Fragmented Urban Spaces? Ethnic Residential Areas in New Zealand Cities, 1996

Residential segregation of ethnic groups is a major feature of cities in multi‐cultural societies such as New Zealand's. Measuring the degree of segregation has been the focus of much attention over the last half century, but there are difficulties with comparative studies using most of the measures adopted. An alternative procedure which classifies residential areas according to a homogeneity‐heterogeneity continuum is applied here to 1996 census small‐scale data for each of New Zealand's 33 main urban areas, to identify the degree of segregation of various ethnic groups. It reports that most New Zealanders from all ethnic groups (defined by self‐identity) live in mixed rather than exclusive residential areas. The main ethnic enclaves/ghettos are occupied by Maori and Pacific Islanders, and are entirely concentrated in the North Island, especially in urban areas which have substantial Polynesian populations.     

Na-rich Partial Melts from Newly Underplated Basaltic Crust: the Cordillera Blanca Batholith, Peru

The late Miocene Cordillera Blanca Batholith lies directly overthick (50 km) crust, inboard of the older Cretaceous CoastalBatholith. Its peraluminous ‘S’ type mineralogyand its position suggest recycling of continental crust, whichis commonly thought to be an increasingly important componentin magmas inboard of continental margins. However, the peraluminous,apparent ‘S’ type character of the batholith isan artefact of deformation and uplift along a major crustallineament. The batholith is a metaluminous ‘I’ typeand the dominant high-silica rocks (>70%) are Na rich withmany of the characteristics of subducted oceanic slab melts.However, the position of the batholith and age of the oceaniccrust at the trench during the Miocene preclude slab melting.Instead, partial melting of newly underplated Miocene crustis proposed. In this dynamic model newly underplated basalticmaterial is melted to produce high-Na, low HREE, high-Al ‘trondhjemitic’type melts with residues of garnet, clinopyroxene and amphibole.Such Na-rich magmas are characteristic of thick Andean crust;they are significantly different from typical cole-alkaline,tonalite-grano-diorite magmas, and their presence along thespine of the Andes provokes questions about models of trondhjemitegenesis by melting of subducted oceanic crust, as well as anygeneralized, circum-Pacific model involving consistent isotopicor chemical changes inboard from the trench. KEY WORDS: batholith; modified ‘I’ type granite; Na-rich magma; thick crust ^* Corresponding author.     

COMPARISON OF PALEOTEMPERATURE RECONSTRUCTIONS AS EVIDENCE FOR THE PALEO-ANALOG HYPOTHESIS

There is an apparent inconsistency between the estimated low accuracy reported by Kheshgi and Lapenis (1996; hereinafter KL) for the reconstructed zonal-mean annual paleotemperatures for the mid-Holocene, and the high agreement reported by Shabalova and Können (1995; hereinafter SK) between the normalized temperature anomalies of these reconstructed paleotemperatures and those reconstructed for earlier epochs. The reasons for this inconsistency could be: (i) overestimation by KL of the reconstruction errors by more than a factor of two, (ii) significant smoothing of the paleodata resulting in a reduction in the number of independent pieces of information represented by the zonal-mean temperatures, or (iii) bias of the paleotemperature reconstruction by prior knowledge of the expected patterns of climate change. Because it is unlikely that the errors involved in producing the reconstructed mid-Holocene temperatures have been overestimated by more than a factor of two, one or both of the other reasons is the likely explanation for the inconsistency. If this holds true, then support for the paleo-analog hypothesis provided by the mid-Holocene paleotemperature reconstruction is severely weakened.     

