Island Granite Research Articles

High-precision U–Pb geochronology of the Lundy igneous complex: implications for North Atlantic volcanism and the far-field Paleocene–Eocene ash record

The Lundy Island (Bristol Channel, UK) granite is a felsic expression of the southernmost igneous centre of the North Atlantic Igneous Province that emplaced millions of cubic kilometres of magma during the Paleogene. The granite's distinctive S-type, peraluminous, two-mica ± garnet ± tourmaline composition has led to the hypothesis that eruptions from the Lundy volcanic centre may be the source of thick felsic ash layers within the early Eocene Fur Formation (Denmark) that act as key marker horizons for the onset and duration of the Paleocene–Eocene Thermal Maximum. This paper presents high-precision zircon U–Pb emplacement ages of 57.24 ± 0.11/0.12/0.13 Ma for the granite and 55.970 ± 0.021/0.030/0.070 Ma for a felsic ‘lundyite’ dyke. Trace and rare earth element patterns indicate close similarities between late-stage Lundy activity and ash layer −33 in Denmark. This ash layer was deposited during the Paleocene–Eocene Thermal Maximum carbon isotope excursion, suggesting that the Lundy volcanic centre is likely to be the source of this key ash horizon and that magmatism at Lundy likely continued into the early Eocene.

Geometry Analysis of Penang Island Faults Based on Satellite Gravity Data

Abstract The gravity method is commonly used in geophysics for fault mapping. This method provides an overview of the subsurface density distribution. When applied to fault areas, the density gradient contrast can be used to map the fault geometry structure. The research area for this study is Penang Island, Malaysia, which is predominantly composed of granite. The Penang Island granite formation was created during the Triassic period, approximately 209 to 22 million years ago (Kwan and Yap, 1986). It underwent uplift during the Oligocene-Miocene period, exposing the previously buried granite. The second formation is a quaternary sediment that is believed to have formed during the Pleistocene and Holocene (Kamaluddin, 1990). Penang Island has 14 faults scattered throughout, with 5 main faults in the middle (Ong, 1993). The study aimed to determine the fracture geometry using the gravity method. The analysis revealed two main rock types: granite with a density range of 2-2.8 gr/cm3 and quaternary sediment with a density of 1.85-2 gr/cm3. Fault geometry analysis was conducted using the SVD method on four passages in Penang Island to identify fault types. The results showed six normal faults and seven thrust faults. Faults No. 3, 4, 8, 9, 12, and 13 are predicted to be normal faults, while Faults No. 1, 2, 5, 7, 10, 11, and 14 are predicted to be thrust faults. The dominant direction of the fault is North-South. The fault geometry on North Pluton has an average depth of 250 m with an average dip angle of 68°, while the fault on South Pluton has a depth of over 400 m with an average dip angle of 65°.

Geological insights from the newly discovered granite of Sif Island between Thwaites and Pine Island glaciers

AbstractLarge-scale geological structures have controlled the long-term development of the bed and thus the flow of the West Antarctic Ice Sheet (WAIS). However, complete ice cover has obscured the age and exact positions of faults and geological boundaries beneath Thwaites Glacier and Pine Island Glacier, two major WAIS outlets in the Amundsen Sea sector. Here, we characterize the only rock outcrop between these two glaciers, which was exposed by the retreat of slow-flowing coastal ice in the early 2010s to form the new Sif Island. The island comprises granite, zircon U-Pb dated to ~177–174 Ma and characterized by initial ɛNd, 87Sr/86Sr and ɛHf isotope compositions of -2.3, 0.7061 and -1.3, respectively. These characteristics resemble Thurston Island/Antarctic Peninsula crustal block rocks, strongly suggesting that the Sif Island granite belongs to this province and placing the crustal block's boundary with the Marie Byrd Land province under Thwaites Glacier or its eastern shear margin. Low-temperature thermochronological data reveal that the granite underwent rapid cooling following emplacement, rapidly cooled again at ~100–90 Ma and then remained close to the Earth's surface until present. These data help date vertical displacement across the major tectonic structure beneath Pine Island Glacier to the Late Cretaceous.

