ALBERT

Description: U-Pb zircon data from the uppermost Cottons Breccia, representing the Marinoan glacial-postglacial transition on King Island, Tasmania, provide the first direct age constraint on the Cryogenian-Ediacaran boundary in Australia. Zircons in four samples from the topmost meter of the Cottons Breccia, dated by sensitive high-resolution ion microprobe, exhibit two modes ca. 660 Ma and ca. 635 Ma. The younger component predominates in the uppermost sample, a possibly volcanolithic dolomitic sandstone, apparently lacking glacially transported debris, in the transition to cap carbonate. Chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb dating of euhedral zircons from that sample yields a weighted-mean age of 636.41 ± 0.45 Ma. Equivalence to published TIMS ash bed dates from Cryogenian-Ediacaran transitional strata in Namibia (635.51 ± 0.82 Ma, within glacial deposit) and China (635.23 ± 0.84 Ma, 2 m above glacial deposit) supports correlation of those strata to the Australian type sections and globally synchronous deglaciation at the end of the Cryogenian Period.

Description: The Pussy Cat Group rhyolites of the Mesoproterozoic west Musgrave Province of central Australia, a constituent part of the Bentley Supergroup, were deposited during the c . 1085–1040 Ma Ngaanyatjarra Rift and Giles events, and are related to the Warakurna Large Igneous Province. This study focuses on the two silicic components of the Pussy Cat Group, the Kathleen Ignimbrite and the Rowland Suite. These silicic rocks are A-type, metaluminous (to slightly peraluminous) rhyolites and are enriched in the rare earth elements (REE) relative to average crustal abundances. The rhyolitic Kathleen Ignimbrite records an explosive caldera fill-sequence and contains, amongst others, a thick (≤500 m), initially subaqueously emplaced, rheomorphic, intra-caldera ignimbrite unit, whereas the Rowland Suite consists of a number of mineralogically and geochemically related porphyritic rhyolites that intrude throughout the Pussy Cat Group. Whole-rock geochemistry, Rb–Sr, Sm–Nd and in situ zircon Lu–Hf isotope data are indicative of a dominantly mantle-derived source for the magmas that formed the Pussy Cat Group rhyolites. Secondary ion mass spectrometry U–Pb dating of these units yields ages of 1062 ± 8, 1071 ± 5, 1076 ± 5, and 1078 ± 5 Ma. The magmas that formed these units were formed by extreme fractional crystallization of a mantle-derived basaltic magma, with minimal crustal contamination, during a failed intra-plate extensional rifting event. This involved three main stages of fractional crystallization: early fractionation of plagioclase, olivine, clinopyroxene and magnetite from a basaltic magma to reach an intermediate composition, subsequent fractionation of plagioclase, K-feldspar and quartz to form a proto-Rowland Suite-type magma at mid- to upper-crustal levels that migrated into the shallow upper crust and formed a magma chamber, and final fractionation of quartz, K-feldspar, plagioclase, magnetite and biotite ± minor REE-enriched accessory phases from the Rowland Suite magma resulting in the evolved Kathleen Ignimbrite magmas. This final phase of fractionation generated the most evolved silicic rock suite identified to date within the entire west Musgrave Province. The new petrographic, geochronological, geochemical, and isotopic data presented within this study indicate that these two units are coeval and comagmatic, suggesting a common source for the Kathleen Ignimbrite and the entire Rowland Suite. In addition, these data suggest that the crystal-rich, porphyritic rhyolite intrusions of the Rowland Suite represent a primitive cumulate end-member of the magmatic system, whereas the varying crystal-poor to crystal-rich Kathleen Ignimbrite eruption sequence represents the evolved and highly fractionated end-member of the system that formed thorough the evacuation of a shared or at least partly linked, compositionally zoned and differentiated source magma chamber or chambers.

