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。     

UHT metamorphism on Seram,eastern Indonesia: reaction microstructures and P–T evolution of spinel‐bearing garnet–sillimanite granulites from the Kobipoto Complex

The island of Seram, part of the northern limb of the Banda Arc in eastern Indonesia, exposes an extensive Mio‐Pliocene granulite facies migmatite complex (the Kobipoto Complex) comprising voluminous leucosome‐rich diatexites and scarcer Al–Fe‐rich residual granulites. The migmatites are intimately associated with ultramafic rocks of predominantly lherzolitic composition that were exhumed by substantial lithospheric extension beneath low‐angle detachment faults; heat supplied by the lherzolites was evidently a major driver for the granulite facies metamorphism and accompanying anatexis. Residual garnet–sillimanite granulites sampled from the Kobipoto Mountains, central Seram, contain scarce garnet‐hosted inclusions of hercynite spinel (~1.5 wt% ZnO) + quartz (± ilmenite) in direct grain‐boundary contact – an assemblage potentially indicative of metamorphism under ultrahigh‐temperature (UHT) conditions. thermocalc ‘Average P–T’ reactions and melanosome‐specific thermocalc , T–M_O, and P–T pseudosections in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) chemical system, supported by Ti‐in‐garnet thermobarometry, are permissive of the rock having experienced a clockwise P–T path peaking at 925 °C and 9 kbar – thus narrowly reaching UHT conditions – before undergoing near‐isothermal decompression to ~750 °C and ~4 kbar. Spinel + quartz assemblages are interpreted to have formed at or just after the metamorphic peak from localized reactions between sillimanite, ilmenite and surrounding garnet. Further decompression of the rock resulted in the formation of complex reaction microstructures comprising cordierite ± plagioclase coronae around garnet, and symplectic intergrowths of cordierite + spinel + ilmenite around sillimanite. Small grains of sapphirine + corundum developed subsequently within spinel by localized quartz‐absent reactions. The post‐peak evolution of the granulites may be related to previously published U–Pb zircon and ⁴⁰Ar/³⁹Ar ages of c. 16 Ma, further substantiating the claim for the Kobipoto Complex granulites having recorded Earth's youngest‐identified episode of UHT metamorphism, albeit at slightly lower temperature and higher pressure than previously inferred. The Kobipoto Complex granulites demonstrate how UHT conditions may be achieved in the ‘modern’ Earth by extreme lithospheric extension, which, in this instance, was driven by slab rollback of the Banda Arc.     

Grampian migmatites in the Buchan Block,NE Scotland

Rocks exposed along the Scottish coast between Fraserburgh and Inzie Head contain information critical to understanding the evolution of the Buchan Block, the type locality for low‐P, high‐T regional metamorphism, and its relationship with the rest of the Grampian terrane, one of the major tectonostratigraphic components of the Scottish Caledonides. The ~8 km long section traverses a regional network of shear zones and, at the highest grades around Inzie Head, passes into the core of the Buchan Anticline, a large‐scale open fold that is commonly regarded as a late structure, post‐dating metamorphism. The metasedimentary rocks increase in grade from upper amphibolite to granulite facies and preserve unequivocal evidence for partial melting. The diatexite migmatites around Inzie Head, along with other gneissose units within the Buchan Block, have been regarded as allochthonous Precambrian basement rocks that were thrust into their current position during the Grampian orogenesis. However, field observations show that the onset of in situ partial melting in metapelitic rocks, which was associated with the formation of garnet‐bearing aplites and associated pegmatites, occurred around Fraserburgh, where shear fabrics are absent. Thus, the rocks preserve a continuous metamorphic field gradient that straddles the shear zone network. This observation supports an alternative interpretation that anatexis was the result of mid‐Ordovician (Grampian) metamorphism, rather than an older tectonothermal event, and that the Inzie Head gneisses are autochthonous. Using an average mid‐Dalradian pelite as a plausible representative protolith, phase equilibria modelling satisfactorily reproduces the observed appearance and disappearance of key minerals providing that peritectic garnet produced with the first formed melts (represented by the garnet‐bearing aplites) depleted the source rocks in Mn. The modelled metamorphic field gradient records a temperature increase of at least 150 °C (from ~650 °C near Fraserburgh to in excess of 800 °C at Inzie Head) but is isobaric at pressures of 2.7–2.8 kbar, suggesting the Buchan Anticline developed synchronous with partial melting. The Buchan Anticline is likely an expression of crustal thinning and asthenospheric upwelling, which produced voluminous gabbroic intrusions that supplied the heat for Buchan metamorphism.     

