Palaeozoic Granites Research Articles

Fingerprinting windblown dust in south-eastern Australian soils by uranium-lead dating of detrital zircon

The U-Pb ages of fine-grained zircon separated from 2 dust-dominated soils in the eastern highlands of south-eastern Australia and measured by ion microprobe (SHRIMP) revealed a characteristic age ‘fingerprint’ from which the source of the dust has been determined and by which it will be possible to assess the contribution of dust to other soil profiles. The 2 soils are dominated by zircon 400–600 and 1000–1200 Ma old, derived from Palaeozoic granites and sediments of the Lachlan Fold Belt, but also contain significant components 100–300 Ma old, characteristic of igneous rocks in the New England Fold Belt in northern New South Wales and Queensland. This pattern closely matches that of sediments of the Murray-Darling Basin, especially the Mallee dunefield, suggesting that weathering of rocks in the eastern highlands has contributed large quantities of sediment to the arid and semi-arid inland basins via internally draining rivers of the present and past Murray–Darling River systems, where it has formed a major source of dust subsequently blown eastwards and deposited in the highland soils of eastern Australia.

Evidence for a major crustal dislocation in the Hodgkinson Province, North Queensland

Several Late Palaeozoic granites which intrude strata of the Silurian‐Devonian Hodgkinson Province, north Queensland, display pronounced west‐northwest‐east‐southeast orientations, as do a suite of brittle structures that have affected both the plutons and country rocks. These features define a 20 km‐wide, west‐northwest‐trending zone, here named the Desailly Structure, which traverses the Hodgkinson Province and extends west across the Palmerville Fault into the Proterozoic Yambo Inlier. Deformation within the Desailly Structure was heterogeneously partitioned into zones of west‐northwest‐east‐southeast faulting separated by tracts of competent country rock. The latter contain a pervasive north‐south‐trending structural grain which locally controlled pluton emplacement and resulted in a meridional orientation of many granitoid bodies. Initiation of the Desailly Structure is attributed to have occurred syn‐ to post‐D2 of the regional deformation history. It was reactivated in the Hunter‐Bowen Orogeny (D4), with the zone expressing an overall sinistral sense of displacement.

Complex trace-element effects of mixing-fractional crystallization composite processes: applications to the Alaer granite pluton, Altay Mountains, Xinjiang, northwestern China

Complex patterns of intersection of linear with power-function curvilinear data point arrays are observed on ratio-ratio correlation plots of trace elements in the Alaer Palaeozoic granite pluton, Altay Mts., Xinjiang, northwestern China. Data points in the linear array are relatively lower in SiO 2 content and differentiation index (D.I.), and they become progressively decreased rightwards along the array in abundances of Zr and Hf, which could be better explained by mixing between zircon-rich restite and the granite melt. Data points in curvilinear arrays are higher in SiO 2 content and D.I. value and, therefore, have presumably suffered more extensive fractional crystallization. The complex patterns cannot be resolved with the classical models. Two models for the complex trace-element effects of mixing-fractional crystallization are developed in the paper, with which the patterns can be resolved. In all models, we begin with a linear data-array formed by simple two-component mixing, and then superpose fractional crystallization trends to generate a family of curves. Model (1) ( D 1 ≤ D 3, D 2 ≤ D 3 or D 1 ≥ D 3, D 2 ≥ D 3): in the general situation, fractional crystallization curves assume a cluster of concave-up or convex-up power-function curves which intersect the initial mixing line sloping positively. Special cases 1–3 ( D 1 = D 2 ≠ D 3 or D 1 ≠ D 2 = D 3 or D 2 ≠ D 1 = D 3): the fractional crystallization curve is transformed into a straight line. Special case 4 ( D 1 = D 2 = D 3): the fractional crystallization line coincides with the initial mixing line. Model (2) ( D 1 > D 3 > D 2 or D 1 < D 3 < D 2): fractional crystallization curves assume the form of hyperbolic curves which intersect the initial mixing line sloping negatively. In all of the two models the iso-fraction lines are uniformly steeper ( D 1 < D 2) or more gentle ( D 1 > D 2) than the initial mixing line. Iso-fraction lines increase ( D 1 < D 2) or decrease ( D 1 > D 2) progressively in slope with the decrease of residual liquid mass fraction F. Through modelling of the data point arrays on ratio-ratio correlation plots of trace elements, one can discern and distinguish between simple mixing, pure fractional crystallization and mixing-fractional crystallization composite processes. Initial compositions of the magma and the mixing modes can also be constrained. Through modelling the functional form, concavity or convexity, and the power index of the fractional crystallization curves, the relative incompatibility of trace elements can be estimated semi-quantitatively and the incompatibility sequence can be established. It is demonstrated through modelling that the Alaer granite pluton experienced mixing-fractional crystallization composite processes. The mixing mode is probably described by the restite model. The incompatibility sequence of the 7 concerned trace elements is D Th < D La < D Y ≈ D Nb ≈ D Ta < D Hf < D Zr. This incompatibility sequence may be applicable to the anatectic granites.

