内蒙乌拉山金矿床钾长石的矿物化学及有序度的分析

采用电子探针显微分析(EMPA)和粉末X射线衍射(XRD)分析了采自乌拉山金矿床含金钾长石石英脉、石英脉以及其他类型岩石中的10 0多个钾长石样品的化学成分和结果状态,并采用R和Q模式聚类分析、Spearman等级相关分析方法对实验数据进行了统计分析。结果表明,含金矿脉、岩浆热液脉和蚀变花岗岩中的钾长石为中等到最大微斜长石,其特征为K2 O含量高,但相对而言,Na2 O、CaO和BaO的含量低。其他岩石类型中的钾长石的化学成分和结果状态变化很大,可以从透长石、正长石到微斜长石,其特征为K2 O的含量相对较低,但Na2 O、CaO和BaO的含量相对较高。含金样品中的钾长石通常更富K2 O ,表明金的成矿作用与富钾的热液流体和碱质交代作用有关。乌拉山金矿床的成矿作用分为两个阶段,主要的含金钾长石石英脉中的钾长石富K2 O ,形成温度为30 7～379℃,平均为35 3℃;第二阶段含金石英脉中的钾长石含K2 O较低,形成温度为2 6 0～318℃,平均为2 81℃。这些结果表明成矿流体与岩浆热液作用有关,流体朝温度降低、K2 O含量降低的方向演化,K2 O含量高的热液流体和2 6 0～380℃的形成温度有利于金的成矿作用。     

钾稳定同位素在水文地球化学领域的研究进展与展望

地表硅酸盐岩矿物风化通常是水体中钙、镁、钠、钾等元素的重要来源,然而相比于水体中的钙、镁和钠,目前对钾的水文地球化学行为的认识仍十分有限。表生地球化学领域最新研究证明风化、吸附等多种水岩反应伴随着较大的钾同位素分馏,表明钾同位素技术可以用于示踪地下水中钾的来源及迁移转化。文章通过系统总结上地壳、水圈和其他地表储库（植物、肥料）的钾同位素组成,发现水圈普遍比大陆上地壳富集41K,为识别地下水的钾来源提供了基础;通过总结钾同位素在常见的水岩作用过程（硅酸盐岩矿物溶解、次生黏土形成、吸附作用、离子交换反应）中的分馏行为,发现硅酸盐岩矿物溶解分馏有限,次生黏土矿物形成引起水体富集41K,表面吸附和离子交换使水体富集39K,不同水岩反应中K同位素行为差异为示踪地下水中钾的迁移转化过程提供了基础;列举了应用钾同位素示踪硅酸盐岩风化和水体污染的最新研究成果。由于钾同位素是硅酸盐岩风化的良好示踪剂,可以利用钾同位素揭示CO2较充足含水层中钾元素释放及迁移转化机理;由于表面吸附和离子交换控制的钾同位素分馏方向与风化控制的钾同位素分馏方向不同,可以利用钾同位素识别出地下水循环过程中多种水岩反应对钾迁移转化的共同控制。在此基础上,对钾同位素在水文地球化学领域的应用进行了展望：（1）开展研究区多端元控制下地下水钾来源贡献的研究;（2）开展地下水漫长循环过程中钾迁移转化的定量研究;（3）联合使用多种同位素示踪碳循环相关的过程。     

