Limb brightening on Uranus: The visible spectrum,II

New narrow-band (100 Å) photoelectric area-scanning photometry of the Uranus disk is reported. Observations were concentrated on the two strong CH₄ bands at λ 6190 and 7300 Å. Adjacent continuum regions at λ 6400 and 7500 Å were also measured for comparison. Both slit and pinhole scans were made in orthogonal directions. Disk structure in each waveband is apparent through lack of circular symmetry in the intensity distribution over the Uranus image. Polar brightening is especially prominent in the λ 7500-Å waveband. Coarse quantitative determinations of the true intensity distribution over the Uranus disk were made. For the λ 6190-Å CH₄ band, Uranus exhibits a disk of essentially uniform intensity except for a hint of polar brightening. For the λ 7300-Å CH₄ band, moderate limb brightening is apparent. Specifically, the true intensities at the center and limb of the planetary disk are approximately in the proportion 1:2. Extreme limb brightening, with a corresponding intensity ratio greater than 1:4, is not permitted by the observational data.     

松辽盆地西缘边界断裂带中北段尼尔基L型构造岩构造年代学及其构造意义

松辽盆地西缘与大兴安岭东缘的边界断裂带,又称嫩江-八里罕断裂带,位于东北地区中部,中亚造山带东段。断裂带呈NNE走向,沿内蒙古自治区、黑龙江省与吉林省的交界,一直向南延伸至河北省境内与平场-桑园大断裂相接,全长1200km。该断裂带传统认识上被认为是一条NNE向的大型正断层或拆离断层。目前对于断裂带性质的研究主要集中在其北段的嫩江断裂,但侧重区域地球物理方面,通过布格重力异常、航磁异常、深反射剖面研究确定了断裂带的走向、展布位置,及伸展拆离断层或大型低角度正断层的性质;其南段断裂带(红山-八里罕断裂)的研究,主要与喀喇沁隆起的形成时间、性质、演化阶段等方面相联系;然而对断裂带中段的嫩江-八里罕断裂的研究少有报道。近期,我们报导了嫩江-白令海断裂带岭下及白城地区发现的具有左旋走滑性质的韧性剪切带,并同断裂带南段的红山-八里罕断裂带楼子店地区进行系统的对比。通过对岭下地区韧性剪切带详细的野外调查、构造要素测量、显微构造研究、同构造变形岩石的有限应变分析、石英EBSD组构分析以及白云母40Ar/39Ar年代学研究,我们认为岭下韧性剪切带与楼子店韧性剪切带的变形特征一致,证实了红山-八里罕断裂带至少可以向北延伸至中段岭下地区,其性质同为左行走滑韧性剪切带,并具有相同的早期走滑时限(~130Ma)。本次,我们进一步对嫩江-白令海断裂带中北段尼尔基地区韧性剪切带进行研究,通过对带内韧性变形岩石构造要素系统测量和统计、显微构造观察、有限应变测量等方面的研究,表明其岩石类型属L型构造岩,具左行剪切特征。综合分析发现除构造岩的类型(岭下与楼子店地区发育S-L型构造岩),其余与岭下与楼子店地区韧性剪切带性质相同。另外,年代学表明L型花岗质岩石中发育的锆石,1)呈自形具有致密的韵律环带,且Th/U比值介于0.06至1.25之间,显示了岩浆成因特点;2)年龄大体分为两个区间258~403Ma(n=5)和207~174Ma(n=15,加权平均年龄为190.0±6.1Ma);3)较老年龄258~403Ma的锆石普遍具有"核-边"结构,证实了该区域存在多期古生代岩浆事件。207~174Ma的锆石中,两粒锆石呈现~190Ma核部年龄,反映了区域内曾经历了早侏罗世的一次强烈的岩浆侵入事件,此后尼尔基地区韧性剪切带形成于中侏罗世(~170Ma),而并非是前人认为的二叠纪。白云母激光40Ar/39Ar年代学显示了一个稳定的年龄坪为158.99±0.61Ma。通过显微构造的观测发现长石残斑与S-C组构指示了左行韧性剪切特征,其动态重结晶主要为膨凸式特点(BLG)。而石英颗粒边界相嵌现象明显,为亚晶粒旋转(SR)向颗粒边界迁移(GBM)重结晶过渡阶段。综合长石和石英的重结晶特征指示,其变质条件为高绿片岩相,变形温度在500℃左右,显然高于白云母的封闭温度(300~400℃),故此我们认为尼尔基左行韧性剪切带的快速隆升时间应为中晚侏罗世(158.99±0.61Ma)。结合最新发表的东北地区黑龙江蓝片岩(前人称之"黑龙江群")与郯庐断裂带北段两个分支(佳依断裂与敦密断裂)的年代学数据,以及西太平洋板块的俯冲速度和方向的相关证据,我们认为1)嫩江-八里罕断裂可能经历了三期变形演化阶段,包括中-晚侏罗世左行伸展走滑阶段(~160Ma),早白垩世中期左行走滑/斜滑剪切阶段(~130Ma)及早白垩世晚期的伸展阶段;2)两期走滑剪切阶段与郯庐断裂带及其北段两个分支具有相似性,其中早期~160Ma的走滑剪切事件相对于佳木斯地块与松嫩地块的拼合时间(~170Ma)晚近10Myr;3)松辽盆地形成早期受到中-晚侏罗世的左行走滑断裂的控制,该左行走滑断裂的成因应归功于侏罗纪时西太平洋板块向欧亚大陆下快速的斜向俯冲有关。     

湖南沃溪钨-锑-金矿床的矿石组构学特征及其成因意义

通过对湖南沃溪矿床的宏观至微观尺度上的矿石组构学研究，揭示出矿床系同生热水沉积成因。层状矿体、细脉状矿化以及围岩蚀变之间的空间关系，指示了矿石与其所赋存的围岩同时形成。矿床形成后的变质—变形作用，主要使矿物发生重结晶、碎裂、位错以及小范围的再活化等。     

Cenozoic intra-plate magmatism in the Darfur volcanic province: mantle source, phonolite-trachyte genesis and relation to other volcanic provinces in NE Africa

Chemical and Sr, Nd and Pb isotopic compositions of Late Cenozoic to Quaternary small-volume phonolite, trachyte and related mafic rocks from the Darfur volcanic province/NW-Sudan have been investigated. Isotope signatures indicate variable but minor crustal contributions. Some phonolitic and trachytic rocks show the same isotopic composition as their primitive mantle-derived parents, and no crustal contributions are visible in the trace element patterns of these samples. The magmatic evolution of the evolved rocks is dominated by crystal fractionation. The Si-undersaturated strongly alkaline phonolite and the Si-saturated mildly alkaline trachyte can be modelled by fractionation of basanite and basalt, respectively. The suite of basanite–basalt–phonolite–trachyte with characteristic isotope signatures from the Darfur volcanic province fits the compositional features of other Cenozoic intra-plate magmatism scattered in North and Central Africa (e.g., Tibesti, Maghreb, Cameroon line), which evolved on a lithosphere that was reworked or formed during the Neoproterozoic.     

Micro-XANES determination of ferric iron and its application in thermobarometry

Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe³⁺/Fe_tot (0.2–0.6), whereas only very low contents are present in garnet (Fe³⁺/Fe_tot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe³⁺/Fe_tot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P–T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe³⁺ and the actual effect on Mg–Fe²⁺ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods.     

Spinel inclusions in olivine of peridotite xenoliths from TUBAF seamount (Bismarck Archipelago/Papua New Guinea): evidence for the thermal and tectonic evolution of the oceanic lithosphere

Olivine in spinel peridotite xenoliths from the Bismarck Archipelago northeast of Papua New Guinea, which were transported to the surface by Quaternary basalts, shows spinel inclusions up to 25 μm long and 200 nm wide. These inclusions mainly occur as inhomogeneously distributed needles and subordinately as octahedral grains in olivine of veined metasomatic peridotites as well as peridotites without obvious metasomatism. The needles very often occur in swarms with irregular spacing in between them. Similar spinel inclusions in olivine have only previously been reported from ultramafites of meteoritic origin. Composition and orientation of the spinel inclusions were determined by transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Both the needles and the grains display a uniform crystallographic orientation in the host olivine with [001]^O1//[1ˉ10]_Spl and (100)_Ol// (111)_Spl. The needles eare elongated parallel [010] in olivine, which is the same in all olivine grains. As these needles have no relation to the metasomatic sections in the peridotite, it is concluded that they are primary features of the rock. Although the composition of the spinel needles is often very similar to the large chromian spinel octahedra in the matrix, the small octahedral spinel inclusions in olivine are in part Mg-rich aluminous spinel and sometimes almost pure magnetite. The spinel needles are suggested to have formed by exsolution processes during cooling of Al- and Cr-rich, high-temperature olivine during the initial formation of the lithospheric mantle at the mid-ocean ridge. The Al-rich spinel octahedra probably formed by the breakdown of an Al-rich phase such as phlogopite or by metasomatism, whereas the magnetite was generated by oxidizing fluids. These oxidizing fluids may either have been set free by dehydration of the underlying, subducted plate or by the Quaternary magmatism responsible for the transport of the xenoliths to the seafloor. Received: 25 May 2000 / Accepted: 12 July 2000