Metamorphism and metamorphic K–Ar ages of the Mesozoic accretionary complex in Northland, New Zealand

Abstract A southwest dipping Mesozoic accretionary complex, which consists of tectonically imbricated turbiditic mudstone and sandstone, hemipelagic siliceous mudstone, and bedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island, New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. The grade of subduction‐related metamorphism increases from northeast to southwest, indicating an inverted metamorphic gradient dip. Three metamorphic facies are recognized largely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite, prehnite‐pumpellyite and pumpellyite‐actinolite. From the apparent interplanar spacing d₀₀₂ data for carbonaceous material, which range from 3.642 to 3.564 Å, the highest grade of metamorphism is considered to have attained only the lowermost grade of the pumpellyite‐actinolite facies for which the highest temperature may be approximately 300°C. Metamorphic white mica K–Ar ages are reported for magnetic separates and <2 µm hydraulic elutriation separates from 27 pelitic and semipelitic samples. The age data obtained from elutriation separates are approximately 8 m.y. younger, on average, than those from magnetic separates. The age difference is attributed to the possible admixture of nonequilibrated detrital white mica in the magnetic separates, and the age of the elutriation separates is considered to be the age of metamorphism. If the concept, based on fossil evidence, of the subdivision of the Northland accretionary complex into north and south units is accepted, then the peak age of metamorphism in the north unit is likely to be 180–130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas that in the south unit is 150–130 Ma; that is, late Late Jurassic to early Early Cretaceous. The age cluster for the north unit correlates with that of the Chrystalls Beach–Taieri Mouth section (uncertain terrane), while the age cluster for the south unit is older than that of the Younger Torlesse Subterrane in the Wellington area, and may be comparable with that of the Nelson and Marlborough areas (Caples and Waipapa terranes).     

构造物理化学条件对煤变质作用的控制

煤是对温度和压力等地质因素十分敏感的有机岩,各种构造-热事件控制下的物理化学条件,是促进煤岩演化的根本动力。本文对煤变质作用过程的研究现状进行了综述,着重讨论了煤岩在高煤阶-石墨演化阶段的控制因素、演化过程和演化机制。煤变质作用包括煤化作用阶段和石墨化作用阶段,共同构成一个连续的有机质演化过程,总体趋势是分子结构有序化、化学成分单一化,最终演变为以碳元素为主、三维有序结构的石墨。温度和压力(应力)是控制煤变质作用两大因素,在不同的演化阶段,这两大因素所起的作用和演变机理都有所差异。在低、中煤阶演化阶段,温度是煤化学结构演化的主要控制因素,为化学键断裂提供活化能,应力缩聚和应力降解则对煤化学结构演化具有催化作用。高煤阶-石墨化阶段的主要机制是导致基本结构单元BSUs之间相互联结使短程有序化范围增大的拼叠作用,构造应力在其中起到关键作用,BSUs定向和面网间距不断减小,促进大分子物理结构演化。加强煤变质作用的高级阶段-石墨演化过程的研究,将丰富和深化对煤-石墨物理化学结构完整演化序列的认识。煤系石墨成矿机制的高温高压模拟实验,则为煤变质作用构造物理化学条件研究提供了可行的技术手段。     

煤基石墨微结构的高分辨电镜研究

煤体经热变质作用形成的石墨叫煤基石墨。湖南省鲁塘地区煤基石墨,经与之共生的红柱石的相图分析,它属于低压相系矿物,在200—800℃和5.5×10~5Pa以下条件下形成。它的高分辨电子显微象显示,煤基石墨按其石墨化程度有四种类型微结构。因而。将煤基石墨划分为相继的四个石墨化阶段。同时有四种石墨产物,它们是:前石墨化阶段——芳层石墨;初石墨化阶段——微柱石墨;中石墨化阶段——柔绉石墨;高石墨化阶段——平直石墨     

