to set a record for the absolute age of deposits found in currently open caves.

Spinel to garnet lherzolite transition in relation to high temperature palaeogeotherms, eastern Australia∗

Eastern Australian xenolith suites and lithospheric transition zones are re‐evaluated using new mineral analyses and thermo‐barometry. Some suites, including that defining the southeastern Australian geotherm, are not fully equilibrated. New pressure‐temperature estimates, based on experimental calibrations that allow for Cr and Ti in pyroxenes, differ from earlier results by up to 0.6 GPa and 250°C. The preferred Brey and Kohler 1990 thermo‐barometer indicates a shallower cooler garnet lherzolite transition under Mesozoic New South Wales (50 km depth at 980° C) than for Tertiary Tasmania (60 km depth at 1090°C). Deviations between palaeogeotherms may reflect: (i) higher temperature gradients for Tasmania and New South Wales (by 100°C/0.1 GPa) related to abnormally hot mantle; (ii) higher temperature gradients linked to more voluminous magmatism, largely Cenozoic in age; and (iii) complex temperature perturbations linked to different levels of magmatic intrusion. These deviations blur reconstructions of l...

Vibrational spectroscopic analysis of the mineral crandallite CaAl3(PO4)2(OH)5·(H2O) from the Jenolan Caves, Australia

The mineral crandallite CaAl(3)(PO(4))(2)(OH)(5)·(H(2)O) has been identified in deposits found in the Jenolan Caves, New South Wales, Australia by using a combination of X-ray diffraction and Raman spectroscopic techniques. A comparison is made between the vibrational spectra of crandallite found in the Jenolan Caves and a standard crandallite. Raman and infrared bands are assigned to PO(4)(3-) and HPO(4)(2-) stretching and bending modes. The predominant features are the internal vibrations of the PO(4)(3-) and HPO(4)(2-) groups. A mechanism for the formation of crandallite is presented and the conditions for the formation are elucidated.

Multiple felsic events within post-10 Ma volcanism, Southeast Australia: inputs in appraising proposed magmatic models

Felsic episodes in young SE Australian volcanism were studied using new combined zircon U–Pb, feldspar 40Ar–39Ar and fission-track dating. Trachytes, xenocrysts in basalts and derived detrital crystals yielded an 8 Ma range for felsic sequences in the Macedon–Trentham (ca 8–5 Ma) and Western District (< 5–0.0 Ma) provinces of Victoria. At Newham, zircon and feldspar ages of 6.3–6.1 ± 0.1 Ma agree with the local basalt stratigraphy, while near Trentham zircon dating suggests felsic activity at ca 8.3 Ma and 6–5 Ma. Zircons crystallised in high-temperature crustal trachytes that evolved from alkali basalts, following amphibole crystallisation in the mantle (6.3 Ma Brimbank complex). The 8–5 Ma felsic episodes are attributed to lithospheric passage over an asthenospheric plume-like upwelling, now centred under Bass Strait. The Western District Province includes quartz-normative trachyte near Creswick (40Ar–39Ar age ca 2.4 ± 0.4 Ma), zircon xenocrysts in basalt near Daylesford (U–Pb age 1.8 ± 0.3 Ma) and zircon megacrysts in tuff at Bullenmerri maar (U–Pb zircon age 0.28 ± 0.04 Ma). The Creswick and Daylesford felsic phases may represent fractionation of basaltic icelandites during peak Western District volcanic activity. Bentonitic beds of trachyandesite affinities in NW Victoria–SE New South Wales lie in strata dated at ca 2 Ma and may mark a separate distal phase of peak Western District felsic volcanism. The E–W trend of post-5 Ma Western District basaltic activity has been attributed to lithospheric edge-driven or Tasman Fracture Zone fault-driven magmatic up-wells. However, new tomographic modelling of sublithospheric upper mantle suggests that Bassian asthenospheric inputs may explain young felsic components in adjacent basalts. Multiple felsic inputs allow greater appraisal of the young volcanic genesis and eruptive risks for the area.