Human activities at night negatively impact Little Penguin (Eudyptula minor) numbers and behaviours

The presence of humans within the natural environment is increasing worldwide. Assessing the impact of such activities on wildlife is crucial for declining populations where human disturbance adds to existing pressures. Here, we investigated how human activities at night influenced Little PenguinEudyptula minornumbers and behaviours (specifically return time, number of vocalizations and time spent in vigilance) on Granite Island, a declining population in South Australia, Australia. We combined data from regular night surveys with continuous video and audio monitoring to assess the impact of human activities on the Little Penguins. The use of white light (i.e. from torches or camera flashes) by people was the most frequent activity recorded at night (recorded on 65% of the monitored nights). Fewer penguins were found on land at night when DogsCanis lupus familiariswere present, but not when the number of people increased, when concerts occurred, or when white lights were used. Little Penguins were observed more often returning late from sea at night when Dogs were present and when white lights were used, but not when concerts occurred. An increase in penguin vocalizations at night correlated with the presence of Dogs and the occurrence of concerts, whereas penguins vocalized less when white lights were used. The time Little Penguins spent in vigilance did not correlate with any of the disturbances analysed. Our study also highlights the impact of coronavirus disease 2019 (COVID‐19) on wildlife, as the occurrence of human activities increased significantly following the implementation of the COVID‐19 health protection measures. These results add to a growing body of literature suggesting that human activities on land, and their consequent disturbance(s), may affect the numbers and behaviours of wildlife and that appropriate measures need to be developed to limit such impacts.

The Alteration of Giglio Island Granite: Relevance to the Conservation of Monumental Architecture

The research examines the alteration phenomena of Giglio island granite, a rock quarried by Romans from the 3rd century, used for columns in the Italian peninsula and later reemployed in many Christian religious buildings. The study has shown that already in the bedrock there are small percentages of clay minerals. Starting from this condition, the alteration develops by an increase in porosity, which in turn favours the establishment of a slight hydrolysis of the silicates with a decrease in Na, Ca, and K, in accordance with the sericitisation process. The alteration proceeds with a further increase in porosity, apparently not related to a real loss of cohesion, which, however, occurs shortly after, highlighting the necessity of a continuous monitoring of the state of conservation of the material in the architectural heritage.

Correlation between Uniaxial Compressive Strength and Point Load Strength of Penang Island Granites

Uniaxial compressive strength (UCS) is one of the most widely used strength parameters in geotechnical design. The value is obtained through direct testing method, carried out through careful procedures of testing in a laboratory setting. As such, it is relatively more expensive, more tedious and requires longer time to complete. Considering that UCS is highly important, Engineers often utilize indirect methods, namely the Point Load Test, to obtain the Point Load Index Strength (Is(50)). From this test, the results are correlated in order to predict the UCS value. One of the general correlations is provided by ISRM where UCS=20-25 Is(50). However, it is highly contested as it is not universal for all types of rocks. The main focus of this research is to find the correlation of UCS and Is(50) in the Penang Island area. This research would also attempt to classify the granites according to strength. This study showed that the Penang Island Granites are “weak to very strong” granites with UCS values between 24.76 to 156.82MPa. The granites from the North Pluton are classified as “medium strong to very strong” with UCS values between 37.11 to 156.82MPa. The granites from the South Pluton are classified as “weak to very strong” with UCS values between 24.76 to 141.59 MPa. The recommended correlation between UCS and Is(50) are (1) Overall: 8.385 Is(50) + 30.016 (2) North Pluton: 7.93 Is(50)+ 35.606 (3) South Pluton: 9.03Is(50)+20.138.

