应用沉降和水位数据计算上海地区弱透水层的参数

弱透水层参数的确定是准三维或三维地下水流数学模型应用时的难点,而应力-应变图解法是一种估计弱透水层参数的有效方法。在介绍该法的基本原理之后,结合上海沉降和水位观测资料,求出了上海地区弱透水层的弹性贮水率、非弹性贮水率和垂向渗透系数,为上海地面沉降的数值模拟提供了较好的初始参数值。对于弱透水层,抽水试验的方法是不可取的,如果又没有条件取土样作室内试验时,若有沉降资料和相应的水位资料,则应力-应变图解法是确定近似参数值的一种比较好的方法。     

Methods for determining the in situ hydraulic conductivity of shallow aquitards – an overview

Shallow clay-rich aquitards limit groundwater recharge to underlying aquifers, but they also protect the aquifers from contamination. The bulk hydraulic conductivity of such shallow aquitards can range from less than 1 mm/year to more than 100 m/year and may be much greater than the hydraulic conductivity of small intact samples of the aquitard material. This enhanced hydraulic conductivity diminishes the qualities of the aquitards for the protection of underlying aquifers but allows a higher rate of recharge. For aquifers that are overlain by aquitards, management and protection of groundwater resources may be critically dependent on reliable determinations of aquitard permeability. A variety of methods for determining bulk hydraulic conductivities of shallow clay aquitards is available; each has drawbacks and advantages, and each is based on simplifying assumptions. These methods include slug tests, pumping tests, response of the aquitard to mechanical loading, and analysis of natural pore-pressure fluctuations. Several of the commonly used methods require an independent measurement of specific storage. Laboratory methods for determining specific storage are probably not representative of in situ conditions and may lead to overestimation of aquitard permeability. Much of the theory developed to date depends on the assumption that horizontal displacement of the solid material is negligible, and this may not be a valid assumption for highly deformable media such as clay aquitards. However, with judicious selection of the most suitable methods for a particular site, good test design, careful instrumentation, and respect for the underlying assumptions, reliable determinations of aquitard permeability can be obtained. Electronic Publication     

In situ stress field of eastern Australia

New in situ data based on hydraulic fracturing and overcoring have been compiled for eastern Australia, increasing from 23 to 110 the number of in situ stress analyses available for the area between and including the Bowen and Sydney Basins. The Bowen Basin displays a consistent north‐northeast maximum horizontal stress (σ_H) orientation over some 500 km. Stress orientations in the Sydney Basin are more variable than in the Bowen Basin, with areas of the Sydney Basin exhibiting north‐northeast, northeast, east‐west and bimodal σ_H orientations. Most new data indicate that the overburden stress (σ_V) is the minimum principal stress in both the Bowen and Sydney Basins. The Sydney Basin is relatively seismically active, whereas the Bowen Basin is relatively aseismic. Despite the fact that in situ stress measurements sample the stress field at shallower depth than the seismogenic zone, there is a correlation between the stress measurements and seismicity in the two areas. Mohr‐Coulomb analysis of the propensity for failure in the Sydney Basin suggests 41% of the new in situ stress data are indicative of failure, as opposed to 13% in the Bowen Basin. The multiple pre‐existing structural grains in the Sydney Basin further emphasise the difference between propensity for failure in the two areas. Previous modelling of intraplate stresses due to plate boundary forces has been less successful at predicting stress orientations in eastern than in western and central Australia. Nonetheless, stress orientation in the Bowen Basin is consistent with that predicted by modelling of stresses due to plate boundary forces. Variable stress orientations in the Sydney Basin suggest that more local sources of stress, such as those associated with the continental margin and with local structure, significantly influence stress orientation. The effect of local sources of stress may be relatively pronounced because stresses due to plate boundary forces result in low horizontal stress anisotropy in the Sydney Basin.     