Zeolite crystal habits, compositions, and paragenesis; Blackhead Quarry, Dunedin, New Zealand

Abstract Blackhead Quarry exploits a small Miocene (~10 Ma) basanitic volcanic centre in the Dunedin Volcanic Group, New Zealand. Vesicles near the quarry top contain Ca- and Na-rich zeolites, abundant calcite and rare pyrite resulting from localized low-T hydrothermal alteration (<100ºC). Mineral assemblages were characterized by EDS and laser Raman spectroscopy with the latter the most useful in determining the identity of fibrous zeolite species. Secondary mineral assemblages crystallized during final stages of lava cooling from aqueous solutions enriched in Ca, Na, K, Ba and (CO3)2- leached from surrounding calcareous rocks and basanite. Phillipsite-K (generation I) crystallized first (in some places directly on fresh basanite) followed by the Ca-rich zeolites, chabazite-Ca, calcian phillipsite-K (generation II), gismondine and thomsonite. Later Na-rich fluids crystallized gonnardite and natrolite, and finally calcite from late Ca-rich fluids. Zeolite composition is not reflected by morphology. For example, both phacolitic and pseudocubic chabazite are chabazite-Ca, and although all phillipsite crystals have a similar habit, their composition varies widely. Various lithologies comprising the Blackhead volcanic centre have unique secondary mineral paragenetic sequences controlled largely by the rock structure and solution chemistry.

Vibrational spectroscopic analysis of taranakite (K,NH4)Al3(PO4)3(OH)·9(H2O) from the Jenolan Caves, Australia.

Many phosphate containing minerals are found in the Jenolan Caves. Such minerals are formed by the reaction of bat guano and clays from the caves. Among these cave minerals is the mineral taranakite (K,NH(4))Al(3)(PO(4))(3)(OH)·9(H(2)O) which has been identified by X-ray diffraction. Jenolan Caves taranakite has been characterised by Raman spectroscopy. Raman and infrared bands are assigned to H(2)PO(4), OH and NH stretching vibrations. By using a combination of XRD and Raman spectroscopy, the existence of taranakite in the caves has been proven.

Cumulate-rich xenolith suite in Late Cenozoic basaltic eruptives, Hepburn Lagoon, Newlyn, in relation to western Victorian lithosphere*

Cumulate‐textured rocks form 90 vol% of xenoliths erupted with Hy‐normative olivine basalt at Hepburn Lagoon, Victoria. The phreatomagmatic host eruption punched through Late Cenozoic (<3 Ma) hawaiites. More than 80% of the xenoliths are pyroxenites, largely websterites, and contain Al–and Ti–enriched augites (Mg# 0.76–0.87) and orthopyroxene (Mg# 0.75–0.79), ±altered olivine and spinel (Mg# 0.52–0.60). Subordinate gabbros, gabbronorites and norites contain sodic to calcic plagioclase (An45–64). The xenoliths include minor Mg–rich metaperidotites (olivine Mg# 0.86–0.90) and metapyroxenites and granulites (± altered garnet or spinel).Thermobarometry for the main cumulates suggests subliquidus equilibration between 1000 and 1100°C at ∼1 ± 0.25 GPa, compatible with observed cumulates in cross‐cutting veins in mantle metaperidotites. The re‐equilibrated granulites and mantle metaperidotites yield lower temperatures (840–980°C) than the cumulates. Gabbroic banding in some pyroxenites suggests that a common parental mafic melt was involved. A melt composition was reconstructed by combining websterite and gabbronorite analyses in the observed 8:1 pyroxenite/gabbro ratio. This yields a Hy‐normative parental basaltic melt (Mg# 0.73–0.76), with oceanic basalt trace‐element contents. The overall range in cumulate compositions and in the mantle‐normalised incompatible trace‐element patterns suggests that some parental melts were Ne‐normative. The Hepburn Lagoon xenolith suite is notable for its little‐modified, relatively anhydrous pyroxene‐rich cumulate assemblages, both in the Victorian context and in intraplate basalt settings elsewhere. It differs markedly from more recrystallised and metasomatised mantle‐crust columns represented in xenolith suites to the southwest (Bullenmerri) and southeast (The Anakies). This may reflect its position within the Lachlan Fold Belt, between the older Delamerian (west) and Selwyn Block (east) basements.