Little penguins are more aggressive on islands that experience greater unregulated human disturbance

Animals living near urban habitats face many pressures linked with human disturbance. While individuals vary in their ability to cope with disturbance, it is predicted that individuals in highly disturbed environments will be bolder and more aggressive than those in less disturbed environments. Using little penguins, Eudyptula minor , we investigated whether individual aggressiveness was consistent across trials and whether aggressiveness was greater in colonies that experience higher levels of unregulated human disturbance. We scored aggressiveness at the nest, based on the penguins’ response to a standardized nest intrusion experiment, across four penguin colonies in South Australia. Nest defence scores were significantly repeatable over two trials, suggesting that it was a good indicator of personality in little penguins. Island was a significant predictor of nest defence score, with little penguins on Granite Island (the island with the greatest amount of unregulated human disturbance) being more aggressive than those on Troubridge Island (the island with the lowest amount of human disturbance). Considering that humans increasingly interact with animals, whether directly or indirectly, understanding the mechanisms behind nonrandom distribution of behaviours may have important implications for population management and conservation. • Animals in highly disturbed environments should be bolder and more aggressive. • In little penguins, nest defence scores were repeatable. • Aggressiveness correlated with levels of unregulated human disturbance. • Behavioural variation in relation to disturbance may be important for conservation.

Petrogenesis of the Cretaceous granitoids in Zhejiang, northeast South China Block and their implications for episodic retreat and roll-back of the Paleo-Pacific Plate

Abstract Two Cretaceous granitoid belts (i.e., the northwest and southeast belts) have been identified in Zhejiang, northeast South China Block. In this study, seven granitoid plutons from both the two belts were collected for zircon U-Pb dating, whole-rock geochemistry, Sr-Nd isotope, and zircon Hf isotope analyses. Chronologically, the Longyou (132 Ma), Sucun (136 Ma), Shanghekou (131 Ma), and Huangshitan (ca. 126 Ma) plutons from the northwest belt display older magma crystallization age than those of the Xiaoxiong (100 Ma), Zhujiajian (108 Ma), and Qingbang island (108 Ma) plutons from the southeast belt. The Sucun quartz monzonite and the Longyou, Shanghekou, Zhujiajian, and Qingbang island granites therein are fractionated I-type granites (i.e., partial melting of meta-igneous rocks) with relatively moderate-low Zr saturation temperature (723–823 °C) and pronouncedly evolved Nd and Hf isotopic compositions (εNd(t) = –8.17 to –5.67 and εHf(t) = –15.07 to –5.67), indicating that they are derivatives of ancient crustal melt-dominated magmas. The Huangshitan granite shows A-type granitic (i.e., granites that are alkaline and anhydrous and from anorogenic setting) features with high Ga/Al (3.47–5.58), rare earth element (REE) content (271–402 ppm), and Zr saturation temperature (781–889 °C). It holds less enriched Nd and Hf isotopic compositions (εNd(t) = –4.13 to –3.60 and εHf(t) = –5.90 to –2.16) and is attributed to partial melting of mature crustal materials with minor basaltic magma incorporation. The Xiaoxiong (quartz) syenitic porphyry is characterized by moderate SiO2 content (60.68–69.92 wt%), high alkali (9.03–11.66 wt%) and REE contents with fractionated REE pattern [(La/Yb)N = 13.8–26.1]. Its relatively depleted Nd and Hf isotopic compositions (εNd(t) = –3.67 to –3.42 and εHf(t) = –5.76 to –2.25) imply that it could be a derivative of basaltic magma from K-rich metasomatized mantle. Available geochronological data indicate that there were two episodic magmatic pulses at ca. 140–120 Ma and ca. 110–85 Ma associated with the Paleo-Pacific Plate underthrusting beneath the northeast South China Block. Here we put forward an episodic slab retreat and roll-back model to account for generation of these magmatic rocks. Firstly, the subducting Paleo-Pacific slab roll-back initiated at ca. 140 Ma and reached climax at ca. 130–120 Ma, which led to formation of the Longyou, Sucun, and Shanghekou I-type granites and the Huangshitan A-type granite, respectively. Subsequently, a flat slab subduction stage occurred with eastward trench retreat, causing a period of magmatic quiescence from ca. 120 to 110 Ma. The following second slab roll-back started at ca. 110 Ma and reached climax at ca. 100 Ma, giving rise to the earlier Zhujiajian and Qingbang island I-type granites and the later Xiaoxiong (quartz) syenitic porphyry.

