Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge,Saxony, Germany

The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle. The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities yielded identical ²⁰⁷Pb/²⁰⁶Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published ³⁹Ar/⁴⁰Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing continental collision and orogenic collapse. Received: 5 June 1997 / Accepted: 7 January 1998     

熔体的形态与分布特征对岩石流变的影响

熔体的形态与分布研究表明,在静态条件下,熔融程度比较低时,熔体主要分布于三个矿物颗粒之间,形成三角形状熔体结构,熔体二面角在0°~60°;熔融程度比较高时,熔体沿多个颗粒边界形成孤立的三角形或四边形结构,熔体三联点的二面角接近60°或大于60°。在动态条件下,在部分或全部矿物颗粒边界出现熔体薄膜,把熔体三角形连通,形成局部熔体网络,熔体三联点的二面角接近0°。如果熔体呈孤立的三角形或四边形结构时,熔体对岩石流变的影响比较小:当熔体含量小于2%~3%,熔体对岩石流变基本没有影响;只有熔体含量接近或超过3%~5%,熔体对流变强度的弱化作用才出现,当熔体含量达到10%时,流变强度弱化增加3倍左右。如果矿物颗粒边界出现熔体薄膜,微量熔体(小于1%)就对岩石流变强度有显著的弱化作用。流变实验表明,在颗粒边界含有小于1%的熔体时,熔体对流变强度的弱化达到4倍,当颗粒边界含有3%的熔体时,这种弱化作用达到10倍。     

Microstructures of melt inclusions in anatectic metasedimentary rocks

The occurrence of crystallized and glassy melt inclusions (MI) in high‐grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly ≤15 μm, and only rarely reaching 30 μm; they occur in clusters. In most cases inclusions are crystallized (‘nanogranites’) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K‐feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (≤8 μm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re‐homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.     

The melt rocks of the Boltysh impact crater,Ukraine, USSR

The 100±12 m.y., 25 km diameter Boltysh impact crater was formed in Precambrian granites and granite gneisses of the Ukrainian Shield. The crater deposits have undergone minimal post-impact erosion and it is possible to study a complete vertical section of the underlying 200 m thick melt sheet. The melt rocks, as sampled in two drill holes, can be subdivided into two major textural classes: microcrystalline and glassy. The microcrystalline melt rocks form an uppermost and two lowermost units, with the glassy variety occupying the middle of the melt sheet. The microcrystalline units contain 25% zoned plagioclase phenocrysts set in a microcrystalline matrix of intergrown alkali feldspar and quartz. Pyroxene has been replaced by sheet-silicates. Mineral and lithic clasts make up 5–15% and show varying degrees of shock and resorption. The glassy melt rocks are characterized by 10–30% zoned plagioclase and 5–10% orthopyroxene set in a fresh to partially devitrified glassy matrix. Clast content is <5%. Chemically, the melt rocks are relatively homogeneous and correspond to a mixture of Kirovograd granites and gneisses in the ratio of 5 to 1, with Ni, Ir and Cr showing slight enrichments over the target rocks. There are minor differences in the Fe₂O₃/FeO ratio and the alkalis between the microcrystalline and glassy varieties. The increase in matrix crystallinity at the upper and lower contacts is contrary to observations at other impact melt sheets, where greater matrix crystallinity occurs in the interiors of the melt sheets. One possible explanation is that the melt matrix was originally glassy throughout, due to its high SiO₂ content, and the microcrystalline matrix is the result of extensive devitrification involving minor alkali exchange with circulating ground-waters.Contribution from the Geological Survey of Canada 40986     

The formation of graphite upon the interaction of subducted carbonates and sulfur with metal-bearing rocks of the lithospheric mantle

Experimental studies of the Fe⁰–(Mg, Ca)CO₃–S system were carried out during 18–20 h at 6.3 GPa, 900–1400°C. It is shown that the major processes resulting in the formation of free carbon include reduction of carbonates upon redox interaction with Fe⁰ (or Fe₃C), extraction of carbon from iron carbide upon interaction with a sulfur melt/fluid, and reduction of the carbonate melt by Fe–S and Fe?S–C melts. Reconstruction of the processes of graphite formation indicates that carbonates and iron carbide may be potential sources of carbon under the conditions of subduction, and participation of the sulfur melt/fluid may result in the formation of mantle sulfides.     