Description: The Talbot Sub-basin is one of several bimodal volcanic depositional centres of the Mesoproterozoic Bentley Basin in central Australia. It is dominated by rocks of rhyolitic composition and includes ignimbrites, some forming large to super-eruption size deposits. Ferroan, incompatible trace element enriched, A-type compositions, anhydrous mineralogy and clear evidence for local rheomorphism indicate high eruption temperatures, with apparent zircon-saturation temperatures suggesting crystallization at 〉900°C. Comagmatic basalt is of mantle origin with minor Proterozoic basement contamination. The rhyolites cover the same range of Nd isotope compositions ( Nd(1070) +1·24 to –0·96) and La/Nb ratios (1·2–2·1) as the basalts ( Nd(1070) +2·1 to –1·1: La/Nb 1·2–2·3) and are compositionally far removed from all older basement and country-rock components (average Nd(1070) = –4, La/Nb = 10). The rhyolites and basalts are cogenetic through a process probably involving both fractional crystallization of mafic magmas and partial melting of recently crystallized mafic rock in a lower crustal intraplate, extraction of dacitic magmas to a voluminous upper crustal chamber system, and separation of rhyolite by processes involving rejuvenation and cannibalization of earlier chamber material. More than 230 000 km 3 of parental basalt is required to form the 〉22 000 km 3 of preserved juvenile rhyolite in the Talbot Sub-basin alone, which represents one of the most voluminous known felsic juvenile additions to intracontinental crust. Zircon U–Pb age components are complex and distinct from those of basement and country rock and contain antecrystic components reflecting dissolution–regrowth processes during periodic rejuvenation of earlier-emplaced chamber material without any significant interaction with country rock. The overall duration of magmatism was 〉30 Myr but can be divided into between two and four separate intervals, each probably of a few hundred thousand years’ duration and each probably reflecting one of the distinct lithostratigraphic groups defined in the sub-basin. Neither the composition nor style of felsic and mafic volcanism changes in any significant way from one volcanic event to the next and the range of zircon U–Pb ages indicates that each period utilized and cannibalized the same magma chamber. This volcanism forms a component of the 1090–1040 Ma Giles Event in central Australia, associated with magma-dominated extension at the nexus of the cratonic elements of Proterozoic Australia. This event cannot be reasonably reconciled with any putative plume activity but rather reflects the 〉200 Myr legacy of enhanced crustal geotherms that followed the final cratonic amalgamation of central Australia.

Description: The Musgrave Province lies at the convergence of major structural trends formed during the Proterozoic amalgamation of the North, West and South Australian Cratons prior to c. 1290 Ma. The Musgrave Orogeny, one of three Mesoproterozoic orogenies to affect the province, produced the granites of the Pitjantjatjara Supersuite, which dominate the outcrop. This orogeny was an intracontinental and dominantly extensional event in which ultrahigh-temperature (UHT) conditions persisted from c. 1220 to c. 1120 Ma. The onset of UHT conditions is heralded by a change from low-Yb granites to voluminous Yb-enriched granites, reflecting a rapid decrease in crustal thickness. The Pitjantjatjara granites are ferroan, calc-alkalic to alkali-calcic rocks and include charnockites with an orthopyroxene-bearing primary mineralogy. They were emplaced at temperatures ≥1000°C from c. 1220 to c. 1150 Ma. Their geochemical and Nd and Hf isotopic homogeneity over a scale of 〉15 000 km 2 reflects a similarly homogeneous source. This source included an old enriched felsic crustal component. However, the bulk source was mafic to intermediate in composition. The long-lived UHT regime, and thermal limits on the amount of crust sustainable below the level of intrusion, indicates a significant (〉50%) mantle-derived source component. However, a positive correlation between Mg-number and F suggests that many Pitjantjatjara granites formed through the breakdown of F-rich biotite in a crustal granulite. We suggest that under- and intraplated mafic magmas assimilated the limited available felsic crust into lower crustal MASH (melting, assimilation, storage, homogenization) domains. These partially cooled but were remobilized during subsequent under- and intra-plating events to produce the Pitjantjatjara granites. The duration of UHT conditions is inconsistent with a mantle plume. It reflects an intracontinental lithospheric architecture where the Musgrave Province was rigidly fixed at the nexus of three thick cratonic masses. This ensured that any asthenospheric upwelling was focused beneath the province, providing a constant supply of both heat and mantle-derived magma.

