The Tennant Creek porphyry revisited: A synsedimentary sill with peperite margins,Early Proterozoic,Northern Territory

Isolated outcrops of coarse‐ and medium‐grained, quartz‐feldspar porphyry occur for some 60 km from Nobles Nob to near Warrego in the Early Proterozoic Tennant Creek Block of the central Northern Territory. The outcrops appear to be part of an approximately conformable sheet of variable thickness (several tens to a few hundred metres), enclosed by sedimentary rocks of the Warramunga Group. The porphyry is characterized by euhedral, complete, evenly distributed crystals up to 2 cm across, in a microcrystalline groundmass. In some samples the groundmass shows relict perlitic cracks and was formerly coherent glass. In detail, upper and lower contacts of the porphyry sheet are highly irregular: porphyry contains wisps, blocks and large rafts of sandstone, and sandstone encloses single euhedral crystals and blebs, tongues and lobes of porphyry. Clasts of each rock type commonly have fluidal shapes. Sedimentary rocks at both the upper and the lower contacts are indurated and silicified. Close to the contacts, bedding is not easily identified, or else is disturbed. The quartz‐feldspar porphyry is interpreted to have cooled and solidified from phenocryst‐rich magma that intruded approximately parallel to bedding in the enclosing sedimentary sequence. The peperite margins of the sill suggest that the porphyritic magma invaded relatively weak, poorly consolidated, wet sediments rather than solid sedimentary rock. Rapid expansion and movement of heated pore fluid would have temporarily disrupted particle packing in the sediments at the contacts, and allowed intricate penetration of magma accompanied by quenching and fragmentation. In addition to relatively short‐term and local effects on the pore fluid chemical and physical properties, the intrusion was probably responsible for premature and permanent dewatering of the adjacent sediments. The sill together with its bordering zone of peperite and indurated sedimentary rocks may have constituted a significant physical barrier to any subsequent fluid transport in the enclosing sedimentary sequence.     

State of the Society address

Jocelyn Bell Burnell looks back over her first year as President of the Royal Astronomical Society.     

Pliers, pulsars and extreme physics

Jocelyn Bell Burnell looks back at the discovery of pulsars 35 years ago, in her Presidential Address of 2003.     

Origin of high-Si dacite from rhyolitic melt: evidence from melt inclusions in mingled lavas of the 1.6 Ga Gawler Range Volcanics, South Australia

Summary ?Part of the Mesoproterozoic (1.6 Ga) Gawler Range Volcanics in South Australia is composed of mingled feldspar- quartz- phyric dacite, rhyodacite and rhyolite lavas. Field relationships suggest that dacite erupted first, locally grading into rhyodacite, followed by mingled dacite and rhyolite or rhyodacite and rhyolite, and finally in some areas rhyolite, and imply that the three lithofacies co-existed in a compositionally stratified magma chamber. Data on the bulk rock, groundmass and melt inclusion compositions suggest that post-eruption alteration has had very little effect on the original rock compositions. Melt inclusions in quartz from rhyolite and rhyodacite-dacite, respectively, belong to two compositional populations. Inclusions in the rhyolitic quartz have less evolved compositions with lower SiO₂ (72–76.4 wt %) and higher Al₂O₃ (13.2–15.6 wt%) and Na₂O (2.5–4.2 wt%) abundances. In contrast, melt inclusions in quartz from the rhyodacite-dacite are more “evolved” (i.e., 75.5–78.3 wt% SiO₂, 11.2–12.7 wt% Al₂O₃ and 1.7–2.2 wt% Na₂O). The two melt populations define a single compositional trend towards groundmass compositions, which are essentially similar in all three lithofaci es (77.8–80.5 wt% SiO₂, 9.9–11.1  wt% Al₂O₃ and 2.2–2.4 wt% Na₂O). This trend is consistent with the derivation of the groundmass melt from a single precursor melt of rhyolitic composition by means of crystallisation of dominant plagioclase, K-feldspar and minor quartz. Plagioclase-enriched dacite-rhyodacite magma comprises a mixture of the residual melt and plagioclase phenocryst s that accumulated in the upper part of the magma chamber and erupted first. Similar residual melt containing quartz and K-feldspar phenocrysts was present deeper in the magma chamber and erupted later to form quartz-, K-feldspar-phyric rhyolite.