Evolution of the Tasmanides,eastern Australia

The Beltana zinc‐lead deposit, the largest of several such small deposits in the Beltana area, owes its formation to intense localised marginal faulting and brecciation of Ajax Dolomite at its contact with the Beltana Diapir. Considerable frictional heat was generated by the upward movement of this diapir. The enormous stresses set up in it squeezed out hot interstitial saline mineralising fluids into the brecciated dolomite footwall to form initially a prominent lead arsenate‐rich horizon containing hedyphane, finnemanite, vanadinite and several minor components. Sporadic, but significant manganese mineralisation also occurred during this phase resulting in the precipitation of coronadite and hollandite. During the final and most prominent stage in the paragenetic sequence, the mineralising fluids became more siliceous and zinciferous leading to extensive replacement of earlier arsenates and precipitation of willemite. Late‐stage alteration is largely confined to the earlier arsenate‐rich zone, which has been carbonated and leached by meteoric waters.     

Nonlinear consolidation in randomly heterogeneous highly compressible aquitards

Severe land subsidence due to groundwater extraction may occur in multiaquifer systems where highly compressible aquitards are present. The highly compressible nature of the aquitards leads to nonlinear consolidation where the groundwater flow parameters are stress-dependent. The case is further complicated by the heterogeneity of the hydrogeologic and geotechnical properties of the aquitards. The effect of realistic vertical heterogeneity of hydrogeologic and geotechnical parameters on the consolidation of highly compressible aquitards is investigated by means of one-dimensional Monte Carlo numerical simulations where the lower boundary represents the effect of an instant drop in hydraulic head due to groundwater pumping. Two thousand realizations are generated for each of the following parameters: hydraulic conductivity (K), compression index (C _c), void ratio (e) and m (an empirical parameter relating hydraulic conductivity and void ratio). The correlation structure, the mean and the variance for each parameter were obtained from a literature review about field studies in the lacustrine sediments of Mexico City. The results indicate that among the parameters considered, random K has the largest effect on the ensemble average behavior of the system when compared to a nonlinear consolidation model with deterministic initial parameters. The deterministic solution underestimates the ensemble average of total settlement when initial K is random. In addition, random K leads to the largest variance (and therefore largest uncertainty) of total settlement, groundwater flux and time to reach steady-state conditions.     

The distribution of saline groundwater and its relation to the hydraulic conditions of aquifers and aquitards: examples from Israel

The effect of separation by aquitard layers on the distribution of saline groundwater in coastal aquifers has been demonstrated in two Israeli coastal aquifers: the Mediterranean and the Dead Sea aquifers. There is vertical separation in the Dead Sea area, even where the clayey aquitard layers are <1?m thick, exhibited by large differences in hydraulic head (2?C5?m), salinity (TDS of 50?C340?g/L) and chemical composition (e.g. Na/Cl range 0.28?C0.55). Similar features are found in the Mediterranean coastal aquifer, where the separating aquitard layers are thicker (??5?C10?m). Here, the different subaquifers host fresh and saline groundwater of different ages (tritium and ¹⁴C ages range from tens to thousands of years), as well as different chemical compositions. This high resolution of results can be obtained only by drilling without fluids; otherwise, the spatial information may lead to incorrect representation of the studied aquifer. This is especially important in saline systems where only partial flushing occurs and, thus, large variations in salinity and chemical composition are expected. The main factors controlling the salinity of groundwater in subaquifers in coastal aquifers are their connection to the sea or saline lakes, existence of brines, salinization and flushing rates, and separation by aquitard layers.     

Marble weathering in an industrial environment,eastern Australia

Qualitative evidence from monuments and buildings in industrialized countries indicates that rates of stone deterioration rise in the presence of urban and industrial pollutants. Measurements presented here on surface reduction of marble tombstones show that mean weathering rates have increased over the period 1885 to 1955. Weathering rates were lower before the establishment of sulfur dioxide-emitting plants. Marble weathering involved solution etching along grain boundaries and within individual grains, and proceeded by surface roughening and dislodgement of grains with little apparent accumulation of gypsum. Absolute surface reduction of marble was less than that recorded in industrial environments in Europe and North America.     