Identification of terrestrial predators at two Little Penguin colonies in South Australia

Identifying predators and determining their influence on bird population declines can be crucial in the implementation of appropriate conservation measures. In this study, we focus on the Little Penguin Eudyptula minor , a flightless seabird whose numbers have seriously declined in some populations across Australia and New Zealand, for a variety of reasons, some not fully understood. Using a combination of motion-camera monitoring at active burrows and postmortem examinations of carcasses, we present data on the identity, occurrence and impact of terrestrial predators at two colonies in South Australia: Granite Island and Emu Bay (Kangaroo Island). Terrestrial predation varied from 20% for carcasses found on Granite Island to 44% at Emu Bay. There was no evidence for predation at burrows on Granite Island but, at Emu Bay, we identified goannas (Heath Monitor Varanus rosenbergi) as predators of chicks (by using cameras), and Cats Felis catus as predators of chicks and juveniles (from postmortem examination of carcasses). In addition, 45% of the carcasses found at Emu Bay were categorised as chicks. We discuss potential biases in identifying causes of death from carcasses only and the importance of combining both direct and indirect evidence.

Low survival rather than breeding success explains little penguin population decline on Granite Island

Over the past decade, populations of little penguins (Eudyptula minor) have been seriously declining across South Australia for reasons still not fully understood. In the present study, I investigated breeding performance and return rates of little penguins on Granite Island for 17 years in relation to patterns of population decline. I focussed on the impacts of human disturbance, breeding site, abandonment and predation on breeding success. The average breeding success was 1.05±0.12 fledglings per pair; and breeding success increased since 1990, despite population decline. Breeding site was the main factor affecting breeding success. I found no effect of predation or abandonment on breeding success, but there was a negative effect of predation on the number of dead chicks found. In addition, I found a negative effect of human disturbance. Despite the observed increase in breeding success, return rates were extremely low for both adults (16.3%) and fledglings (2.3%). Population modelling confirmed the observed population decline on Granite Island, with subadult survival being the most critical variable affecting population growth. The present study thus highlighted the need for further studies into factors affecting survival of adults and subadults.

New age data refine extent and duration of Paleozoic and Neoproterozoic plutonism at Ganderia–Avalonia boundary, Newfoundland

Devonian and younger plutonic suites of the Appalachian orogen in Newfoundland form a curvilinear belt straddling the boundary between Ganderia and Avalonia, and exhibit a general pattern of younging to the southeast. However, many individual plutonic suites within this belt remain undated or have ages that are poorly constrained. This study reports U–Pb sensitive high-resolution ion microprobe (SHRIMP) data from zircon and 40Ar–39Ar data from hornblende, biotite, and muscovite, which collectively improve definition of post-tectonic plutonic events in southeastern Newfoundland, but at the same time raise new questions for further study. U–Pb zircon SHRIMP determinations provide valuable new geochronological information for the Deadman’s Bay Granite (390 ± 4 Ma), the Gander Lake Granite (378 ± 4 Ma), the Terra Nova Granite (384 ± 4 Ma), the Old Woman stock (376 ± 4 Ma), and the easternmost unit (Tolt phase) of the composite Ackley Granite (372 ± 4 Ma). The Pass Island Granite, long considered part of this age group, gave a Silurian age of 423 ± 4 Ma and provides the first confirmation of Silurian granitoid magmatism in Avalonia of Newfoundland. The Berry Hills Granite on the Burin Peninsula, previously designated as Devonian, instead gave a Neoproterozoic age of 570 ± 6 Ma, and a similar age of 575 ± 6 Ma was obtained from granite that is intruded by the ca. 376 Ma Old Woman stock. These results indicate that regional geological context, deformation state, and (or) petrographic–geochemical data are not reliable indicators of emplacement age in this region. 40Ar–39Ar data from the Paleozoic granites provide mixed results in comparison to the U–Pb emplacement ages, and in some cases Ar systematics appear to have been disturbed by later hydrothermal alteration. In other cases, correspondence between the U–Pb and 40Ar–39Ar data sets is consistent with rapid postemplacement exhumation. Granophile-type mineralization (Mo, W, F, base metals) is present within both Ganderia and Avalonia examples, but is generally restricted to the youngest plutons, in which it is associated with high-level, near-roof, or apical settings. This age versus mineralization pattern is consistent with an interpretation in which mineralized environments within older suites have been preferentially eroded as a consequence of greater exhumation. The identification of a Silurian (ca. 423 Ma) granite in an area previously assigned to Avalonia may be important in constraining tectonic models for Ganderia–Avalonia interaction, which has generally been considered Silurian or younger. Given that many other plutonic suites in Avalonia are not well constrained by regional geology and remain undated, more work is needed to better define the extent of such magmatism and evaluate its implications for broader models of orogenic development.