A palaeogeographic link between Australia and eastern North America: a New England connection?

Abstract. This research tests the hypothesis that Australia and eastern North America met in rotational collision during the Palaeozoic with the corollary that the New England fold belt of Australia, rather than the Reguibat promontory of Africa, collided with the Alleghanian orogen in the central Appalachians. Identical Lancefieldian‐stage, zone 1 (Early Ordovician) graptolites of the Anisograptid family are found in identical environments in Newfoundland, Norway, and the Lachlan fold belt of Australia. Palaeozoic granites are consistent with a tectonic model in which the Lachlan fold belt caused mechanical deformation of the Canadian Appalachians. The Lachlan and New England fold belts of Australia and the Alleghanian orogen of North America are tectonically consistent with the east coast hypothesis. Major deformations and magmatic episodes are coeval from Silurian to Permian. The tectonic, palaeontologic, lithologic, and geometric evidence for this position is more abundant and precise than the stratigraphic evidence for a west coast location of Australia relative to North America.

Ion microprobe UPb zircon geochronology of granitic magmatism in the Western Province of the South Island, New Zealand

Zircon UPb isotopic systems are described for three mid-Palaeozoic and four late Mesozoic granites from the Western Province of the South Island, New Zealand. The granites chosen are representative of the major plutons in this region. Two distinct episodes of Palaeozoic magmatism have been identified: Middle Devonian (∼ 380 Ma) and Early Carboniferous (∼ 330 Ma). The Mesozoic granites are all mid-Cretaceous in age, and range from ∼ 120 to ∼ 110 Ma. The O'Sullivans Granite, forming a large part of the Karamea Batholith, has a crystallisation age of 377.7 ± 4.1 Ma, and the Windy Point Granite from the Paparoa Batholith has an age of 328.6 ± 4.4 Ma. Further west, the granite at Cape Foulwind has a similar age of 327.3 ± 6.2 Ma. From east to west, the Cretaceous granites appear to decrease in age. The Separation Point Batholith is dated at ∼ 118 Ma. The oldest phase at Separation Point gave an age of 116.6 ± 1.9 Ma and the youngest phase, the Pearse Granodiorite, gave the same age within error at 119.4 ± 2.3 Ma. The Mount Olympus pluton in NW Nelson gave an age of 111.4 ± 2.0 Ma and the same age is found for the Buckland Granite, which forms most of the Paparoa Batholith, at 109.6 ± 1.7 Ma. Inherited zircon components as old as 1700 Ma have been identified in many of the granites, and distinct components are present at 1000 and 500–600 Ma. This pattern of zircon ages has also been observed in granites from the Lachlan Fold Belt and in Ordovician sediments from SE Australia and New Zealand. However, it is not clear whether the old zircons are derived from the lower crust, from the incorporation of upper-crustal material, or both. The Palaeozoic granites can be broadly correlated with rocks of similar age in Antarctica, SE Australia and Tasmania that formed part of the Gondwana supercontinent. The Cretaceous granites are probably part of an extensive Mesozoic magmatic arc that can be traced from South America through West Antarctica and on into New Zealand.

Early Palaeozoic terranes in New Zealand and their relationship to the Lachlan Fold Belt

