Deformation of granitic rocks across the brittle-ductile transition

A microstructural analysis has been carried out on mylonites and mylonitic gneisses of the Eastern Peninsular Ranges Mylonite Zone, which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies. Composite planar fabrics in the form of C and S planes are found at all metamorphic grades. Fractured feldspars, kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites. At mid-upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery. Biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns. At epidote-amphibolite grade and above, recovery processes and annealing recrystallization predominate in all minerals. Residual orthoclase porphyroclasts show strain-related myrmekite formation along those sides of the grains that face the instantaneous shortening direction. Myrmekite formation due to replacement reactions cannot explain this geometry. It is proposed that the myrmekites formed due to a combination of exsolution, replacement and strain-enhanced diffusion.     

Textural variation in the pyrite-rich ore deposits of the Røros district, Trondheim Region, Norway: implications for pyrite deformation mechanisms

The Røros district is a pyrite-rich polymetallic sulfide orefield in the southeastern part of the Trondheim region, Central Norwegian Caledonides. All of the ore deposits at Røros are hosted within a Cambrian to Silurian succession that was deformed and metamorphosed at lower greenschist to lower amphibolite facies conditions during the Caledonian orogeny. Samples from five individual deposits across the orefield have been analyzed using a combination of reflected light petrographic observation, orientation contrast imaging, and electron backscatter diffraction. Results indicate that, whereas samples from each ore deposit have a variety of different textures, all of them preserve plastic deformation in pyrite grains that occurred at peak metamorphic conditions characterized by the development of internal lattice misorientation within pyrite grains and low-angle (～2°) dislocation walls. These observations indicate that the principal deformation mechanisms at peak metamorphic conditions were dislocation glide and creep. The preservation of brittle fracturing represents later overprinting events.     

吉林南部地区老岭群变质作用特征研究

吉林南部地区老岭群变质矿物较为发育,本文通过对其中发育的变质矿物进行详细鉴定和大量探针分析,将老岭群下亚群主要变质矿物划分为两个世代,分别代表两期变质作用M1和M2.结合研究区老岭群变质矿物组合、分布特征以及变质相带的研究,认为M1为中-低温区域动力热流变质作用,可以划分为低绿片岩相和高绿片岩相,而M2为局部热变质作用...     

甘肃阳山金矿区安昌河—观音坝断裂带显微构造特征及地质意义

甘肃阳山金矿是我国最大的金矿床,位于西秦岭造山带的陕甘川"金三角"地区。金矿成矿时代为早侏罗世,与燕山期斜长花岗斑岩有密切的成因联系。基于野外地质调查,本文对安昌河—观音坝断裂带构造岩进行了细致的显微构造研究,以期通过微观构造特征认识宏观断裂构造的活动规律。镜下观察表明断裂带内兼具大量的脆性与塑性显微变形,主要发育左行剪切,暗示该断裂为左行韧-脆性剪切带。断裂带内构造岩经历了高绿片岩相、低绿片岩相及低于绿片岩相的变质-变形过程,且断裂带内至少存在过三到四期构造变形,为断裂带曾发育"多期构造变形"提供依据。显微构造应力分析及岩层产状等密度图显示区域主压应力方向为NNW-NNE,是对印支期以来多期主应力方位的综合反映。据亚颗粒法及动态重结晶法计算的成矿前古应力差值为128.6～95.8 MPa,成矿期古应力差值为74.9～69.3 MPa,成矿后古应力差值为65.8 MPa。综合分析认为中—晚三叠世以来安昌河—观音坝断裂带变质相相变为高绿片岩相→低绿片岩相→低于绿片岩相,变形序列为韧性→韧-脆性→脆性,区域主应力大小发生了大→小的转变,主应力方位经历了SN向挤压→NE向挤压→NW及SN向挤压的转换。安昌河—音坝断裂带构造演化特征反映其经历了从深部到浅部逐渐抬升的过程。     