Interaction between a mafic melt and host rocks during formation of the Kivakka layered intrusion,North Karelia

The study of interaction between mantle melts and crustal rocks is of great importance for deciphering the evolution of the Earth’s crust and for better understanding the composition of mantle sources, in particular, the degree of their compositional heterogeneity. This work presents the results of Rb-Sr and Sm-Nd isotopic studies of 37 samples taken from the Kivakka layered intrusion, host rocks, and rocks at the contact. The studies were aimed at verifying the hypothesis of possible crustal contamination of mafic melt during magma chamber crystallization. It was found that the section of the Kivakka layered massif is characterized by initial Sr and Nd isotopic heterogeneity, with negative correlation between initial Nd isotopic ratio and its content. The rocks of the massif have low ɛ_Nd(T) values.     

Thermal history of impact melt rocks in the Fennoscandian shield

Contributions to Mineralogy and Petrology - Shock-melted rocks from the Fennoscandian shield commonly contain fragments of quartz which appear to represent paramorphs of quartz after lechatelierite...     

The reservoir properties of the rocks of the bazhenovskaya formation

This work is dedicated to interpretation of the results of complex laboratory petrophysical studies of core samples of the rocks of the Bazhenovkaya Formation from several wells at Western Siberian deposits. The major aim of this study is the comparison and substantiation of analytical results that were obtained by various methods of determination of petrophysical rock properties, which is necessary to reveal the methods of complex petrophysical studies that are the most correct and applicable for the rocks of the Bazhenovskaya Formation.     

混合岩化作用中交代岩和铁矿形成的实验研究

通过黑龙江西麻山、天宝山和河南桐柏山混合岩化交代铁矿床的野外地质工作,并进行了高温高压下系统的模拟实验。实验结果证明了在380～650℃的温度和300×10~5～700×10~5Pa的压力下,含K、Na卤化物的弱酸性混合岩化的热液,与太古代的变质岩经历了120个小时的持续交代作用,形成了与这些矿区基本相一致的交代岩和铁矿,揭示了混合岩化作用中形成了交代岩和铁矿的物理化学条件。     

Grain-scale melt distribution in two contact aureole rocks: implications for controls on melt localization and deformation

The grain‐scale spatial arrangement of melt in layer‐parallel leucosomes in two anatectic rocks from two different contact aureoles located in central Maine, USA, is documented and used to constrain the controls on grain‐scale melt localization. The spatial distribution of grain‐scale melt is inferred from microstructural criteria for recognition of mineral pseudomorphs after melt and mineral grains of the solid matrix that hosted the melt. In both rocks, feldspar mimics the grain‐scale distribution of melt, and quartz is the major constituent of the solid matrix. The feldspar pockets consist of individual feldspar grains or aggregates of feldspar grains that show cuspate outlines. They have low average width/length ratios (0.54 and 0.55, respectively), and are interstitial between more rounded and equant (width/length ratios 0.65 for both samples) quartz grains. In two dimensions, the feldspar pockets extend over distances equivalent to multiple quartz grain diameters, possibly forming a connected three‐dimensional intergranular network. Both samples show similar mesoscopic structural elements and in both samples the feldspar pockets have a shape‐preferred orientation. In one sample, feldspar inferred to replace melt is aligned subparallel to the shape‐preferred orientation of quartz, indicating that pre‐ or syn‐anatectic strain controlled the grain‐scale distribution of melt. In the other sample, the preferred orientation of feldspar inferred to replace melt is different from the orientations of all other mesoscopic or microscopic structures in the rock, indicating that differential stress controlled grain‐scale melt localization. This is probably facilitated by conditions of higher differential stress, which may have promoted microfracturing. Grain‐scale melt distribution and inferred melt localization controls give insight into possible grain‐scale deformation mechanisms in melt‐bearing rocks. Application of these results to the interpretation of deep crustal anatectic rocks suggests that grain‐scale melt distribution should be controlled primarily by pre‐ or syn‐anatectic deformation. Feedback relations between melt localization and deformation are to be expected, with important implications for deformation and tectonic evolution of melt‐bearing rocks.     

