A paired metamorphic belt in a subduction‐to‐collision orogen: An example from the South Qilian–North Qaidam orogenic belt,NW China

The early Palaeozoic South Qilian–North Qaidam orogenic belt in northwestern China records a nearly complete history of early‐stage long‐lived oceanic subduction–accretion followed by late‐stage continental collision. Most previous studies have focused on low dT/dP metamorphism (HP–UHP) in this belt whereas the paired high dT/dP belt in the hinterland has received little attention. In this contribution, phase equilibrium modelling is combined with zircon petrochronology to determine the P–T–t evolution of granulites in the North Wulan gneiss complex in the high dT/dP hinterland of the South Qilian–North Qaidam orogen. Granulites record a clockwise P–T path with near‐peak temperatures of ~800–900°C at 5.5–7 kbar. Peak metamorphism was followed by high‐T decompression. Zircon petrochronology reveals protracted zircon growth from c. 474 to 446 Ma during the high‐T portion of the P–T path. High dT/dP metamorphism in the North Wulan gneiss complex was likely the result of heat transfer from the underlying hot asthenosphere and minor coeval magmatism in an arc–back‐arc system during slab retreat and roll‐back of the South Qilian oceanic plate. Broadly contemporaneous but slightly younger HP–UHP metamorphism in the foreland of the South Qilian–North Qaidam orogenic belt indicates that the region records an early Palaeozoic paired metamorphic belt. This early Palaeozoic paired metamorphic belt provides a detailed example of dual thermal regimes in a modern‐style orogenic system that can be applied to understanding the time‐scales and P–T conditions of high dT/dP metamorphism that accompany subduction in Phanerozoic and Precambrian orogenic belts.     

13C Record of benthic foraminifera in the last interglacial ocean: Implications for the carbon cycle and the global deep water circulation

The ¹³C/¹²C ratios of Upper Holocene benthic foraminiferal tests (genera Cibicides and Uvigerina) of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide (TCO₂) measured during the GEOSECS program. The δ¹³C difference between benthic foraminifera and TCO₂ is 0.07 ± 0.04‰ for Cibicides and ?0.83 ± 0.07‰ for Uvigerina at the 95% confidence level. δ¹³C analyses of the benthic foraminifera that lived during the last interglaciation (isotopic substage 5e, about 120,000 yr ago) show that the bulk of the TCO₂ in the world ocean had a δ¹³C value 0.15 ± 0.12‰ lower than the modern one at the 95% confidence level, reflecting a depletion, compared to the present value, of the global organic carbon reservoir. Regional differences in δ¹³C between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one: the Antarctic Bottom Water production was higher than today during the last interglaciation, but the eastward transport in the Circumpolar Deep Water was lower.     

Constraints on the genesis of gold mineralization at the Homestake Gold Deposit, Black Hills, South Dakota from rhenium–osmium sulfide geochronology

The Homestake gold deposit, located in the Black Hills, South Dakota, USA, is one of the largest known hydrothermal gold deposits globally, with total mining production exceeding 40 Moz Au. Rhenium–osmium geochronology of ore-associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern Black Hills. Arsenopyrite yields a rhenium–osmium (Re–Os) age of 1,736 ± 8 Ma (mean squared weighted deviation = 1.6), consistent with existing age constraints for gold mineralization, whereas Re–Os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age. This is taken to indicate that the Re–Os arsenopyrite chronometer is robust to at least 400°C, whereas the Re–Os pyrrhotite chronometer is likely disturbed by temperatures of 300–350°C. The Re–Os arsenopyrite age and initial Os ratio (0.28 ± 0.15) are interpreted to indicate that gold was introduced at ca. 1,730 Ma, coincident with the onset of exhumation of crustal blocks and, possibly, the earliest intrusive phases of Harney Peak granite magmatism. New in situ U–Pb monazite analyses from an aplite dike in the east-central Black Hills indicate that granite magmatism was a protracted event, persisting until at least ca. 1,690 Ma.     

Does the Agulhas Current amplify global temperatures during super‐interglacials?