Molybdenite in Pomona Island Granite at Lake Manapouri, Fiordland

A small occurrence of molybdenite (MoS2) mineralisation has been discovered in the weakly A-type Pomona Island Granite on the shorelines of Lake Manapouri in eastern Fiordland. The disseminated appearance of molybdenite and the absence of quartz veins indicates that mineralisation is probably the product of magmatic and/or hydrothermal activity related to pluton crystallisation at c. 157 Ma, and not younger (c.128–116 Ma) shear zone-related mesothermal mineralisation as has been recently described from the Murchison Mountains to the north and Stewart Island to the south. Although apparently not of economic grade, the Pomona Island Granite locality is regionally important because it is the first direct pluton-related Mo-mineralisation event to be recognised in eastern Fiordland. This occurrence adds to the growing number and known styles of base metal occurrences within the Jurassic–Cretaceous magmatic arc (Outboard Median Batholith) that formed on or near the New Zealand Gondwana margin. Furthermore, the wide distribution of essentially uninvestigated A-type granites in the Outboard Median Batholith means that there may be further Mo-mineralised localities awaiting discovery.

From richer to poorer: zircon inheritance in Pomona Island Granite, New Zealand

Previous U–Pb zircon dating of the Pomona Island Granite (PIG) pluton (South Island, New Zealand) yielded either Permo-Carboniferous or Late Jurassic ages for five samples essentially indistinguishable in their field, petrographic, and geochemical characteristics. Detailed cathodoluminescence imaging and LA-ICP-MS dating of zircon in new and previously dated samples reveal that portions of the pluton contain either delicately oscillatory-zoned Late Jurassic zircon grains with rare Permo-Carboniferous cores, or Permo-Carboniferous grains with ubiquitous but thin Late Jurassic rims. Based on zircon dissolution-overgrowth textures, zircon rim and core trace element compositions, and the limited extent of sub-solidus rock recrystallisation textures, the bipartite age distribution is unlikely to reflect variable Pb-loss or metamorphic re-equilibration. Magmatic Zr-saturation temperatures were ≥851°C for samples dominated by Jurassic zircon and ≤809°C for samples with a predominance of Permo-Carboniferous zircon. Together, these data are consistent with PIG magmas having been derived from partial melting of a Permo-Carboniferous felsic igneous source at variable temperature wholly in the Late Jurassic (157 ± 3 Ma). The lowest temperature melts would have been incapable of dissolving significant amounts of pre-existing zircon and consequently generated inheritance-rich magmas, with the very thin rims on the pre-existing zircon grains the only evidence of the Late Jurassic magmatic age. As the partial melting temperature increased and nearly all pre-existing zircon grains dissolved into the magma, an inheritance-poor batch of melt was generated, which precipitated new zircon grains upon crystallisation. Concentrations of major and many trace elements in both magma batches may have been buffered by retention of residual quartz and feldspar in the source, which would explain the limited geochemical differences between inheritance-rich and inheritance-poor portions.

Passing of Sovereignty: The Malaysia/Singapore Territorial Dispute before the ICJ

The 2008 Malaysia/Singapore case before the International Court of Justice relates to the dispute concerning territorial sovereignty over three maritime features in the Straits of Singapore, namely Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge. Pedra Branca/Pulau Batu Puteh (hereafter Pedra Branca) is a granite island, measuring 137 m long, with an average width of 60 m. It is situated at the eastern entrance of the Straits of Singapore – one of the busiest maritime passages in the world. It lies approximately 24 nautical miles to the east of Singapore, 7.7 nautical miles to the south of the Malaysian state of Johor and 7.6 nautical miles to the north of the Indonesian island of Bintan. Middle Rocks consists of two clusters of small rocks that are permanently above water, and is located 0.6 nautical miles to the south of Pedra Branca.

