Some remarks on high-temperature—low-pressure metamorphism in convergent orogens

Many high-temperature–low-pressure (high- T –low- P ) metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating–cooling cycle. Such terranes are not readily explained as the conductive response to crustal thickening since the resulting Moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric-scale strains must greatly outlast the deformation. Consequently, high- T –low- P metamorphism may be generated during crustal thickening only when significant heat is advected within the crust, as for example may occur during the segregation of granitic melts. We show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially, for example, by removing the mantle part of the lithosphere. A simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust, _c, and the lithosphere, ₁ respectively, allows the potential energy of the evolving orogen to be readily evaluated. Using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high- T –low- P metamorphism during crustal thickening (with _c1) is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations (f_c= f₁) for the same initial lithospheric configuration.     

碰撞造山带超高温变质作用及构造意义——以泛非造山带为例

超高温（≥900℃）变质作用发生在自太古代以来的各个地质历史时期,目前极可能也正发生在青藏高原地壳深部。同时,它也是以冈瓦纳为代表的超大陆在最终拼合时的显著标识,这一关联指示了超高温变质作用与碰撞造山带的密切关系。本文总结了东冈瓦纳内与泛非造山作用有关的典型超高温变质岩的分布、岩石学特征、峰期变质条件、P-T轨迹及形成时代,并简要介绍我们在柴达木地块西段新识别出的泛非期超高温变质作用的基本特征。结合东冈瓦纳超高温变质作用特征和造山带热模拟研究的新进展,本文获得以东冈瓦纳超高温变质作用为代表的碰撞造山带超高温变质作用的几点认识：1）东冈瓦纳麻粒岩地块中的超高温变质岩和普通麻粒岩记录了相似的变质年龄、P-T轨迹以及呈过渡变化的峰期温度,两者可能是同一构造事件的产物,共同组成一个高温-超高温变质岩单元;2）超高温变质作用在东冈瓦纳内部持续了至少超过30Myr,但未见呈大规模的同期或近同期基性岩岩浆出露,指示此处需要的长期热源不是地幔来源岩浆;3）虽然数值模拟能成功呈现加厚地壳被放射元素衰变热加热至超高温条件的情况,且加热及持续时间与东冈瓦纳超高温变质约束的结果相当,但是模拟中需要的高生热值暗示,在自然界中,完全只靠放射性元素衰变生热或许不能让碰撞造山带内达到超高温条件;4）碰撞造山带经历了长期的构造演化,这一过程中,造山带内地壳不太可能同时达到超高温变质条件,这一特征可能反映在P-T-t轨迹的差异上,对这些轨迹的系统研究有助于对超高温变质作用的构造-热过程的理解。     

Ultrahigh-pressure metamorphism and exhumation of garnet-bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.     

显生宙碰撞造山带超高温变质作用的加热机制：来自二维数值模拟的约束

大量的岩石学证据表明: 碰撞造山带中常发育900~1100℃的超高温变质作用。然而, 碰撞造山带中如何出现如此极端的超高温条件仍然存在争议。为了更好地理解超高温变质作用加热机制和碰撞造山带中主要热源的相对贡献, 我们建立了一系列高分辨率二维热-动力学模型, 借此探讨了俯冲大陆岩石圈密度亏损程度、大陆地壳放射性生热率和大陆汇聚速率等因素对碰撞造山过程中超高温变质主要热源的影响。当大陆岩石圈密度亏损(Δρ=ρ软流圈地幔-ρ岩石圈地幔)大于50kg/m3时, 有利于发生大陆平板俯冲, 软流圈地幔无法上涌为地壳物质提供热源; 此时, 具有较高放射性生热率(>3μW/m3)的地壳可以发生"浅俯冲-折返"型超高温变质作用。而当大陆岩石圈密度亏损小于10kg/m3时, 大陆上地壳在深俯冲阶段首先发生超高压榴辉岩相变质作用, 随后伴随着大陆岩石圈地幔后撤和软流圈上涌, 进而出现以异常高的地幔热流加热为主的"深俯冲-折返"型超高温变质作用。此外, 较低的大陆汇聚速率(< 1cm/yr)更有利于"深俯冲-折返"型超高温变质作用的产生。将数值模拟结果与特提斯构造域的变质岩石数据和地球物理观测进行对比, 我们认为在现今板块构造体制下, 由具有密度亏损程度较高的大陆岩石圈平俯冲有利于"浅俯冲-折返"型超高温变质作用的发生, 而由密度亏损程度较低的大陆岩石圈俯冲可能导致"深俯冲-折返"型超高温变质作用的产生。     