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Zusammenfassung ?Die Bildung von Si-reichem Dacit aus rhyolitischer Schmelze: Evidenz aus Schmelzeinschlüssen in Laven der 1.6 Ga Gawler Range Volcanics, Südaustralien Ein Teil der mesoproterozoischen (1.6 Ga) Gawler Range Volcanics in Südaustralien setzt sich aus “mingled” Feldspat- Quarz-phyrischen dacitischen, rhyodacitischen und rhyolithischen Laven zusammen. Gel?ndebefunde legen nahe, da? die Dacite, die lokal in Rhyodacite übergehen, zuerst eruptierten, gefolgt vom “mingled” Dacit und Rhyolith oder Rhyodacit und Rhyolith. Schlie?lich bildeten sich in einigen Gebieten Rhyolithe. Diese Beobachtungen lassen die Schlu?folgerung zu, da? die drei Lithofazies in einer geschichteten Magmenkammer koexistierten. Die Daten der Gesamtgesteins-, Grundmasse- und Schmelzeinschlu?- Zusammensetzungen zeigen, da? Alterationsvorg?nge nach der Eruption einen sehr minimalen Effekt auf die ursprüngliche Gesteinszusammensetzung hatten. Die Schmelzeinschl üsse in den Rhyolithen und Rhyodaciten geh?ren zwei unterschiedlich en Populationen an. Die Schmelzeinschlüsse in Quarz der Rhyolithe sind weniger deutlich “entwickelt” mit niedrigeren SiO₂ (72–76.4 Gew.%) und h?heren Al₂O₃ (13.2–15.6 Gew.%) und Na₂O-(2.5–4. 2 Gew.%) Gehalten. Im Unterschied dazu sind die Einschlüss e in Quarz aus Rhyodacit-Dacit st?rker “entwickelt” (i.e., 75.5–78.3 Gew.% SiO₂, 11.2–12.7 G ew.% Al₂O₃ und 1.7–2.2 Gew.% Na₂O). Die beiden Populationen von Schmelzeinschlüssen definieren einen einzigen Entwicklungstrend hin zur Zusammensetzung der Grundmasse, die in allen drei Lithofazies ?hnlich ist (77.8–80.5 Gew.% SiO₂, 9.9–11.1 Gew.% Al₂O₃ und 2.2–2.4 Gew.% Na₂O). Dieser Trend ist mit der Herkunft der Grundmasse-bildenden Schmelze aus einer einzigen Ausgangsschmelze rhyolithischer Zusammensetzung infolge der Kristallisation von haupts?chlich Plagioklas, Alkalifeldspat und untergeordnet Quarz konsistent. Dacit-Rhyodacitmagmen, die an Plagioklas angereichert sind, stellen eine Mischung der Residualschmelze mit Plagioklas- Ph?nokristallen, die sich in den oberen Teilen der Magmenkammer akkumuli ert haben, dar; sie eruptierten zuerst. ?hnliche residuale Schmelzen mit Quarz und Akalifeldspat-Ph?nokristallen waren auf die tieferen Teilen der Magmenkammer beschr?nkt; sie eruptierten sp?ter und bildeten die Quarz- und Akalifeldspat-phyrischen Rhyolithe.

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Received April 1, 1999;/revised version accepted July 27, 1999     

Calcite uranium–lead geochronology applied to hardground lithification and sequence boundary dating

Hardground discontinuities within carbonate platforms form important stratigraphic surfaces which can be used at basin scale to correlate sequence boundaries. Although these surfaces are commonly used in sequence strati‐graphy, the timing and duration of hardground lithification and the crystallization of early cements remain unexplored. Here, early calcite cements were dated by U‐Pb geochronology for five Jurassic hardgrounds, interpreted as third‐order sequence boundaries, situated within a well‐constrained petrographic, sedimentological and stratigraphic framework. The consistency or the slight deviation between the age of the cements and the stratigraphic age of deposition of the formations illustrate that cementation occurred early in the diagenetic history. The ages obtained on dogtooth cements, replacing aragonite in gastropod shells and pendant cements in intergranular spaces, match those of the standard Jurassic biostratigraphic ammonite Zones, making calcite U‐Pb geochronology a promising method for dating third‐order sequence boundaries of depositional sequences and refining the Jurassic time scale in the future.     

Secondary welding of submarine,pumice-lithic breccia at Mount Chalmers,Queensland, Australia

Very thick units of massive pumice and lithic clast-rich breccia in the Early Permian Berserker beds at Mount Chalmers, Queensland, are deposits from cold, water-supported, volcaniclastic mass flows emplaced in a below-wave base submarine setting. Adjacent to syn-volcanic andesitic and rhyolitic sills and dykes, the pumice-lithic breccia shows a well-developed eutaxitic texture. The eutaxitic foliation is parallel to intrusive contacts and extends as far as a few metres away from the contact. At these sites, pumice clasts are strongly flattened and tube vesicles within the pumice clasts are compacted and aligned parallel to the direction of flattening. Some lenticular pumice clasts contain small (2 mm), round, quartz-filled amygdales and spherulites. Further away from the sills and dykes, the pumice clasts have randomly oriented, delicate tube vesicle structure and are blocky or lensoid in shape. Round amygdales were generated by re-vesiculation of the glass and the spherulites indicate devitrification of the glass at relatively high temperatures. The eutaxitic texture is therefore attributed to re-heating and welding compaction of glassy pumice-lithic breccia close to contacts with intrusions. In cases involving sills, secondary welding along the contacts formed extensive, conformable, eutaxitic zones in the pumice-lithic breccia that could be mistaken for primary welding compaction in a hot, primary pyroclastic deposit.     

Unsupervised separation of seismic waves using the watershed algorithm on time-scale images

This paper illustrates the use of image processing techniques for separating seismic waves. Because of the non‐stationarity of seismic signals, the continuous wavelet transform is more suitable than the conventional Fourier transforms for the representation, and thus the analysis, of seismic processes. It provides a 2D representation, called a scalogram, of a 1D signal where the seismic events are well localized and isolated. Supervised methods based on this time‐scale representation have already been used to separate seismic events, but they require strong interactions with the geophysicist. This paper focuses on the use of the watershed algorithm to segment time‐scale representations of seismic signals, which leads to an automatic estimation of the wavelet representation of each wave separately. The computation of the inverse wavelet transform then leads to the reconstruction of the different waves. This segmentation, tracked over the different traces of the seismic profile, enables an accurate separation of the different wavefields. This method has been successfully validated on several real data sets.