酸性岩的变质相

在沸石相变质条件下,花岗岩里浊沸石交代了斜长石和石英,在酸性火山岩里产生明矾石、埃洛石或高岭石。经受绿纤石-葡萄石相变质的花岗岩,其中黑云母变为钙铝榴石、帘石、绿纤石和葡萄石集合体,同时斜长石发生绢云母化。绿片岩相内酸性岩的浅色矿物有石英、微斜长石、钠长石和绿帘石,暗色矿物有绿泥石和黑云母。在角闪岩相变质的酸性岩中,开始出现中、基性斜长石,其中暗色矿物黑云母的镁铁比值要大于角闪石的镁铁比值。经受麻粒岩相变质后,紫苏花岗岩的矿物组成没有变化,但有铀、钍和钾的迁出。     

Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt,Syros (Greece)

Is metamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt (CBB). The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the CBB on Syros. The majority are from samples that have well‐preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preserves an age of 35 Ma while post‐tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro‐sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectonometamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Belt phengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under greenschist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post‐tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.     

Accretion and post-accretion metamorphism in subduction complex terranes of the New England Fold Belt, eastern Australia

Abstract Regional metamorphic rocks that form Late Palaeozoic subduction complexes in central Queensland, Australia, are products of two metamorphic episodes. Synaccretion metamorphism (M1) gave rise to prehnite-pumpellyite and greenschist facies rocks, whereas a subsequent episode (M2) at about 250 Ma formed upper greenschist to upper amphibolite facies rocks of both intermediate- and low-pressure type, probably in a compressive arc or back-arc setting. A similar pattern can be recognized for 1000 km along the New England Fold Belt, although at several localities, where higher grade rocks are exposed, metamorphism was essentially continuous over the M1-M2 interval, with a rapid rise in geothermal gradient at the end of accretion. Where out-stepping of tectonic elements has occurred at long-lived convergent margins elsewhere, similar overprinting of high- by lower-pressure facies series is anticipated, complicating the tectonic interpretation of metamorphism. The discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out-stepping are followed by the subduction of a major heat source.     

晋北早前寒武纪变质岩区构造相研究

晋北地区出露有不同层次的地壳,是研究构造相最理想的地区之一。本文以构造分析为主线,结合当前地壳流变学研究进展,对该区早前寒武纪构造特征进行了分析,初步建立了早前寒武纪地质事件序列。提出了该区北部（大同-集宁）麻粒岩相变质岩代表下地壳的物质组成,中部（恒山）中深变质岩代表典型的中下地壳过渡带的特征（早前寒武纪一个重要的软...     

Pyrite deformation in stratiform lead-zinc deposits of the Canadian Cordillera

Pyrite textures in five stratiform lead-zinc deposits from lower to upper greenschist facies environment of the Canadian Cordillera are described and discussed in terms of deposition/early diagenesis, deformation, metamorphism and hydrothermal alteration processes. Overgrowth is an important process during both diagenesis and deformation. Diagenetic and deformational overgrowths can be distinguished. Diffusive mass transfer, involving pressure solution and oriented overgrowth of pyrite is the main deformation mechanism in pyrite deposits at low metamorphic grades. Although diffusive mass transfer favours fine-grained mineral aggregates, its effect on coarse pyrite grains has also been identified. Ore minerals dissolved by pressure solution may be transported, with the assistance of pore fluids within fractures and grain boundaries, over distances significantly greater than the scale of individual grains to give a range of pressure solution/overgrowth textures. The textural modification of pyritic ores from the early stages of diagnesis, through metamorphism and deformation, to post deformation thermal annealing, has important implications for the distribution of trace elements and isotopic compositions in pyritic ores.     