Future projections of climate suggest our planet is moving into a ‘super‐interglacial’. Here we report a global synthesis of ice, marine and terrestrial data from a recent palaeoclimate equivalent, the Last Interglacial (ca. 130–116 ka ago). Our analysis suggests global temperatures were on average ～1.5°C higher than today (relative to the AD 1961–1990 period). Intriguingly, we identify several Indian Ocean Last Interglacial sequences that suggest persistent early warming, consistent with leakage of warm, saline waters from the Agulhas Current into the Atlantic, intensifying meridional ocean circulation and increasing global temperatures. This mechanism may have played a significant positive feedback role during super‐interglacials and could become increasingly important in the future. These results provide an important insight into a future 2°C climate stabilisation scenario. Copyright © 2010 John Wiley & Sons, Ltd.     

Reappraising the P–T evolution of the Rogaland–Vest Agder Sector,southwestern Norway

The Rogaland-Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ~1000 km~2 Rogaland Igneous Complex(RIC).New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances(30 km,10 km and ~ 10 m) from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P-T-t models for the metamorphic evolution of the area.The results are consistent with a revised two-phase evolution.Regional metamorphism followed a clockwise P-T path reaching peak conditions of ~ 850-950 ℃ and ~7-8 kbar at ~1035 Ma followed by high-temperature decompression to ~5 kbar at ~950 Ma,and resulted in extensive anatexis and melt loss to produce highly residual rocks.Subsequent emplacement of the RIC at ~930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite.This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet,and in the intermediate(10 km) sample by replacement of sapphirine by coarse intergrowths of cordierite,spinel and biotite.The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma.Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.     

紫硫镍矿交代镍黄铁矿的水热反应机理及动力学

本工作首次在实验室条件下对浅生区紫硫镍矿(Ni,Fe)3S4交代镍黄铁矿(Ni,Fe)9S8水热反应的机理及动力学进行了研究。起始反应矿物采用高纯自然镍黄铁矿,合成纯镍黄铁矿或合成镍黄铁矿-磁黄铁矿集合体。反应pH值采用0.2M醋酸-醋酸纳缓冲溶液控制在3～5的范围内。反应进程由X-射线衍射物相定量分析及扫描电镜观察进行跟踪。结果表明,当反应温度恒定在80℃时,交代20(4)%的镍黄铁矿需792h。相同条件下加入少量H2S可将反应速率提高一倍。当反应在125℃饱和蒸汽压水热环境下进行时,完全交代纯镍黄铁矿需约168h。此过程由于磁黄铁矿的存在而被催化,交代集合体中的镍黄铁矿仅需68h,进一步反应磁黄铁矿被交代成白铁矿。磁黄铁矿的催化作用可能源于溶解产生的微裂纹加速了流体的传质过程。当反应温度升高至145℃时,速率反而下降,不遵循Arrhenius经验规律。动力学分析得80℃速率常数介于5.8×10-8～3.0×10-7/s之间,125℃及145℃速率常数分别介于2.8×10-6～2.08×10-5/s及1×10-6～5.1×10-6/s之间,远高于同温度下固相扩散反应的速率常数,表明该反应在地质时标上为一快速反应。此外,用背散射电子显微技术对矿物表面形貌进行了分析,发现交代产物紫硫镍矿具有颗粒细小及存在微裂纹等特征,与自然界浅生矿床中的紫硫镍矿非常相似;电镜实验还表明该交代作用是一个典型的耦合溶解-再沉淀反应。其耦合机制的驱动力可能与反应界面处微空隙对流体饱和度的控制有关。     

The Nucleus Deposit: Superposed Au–Ag–Bi–Cu Mineralization Systems at Freegold Mountain,Yukon, Canada

In this paper we present titanite U–Pb (both single crystal CA ID‐TIMS and in situ LA ICP‐MS) data, coupled with ore and gangue mineralogy and geochemical (both lithogeochemistry and microanalysis) data from the Nucleus Au–Ag–Bi–Cu deposit, in the Yukon (Canada) portion of the Tintina Au province. Arsenic‐bearing Au–Ag–Bi–Cu mineralization at Nucleus consists of two distinct styles of mineralization including: (i) reduced Au skarn and sulfide replacement; and (ii) a relatively shallow‐emplaced (as supported by textures and temperature of formation), vein‐controlled mineralization occurring mainly as veins and veinlets of various shapes (sheeted, single, stockworks, and crustiform), breccias, and disseminations. Whereas Au, Bi, and Cu mineralization from skarn is associated with hydrous retrograde alteration phases (actinolite, ferro‐actinolite, hastingsite, cannilloite, and hornblende), numerous alteration types are associated with the vein‐controlled style of mineralization and these include: biotite, phyllic, argillic, propylitic, carbonate, and quartz (silicification) alterations. The mineralization–alteration processes took place over a wide temperature range that is bracketed between 340 and 568°C, as indicated by chlorite and arsenopyrite geothermometers. The Au‐rich Nucleus deposit is characterized by anomalously high content of As and Bi (as much as 1 %), and whereas Au moderately correlates with Bi (r = 0.40) in the skarn mineralization style (where native Au is spatially associated with native Bi and Bi‐bearing sulfides), the two elements correlate poorly (r = 0.14) in the vein‐controlled type, in which native Bi‐ and Bi‐sulfide‐bearing veins are locally observed. Sphalerite from the vein‐controlled mineralized type is Fe‐rich (9.92–10.54 mol % FeS) indicative of low sulfidation conditions, as well as high temperature, with the latter further supported by arsenopyrite geothermometry (up to 491°C), low Ag content (3–7 wt.%) in Au, and the high gold fineness (926–964). Whereas molybdenite Re–Os ages from quartz‐molybdenite veins range from 75.8 to 76.2 ± 0.3 Ma, titanite from the skarn type mineralization recorded CA ID‐TIMS and LA ICP‐MS U–Pb ages of 182.6 ± 2.4 Ma and 191.0 ± 1.5 Ma, respectively, thus precluding any genetic link between the two spatially associated styles of mineralization from the Nucleus deposit area. The Au–Ag–Bi–Cu Nucleus deposit is therefore regarded as a superposed system in which two mineralization types, without any petrogenetic relationship, overlapped, possibly with remobilization of early‐formed mineralization.     

新疆叶城晚新生代山前盆地演化与高原北缘的隆升—Ⅰ地层学与岩石学证据

叶城晚新生代山前盆地的岩性主要由中新世的细粒泥岩和砂岩（乌恰群）,上新世的砂岩夹薄层砾岩（阿图什组）及上新世－更新世的粗粒砾岩（西域组）构成。中新世的沉积以细颗粒泥砂岩为主,表明物源区较远,古流域坡度较小,搬运距离较长。古流向分析显示物源区位于南和偏南方,此时昆仑山的地势起伏尚不大。到上新世的阿图什组沉积时,开始出现砾石沉积,反映西昆仑山已经开始有规模地隆升。西域砾岩的沉积标志着作为物源区的西昆仑山民有相当的高度,随着山系的隆升,基底岩石被暴露和剥蚀。     

Deprivation,resources and planning: Some implications for applied rural geography

Recent concern over social deprivation in rural areas of the U.K. has provided a new focus for applied rural geographers. However, the concept of deprivation may be broadened to include many other aspects of the rural environment, and therefore it offers a framework for more integrated approaches to planning in the countryside. A more dynamic strategy of rural research, involving greater cooperation between academic geographers and practising planners, and focussing upon problems of rural deprivation, is advocated in this paper.     

Snow Avalanche Hazard in Canada – a Review

Snow avalanches affect recreation, transportation, resource industries and property. During the 1990s an average of 12.5 persons per year were killed in avalanches in Canada. The snow avalanche hazard has affected people and facilities in B.C, Alberta, Yukon, NWT, Nunavut, Ontario, Quebec and Newfoundland. Avalanche risk may be voluntary, for example skiing and snowmobiling, or involuntary, for example public transportation corridors. A worst-case avalanche scenario is most likely to occur in the Western Cordillera, resulting from a single large-scale weather pattern, where a cold period resulting in the development of a weak layer in the snowpack is followed by a series of major mid-winter storms. Emergency preparedness for avalanches is most advanced in western Canada. New education and information initiatives in Quebec and Newfoundland are aimed at improving preparedness there. Current research is focused on avalanche forecasting, weather forecasting for avalanche prediction, avalanche failure characteristics, forestry and avalanches and geomorphology and avalanches. An important area of future research is the impact of climate change on avalanches, particularly in northern Canada.