A Nd- and O-isotope study of the REE-rich peralkaline Strange Lake granite: implications for Mesoproterozoic A-type magmatism in the Core Zone (NE-Canada)

It is well established that A-type granites enriched in high field strength elements, such as Zr, Nb and the REE, form in anorogenic tectonic settings. The sources of these elements and the processes controlling their unusual enrichment, however, are still debated. They are addressed here using neodymium and oxygen isotope analyses of samples from the 1.24 Ga Strange Lake pluton in the Paleoproterozoic Core Zone of Québec-Labrador, an A-type granitic body characterized by hyper-enrichment in the REE, Zr, and Nb. Age-corrected εNd values for bulk rock samples and sodic amphiboles (mainly arfvedsonite) from the pluton range from ?0.6 to ?5.7, and ?0.3 to ?5.3, respectively. The εNd values for the Napeu Kainiut quartz monzonite, which hosts the pluton, range from ?4.8 to ?8.1. The ¹⁴⁷Sm/¹⁴⁴Nd ratios of the suite and the host quartz monzonite range from 0.0967 to 0.1659, large variations that can be explained by in situ fractionation of early LREE-minerals (Strange Lake), and late hydrothermal HREE remobilization. Oxygen isotope analyses of quartz of both Strange Lake and the host yielded δ¹⁸O values between +8.2 and +9.1, which are considerably higher than the mantle value of 5.7 ± 0.2‰. Bulk rock oxygen isotope analyses of biotite-gneisses in the vicinity of the Strange Lake pluton yielded δ¹⁸O values of 6.3, 8.6 and 9.6‰. The negative εNd values and positive δ¹⁸O values of the Strange Lake and Napeu Kainiut samples indicate that both magmas experienced considerable crustal contamination. The extent of this contamination was estimated, assuming that the contaminants were sedimentary-derived rocks from the underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low εNd and high δ¹⁸O values. Mixing of 5–15% of a gneiss, having an εNd value of ?15 and a δ¹⁸O value of +11, with a moderately enriched mantle source (εNd = +0.9, δ¹⁸O = +6.3) would produce values similar to those obtained for the Strange Lake granites. Based on analogies between the Nain Plutonic Suite and the Gardar alkaline igneous province (SW-Greenland), we conclude that the Strange Lake pluton and associated REE-mineralized anorogenic bodies formed from a combination of subduction-induced fertilization of the sublithospheric mantle, crustal extension and in situ magma evolution.     

Redefinition of Lemanskiite: New Mineralogical Data,Crystal Structure,and Revised Formula NaCaCu5(AsO4)4Cl · 3H2O

Geology of Ore Deposits - The crystal structure of lemanskiite is determined for the first time (R = 0.019) and the mineral is redefined. Its chemical formula, crystal system, space group and...     

Petrology and Geochemistry of MORB from 25?E to 46?E along the Southwest Indian Ridge: Evidence for Contrasting Styles of Mantle Enrichment

The 1984 PROTEA expedition, leg 5, to the central SouthwestIndian Ridge recovered basaltic lavas from fracture zones andridge segments between 25?E and 48?E. In terms of petrographyand major element variations the samples are unremarkable forocean ridge basalts and range from aphyric to highly plagioclasephyric and from primitive (mg-number = 70) to moderately evolved(mg-number = 40) in composition. Multiply saturated (i.e., olivine,plagioclase, and clinopyroxene) basalts are common within thisregion. There is no systematic difference in compositional characteristicsbetween basalts dredged from fracture zone walls and those dredgedfrom ridge segments, and fractional crystallization has playedan important role in controlling the overall range in lava compositionin both tectonic environments. Incompatible element abundance ratios in the basalts are morenotable and distinguish between geochemically depleted (N-type)MORB with high Zr/Nb (1668) and Y/Nb (4?723) ratios and low(La/Sm)^m, ratios (0-?76–1?00), and geochemically enriched(E-type) MORB with low Zr/Nb (3?4–15?8) and Y/Nb (0?5–8?8)and high (La/Sm). ratios (1?07–3?8). N-type MORB appearsto be absent in the immediate vicinity of Marion Island, butoccurs further along the ridge to the northeast and southwest.Geochemically enriched MORB occurs at scattered localities alongthe ridge but is particularly abundant along the section ofthe ridge closest to the Marion hotspot. In detail, two distinct varieties of E-type MORB can be recognized.The one type has incompatible element and isotopic ratios similarto, although slightly less enriched than, those characteristicof the Marion hotspot (Zr/Nb=5?8–8?6; Y/Nb=0?5–0?8;Ba/Nb=5?1–9?0). The second type can be distinguished byhaving high Ba/Nb ratios (9–22), unlike any lavas directlyassociated with the Marion hotspot, but similar to those characteristicof DUPAL ocean island basalts (OIB). A single sample from thisgroup for which there are isotopic data indicates derivationfrom an isotopically anomalous source region. A model is proposed whereby the sub-oceanic mantle below thisportion of the southwest Indian Ocean has experienced at leasttwo distinct enrichment events. The one is associated with theupwelling of the Marion mantle plume (geochemically characterizedby having low Ba/Nb ratios and normal OIB isotopic ratios).The other is associated with upwelling from a DUPAL source (characterizedby having high Ba/Nb ratio and unusual isotopic ratios) whichhas been proposed to exist beneath this portion of the southwestIndian Ocean (Hart, 1984). On the basis of Ba/Nb and Nb/U ratios,recycled oceanic lithosphere is favoured as a source for theMarion hotspot, while recycled oceanic lithosphere plus ancientpelagic sediment appears to be the most likely source for theDUPAL anomaly and the DUPAL E-type MORB in this region.     

