Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating

Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanite–sillimanite–staurolite–garnet–muscovite–biotite schistsfrom Lord Nunatak show prograde growth zonation, and give Sm–Ndgarnet, U–Pb monazite and Rb–Sr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and P–T pseudo-section calculationsin the model system CaO–Na₂O–K₂O– TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent P–T decrease to <650°C at <6kbar. All data indicate that rocks from Lord Nunatak were affectedby a single orogenic cycle. In contrast, garnet porphyroblastsin K-poor kyanite–sillimanite– staurolite–garnet–cordierite–biotite-schistsfrom Meade Nunatak show two growth stages and diffusion-controlledzonation. Two distinct age groups were obtained. Laser ablationplasma ionization multicollector mass spectrometry in situ analysesof monazite, completely enclosed by a first garnet generation,yield ages of c. 1700 Ma, whereas monazite grains in open garnetfractures and in most matrix domains give c. 500 Ma. Both agegroups are also obtained by U–Pb thermal ionization massspectrometry analyses of matrix monazite and zircon, which fallon a discordia with lower and upper intercepts at 502 ±1 and 1686 ± 2 Ma, respectively. Sm–Nd garnet datingyields an age of 1571 ± 40 Ma and Rb–Sr biotiteanalyses give an age of 504 ± 1 Ma. Integrated geochronologicaland petrological data provide evidence that rocks from MeadeNunatak underwent a polymetamorphic Barrovian-type metamorphism:(1) garnet 1 growth and subsequent diffusive garnet annealingbetween 1700 and 1570 Ma; (2) garnet 2 growth during the RossOrogeny at c. 500 Ma. During the final orogenic event the rocksexperienced peak P–T conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; P–T pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica     

安太堡露天煤矿排土场基底黄土的微结构演化与弱层的形成机理

本文根据安太堡露天煤矿排土场基底黄土的粘土矿物组成的微结构及其在分级荷载下微结构变化的研究，论述了排土场基底黄土内弱层的形成机理，提出弱层是在上覆排土压力下由于黄土的微结构变化而形成的演化弱层。并指出弱层的形成是导致排土场失稳滑坡的主要控制因素，这种弱层在新排土场设计时期并不存在，极具隐蔽性，但在设计时必须考虑在排土增至一定高度后，弱层的形成对排土场稳定性的影响，否则必将会存在重大滑坡隐患。     

中太平洋海山区富钴结壳构造与地球化学特征及意义

为了释读蕴藏在富钴结壳中的环境变化信息,运用电子探针技术对中太平洋海山CXD05结壳进行了详细的构造和地球化学研究.结果表明:1)CXD05结壳约从19.4 Ma BP(早中新世)开始发育,从基部至顶部依次发育了斑杂构造、柱状构造和纹层构造;2)结壳中亲氧元素和碎屑组分从老至新发生规律性变化.结壳构造序列反映了其生长过程中海洋动力环境能量由高到低的变化;而壳层亲氧元素和碎屑组分的变化则记录了海水的氧化程度和风尘影响强度的变化.     

永登5.8级地震极震区黄土微结构孔隙性的定量研究

通过电镜扫描手段和计算机图像处理,对1995年永登5．8级地震极震区遭受地震影响和未遭受地震影响的黄土微结构变化的各项指标进行了对比试验和分析,并将分析结果绘成受震与未受震两种孔隙分布曲线。从这些曲线可以看出,不同地点的黄土孔隙分布特征不同;同一深度不同地点的孔隙分布亦有明显差异。对未受震(粒状架空孔隙结构)与受震(粒状镶嵌孔隙结构)黄土的微观结构进行了定量处理,其分析结果较好地解释了土体的工程性质。同时介绍了黄土微结构研究的方法和要求,通过对黄土微观结构的定量处理,揭示了黄土震陷性与黄土孔隙含量所占比例大小的内在关系,达到了确定黄土场地震陷量的目的,验证了利用微结构研究黄土结构变化的可行性。     

Hydrochemistry and sediment characteristics of polar periglacial lacustrine environments on Fisher Island and Broknes Peninsula,East Antarctica

Fisher Island and Broknes Peninsula in the Larsemann Hills constitute part of a polar lowland periglacial environment between marine and glacial ecosystems. The landscape is characterized by gently rolling hills and broad valleys interspersed with lakes formed in glacially scoured basins. We analyzed the physieochemical parameters and the ionic constituents of water samples from 10 lakes in each of these two locations. Our results showed considerable differences between the two regions and demon- strated the influence of lithology and processes including weathering, evaporation, and atmospheric precipitation. All major cations and anions in the lake waters showed positive correlations indicating balanced ionic concentrations. Unconsolidated sediments were sparsely distributed and scattered over glacial deposits, valley fills, and occasional moraine ridges. The type and rate of sedi- mentation was mainly controlled by surface run-off and aeolian influx. The sediment samples from lake beds and the catchment area on Fisher Island were immature and poorly to very poorly sorted, consisting of gravelly sand with negligible silt and finer fractions. Sediments had a polymodal grain size distribution with the two major populations lying between -2 and 1 phi and be- tween 0 and 1.5 phi. The sediments were lithic arenite to arkosic in composition and the microtextures imprinted over quartz grains were dominated by mechanical textures resulting from several stages of glacial crushing and grinding. The presence of deep disso- lution cavities, cryptocrystalline precipitation, and euhedral crystal growth signified the effect of chemical activity after the deposition of grains in the lacustrine environment.     

胶东三山岛金矿床黄铁矿As富集机制及其对金成矿作用的指示

胶东是中国最大的金矿集区,其金矿成因目前还存在较大争议.作为最重要的载金矿物,黄铁矿显微结构及元素-同位素组成能够很好地示踪成矿物质来源和成矿过程.利用SEM、EPMA和LA-ICP-MS微区原位分析方法对胶东代表性蚀变岩型金矿—三山岛金矿的黄铁矿开展详细的BSE显微结构以及As含量和S同位素耦合关系研究,发现矿区至少存在3种不同结构和As含量的黄铁矿:①含金石英-黄铁矿细脉中具交代残余结构的低As黄铁矿(Py-1),在BSE图像中显示暗色的核部(Py-1a)和亮色的边部(Py-1b),Py-1a和Py-1b的w(As)分别为48×10-6～524×10-6和183×10-6～1134×10-6,δ34S值分别为10.4‰～10.8‰和11.6‰～11.9‰;②含金石英-黄铁矿细脉中具韵律环带结构的高As黄铁矿(Py-2),在BSE图像中显示富含矿物包裹体并多孔的核部(Py-2a)以及很少含矿物包裹体的"干净"边部(Py-2b),Py-2a和Py-2b的w(As)分别为0.14％～0.31％和0.47％～0.97％;③含金石英-多金属硫化物细脉中具核边结构且As含量变化大的黄铁矿(Py-3),其核部Py-3a富含黄铜矿、闪锌矿等矿物包裹体并贫As,w(As)仅为1×10-6～10×10-6,δ34S值为4.2％～5.8‰,边部Py-3b几乎不含矿物包裹体但富As,w(As)为8877×10-6～17839×10-6,δ34S值为7.7％～10.1‰.上述黄铁矿中,从Py-1a到Py-1b,w(As)和δ34S值缓慢升高,可能主要受水岩相互作用的控制;Py-2具韵律环带且核部多孔、多矿物包裹体,指示了较为剧烈的流体沸腾作用;Py-3a和Py-3b具有截然不同的w(As)和δ34S值,指示了外来富As流体的加入.这些过程有利于As和Au的进一步富集,多种形式的富As和Au机制表明胶东金矿存在复杂的矿化过程和物质来源,特别是流体在迁移过程中萃取围岩中的成矿物质不可忽视.     

新西兰碧玉的宝石学特征

以产自新西兰的碧玉样品作为研究对象,采用常规的宝石学测试、傅里叶变换红外光谱仪、环境扫描电子显微镜、等离子体质谱仪、X射线粉晶衍射仪,对其表面微观形貌、矿物组成、化学成分特征进行了较全面的研究。研究表明,新西兰碧玉的显微结构致密程度不高,矿物结晶颗粒粗细不一,局部具有定向性,其主要的矿物组成为角闪石,内含黑色的铬铁矿杂质。将新西兰碧玉的化学成分特征与新疆一带所产的碧玉相比较,主量元素和微量元素的含量均有所差别。     

长白山天文峰剖面全新世火山喷发物长石风化溶蚀特征及地质意义

利用扫描电镜和能谱分析研究了长白山天池火山天文峰剖面全新世喷发物中长石表面风化溶蚀显微形貌和化学组成,结果显示: 天文峰剖面从顶部黑色浮岩向下到暗灰色浮岩中,长石表面发育风化溶蚀不同的显微形貌结构。其特征有,风化初期的溶蚀作用主要在双晶缝、解理缝等结构薄弱部位,形成规模较小的溶孔和溶缝等,随着风化溶蚀程度的增加,已经形成的溶蚀痕迹进一步扩大、合并、相互贯通而形成矩形溶孔或棱柱状溶孔。长石晶体上溶蚀痕迹的扩大受长石晶体各向异性的控制。天文峰剖面火山喷发物时代越早,长石颗粒表面风化溶蚀程度越强,结构越复杂; 火山喷发物时代越晚,长石颗粒表面风化溶蚀程度越弱,结构越简单。即长石表面风化溶蚀程度与火山喷发的时代具有一定的相关性。根据长石颗粒表面的显微形貌结构,认为漫江林场浮岩与天文峰剖面中灰色浮岩应是火山同一次喷发的产物。长石表面风化溶蚀度可以作为判定火山喷发精细序列的一个代用指标。能谱分析显示,较新鲜的长石表面与风化溶蚀的长石化学组成不同。长石风化溶蚀的显微形貌和化学组成反映了它是天然风化、溶解、淋滤,最后又有物质沉淀的自然成因。Fe元素则可能与微生物作用有关。详细研究长石表面的风化溶蚀特征,它将有可能成为从岩石微观方面探讨火山喷发后环境变化和火山喷发期次的一种新方法。     

“Spherulite-like” jadeite growth in shock-melt veins of the Novosibirsk H5/6 chondrite

The Novosibirsk H5/6 ordinary chondrite has signs of shock metamorphism, such as dark shock-melt veins (SMVs) crossing the chondrite host rock. The plagioclase composition grains (Ab₇₈An₁₄Or₇) with jadeite were found in the host-rock fragments inside the SMVs. Jadeite has an unusual radial-concentric spherulite-like microtexture. The spherulite-like jadeite formed from the molten plagioclase grain under high-pressure and high-temperature conditions during an impact event. The crystallization was accompanied by sodium-potassium differentiation between coexisting jadeite and residual melt. The PT-conditions of jadeite formation were estimated to be 3-14 GPa and 1400-2150 °C. Jadeite crystallization, Na-K differentiation, and the pressure-temperature estimates of jadeite formation in the Novosibirsk chondrite are very close to those in the Chelyabinsk LL5 chondrite. The spherulite-like microtexture and jadeite-glass coexistence, most likely, point to a high cooling rate of the SMVs at the pressure release stage of the metamorphic process.