Role of aquitards in hydrogeochemical systems: a synopsis

Aquitards exert significant influence on the hydrogeochemistry of aquifer systems. This influence is manifested somewhat differently depending on the relative position of aquitards within a system. In the deeper regimes, they are influential in the origin and distribution of brines and the development of geopressured zones. In intermediate regimes, they form multi-layered aquifer systems and provide a source of reactive minerals and exchangeable ions. In shallow regimes, aquitards can influence the topography and drainage patterns; this influences the relationship between the water table and the potentiometric surface of confined aquifers, controls the rates of infiltration and discharge, and controls whether the geochemical system is open or closed to exchange of carbon dioxide gas. In coastal areas, aquitards can determine the depth of the saltwater-freshwater interface, its distance from the shoreline, and the position of the mixing zone that causes geochemical alteration of minerals and development of porosity.     

Physical Modelling of Stress-dependent Permeability in Fractured Rocks

This paper presents the results of laboratory experiments conducted to study the impact of stress on fracture deformation and permeability of fractured rocks. The physical models (laboratory specimens) consisted of steel cubes simulating a rock mass containing three sets of orthogonal fractures. The laboratory specimens were subjected to two or three cycles of hydrostatic loading/unloading followed by the measurement of displacement and permeability. The results show a considerable difference in both deformation and permeability trends between the first loading and the subsequent loading/unloading cycles. However, the micrographs of the contact surfaces taken before and after the tests show that the standard deviation of asperity heights of measured surfaces are affected very little by the loadings. This implies that both deformation and permeability are rather controlled by the highest surface asperities which cannot be picked up by the conventional roughness characterization technique. We found that the dependence of flow rate on mechanical aperture follows a power law with the exponent n smaller or larger than three depending upon the loading stage. Initially, when the maximum height of the asperities is high, the exponent is slightly smaller than 3. The first loading, however, flattens these asperities. After that, the third loading and unloading yielded the exponent of around 4. Due to the roughness of contact surfaces, the flow route is no longer straight but tortuous resulting in flow length increase.     

Stress-dependent morphological features on fracture surfaces of quartz and glass

Stress dependent morphological features have been found on tensile fracture surfaces of natural Brazilian quartz and synthetic quartz specimens which were flexed in air at room temperature. These features are similar to stress dependent morphological features found on fracture surfaces of pyrex, soda-lime and lead glass when deformed in similar experiments. For glass, the relevant morphological feature is the fracture “mirror”, a planar region surrounding the initial flaw. Stress dependent index features chosen for measurement in quartz depend upon the crystallographic orientation on the fracture surface. Their development is related to the crack-system energy but, in contrast to glass, the features are not directly related to crack branching. The experimental results follow an equation of the form: where σ_f is the fracture stress, r* is the size of a particular morphological feature (e.g. the “mirror” radius in the case of glass) and A* is a constant.From the magnitudes of A* and the fracture toughness determined in these experiments we conclude that the natural Brazilian quartz used in this study has significantly higher resistance to fracture than synthetic quartz.     

Refining accretionary orogen models for the Tasmanides of eastern Australia

The well-known southwest-to-northeast younging of stratigraphy over a present-day cross strike distance of >1500 km in the southern Tasmanides of eastern Australia has been used to argue for models of accretionary orogenesis behind a continually eastwards-rolling paleo-Pacific plate. However, these accretionary models need modification, since the oldest (ca 530 Ma) outcrops of Cambrian supra-subduction zone rocks occur in the outboard New England Orogen, now ～900 km east of the next oldest (520–510 Ma) supra-subduction zone rocks. This is not consistent with simple, continuous easterly rollback. Instead, the southern Tasmanides contain an early history characterised by a westwards-migrating margin between ca 530 and ca 520 Ma, followed by rapid eastwards rollback of the paleo-Pacific plate from 520 to 502 Ma that opened a vast backarc basin ～2000 km across that has never been closed. From the Ordovician through to the end of the Carboniferous, the almost vertical stacking of continental margin arcs (within a hundred kilometres of each other) in the New England Orogen indicates a constant west-dipping plate boundary in a Gondwana reference frame. Although the actual position of the boundary is inferred to have undergone contraction-related advances and extension-related retreats, these movements are estimated to be ～250 km or less. Rollback in the early Permian was never completely reversed, so that late Permian–Triassic to Cretaceous arcs lie farther east, in the very eastern part of eastern Australia, with rifted fragments occurring in the Lord Howe Rise and in New Zealand. The northern Tasmanides are even more anomalous, since they missed out on the middle Cambrian plate boundary retreat seen in the south. As a result, their Cambrian-to-Devonian history is concentrated in a ～300 km wide strip immediately west of Precambrian cratonic Australia and above Precambrian basement. The presence in this narrow region of Ordovician to Carboniferous continental margin arcs and backarc basins also implies a virtually stationary plate boundary in a Gondwana frame of reference. This bipolar character of the Tasmanides suggests the presence of a segmented paleo-Pacific Plate, with major transform faults propagating into the Tasmanides as tear faults that were favourably oriented for the formation of local supra-subduction zone systems and for subsequent intraplate north–south shortening. In this interpretation of the Tasmanides, Lower–Middle Ordovician quartz-rich turbidites accumulated as submarine fan sequences, and do not represent multiple subduction complexes developed above subduction zones lying behind the plate boundary. Indeed, the Tasmanides are characterised by the general absence of material accreted from the paleo-Pacific plate and by the dominance of craton-derived, recycled sedimentary rocks.     