我国煤系石墨研究及资源开发利用前景

煤系石墨属于特殊的非典型晶质矿物,多为隐晶质石墨,既是石墨矿产的重要组成部分,也是煤系综合矿产的一种。煤系石墨与煤层为同层异矿,煤向石墨演化的实质在元素组成上表现为富碳、去氢和脱氧,在分子结构上表现为有序化增强和石墨晶体结构的逐渐形成。煤岩组分、岩浆热和地质构造等均对煤成石墨化作用具有重要的影响,在多因素综合作用下,煤系石墨成矿常表现出差异石墨化特征。从煤系石墨成矿机理入手,提出了以化学组成参数为基础指标,以结构参数为精确指标的煤系石墨鉴别指标体系;从资源评价需求出发,将不同演化程度的煤系石墨划分为Ⅰ级（石墨）、Ⅱ级（半石墨）和Ⅲ级（石墨化无烟煤）等3类。煤系石墨的形成与岩浆活动和挤压性构造环境密切相关,受区域性构造-岩浆带控制,煤系石墨分布具有方向性、递变性、集中成带的特点,成矿区带呈现出“一纵三横”的分布特点,划分为滨太平洋成矿域、南岭成矿域、秦岭-大别山成矿域和阴山-燕山成矿域和9个成矿带。分析了我国煤系石墨资源现状,指出由于我国煤炭资源丰富和赋煤区构造-热叠加作用显著,煤系石墨资源潜力巨大。煤系石墨开发利用对于增强石墨矿产的战略保障能力、促进煤系矿产资源合理开发利用和推动煤炭企业转型升级等均具有重要意义。分析了当前煤系石墨矿产资源开发利用方面存在的问题,提出了相应的政策建议。      

The association of poorly ordered graphite, coke and bitumens in greenschist facies rocks of the Poniklá Group, Lugicum, Czech Republic: the result of graphitization of various types of carbonaceous matter

Greenschist facies rocks of the Poniklá Group (Ordovician-Silurian), Czech Republic, contain several types of carbonaceous matter that differ in their morphology, texture, reflectance and Raman characteristics. The first type consists of large (up to 3 mm) irregularly bound particles of low reflectance (R_omin = 0.9%; R_omax = 5.6%). The area ratio of the 1585 cm^-1 to 1350 cm^-1 Raman peaks (1.08–1.17) indicates an intermediate degree of graphitization. The formation of this type of highly porous particle, displaying a texture reminiscent of regular or needle coke, is attributed to the thermal alteration of the amorphous (structureless) kerogen of the precursor sediments. The second type consists of lamellar particles up to 30 μm thick, which can be associated with the latter or can occur independently in white mica-rich laminae. This type is characterized by high bireflectance (R_omin = 0.6%; R_omax = 11.9%) and by lower ratios (0.70–0.82) of the Raman peak areas. These particles are interpreted as the product of solid-state, diffusion-controlled graphitization of a chemically homogeneous organic material, e.g. of graptolite periderms. The third type consists of isometric, up to 2 mm large, commonly fractured grains and fragments which mainly occur in quartz-rich laminae. In reflected light, the texture is either homogeneous or consists of various types of anisotropic mozaics. The Raman peak area ratios (0.75–1.14) indicate a highly variable degree of structural ordering. These particles are considered as the remains of metamorphosed bitumens, accumulated in the sandy laminae of the original sediments. The fourth type consists of small particles of carbonaceous matter (maximum length 25 μm, thickness 1-2 μm), which occur adjacent to crystal faces of white micas. This type is probably the product of epitaxial growth of graphite from the gaseous phase. The results of this work indicate that the differences in the degree of graphitization of the carbonaceous matter in low-grade metamorphosed rocks can be mainly related to the initial nature of the sedimentary organic matter and to its premetamorphic history.     

