新疆阿希低硫型金矿稀有气体同位素地球化学及其成矿意义

阿希是一个赋存在新疆西天山古生代陆相火山岩中的大型低硫型(冰长石-绢云母型)浅成低温热液金矿床.本文利用高真空惰性气体同位素质谱仪测定了阿希金矿黄铁矿中流体包裹体的稀有气体同位素组成,结果显示其~3He/~4He 比值为0.0218～0.138R_a,平均为0.044R_a;~(40)Ar/~(36)Ar 为317.7～866.0,平均为467.3,略高于大气中~(40)Ar/~(36)Ar 比值(295.5);~(20)Ne/~(22)Ne 和~(21)Ne/~(22)Ne 的比值分别为8.08～13.00和0.028～0.063,~(82)Kr/~(84)kr、~(83)Kr/~(84)kr、~(86)Kr/~(84) Kr 的比值分别为0.159～0.278、0.171～0.238、0.280～0.348,~(129)Xe/~(132)Xe 和~(134)XP/~(132)Xe 分别为0.96～1.017、0.401～0.443。流体包裹体~3He/~4He 显示成矿流体来自壳源,不舍幔源 He;流体中除壳源放射性成因~(40)Ar~*外,有大气 Ar 的参与,放射性成因~(40)Ar~*的含量为7.0%～65.9%,平均为29.0%,大气~(40)Ar 的贡献为43.0～93 0%,平均为71.0%,稀有气体同位素组成反映出阿希金矿成矿流体为循环的大气降水。F~(84)Ke 和 F~(129)Xe 值显示流体在成矿作用过程中曾发生过沸腾作用。     

Porphyroblast inclusion trails: the key to orogenesis

Detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral-shaped inclusion trails are common indicates that spiral-shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates. They formed instead by progressive growth by porphyroblasts over several sets of near-orthogonal foliations that successively overprint one another. The orientations of these near-orthogonal foliations are alternately near-vertical and near-horizontal in all porphyroblasts examined. This provides very strong evidence for lack of porphyroblast rotation.
The deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two-stage cycle of: (1) crustal shortening and thickening, with the development of a near-vertical foliation with a steep stretching lineation; followed by (2) gravitational instability and collapse of this uplifted pile with the development of a near-horizontal foliation, gravitational spreading, near-coaxial vertical shortening and consequent thrusting on the orogen margins. Correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen. This information, combined with information about chemical zoning in porphyroblasts, provides details about the structural/metamorphic ( P-T-t ) paths the rocks have followed.
The ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical, ductile tear/transcurrent faults across which there is no component of bulk shortening or transpression.     

Early Cretaceous fish trails from La Rioja, Spain

We report a new find of the fish trail Undichna unisulca in fluvial sediments of the Lower Cretaceous Oliván Group of the Cameros Basin (Aptian-Albian; Valtrujal, La Rioja, northern Spain). It is the second Mesozoic record of Undichna unisulca after the first discovery in the Lower Cretaceous of central Spain (Las Hoyas, Cuenca, central Spain). The trails are characterized by single sinusoidal waves which in this case are often surrounded by well-preserved lateral levees that rule out an undertrail hypothesis.The continental depositional setting, a floodplain fluvial system, reflects the known broad palaeoenvironmental distribution of Undichna and confirms its occurrences in freshwater settings.     

陕北横山晚三叠世造迹鱼类恢复

运用遗迹化石恢复造迹动物的形态结构是古遗迹学研究的难点.近年来在陕北横山晚三叠世地层中发现大量完好的鱼类游泳遗迹, 能较好地恢复造迹鱼类鳍的着生位置及相关形态结构, 确定造迹鱼的类型.通过游泳遗迹的形态结构与鱼类形态结构相互关系的分析, 建立了游泳遗迹形态结构参数与造迹鱼类形态结构参数之间的数学关系式.通过计算显示同一遗迹种的鱼鳍间距与鱼体长度比值一定, 这与现代鱼类形体特征一致.应用该方法恢复了研究区两遗迹种的造迹鱼类, 其中Undichna britannica造迹鱼类的尾鳍和臀鳍间距与体长之比为0.3, 臀鳍和胸鳍间距与体长之比为0.45~0.5, 尾鳍和腹鳍间距与体长之比为0.49~0.5, 与邻区相同层位发现的鱼类实体化石Wayaobulepis zichangensis相关特征一致.另一遗迹种Undichna tricosta恢复的尾鳍和臀鳍间距与体长之比约为0.35, 可能与本区相同层位发现的Saurichthys huanshenensis特征相似.      

Deformation partitioning and porphyroblast rotation in meta-morphic rocks: a radical reinterpretation

Abstract Most porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.     

STUDY ON RECHARGE OF GROUNDWATERS AND INTERRELATIONS BETWEEN VARIOUS WATERS BY MEANS OF ENVIRONMENTAL ISOTOPES

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The relationship of mantle-derived fluids to gold metallogenesis in the Jiaodong Peninsula: Evidence from D–O–C–S isotope systematics

The largest gold district in China is the Jiaodong Peninsula, where three types of gold deposits are recognized: quartz vein, fracture-altered and breccia types. The first two developed along a group of NE-trending faults and are hosted by granitic intrusions, dated at 160 to 150 Ma (biotite granite) and 130 to 126 Ma (granodiorite), and by metamorphic rocks of the Precambrian crystalline basement. The breccia-type gold system is mainly located around the northern margin of the Jiaolai Cretaceous basin, where mineralisation is controlled by both detachment fractures and NE-trending faults. This study is based on stable isotope determinations from ten gold deposits, including Linglong, Jiaojia, Sanshandao, Cangshang, Wang'ershan, Dayigezhuang, Denggezhuang, Pengjiakuang, Fayunkuang and Dazhuangzi, as well as the Linglong Jurassic biotite granite, Guojialing Cretaceous granodiorite and Archean gneiss. The stable isotope systematics reflect the style of the three types of gold deposits, but also show that they belong to the same metallogenic system, in which the hydrothermal fluids were derived from a mantle fluid reservoir and mixed with crustal fluids. The ore-forming age is later than both the Jurassic biotite granite and Cretaceous granodiotite, but overlaps with the 121 to 114 Ma ages of lamprophyre and diabase dykes. The hydrothermal fluids that were responsible for both gold mineralisation and the retrograde alteration of the diabase and lamprophyre dykes are similar, and represent a CO₂ and potassium-rich system. This fluid system is interpreted to be the consequence of Cretaceous lithospheric thinning, asthenospheric upwelling and mantle degassing in Eastern China.     

Isotopes of deuterium and oxygen-18 in thermal groundwater in China

Compositions of deuterium and ¹⁸O isotopes of 90 representative samples indicate that thermal groundwater in most parts of China is meteoric in origin. Latitude, altitude, and continent effects have significant bearing on the values of δD and δ¹⁸O of the hot water samples. Oxygen-18 shift is not significant in most of the thermal groundwater, especially the hot water of low-to-moderate temperature. Slight oxygen-18 shift is only found in some hot springs of high temperature in Tibet and western Yunnan and in thermal groundwater of low-to-moderate temperature in the deep-seated carbonate aquifers in the northern North China Plain (including the Tianjin area). Near-surface boiling may causes the shift of the former and the latter may be attributed to exchange of oxygen-18 between water and carbonates in the geothermal systems of taphrogenic basin-type. Hot springs in Tibet and western Sichuan have very low contents of δD and δ¹⁸O, possibly due to recharge of precipitation and snow-melting water of extremely depleted δD and δ¹⁸O values at high latitudes of several thousands of meters.     

Rotation of porphyroblasts in non-coaxial deformation: insights from computer simulations

Porphyroblast inclusion trails have the potential to provide critical information about tectonometamorphic events. Recently, however, traditional interpretations of inclusion trails have been called into question by the suggestions that porphyroblasts do not rotate during non-coaxial deformation and that apparent spiral inclusion trails can be generated in coaxial deformation. We present a new computer model that simulates inclusion trail development. Model results suggest: (1) that the extent of porphyroblast rotation is controlled by conditions at the porphyroblast-matrix boundary; (2) that curved inclusion trails may develop in unrotated porphyroblasts; (3) that classic "snowball" inclusion trails are most simply explained by rotational growth histories; and (4) that some of the observations used to support the view that porphyroblasts do not rotate (e.g. weakly sigmoidal inclusion trails, apparent truncations of inclusion trails) can be accounted for by variations in the growth rate of rotating porphyroblasts.