Transformation of garnet epidote amphibolite to eclogite, western Dabie Mountains, China

Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H₂O, and Amp + Ab = Omp ± Czo + Qtz + H₂O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H₂O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.     

Le Silurien de la région d'Oulad Abbou (Meseta occidentale,Maroc) : une sédimentation péritidale sous contrôle tectonique

In the Oulad Abbou syncline, western coastal Meseta, the Silurian deposits exhibit siliciclastic or mixed siliciclastic/carbonate tidal facies that recorded alkaline basalt flows and syn-sedimentary deformations. These facies are staked into peritidal shallowing upward sequences reflecting the evolution from an infratidal to a supratidal environment. These sequences recorded low-amplitude and high-frequency sea-level variations. The built-up of these rhythmic sequences is related to distensive tectonic that allowed the development of isolated platform from extensive siliciclastic influx. This tectonic event is well recorded in the palaeogeographic evolution of the northern Gondwana platform during the Lower Palaeozoic time. To cite this article: A. Attou, N. Hamoumi, C. R. Geoscience 336 (2004).     

利用地震属性研究准噶尔盆地西北缘拐19井区下侏罗统三工河组沉积环境

在准噶尔盆地西北缘，寻找岩性圈闭油气藏已是至关重要的问题。但人们几乎还是用常规的手段来寻找岩性油气藏，没有实质性进展。本文是以利用宽方位角采集的地震数据为基础，对数据进行高保真资料处理和参考标准层的层拉平解释。利用前人的区域地质研究成果和钻井解释成果对侏罗系的沉积环境进行了精细的描述，确定了拐19井区在下侏罗统三工河组的出油层段附近的沉积环境由湖泊相－三角洲前缘相－河流相－湖泊相的演化过程。利用地震属性解释的结果也能较好地反映目标区的沉积环境变迁，并且与区域地质、测井解释结果相吻合，为寻找油气提供了很好的依据。由此得出利用地震属性可以进行沉积环境变迁分析，为油田寻找岩性圈闭及油气藏做出贡献。     

珠江三角洲江村ZK2钻孔沉积物粒度的环境意义

运用MALVERN公司2000型粒度仪对珠江三角洲地区的江村ZK2钻孔作粒度分析，江村钻孔按粒度的偏态值可划分为两个河相与海相或湖泊沼泽相的沉积交替过程，同时与粒度参数、年代数据以及前人所做的孢粉等证据相结合，大致看出研究区气候变化的4个千年尺度的气候波动：第一阶段为较长时间的冷干期，该段时间约为20-10．7kaB．P．；第二阶段为回暖期，时间大致在10．7-7．5kaB．P.,总体比较湿润；第三阶段为升温期，该时期约在7．5-5kaB．P．之间，在此期间各有一次干湿交替；第四阶段为降温期，时间大约出现在5kaB．P．至今，这是一个波动性较大的时期，也各有一次干湿交替。     

STRUCTURAL GEOLOGY

正20141283 Bai Daoyuan(Hunan Institute of Geological Survey,Changsha 410016,China);Zhong Xiang Nature,Origin and Tectonic Setting of Jinzhou Basin in the South Segment of Xuefeng Orogen(Geology in China,ISSN1000-3657,CN11-1167/P,40(4),2013,p.1079-1091,10 illus.,47 refs.)Key words:foreland basins,strike-slip faults,Hunan Province     

GEOPHYSICAL EXPLORATION

正20141074Bao Xijie(Research Institute of Exploration and Development,Daqing Oilfield Company,PetroChina,Daqing 163712,China)Gather Optimal Processing and Application Effect of Prestack AVA Instantaneous Inversion     

Carboniferous eclogite and garnet–omphacite granulite from northeastern Hainan Island,South China: Implications for the evolution of the eastern Palaeo‐Tethys

High‐P (HP) eclogite and associated garnet–omphacite granulite have recently been discovered in the Mulantou area, northeastern Hainan Island, South China. These rocks consist mainly of garnet, omphacite, hornblende, quartz and rutile/ilmenite, with or without zoisite and plagioclase. Textural relationships, mineral compositions and thermobarometric calculations demonstrate that the eclogite and garnet–omphacite granulite share the same three‐stage metamorphic evolution, with prograde, peak and retrograde P?T conditions of 620–680°C and 8.7–11.1 kbar, 820–860°C and 17.0–18.2 kbar, and 700–730°C and 7.1–8.5 kbar respectively. Sensitive high‐resolution ion microprobe U–Pb zircon dating, coupled with the identification of mineral inclusions in zircon, reveals the formation of mafic protoliths before 355 Ma, prograde metamorphism at c. 340–330 Ma, peak to retrograde metamorphism at c. 310–300 Ma, and subsequent pegmatite intrusion at 295 Ma. Trace element geochemistry shows that most of the rocks have a MORB affinity, with initial ε_Nd values of +2.4 to +6.7. As with similar transitional eclogite–HP granulite facies rocks in the thickened root in the European Variscan orogen, the occurrence of relatively high P?T metamorphic rocks of oceanic origin in northeastern Hainan Island suggests Carboniferous oceanic subduction leading to collision of the Hainan continental block, or at least part of it, with the South China Block in the eastern Palaeo‐Tethyan tectonic domain.     