Subduction and accumulation of lawsonite eclogite and garnet blueschist in eastern Australia

Lawsonite eclogite and garnet blueschist occur as metre-scale blocks within serpentinite mélange in the southern New England Orogen (SNEO) in eastern Australia. These high-P fragments are the products of early Palaeozoic subduction of the palaeo-Pacific plate beneath East Gondwana. Lu–Hf, Sm–Nd, and U–Pb geochronological data from Port Macquarie show that eclogite mineral assemblages formed between c. 500 and 470 Ma ago and became mixed together within a serpentinite-filled subduction channel. Age data and P–T modelling indicate lawsonite eclogite formed at ~2.7 GPa and 590°C at c. 490 Ma, whereas peak garnet in blueschist formed at ~2.0 GPa and 550°C at c. 470 Ma. The post-peak evolution of lawsonite eclogite was associated with the preservation of pristine lawsonite-bearing assemblages and the formation of glaucophane. By contrast, the garnet blueschist was derived from a precursor garnet–omphacite assemblage. The geochronological data from these different aged high-P assemblages indicate the high-P rocks were formed during subduction on the margin of cratonic Australia during the Cambro-Ordovician. The rocks however now reside in the Devonian–Carboniferous southern SNEO, which forms the youngest and most outboard of the eastern Gondwanan Australian orogenic belts. Geodynamic modelling suggests that over the time-scales that subduction products accumulated, the high-P rocks migrated large distances (~>1,000 km) during slab retreat. Consequently, high-P rocks that are trapped in subduction channels may also migrate large distances prior to exhumation, potentially becoming incorporated into younger orogenic belts whose evolution is not directly related to the formation of the exhumed high-P rocks.     

Analysis of Stress-dependent Permeability in Nonorthogonal Flow and Deformation Fields

Summary The stress-dependent permeability of porous-fractured media is examined where principal stresses do not coincide with the principal permeabilities. This condition is the norm, and may arise when either flow is controlled at the local level due to the presence of inclined bedding partings or oblique fractures, or as a result of the evolving loading environment. Permeability response is controlled by shear and normal stiffnesses of fractures, frictional dilation coefficients, skeletal and grain modulii, initial permeabilities and stress state. For parameters representative of intact and fractured rocks, hydrostatic loading modes are shown to have the greatest effect in the pre-failure regime. Shear dilation effects are small, primarily controlled by the selected magnitudes of shear stiffnesses and dilation coefficients. The resulting stress-permeability relationships, which cover both fractured and intact media, are examined in a numerical study of fluid flow injected across the diameter of a cylindrical core with inclined fabric, subjected to various loading configurations. This is used to produce relationships that allow one to reduce flow test data in non-standard specimen geometries, where effective stress changes are simultaneously applied. These results confirm the significant impact of inclination of the rock fabric with respect to both flow and loading geometry on the evolving permeability field.     