庐山变质核杂岩的变质变形及构造意义

庐山出露的变质核杂岩的构造层次具有典型的三层结构,即最上面的未变质沉积盖层、中间韧性流变层、核部的变质基底,其间分别以上剥离断层和拆离断层为分界线。上部沉积盖层以脆性变形为主,中间韧性流变层变形程度不一,但远离拆离带变质变形逐渐减弱,而核部的变质基底以强塑性变形为特征。对变质核杂岩南北两条地质剖面的分析,得到三个构造层次的岩石变质变形及岩性变化特征。区域动力学分析反映变质核杂岩东部原拆离带在五里正断层附近。拆离带中多硅白云母压力计得出的变质压力为0.6~1.1 GPa;锆石U-Pb年龄测试得到变质核杂岩的隆升时代为早白垩世。     

拉萨地块松多榴辉岩的变质演化过程:NCKMnFMASHTO体系中的相平衡关系

拉萨地块松多榴辉岩主要矿物组合为石榴子石、绿辉石、角闪石、多硅白云母、绿帘石、金红石。石榴子石环带不明显,核部成分均一,从核部到边部,镁铝榴石和钙铝榴石含量降低,可能分别记录了榴辉岩峰期及退变质过程信息。绿辉石显示微弱的成分环带,硬玉含量从核部到边部略有升高,部分绿辉石边部发育韭闪石退变质边,反映了在减压过程中外来流体进入体系的过程。多硅白云母具有高的Si含量(3.5~3.6),其中石榴石包体中的多硅白云母相对基质中的白云母有更高的Si值。本文利用Thermocalc变质相平衡模拟软件,结合详细岩相学观察,在NCKMn FMASHTO体系下,模拟松多含多硅白云母榴辉岩的变质演化过程。其中,榴辉岩峰期矿物组合为g+o+law+phn+ru,石榴石核部最大镁铝榴石值和石榴子石包体中多硅白云母最大Si值确定的榴辉岩峰期温压条件约为620℃,32×105Pa,榴辉岩经历了近等温降压的退变质过程。相平衡模拟结果表明拉萨地块松多榴辉岩经历了超高压变质作用过程,并经历了相对快速的折返过程到中部地壳层次。     

Microstructure,metamorphism, thermochronology and P-T-t-deformation history of the Port aux Basques gneisses,south-west Newfoundland,Canada*

Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW-directed reverse shearing. The NE-trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright-to-steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3). Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast-matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K-feldspar, and the development of an increasing volume of anatectic migmatite in south-eastern sectors of the area record syn- to late-D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P-T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east. Titanite from an upper amphibolite facies calc-silicate gneiss yields U-Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. ⁴⁰Ar/³⁹Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south-east to north-west associated with displacement on major shear zones. ⁴⁰Ar/³⁹Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation. The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.     

The origin of strain patterns resulting from contemporaneous deformation and metamorphism in the Sambagawa metamorphic belt

Abstract The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high- P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a₂/a₃ ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a₁/a₂ ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high- P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high-grade zones represent a deep-seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate.     

Microstructural features of albite porphyroblasts as indicators of sequential Barrovian metamorphic mineral growth in the Caledonides of the SW Scottish Highlands

Inclusion – porphyroblast and porphyroblast – porphyroblast relationships show that abundant albite in mica schists in the Caledonides of the SW Scottish Highlands are part of the Barrovian metamorphic assemblage. Growth early in the D2 deformational phase of porphyroblast cores followed the growth of Mn‐rich garnet but preceded the growth of porphyroblasts of the index mineral almandine. Two sets of inclusion trails in the albite correspond to the regionally expressed S1 and S2. Straight trails of muscovite, chlorite, quartz, epidote and the earliest growth of biotite make up S1. Crenulated trails express deformation of S1 early in D2 with muscovite, chlorite, biotite, quartz, epidote and the Mn‐rich garnet associated with the development of S2 crenulation cleavage. The geometries of these trails uniquely record early stages of D2 deformational history. An _0?3 growth is related to the temporal coincidence of the formation of S1–S2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component. The main characteristics of the regionally expressed D2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth. Essentially inclusion‐free An _4?19 rims grew on the inclusion‐containing cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite, biotite and chlorite indicate that mineral growth extended from the later stages of D2 to post‐D2. Previous interpretations of the albite porphyroblast growth having been during D4 to post‐D4 contemporaneous with retrogression are inconsistent with the microstructural evidence.     

对二郎坪群火神庙组物质组成的质疑与再认识

经研究证明，前人建立的二郎坪群火神庙组确有少量的亚碱性双峰式火山岩。这些火山岩零星地分布在几个地区，走向不连续，出露宽度一般为数百米。构成该组主体的是一条规模宏伟的侵入岩带，其岩石组合为辉长岩－闪长岩－石英闪长岩－斜长花岗岩－花岗岩，总体上表现为正序岩浆演化序列。这些侵入岩普遍经历了三个阶段的变形变质作用，即第一阶段为韧性变形；第二阶段为静态重结晶；第三阶段为带状褶劈理，表现了复杂而漫长的构造演化史。     

Phase petrology and P-T conditions of mafic blueschists from the Meliata unit, West Carpathians, Slovakia

Abstract Blueschists occurring as layers in calcite marbles of the Meliata unit occur along the so-called Roznava tectonic line situated in the southern part of the Gemericum, Slovakia. Mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated. The most common minerals in the blueschists are blue amphibole, epidote and albite. Some Fe²⁺- and Al-rich rocks also contain garnet and chloritoid, respectively. Na-pyroxene with a maximum 50% jadeite component was also found. The blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples. Almandine- and spessartine-rich garnet has very low MgO content (<3 wt%). The Si content in phengite ranges between 3.3 and 3.5 pfu calculated on the basis of 11 oxygens. The zoning patterns of blue amphibole, garnet and chloritoid suggest their formation during a prograde stage of metamorphism. The P-T conditions of metamorphism are estimated to be about 380–460° C and 10–13 kbar. Pressures of 7.5–8.5 kbar and temperatures of 350–370° C were obtained for some actinolite- and aegirine-rich rocks. Apart from chlorite, other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite.