Protolith ages and deformation mechanism of metamorphic rocks in the Zhangbaling uplift segment of the Tan-Lu Fault Zone

Protolith ages and Indosinian deformation mechanism of metamorphic rocks in the Zhangbaling uplift segment of the Tan-Lu Fault Zone are important, unsolved problems. Our LA-ICP-MS zircon dating work indicates that protolith ages of the greenschist-facies Zhangbaling Group are 754–753 Ma, and those of the amphibolite-facies Feidong Complex are 800–745 Ma. These rocks belong to the earliest cover of the Yangtze Plate. Their ages and metamorphic features suggest that the rocks did not come from the Dabie Orogen. The Indosinian structures in the Zhangbaling Group and lower Sinian strata formed in a flatlying ductile detachment zone with a shear sense of top-to-the-SSW whereas those in the underlying Feidong Complex are characterized by ENE-WSW inclined folds developed under a ductile regime. It is suggested therefore that the sinistral Tan-Lu Fault Zone of the Indosinian period is buried under the Hefei Basin west of the Zhangbaling uplift segment and the uplift segment is a displaced block neighboring the fault zone. Detachment deformation between the upper rigid and lower ductile crust during displacement of the Zhangbaling uplift segment resulted in the formation of the flat-lying ductile detachment zone and its underlying drag fold zone of a ductile regime. The protolith ages and deformation mechanism in the Zhangbaling uplift segment further prove sinistral origination of the Tan-Lu Fault Zone during the continent-continent collision of the North China and Yangtze plates and support the indentation model for the two-plate collision that considers the Tan-Lu Fault Zone as an oblique convergence boundary.     

A laser ablation inductively coupled plasma mass spectrometry study of the distribution of chalcophile elements among sulfide phases in sedimentary and magmatic rocks of the Duluth Complex,Minnesota, USA

Nickel-copper sulfide deposits occur in the basal unit of the Partridge River Intrusion, Duluth Complex (Minnesota, USA). Many lines of evidence suggest that these sulfides are formed after assimilation of the proterozoic S-rich black shales, known as the Bedded Pyrrhotite Unit. In addition to S, black shales are enriched in Te, As, Bi, Sb and Sn (TABS) and the basaltic magma of the intrusion is contaminated by the partial melt of the black shales. The TABS are chalcophile and together with the platinum-group elements, Ni and Cu partitioned into the magmatic sulfide liquid that segregated from the Duluth magma. The TABS are important for the formation of platinum-group minerals (PGM) thus their role during crystallization of the base metal sulfide minerals could affect the distribution of the PGE. However, the concentrations of TABS in magmatic Ni-Cu-PGE deposits and their distribution among base metal sulfide minerals are poorly documented. In order to investigate whether the base metal sulfide minerals host TABS in magmatic Ni-Cu-PGE deposits, a petrographic and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) study has been carried out on base metal sulfide and silicate phases of the Partridge River Intrusion, Duluth Complex.Petrographic observations showed that the proportions of the base metal sulfide minerals vary with rock type. The sulfide assemblage of the least metamorphosed Bedded Pyrrhotite Unit from outside the contact metamorphic aureole consists of pyrite with minor pyrrhotite plus chalcopyrite (<5%), whereas within the contact aureole the sulfide assemblage of the Bedded Pyrrhotite Unit rocks consists dominantly of pyrrhotite (>95%) with small amount of chalcopyrite (<2%). The sulfide mineral assemblage in the xenoliths of the Bedded Pyrrhotite Unit and in the mafic rocks of the basal unit contains two additional sulfides, pentlandite and cubanite.Our LA-ICP-MS study shows that sulfides of the Bedded Pyrrhotite Unit are rich in TABS; consistent with these S-rich black shales being the source of TABS that contaminated the mafic magma. Most of the TABS are associated with sulfides and platinum-group minerals in the rocks of the Bedded Pyrrhotite Unit from the contact aureole, the Bedded Pyrrhotite Unit xenoliths and the mafic rocks of the Duluth Complex. In addition to these phases the laser maps show that silicate phases, i.e., orthopyroxene and plagioclase contain Sn and Pb respectively. In contrast, in the least metamorphosed samples of the Bedded Pyrrhotite Unit from outside the contact aureole although the pyrite contains some TABS mass balance calculations indicates that most the TABS are contained in other phases. In these rocks, galena hosts significant amounts of Te, Bi, Sb, Sn and Ag and few very small grains of Sb-rich phases were also observed. The host phases for As were not established but possibly organic compounds may have contributed.     

