Corona textures between kyanite, garnet and gedrite in gneisses from Errabiddy, Western Australia

Abstract Corona textures, which developed in alternating layers in rocks in a supracrustal belt at Errabiddy, Western Australia, involved:
(a) The production of staurolite, cordierite and quartz or sapphirine between Kyanite and/or sillimanite and gedrite; and
(b) The production of cordierite between garnet and gedrite.
These textures are inconsistent with development along the same pressure–temperature path in the system FeO–MgO–Al₂O₃–SiO₂–H₂O, but can be accounted for if CaO, mainly in garnet, is taken into account. The sapphirine-bearing kyanite–gedrite textures are explained by lower a (SiO₂) during their development. The assemblages indicate a consistent pressure–temperature ( P–T ) trajectory involving substantial uplift with only a slight decrease in temperature. The history of these rocks includes reheating of originally high-grade rocks that had cooled to a stable conductive geotherm, followed by substantial, essentially isothermal uplift. The tectonic environment for this was presumably the one responsible for emplacement of the high-grade terrain in the upper crust.     

Ophiolites

An ophiolite is an association of igneous and metamorphic rocks, including basalts, dolerites, gabbros and peridotites, which may be a fragment of crust and mantle formed by spreading at a ridge beneath an ancient ocean. This remarkable conclusion is supported by many features of the structure, field relationships, petrology and geochemistry of ophiolites. Although recent research has led to a degree of disillusionment with the idea that ophiolites are fragments of typical ocean floor, these rocks have a good deal to tell us about processes of ocean floor construction and mountain building.     

Tidal-shelf sedimentation: an example from the Scottish Dalradian

The Jura Quartzite, a formation of probably late Precambrian metasediments over 5 km thick from the Caledonian belt in Southwest Scotland, has been divided into a coarse and three fine facies. The former comprises cross-bedded sands with some laminated sands and silt horizons, interpreted as the deposits of shallow marine tidal dunes and other bedforms together with some beach units. Deposition from suspension of silt and sand formed climbing dunes while largescale erosion produced flat or channelled surfaces. The fine facies comprise laterally persistent, parallel and cross-laminated sand units from millimetres to decimetres thick, interbedded with muds. The coarse and fine facies can be finely interbedded, the former sometimes filling decimetre deep, straight channels, cut in the latter. The fine facies exhibit structures indicative of deposition from decelerating currents and are interpreted as shallow marine storm deposits. The facies are compared with a model developed from published observations on modern shelf areas. Zones of erosion, large and small dunes, flat bedded sand and mud are considered to be the end product of a wide spectrum of tidal and storm conditions. During severe storms the fair weather tidal dunes may be modified or washed out, new dunes may be initiated downcurrent of the normal dune field while storm-sand layers are deposited in the distal zones. Hence, the nature of the preserved sediment blanket reflects the rare severe storm event rather than normal tidal conditions. The Jura Quartzite was deposited in a tidal gulf intimately connected with an ocean basin. The north-northeast directed palaeocurrent modes are probably roughly parallel to the coastline.     

Computer program for presentation of grain-size data by the graphic method

A computer program is described for a Hewlett-Packard desk-top Calculator (Model 9820A)-Plotter (Model 9862A) which plots a histogram, frequency polygon and/or cumulative curve, as well as standard statistical parameters and percentages of gravel, sand, silt, and clay from raw weight data from a sediment grain-size analysis. The program utilizes the graphic method for calculating statistical parameters in contrast to other commonly used computer programs derived for the method of moments. This program is advantageous because (1) it is rapid, (2) it allows direct comparison with grain-size data in the literature which have been computed by the graphic method, (3) either a complete presentation of a distribution can be obtained on one sheet of paper or a series of cumulative curves or frequency polygons can be prepared on a single graph which is ready for publication photocopying, (4) automatic extrapolation of data points from a cumulative curve eliminates inconsistencies arising from manual extrapolation, and (5) the desk-top calculator is more convenient to use, eliminates punching of data cards, and can be operated by someone without a knowledge of computers and programming. Disadvantages of the program are: (1) some authors feel critical percentiles are more accurately read off cumulative curves plotted on a probability ordinate than on an arithmetic ordinate, (2) the cumulative curve is a series of straight-line segments, (3) the frequency polygon is not as accurate a representation of grain-size distribution as a frequency curve and (4) size class intervals for the histogram and frequency polygon are standardized at 0.5φ, even though gravel and mud fractions are analysed at whole φ intervals. For most studies, the advantages of the program outweigh the disadvantages.     