Written in Granite: An Illustrated History of the Ether Monument.

Written in Granite: An Illustrated History of the Ether Monument.

Neoproterozoic and Cambrian U–Pb (zircon) ages from Grand Manan Island, New Brunswick: implications for stratigraphy and northern Appalachian terrane correlations

Pre-Mesozoic rocks are exposed on the southeastern third of Grand Manan Island and adjacent offshore islands in the Bay of Fundy. U–Pb (zircon) ages reported here demonstrate that most of these rocks are late Neoproterozoic to Early Cambrian. The oldest dated unit is the Ingalls Head Formation, from which two felsic tuff samples yielded ages of 617.6 ± 3.2 and 618.3 ± 2.8 Ma. The Three Islands granite, exposed only on offshore islands, is younger than the Ingalls Head Formation at 611.1 ± 2.4 Ma. It provides a minimum age for marble of the Kent Island Formation, which occurs as large xenoliths in the granite. The High Duck Island granite yielded an age of 547.3 ± 1.1 Ma, providing a minimum late Neoproterozoic age for the host Long Island Bay Formation. An age of 539.0 ± 3.3 Ma from a dacitic lithic-crystal tuff demonstrates that the Priest Cove Formation is earliest Cambrian; this supersedes a Silurian age previously suggested on the basis of poorly documented fossil evidence. The Long Pond Bay Formation is considered to be the youngest pre-Mesozoic unit on Grand Manan Island, based on petrological features, but a felsic intrusive sample from the unit yielded ca. 588 Ma xenocrystic zircon grains that provide only a maximum age for the unit. Although lack of exposed contacts or U–Pb ages from a few units precludes a complete stratigraphic picture, the ages reported here deomonstrate that most pre-Mesozoic units on Grand Manan Island are not Paleozoic as previously assumed. Some units on Grand Manan Island show similarities in rock types and ages to components in the Islesboro block in Penobscot Bay in coastal Maine. Overall, the range of ages and rock types suggests that they correlate with the New River and Mascarene terranes of southern New Brunswick, requiring an offset of at least 40 km between southern New Brunswick and adjacent Maine along the Grand Manan – Oak Bay fault system. Hence, both the Ganderia–Avalonia and Avalonia–Meguma boundaries lie farther offshore than Grand Manan Island.

Surgeon island granite SHRIMP zircon ages: a clue for the Cambrian tectonic setting and evolution of the Palaeopacific margin of Gondwana (northern Victoria Land, Antarctica)

AbstractIn northern Victoria Land (NVL), Antarctica, the palaeopacific margin of Gondwana is made up of the inboard Wilson (WT) and the two outboard Bowers (BT) and Robertson Bay (RBT) terranes. The occurrence of a Cambro‐Ordovician magmatic arc in the WT argues for a southwestward subduction leading to the final configuration of this margin during the Ross–Delamerian Orogeny. A U‐Pb SHRIMP crystallization age (511.7 ± 2.9 Ma) obtained on the Surgeon Island Granite (SIG), located at the eastern end of the RBT, indicates that the SIG also belongs to the Ross cycle, and provides evidence for multiple subduction zones during the Ross Orogeny. Structural observations show that the SIG and its country rocks are basement to the RBT turbidites. SIG inherited zircon ages indicate the occurrence of Proterozoic crust east of the RBT and constrains the location of the Proterozoic–Palaeozoic boundary in Cambrian Gondwana.

Provenance analysis using conglomerate clast lithologies: a case study from the Pahau terrane of New Zealand

The field of provenance analysis has undergone a revolution with the development of single-crystal isotope dating techniques using mainly silt to sand sized single minerals. This study focuses on coarser-grained rocks, notably conglomerates, which involve much shorter transport distances and therefore may be used to trace proximal sources. The Torlesse terranes—part of the New Zealand Eastern Province—are accretionary complexes that comprise an enormous volume of quartzofeldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages. Two terranes are recognised in the South Island, the Permian to Late Triassic Rakaia terrane and the Late Jurassic to Early Cretaceous Pahau terrane. Geochronology, geochemistry and Sr–Nd isotopes of igneous clasts from Pahau terrane conglomerates identify the Median Tectonic Zone (s.l.) as a major contributor of detritus to the Pahau depositional basin. SHRIMP U–Pb zircon ages of igneous clasts range from 128 to 123 Ma and from 147 to 135 Ma. Calc-alkaline clasts of the latter range are indistinguishable in age, chemical composition, and petrogenesis from the calc-alkaline granitoids of the Darran Suite, whereas alkaline clasts correlate with the Electric Granite, both parts of the Median Tectonic Zone. An Early Jurassic calc-alkaline rhyolite clast from Kekerengu (188±3 Ma) correlates with the Bounty Island Granite, Campbell Plateau. The petrography and geochemistry of Pahau terrane sandstone clasts indicate the recycling of Permian to early Late Triassic older Rakaia terrane rocks. This is further supported by the detrital zircon age data. Igneous conglomerate clasts are capable of placing exceptionally tight constraints on Pahau provenance and on its Mesozoic tectonic setting within the Southwest Pacific margin of Gondwana. They provide the best evidence yet that the Pahau terrane is locally derived.

A barren part of the main?: the vegetation of Hunter Island, Fleurieu Group, Bass Strait

Hunter Island, visited over millennia by Aboriginal people and settled in the nineteenth century by sealers and later by a succession of grazing lessees, has preserved in its vegetation pattern the effects of a long history of firing and clearing. The vegetation also reflects the substrate and topographic features of the island with a major dichotomy between the heaths and scrub on the highly acid quartzites that underlie most of the island, and woodlands and grasslands on the highly alkaline sands that are rafted onto its western part. Eucalyptus viminalis occurs on alkaline sands on Hunter Island, in contrast to the absence of the species on calcareous coastal sands in the rest of its range in southeastern Australia. Heathlands make up 380/0 and scrub 37% of the island. Other vegetation types recorded are swamp forests, button grass moorland, exotic and native grasslands, Eucalyptus viminalis woodland, wetlands, mutton-bird rookery vegetation and lichen-fields. The flora has strong affinities with the northwestern coast of the Tasmanian mainland and a weaker affinity with Three Hummock Island, which is a granite island rather than a quartzite one. A Bassian floristic element occurs on the alkaline substrates; however, it is not as strong as that in the island flora of eastern Bass Strait. The flora and vegetation have components that are significant for nature conservation such as remnant Eucalyptus viminalisstands on the sands. The species ofconservation significance include rare and threatened species: Pterostylis cucullata, Calochilus herbaceous, Cyrtostylis robusta, Parietaria debilis, Ranunculus amphitrichus and Cotula vulgaris var. australasica.

The Magondi belt in northeast Botswana: regional relations and new geochronological data from the Sua Pan area

Available data bearing on the extent of the 2.0-1.8 Ga Magondi Orogenic Belt beneath Phanerozoic cover in Botswana are reviewed. Also, new U-Pb and Sm-Nd geochronological data from isolated outcrops of granite on Kubu Island in Sua Pan, ca 60 km west of the main exposures of Precambrian basement in northeast Botswana, are presented. The Kubu Island Granite previously has been considered as a continuation of the Archæan Zimbabwe Craton exposed to the east. U-Pb isotopic analyses of single zircons from the granite yield a crystallisation age of 2039.2 ± 1.4 Ma, indicating that the granite was intruded during Palæoproterozoic orogenesis. One zircon grain interpreted as a xenocryst has a 207Pb/ 206Pb data of 2673 Ma. Sm-Nd isotopic data for the granite yield a T dm model age of 2682 Ma. These data indicate the presence of Archæan crustal components in the lithospheric column sampled by the granite magma, consistent with intrusion of the granite near the edge of the Zimbabwe Craton. Together with regional geophysical evidence, the U-Pb zircon age for the Kubu Island Granite constrains the eastern margin of the Magondi Orogenic Belt to lie between Kubu Island and exposed parts of the Zimbabwe Craton farther east.