北大别的高温超高压变质作用与多阶段折返

近期的变质岩石学、地球化学及同位素年代学研究表明,北大别整体经历了高温超高压变质作用和多阶段折返历史,因而表现为广泛发育的多期减压结构和极少保留早期的超高压变质记录。北大别榴辉岩以高温变质作用以及折返期间因麻粒岩相和角闪岩相退变质变质作用而形成的多期后成合晶为显著特征。石榴子石中伴有放射状胀裂纹的单晶和多晶石英包体指示早期柯石英的转变结果,这已被锆石中发现的柯石英残晶所证实。结合北大别北部榴辉岩和片麻岩中发现的金刚石等超高压证据以及三叠纪变质记录,由此证明北大别整体经历过深俯冲和印支期超高压变质作用。北大别榴辉岩的多阶段高温条件主要来自石榴子石-绿辉石矿物对温度计、单斜辉石中紫苏辉石+石英针状矿物出熔体以及金红石中较高的Zr含量和变质锆石中较高的Ti含量等得出的温度证据。此外,多期后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个多阶段、快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩经历了长时间的高温变质演化与缓慢冷却过程。长时间的高温变质作用与缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压记录的重要原因。因此,这些成果为大别山三个不同超高压岩片的差异折返模型的建立提供了新的证据。     

新疆阿尔泰西北部白哈巴一冲乎尔一带加里东和海西两期区域变质作用的厘定

通过对新疆西北部阿尔泰白哈巴—冲乎尔一带详细的野外地质填图和综合研究,在原划分的奥陶纪、震旦—寒武纪地层中分别采获了早泥盆世双壳化石,获得了加里东期、海西期不同期次的同位素地质年龄,在浅变质岩中划分出了石英岩状结构亚带和刺状结构亚带,在区域动力热流变质岩中划分出蓝晶石带、矽线石带、铁铝榴石带、十字石带等。新的证据表明,研究区白哈巴地区发育加里东期和海西期两期区域低温动力变质,南东部发育两期区域动力热流变质,海流滩为加里东期,冲乎尔则为海西期,沿库根断裂带产生叠加。这一划分使争论了近半个世纪的该区变质类型和变质世代的问题有了新的答案。     

Microstructure, metamorphism and tectonics of the western Cape Breton Highlands, Nova Scotia

ABSTRACT Microstructural and petrological data from the Jumping Brook metamorphic suite, western Cape Breton Highlands, suggest that a single episode of syntectonic prograde metamorphism, followed by uplift, cooling and associated retrogression, affected these rocks during mid-Palaeozoic times. Microstructures indicative of progressive crenulation foliation development can be traced from low-grade (chlorite zone) through high-grade (kyanite zone) rocks, allowing a clear sequence of porphyroblast growth to be established. Metamorphic reactions and P-T calculations suggest metamorphic conditions of 700-750°C at 8-10 kbar were achieved in kyanite zone rocks. Although a complete P-T-t path was not defined, combined petrological and geochronological data can be used to constrain computed P-T-t models. These models suggest that a component of post-metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift. The microstructural and petrological data to not support the interpretation that the high-grade rocks represent pre-existing crystalline basement. Indeed, the metamorphic history, geochronology and computed tectonic models all point to a single, short-lived episode of Silurian-Devonian volcanism, intrusion, convergence, regional metamorphism and uplift, probably resulting from collision tectonics at an irregular continental margin.     

Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.     