COAL GEOLOGY

正20141574 Chen Hao(Exploration and Development Research Institute,Daqing Oilfield Company,Daqing 163712,China)High-Resolution Sequences and Coal Accumulating Laws in Nantun Formation of Huhe Lake Sag(Petroleum GeologyOilfield Development in Daqing,ISSN1000-3754,CN23-1286/TQ,32(4),2013,p.15-19,5 illus.,15 refs.)Key words:coal accumulation regularity,coal     

Stromatoporoid growth forms and Devonian reef fabrics in the Upper Devonian Alexandra Reef System,Canada – Insight on the challenges of applying Devonian reef facies models

Existing facies models for Devonian reef systems can be divided into high‐energy and low‐energy types. A number of assumptions have been made in the development of these models and, in some cases, criteria that distinguish important aspects of the models are poorly defined. The Upper Devonian Alexandra Reef System contains a variety of reef fabrics from different depositional environments and is ideal for studying the range of environments in which stromatoporoids thrived and the facies from these different environments. A wide variety of stromatoporoid growth forms including laminar, tabular, anastamosing laminar and tabular, domal, bulbous, dendroid, expanding conical, concave‐up whorled‐laminar, concave‐up massive tabular and platy‐multicolumnar are present in the Alexandra Reef System. The whorled‐laminar and massive tabular concave‐up growth forms are virtually undocumented from other Devonian reefs but were common in the reef front of the Alexandra, where they thrived in a low‐energy environment around and below fair‐weather wave base. In contrast, high‐energy parts of the reef margin were dominated by bioclastic rubble deposits with narrow ribbon‐like discontinuous bodies of laminar stromatoporoid framestone. In the lagoon, laminar stromatoporoids formed steep‐sided sediment‐dominated bioherms in response to sea‐level rise and flooding. Relying mostly on the different reef facies in the Alexandra system, a new classification scheme for Devonian reef fabrics has been developed. Devonian reef fabrics can be classified as being: (i) sediment‐laden metazoan dominated; (ii) metazoan–microbial dominated (boundstone); (iii) metazoan dominated (framestone); or (iv) metazoan–marine cement dominated. Distinction of these fabrics carries important sedimentary and palaeoecological implications for reconstructing the depositional environment. With examples from the Alexandra Formation, it is demonstrated that reef facies accumulated in a range of depositional environments and that the simple observation of massive stromatoporoids with or without microbial deposits does not automatically imply a high‐energy reef margin, as otherwise portrayed in a number of the existing facies models for these systems.     

Strain partitioning into dry and wet zones and the formation of Ca-rich myrmekite in syntectonic syenites: A case for melt-assisted dissolution-replacement creep under granulite facies conditions

The formation of Ca-rich myrmekites is described in syntectonic syenites crystallized and progressively deformed under granulite facies conditions. The syenites are found in high- and low-strain zones where microstructure and mineral composition are compared. Heterogeneously distributed water-rich, late-magmatic liquids were responsible for strain partitioning into dry and wet high-strain zones at outcrop scale, where contrasting deformation mechanisms are reported. In dry high-strain zones K-feldspar and clinopyroxene are recrystallized under high-T conditions. In wet high-strain zones, the de-stabilization of clinopyroxene and pervasive replacement of relatively undeformed K-feldspar porphyroclasts by myrmekite and subordinate micrographic intergrowths indicate dissolution-replacement creep as the main deformation mechanism. The reworking of these intergrowths is observed and is considered to contribute significantly to the development of the mylonitic foliation and banding. A model is proposed for strain partitioning relating a positive feedback between myrmekite-forming reaction, continuous inflow of late-magmatic liquids and dissolution-replacement creep in the wet zone at the expenses of original mineralogy preserved in the dry zones. Melt-assisted dissolution-replacement creep in syntectonic environments under granulite-facies conditions may extend the field of operation of dissolution-replacement creep, changing significantly the rheology of the lower continental crust.