A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China

The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. This indicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result of Caledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpeilyite to lower greenschist facies for the lower part of the Middle Ordovician vol     

Raman characterization of carbonaceous material in the Macraes orogenic gold deposit and metasedimentary host rocks,New Zealand

Raman spectroscopic and petrographic analyses were performed on samples collected from zones distal and proximal to the Macraes gold deposit in the Otago Schist of New Zealand to characterize the features and possible origins of Carbonaceous Material (CM) and to assess the potential role of CM in the formation of gold deposits. CM is a common component in meta-sedimentary orogenic gold deposits, and it has been proposed that CM contributes to gold mineralization processes, but the details of the mechanisms responsible are not fully understood. Documentation of the origins of the Otago schist CM will improve our understanding of the role of CM in gold deposits.This work has identified four types of CM of varying thermal maturity and origins from prehnite–pumpellyite grade to lower greenschist grade samples. In prehnite–pumpellyite and pumpellyite–actinolite grade rocks, low-maturity CM 1 coexists with framboidal pyrite, indicating an in-situ, sedimentary origin, with a potential association with the source of gold. Low crystallinity CM 2 is also found in low grade samples and is likely to have been deposited from fluids unrelated to gold mobilization. CM 3 is the highest maturity CM recognized. CM 3 is found in samples from the highest metamorphic grades studied (lower greenschist facies), where bands of CM 3 cross cut the foliation, CM 3 is therefore thought to have been transported by fluids, though possibly only at short length scales. CM 4 is less mature than CM 3 and is found in mineralized rocks in association with sulfide minerals and gold. CM 4 is likely to have a depositional origin but its precise role with respect to gold mineralization has not been identified.     

Pumpellyite in Low-grade Metamorphism

The regional metamorphism of the Kant? Mountains in Japan producedrocks of the following facies, with rising temperature: chloritefacies, pumpellyite-chlorite facies, glauco-phane-schist facies,and greenschist facies. This relationship is compared with theprogressive metamorphic zones in other regions where pumpellyitehas been found. Almost similar relations appear to hold in manymetamorphic terrains. Pumpellyites in glaucophanitic metamorphicterrains have, generally, low Fe'/R'R ratios. Physical andchemical conditions responsible for the formation of pumpellyiteare also discussed.     

Raman spectrum of carbonaceous material: a possible metamorphic grade indicator for low-grade metamorphic rocks

Raman spectral analyses of carbonaceous material (CM) extracted from pelitic samples along two sections traversing the metamorphic belt of Taiwan were carried out in the present study. The results show similar spectral variations of CM with metamorphic grade as those documented in the literature. However, continuous sampling from zeolite facies through prehnite–pumpellyite facies to greenschist facies metamorphic rocks in the present study does reveal some interesting features on the Raman spectra of CM that were not noted before. Both the Raman D (disordered-)/O (ordered-) peak area (i.e. integrated intensity) ratio and the D/O peak width (i.e. full width at half maximum, FWHM) ratio of the CM decrease with progressive metamorphism, but the most prominent change in the D/O peak area ratio occurs in samples of lower greenschist facies metamorphic grade, while the most significant decrease in the D/O peak width ratio occurs in samples near the boundary of prehnite–pumpellyite facies and greenschist facies. This phenomenon is interpreted as a result of the decoupling of the changing rates of in-plane crystallite size and degree of defects of CM with progressive metamorphism. It is postulated that the Raman spectrum of CM can serve as a metamorphic grade indicator to distinguish samples of prehnite–pumpellyite facies metamorphic grade from those of greenschist facies metamorphic grade.     

Gold mineralization without quartz veins in a ductile-brittle shear zone, Macraes Mine, Otago Schist, New Zealand