Review and classification of structural controls on fluid flow during regional metamorphism

Mechanisms for kilometre-scale, open-system fluid flow during regional metamorphism remain problematic. Debate also continues over the degree of fluid flow channellization during regional metamorphism, and the mechanisms for pervasive fluid flow at depth. The requirements for pervasive long-distance fluid flow are an interconnected porosity and a large regional gradient in fluid pressure and hydraulic head (thermally or structurally controlled) that dominates over local perturbations in hydraulic head due to deformation. In contrast, dynamic or transient porosity interconnection and fluid flow accompanying deformation of heterogeneous rock suites should result in moderately to strongly channellized flow at a range of scales, of which there are many examples in the literature. Classification of fluid flow types based on scale and degree of equilibration between fluid and rock, wallrock permeability, and mode of fluid transport contributes to an understanding of key factors that control fluid flow. Closed-system fluid behaviour, with restricted fluid flow in microcracks or cracks and limited fluid–rock interaction, occurs over a range of strains and crustal depths, but requires low permeabilities and/or small fluid fluxes. Long-distance, open-system fluid flow in channels is favoured in heterogeneous rocks at high strains, moderate (but variable) permeabilities, and moderate to high fluid fluxes. Long-distance, broad, pervasive fluid flow during regional metamorphism requires that the rocks are not accumulating high strains and have high permeabilities, low permeability contrasts, and high fluid fluxes. The ideal situation for such fluid flow is in situations where the rocks are undergoing stress relaxation immediately after a major deformation phase. In the mid-crust, fairly specific conditions are thus required for pervasive fluid flow. During active orogenesis, structurally controlled fluid flow (with focused open systems surrounding regions of closed-system behaviour) predominates in most, but not all, regional metamorphic situations, at a range of scales.     

Pressure–temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high-pressure rocks in the Central Aegean

The blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km^?1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.     

New, high-precision P–T estimates for Oman blueschists: implications for obduction, nappe stacking and exhumation processes

Oman blueschists and eclogites lie below the obduction nappe of the Semail ophiolite in one of the key areas on Earth for the study of plate convergence. Here new metamorphic and tectonic constraints are provided for the central, yet poorly constrained Hulw unit, sandwiched between the low‐grade units (～10 kbar, <300 °C) and the As Sifah eclogites (P_max ～ 23 kbar; T_max ～ 600 °C). TWEEQU multi‐equilibrium thermobarometry, using both compositional mapping and spot analyses, and Raman spectroscopy of carbonaceous material yield a high‐precision P–T path for the Hulw and As Sheikh units and reveal that they shared a common P–T history in four stages: (i) a pressure decrease from 10–12 kbar, 250–300 °C to 7–9 kbar, 300–350 °C; (ii) almost isobaric heating at ～8–10 kbar from 300–350 °C to 450–500 °C; (iii) a pressure decrease at moderate temperatures (～450–500 °C); and (iv) isobaric cooling at ～5–6 kbar from 450–500 to 300 °C. No significant pressure or temperature gap is observed across the upper plate–lower plate discontinuity to the north and west of the Hulw unit. The combination of tectonic and P–T data constrains the stacking chronology of the three main metamorphic units comprising the Saih Hatat window (i.e. the Ruwi‐Quryat, the Hulw‐As Sheikh and the Diqdah‐As Sifah units). These results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple, N‐vergent continental subduction of the passive Arabian margin below the obduction nappe along a cold P–T gradient.     

北大别超高压榴辉岩的快速折返与缓慢冷却过程

岩石学研究表明,北大别超高压榴辉岩经过了超高压和高压榴辉岩相变质作用以及麻粒岩相叠加和角闪岩相退变质作用.其中,高压麻粒岩相和角闪岩相变质阶段形成的后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩在峰期超高压变质作用之后又经历了一个缓慢冷却过程.超高压岩石折返期间的缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压证据的重要原因.因此,本文为大别山不同超高压岩片的差异折返模型的建立提供了新的证据.     