Description: Paulsens is a mesothermal orogenic gold deposit located in the Wyloo Inlier on the southern margin of the Pilbara craton of Western Australia. Gold occurs in quartz-sulfide veins hosted within a folded and faulted gabbro dike, from which baddeleyite yields a U-Pb crystallization age of 2701 ± 11 Ma. Monazite and xenotime in the veins and from hydrothermally altered country rocks yield three distinct U-Pb dates of ca. 2400, 1730, and 1680 Ma. Textural relationships between euhedral xenotime and pyrite with rounded native gold inclusions from within the quartz-sulfide veins show that the primary gold mineralization was synchronous with xenotime crystallization at 2403 ± 5 Ma, and coeval with pervasive alteration of the host rocks, which yield monazite ages of 2398 ± 37 and 2403 ± 38 Ma. Regional-scale hydrothermal events at ca. 1730 and 1680 Ma are linked to the growth of monazite within phyllitic rocks at 1730 ± 28 and 1721 ± 32 Ma, carbonate veining at 1655 ± 37 Ma, and gold remobilization or introduction of new gold at 1680 ± 9 Ma. The ca. 2400 Ma age for mineralization and hydrothermal alteration does not correspond with any known deformation event in the region, indicating a significantly different and more complicated low-temperature tectonothermal evolution for the southern Pilbara region than previously recognized. The in situ secondary ion mass spectrometry dating of monazite and xenotime employed here will lead to better targeting of orogenic gold deposits in the northern Capricorn Orogen, and these techniques can be utilized for orogenic gold exploration worldwide.

Description: Neoarchean rocks of the Tropicana Zone, including granites with subduction-zone affinities, formed in a terrane adjacent to, or on the margin of, the Yilgarn Craton at the commencement of a long-lived, amphibolite to granulite facies event – the 2722–2554 Ma Atlantis Event. Early stages of this event overlap with extensive komatiite emplacement within the Eastern Goldfields Superterrane (Yilgarn Craton), suggestive of a plume-related rift environment, which was followed by 2660–2630 Ma greenschist facies, orogenic gold mineralization. This indicates differences in the tectonic evolution of the Tropicana Zone compared with within the craton, although isotopic data show similarities in crustal sources. At c. 2520 Ma, the Tropicana Zone was retrogressed to greenschist facies as it was thrust onto the Yamarna Terrane (Yilgarn Craton), forming a northwesterly directed fold-and-thrust belt above the flat-lying Plumridge Detachment. This fold-and-thrust belt is host to the c. 2520 Ma, Tropicana gold deposit. The Plumridge Detachment may extend north to the Yamarna greenstone belt, linking to the Yamarna Shear Zone – the boundary between the Burtville and Yamarna Terranes. The fertility of the Tropicana Zone is related to its Neoarchean geodynamic setting within a continental arc environment, implying that deformed margins of Archean cratons may be prospective for Neoarchean Au deposits.

Description: The voluminous Kalkarindji flood basalts erupted in Australia during the Cambrian and covered 〉2 x 10 6 km 2 . New U-Pb and 40 Ar/ 39 Ar age data from intrusive rocks and lava flows yielded statistically indistinguishable ages at ca. 511 Ma, suggesting a relatively brief emplacement for this province. A zircon age of 510.7 ± 0.6 Ma shows that this province is temporally indistinguishable at the few-hundred-thousand-year level from the Early–Middle Cambrian (Stage 4–5) boundary age of 510 ± 1 Ma, which marks the first severe extinction of the Phanerozoic and an extended marine anoxia period. Sulfur concentration measurements ranging from 〈50 to 1900 μg/g, and fractal analysis of extensive explosive volcanic breccias, suggest that blasts and phreatomagmatic explosions have contributed to injection of large amounts of sulfur into the stratosphere. In addition, magma intrusions in oil, gas, and sulfate deposits may have generated significant emission of CH 4 and SO 2 which, along with volcanic gases, would have combined to cause an oscillation of the climate and led to the Cambrian extinction.