Greenschist facies schist which hosts the Macraes Mine in East Otago, New Zealand has been pervasively altered by post-metamorphic (lower greenschist facies) fluids over a 120 m thick section perpendicular to foliation. Metamorphic titanite has been replaced by rutile, and epidote has been replaced by a variety of metamorphic minerals including siderite, chlorite, muscovite and calcite. The early stages of this alteration occurred during development of a ductile cleavage associated with kilometre scale recumbent folding. The cleavage was widely overprinted by a subparallel set of spaced (mm scale) microshears which are locally enriched in rutile and hydrothermal graphite. Strain was then concentrated into narrow (m scale) zones where more intensely deformed portions of the rock are crossed and highly disrupted by closely spaced (100 μm scale) microshears. The highly strained rocks show a combination of mylonitic and cataclastic microstructures, including crystal-plastic grain size reduction and recrystallization of micas to form a new foliation. Muscovite has grown at the expense of albite in the mylonitic cataclasites. Hydrothermal alteration was accompanied by addition of pyrite, arsenopyrite and gold without development of quartz veins. Gold precipitated with sulphides during reduction of the fluid by hydrothermal graphite. The whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold, scheelite, rutile, pyrite and arsenopyrite. This deposit displays a continuum of post-metamorphic processes and hydrothermal fluid flow which occurred during uplift of the schist belt. Received: 4 December 1997 / Accepted: 21 September 1998     

Windows of metamorphic sulfur liberation in the crust: Implications for gold deposit genesis

Understanding the source of metamorphic sulfur is critical to clarifying the complete cycle of ore genesis, from source to sink, for several mineral deposit types. In this study, a mass balance approach and the thermodynamic computer programs Thermocalc and PerpleX were used to constrain the P-T range of pyrite breakdown to pyrrhotite (which liberates sulfur) in common metamorphic lithologies. The results suggest that most of the continental crust’s metamorphic sulfur is liberated in a relatively narrow temperature-pressure window corresponding to the terminal breakdown of chlorite at moderate to low pressures. This is because pyrite stability is controlled partly by temperature and pressure, and partly by the amount of H₂O present. During prograde metamorphism from the greenschist to the amphibolite facies, metamorphic H₂O is produced primarily through chlorite breakdown in mafic to pelitic bulk compositions. As temperature increases, more sulfur is required from pyrite to maintain equilibrium proportions of H₂O, H₂S and SO₂ in the fluid, and in addition, progressively more sulfur is required at lower pressures. At low temperatures, little sulfur is required by metamorphic fluid released during initial chlorite breakdown, whereas at higher temperatures coinciding with the terminal breakdown of chlorite, not only is more fluid present, but the fluid’s sulfur requirement has also increased dramatically. In this way, metamorphic dehydration drives pyrite breakdown and generation of sulfur-rich hydrothermal fluids at mesothermal conditions. Beyond the chlorite stability field there is minimal metamorphic fluid production, except at low pressures and high temperatures where muscovite can break down without causing melting; conditions that are a long way from typical crustal geotherms. However, deformation also plays a key role in pyrite breakdown. Without deformation, small amounts of fluid in chemical communication with individual pyrite grains will quickly acquire equilibrium concentrations of the sulfur species and minimal pyrite breakdown is necessary. Whereas during deformation, there may be a continuous fluid flux past pyrite grains, promoting ongoing sulfur liberation. In this way, periods of deformation may be the major sulfur-liberating episodes during a metamorphic cycle. Since hydrothermal fluids are inherently buoyant and consequently tend to migrate upwards and towards cooler temperatures through the crust, these results imply that orogenic gold deposits are most likely to form at lower-amphibolite to prehnite-pumpellyite facies conditions, and unlikely to form at higher temperatures. The pressure constraint on metamorphic sulfur liberation implies that tectonic settings that allow prograde metamorphism to follow low pressure P-T-t paths in an occasionally compressional or transpressional environment are necessary. Settings that promote extensive injection of felsic magma into a mid-crust that contains a significant proportion of pyritic carbonaceous metasediment are shown to be ideal for orogenic gold deposit genesis. Inverted back-arc basins are interpreted to be the most favourable of these.     