Late Jurassic intraplate faulting in eastern Australia: A link to subduction in eastern Gondwana and plate tectonic reorganisation

Eastern Gondwana was subjected to subduction processes during the Middle-Late Jurassic, but how these processes affected intraplate deformation in eastern Australia is poorly understood. Here we present ⁴⁰Ar/³⁹Ar, K-Ar, and Rb-Sr geochronological data from illitic clay-bearing fault gouges associated with the northern part of the 200 km long, N-striking, dextral strike-slip, Demon Fault in eastern Australia. We show a major range of geochronological ages at 162.99 ± 0.74–152.1 ± 4.8 Ma, indicating that the Demon Fault was active during the Late Jurassic. This period partially coincides with the Middle-Late Jurassic deposition of widespread ash-fall tuffs in the Clarence-Moreton, Surat, and Eromanga basins. We propose that Middle-Late Jurassic intraplate tectonism in eastern Australia was influenced by subduction processes farther east, which produced extensive calc-alkaline magmatism in New Zealand from ~170 Ma. A global plate reorganisation event, related to the development of Early-Middle Jurassic sea-floor spreading of the Pacific Plate, possibly acted as the driving mechanism responsible for the intensification of magmatism and intraplate faulting in eastern Gondwana.     

Paleozoic terranes of eastern Australia and the drift history of Gondwana

Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps.     

Rare earth element-enriched cobaltiferous wads,Bungonia, eastern Australia

Wad occurs as a cement to fluvial sediments which unconformably overlie a complex Palaeozoic basement comprising flysch-type greywacke, limestone, shale, tuff and granodiorite. The Eocene fluvial sediments are variably cemented by Fe oxides, Mn oxides and silica. The area underwent 0.5–0.75 km uplift throughout the Tertiary and was intenseely lateritised in the Late Miocene. The Mn-rich cement comprises cobaltian todorokite — lithiophorite — birnessite in sandstones which contain up to 12% MnO and enrichments in Co (max. 1.8%), total REE (max. 0.5%), Ce (max. 0.2%), Nd (max. 0.1%), Ni (max. 0.3%), Cu (max. 0.2%) and Li (max. 0.1%). The total REE content, Ce content and NASC-normalized La/Lu ratio are positively correlated with the MnO/Fe₂O₃ ratio. Enrichment of REE (especially LREE) and other elements derives from Miocene lateritisation and resultant mobilization of elements from basalt (Fe, Co, Ni, Cu, REE), shale (REE), limestone (Mn) and granite (Li). These warm acid fluids were focussed in Eocene fluvial channels which acted as aquifers and redox barriers by the oxidative precipitation of Fe and Mn oxide cements. Further oxidation of Ce and adsorption of REE, Co etc. by todorokite have resulted in the extraordinary enrichment of REE in wad-cemented fluvial sandstones.

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Zusammenfassung Wads findet man als Zement fluvialer Sedimente, die ein komplexes paläozoisches Basement aus flyschartiger Grauwacke, Karbonaten, Schiefern, Tuffen und Granodioriten diskordant überlagern. Die eozänen fluvialen Sedimente sind unterschiedlich von Fe-Oxiden, Mn-Oxiden oder kieselig zementiert. Das gesamte Gebiet wurde während des Tertiärs um 0,5 bis 0,75 km gehoben und während des Obermiozäns intensiv lateritisiert. Der Mangan-reiche Zement besteht aus kobalthaltigem Todokorit-Lithiophorit-Birnessit in Sandsteinen, die bis zu 12% MnO und Anreicherungen in Co (max. 1,8%), REE insgesamt (max. 0,5%), Ce (max. 0,2%), Nd (max. 0,1%), Ni (max. 0,3%), Cu (max. 0,2%) und Li (max. 0,1%) enthalten. Der Gehalt an REE, Ce und das NASC-korrigierte La/Lu Verhältnis steht in einer positiven Korrelation mit dem MnO/Fe₂O₃ Verhältnis. Die Anreicherung von REE (besonders von LREE), sowie weiteren Elementen wird auf die miozäne Lateritisierung und die daraus resultierende Mobilisation von Elementen aus Basalt (Fe, Co, Ni, Cu, REE), Schiefer (REE), Karbonaten (Mn) und Granit (Li) zurückgeführt. Die warmen, sauren Lösungen bewegten sich in eozänen fluvialen Kanälen, die als Wassertransportwege und Redoxbarriere durch die Fällung von Fe- und Mn-Oxid-Zementen wirkten. Eine weitere Oxidation des Ce als auch die Adsorption von REE, Co etc. durch Todokorit führte zu der außergewöhnlichen Anreicherung der REE in Wad-zementierten fluvialen Sandsteinen.

