河北承德黑山铁矿床热液成矿特征及流体包裹体研究

黑山大型钒钛磁铁矿矿床产于大庙斜长岩杂岩体中,是承德地区最重要的"大庙式"岩浆型铁矿床。笔者在矿区野外地质观察过程中发现,穿插于斜长岩中的铁磷矿脉、磁铁矿硫化物矿脉有热液成矿作用的显示,表明黑山铁矿床成因除传统认为的岩浆期结晶、熔离、矿浆贯入成矿作用外,还有热液期的成矿作用发生。本文对热液成矿期铁磷矿石中磷灰石和矿化蚀变石英中的流体包裹体进行了显微测温和激光拉曼光谱分析。结果表明,磷灰石中原生包裹体可以分为气液两相包裹体、含子矿物包裹体、含液态CO2三相包裹体、单液相、单气相包裹体5类,均一温度主要集中于180～420℃,盐度主要集中于6.2%～38.9%NaCleq,流体包裹体气相成分主要为CO2、N2和CH4,液相成分主要为H2O,固相成分主要为方解石、石盐、白云石及铁氧化物子矿物。石英中流体包裹体类型和成分与磷灰石中的类似,但固相成分未发现石盐和不透明金属子矿物,均一温度变化于149～422℃,盐度变化于5.7%～22.9%NaCleq。成矿流体为CaCl2-NaCl-H2O-CO2体系,均一温度和盐度呈现正相关连续渐变的特征。铁磷矿石的磷灰石中原生包裹体为流体包裹体,盐度高,子矿物种类复杂,组成中富含CO2和CH4等,这些特征显示成矿流体以岩浆热液为主;成矿机理可能与大气降水对岩浆热液的稀释有关。     

Petrogenesis of Franciscan glaucophane schists and associated metamorphic rocks,California

Rocks of the glaucophane-schist facies are widely though irregularly developed in the Franciscan formation of California. Minerals critical of the facies are lawsonite, aragonite, jadeite and omphacitic pyroxenes associated with quartz; amphiboles of the glaucophane-crossite series are almost ubiquitous. The most widely distributed rock, occurring over areas of many square kilometers, is jadeite-lawsonite metagraywacke, commonly veined with aragonite. More spectacular, but occurring mainly in isolated blocks are coarse-grained glaucophane-lawsonite Schists of many kinds. Commonly, but by no means invariably, they are closely associated with bodies of serpentinite. Also common in the vicinity of serpentinite masses are blocks of amphibolite and eclogite.All the metamorphic rocks are considered to be Franciscan sediments and basic volcanics metamorphosed and metasomatized in the deep levels of a folded geosynclinal prism. Experimental data on the stability fields of jadeite-quartz, aragonite, and lawsonite show that the glaucophane-schist facies represents metamorphism at pressures of between 5 and 10 kb and temperatures of 150–300° C. Such conditions could develop at depths greater than 15 km provided a very low geothermal gradient (10°/km) were maintained. The metagray-wackes are considered to represent a regional response to such conditions.The role of serpentinites in glaucophane-schist metamorphism is discussed in terms of a tentatively proposed model: — In very deep levels — perhaps at depths as great as 30 km, bodies of hot ultramafic magma develop restricted aureoles' in which temperatures of 400–600° C are maintained fer perhaps 100–1000 years. The products of metamorphism, which also involves desilication under the influence of the ultramafic magma, are eclogite and amphibolite. Later, and perhaps at higher levels serpentinization of the now solid ultramafic masses (near 400° C), causes renewed metamorphism at lower grades. Marginal development of glaucophane Schists and prehnite and hydrogarnet rocks, and retrogressive alteration of eclogite and amphibolite to glaucophane-schist assemblages is attributed to this period.     

On the role of the unresolved eddies in a model of the residual currents in the central strait of Georgia,B.C.: Research Note

Abstract

A depth‐independent numerical model of the Juan de Fuca/Strait of Georgia system reproduces the broad structure of the observed depth‐averaged residual circulation in the Central Strait of Georgia but underestimates its magnitude (Marinone and Fyfe, 1992). Here we present some new calculations based on a re‐parameterization of the unresolved eddies in terms of “statistical dynamical tendencies” instead of the previous eddy‐viscosity treatment. With the new parameterization, the simulated time‐mean flow is closer to the observed circulation both in structure and magnitude. While not specifically designed to do so, the new parameterization also leads to a modest improvement in the low‐pass filtered component of the flow. Based on these results, the depth‐averaged residual currents in the region are conjectured to involve a four‐way balance between the hitherto ignored effect of “statistical dynamical tendencies” and conventional tidal, atmospheric and buoyancy forcing.     