Two tectonostratigraphic terranes of Palaeozoic age are recognised in the Western Province of New Zealand, separated by the north-trending Anatoki Thrust. The Buller (western) terrane is composed largely of a relatively uniform suite of quartz-rich elastics (mainly turbiditic sandstone), siltstone and black shale and ranges from basal Ordovician to Late Ordovician in age. It is an equivalent of the Ballarat-Bendigo belt of Victoria. The Takaka (eastern) terrane contains a varied assemblage of Middle and Late Cambrian volcanics, volcaniclastics, carbonates and turbidites, Ordovician carbonate, shale and sandstone, and Silurian quartzite. The closest equivalent in Australia is Early Palaeozoic rocks of west Tasmania. The main deformational events, in the latest Ordovician or Silurian (Buller terrane), latest Silurian (Takaka terrane) and ?Middle Devonian (both terranes), can be equated with the Benambran, Bowning and Tabberabberan Orogenies of eastern Australia respectively. Palaeozoic granites are restricted to Buller terrane, and are spread over a belt 300–450 km wide. Age range (on sparse isotopic data) is 370−310 Ma (Late Devonian-Carboniferous), with the bulk in the Early Carboniferous. S-type granite predominates, with several A-type granites and a single I-type. The similarity in the nature and timing of the main events in the sedimentary, tectonic and volcanic history of New Zealand clearly link it with the Lachlan Fold Belt. On the other hand, the age and chemistry of the S-type New Zealand Palaeozoic granites make it unlikely that they are the direct equivalents of granite terranes of the Lachlan Fold Belt or Antarctica.

Proterozoic granite types in Australia: implications for lower crust composition, structure and evolution

ABSTRACTGranites and their associated comagmatic felsic volcanic rocks occur in most Proterozoic provinces of Australia. Using multi-element, primordial-mantle-normalised abundance diagrams and various petrological characteristics, Australian Proterozoic granites can be subdivided into five groups: (i) I-type, Sr-depleted, Y-undepleted, restite-dominated, (ii) I- type, Sr-depleted, Y-undepleted, fractionated, low in incompatible elements, (iii) I-type Sr-depleted, Y-undepleted, enriched in incompatible elements (anorogenic granites), (iv) I-type, Sr-undepleted, Y-depleted, (v) S-type, Sr-depleted, Y-undepleted. The four Sr-depleted groups dominate, and group (iv) is of very limited extent. A comparison of these Proterozoic granites with Australian and Papua New Guinean granites of other time periods shows that these characteristic Sr-depleted Y-undepleted patterns are also dominant in early Palaeozoic granites. They are significantly different from those of granites in modern island arcs associated with subduction, and with most granites from Archaean terranes, where the multi-element diagrams are dominated by Sr-undepleted, Y-depleted patterns.The Sr-depleted, Y-undepleted patterns are thought to indicate source regions that contained plagioclase but not garnet, whilst the Sr-undepleted, Y-depleted patterns are taken to correspond with the presence of garnet, but not plagioclase, in the source rocks. The Sr-depleted, Y-undepleted patterns also only occur in regions where the lower crustal structure is dominated by an underplated mafic layer with a P-wave velocity of 7·2-7·-4 km/s. In contrast, in regions where the granites are dominated by Sr-undepleted, Y-depleted patterns, such as in the Archaean and in Cainozoic island arcs, this intermediate velocity layer is not present, and the crust-mantle boundary is very sharp.Two other distinctive compositional changes have been noted among the I-type granites of different age. Firstly, Na is highest in Archaean and Cainozoic granites, and lowest in early Proterozoic granites; Palaeozoic and Mesozoic granites have intermediate values. Secondly, late Archaean and Proterozoic granites are the most enriched in K, Th and U, while the Cainozoic and early Archaean tonalites are the most depleted; Palaeozoic and Mesozoic granites again contain intermediate amounts of those elements.

Zircon inheritance in mafic inclusions from Bega batholith granites, southeastern Australia: An ion microprobe study

Ion microprobe U‐Th‐Pb dating zircons from four mafic inclusions from the Palaeozoic granites of the Glenbog and Blue Gum suites, Bega Batholith, shows that while most of the zircons are probably magmatic, the cores of some are premagmatic. The mean 206Pb/238U ages of the magmatic zircons are 412±3 Ma and 414±2 Ma for two inclusions from the Glenbog Granodiorite, 406±6 Ma for an inclusion from the Anembo Granodiorite, and 377±3 Ma for an inclusion from the Blue Gum Tonalite. The mafic inclusions and inherited zircons come from the granites' source rocks. The abundance of inherited zircons in the inclusions is less than in the granites. Most of the inherited zircons, as in the granites, are 430–630 Ma old, which probably represents the time interval of underplating of the crust prior to granite genesis, or a period of prolonged metamorphism of earlier underplated material. Rare Proterozoic inherited zircons are probably from a sedimentary component in the source. The inclusions do not appear to contain inherited zircons ∼1000 Ma old, which are common in the granites.