Raman spectroscopic study of the mineral arsenogorceixite BaAl3AsO3(OH)(AsO4,PO4)(OH,F)6

Arsenogorceixite BaAl(3)AsO(3)(OH)(AsO(4),PO(4))(OH,F)(6) belongs to the crandallite mineral subgroup of the alunite supergroup. Arsenogorceixite forms a continuous series of solid solutions with related minerals including gorceixite, goyazite, arsenogoyazite, plumbogummite and philipsbornite. Two minerals from (a) Germany and (b) from Ashburton Downs, Australia were analysed by Raman spectroscopy. The spectra show some commonality but the intensities of the peaks vary. Sharp intense Raman bands for the German sample, are observed at 972 and 814 cm(-1) attributed to the ν(1) PO(4)(3-) and AsO(4)(3-) symmetric stretching modes. Raman bands at 1014, 1057, 1148 and 1160 cm(-1) are attributed to the ν(1) PO(2) symmetric stretching mode and ν(3) PO(4)(3-) antisymmetric stretching vibrations. Raman bands at 764 and 776 cm(-1) and 758 and 756 cm(-1) are assigned to the ν(3) AsO(4)(3-) antisymmetric stretching vibrations. For the Australian mineral, the ν(1) PO(4)(3-) band is found at 973 cm(-1). The intensity of the arsenate bands observed at 814, 838 and 870 cm(-1) is greatly enhanced. Two low intensity Raman bands at 1307 and 1332 cm(-1) are assigned to hydroxyl deformation modes. The intense Raman band at 441 cm(-1) with a shoulder at 462 cm(-1) is assigned to the ν(2) PO(4)(3-) bending mode. Raman bands at 318 and 340 cm(-1) are attributed to the (AsO(4))(3-)ν(2) bending. The broad band centred at 3301 cm(-1) is assigned to water stretching vibrations and the sharper peak at 3473 cm(-1) is assigned to the OH stretching vibrations. The observation of strong water stretching vibrations brings into question the actual formula of arsenogorceixite. It is proposed the formula is better written as BaAl(3)AsO(3)(OH)(AsO(4),PO(4))(OH,F)(6)·xH(2)O. The observation of both phosphate and arsenate bands provides a clear example of solid solution formation.

Spectroscopic research on ultrahigh pressure (UHP) macrodiamond at Copeton and Bingara NSW, Eastern Australia.

Millions of macrodiamonds were mined from Cenozoic placers across Eastern Australia, 98% from within the Copeton and Bingara area (85 km across) in the Phanerozoic New England region of New South Wales (NSW). Raman spectroscopy of inclusions in uncut diamond, from the Copeton and Bingara parcels, identifies them as ultrahigh pressure (UHP) macrodiamond formed during termination of subduction by continental collision. Infrared spectral properties of the two parcels are critically similar in terms of nitrogen abundance (low in zoned diamond, high in unzoned diamond), requiring a pair of different growth mechanisms/protoliths. Within each parcel, the degrees of nitrogen aggregation are relatively strong and coherent, but they are so different from each other (moderate aggregation for Bingara, strong for Copeton) that the two parcels require separate primary and local sources. The local sources are post-tectonic alkali basaltic intrusions which captured UHP minerals (garnet, pyroxene, diamond) from eclogite-dominated UHP terranes (density stranded at depth-mantle, lower crust). X-ray diffraction studies on Copeton diamond indicate a normal density, despite previous reports of anomalously high density. For non-fluorescent diamond, a 2nd order Raman peak, which is prominent in theoretical perfect diamond and in African cratonic diamond, is suppressed in Copeton and Bingara UHP macrodiamond. Pervasive deformation during macrodiamond growth probably causes this suppression, the strong nitrogen aggregation, and the exceptional durability documented through industrial use.