青藏高原东缘晚新生代成都盆地物源分析与水系演化

成都盆地位于青藏高原东缘,夹于龙门山与龙泉山之间,盆地中充填了3.6Ma以来的大邑砾岩、雅安砾石层和晚更新世—全新世砾石层,其物源均来源于盆地西侧的龙门山,具横向水系和单向充填的特征。本次以物源区分析作为切入点,以岷江和青衣江水系为重点,采用砾岩成分分析、砂岩岩屑成分分析、重矿物分析和砾石的地球化学分析等基本方法,开展青藏高原东缘晚新生代以来的古水系重建工作,研究结果表明,成都盆地主要有两个物源区,其中成都盆地北部的都江堰街子场、崇州白塔山、大邑白岩沟、大邑氮肥厂、彭州丁家湾、彭州葛仙山等剖面中的砾石层在碎屑成分、重矿物和花岗岩砾石的地球化学成分等方面相似,应为古岷江的产物,而其与现代岷江在砾岩成分和重矿物特征等方面的差异性则表明古岷江可能存在改道的现象;成都盆地南部的庙坡剖面和熊坡东剖面中的砾石层在碎屑成分、重矿物和花岗岩砾石的地球化学成分等方面相似,应为古青衣江的产物,但其流向却与现代青衣江的流向不同,表明熊坡背斜是在大邑砾岩沉积之后隆起的,它的隆起迫使古青衣江改道。     

On the transport of small river run off by an alongshore baroclinic sea current

A numerical simulation has been undertaken to study the process of the transport of small river runoff by alongshore baroclinic sea currents. The study is based on the implementation of the Princeton Ocean Model (POM) under the conditions of a circular stratified basin whose surface is exposed to a transient tangential wind stress to form an alongshore baroclinic current. A baroclinic current of the downwelling type (in the Northern Hemisphere directed to the left to a sea-viewing observer) was shown to provide the carrying out of the river discharge from estuary’s vicinity more effectively than that of the upwelling type (in the Northern Hemisphere directed to the right to a sea-viewing observer).     

Main results of recent investigations into the physical mechanisms of the interaction of tropical cyclones and the ionosphere

Izvestiya, Atmospheric and Oceanic Physics - International investigations into the correlation between tropical cyclones (TCs) and the ionosphere are associated with great difficulties in proving...     

A field test of a linear stability model for crescentic bars

A morphodynamical linear stability analysis is used to predict the natural development of crescentic bed patterns and rip channels. The purpose is to investigate whether this technique, which is useful for understanding the physics of emerging bed-forms, can be used to make quantitative predictions in the field, which may then be of use for coastal engineers.     

Agricultural land use dynamics in the Brazilian Amazon based on remote sensing and census data

The potential impact of deforestation in the Brazilian Amazon on greenhouse gas emissions to the atmosphere calls for policies that take account of changes in forest cover. Although much research has focused on the location and effects of deforestation, little is known about the distribution and reasons for the agricultural uses that replace forest cover. We used Landsat TM-based deforestation and agricultural census data to generate maps of the distribution and proportion of four major agricultural land uses throughout the Brazilian Amazon in 1997 and 2007. We built linear and spatial regression models to assess the determinant factors of deforestation and those major agricultural land uses - pasture, temporary agriculture and permanent agriculture - for the states of Pará, Rondônia, and Mato Grosso. The data include 30 determinant factors that were grouped into two years (1996 and 2006) and in four categories: accessibility to markets, public policies, agrarian structure, and environment. We found an overall expansion of the total agricultural area between 1997 and 2007, and notable differences between the states of Pará, Rondônia, and Mato Grosso in land use changes during this period. Regression models for deforestation and pasture indicated that determinant factors such as distance to roads were more influential in 1997 than in 2007. The number of settled families played an important role in the deforestation and pasture, the effect was stronger in 2007 than 1997. Indigenous lands were significant in preventing deforestation in high-pressure areas in 2007. For temporary and permanent agricultures, our results show that in 1997 the effect of small farms was stronger than in 2007. The mapped land use time series and the models explain empirically the effects of land use changes across the region over one decade.