The Sabzevar blueschists of the North-Central Iranian micro-continent as remnants of the Neotethys-related oceanic crust subduction

The Sabzevar ophiolites mark the Neotethys suture in east-north-central Iran. The Sabzevar metamorphic rocks, as part of the Cretaceous Sabzevar ophiolitic complex, consist of blueschist, amphibolite and greenschist. The Sabzevar blueschists contain sodic amphibole, epidote, phengite, calcite ± omphacite ± quartz. The epidote amphibolite is composed of sodic-calcic amphibole, epidote, albite, phengite, quartz ± omphacite, ilmenite and titanite. The greenschist contains chlorite, plagioclase and pyrite, as main minerals. Thermobarometry of a blueschist yields a pressure of 13–15.5 kbar at temperatures of 420–500 °C. Peak metamorphic temperature/depth ratios were low (~12 °C/km), consistent with metamorphism in a subduction zone. The presence of epidote in the blueschist shows that the rocks were metamorphosed entirely within the epidote stability field. Amphibole schist samples experienced pressures of 5–7 kbar and temperatures between 450 and 550 °C. The presence of chlorite, actinolite, biotite and titanite indicate greenschist facies metamorphism. Chlorite, albite and biotite replacing garnet or glaucophane suggests temperatures of >300 °C for greenschist facies. The formation of high-pressure metamorphic rocks is related to north-east-dipping subduction of the Neotethys oceanic crust and subsequent closure during lower Eocene between the Central Iranian Micro-continent and Eurasia (North Iran).     

Horizontal contraction or horizontal extension? Heterogeneous Late Eocene and early Oligocene general shearing during blueschist and greenschist facies metamorphism at the Pennine-Austroalpine boundary zone in the Western Alps

Mylonitic structures related to two orogenic events are described from the upper and lower contacts of the Combin zone and the immediately overlying upper Austroalpine Dent Blanche nappe/Mont Mary klippe and the directly underlying lower Austroalpine Etirol-Levaz slice. The first event, Late Eocene in age, commenced during blueschist facies P-T conditions, but pre-dated the peak of subsequent greenschist facies overprint. The second event, Early Oligocene in age, took place during retrograde greenschist facies conditions. Most sense of shear indicators associated with the retrograde mylonites indicate top SE shearing, but subordinate top NW displacing shear sense indicators have also been mapped. Mylonitic top SE shearing appears to be restricted to the Combin zone and its upper and lower contacts. Within the Dent Blanche nappe and Mont Mary klippe and at the base of the Etirol-Levaz slice, structures were observed which developed during blueschist/greenschist facies conditions and are, in conjunction with the P-T-t history of these rocks, inferred to be older. Associated kinematic data indicate a top NW shear sense. Comparable blueschist/greenschist facies shear sense indicators have not been observed in the Combin zone. Nonetheless, the foliation in the Combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions. This indicates that the Combin zone and the immediately over- and underlying Austroalpine units shared a common tectono-metamorphic evolution since the Late Eocene. Finite strain data reveal oblate strain fabrics, which are thought to result from a true flattening strain geometry. Flow path modelling reveals a general non-coaxial deformation régime and corroborates significant departures from a simple shear deformation. In the study area, mylonitic top SE shearing in the Combin zone is attributed to Early Oligocene backfolding and backthrusting of the Mischabel phase. Temperature-time curves suggest slight reheating in the Monte Rosa nappe underneath and cooling in the Dent Blanche nappe above the Combin zone, hence confirming a thrust interpretation for this event. The top NW displacing structures are thought to result from Late Eocene emplacement of the Dent Blanche nappe and the Combin zone onto the Middle Pennine Barrhorn series along the Combin fault. As related structures initiated during mildly blueschist facies conditions in the Dent Blanche nappe and the underlying Combin zone and both were emplaced together onto the greenschist facial Barrhorn series, it is concluded that the structures developed as the nappes moved upward relative to the earth's surface. Thus the Combin fault is regarded as a thrust. The geometry of this structure indicates that the Combin fault is an out of sequence thrust that locally cut down section. Hence, top NW out of sequence thrusting caused local thinning of the metamorphic/structural section in association with horizontal shortening. Out of sequence thrusts cutting down section, and back-thrusts, offer the possibility of explaining the pronounced break in the grade of metamorphism across the Combin fault, i.e. the contact between the eclogite facial Zermatt-Saas zone and the overlying lower grade Combin zone, by contractional deformation.     