The influence of Precambrian source components on the U-Pb zircon age of a Palaeozoic granite from Northern Victoria land, Antarctica

Zircons from an S-type granite in Antarctica originally analysed by conventional U-Pb procedures have been reanalysed by ion-microprobe. The new data show that the Proterozoic age previously derived for granitoid emplacement is not correct but reflects a meaningless value between the magmatic age of the granite (544±4 Ma) and that of the provenance of its source rocks. Subsequent growth of metamorphic zircon has compounded the complexity. A total of six different episodes of zircon growth are documented within this rock. These occurred at 469 ± 4 Ma, 544 ± 4 Ma, 1130 ± 50 Ma, about 2000 Ma, about 2500 Ma and 2825 ± 100 Ma. The data illustrate the ever-present danger of extrapolating isotopic data alignments to concordia, especially when they form a ‘reverse’ discordia, in which the data points more closely approach the younger part of the concordia than the upper part. Although not itself of Precambrian age, the granite, which forms part of the Wilson Plutonic Complex of northern Victoria Land, Antarctica, formed by melting of rocks of currently unknown depositional age, but which were apparently derived from a dominantly ∼ 1100 Ma provenance.

The petrology and geochemistry of granitic gneisses from the East Arunta inlier, central Australia: implications for Proterozoic crustal development

The Entia Gneiss Complex is an Early to Middle Proterozoic inlier composed of upper amphibolite facies orthogneisses and paragneisses in the eastern end of the Arunta Block in central Australia. It is separated from overlying, structurally distinct, compositionally different and probably younger, gneisses of the Harts Range Complex, by a subhorizontal detachment zone. This paper discusses the petrology of four major granitic gneiss bodies. One of these (the Bruna Gneiss) has intruded this detachment zone, the other three are components of the structurally underlying Entia Gneiss Complex. The Entia Gneiss Complex has a distinctive subhorizontally layered character with recumbent folds and flat-lying schistosity. The granitic gneisses that are components of this complex appear to have intruded an actively deforming terrain and are folded sheet-like bodies. The orthogneisses of the Entia Complex range from gabbro to true granite in composition and the granites are I-type on both geochemical and petrographic criteria. They are significantly different from the relatively alkali-rich granites of equivalent age which occur in some of the other northern Australian Proterozoic Blocks and are most like the granites of the modern cordilleran belts, particularly in the association of diorite and gabbro with granodiorite. It is suggested they are formed in narrow collisional belts formed as a result of closure of intracratonic rift basins. The high surface-area to volume ratio of these granites, which is a consequence of the deformational environment into which they were intruded, has resulted in considerable wall rock exchange. In the more differentiated end members of the suites this has led to contamination by a net gain of K 2O, Rb and Ba and loss of relatively compatible elements including the REE, Y, Nb and TiO 2. Compared with the syn-tectonic Entia granites, the Bruna Granite Gneiss is post-tectonic and shows geochemical affinities with A-type or anorogenic Palaeozoic granites. Most of this granite is confined to the detachment zone, but the very latest phases also intrude the Harts Range Cover sequence. An important conclusion of this study is that detachment zones of the type described from this area of the eastern Arunta Block, exercise strong control on plutonism into the Early to Middle Proterozoic crust. As long as the very mobile style of Early Proterozoic tectonics continues, the intracrustal dislocation zone confines syn-tectonic granitic magmatism to deeper levels.