Lithospheric tectonic structures developed under high-grade metamorphism in the Southern part of Madagascar

Abstract

This paper describes the tectono-metamorphic evolution of a segment of the Precambrian deep crust, in the southern Madagascar island. This crust corresponds to an Archaean basement reworked by a widespread, late panAfrican event (550–580 Ma) during the formation of the Mozambican belt. The finite geometry and associated metamorphism are depicted by satellite imaging, field mapping and P-T estimations using both conventional thermobarometric methods and TWEEQ software program with internally consistent thermodynamic data and uniform set of solution models. The structural pattern developed during high-grade metamorphism shows the juxtaposition of domains with complex fold geometries separated by a 15 km wide ductile shear zone. Within the folded domains, kilometre scale interference patterns associated with strongly dipping metamorphic stretching lineations can be described as superposed folding (F1 and F2 folds). The tight and upright F2 folds result from East-West horizontal shortening. The shear zone is defined by homogeneous orientations of steep foliations, sub-horizontal stretching lineations, and kilometre scale strain gradient. Within the shear zone, we observe dominant non-coaxial criteria at various scales that are consistent with a sinistral strike-slip system during D2 deformation stage. Nevertheless, we have also found in the shear zone, geometries typical of a horizontal shortening. Such a strain pattern is characteristic of transpression tectonics.

The synkinematic metamorphic conditions are estimated on mafic garnetiferous metabasites. Results show that regional transpression tectonics has developed under very high and constant thermal regime (about 800°C). A pressure gap, of about 3 kbar between the domains separated by the shear zone is identified. This implies tectonic coupling of two different structural levels during tranpressive tectonic.     

High-pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east-central Alaska

Abstract Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska. These terranes differ in protoliths, structural level and cooling ages. A structurally complex zone of gently north-dipping tectonites separates the two terranes. The northern, structurally higher Taylor Mountain terrane includes garnet amphibolite, biotite ± hornblende gneiss, marble, quartzite, metachert, pelitic schist and cross-cutting granitoids of intermediate composition (including the Late Triassic to Early Jurassic Taylor Mountain batholith). Lithological associations and isotopic data from the granitoids indicate an oceanic or marginal basin origin for the Taylor Mountain terrane. ⁴⁰Ar/³⁹Ar metamorphic cooling ages from the Taylor Mountain terrane are latest Triassic to earliest Middle Jurassic. The southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane is made up of quartz-biotite schist and gneiss, augen gneiss, pelitic schist, garnet amphibolite and quartzite; we interpret it to comprise a continental margin and granitoid belt built on North American crust. Metamorphic cooling ages from the Lake George subterrane are almost entirely Early Cretaceous. Geothermobarometric analysis of garnet rims and adjacent phases in garnet amphibolite and pelitic schist from the Taylor Mountain terrane and Lake George subterrane indicate peak metamorphic conditions of 7.5-12 kbar at 555-715° C in the northern part of the Taylor Mountain terrane, in which NNE-vergent shear fabrics are preserved; 6.5-10.8 kbar at 520-670° C within the contact zone between the two terranes, in which NW-vergent shear fabrics predominate; and 6.8-11.8 kbar at 570-700° C in the Lake George subterrane of the Yukon-Tanana terrane, in which NW-vergent shear is recorded in the northern part of the study area and SE-vergent shear in the southern part. Where the two shear-sense directions occur together in the northern Lake George subterrane and, locally, in the contact zone, fabrics that record NW-vergent shear are more penetrative and preceded fabrics that record SE-vergent shear. We interpret the pressure, temperature, kinematic and age data to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. The difference in metamorphic cooling ages between the Taylor Mountain terrane and adjacent parts of the Lake George subterrane is best explained by Early Cretaceous unroofing of the Lake George subterrane caused by crustal extension, recorded in its younger top-to-the-SE fabric.     

Stable isotopic evidence for fluid infiltration during contact metamorphism in a multiply-metamorphosed terrane: the Reynolds Range, Arunta Block, central Australia