乌伦古地区巴山组烃源岩发育特征

正乌伦古地区位于准噶尔盆地东北缘,登记面积约7.4×10~3 km,油气源分析表明,上石炭统巴山组是主要的油气来源之一。烃源岩是新区勘探的首要问题,也是新区油气勘探的重要对象之一(颜小宁,2011)。针对乌伦古地区巴山组烃源岩受火山活动影响强烈导致烃源岩生烃潜力存在差异的现状,笔者在对乌伦古地区及周缘多条野外露头进行观测、取样及分析化验的基础上,结合钻井、岩芯等资料,对巴山组同一层段不同岩石组合特征的烃源岩进     

Trace-element zonation in garnets from The Thumb: heating and melt infiltration below the Colorado Plateau

Proton microprobe (PIXE) analysis of garnet, pyroxene, and olivine for Zr, Y, Ga, Ge, Sr, Ni, Mn and Zn has been combined with electron-probe and petrographic analysis to interpret the histories of garnetperidotite xenoliths from the minette neck at The Thumb on the Colorado Plateau. Garnet in seven rocks contains 10–110 ppm Zr and 25–95 ppm Ni. Substantial parts of these ranges are preserved in single, zoned garnets (Zr, 25–90 ppm; Ni, 25–60 ppm). Pyroxene and olivine are more homogeneous and equilibrated more quickly than granet to changing temperatures and metasomatic fluxes. The distribution coefficient of Ti between pyroxene and garnet rims may be sufficiently sensitive to pressure to be used as a geobarometer. Zirconium and Ti appear to have behaved similarly during melt infiltration and diffusion within garnet. Nickel in garnet is a sensitive recorder of temperature. A temperature of 900° C or less calculated from Ni in the cores of large garnets in one rock is at least as cool as that calculated for the Archaean Kaapvaal craton at similar depth, and the low temperature may be due to cooling of the Plateau lithosphere by the subducted Farallon plate. The zonation of these garnets to Ni-enriched rims has been simulated numerically by heating 260° C at 0.02°/year, followed by overgrowth of a rim and short annealing. Garnet in another rock records a temperature decrease of about 70° C, but Ni is more homogeneous in garnets in the other five rocks. The diverse temperature histories are attributed to local melt-mantle interactions. Calculated pressures and temperatures of xenoliths from The Thumb form a grouping similar to those for high-temperature parts of inflected geotherms in other xenolith suites, and the similarity is evidence for both the reality and the transients nature of the calculated inflections. Garnet that is zoned in Zr, Y, Ti and other elements preserves evidences for grain growth in response to melt infiltration in four of seven rocks. The ranges of both major and trace elements in the xenolith suite may be due largely to enrichment processes following earlier depletion.     

The subsolidus segregation of layer-parallel quartz-feldspar veins in greenschist to upper amphibolite facies metasediments

Abstract Layer-parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (D₂) compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer-parallel veins show the following trends: (1) an increase in thickness and internal complexity, the latter due to successive boudinage; (2) low-grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz-rich than younger. Mass balance calculations and whole-rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress-induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine-scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing plagioclase/quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagioclase component.     

Evidence for stable grain boundary melt films in experimentally deformed olivine-orthopyroxene rocks