Variability of baroclinic tidal currents on the Mackenzie Shelf,the Southeastern Beaufort Sea

Semidiurnal tidal currents on the outer shelf of the Mackenzie Shelf in the Beaufort Sea were found to be strongly influenced by the locally generated baroclinic tide. Two primary factors are involved in this process: (1) the sharp shelf break along the northeastern Mackenzie Shelf, promoting the generation of vigorous internal tidal waves; and (2) the proximity to critical latitudes for M₂ and N₂ motions locking these waves and preventing them from leaving the source region. As a result, internal tides are resonantly trapped between the shelf and critical latitudes. The physical properties and temporal variations of tidal motions were examined using current meter measurements obtained from 1987–1988 at four sites (SS1, SS2, SS3, and SS4) offshore of the shelf break at depths of ∼200 m. Each mooring had Aanderaa RCM4s positioned at ∼35 m below the surface and ∼50 m above the bottom. Complex demodulation was used to compute the envelopes (amplitude modulation) of these components. A striking difference in the variability of clockwise (CW) and counterclockwise (CCW) tidal currents was found. The CW tides are highly variable, have greater amplitude, exhibit a burst-like character associated with wind events and contain about 80% of the total energy of the semidiurnal tidal currents. In contrast, the CCW components have a more regular temporal regime with distinct monthly, fortnightly and 10-day modulation at astronomical periodicities associated with frequency differences M₂–N₂ (0.03629 cpd), S₂–M₂ (0.06773 cpd), and S₂–N₂ (0.10402 cpd). Significant horizontal correlation of the CW current envelopes was found only between stations near the northeast Mackenzie Shelf, indicating this to be the main area of baroclinic internal wave generation.     

河北阜平杂岩长英质片麻岩中一些浅色体的地球化学性质及其地质意义

阜平杂岩中的浅色体主要分为2种类型,早期浅色体基本沿片麻理分布,较富钾长石（微斜长石）、石英,贫斜长石,形成于低水深熔或压溶条件下,水含量较低,局部脱水但尚未达到整体脱水的程度;晚期浅色体切割现有的片麻理,较富斜长石、石英,贫钾长石,形成于有水熔融条件下,平阳奥长花岗岩的出现即与晚期浅色体相对应。南营片麻岩、南营浅色花岗片麻岩的地球化学性质与其他片麻岩具有明显的不一致性。浅色体的稀土元素配分型式较复杂,根据对元素相对富集或亏损的分析得知,大多数浅色体的形成受围岩成分的控制,从早期浅色体向晚期浅色体发展,稀土元素含量向LREE相对亏损、HREE相对富集的方向转化,某些微量元素有明显的继承性。与地球化学特征相比,矿物组合的确定对浅色体阶段的归属判定更为重要。阜平杂岩变质演化不是简单的进变质脱水反应,其水分含量的变化十分复杂,对其深入研究有助于更好地了解该区地质和构造的演化过程。     

Extensional deformation along the late Precambrian-Cambrian Baltoscandian passive margin: the Sarektjåkkå Nappe,Swedish Caledonides

The structural evolution of a part of the late Precambrian Baltoscandian passive margin just before the inception of seafloor spreading is described, recording the change from deformation by faulting to dominantly magmatic extension of the crust. The allochthon of the Scandinavian Caledonides contains the imbricated passive margin of continental Baltica overlain by various exotic terranes. The Sarektjåkkå Nappe in the Seve Nappe Complex, which contains the outer parts of Baltica's passive margin, consists of sedimentary rocks, occurring as screens between Vendian (573±74 Ma) diabase dykes. These dykes constitute 70–80% of the nappe and locally form sheeted dyke complexes. The Sarektjåkkå Nappe largely escaped penetrative Caledonian deformation and preserves igneous, metamorphic and structural elements that are linked to the evolution of a pre-Caledonian rift to a passive continental margin. Extensional deformation before dyke emplacement is recorded by normal faults, pull-apart structures and folds. Unconformities, dykes affected by brittle deformation, and fluidization of sediments during dyke emplacement indicate close relations between the deposition of sediments, extensional deformation and dyke emplacement. The Sarektjåkkå Nappe is compared with other parts of the Baltica's passive margin and its tectonic evolution is discussed.     

A local groundwater divide in a regional system

This paper presents an analytical solution of a groundwater flow problem involving a local drainage system embedded in a regional system. As an extension of the usual complex variable methods it combines the planes of the hodograph and the Zhukovsky function. This combination of basic parameters appears to be useful for problems involving a free water table and horizontal potential lines. The solution may elucidate the relation between local and regional water divides.     

Early applications of geographical information systems at the Institute of Terrestrial Ecology

Abstract

The Institute of Terrestrial Ecology (ITE) Digital Cartographic Service provides a range of services to the scientists of ITE (and others) including various techniques within the field of geographical information systems (GIS). This paper discusses work of this type as it is applied to specific ITE projects. The emphasis is very much on GIS applications to ‘real ecological science’ such as the ‘acid water’ problem and the change of forest areas into heathland. Some comments are made on fundamental GIS techniques, their use and limitations. At ITE, GIS applications are growing out of an existing use of digital mapping and remote sensing technologies; they have not purchased a GIS system ‘off the shelf.’     

