Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

ABSTRACT The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with ⁴⁰Ar/³⁹ Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new ⁴⁰Ar/³⁹Ar data suggest a minimum time-integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma^-1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre-dated the rapid cooling. Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.     

我国地质岩心钻探设备发展历程及方向

通过回顾我国地质岩心钻探设备发展历程，分析岩心钻探设备的现状，优势和不足，提出了对其今后发展的几点看法。     

钻孔岩芯饼状断裂破坏机制

本文首先阐述岩芯饼状断裂及其特征, 进而用断裂力学的理论与方法对岩饼破坏进行了断裂扩张机制的分析, 不但合理地解释了断裂特征及断裂破坏机制, 且以二滩坝址为例, 分析了产生钻孔岩饼的微裂纹条件及应力条件, 为定量地评价坝址地应力状态, 包括其大小及方向提供了一种新的方法。上述不仅具有重要的理论意义, 而且具有重要的工程实际意义。     

探测冰盖和多年冻土层的融冰探头研发

The exploration of ice sheets by melting vertical holes into the ground has some tradition in terrestrial glaciology. Such probes have been used since the 1960's to investigate the vertical structure of the ice in Greenland and Antarctica and in alpine glaciers. In this paper we look into the possibility to develop similar devices for use on extraterrestrial icy bodies, like e.g. the polar areas on Mars or the icy satellites of the outer solar system. We report on some basic experiments performed in the cryo-vacuum laboratory of the Space Research Institute of the Austrian Academy of Sciences, Graz. In these experiments the penetration of a simple melting probe into compact and porous water ice (with a snow-like texture) was monitored, both under vacuum conditions and under air pressure. The observed penetration speeds for a given power supply are compared with a simple mathematical model. We conclude that a miniature melting probe with small overall dimensions and a reasonable power demand could well be part of the payload of a future planetary mission, for example to the poles of Mars. Such missions are currently under discussion in several space agencies. Moreover such probes could also e? ectively be used in terrestrial environments. A possible design is presented at the end of the paper.     

土壤的冷生构造及其对阿拉斯加土地利用的意义

Cryogenic structure (patterns made by ice inclusions) in seasonally frozen and permafrost-af-fected soils result from ice formation during freezing. Analysis of cryogenic structures in soils is essential to our understanding of the cryogenic processes in soils and to formulating land use management interpretations. When soils freeze, the freezing front moves downward and attracts water moving upward resulting in mainly horizontal lenticular ice formation. Platy and lenticular soil structures form between ice lenses in upper active layer. The reticular soil structure usually forms above the permafrost table caused by freeze-back of the permafrost. The upward freeze-back resulted in platy soil structure and the volume changes following the annual freeze-thaw cycle resulted in vertical cracks. The combined result is an ice-net formation with mineral soils embedded in the ice net. The upper permafrost layer that used to be a part of the active layer has an ice content exceeding 50% due to repeated freeze-thaw cycles over time. The mineral soils appear in blocks embedded in an ice matrix. The permafrost layer that never experienced the freeze-thaw cycle often consists of alternate layers of thin ice lens and frozen soils with extreme hard consistence and has relatively lower ice content than the ice-rich layer of the upper permafrost. Ice contents and thaw settling potentials associated with each cryogenic structure should be considered in engineering and land use interpretations.     

用混合和扩散云室对大气冰核浓度的观测分析

继1963年用混合云室对春季冰核浓度观测之后，1994～1996年，在同季节、同地点用混合和静力扩散云室又做了同步观测。对取得的资料进行整理分析，得出了凝华核浓度在不同天气背景下占总冰核浓度的百分比，两种云室测值的对应关系；分析了扩散云室的容积效应及冰核浓度随气压、能见度等气象因子的变化。     

我国主要金矿类型中黄铁矿"电子-空穴心"特征及影响因素

通过对我国主要金矿类型中黄铁矿导电类型的分析表明，黄铁矿“电子。空穴心”与金矿床的成因类型具有密切的联系，华北地台太古代变基性火山岩中的金矿床(绿岩带型)、热水淋滤型(卡林型)金矿床，黄铁矿多为单一的“电子心”型导电。大多数产于中生代岩体中的中深脉状金矿、火山次火山岩中的金矿床，黄铁矿为“电子心”、“空穴心”混合型导电，个别的为单一的“空穴心”导电。黄铁矿的“电子．空穴心”受杂质成分As、Co、Nl在成矿背景中的丰度，进入黄铁矿品格中的替代能力的差异、补偿类质同象现象、成矿时温度以及．f(S2)等多种因素的耦合制约。     

Seismic image of the deep crust at the eastern margin of the Alps (Austria): indications for crustal extension in a convergent orogen

A 39-km-long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the Eastern Alps (Austria). The ESE–WNW-trending, low-fold seismic line crosses Austroalpine basement units and extends approximately from 20 km west of the Penninic window group of Rechnitz to 60 km SSE of the Alpine thrust front.The explosive-source seismic data reveals a transparent shallow crust down to 5 km depth, a complexly reflective upper crust and a highly reflective lowermost crust. The upper crust is dominated by three prominent west-dipping packages of high-amplitude subparallel reflections. The upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low-angle normal shear zones related to the Miocene exhumation of the Rechnitz metamorphic core complex. In the western portion of the upper crust, east-dipping and less significant reflections prevail. The lowermost package of these reflections is suggested to represent the overall top of the European crystalline basement.Along the western portion of the line, the lower crust is characterised by a 6–8-km-thick band of high-amplitude reflection lamellae, typically observed in extensional provinces. The Moho can be clearly defined at the base of this band, at approximately 32.5 km depth. Due to insufficient signal penetration, outstanding reflections are missing in the central and eastern portion of the lower crust. We speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower Moho in the east, preferable at about 30.5 km depth.The high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust, the latter interpreted to represent broad extensional mylonite zones, emphasises the latest extensional processes in accordance with eastward extrusion.     

L'archivage des activités humaines par les neiges et glaces polaires : le cas du plomb

The investigation of the occurrence of lead in dated snow and ice from Greenland and Antarctica has played a major role in our understanding of the history of the pollution of the atmosphere of our planet by this metal. Such studies have however proved to be very demanding, mainly because of the extreme purity of polar snow and ice. Reliable measurements can be obtained only if ultra-clean and highly sensitive procedures are used, as pioneered by Clair Patterson. The Greenland data show evidence of large-scale pollution of the atmosphere of the Northern Hemisphere for lead as early as two millennia ago during Greco–Roman times, especially because of mining and smelting activities in southern Spain. It peaked at the end of the 1960s, with lead concentrations in snow about 200 times higher than natural values, before declining during recent times because of the fall in the use of leaded gasoline. Lead pollution in Antarctica was already significant at the end of the 19th century as a consequence of whaling activities, the traffic of coal-powered ships crossing the Cape Horn, and mining activities in South America, South Africa and Australia. After declining because of the opening of the Panama Canal, the great economic depression and World War II, it reached a maximum during the 1980s, with lead concentrations 20 times higher than natural values. Other studies focus on past natural variations of lead in ancient ice dated from the last climatic cycles. To cite this article: C. Boutron et al., C. R. Geoscience 336 (2004).     

时空多点连续定向取心技术应用研究

为解决以往定向仪不抗振、岩样采取率不高等问题，利用时间与钻孔空间位置对应的关系，研制成功了KD型定向仪。详细介绍了该定向仪的基本构成、技术参数及现场应用情况。