The Rio Narcea gold belt intrusions: geology, petrology, geochemistry and timing

This paper discusses the petrographical, mineralogical and geochemical characteristics of the intrusive rocks located along the Rio Narcea Gold Belt, and the timing of formation of the El Valle-Boinás deposit. Rocks in the belt range from quartz-monzodiorites through quartz-monzogranites to monzogranites. The former are made up of pyroxene (clino and ortho), amphibole (magnesiohornblende), biotite, zoned plagioclase (An35-70), and to a lesser degree quartz and K-feldspar. The monzogranites consist of biotite, zoned plagioclase (An30-60), quartz and K-feldspar. All igneous rocks are characterized by the presence of ilmenite and the lack or scarce presence of magnetite indicating their formation under reducing conditions. The granitoids are calc-alkaline I type, potassium-rich and highly reducent with more ferrous than ferric iron. Their characteristics are like the plutons associated with gold and copper (zinc) skarns, but their characteristics reflect more reducent formation conditions, increasing their capacity to form gold skarns.The Boinas granitoid emplacement occurred at about 303±6 Ma and generated calcic and magnesic skarns at the contact with limestone and dolostones of the Láncara Formation. Skarns and granitoids were first altered to amphibole and sericite, respectively, and mineralized at 302±9 Ma. The intrusion of subvolcanic porphyritic dikes produced a second period of alteration at 285±4 Ma, characterized by carbonatization and sericitization of the monzogranites and chloritization and serpentinization of the skarns. The later intrusion of diabasic dikes at 255±6 Ma produced limited carbonatization, silicification and sericitization and hypogene oxidation of the previous stages. Supergene oxidation then occurred at the top of the ore and along fractures and breccias.     

Yaxcopoil-1 and the Chicxulub impact

CSDP core Yaxcopoil-1 was drilled to a depth of 1,511 m within the Chicxulub crater. An organic-rich marly limestone near the base of the hole (1,495 to 1,452 m) was deposited in an open marine shelf environment during the latest Cenomanian (uppermost Rotalipora cushmani zone). The overlying sequence of limestones, dolomites and anhydrites (1,495 to 894 m) indicates deposition in various carbonate platform environments (e.g., sabkhas, lagoons). A 100-m-thick suevite breccia (894–794 m) identifies the Chicxulub impact event. Above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of Plummerita hantkeninoides zone CF1, which spans the last 300 ky of the Maastrichtian. An erosional surface 50 cm above the breccia/dolomite contact marks the K/T boundary and a hiatus. Limestones above this contact contain the first Tertiary planktic foraminifera indicative of an upper P. eugubina zone P1a(2) age. Another hiatus 7 cm upsection separates zone P1a(2) and hemipelagic limestones of planktic foraminiferal Zone P1c. Planktic foraminiferal assemblages of Zone Plc to P3b age are present from a depth of 794.04 up to 775 m. The Cretaceous carbonate sequence appears to be autochthonous, with a stratigraphic sequence comparable to late Cretaceous sediments known from outside the Chicxulub crater in northern and southern Yucatan, including the late Cenomanian organic-rich marly limestone. There is no evidence that these sediments represent crater infill due to megablocks sliding into the crater, such as major disruption of sediments, chaotic changes in lithology, overturned or deep dipping megablocks, major mechanical fragmentation, shock or thermal alteration, or ductile deformation. Breccia units that are intercalated in the carbonate platform sequence are intraformational in origin (e.g., dissolution of evaporites) and dykes are rare. Major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center. Yaxcopoil-1 may be located outside the collapsed transient crater cavity, either on the upper end of an elevated and tilted horst of the terrace zone, or even outside the annular crater cavity. The Chicxulub site thus records a large impact that predates the K/T boundary impact and mass extinction.     

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa

Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K₂O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H₂O, K₂O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H₂O and K₂O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K₂O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.     

Accretion and exhumation at a Variscan active margin, recorded in the Saxothuringian flysch

The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370?Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Münchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380?Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Viséan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380?Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Viséan), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30?mm per year, and rates of exhumation of ca. 3?mm/a, and possibly more.     

Emplacement of the Ardara pluton (Ireland): new constraints from magnetic fabrics, rock fabrics and age dating

