Trace element modelling of pelite-derived granites

The presence or absence of a vapour phase during incongruent-melt reactions of muscovite and biotite together with the composition of the protolith determines the trace-element characteristics of the resulting melt, provided that equilibrium melting occurs for those phases that host the tracc elements of interest. For granitic melts, Rb, Sr and Ba provide critical constraints on the conditions that prevailed during melting, whereas REE are primarily controlled by accessory phase behaviour. Mass-balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction, and deplete the restite in feldspar. Hence the melt will be characterized by low Rb/Sr and high Sr/Ba ratios. In contrast, vapour-absent melting will result in a smaller melt fraction, and an increase in the restitic feldspar. Consequently high Rb/Sr and low Sr/Ba ratios are predicted. Vapour-absent melting will also enhance the negative Eu anomaly in the melt. Granites that result from the incongruent melting of biotite in the source will be characterized by higher Rb concentrations than those that result from the incongruent melting of muscovite. The Himalayan leucogranites provide an example of unfractionated, crustally derived eutectic melts that are enriched in Rb but depleted in Sr and Ba relative to their metasedimentary protoliths. These compositions may be generated by the incongruent melting of muscovite as a low melt fraction (F0.1) from a pelitic source under vapour-absent conditions.     

A Younger Dryas Ash Bed in western Norway,and its possible correlations with tephra in cores from the Norwegian Sea and the North Atlantic

A bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western Norway. It is formally mamed the Vedde Ash Bed, and its age is approximately 10,600 yr B.P., i.e., mid-Younger Dryas. The bed consits of pure glass having a bimodal basaltic and rhyolitic somposition. The geochemistry of the glass shards suggests an Icelandic source. By means of stratigraphic position and geochemistry, the ash is correlated with ash zones found in cores from the continental shelf, the Norwegian Sea, and the North Atlatic.     

Late Quaternary sediments and stratigraphy on the continental shelf off Troms and west Finnmark, northern Norway

Lithologic, paleontologic, and chronostratigraphic investigation of 13 gravity cores indicates the following environmental evolution: a high- (mid-) arctic period with a slight influx of ice-rafted debris occurred during the early middle Weichselian followed by a mid- (high-) arctic environment with a high influx of iceberg-rafted debris during the remainder of the middle Weichselian. The continental ice sheet probably did not extend beyond the inner shelf during middle Weichselian and a minimum relative sea level was ca. −120 m. A low-arctic environment occurred during (parts of) the late Weichselian with an initial winnowing of the sediments. The Norwegian Current entered the area during this substage. A high- (mid-) boreal environment occurred during the Holocene with high winnowing activity in the early Holocene. Winnowing is still very active on the shallower banks in contrast to the deeper banks where it has ceased. Relatively high percentages of carbonate in the form of biogenic skeletal remains occur in the Holocene sediments.     

The proximity of sexual violence to schools: evidence from a township in South Africa

GeoJournal - Schools are unique places that greatly influence the daily lives of learners. Examining any factors that may be contributing to school absenteeism and dropout rates is vital as gaining...     

Holocene floristic diversity and richness in northeast Norway revealed by sedimentary ancient DNA (sedaDNA) and pollen

We present a Holocene record of floristic diversity and environmental change for the central Varanger Peninsula, Finnmark, based on ancient DNA extracted from the sediments of a small lake (sedaDNA). The record covers the period c. 10 700 to 3300 cal. a BP and is complemented by pollen data. Measures of species richness, sample evenness and beta diversity were calculated based on sedaDNA sampling intervals and 1000‐year time windows. We identified 101 vascular plant and 17 bryophyte taxa, a high proportion (86%) of which are still growing within the region today. The high species richness (>60 taxa) observed in the Early Holocene, including representatives from all important plant functional groups, shows that modern shrub‐tundra communities, and much of their species complement, were in place as early as c. 10 700 cal. a BP. We infer that postglacial colonization of the area occurred prior to the full Holocene, during the Pleistocene‐Holocene transition, Younger Dryas stadial or earlier. Abundant DNA of the extra‐limital aquatic plant Callitriche hermaphroditica suggests it expanded its range northward between c. 10 200 and 9600 cal. a BP, when summers were warmer than present. High values of Pinus DNA occur throughout the record, but we cannot say with certainty if they represent prior local presence; however, pollen influx values >500 grains cm^?2 a^?1 between c. 8000 and 7300 cal. a BP strongly suggest the presence of pine woodland during this period. As the site lies beyond the modern tree limit of pine, it is likely that this expansion also reflects a response to warmer Early Holocene summers.     