The Lander Rock Beds form the local basement of the Reynolds Range in the Arunta Inlier of central Australia. These dominantly quartzose and pelitic lithologies underwent low-grade ( c.   400  °C) regional metamorphism prior to contact metamorphism ( c.   2.5  kbar) around S-type megacrystic granitoids at 1820–1800  Ma. The Lander Rock Beds are overlain by metasediments of the Reynolds Range Group, which were subsequently intruded by granitoids at c. 1780  Ma. Regional metamorphism at 1590–1580  Ma produced grades varying from greenschist (400  °C at 4–5  kbar) to granulite (750–800  °C at 4–5  kbar) from north-west to south-east along the length of the Reynolds Range. Oxygen isotope ratios of the Lander Rock Beds were reset from 13.4±0.8 to as low as 6.7 adjacent to the contacts of the larger plutons, and to 10.3±1.1 around the smaller plutons. Biotite in all the major rock types found in the aureoles has δD values between −52 and −69, probably reflecting resetting by a cooling igneous+metamorphic fluid near the plutons. Sapphirine-bearing and other Mg- and Al-rich rock types have low δ¹⁸O values (4.0±0.7). The precursors to these rocks were probably low-temperature ( c. 200  °C) diagenetic–hydrothermal deposits of Mg-rich chlorite, analogous to those in Proterozoic stratiform precious metal and uranium deposits that form by the infiltration of basin brines or seawater. As in the overlying Reynolds Range Group, regional metamorphism involved little fluid–rock interaction and isotopic resetting.     

湖南新化地区煤变形变质与构造环境特征

采用煤质化验、XRD、SEM等测试手段,分析煤的化学组分、结构特征、显微构造等特征,结合古应力估算值、古温度和应变速率等结果,探讨了湖南新化地区煤变形变质特征与构造环境的关系。结果表明：煤样的d₀₀₂值为3.36~3.39 nm,受天龙山构造挤压的局部应力场及叠加岩浆构造热的作用,煤逐渐转变为隐晶质石墨,结晶颗粒是由细小的鳞片组成的集合体,粒径集中在50~250 nm;石墨3R型含量随着煤化程度升高,石墨晶体开始增加,Rh相的数量减少;构造变形环境属于低温低围压高应变速率的浅层脆性-韧脆性构造域;天龙山岩体提供了构造热源,侧向挤压应力使寒婆坳向斜煤大分子结构有序化,层面间距减小,形成隐晶质石墨。     

Domal structures and high-grade metamorphism in the Higher Himalayan Crystalline, Zanskar Region, north-west Himalaya, India

ABSTRACT In the main Himalayan range in the Ladakh-Zanskar area, domal structures have been observed at structurally deeper levels in the tectonic unit of the Higher Himalayan Crystalline. Their formation occurred during a second, temperature-dominated phase (M₂) of high-grade regional metamorphism, characterized by the semipelitic paragenesis of sillimanite-K-feldspar and incipient anatexis. The doming event reveals a local system of synmetamorphic uplift superimposed on a regional system of northeast-southwest trending compression. In the main Himalayan range the development of the dominant S₂ foliation is related to deformation during the doming phase, which started early in the M₂ event. The deformation propagated continuously north-east and south-west with time. In the north-east, on the northern slopes of the main Himalayan range, this deformation is expressed by extensional shear movements of the upper tectonic levels finally leading to the late- to postmetamorphic normal fault system of the Zanskar shear zone. Towards the south-west, deformation is expressed by compressional movements, e.g. at the Main Central Thrust (MCT) in the Kishtwar window area. The observed compression and extension is inferred to relate to an increased uplift of the domal bulges of the tectonic Kishtwar window and of the whole main Himalayan range.     

煤岩构造变形与动力变质作用

煤岩是一种对温度、压力等地质环境因素十分敏感的有机岩,地质演化过程中的各种构造-热事件必然导致煤岩发生一系列物理与化学结构的变化,并形成不同类型的构造煤。在构造应力作用下,煤岩不仅发生脆性和韧性变形,而且还产生不同程度的动力变质作用。因而,关于煤岩构造变形与动力变质作用的研究不仅具有重要的科学意义,而且在煤层气资源评价以及煤与瓦斯突出危险性预测方面也具有重要的实际意义。文中在已有研究成果基础上,通过对构造煤系列Ro,max、XRD和NMR(CP/MAS+TOSS)等测试和实验方法的对比研究,深入分析了煤岩不同构造变形和动力变质特征,进一步探讨了构造应力下煤岩动力变质作用的机理。研究成果表明,在构造应力作用下,煤岩脆性变形主要是通过破裂面上快速机械摩擦转化为热能而引起煤岩化学结构与其成分的改变;而韧性变形煤主要是通过局部区域应变能的积累而引起煤岩化学结构的破坏,从而发生不同机制的动力变质作用。     

