An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

Spectacular shallow-level migmatization of ferrogabbroic rocks occurs in a metamorphic contact aureole of a gabbroic pluton of the Tierra Mala massif (TM) on Fuerteventura (Canary Islands). In order to improve our knowledge of the low pressure melting behavior of gabbroic rocks and to constrain the conditions of migmatization of the TM gabbros, we performed partial melting experiments on a natural ferrogabbro, which is assumed as protolith of the migmatites. The experiments were performed in an internally heated pressure vessel (IHPV) at 200 MPa, 930–1150 °C at relatively oxidizing conditions. Distinct amounts of water were added to the charge.

From 930 to 1000 °C, the observed experimental phases are plagioclase (An_60–70), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe–Ti oxides, and a basaltic, K-poor melt. Above 1000 °C, amphibole is no longer stable. The first melts are very rich in normative plagioclase (>70 wt.%). This indicates that at the beginning of partial melting plagioclase is the major phase which is consumed to produce melt. In the experiments, plagioclase is stable up to high temperatures (1060 °C) showing increasing An content with temperature. This is not compatible with the natural migmatites, in which An-rich plagioclase is absent in the melanosomes, while amphibole is stable. Our results show that the partial melting of the natural rocks cannot be regarded as an “in-situ” process that occurred in a closed system. Considerable amounts of alkalis probably transported by water-rich fluids, derived from the mafic pluton underplating the TM gabbro, were necessary to drive the melting reaction out of the stability range of plagioclase. A partial melting experiment with a migmatite gabbro showing typical “in-situ” textures as starting material supports this assumption.

Crystallization experiments performed at 1000 °C on a glass of the fused ferrogabbro with different water contents added to the charge show that generally high water activities could be achieved (crystallization of amphibole), independently of the bulk water content, even in a system with very low initial bulk water content (0.3 wt.%). Increasing water contents produce plagioclase richer in An, reduces the modal proportion of plagioclase in the crystallizing assemblage and extends the melt fraction. High melt fractions of >30 wt.% could only be observed in systems with high bulk water contents (>2 wt.%). This indicates that the migmatites were generated under water-rich conditions (probably water-saturated), since those migmatites, which are characterized as “in-situ” formations, show generally high amounts of leucosomes (>30 wt.%).     

Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif

The peak metamorphic conditions of subducted continental crust in the Dora-Maira massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrologic information from whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage garnet+phengite+kyanite+coesite±talc originates from the reaction kyanite+talc↔garnet+coesite+liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at 730 °C. At lower pressures, talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid contains probably similar amounts of H₂O and dissolved granitic components. The composition of the liquid in the whiteschists at peak metamorphic conditions, a major unknown in earlier studies, was probably very similar to the liquid composition produced in the experiments. Therefore, the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists. Experimentally determined Si-isopleths for phengite further constrain peak pressures to 43 kbar for the measured Si=3.60 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond-facies conditions.

The fluid-absent equilibrium 4 kyanite+3 CELADONITE=4 coesite+3 muscovite+pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra-high-pressure (UHP) metapelites. For graphite-bearing metapelites and kyanite–phengite eclogites, forming the country rocks of the whiteschists, peak metamorphic pressures of about 44±3 kbar were calculated from this barometer for temperatures of 750 °C estimated from garnet–phengite thermometry. Therefore, the whole ultra-high-pressure unit of the Dora-Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. While graphite is common in the metapelites, diamond has not been found so far. The absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism, the absence of a free fluid phase in the metapelites and a short residence time in diamond-facies conditions resulting in kinetic problems in the conversion of graphite to diamond.     

A Linear Programming Approach to Determine the Normative Composition of Sedimentary Rocks

The analysis of modal and normative composition of sedimentary rocks is widely used for studying their sources and tectonic settings. The normative calculation of the mineral composition of rocks in this study is formulated as a linear programming problem and is solved by means of the simplex method. This enables both simultaneous and successive subtraction of a set of basic minerals from a rock sample represented by its chemical composition {SiO₂...LOI}. Such an approach provides a more exact calculation of the contribution of basic minerals in the rock. This mathematical approach is used to study two representative sets of sandstones and fine-grained rocks from a Meso- to Neoproterozoic marginal basin of southeastern Siberia (Uchur–Maya region, Yakutia) and a Pennsylvanian-Lower Permian uplifted continental block in Colorado, USA. The calculated normative mineral compositions of the Siberian sandstones are consistent with the observed modal compositions. These sandstones vary from K- Feldspar rich arkoses at the base of the sequence (the Uchur Group, lower Riphean) to quartz arenites or lithic sandstones and wacke in transgressive successions of the middle-upper Riphean. Arkoses and quartz arenites are dominant in Meso- to Neoproterozoic Siberia. These samples represent craton interior uplifted basement and quartzose, recycled orogen provenance of a stable craton in Rodinia. There are higher but consistent discrepancies between the calculated and observed compositions for the Pennsylvanian to Lower-Permian arkoses and quartz arenites (Sangre de Cristo, Belden, and Maroon Formations). The differences between the predicted and observed mineralogy may be due to uncertainties in the modes in the matrix and cement of the sandstones. This normative program should supplement modal calculations and provide better genetic constructions, especially in case of matrix-rich sandstones.     

Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada

The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near‐isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well‐preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate‐pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.     

楚雄盆地下侏罗统磨拉石楔沉积与含油性研究

楚雄盆地为一中生代周缘型走滑前陆盆地，通过对下侏罗统冯家河组沉积特征的详细研究与对比，作者认为早侏罗世是楚雄前陆盆地强烈坳陷与沉降的主要发展时期从早期到晚期经历了一个变深又变浅的典型前陆盆地充填过程。在前陆冲断块体的不断逆冲和加载条件下，楚雄盆地西部早侏罗世沉积物的来源主要为哀牢山造山带隆起，其沉积环境以滨，浅湖或湖泊三角洲为主。     

安徽南部晚二叠世龙潭期岩相古地理研究

本文系统地讨论了皖南地区晚二叠世早期沉积层发育特征，确定了龙潭期地层的顶、底界线。以单因素图件为基础，结合其它成因标志，编制了龙潭早期（三个亚期）和龙潭晚期岩相古地理图，由此可以看出龙潭期的岩相古地理特征及其演化历史     

辽河小洼油田东营组沉积环境条件的研究

陆相盆地沉积作用主要受构造作用及古气候的控制，所以从区域构造背景出发，结合地层、古生物、沉积、地球化学等方面资料，对小洼油田东营组的沉积环境条件进行综合分析。确定了沉积物源来自北东方向的中央凸起，古气候属于半干旱—半潮湿的温带气候条件，水介质是弱还原性的淡水滨浅湖泊沉积环境。此时中央凸起上丰富的风化剥蚀产物经过河流的搬运和分异作用，进入湖盆中形成三角洲沉积体     

湘鄂赣二叠纪岩相古地理研究

湘鄂赣二叠纪沉积盆地是在华南陆块基础上发育而成，具有扬子克拉通北缘被动大陆边缘盆地，扬子克拉通盆地，湘赣板内拉张盆地和华夏克拉通边缘盆地等四类。根据古暴露等标志，建立了各类盆地的沉积层序，并进行了区域对比。     

Striking liptinitic bark remains peculiar to some Late Permian Chinese coals

An unusual liptinite coal component has been reported in the Chinese literature over the past sixty years. It has been described as a maceral in the Chinese National Standard (1991), but it has not been named internationally. In Chinese literature it is called “barkinite”, on the basis of its morphological features and because it is believed to have originated as bark tissue.“Barkinite” occurs in Late Permian, marine-influenced coals and is best represented in the Changguang, Leping and Shuicheng Basins of southern China.The material originates from plant periderm or the bark of higher plants. However, “bark” contains a variety of substances, including resin and suberin, which are recognised as the precursors of the resinite and suberinite macerals. “Barkinite” is distinguished by (i) its thickness; individual pieces can be more than ten cells thick and several centimetres long and (ii) it fluoresces strongly at 0.6% vitrinite reflectance and loses its fluorescence at about 1.1% vitrinite reflectance.The reporting of “barkinite” from only Chinese coals may be due to its origin from Lepidodendron and Psaronius flora, which was common in the Northern Hemisphere during the Carboniferous, but which was isolated to China by the Late Permian. It is proposed that the remnant flora evolved into unique forms in China by the Late Permian. Lepidodendron and Psaronius remains, coupled with a strongly marine-influenced, peat-forming environment have given rise to “barkinite” and to its restricted distribution.