Role of plagioclase and reaction softening in a metagranite shear zone at mid‐crustal conditions (Gotthard Massif,Swiss Central Alps)

A lower amphibolite Alpine shear zone from the Fibbia metagranite (Gotthard Massif, Central Alps) has been studied to better understand the parameters controlling strain localization in granitic rocks. The strain gradient on the metre‐scale shows an evolution from a weakly deformed metagranite (Qtz_I–Kfs_I–Ab_I–Bt_I ± Pl_II–Zo_I–Phg_I–Grt) to a fine banded ultramylonite (Qtz_II–Kfs_II–Ab_II–Pl_II–Bt_II–Phg_II ± Grt–Zo_II). Strain localization is coeval with dynamic recrystallization of the quartzofeldspathic matrix and a modal increase in mica, at the expense of K‐feldspar. The continuous recrystallization of plagioclase during deformation into a very fine‐grained assemblage forming anastomosed ribbons is interpreted as the dominant process in the shear zone initiation and development. The shear zone initiated under closed‐system conditions with the destabilization of metastable Ab_I–Zo_I porphyroclasts into fine‐grained (20–50 μm sized) Ab_II–Pl_II aggregates, and with minor crystallization of phengite at the expense of K‐feldspar. The development of the shear zone requires a change in state of the system, which becomes open to externally derived fluids and mass transfer. Indeed, mass balance calculations and thermodynamic modelling show that the ultramylonite is characterized by gains in CaO, FeO and H₂O. The progressive input of externally derived CaO drives the continuous metamorphic recrystallization of the fine‐grained Ab_II–Pl_II aggregate into a more Pl_II‐rich and finer aggregate. Input of water favours the crystallization of phengite at the expense of K‐feldspar to form an interconnected network of weak phases. Thus, recrystallization of 50% of the bulk rock volume would induce a decrease of the strength of the rock that might contribute to the development of the shear zone. This study emphasizes the major role of metamorphic reactions and more particularly plagioclase on strain localization process. Plagioclase represents at least one‐third of the bulk rock volume in granitic systems and forms a stress‐supporting framework that controls the rock rheology. Therefore, recrystallization of plagioclase due to changes in P–T conditions and/or bulk composition must be taken into account, together with quartz and K‐feldspar, in order to understand strain localization processes in granites.     

浙江大桥坞铀矿床斑岩体的“双层结构”与铀矿化

大桥坞斑岩型铀矿床产于中生代蒙山破火山口中一石英斑岩体的内外接触带及其隐爆角砾岩筒中。铀成矿主要与斑岩体的“双层结构”有关,即”双层结构”成矿模式。浅部石英斑岩为含矿岩体,深部花岗斑岩为矿源岩体。矿化以裂隙型、角砾型为主,矿体的形态主要呈细脉状、浸染状,与围岩界线不清。成岩物质来源于壳幔混源的富铀钙碱性岩浆。成矿物质主要来源于该深部富铀花岗岩浆冷凝结晶分异产生的富铀岩浆期后热液。其成矿过程主要是富铀岩浆期后热液流体沿断裂构造系统上升、运移,在浅部石英斑岩体内外接触带发生隐爆,成岩成矿。早期铀-赤铁矿化阶段形成贫铀矿化,晚期铀-金属硫化物矿化阶段,形成富铀矿化,两期叠加作用致富。     

松嫩平原湖泊鱼类群聚结构

根据2008-2009年的湖泊渔业资源调查,分析松嫩平原20个湖泊鱼类群聚结构特征.结果表明,20个湖泊鱼类群聚由44种鱼类组成,隶属于4目9科34属.21种鱼类的优势度指数在10000以上,17种鱼类为优势种.群聚Margalef指数(dMa值)为0.908-3.096(1.615±0.615),Simpson指数(λ值)为0.090-0.568(0.222±0.110),Gini指数(D值)为0.432-0.910(0.778±0.107),Shannon-Wiener指数(H值)为1.012-2.690(1.878±0.431),Pielou指数(J值)为0.407-0.915(0.777±0.107).dMa值与λ值、D值、H值和J值的相关系数分别为-0.670、0.665、0.867和0.340;λ值与D值、H值和J值的相关系数分别为-0.631、-0.924和-0.896;D值与H值和J值的相关系数分别为0.924和0.896;H值和J值的相关系数为-0.639.总体上,20个湖泊鱼类群聚结构处在不稳定中.     

Sidewall erosion: Insights from in situ-produced 10Be concentrations measured on supraglacial clasts (Mont Blanc massif,France)

Sidewall erosion because of rockfalls is one of the most efficient erosional processes in the highest parts of mountain ranges; it is therefore important to quantify sidewall erosion to understand the long-term evolution of mountainous topography. In this study, we analyse how the ¹⁰Be concentration of supraglacial debris can be used to quantify sidewall erosion in a glaciated catchment. We first analyse, in a glaciated catchment, the cascade of processes that move a rock from a rockwall to a supraglacial location and propose a quantitative estimate of the number of rockfalls statistically mixed in a supraglacial sand sample. This model incorporates the size of the rockwall, a power law distribution of the size of the rockfalls and the mean glacial transport velocity. In the case of the Bossons glacier catchment (Mont Blanc massif), the ¹⁰Be concentrations obtained for supraglacial samples vary from 1.97 ± 0.24 to 23.82 ± 1.68 × 10⁴ atoms g⁻¹. Our analysis suggests that part of the ¹⁰Be concentration dispersion is related to an insufficient number of amalgamated rockfalls that does not erase the stochastic nature of the sidewall erosion. In the latter case, the concentration of several collected samples is averaged to increase the number of statistically amalgamated rockfalls. Variable and robust ¹⁰Be-derived rockwall retreat rates are obtained for three distinct rockfall zones in the Bossons catchment and are 0.19 ± 0.08 mm year⁻¹, 0.54 ± 0.1 mm year⁻¹ and 1.08 ± 0.17 mm year⁻¹. The mean ¹⁰Be retreat rate for the whole catchment (ca. 0.65 mm year⁻¹) is close to the present-day erosion rate derived from other methods. © 2019 John Wiley & Sons, Ltd.     