Leucogranites from the Eastern Part of the South Mountain Batholith, Nova Scotia

The South Mountain Batholith is a peraluminous granitic complexranging in composition from biotite granodiorite to muscovite-topaz‘leucogranite’. Leucogranitic rocks (with generally<2% biotite) form a minor part ({small tilde}1•5%) ofthe batholith, and are of two types: (1) ‘associated leucogranites’occurring as relatively small zones in fine-grained leucomonzogranites;and (2) ‘independent leucogranites’ forming generallylarger bodies having no particular spatial association withother rock types. Mean chemical compositions of these two typesof leucogranite are as follows (associated, independent): Na₂O(3•46,3•83),K₂O(4•40,4•09),andP₂O₅ (0•26, 0•45)in wt.%;Li(149, 281), F(1199, 2712),Rb (393, 725), U (7•4, 4•4), Nb (12•8, 23•4),Ta (2•9, 7•1), and Zr (52, 31) in ppm. Rare earthelements also differ between the two types (associated, independent):REE (34•1 ppm, 19•9 ppm); and in the degree and variabilityof heavy REE fractionation (Gd_N/Yb_N=4•62•2, 2•00•7).In addition, associated leucogranite has REE compositions similarto those of its host rocks. Mean ¹⁸O values (associated +ll•21•2,independent +ll•40•5; relative to SMOW) are comparablewith the mean for the entire South Mountain Batholith (+l0•80•7).Radiometric dating (⁴⁰Ar/³⁹Ar on muscovite) shows that bothtypes of leucogranite have identical ages of 3723 Ma, equivalentto ages determined by other techniques for granodiorite andmonzogranite samples elsewhere in the batholith. Field relationsand geochemistry suggest that the associated leucogranite resultsfrom an open-system interaction between a fluid and its hostleucomonzogranite, whereas the independent leucogranite bodiesare discrete intrusions of highly fractionated melts that underwentclosed-system, late-magmatic to post-magmatic fluid alteration.Where mineralized, the associated leucogranite characteristicallyhosts greisen-type or disseminated polymetallic mineralization,whereas the independent leucogranite hosts pegmatitic or disseminatedpolymetallic mineralization.     

Melting Behavior of Simplified Lherzolite in the System CaO-MgO-Al2O3-SiO2-Na2O from 7 to 35 kbar

Phase equilibrium data have been collected for isobaricallyunivariant melting of simplified Iherzolite compositions inthe system CaO-MgO-Al₂O₃ SiO₂-Na₂O over a pressure range of7–35 kbar. These data permit the melting behavior of awide variety of model lherzolite compositions to be determinedquantitatively by algebraic methods. Two P-T univariant meltingreactions, corresponding to plagioclase to spinel lherzoliteand spinel to garnet lherzolite, are identified as peritectic-typetransitions and have positive Clapeyron slopes. The univariantcurves move to higher pressures and temperatures with increasingNa₂O in the liquid. The effect of the univariant curves on meltingis to produce low-temperature regions and isobarically invariantmelting intervals along lherzolite solidi. In the plagioclaselherzolite stability field, melting of four-phase model lherzoliteis pseudo-invariant, occurring over small temperature intervals(5C) and producing liquids that are quartz tholeiites at <8kbar and olivine tholeiites at >8 kbar. Calculated equilibriumconstants for plagioclase-liquid equilibria show both temperatureand pressure dependence. Plagioclase with anorthite content(AN) >90 mol%, as observed in some oceanic basalts, can crystallizefrom liquids with <1% Na₂O. Melting of spinel lherzoliteis not pseudo-invariant but occurs over large temperature intervals(15–60 C), producing a wide range in liquid compositions,from alkali basalts and alkali picrites at low to moderate degreesof melting (<1–10%) to olivine tholeiites and picritesat higher degrees of melting (>10%). On the basis of limiteddata in the garnet Iherzolite field, melts from garnet lherzoliteare more silica rich for a given degree of melting than meltsfrom spinel lherzolite, and liquid compositions trend towardenstatite with increase in pressure. Source fertility (especiallyNa₂O content) has a strong control on the temperature of meltingand liquid composition. Less fertile sources produce smalleramounts of liquids richer in normative silica. For certain bulkcompositions (high SiO₂ and low Al₂O₃), spinel is not a stablephase along the lherzolite solidus.     

Palaeoclimatic inferences from upper Palaeozoic siltstone of the Earp Formation and equivalents, Arizona-New Mexico (USA)

The Late Palaeozoic configuration of Pangaea contributed to a palaeoclimatic extreme that was characterized by both icehouse and monsoonal conditions. This study uses sedimentological, geochemical, and provenance data from silty facies of the Earp and equivalent Supai Formations (Arizona, New Mexico) to shed light on atmospheric circulation and glacial–interglacial climate change in westernmost equatorial Pangaea. Five silt‐rich facies comprise both loessite and marine and fluvially reworked loessite. An initial aeolian origin for the silt is indicated by the remarkably invariant grain size and the laterally continuous, sheet‐like geometry of beds. The silt‐rich facies occur in repetitive facies associations (1–20 m scale) that form mixed continental‐marine (loess, marine‐reworked loess), shallow‐marine, and continental (loess, palaeosol) ‘sequences’. Facies repetitions of both mixed continental‐marine and shallow‐marine sequences reflect a linked glacioeustatic–glacioclimatic control, whereas the continental (loess–palaeosol) couplets reflect a primary glacial–interglacial climatic cyclicity linked to glacioeustasy. Stratigraphic interpretations suggest that aeolian silt flux maximized during glacial to incipient interglacial stages (lowstand to early transgression), and decreased significantly or ceased during interglacials (highstand to early falling stage). Detrital‐zircon geochronological data indicate a transition from dominantly north‐easterly winds during the Middle Pennsylvanian to north‐westerly and south‐easterly winds by the Early Permian, which trend is inferred to reflect the onset of monsoonal circulation in western Pangaea. Relative grain‐size data support the detrital‐zircon data, and exhibit a significant decrease from the Sedona arch/Central Arizona shelf (north) to the Pedregosa basin (south) sections. Whole‐rock geochemical data suggest a relatively unweathered source for the silt in the north, and detrital‐zircon data indicate significant silt was derived from the local basement. These large piles of silt(stone) preserve valuable information for reconstructing both long‐term evolution in atmospheric circulation and short‐term fluctuations in glacial–interglacial climate. Many such indicators for long have been applied to ‘recent’ (Plio‐Pleistocene) loess, but are equally applicable to ‘deep‐time’ strata.     