Cretaceous plutonism in Central Tibet: an example of post-collision magmatism?

The magmatic province of the northern Lhasa Terrane includes an Early Cretaceous (120–130 Ma) plutonic event, and a Late Cretaceous (80–110 Ma) volcanic event. The plutonic association constitutes an older suite of granodiorites, monzogranites and tonalites and a younger peraluminous leucogranite facies. Plutonism occurred about 20 Ma after obduction of the Banggong ophiolite, following closure between the Lhasa and Qiantang Terranes.The earlier suite is of broadly calc-alkaline in composition but differs from arc-related magmas in that only more evolved compositions are represented (SiO₂ > 58%) and Rb/Zr ratios are elevated relative to the Gangdese batholith to the south. Trace-element and isotopic constraints are consistent with derivation from a Late Proterozoic amphibole-bearing crustal source requiring temperatures > 950°C during anatexis. The leucogranites require a pelitic source which is tentatively identified as the Nyaingentanglha basement exposed south of the plutonic province. Unlike the High Himalaya leucogranites, trace elements and field relations require a high degree of melting at source (> 50%) suggesting fluid-absent melting at temperatures > 850°C. Such high crustal temperatures indicate convective heat transfer from the mantle.Thermal constraints together with a tectonic setting of post-emplacement uplift followed by a marine transgression in the northern Lhasa Terrane can not be reconciled with a model of tectonically thickened crust but are consistent with post-collision attenuation of the lithosphere.     

Kinematic reworking and exhumation within the convergent Alpine Orogen

Kinematic data from the internal zones of the Western Alps indicate both top-to-SE and top-to-NW shearing during synkinematic greenschist facies recrystallisation. Rb/Sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 °C. The data indicate an initial phase of accretion and foreland-directed thrusting at ca. 60 Ma followed by almost complete reworking of thrust-related deformation by SE-directed shearing. This deformation is localised within oceanic units of the Combin Zone and the base of the overlying Austroalpine basement, and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike. The timing of deformation in this shear zone spans 9 Ma from 45 to 36 Ma. The SE-directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy, and such structures have been interpreted in the past as backthrusts in response to ongoing Alpine convergence. However, on a regional scale, the top-to-SE deformation is related to crustal extension, not shortening, and is coincident with exhumation of eclogites in its footwall. During this extension phase, deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking. Top-NW kinematics preserved within the Combin Zone show a range of ages. The oldest (48 Ma) may reflect the final stages of emplacement of Austroalpine Units above Piemonte oceanic rocks prior to the onset of extension. However, much of the top-to-NW deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone.⁴⁰Ar/³⁹Ar data from the region are complicated due to the widespread occurrence of excess ⁴⁰Ar in eclogite facies micas and partial Ar loss during Alpine heating. Reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 Ma. These ages are slightly younger than Rb/Sr deformation ages and suggest that cooling below ca. 350 °C occurred after juxtaposition of the units by SE-directed extensional deformation.Our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the Alpine Orogen. To constrain the palaeogeographic and geodynamic evolution of the Alps requires that these data be integrated with data from the more external zones of the orogen. Complexity such as that described is unlikely to be restricted to the Western Alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens. Recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens.     

The poor cluster of galaxies S639

We have studied the poor southern cluster of galaxies S639. Based on new Strömgren photometry of stars in the direction of the cluster, we confirm that the Galactic extinction affecting the cluster is large. We find the extinction in Johnson B to be A_B =0.75±0.03. We have obtained new photometry in Gunn r for E and S0 galaxies in the cluster. If the Fundamental Plane is used for determination of the relative distance and the peculiar velocity of the cluster, we find a distance, in velocity units, of (5706±350) km s⁻¹, and a substantial peculiar velocity, (839±350) km s⁻¹. However, the colours and the absorption line indices of the E and S0 galaxies indicate that the stellar populations in these galaxies are different from those in similar galaxies in the two rich clusters Coma and Hydra I. This difference may severely affect the distance determination and the derived peculiar velocity. The data are consistent with a non-significant peculiar velocity for S639 and the galaxies in the cluster being on average 0.2 dex younger than similar galaxies in Coma and Hydra I. The results for S639 caution that some large peculiar velocities may be spurious and caused by unusual stellar populations.