The Ardara pluton as part of the Donegal batholith was intruded into Neoproterozoic metasediments and metadolerites at mid-crustal levels. The emplacement mechanism of the Ardara granite is very controversial, and mechanisms ranging from diapirism, ballooning and stoping followed by nested diapirism have been proposed. Magnetic fabrics, rock fabrics and K/Ar dating of micas are used here to constrain the emplacement history. The compositional zoning of the Ardara pluton is clearly reflected in the different bulk magnetic susceptibilities between the outer quartz monzodiorite and the central granodiorite, whereas the intervening tonalite is of intermediate nature. The magnetic carriers are characterized by the anisotropy of the magnetic susceptibility (AMS), thermomagnetic measurements and through high field analyses (HFA). The separation of the ferrimagnetic and paramagnetic contributions revealed that biotite and magnetite control the AMS in the quartz monzodiorite. Both minerals are oriented in such a way that their summed contribution is constructive and originates from the shape fabric of magnetite and the texture of biotite. Biotite is responsible mainly for the AMS in the tonalite and granodiorite. The magnetic foliation can be directly related to the macroscopic foliation and also to the D4 structures in the country rocks. The foliation is consistent with the geometry of the roughly circular shape and has a mostly steep to vertical dip. Towards the central granodiorite the magnetic foliation dies out, although plagioclase texture measurements indicate a weak magmatic shape fabric. With the exception of the tail, the K_max axes (magnetic lineation) vary from steeply to gently plunging. The so-called lineation factor is approximately 1.01 and therefore points to a less significant axial symmetry. These observations coincide with strain estimates on mafic enclaves that show a very consistent pattern of K ∼0 flattening strain. Texture analyses of biotite and quartz additionally support the observations made by the strain analyses and the magnetic fabric data. Microstructural investigations give evidence that the fabrics are associated with the emplacement over a range of temperatures from truly magmatic to high-temperature solid-state conditions. The age of the intrusion is still under discussion, but a new cooling age was determined by K/Ar dating of biotite at 403.7±8 Ma corresponding to a temperature range between 450 and 300°C. For a mylonite along the southern contact between the Ardara pluton and the country rock a K/Ar muscovite age of 378.8±7 Ma indicates a minimum age for the shear zone when the Ardara pluton must have already been cooled down below 350±50°C. Received: 28 January 1999 / Accepted: 28 December 1999     

中酸性侵入岩氧化钾平面分布规律及其与地壳厚度的关系

燕山地区中生代陆内造山作用强烈，形成数百个中生代中酸性侵入体。本区侵入岩的氧化钾分布具有南北分区、东西分带的特征。燕山北区中酸性侵入岩Ｋ２Ｏ含量普遍较高，平均为５．０％，而燕山南区中酸性侵性岩Ｋ２Ｏ含量平均为４．０％，统计分析表明，燕山陆内造山带中生代中酸性侵入岩氧化钾与地壳厚度呈线性相关，；地壳厚度是决定燕山地区中生代侵入岩氧化钾空间分布的主要因素，与板块边缘岛弧带火山岩氧化钾空间分布的主要制药     

Oligocene–Miocene geodynamic evolution of the central part of Urumieh-Dokhtar Arc of Iran

Basic volcanic rocks from Tafresh, west Kashan, and west Nain volcanic successions in the central part of Urumieh-Dokhtar Magmatic Assemblage (UDMA) of Iran yield K–Ar ages ranging from 26.8 to 18.2 Ma. These ages indicate significant Late Oligocene–Early Miocene basic volcanism in the UDMA. These ages, combined with K–Ar ages of 26.0 and 14.1 Ma, respectively, for associated low-silica and high-silica adakites, help constrain reconstructions of the UDMA geodynamic evolution. Late Oligocene–Early Miocene slab roll-back associated with an asthenospheric mantle influx are suggested as the major processes responsible for concurrent volcanism showing Nb–Ta-depleted, Nb–Ta-enriched and low-silica adakite signatures. Slab roll-back, the likely consequence of a decrease in subduction velocity, led to partial melting of the subducted slab and produced Early–Middle Miocene high-silica (dacitic) adakites. Oligocene to Miocene volcanic rocks do not conform to the Oligocene continental collisional model for the UDMA, rather they suggest a decrease in the subduction rate that prompted the asthenospheric mantle influx.     

东亚地区天气吸引子特征的研究

利用东亚地区１１个有代表性测站（１９７９－１９８８年）连续１０年的５００ｈＰａ逐日位势高度时间序列，全面计算了它们的浑沌特征量。结果表明，各测站相应的关联维数都是分数；完整的Ｌｙａｐｕｎｏｖ指数谱中，不仅都有正值、零和负值，而且Ｌｙａｐｕｎｏｖ指数之和都小于零。因此，从整体来看，相应于东亚地区天气尺度的吸引子是由一个有限窨支撑起来的。东亚地区的天气吸引子所显示的系统平均Ｋｏｌｍｏｇｏｌｏｖ熵约为０