The Lachlan Orogen,like many other orogenic belts,has undergone paradigm shifts from geosynclinal to plate-tectonic theory of evolution over the past 40 years. Initial plate-tectonic interpretations were based on lithologic associations and recognition of key plate-tectonic elements such as andesites and palaeo-subduction complexes. Understanding and knowledge of modern plate settings led to the application of actualistic models and the development of palaeogeographical reconstructions, commonly using a non-palinspastic base. Igneous petrology and geochemistry led to characterisation of granite types into ‘I’ and ‘S’, the delineation of granite basement terranes, and to non-mobilistic tectonic scenarios involving plumes as a heat source to drive crustal melting and lithospheric deformation. More recently, measurements of isotopic tracers (Nd, Sr, Pb) and U–Pb SHRIMP age determinations on inherited zircons from granitoids and detrital zircons from sedimentary successions led to the development of multiple ...

Tectonic evolution of the Lachlan Orogen, southeast Australia: Historical review, data synthesis and modern perspectives

Summary Information on pressure (P) and temperature (T) is a fundamental aspect of research on metamorphic terrains. Unfortunately, many workers employ thermobarometers that are not experimentally calibrated, are insensitive or too sensitive to P-T changes, depend on a priori assumptions of water pressure (such as most petrogenetic grids), or are rapidly reset on cooling. Many systems are based on inaccurate thermodynamic data, involve solids with inadequately characterized structural states, neglect effects of thermal expansion and compressibility, or require long extrapolations in P-T-X space. For instance, application of the widely used garnet-clinopyroxene KD thermometer may require extrapolation to temperatures where current thermodynamic models of pyroxenes and garnets remain uncertain. Current versions of the Mg/Fe exchange thermometer for biotite-garnet involve substantial compositional extrapolations for many applications and the biotite is easily reset while cooling from higher T. The most widely employed barometer is based on dilution of the reaction grossular + kyanite + quartz = anorthite, but failure to correct molar volumes for P-T-X may yield systematic errors of 1–2 kbar for barometry of crustal metamorphites. Application of this barometer to rocks equilibrated at T < 600–650°C is presently unwarranted in view of unknown a-X relations of garnets and plagioclases at these T. However, by careful selections, thermobarometry may be accurate to ±50°C and ±1 kbar in many metamorphic terrains if a variety of different equilibria can be applied. Well-calibrated barometers that are useful for T τ 600–650°C rely on continuous reactions based on equilibria such as almandine + rutile = ilmenite + sillimanite + quartz, garnet + quartz = ferrosilite + plagioclase, garnet + rutile = ilmenite + anorthite + quartz, and almandine + sillimanite = hercynite + quartz. An extensive survey of the recent literature on thermobarometry of individual metamorphic facies reveals the range of P-T encountered in each facies. Temperature estimates are in good agreement with the inferences of Turner (1968). Barometry reveals that the blueschist, amphibolite and granulite facies give way to the eclogite facies over the pressure range of 12–16 kbar.

The current status of thermobarometry in metamorphic rocks

The Internal Zone of the Betic Cordilleras consists of several superimposed major thrust sheets with different P-T-t evolutions. On the basis of an integrated field, miCroscopic and laboratory study, the tectono-metamorphic history of the Mulhacen Complex and Almanzora Unit has been reconstructed in detail. The Mulhacen Complex has been af€ected by at least five phases of penetrative deformation, which have been labelled Dx-,, D., DX+,, D,,, and DX++ D,-, and D, are related to continent-continent collision, which is indicated by high pressure-low temperature (WILT) and subsequent intermediate PIT metamorphic conditions. D,,, is related to crustal thinning and heterogeneous extension. During this event the ALmanzora Unit was juxtaposed against the Mulhacen Complex. This phase was succeeded by the establishment of low pressure-high temperature (LPIHT) conditions and at least two phases of folding and overthrusting. The Almanzora Unit shows a comparable tectono-metamorphic evolution pt D,,,. However, the PIT conditions prior to D.,, indicate a higher crustal position with respect to the Mulhacen Complex during the collisional event. Key w& Alpine orogeny; Betic Cordilleras, Spain; continent-continent collision; crustal thickening; crustal thinning; deformation structures; geothermobarometry; P-T-t path.

The geodynamic evolution of the Internal Zone of the Betic Cordilleras (south-east Spain): a model based on structural analysis and geothermobarometry

A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo-silicate mineralogy, illite crystallinity and bo parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low-pressure facies series type, with a geothermal gradient of ∼ 40°Ckm-1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.



Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite-pumpellyite assemblages in metabasites and it is suggested that the latter represent non-buffered, and therefore non-diagnostic, assemblages.

Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

The South Portuguese Zone (see Fig. 1) of the Iberian Variscan chain consists of an Upper Paleozoic sedimentary series with intercalations of felsic to mafic volcanics which were metamorphosed under low-grade conditions during Hercynian times. The metamorphic phenomena is complex and reflects the superposition of a regional (dynamothermal) event onto an early hydrothermal alteration stage, contemporaneous with massive sulphide ore formation and cooling of the Pyrite Belt volcanic piles.

Metamorphic Evolution of the South Portuguese/Pulo Do Lobo Zone

The b
0 of the potassic white micas was measured in 410 samples of low-grade pelitic schists from the Central Pyrenees, the Abukuma belt, Northern New Hampshire, the Eastern Alps, Otago, the Sanbagawa belt and the Central Pontic Chain. Temperature and rock bulk composition can be considered substantially constant all over the sample range, so that the differences in the b
0 values depend primarily on differences in pressure. The analytical data demonstrate that the b
0 increases for increasing pressure, and that it is practically identical in the micas from very different regions belonging to the same baric type of metamorphism. With the help of other analytical data (about 600), a b
0 scale was prepared, which makes the distinction of six different facies series possible. The writers, proposing this very rapid new method of barometric analysis, draw attention to its limitations and point out the main fields of its application.

F. P. Sassi

Papers

The b 0 value of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists