U-Pb dating of zircon fromthe bed parallel anatectic granitic intrusion in the Baoban group in Hainan Island and the tectonic implication

1 Geological setting Hainan Island is situated in the conjunction region between the Euro-Asian plate, the Indian-Australian plate and the Pacific plate, its tectonic setting and evolution is implicated in understanding the continen-tal margin accretion and evolution of East Asia and the formation of the South China sea. The Jiusuo-Lingshui fault zone divides Hainan Island into the Yaxian Pa-leozoic massif in the south and the Qiongzhong Pa-leozoic massif in the north (Fig. 1), they con…     

Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire,French Massif Central)

In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension-driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high-pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well-preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T  ~  700 ± 20°C, but differ from each other in whole-rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome-margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome-core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low-P/high-T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome.     

国产地球辐射表长波滤光罩性能的测试研究

滤光罩是地球辐射表的关键部件, 其性能直接影响着该仪器的性能与质量.利用LOWTRAN 7和国产各种滤光罩实测的光谱透射比数据, 对不同的模式大气下各滤光罩有效透射比等性能指标进行计算, 并与Eppley罩的研究情况作了对比.结果表明, 国产罩的各项性能指标并不次于Eppley罩.     

Effects of eruption history and cooling rate on lava dome growth

To better understand the factors controlling the shapes of lava domes, laboratory simulations, measurements from active and prehistoric flows and dimensional analysis were used to explore how effusion history and cooling rate affect the final geometry of a dome. Fifty experiments were conducted in which a fixed volume of polyethylene glycol wax was injected into a tank of cold sucrose solution, either as one continuous event or as a series of shorter pulses separated by repose periods. When the wax cooling rates exceeded a critical minimum value, the dome aspect ratios (height/diameter) increased steadily with erupted volume over the course of a single experiment and the rate at which height increased with volume depended linearly on the time-averaged effusion rate. Thus the average effusion rate could be estimated from observations of how the dome shape changed with time. Our experimental results and dimensional analyses were compared with several groups of natural lava flows: the recently emplaced Mount St Helens and Soufrière domes, which had been carefully monitored while active; three sets of prehistoric rhyolite domes that varied in eruptive style and shape; and two sets of Holocene domes with similar shapes, but different compositions. Geometric measurements suggest that dome morphology can be directly correlated with effusion rate for domes of similar composition from the same locality, and that shape alone can be related to a dimensionless number comparing effusion rate and cooling rate. Extrapolation to the venusian pancake domes suggests that they formed from relatively viscous lavas extruded either episodically or at average effusion rates low enough to allow solidified surface crust to exert a dominating influence on the final morphology.     

湘中白马山-龙山金矿带穹隆控矿规律分析

期望通过对湘中白马山-龙山金矿带的穹隆和发育于其核部的次级断裂与金(锑)矿成矿关系的深入研究来拓宽找矿思路.在野外调研的基础上,总结矿带上古台山、大乘山、龙山3个主要穹隆的地质特征及其对金(锑)矿的控制作用,认为穹隆为矿带的主要控矿构造,其核部的放射状断裂为容矿构造,并在此基础上建立了矿带的穹隆控矿模式.在金(锑)矿床勘查与找矿预测方面,提出应从穹隆的整体出发,重视对不同方向断裂的控矿性研究.通过对成矿带上不同穹隆剥蚀度的研究认为,大乘山穹隆为有利的找矿部位.     

Tectonic Controls on the Formation of the Liwu Cu-rich Sulfide Deposit in the Jianglang Dome, S W China

Abstract. The Liwu Cu‐rich sulfide deposit occurs within the Jianglang dome in the eastern margin of the Tibetan plateau. The dome consists of a core, a middle slab and a cover sequence. The main deposit is hosted in the core with minor ore bodies in the middle slab. The protolith of the core consists of clastic sedimentary rocks with inter‐layered volcanic rocks. All of the ore bodies are substantially controlled by an extensional detachment fault system. The ore bodies within the core are distributed along the S₂ foliation in the hinge of recumbent fold (D₂), whereas ore bodies with en echelon arrangement are controlled by the mylonitic foliation of the lower detachment fault. Ore bodies within the middle slab are oriented with their axes parallel to the mylonitic foliation. Pyrite and pyrrhotite from the ores contain Co ranging from 37 to 1985 ppm, Ni from 2.5 to 28.1 ppm, and Co/Ni ratios from 5 to 71. These sulfides have δ³⁴S values ranging from 1.5 to 7.5 % whereas quartz separates have δ¹⁸O values of 11.9 and 14.3 % and inclusion fluid in quartz has δD value of‐88.1 %. These features suggest that the deposit was of hydrothermal origin. Two ore‐forming stages are recognized in the evolution of the Jianglang dome. (1) A low‐temperature ore‐forming process, during the tectonic transport of the upper plate above the lower detachment, and the initial phase of the footwall updom‐ing at 192–177 Ma. (2) A medium‐temperature ore‐forming stage, related to the final structural development of the initial detachment at 131–81Ma. Within the core, the ore bodies of the first stage were uplifted to, or near, the brittle/ductile horizon where the ore‐forming metals were re‐concentrated and enriched. A denudation stage in which a compressional tectonic event produced eastward thrusting overprinted the previous structures, and finally denuded the deposit. The Liwu Cu‐rich sulfide deposit was formed during a regional extensional tectonic event and is defined as a tectono‐strata‐bound hydrothermal ore deposit.     

藏南确当地区高Sr/Y比值二云母花岗岩的形成机制及其构造动力学意义

最近的研究表明,喜马拉雅造山带普遍经历了大于30Ma的高温变质和部分熔融作用,形成了不同规模的混合岩或花岗岩.厘定这些地壳深熔岩浆的地球化学性质及其形成机理对于深化认识大型碰撞造山带的岩浆作用及其构造物理效应具有重要意义.在雅拉香渡穹隆核部及其南部发育一系列类似的二云母花岗岩岩体,其中确当花岗岩侵入到二叠纪(?)复理石地层内.全岩地球化学测试结果表明,确当花岗岩具有与雅拉香波花岗岩、打拉花岗岩类似的特征,即①高SiO_2(68.2%～69.3%)和铝(Al_2O_3>15.14%).低铁(TFeO<2.0%)和镁(MgO<1.5%),为富钠过铝质花岗岩;②富集LREE,亏损HREE,平坦的Ho到Lu稀土元素分布样式(Ho/Yb)_N=1.2～1.4),无或微弱Eu负异常(Eu/Eu=0.9～1.0);③较低的Y(<8.1×10~(-6))和Yb(<0.7×10~(-6))含量,较高的Sr/Y(>37.5)和La/Yb(>29.3)比值.这些特征表明,确当花岗岩是以角闪岩为主和变泥质岩为辅组成的源区发生部分熔融的结果.     

Contrasting textures in metamorphic and anatectic migmatites: an example from the Scottish Caledonides

Abstract. A method for the quantitative analysis of the spatial relations of minerals is described. Dispersed distributions are formed by annealing and destroyed in post-tectonic migmatization. Aggregate distributions characterize solid-state differentiation, whereas leucosomes formed in systems of high fluid:rock ratio (in the examples studied, anatectic melts) show random distributions.
Quantitative textural analysis can be used to indicate whether migmatization was post-tectonic or earlier, though caution is necessary if post-migmatite cooling is slow or if there is some minor deformation. More importantly, it can be used to discriminate melt-present from melt-absent leucosomes; this is exemplified by a suite of metamorphic and anatectic migmatites from the Scottish Caledonides.
The textural evolution of anatexites with increasing melt percentage is traced. Initial feldspar porphyroblastesis occurs by Ostwald ripening via grain boundary melts; subsequently ophthalmites develop with fabrics and chemistry inherited from the palaeosome. At greater than 30% melt these inherited fabrics are wholly destroyed. Deformation prompts segregation into melanosome and leucosome; resultant leucosomes contain no inherited crystals. The scale of anatectic systems is fixed at the point at which segregation begins; ophthalmites provide evidence for melt and crystal transfer beyond original palaeosome boundaries. In contrast, metamorphic migmatites are necessarily small-scale systems because of diffusive constraints, and melanosomes are invariably produced.     

Tree-ring evidence for 1842–1843 eruptive activity at the Goat Rocks dome,Mount St. Helens,Washington

Until the 18 May 1980 eruption of Mount St. Helens, a debris fan and adjacent forest downslope from the dacitic Goat Rocks dome, on the north flank of the volcano, contained evidence that the dome was active in 1842 or 1843. The fan was destroyed by the debris avalanche of 18 May. Before 1980, the oldest tree cored on the debris fan showed that the fan predated 1855 by a few years. The young age of this tree suggests that the dome was active several decades after extrusion of the nearby andesitic floating island lava flow, dated to 1800. An anomalous series of narrow rings that starts with the 1843 ring is present in cores from two older trees adjacent to the fan. These ring-width patterns imply that these trees were damaged in late 1842 or early 1843 by flowage material from the dome; the trees were probably singed by an ash-cloud surge that originated on the dome as a hot-rock avalanche. Several lines of evidence suggest that the anomalous ring patterns record tree injury by surge, rather than by lahars or nonvolcanic causes (climate or insects). First, comparable ring patterns formed in all sampled trees that survived the 18 May surge, but formed in only a few sampled trees abraded or partially buried by 18 May lahars. Second, a 13-cm fine-ash layer, consistent with either tephra fall or surge emplacement, was present on the 1840s forest floor; yet the lack of similar tree-ring responses to 1980 tephra fall shows that such minor tephra fall could not have caused the ring patterns. Third, identical 1843 narrow-ring patterns are absent in control trees further from the volcano. The ring patterns of the trees adjacent to the Goat Rocks fan provide the first field evidence that the dome was active in late 1842 or early 1843. Thus, the new tree-ring dates confirm stratigraphic evidence for the youth of the activity of the Goat Rocks dome. They also link historical accounts of mid nineteenth century volcanism at Mount St. Helen with continuing dome extrusion. The dates additionally corroborate and revise the dacite-andesite-dacite petrologic cycle interpretation of Mount St. Helens' Goat Rocks eruptive period (1800–1857). They constrain the cycle to no more than 43 years. Lastly, the dates support the notion that the vent that erupted the 1800 dacitic T tephra was different from the one that produced the Goat Rocks dome. We infer that the magma that formed the floating-island lava flow plugged the T tephra vent. This forced residual magma from the compositionally zoned magma chamber into an alternate conduit. The second conduit produced the unnamed 1842 lithic tephra and the Goat Rocks dome.     

中祁连山东段史纳村一带湟源岩群表壳岩的变质作用及地化特征

甘、青交界处民和县史纳村一带早元古宙湟源岩群表壳岩属一套“有层无序”地层,岩石组合为石英岩、绢云（二云）石英片岩、黑云斜长石英片岩、含榴（十字）二云石英片岩、角闪片岩等。晋宁旋回主要发生了区域动力热流变质作用,出现了从低绿片岩相—低角闪岩相的递增变质带,高峰期变质条件Ｐ＝３．８×１０８～４．８×１０８Ｐａ,Ｔ＝５５８～６４５℃。该套岩层具“褶叠层”性质,新测得Ｓｍ－Ｎａ全岩等时线年龄为１９００Ｍａ,为古元古代地质事件。变质作用Ｐ－Ｔ－ｔ轨迹具逆时针特征,表明此时为活动大陆边缘或是大陆岛弧环境。地质、地球化学资料表明湟源岩群表壳岩形成于大陆边缘裂谷环境,碎屑岩Ｎａ２Ｏ＋Ｋ２Ｏ＝５．２８×１０－２,Ｖ／Ｎｉ＝７,Ｓｃ／Ｎｉ＝１,Ｓｃ／Ｓｒ＝０．１。