Some effects of truncation on topographic instability

Abstract

The topographic stability of forced planetary waves in α β‐channel is investigated using a barotropic model. The equilibrium forced waves are the result of the interaction of a constant mean zonal wind over finite‐amplitude surface orography. Small‐amplitude perturbations of the equilibrium flows are considered that have a wavy part with the same zonal wavenumber as the forcing but an arbitrary meridional structure. The mean zonal part of the perturbations is also taken to be arbitrary. This configuration allows us to (1) isolate those instabilities that depend crucially on topography through form drag and (2) investigate non‐topographic effects on topographic instability that arise from the convergence of Reynolds stresses. A numerical stability analysis is then performed wherein the effects of truncation are emphasized.

This numerical approach casts doubts about the results obtained from some earlier studies involving various ad hoc assumptions. We find, in particular, that unstable long waves (i.e. waves with the zonal wavelength greater than the meridional wavelength) exist under superresonant conditions. This contradicts some previous results that suggest long waves are unstable only when the flow is subresonant. Further, our model reveals the existence of some interesting travelling instabilities. The latter are shown to depend on both form drag and Reynolds stresses in that these two mechanisms alternate in time in supplying the perturbation with the required energy to maintain the exponential growth.     

Global change: anthropogenic forcing—the moving target

If we are to develop valid models for predicting the future of the global environment, anthropogenic forcing must become the major component of study. The technological developments needed to supply food, water and materials for 10 billion humans will dominate environmental change over thenext decades.     

Fluid transport and shear zones at Scourie,Sutherland: Evidence of overthrusting?

Rocks from some shear zones at Scourie are highly oxidised and enriched in potash relative to adjacent undeformed granulites. Oxidation is considered to result from dissociation of water, and the extent of oxidation is evidence for massive fluid flow through the shear zones. The conditions under which oxidation and reduction occur during fluid flow are discussed and it is thought that progessive oxidation of the shear zones at Scourie indicates that fluid flow took place up a thermal gradient. It is proposed that the inverted thermal gradient resulted from overthrusting, and this is consistent with existing interpretations of the structural evolution of the area.     

Archaean hydrothermal talc evidence for high ocean temperatures

Bedded sedimentary talc is abundant in Archaean greenstone belts in proximity to seafloor hydrothermal vents. Trace-element abundances and oxygen-isotope evidence suggest formation from heated ocean water. It is suggested that talc forms by reaction of silica in the hot discharge with metal bicarbonate species in ocean water thus releasing carbon dioxide. The possible CO₂ flux is similar to the present-day combustion flux and may have maintained a global greenhouse with warm oceans as reflected in the oxygen-isotope systematics of cherts. It seems probable that surface-Earth temperatures may have exceeded the boiling point prior to 4 Ga. If this is possible, then the beginning of the geologic record for the Earth may coincide with the first widespread development of the hydrosphere.     

Eclogites and water pressure

Available data on the stability of amphibolites and the basalt-eclogite transition allow an estimate of the stability field of eclogites in wet systems. It appears that if water and load pressures are comparable, eclogites cannot be stable in the crust; the formation of eclogites in the crust requires moderate total pressures and low water pressures. If the reactions forming eclogites occur in essentially dry rocks, their formation may reflect kinetic as well as equilibrium factors.     

Devonian fluviodeltaic sedimentation in Hong Kong

Twenty-three distinct sedimentary facies have been distinguished in the Devonian Bluff Head Formation exposed around the Tolo Channel area of northeastern Hong Kong. These facies have been combined into five facies associations: channel, aeolian, sheetflood, deltaic and palaeosol, with the bulk of the formation belonging to the channel association. The lower part of the Bluff Head Formation, around Plover Cove, consists of upwards-coarsening cycles, each typically 3 m thick. The basal parts of some cycles contain distinctive marine fossils of Early to Middle Devonian age. Sedimentation occurred on a marine-influenced delta plain, with eustatic sea level rises controlling cycle development. The middle and upper parts of the formation contrast strongly with the lower part and show a distinct change to alluvial plain sedimentation. Upward-fining cycles, each approximately 40 m thick, comprise basal multistorey, braided, conglomerate-filled channel deposits at the base passing upwards into finer-grained sheetflood, aeolian and channel sandstones at the top. Palaeosols in the higher parts of the cycles indicate periods of non-deposition under semi-arid climatic conditions. These cycles represent repeated abandonments of the fluvial system which we suggest were controlled by tectonic pulses.