The microstructure of olivine-olivine grain boundaries has been studied in experimentally deformed (1200–1227 °C, 300 MPa) partially molten olivine and olivine-orthopyroxene rocks. In-situ melting produced ∼1 vol% melt in all samples studied. Grain boundary analyses were carried out using a number of transmission electron microscopy techniques. The grain boundary chemistry in undeformed olivine-orthopyroxene starting material showed evidence for the presence of an intergranular phase along some, but not all, of the olivine-olivine boundaries. In the deformed samples, ultrathin Si-rich, Al- and Ca-bearing amorphous films have been observed along all investigated olivine-olivine grain boundaries. The chemistry of the grain boundaries, which is considered to be indicative for the presence of a thin film, was measured with energy-dispersive X-ray spectroscopy (EDX) and energy-filtering imaging. The amorphous nature of the films was confirmed with diffuse dark field imaging, Fresnel fringe imaging, and high-resolution electron microscopy. The films range in thickness from 0.6 to 3.0 nm, and EDX analyses show that the presence of Al and Ca is restricted to this ultrathin film along the grain boundaries. Because thin melt films have been observed in all the samples, they are thought to be stable features of the melt microstructure in deformed partially molten rocks. The transition from the occasional presence of films in the undeformed starting material to the general occurrence of the films in deformed materials suggests that deformation promotes the formation and distribution of the films. Alternatively, hot-pressing may be too short for films to develop along all grain boundaries. A difference in creep strength between the studied samples could not be attributed to grain boundary melt films, as these have been found in all deformed samples. However, a weakening effect of grain boundary melt films on olivine rheology could not be ruled out due to the lack of confirmed melt-film free experiments. Received: 13 April 1999 / Revised, accepted: 10 February 2000     

Application of the Linkam TS1400XY heating stage to melt inclusion studies

Melt inclusions (MI) trapped in igneous phenocrysts provide one of the best tools available for characterizing magmatic processes. Some MI experience post-entrapment modifications, including crystallization of material on the walls, formation of a vapor bubble containing volatiles originally dissolved in the melt, or partial to complete crystallization of the melt. In these cases, laboratory heating may be necessary to return the MI to its original homogeneous melt state, followed by rapid quenching of the melt to produce a homogeneous glass phase, before microanalyses can be undertaken. Here we describe a series of heating experiments that have been performed on crystallized MI hosted in olivine, clinopyroxene and quartz phenocrysts, using the Linkam TS1400XY microscope heating stage. During the experiments, we have recorded the melting behaviors of the MI up to a maximum temperature of 1360°C. In most of the experiments, the MI were homogenized completely (without crystals or bubbles) and remained homogeneous during quenching to room temperature. The resulting single phase MI contained a homogeneous glass phase. These tests demonstrate the applicability of the Linkam TS1400XY microscope heating stage to homogenize and quench MI to produce homogeneous glasses that can be analyzed with various techniques such as Electron Microprobe (EMP), Secondary Ion Mass Spectrometry (SIMS), Laser ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP-MS), Raman spectroscopy, FTIR spectroscopy, etc. During heating experiments, the optical quality varied greatly between samples and was a function of not only the temperature of observation, but also on the amount of matrix glass attached to the phenocryst, the presence of other MI in the sample which are connected to the outside of the crystal, and the existence of mineral inclusions in the host.     

FeNi metal in impact melt rocks of Lake Lappajärvi,Finland

Particles of FeNi metal ranging in size from a few mto 0.1 mm in diameter are a distinctive feature of the shock-generated melt rocks of Lake Lappajärvi. Irondeficient iron sulphide, pyrrhotite, is nearly always associated with the metal. Accessory phases are FeNiCo-sulphides, a phosphorus-bearing mineral (probably apatite), iron-rich aluminosilicates, and ilmenite. The content of Ni in the metal varies from less than 2% to more than 10%. There is evidence that the iron formed from molten globules which crystallized slowly and adjusted to low temperatures by solid state diffusion. The Ni concentration in the largest metal particles which appear to be least affected by alteration processes, and hence may represent the composition of the parent metal phase of the projectile, is, however, lower than in any reported meteoritic iron.     

内蒙古大青山高级变质岩中熔体线理特征

熔体线理由浅色部分熔融物质形成长的集合体或杆状体沿着应变椭球体X轴方向定向排列构成,是高级变质岩特有的一种线状构造.熔体线理发育在深部构造层次韧性变形带中,与矿物拉伸线理一样具有运动方向指向意义.熔体线理形成于伸展构造环境中,是变形作用、变质作用和部分熔融作用共同作用的结果.伸展构造变形导致岩石部分熔融,是熔体线理形成...