The Archaean gold mineralization in Rämepuro, llomantsi greenstone belt, Eastern Finland

The Rämepuro gold mineralization is situated in the Hattu sub-belt of the Archaean Ilomantsi greenstone belt, Eastern Finland. It consists of gold-bearing quartz veins located mainly within the shear zone contact between felsic volcanics and sedimentary rocks. The quartz veins cutting all the types of Archaean rocks are composed mainly of quartz and tourmaline and minor amounts of pyrite, pyrrothite, chalcopyrite, sphalerite, siderite, hedleyite, native bismuth and gold. The highest amounts of gold are found in the quartz-tourmaline veins, with abundant chalcopyrite and sphalerite. Gold has its best correlation with Bi, Te, S, Zn and Cu. The minor mineralogical alterations (sericirization, saussuritizarion, chloritization) are connected with variations in some major elements. The quartz veins were emplaced under greenschist facies conditions after the peak in the lower amphibolite facies metamorphism.     

P–T and structural evolution during exhumation of high-T, medium-P basement rocks in the Barberton Mountain Land, South Africa

The P–T evolution of amphibolite facies gneisses and associated supracrustal rocks exposed along the northern margin of the Paleo to MesoArchean Barberton greenstone belt, South Africa, has been reconstructed via detailed structural analysis combined with calculated K(Mn)FMASH pseudosections of aluminous felsic schists. The granitoid‐greenstone contact is characterized by a contact‐parallel high‐strain zone that separates the generally low‐grade, greenschist facies greenstone belt from mid‐crustal basement gneisses. The supracrustal rocks in the hangingwall of this contact are metamorphosed to upper greenschist facies conditions. Supracrustal rocks and granitoid gneisses in the footwall of this contact are metamorphosed to sillimanite grade conditions (600–700 °C and 5 ± 1 kbar), corresponding to elevated geothermal gradients of ～30–40 °C km^?1. The most likely setting for these conditions was a mid‐ or lower crust that was invaded and advectively heated by syntectonic granitoids at c. 3230 Ma. Combined structural and petrological data indicate the burial of the rocks to mid‐crustal levels, followed by crustal exhumation related to the late‐ to post‐collisional extension of the granitoid‐greenstone terrane during one progressive deformation event. Exhumation and decompression commenced under amphibolite facies conditions, as indicated by the synkinematic growth of peak metamorphic minerals during extensional shearing. Derived P–T paths indicate near‐isothermal decompression to conditions of ～500–650 °C and 1–3 kbar, followed by near‐isobaric cooling to temperatures below ～500 °C. In metabasic rock types, this retrograde P–T evolution resulted in the formation of coronitic Ep‐Qtz and Act‐Qtz symplectites that are interpreted to have replaced peak metamorphic plagioclase and clinopyroxene. The last stages of exhumation are characterized by solid‐state doming of the footwall gneisses and strain localization in contact‐parallel greenschist‐facies mylonites that overprint the decompressed basement rocks.     

越城岭花岗岩体西侧滑脱型韧性剪切带的发现及其形成的构造体制

越城岭花岗岩体西侧片麻状花岗岩带主体是一条由韧性剪切作用形成的复合糜棱岩带,包括早期高绿片岩相条件下形成的糜棱片麻岩和晚期低绿片岩相条件下形成的糜棱岩。高绿片岩相韧性剪切带和低绿片岩相韧性剪切带具有基本一致的运动学性质,为滑脱型韧性剪切带,总体为在向NWW缓倾的糜棱面理上向SW方向滑动,并在平面上表现为左旋效应。根据构造对比研究,认为越城岭西侧滑脱型韧性剪切带形成于后造山阶段的伸展构造体制。