The Role of Tourism in the Economic Transformation of the Central West Queensland Economy

Central West Queensland's predominantly pastoral sector experienced considerable hardship in the decade to 1996 as the consequence of drought and low commodity prices. The social and economic consequences were both large and spatially diverse. Those often remoter shires, characterised by few people and services, experienced sharp decline on both dimensions. In contrast, some of the larger and more accessible towns managed to stabilise or even increase both population and services, assisted by the development of the tourism sector. This, in turn, reflects the region's unique geographical character, features of which include strong historical associations, harsh and seasonably variable climate, vast distances, and sparse populations. Unsurprisingly, the tourist industry has been developed by small-scale local entrepreneurs, focuses on distinct market segments--especially the elderly and adventurous, and is highly seasonal.     

Palaeo-ice streaming in the central sector of the British—Irish Ice Sheet during the Last Glacial Maximum: evidence from the northern Irish Sea Basin

Late Devensian glacial sediments and landforms of the Isle of Man record the advance and deglacial signature of the central sector of the British-Irish Ice Sheet. Evidence from the area, gathered from striae, erratic trains and drift limits, show ice was routed over and around the island in two flow phases post-36 kyr BP. In the south of the island, streamlined depositional bedforms with low elongation ratios suggest low ice-flow velocities resulting from one or more of (i) the up-ice location of the island within a regional onset zone, (ii) flow retardation of ice interacting with the margins of the island and (iii) localized drainage of the deforming bed. The deglacial landform assemblage of lateral marginal sandurs and drainage diversions, coupled with a lack of dead-ice features, suggests ice did not downwaste in situ but retreated intact along the coastal margins as Manx Upland ice thinned. In the north of the island, however, the Bride Moraine complex indicates a change in deglacial ice-sheet dynamics, with temporary re-advance and marginal oscillation causing proglacial tectonism and thrusting of the glacial sediment pile, possibly during the Killard Point Stadial event (18.8-16.4 cal. kyr BP). From a basin-wide perspective, the Irish Sea Basin sector of the British-Irish Ice Sheet had many of the characteristics of an ice stream, such as a zone of flow convergence up-ice, a grounding line in the southern Celtic Sea and recessional limits characterized by proglacially tectonized and thrust dead-ice landscapes indicative of a rapidly oscillating ice margin.     

THE PEATLAND/ICE AGE HYPOTHESIS REVISED, ADDING A POSSIBLE GLACIAL PULSE TRIGGER

Carbon sequestering in peatlands is believed to be a major climate‐regulating mechanism throughout the late Phanerozoic. Since plant life first evolved on land, peatlands have been significant carbon sinks, which could explain significant parts of the large variations in atmospheric carbon dioxide observed in various records. The result is peat in different degrees of metamorphosis, i.e. lignite, hard coal and graphite. During phases of extensive glaciations such as the 330–240 Ma Pangea Ice Age, atmospheric carbon dioxide was critically low. This pattern repeats itself during the Pleistocene when carbon dioxide oscillates with an amplitude of c. 200–300 ppmv. This paper suggests that the ice age cycles during the Pleistocene are generated by the interglacial growth of peatlands and the subsequent sequestering of carbon into this terrestrial pool. The final initiation of ice age pulses towards the end of inter‐glacials, on the other hand, is attributed to the cyclic influx of cosmic dust to the Earth surface, which in turn regulates cloud formation and the incoming shortwave radiation. These shorter cycles have a frequency of c. 1000‐1250 years and might be connected to sunspot or other low frequency solar variations. In a wider context the ice age cycling could be regarded as an interplay between terrestrial life on the high latitudes of the northern hemisphere and the marine subsurface life in the southeast. If the results presented here are correct, the present global warming might just be the early part of a new warm period such as the Bronze Age and the Roman and Medieval Warm periods. This could be caused by entry into another phase of decreasing influx rates of cosmic dust. The increasing concentrations of atmospheric carbon dioxide might have contributed to this warming but, most important of all, it might temporarily have saved us from a new ice age pulse.     

Petrology and Geochemistry of Intraplate Basalts in the South Auckland Volcanic Field, New Zealand: Evidence for Two Coeval Magma Suites from Distinct Sources

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·0–4·6wt %), Nb (7–29 ppm), and (La/Yb)_N (3·4–7·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·3–7·9wt %), Nb (32–102 ppm), and (La/Yb)_N (12–47). GroupA has slightly higher ⁸⁷Sr/⁸⁶Sr, similar _Nd, and lower ²⁰⁶Pb/²⁰⁴Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high ²³⁸U/²⁰⁴Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources     

When downhole logging turns to geochemistry

The procedure of lowering geophysical instruments down boreholes on a wire has, over the past 60 years, been a useful aid in defining the physical nature of the rock formations through which the borehole passes. Current developments, however, herald the advent of the use of such techniques for the measurement of geochemical concentrations in a quantitative way. This opens the door to a whole new series of applications in a wide range of geological disciplines.