Regional conductance map of Andaman and Nicobar region

To image the electrical conductivity distribution, fluxgate magnetometers are operated at five sites in Andaman and Nicobar region. Transfer functions are estimated for the period range 8–128 min, from nighttime transient geomagnetic variations, using robust regression analysis. The observed induction arrows in Andaman Islands are found to point towards east despite deep sea located towards its west. This indicates that fore-arc basin (Andaman–Nicobar deep) is more conducting than the region of outer non-volcanic Island arc.Thin sheet model requires the conductance of 10,000–35,000 S (with increase conductivity towards the south) for explaining the observed induction pattern. The observed induction pattern at Andaman–Nicobar stations can be explained in terms of high conducting Cretaceous–Tertiary sediments filling the Andaman–Nicobar deep. High conductivity over Invisible bank has been attributed to the partial melts/volatile fluids derived from the subducting Indian plate that are intruding into the eastern margin of fore-arc basin through the West Andaman Fault (WAF).The induction pattern at Great Nicobar station (Campbell Bay) may be related to the highly conducting sediments filling the Mergui basin along with mafic intrusions. Also crustal transition occurs below the Mergui Terrace at the Malayan coast contributing to the enhanced conductivity anomaly.     

Self-similar magnetogasdynamic cylindrical shock waves,I

Similarity solutions describing the flow of a perfect gas behind a cylindrical shock wave with transverse magnetic field are investigated in an inhomogeneous medium. The total energy of the shock wave is assumed to be constant. A comparative study has been made between the results with and without magnetic field.     

Engineering geology of loess and loess-like deposits: a commentary on the Russian literature

Loess and loess-like deposits were much studied in the Soviet Union, and are currently under investigation in Russia and surrounding countries. There is a vast literature in Russian, which touches on all aspects of loess science and technology. In particular, the studies of the origin of collapsibility are almost totally in Russian, and of course studies on the various regions of Russia and the countries of the Former Soviet Union FSU appear in Russian. This review looks at the literature in Russian and attempts to pick out key contributors, major topics and works and to identify the critical regions and zones of investigation. Because so many regions of the FSU had people living on loess ground, there is a vast literature on engineering geology and ground engineering topics, and this tends to dominate all the literature on loess in Russian. Following Russian practice, the fine-grained deposits under consideration are divided into loess and loess-like deposits. Three main topics are recognised across the whole spectrum of loess research: formation and distribution of loess deposits; stratigraphy, cyclicity and palaeoclimatology; and engineering topics, in particular hydrocollapse and subsidence, and we concentrate on the engineering geology topics. An attempt is made, based on the map of Abelev and Abelev [Abelev, Yu.M., Abelev, M.Yu., 1968. Fundamentals of design and construction on collapsible macroporous soils, 2nd ed. Stroiizdat, Moscow, 431 pp. (in Russian)] of collapsing loess deposits, to define seven loess regions within the geographical limits of the old USSR. The seven regions are those where geotechnical problems might be expected.     

Research progress and prospect of snow avalanche disaster prevention and control北大核心CSCD

Snow avalanches are a major natural hazard in the cryosphere. It seriously threatens transportation corridors, energy transmission and communication lines, mining and touristic areas in the cold mountainous regions and often causes the destruction of infrastructure and human casualties, hindering the sustainable development of society and economy in mountainous areas. Under climate change and the expansion of human activities to alpine mountains, more population and infrastructure will expose to the risk of avalanches. In order to ensure the sustainable development in mountainous areas, the demand for the prevention and management of avalanche disasters is increasing. Based on the review of the main avalanche research progress in China since 1960 and the avalanche research results all over the world, this paper summarized the progress on the influencing factors and regional distribution of avalanche activities, avalanche formation and movement mechanism, avalanche monitoring and early warning, avalanche risk assessment and engineering prevention, as well as the frontier problems and scientific difficulties that need to be studied. In addition, the impact of climate change on avalanche activities and the interaction between human activities and avalanche activities are discussed. By looking forward to the future needs of avalanche disaster prevention and reduction, including the countermeasures, the research on avalanche in China is promoted. © 2022 by the Author(s).     

High precision intercalibration of 40Ar-39Ar standards

Intercalibration of one intralaboratory and three interlaboratory standards used in ⁴⁰Ar-³⁹Ar dating has been carried out. In order to provide homogeneous values for ${}^{40}\text{Ar}{}^1{}^{40}\text{K}$ the standards were prepared by careful handpicking. To control the neutron fluence in the Herald Reactor (A.W.R.E.) 16 aliquots of the standards were arranged along 0.6 × 60 cm of a single silica tube. The corrections for all known interferences from K, Ca, Cl were carefully assessed. Two of the hornblende standards, Hb3gr and MMHb-1 appear homogeneous at the 0.1% level while the other two standards, LP-6 and FY12a are not completely homogeneous. The mean values of ${}^{40}\text{Ar}{}^1{}^{40}\text{K}$ when referenced to the previously determined value for Hb3gr (turner et al., 1971) are:

     

997.

Unravelling the effects of melt depletion and secondary infiltration on mantle Re-Os isotopes beneath the French Massif Central     

J. Harvey  A. Gannoun  K.W. Burton  P. Schiano  O. Alard 《Geochimica et cosmochimica acta》2010,74(1):293-6182

Spinel lherzolite xenoliths from Mont Briançon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt/fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7-46.1 wt.%; CaO 0.9-3.6 wt.%), and many samples yield unradiogenic bulk-rock Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25% of Nd and 40% of Sr Critically, for Re-Os geochronology, melt/fluid infiltration is accompanied by the mobilisation of sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and/or homogenised average of these multiple events.Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from two populations of sulfide grain: (i) interstitial, metasomatic sulfide with low [Os] and radiogenic ¹⁸⁷Os/¹⁸⁸Os, and (ii) primary sulfides with high [Os] and unradiogenic ¹⁸⁷Os/¹⁸⁸Os, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for >97% of bulk-rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s).     

998.

The Coolgardie Goldfied,Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane     

J. T. Knight  D. I. Groves  J. R. Ridley 《Mineralium Deposita》1993,28(6):436-456

Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west compressional far-field stress régime, but with local heterogeneous stress orientations. This occurred after an earlier period of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase + calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels.     

999.

Effects of diffusional modification of garnet growth zoning on P-T path calculations   总被引：2，自引：0，他引：2  

Frank P. Florence  Frank S. Spear 《Contributions to Mineralogy and Petrology》1991,107(4):487-500

The effect of intragranular diffusion on chemical zoning in garnet and on P-T paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth. Syn-and post-growth diffusion of Mg–Fe–Mn–Ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635°C. Compositions from zoned garnet, as modified by diffusion, hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure-temperature (P-T) paths from isobaric and polybaric model histories. P-T path calculations, based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635°C, show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths. Retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path. The magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition. The effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating-only models that reached peak temperatures approximately 15–30°C higher. Compositions of garnets with radii less than 1 mm, that reached actual peak temperatures of 605°C along temperature-time histories characteristic of regional metamorphism, experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories. Path distortions were significant at heating/cooling rates less than 10°C/Ma, but rapidly diminished for rates faster than this. In polybaric simulations diffusion effects were least noticeable when the actual pressure-temperature conditions changed in a clockwise sense (i.e., convex to higher P and higher T), but apprecciable modification was seen in path models that underwent counterclockwise changes in P and T. Reequilibration of garnet rim compositions occurred during cooling on all paths, and temperature maxima obtained from garnet-biotite geothermometry underestimated actual peak conditions by 40 to 70°C. Calculations suggest that P-T path trajectories calculated from garnets of at least 1 mm size, and that experienced actual thermal maxima below 585°C, are not likely to be distorted by diffusional effects during regional metamorphism. However, P-T path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history. The partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling, seriously complicating the task of quantitatively estimating diffusion effects on path calculations.     

1000.

Granitoids of the Ufalei block (South Urals): Sr-Nd isotope systematics, geodynamic position and genetic reconstructions     

Yu. L. Ronkin  G. Yu. Shardakova  A. V. Maslov  E. S. Shagalov  O. P. Lepikhina 《Stratigraphy and Geological Correlation》2009,17(2):137-145

Petrogeochemical and isotopic-geochronological signatures in granitoids developed in structures with complex geological history represent an important feature for reconstructing paleogeodynamic settings. Granitoids are widespread in the western slope of the Urals, where the Uralian Orogen contacts via a collage of different-age blocks of the east European Platform. The Ufalei block located in the Central Urals megazone at the junction between the South and Middle Urals’ segments represents one such boundary structure with multistage geological evolution. The isotopic ages obtained by different methods for acid igneous rocks range from 1290 to 245 Ma. We determined close Rb-Sr and Sm-Nd ages (317 Ma) for granites of the Nizhnii Ufalei Massif. By their petrochemical parameters, granitoids and host granite-gneisses differ principally from each other: the former are close to subduction-related, while the latter, to continental-riftogenic varieties. The primary ratio (⁸⁷Sr/8⁶Sr)₀ = 0.70428 and ?_Nd ≈ +4 values indicate significant contribution of oceanic (island-arc?) material to the substrate, which served as a source for granites of the Nizhnii Ufalei Massif. Model Nd ages of granites vary from 641 to 550 Ma. Distinct oceanic rocks and varieties with such ages are missing from the surrounding structures. New isotopic dates obtained for ultramafic and mafic rocks from different zones of the Urals related to the Cadomian cycle imply development of unexposed Upper Riphean-Vendian “oceanic” rocks in the central part of the Ufalei block, which played a substantial role in the formation of the Nizhnii Ufalei granitoids. Such rocks could be represented, for example, by fragments of the Precambrian Timanide-type ophiolite association. The analysis of original materials combined with published data point to the heterogeneous composition and structure of the Ufalei block and a significant part of the western segment of the Central Uralian Uplift and extremely complex geological history of the region coupling the Uralian Orogen with the East European Platform in the present-day structure.     

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Hb3gr	hbld	$\text{.08504 ± .05\%}$(±lσ)	1072. m.y.
MMHb-1	hbld	$\text{.03493 ± 05\%}$	518.9 m.y.
LP-6	biot	$\text{.007735 ± .13\%}$	128.5 m.y.
FY12a	hbld	$\text{.02858 ± .25\%}$	435.0 m.y.

Unravelling the effects of melt depletion and secondary infiltration on mantle Re-Os isotopes beneath the French Massif Central

Spinel lherzolite xenoliths from Mont Briançon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt/fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7-46.1 wt.%; CaO 0.9-3.6 wt.%), and many samples yield unradiogenic bulk-rock Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25% of Nd and 40% of Sr Critically, for Re-Os geochronology, melt/fluid infiltration is accompanied by the mobilisation of sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and/or homogenised average of these multiple events.Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from two populations of sulfide grain: (i) interstitial, metasomatic sulfide with low [Os] and radiogenic ¹⁸⁷Os/¹⁸⁸Os, and (ii) primary sulfides with high [Os] and unradiogenic ¹⁸⁷Os/¹⁸⁸Os, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for >97% of bulk-rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s).     

The Coolgardie Goldfied,Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane

Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west compressional far-field stress régime, but with local heterogeneous stress orientations. This occurred after an earlier period of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase + calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels.     

Effects of diffusional modification of garnet growth zoning on P-T path calculations

The effect of intragranular diffusion on chemical zoning in garnet and on P-T paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth. Syn-and post-growth diffusion of Mg–Fe–Mn–Ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635°C. Compositions from zoned garnet, as modified by diffusion, hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure-temperature (P-T) paths from isobaric and polybaric model histories. P-T path calculations, based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635°C, show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths. Retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path. The magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition. The effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating-only models that reached peak temperatures approximately 15–30°C higher. Compositions of garnets with radii less than 1 mm, that reached actual peak temperatures of 605°C along temperature-time histories characteristic of regional metamorphism, experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories. Path distortions were significant at heating/cooling rates less than 10°C/Ma, but rapidly diminished for rates faster than this. In polybaric simulations diffusion effects were least noticeable when the actual pressure-temperature conditions changed in a clockwise sense (i.e., convex to higher P and higher T), but apprecciable modification was seen in path models that underwent counterclockwise changes in P and T. Reequilibration of garnet rim compositions occurred during cooling on all paths, and temperature maxima obtained from garnet-biotite geothermometry underestimated actual peak conditions by 40 to 70°C. Calculations suggest that P-T path trajectories calculated from garnets of at least 1 mm size, and that experienced actual thermal maxima below 585°C, are not likely to be distorted by diffusional effects during regional metamorphism. However, P-T path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history. The partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling, seriously complicating the task of quantitatively estimating diffusion effects on path calculations.     

Granitoids of the Ufalei block (South Urals): Sr-Nd isotope systematics, geodynamic position and genetic reconstructions

Petrogeochemical and isotopic-geochronological signatures in granitoids developed in structures with complex geological history represent an important feature for reconstructing paleogeodynamic settings. Granitoids are widespread in the western slope of the Urals, where the Uralian Orogen contacts via a collage of different-age blocks of the east European Platform. The Ufalei block located in the Central Urals megazone at the junction between the South and Middle Urals’ segments represents one such boundary structure with multistage geological evolution. The isotopic ages obtained by different methods for acid igneous rocks range from 1290 to 245 Ma. We determined close Rb-Sr and Sm-Nd ages (317 Ma) for granites of the Nizhnii Ufalei Massif. By their petrochemical parameters, granitoids and host granite-gneisses differ principally from each other: the former are close to subduction-related, while the latter, to continental-riftogenic varieties. The primary ratio (⁸⁷Sr/8⁶Sr)₀ = 0.70428 and ?_Nd ≈ +4 values indicate significant contribution of oceanic (island-arc?) material to the substrate, which served as a source for granites of the Nizhnii Ufalei Massif. Model Nd ages of granites vary from 641 to 550 Ma. Distinct oceanic rocks and varieties with such ages are missing from the surrounding structures. New isotopic dates obtained for ultramafic and mafic rocks from different zones of the Urals related to the Cadomian cycle imply development of unexposed Upper Riphean-Vendian “oceanic” rocks in the central part of the Ufalei block, which played a substantial role in the formation of the Nizhnii Ufalei granitoids. Such rocks could be represented, for example, by fragments of the Precambrian Timanide-type ophiolite association. The analysis of original materials combined with published data point to the heterogeneous composition and structure of the Ufalei block and a significant part of the western segment of the Central Uralian Uplift and extremely complex geological history of the region coupling the Uralian Orogen with the East European Platform in the present-day structure.