Shuffled-cards structure and different P/T conditions in the Sanbagawa metamorphic belt, Sakuma-Tenryu area, central Japan

The Sanbagawa metamorphic terrain of the study area is divided into two units, the Shirakura and Sejiri units. The metamorphic thermal structure is interpreted on the basis of the degree of graphitization (GD) of carbonaceous material in pelitic schists. The areal variations of the metamorphic grade are presented by the distribution of GD calculated using the Lc and d₀₀₂ of carbonaceous material. As a result, the two units are classified into four metamorphic zones, respectively: A₁, A₂, B₁ and B₂ for the Shirakura Unit; and I₁, I₂, II₁ and II₂ for the Sejiri Unit. The metamorphic grades of A₁, A₂, I₁ and I₂ are included in the chlorite zone, and that of B₁, B₂, II₁ and II₂ in the garnet zone of the Sanbagawa metamorphism. The degree of graphitization at the boundary between A₂ and B₁ zones is the same as that between I₂ and II₁ zones. Detailed study on the variation of GD suggests that the present‐day structure of the study area is best interpreted as a model of shuffled‐cards structure. An estimated minimum thickness of a stack of continuous cards is about 25 m. The compositions of garnet in pelitic schists and of amphibole in basic schists are different from those in the identical metamorphic range of the Shirakura and Sejiri units. It is suggested that rocks of the Shirakura Unit were metamorphosed under higher P/T conditions than those of the Sejiri Unit.     

Effect of oil and gas deposits on dynamic characteristics of reflected waves

ABSTRACT

The preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2–3 mm synthetic diamond monocrystals at 2–4 GPa in СаСО₃ (1550°С) and К₂СО₃ (1450°С) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the surface graphitization rate of diamonds in the presence of carbonate melts at 1450–1550°C increases with decreasing pressure. Decreased pressure alone can graphitize diamond regardless of exhumation rate. Metastable diamond inclusions survive exhumation with little or no graphitization because of excess pressure up to 2 GPa acting on them, and because inclusions are protected from interaction with C-O-H fluid.     

Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece

We present new Raman spectra data of carbonaceous material (CM) to extend the range of the Raman spectra of CM thermometer (RSCM) to temperatures as low as 100 °C. Previous work has demonstrated that Raman spectroscopy is an excellent tool to describe the degree of graphitization of CM, a process that is independent of pressure but strongly dependent on metamorphic temperature. A linear relationship between temperature and the Raman parameter R2 (derived from the area of the defect band relative to the ordered graphite band) forms the basis of a previous thermometer. Because R2 shows little variability in low-temperature samples, 330 °C serves as a lower limit on the existing thermometer. Herein, we present Raman spectra from a suite of low-temperature (100 to 300 °C) samples from the Olympics Mountains and describe other aspects of the Raman spectra of CM that vary over this range. In particular, the Raman parameter R1 (the ratio of heights of the disordered peak to ordered peak) varies regularly between 100 and 350 °C. These data, together with published results from higher-temperature rocks, are used to calibrate a modified RSCM thermometer, applicable from 100 to 700 °C. Application to low-grade metasediments in the Otago region in the South Island of New Zealand gives temperatures consistent with previous estimates, demonstrating the reliability of the modified RSCM thermometer.We apply the modified RSCM thermometer to 53 samples from Crete to evaluate the role of the Cretan detachment fault in exhuming Miocene high pressure/low-temperature metamorphic rocks exposed there. The metamorphic rocks below the detachment (the Plattenkalk and Phyllite-Quartzite units) give metamorphic temperatures that range from 250 to 400 °C, consistent with previous petrologic estimates. We also demonstrate that the Tripolitza unit, which lies directly above the detachment, gives an average metamorphic temperature of about 260 °C. The modest break in metamorphic temperature in central Crete indicates that the Cretan detachment accounts for only 5 to 7 km of exhumation of the underlying HP-LT metamorphic rocks, which were initially accreted at ∼ 35 km. We argue that the bulk of the exhumation (∼ 28 km out of 35 km total) occurred by pervasive brittle stretching and erosion of structural units above the detachment.     

Raman spectra of carbonaceous material in metasediments: a new geothermometer

Metasedimentary rocks generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within‐sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustrés (Western Alps) and Sanbagawa (Japan) cross‐sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross‐section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high‐temperature, low‐pressure to high‐pressure, low‐temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature [T(°C) = ?445 R2 + 641], whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to ± 50 °C in the range 330–650 °C. A few technical indications are given for optimal application.