Changes in SOC and nutrients under intensive tillage in two types of slope landscapes

The net effect of tillage erosion on soil properties would be associated with the spatial variation in soil constituents,and therefore plays an important role in ecological agriculture.We conducted a consecutive tillage by hoeing 15 times during a period with no rainfall in the two slope landscapes(a linear slope and complex slope) of the Yangtze Three Gorges reservoir areas,to examine the relationship between soil erosion rates and the variations in soil chemical properties and compare the effects of soil redistribution on SOC and nutrients between the linear and complex slopes.After the simulated tillage,notable changes in 137 Cs inventories of the soil occurred in the summit and toeslope positions on the linear slope,while there were significant changes in 137 Cs inventories at convex and concave positions on the complex slope.Soil profile disappeared at the summit slope boundary,with the exposure area of 16.0% and 7.6% of the experimental plot,respectively,for the linear and complex slopes due to no soil replacement.Soil organic C and nutrients were completely depleted with the disappearance of soil profiles at soil eroding zones,whereas a remarkable increase in SOC,total N and available nutrient concentrations of the post-tillage surface soil and a decrease in total nutrient concentrations(P and K) were found at depositional zones on the linear slope.For the complex slope,however,changes in SOC and nutrient concentrations of the post-tillage surface soil exhibited a patterndifferent from that on the linear slope,which showed a remarkable decrease in SOC and total nutrient concentrations but a slight increase in available nutrient concentrations after tillage in the toeslope position.Due to the gradual increase in soil depth from top to bottom of the slope,SOC and nutrient inventories in the soil profiles were significantly correlated with soil redistribution rates on both the linear and complex slopes.Tillage causes remarkable changes of soil chemical properties in the surface soil layer and soil profile,and increases SOC and nutrient inventories for the soil profile downslope in steeply sloping landscapes.     

Modelling the stress-strain behaviour of saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements

Measurement of complex electrical conductivity as a function of frequency is an extremely sensitive probe for changes in pore and crack volume, crack connectivity, and crack surface topography. Such measurements have been made as a function of pore fluid chemistry, hydrostatic confining pressure, as well as uniaxial and triaxial deformation. This paper will; (1) describe the effects of triaxial deformation on the complex electrical conductivity of saturated porous rocks, (2) use the electrical data to model the mechanical stress-strain behaviour, and (3) compare the modelled behaviour with the stress-strain behaviour measured during the deformation. Experimental conductivity data tracks how the rock undergoes compaction with progressive loss of crack volume, followed by dilatation due to new crack formation, growth of existing cracks, crack interlinkage, and finally failure, as axial strain is increased. We have used the complex electrical data to produce a direction-sensitive (anisotropic) crack damage parameter, and used it to calculate the effective Young's modulus by employing the models of Walsh and Bruner. Comparison of the synthetic stress-strain curves so produced, with the experimentally derived stress-strain curves shows good agreement, particularly for undrained tests. This modelling is an improvement on similar curves produced using isotropic crack damage parameters derived from acoustic emission data. The improvement is likely to be due to the directional sensitivity of the electrical conductivity measurement, and its ability to discriminate between the formation of isolated cracks, and those cracks that contribute to the inter-connected crack space i.e. those cracks upon which transport properties of the rock such as electrical conductivity, and mechanical properties depend most critically during triaxial deformation.     

藏南吉隆地区大喜马拉雅结晶杂岩的新生代变质作用和构造意义

大喜马拉雅结晶杂岩位于喜马拉雅造山带的核部,其变质作用的研究是揭示碰撞造山作用和动力学的关键。本文对喜马拉雅造山带中部吉隆地区的大喜马拉雅结晶杂岩中的片麻岩和片岩进行了岩石学和锆石U-Pb年代学的研究,表明这些高级变质岩经历了高角闪岩相的变质作用和部分熔融,以及近等温降压的退变质过程。片麻岩和片岩的峰期变质矿物组合为斜长石+钾长石+黑云母+白云母+石英±石榴子石±钛铁矿和石榴子石+蓝晶石+斜长石+黑云母+白云母+钛铁矿+石英,晚期退变质矿物组合为斜长石+钾长石+黑云母+白云母+夕线石+石英±石榴子石和石榴子石+夕线石+斜长石+黑云母+白云母+钛铁矿+石英。相平衡模拟研究表明,岩石的峰期变质条件为685~755℃和9.5~12kbar,退变质作用条件为675~685℃和6~7.4kbar。锆石U-Pb年代学表明,高级变质岩的部分熔融时间为38~16Ma,熔体结晶时间为17~15Ma。本次研究表明,大喜马拉雅结晶杂岩中、上部并没有普遍经历高压麻粒岩相变质作用,部分地区包括吉隆地区经历的是高角闪岩相变质作用。此外,本文在吉隆地区高级变质的大喜马拉雅结晶杂岩中识别出较低级变质的特提斯喜马拉雅岩系单元。结合已发表的藏南拆离断裂的主要活动时间,本文认为构造楔模型更适用于研究区喜马拉雅造山过程的解释。