Origin of infracrustal (I-type) granite magmas

ABSTRACTI-type granites are produced by partial melting of older igneous rocks that are metaluminous and hence have not undergone any significant amount of chemical weathering. In the Lachlan Fold Belt of southeastern Australia and the Caledonian Fold Belt of Britain and Ireland there was a major magmatic event close to 400 Ma ago involving a massive introduction of heat into the crust. In both areas, that Caledonian-age event produced large volumes of I-type granite and related volcanic rocks. Granites of these two areas are not identical in character but they do show many similarities and are markedly different from many of the granites found in Mesozoic and younger fold belts. These younger, dominantly tonalitic, granites have compositions similar to those of the more felsic volcanic rocks forming at the present time above subduction zones. The Palaeozoic granites show little evidence of such a direct relationship to subduction. Within both the Caledonian and Lachlan belts there are some granites with a composition close to the younger tonalites. A particularly interesting case is that of the Tuross Head Tonalite of the Lachlan Fold Belt, which can be shown to have formed from slightly older source rocks by a process that we refer to as remagmatisation which has caused no significant change in composition. Since remagmatisation has reproduced the former source composition in the younger rocks, the wrong inference would result from the use of that composition to deduce the tectonic conditions at the time of formation of the tonalite. Granites, particularly the more mafic ones, will generally have compositions reflecting the compositions of their source rocks, and attempts to use granite compositions to reconstruct the tectonic environment at the time of formation of the granite may be looking instead at an older event. This is probably also the case for some andesites formed at continental margins.Several arguments can be presented in favour of a general model for the production of I-type granite sources by underplating the crust, so that the source rocks are infracrustal. Such sources may contain a component of subducted sediments with the consequence that some of the compositional characteristics of sedimentary rocks may be present in I-type source rocks and in the granites derived from them. The small bodies of mafic granite and gabbro associated with island arc volcanism have an origin that can be related to the partial melting of subducted oceanic crust or of mantle material overlying such slabs and can be referred to as M-type. These rocks have compositions indistinguishable from those of the related volcanic rocks, except for a small component of cumulative material. The tonalitic I-type granites characteristic of the Cordillera are probably derived from such M-type rocks of basaltic to andesitic composition, which had been underplated beneath the crust. Some of the more mafic tonalites of the Caledonian-age fold belts may also have had a similar origin. More commonly, however, the plutonic rocks of the older belts are granodioritic and these probably represent the products of partial melting of older tonalitic I-type source rocks in the deep crust, these having compositions and origins analogous to the tonalites of the Cordillera. In this way, multiple episodes of partial melting, accompanied by fractionation of the magmas, can produce quite felsic rocks from original source rocks in the mantle or mantle wedge. These are essential processes in the evolution of the crust, since the first stages in this process produce new crust and the later magmatic events redistribute this material vertically without the addition of significant amounts of new crust.

Palaeozoic granites of the Umberatana region, South Australia: the role of volatiles in the crystallization of some alkaline-peralkaline granites

Palaeozoic alkaline to peralkaline sodic granites of the Umberatana area of South Australia have high MnO, P5O5, Nb, Ta, Be, F and B abundances as is typical of »A-type« or anorogenic granites. Abundant F and B in the magma permitted primary muscovite to crystallize at pressures that may have been as low as 1 Kbar because of the lowered solidus temperatures. These volatile-rich magmas precipitated both albite and two generations of K-rich alkali feldspar. The hydrothermal fluids released during crystallization of these magmas resulted in considerable element redistribution and recrystallization both in the plutons and the adjacent country rocks.

The Role of Manganese in the Paragenesis of Magmatic Garnet: A Reply

Previous articleNext article No AccessDiscussion and ReplyThe Role of Manganese in the Paragenesis of Magmatic Garnet: A ReplyCalvin F. Miller and Edward F. StoddardCalvin F. Miller Search for more articles by this author and Edward F. Stoddard Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 90, Number 3May, 1982 Article DOIhttps://doi.org/10.1086/628686 Views: 2Total views on this site Citations: 4Citations are reported from Crossref Copyright 1982 University of ChicagoPDF download Crossref reports the following articles citing this article:Pedro Quelhas, João Mata, Ágata Alveirinho Dias Magmatic Evolution of Garnet-Bearing Highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes, Journal of Earth Science 32, no.66 (Apr 2021): 1454–1471.https://doi.org/10.1007/s12583-020-1389-4T. Kebede, C. Koeberl, F. Koller Magmatic evolution of the suqii-wagga garnet-bearing two-mica granite, wallagga area, western Ethiopia, Journal of African Earth Sciences 32, no.22 (Feb 2001): 193–221.https://doi.org/10.1016/S0899-5362(01)90004-1John P. Hogan Insights from igneous reaction space: a holistic approach to granite crystallisation, Earth and Environmental Science Transactions of the Royal Society of Edinburgh 87, no.1-21-2 (Nov 2011): 147–157.https://doi.org/10.1017/S0263593300006568Graham S. Teale, Bernd G. Lottermoser Palaeozoic granites of the Umberatana region, South Australia: the role of volatiles in the crystallization of some alkaline-peralkaline granites, Geologische Rundschau 76, no.33 (Oct 1987): 857–868.https://doi.org/10.1007/BF01821069

Migration of late Cenozoic volcanism in the South Island of New Zealand and the Campbell Plateau.

In late Cenozoic volcanics of eastern Australia, Wellman and McDougall1 have shown a remarkable southward migration pattern of major shield volcanoes of alkaline olivine basalt association which they relate to movement of the continental crust over a fixed mantle hot-spot. Contemporaneous tholeiite lava field provinces do not show a similar pattern. Following their general selection criteria1, I believe a similar situation emerges for New Zealand examples. In the South Island of New Zealand and the adjacent Campbell Plateau and Chatham Rise are several late Cenozoic volcanic centres that rest on a peneplain in Mesozoic and Palaeozoic granites and metamorphic rocks2,3, which is partly mantled by late Cretaceous to mid-Tertiary shallow marine sediments4,5. These major centres of late Cenozoic alkalic volcanism show an eastward decrease in age from ∼28 Myr to ∼0.5 Myr, presumably reflecting movement of a linear mantle source with respect to the Pacific Plate.