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Résumé Les «wads» dont il est question dans cette note constituent le ciment de sédiments fluviatiles d'âge éocène discordants sur un socle paléozoïque complexe formé de grauwackes de type flysch, de calcaire, de shale, de tuf et de granodiorites. Ces sédiments fluviatiles sont cimentés de manières diverses par des oxydes de Fe, des oxydes de Mn et de la silice. La région a subi, au cours du Tertiaire, un soulèvement de 500 à 700 m et a été intensément latéritisée au Miocène supérieur. Le ciment, riche en Mn, est formé de todorokite — lithiophorite — birnessite cobaltifère, dans des grès qui renferment jusqu'à 12% MnO et sont enrichis en Co (max. 1,8%), en terres rares (max. 0,5%), en Ce (max. 0,2%), en Nd (max. 0,1%), en Ni (max. 0,3%), en Cu (max. 0,2%) et en Li (max. 0,1%). La teneur totale en terres rares, la teneur en Ce et le rapport La/Lu normalisé NASC montrent une corrélation positive avec le rapport MnO/Fe₂O₂.L'enrichissement en terres rares (particulièrement en terres rares légères) et en autres éléments est dû à la latéritisation miocène et à la mobilité consécutive d'éléments du basalte (Fe, Co, Ni, Cu, terres rares), du shale (terres rares), du calcaire (Mn) et du granite (Li). Les fluides chauds acides se sont rassemblés dans les chenaux fluviaux éocènes qui ont joué le rôle d'aquifères et de barrières redox lors de la précipitation des ciments d'oxydes de Fe et Mn. L'oxydation ultérieure du Ce et l'adsorption des terres rares, du Co, etc. par la todorokite sont à l'origine de l'extraordinaire enrichisement en terres rares dans les grès fluviatiles à ciment de wad.

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, , . , . 0,5 0,75 , . , , — — , : 12% , 1,8% , 0,5%, 0,2%, Nd 0,1%, Ni ,3%, u 0,2% Li 0,1%. , La/Lu, NASC, MnO/Fe₂O₃. , LREE, (Fe, , Ni, u, REE), (REE), () (Li). , . . , , . , .

     

Metamorphism in the New England Orogen,eastern Australia: a review

Abstract

This paper summarises current knowledge on metamorphism within the entire New England Orogen (NEO) of eastern Australia. Rocks recording metamorphic assemblages characteristic of each of the three metamorphic facies series (high, medium and low P/T) have been identified within the orogen. These include high P/T blueschists and eclogites, mid P/T orogenic metamorphism and low P/T contact aureoles and sub-regional high-temperature–low-pressure (HTLP) metamorphism (regional aureoles). Metamorphism is described as it relates to six tectonic phases of development of the NEO that together comprise two major cycles of compression–extension. Medium–high-grade contact metamorphism spans all six tectonic phases while low-grade burial and/or orogenic metamorphism has been identified for four of the six phases. In contrast, exposure of high P/T eclogites and blueschists, and generation of sub-regional low P/T metamorphism is restricted to extensional phases of the orogen. Hallmarks of the orogen are two newly identified zones of HTLP metamorphism, the older of which extends for almost the entire length of the orogen.

1. KEY POINTS

2. The orogen is dominated by low-temperature rocks while high-temperature amphibolite to granulite facies rocks are restricted to small exposures in HTLP complexes and contact aureoles.

3. Blueschist metamorphism falls into two categories; that associated with subduction during the Currabubula-Connors continental arc phase occurring at depths of ～13–30?km; and the other of Cambrian–Ordovician age, exposed within a serpentinite melange and associated with blocks of eclogite. The eclogite, initially from depths of ～75–90?km, appears to have been entrained in the deep crust for an extended period of geological time.

4. A comprehensive review of contact metamorphism in the orogen is lacking and as studies on low-grade metamorphism are more extensive in the southern part of the orogen than the north, this highlights a second research gap.

     

Early Mesoproterozoic metamorphism in the Barossa Complex,South Australia: links with the eastern margin of Proterozoic Australia

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ～6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.