Timing of deformation and exhumation across the Karari Shear Zone,north-western Gawler Craton,South Australia

In the north-western Gawler Craton of South Australia, the Karari Shear Zone defines a boundary between late-Archean to earliest Paleoproterozoic rocks, which have remained largely undisturbed since the earliest Paleoproterozoic, and younger Paleoproterozoic rocks that have been reworked through multiple late Paleoproterozoic and Mesoproterozoic metamorphic and deformation events. The history of movement across the Karari Shear Zone has been investigated via new U–Pb and ⁴⁰Ar/³⁹Ar geochronology, in combination with pre-existing geochronological and metamorphic constraints, as well as the structural geometry revealed by a recently acquired reflection seismic transect. The available data suggest a complex history of shear-zone movement in at least four stages, with contrasting sense of motion at different times. The first period of movement across the Karari Shear Zone is inferred to have been a period of extension at ca 1750–1720 Ma. This was likely closely followed by reactivation during the Kimban Orogeny between ca 1720 and 1680 Ma, although the sense of movement during this period is unclear. Further reactivation, in a thrust sense, occurred between ca 1580 and 1560 Ma, resulting in significant exhumation of marginal domains of the Gawler Craton to the north of the Karari Shear Zone. A final episode of largely strike-slip shear-zone movement occurred at ca 1450 Ma.     

走滑断裂对超高压变质岩石折返的贡献及青藏高原北部白垩纪隆升之新思考

青藏高原已识别出柴北缘、南阿尔金和高喜马拉雅三条超高压变质带。这些超高压变质带提供了一个不可多得的研究超高压变质岩石形成和折返的机会。柴北缘超高压变质带位于阿尔金断裂的东边,是柴达木—东昆仑地体与祁连—阿尔金微地体和阿拉善—敦煌地体碰撞的产物,由榴辉岩、石榴石橄榄岩和含柯石英片麻岩组成,榴辉岩形成时代500~440Ma,峰期超高压变质年龄440Ma。南阿尔金超高压变质带位于阿尔金断裂带的西边,以产出榴辉岩和石榴石橄榄岩为特征,榴辉岩形成时代为500Ma。南阿尔金超高压变质带被认为是柴北缘超高压变质带的西延,两者被阿尔金断裂左旋位移约400km。阿尔金断裂是巨大的深度>200km的岩石圈走滑断裂,断裂的活动时代至少早到240~220Ma,认为走滑过程中伴随的隆升作用有可能为柴北缘和南阿尔金超高压变质岩石的折返和出露地表做出了贡献,其中阿尔金断裂起到了类似剪刀型断裂的作用。高喜马拉雅超高压变质带在巴基斯坦和印度被发现,以榴辉岩中含柯石英或金刚石为特征,榴辉岩的超高压变质年龄为46Ma,表明超高压变质岩石发生在雅鲁藏布江缝合线关闭后并快速折返。喀喇昆仑断裂走滑过程中伴随的抬升作用则可能对高喜马拉雅地区超高压变质岩石的折返和出露地表做出贡献。在中国东部出露的大别—苏鲁超高压变质带被巨大郯庐断裂左旋走滑位移约500km,可以看作是走滑作用伴随的抬升运动对超高压变质岩石的最后折返和出露地表做出重要贡献的又一例证。青藏高原的隆升通常被认为是印度板块和欧亚大陆新生代以来的碰撞结果。根据高原北部断裂的时代、火山活动和沉积盆地的形成,我们提出高原的隆升是两次俯冲碰撞的结果。第一次发生在中特提斯班公湖-怒江洋盆在白垩纪时期的关闭,其时由于北部来自塔里木盆地和北中国板块及东部来自太平洋板块俯冲产生的抵柱效应,高原北部开始隆升;第二次发生在印度板块的新生代俯冲碰撞作用,造成高原的整体抬升,由此可以解释高原北部平均海拔(5000m)要高于高原南部(平均海拔4000m)。