Pre‐ to syn‐tectonic emplacement of early Palaeozoic granites in southeastern South Australia

Abstract Stratigraphic and structural observations indicate that the Encounter Bay Granites concordantly intruded the youngest formations of the Kanmantoo Group in the Mount Lofty Ranges metamorphic belt prior to the culmination of the first phase of folding and associated schistosity development recorded during the early Palaeozoic Delamerian Orogeny. Metamorphic textures in the metasediments of the Kanmantoo Group suggest that cordierite crystallized locally near the granites prior to and during the F 1 folding, whereas andalusite crystallized on a regional scale during the F 1 folding and in the post‐F 1 and pre‐F 2 static phase. Rb‐Sr isotope data for total‐rock, feldspar, and muscovite samples of the meta‐sediment‐contaminated border facies and the uncontaminated inner facies of the Encounter Bay Granites indicate that the granites were emplaced between 515 ± 8 m.y. and 506 ± 6 m.y. ago in the Late Cambrian epoch. Rb‐Sr and K‐Ar data for biotite from the granites record variable radiogenic Sr loss un...

Part 6. Present plate boundary seismic, volcanic and kinematic processes: Geochemistry of the volcanic rocks of the Sunda Island arc of Indonesia

The Sunda arc represents the collision zone between the India and Asia (or China) plates. The Sunda volcanic arc extends from north of Sumatra, along the southwestern coast of Sumatra, through Java, Bali, Lombok, Sumbawa, Flores and the Lesser Sunda Islands, after which it becomes known as the Banda arc. There are a variety of tectonic environments represented along the arc. In Sumatra the crust is ~40 km thick and Palaeozoic granites show it to be relatively old. The Benioff zone as defined by the location of earthquake foci extends only to relatively shallow depths of ~200 km. Beneath Java the crust is somewhat thinner, younger; the oldest exposed rocks being? Mesozoic, and the Benioff zone extends to ~600 km beneath the Java Sea to the north. Further east, the crust is thinner (~15 km), oceanic in velocity structure and again the Benioff zone extends to great depths.

Palaeozoic granites and their younger components - A study of Mandi and Rakcham granites from the Himachal Himalaya

DOI = 10.3126/hjs.v5i7.1278 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.82-83

Geochemical characters of muscovite from the Pan African Mandi Granite, and its emplacement and evolution

DOI = 10.3126/hjs.v5i7.1291 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.98

Coastal Dune Lakes as Exemplified from King Island, Tasmania

IT is useful to define types of land forms and to classify them genetically, for we thus provide ourselves with a convenient f amework f concepts against which the geomorphological details of particular regions can be matched and which brings out the relationship and comparative importance of the various operative factors. Never? theless it is true that simple types which fall unequivocally into the single categories of the textbooks are rarities in the field; our classificatory systems often conceal, or at least scarcely hint at, the variety of character and complexity of origin of the entities with which we grapple in nature. During a recent study of the coastal geo? morphology of King Island, Tasmania, this observation was emphatically brought home to the writer by the heterogeneity of the numerous coastal dune lakes of the island. Since the enquiry was mainly directed at questions of changes in the relative level of land and sea, no detailed investigations of these lakes were made and the lack of soundings is particularly felt; however, the examples discussed here may serve to illustrate the general point with which this brief account begins. King Island (Fig. i) consists essentially of a faulted and tilted peneplain developed on Precambrian metamorphics, Cambrian volcanics and tillites, and Palaeozoic granites (Debenham, 1910; Waterhouse, 1915; Carey, 1946). The old erosion surface stands highest in the south-east at altitudes of 300 to 550 feet but declines westwards and northwards so that along a good deal of the coast the foundation of country rock lies not much above, and in parts below, sea level. Extensive development of coastal dunes has occurred and around much of the periphery of the island the dunes form a rim rising higher than the country immediately inland. In this context it is not sur? prising to find innumerable swamps, lakes and lagoons * scattered through the island. The dunes fall into different systems of varying age and composition, but for our present purposes it will be sufficient to distinguish two main groups?the New and the Old Dunes.