黑龙江老柞山金矿床地质、地球化学特征及其成矿构造背景

在对老柞山矿床地质特征系统研究的基础上,对与成矿有关的岩石开展了进一步的主、微量元素地球化学测试分析。结果显示与成矿有关的花岗岩具富Sr、低Ti的高钾钙碱性火山弧花岗岩的特征,闪长玢岩具偏铝质钙碱性岩系列特征。岩石样品均呈现出富集轻稀土元素和大离子亲石元素(LILEs)、相对亏损重稀土元素和Nb、Ta、Ti等高场强元素(HFES)的特征,显示了与岛弧或活动大陆边缘岩浆岩相似的属性。流体包裹体特征显示,成矿流体主要来自岩浆水,矿床具中低温(125.6℃~324.5℃)、中浅成(1.0~7.1 km)成矿的特点。与成矿有关岩浆岩显示幔源岩浆特征,可能是地幔交代作用部分熔融所形成的。结合区域上同时期的火成岩时空展布认为,老柞山金矿床成矿可能分别与晚古生代古亚洲洋板块北向俯冲作用和中生代太平洋板块俯冲作用密切相关。     

福建省前寒武纪变质岩块体地球化学特征

利用1∶20万区域化探水系沉积物测量资料,分析福建省前寒武纪变质岩块体的元素组合,对比了各地层小区前寒武纪变质岩地层(岩)组的地球化学特征,并对其时间演化规律进行了探讨。研究表明,表征福建省前寒武纪变质岩地球化学变化的主要元素组合为与岩浆作用有关的铁族元素和稀有、放射性元素。总体上从老到新SiO2/Al2O3比值逐渐升高,铁族元素和K2O含量逐渐降低,发现部分地层(岩)组的地球化学特征较为特殊,为今后该区变质岩研究提供了新的线索。     

The desertification and its rehabilitation inSongnen Sandy Land

IntroductionSongnenSandyLand,locatedinnorthWestofNortheastChinaplain,belongstosub-htaldmeedow-chemozemzoneanditSthedamntallandformisthefluvialplaincoveredbydunes.ThelinefromtheSecondSonghuamvernmningtbrOughtothelowerreachesoftheHuolinmver(i.e.Taolaizhao-Qian'an-Taonan)isthenasthesouthboundaryofthesandylandwhichisnearthenoItheaStofHorqinSandyLandWhilethelinealongNehe1indha-AndaThaomp-TaolaitheoistakenastheeaStboundary.Withalengthof38okinfromnorthtosouthtalwidthof26okInfromwesttoeasLth…     

松嫩平原生态地质环境变迁构造-气候旋回机制探讨

论述了松嫩平原第四纪以来生态地质环境的变迁过程并对其演化机制进行了初步探讨。新构造运动是本区生态环境变迁的主要内动力,直接影响地形起伏和地貌形态,制约水系的变迁和沉积环境的演化。东亚季风作为新构造运动的间接产物,对本区气候的波动产生重要影响。新构造运动和东亚季风共同控制着本区第四纪以来生态地质环境的演化历史。     

松嫩平原西部盐沼的形成与演化

本文运用环境变迁的理论和方法,较系统地分析了松嫩平原西部盐沼的形成与演变机制。本区沼泽形成的主要控制因素是古河道变迁、新构造运动、风力作用、古气候变化。沼泽残余盐碱化与晚更新世松辽大湖的消亡、河道变迁有关,现代盐碱化则主要是水利工程、植被破坏和气候变化引起的,而且呈迅速加剧的趋势。防治沼泽盐碱化的主要措施是实施水稻和芦苇开发、大规律建立蓄洪区和自然保护区。     

SPATIAL VARIATIONS OF SHEET FLOW AND SEDIMENT TRANSPORT ON AN AGRICULTURAL FIELD

This paper presents the spatial and temporal variations of sheet flow and sediment transport on an agricultural field under natural rainfall. The study site is located in western France (Rennes, Brittany) where rainfall amount and intensity are moderate. It is a field of garden-peas presenting a convex–concave gentle slope where only sheet flow is observed. A network of simple, low-cost collectors is used to measure sheet flow and sediment transport along the slope and to estimate the spatial variability for a given position on the slope. The water and sediment discharge was measured for each rainfall event from February to June. The analysis of space and time variations according to the characteristics of the rainfall events, changes in vegetation and soil crusting shows that the sheet flow is low and the runoff coefficient decreases as the slope length increases. Infiltration is particularly active in concave parts of the hillslope whatever the characteristics of the rainfall events. Sediment transport decreases in time as the soil crusts and vegetation develops.