Bedload abrasion and the in situ fragmentation of bivalve shells

The aim of this study was to determine how Unio bivalve shells fragment within the channel of the Sakmara River (southern Urals, Russia). The Sakmara River has an abundant bivalve population and a highly variable flow regime which, at low flow, allowed much of the channel bed to be examined. A large data set of 1013 shells (Unio sp.) was examined and these were shown to have consistent patterns of orientation, aspect, shell abrasion, perforation and fracture. The close spatial relationship between areas of shell abrasion, shell perforation and shell fracture showed that they form part of a continuum whereby areas of abrasion evolve into perforations and perforations coalesce and enlarge into fractures. The mechanism of shell damage proposed is one of abrasion in place, whereby the shell remains stationary on the surface of the point bar and is impacted by bedload. Underpinning this process are the hydrodynamic properties of the bivalve shell, with consistency in the orientation and aspect of the valve in a flowing current producing consistency in the distribution of damage on the shell surface. Valves preferentially lie in a convex‐up position and orientate in the flow such that the umbo faces upstream. The elevated, upstream‐facing umbo region is exposed to particle impact and is the first to be abraded and perforated. The vulnerability of the umbo to perforation is greatly increased by the thinness of the shell at the umbo cavity. The in situ abrasion process is enhanced by the development of an armoured gravel bed which restricts valve mobility and maintains shells within the abrasion zone at the sediment–water interface. The in situ abrasion process shows that broken shells are not a reliable indicator of long distance transport. The study also raises the issue that tumbling barrel experiments, which are generally used to simulate shell abrasion, will not replicate the type of directionally focused sand‐blasting which appears to be the principal cause of shell fragmentation in the Sakmara River.     

Quantifying the Building Blocks of Igneous Rocks: Are Clustered Crystal Frameworks the Foundation?

Most phenocryst populations in volcanic rocks, and those preservedin shallow-level igneous intrusions, are clustered (variouslyreferred to as clots, clumps or glomerocrysts). These clustersof crystals are the building blocks that accumulate to formthe high-porosity, touching crystal frameworks from which igneouscumulates form. Examination of touching crystal frameworks inolivine- (komatiite cumulates and experimental charges) andplagioclase-dominant crystal populations (Holyoke flood basalt,Connecticut, USA) reveal complex, high-porosity, clustered crystalarrangements. Olivine touching frameworks in komatiite flowsare interpreted to form in hundreds of days. Plagioclase frameworksare calculated to have formed in less than 17 years for a crystalgrowth rate of 1 x 10^-10 mm/s to less than 3 years for a growthrate of 5 x 10^-10 mm/s based on crystal size distributions.The origin of crystal clusters is likely to involve either (ora combination of) heterogeneous nucleation, remobilization ofcumulate mushes or crystals sticking together during settlingand/or flow. The spatial distribution pattern of clustered crystalframeworks from both natural and experimental examples constrainsfields on spatial packing diagrams that allow the identificationof touching and non-touching crystal populations, and furtherimprove our understanding of crystal packing arrangements andcluster size distributions. KEY WORDS: cumulates; CSD; komatiite; basalt; spatial packing; textural analysis     

Near-solidus Melting of the Shallow Upper Mantle: Partial Melting Experiments on Depleted Peridotite

We present the results of melting experiments on a moderatelydepleted peridotite composition (DMM1) at 10 kbar and 1250–1390°C.Specially designed experiments demonstrate that liquids extractedinto aggregates of vitreous carbon spheres maintained chemicalcontact with the bulk charge down to melt fractions of