Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)

The effects of high-strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P-T conditions or tectonic position. Transformation of meta-quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H₂O. Bulk-rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H₂O and Si and variable loss of K, Na, Ca and Al. δ¹⁸O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ¹⁸O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress. Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high-strain deformation provided pathways for both synkinematic and post-kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.     

Behavior of isotopic systematics during deformation and metamorphism: a Hf,Nd and Sr isotopic study of mylonitized granite

A combined compositional and Hf, Nd and Sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the Harquahala Mountains, western Arizona, to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks. The 1.4 Ga Harquahala Granite was deformed during Mesozoic thrusting along the Harquahala thrust. Granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite. In most of these mylonites, the protolith is not megascopically recognizable, and can be discerned only by the progressive transition to undeformed granite. Isotopic analyses of Hf, Nd and Sr from the shear zone document the immobility of the Hf and Nd isotopic systems relative to that of the Sr isotopic system during deformation. The Rb–Sr isotopic data show considerable scatter on an isochron plot, exhibiting both gains and losses of Rb and Sr from the whole-rock systems. In contrast, the Sm–Nd and Lu–Hf isotopic systematics are mostly well behaved on isochron diagrams, plotting mostly in tight clusters or along 1.4 Ga isochrons. These results show that while the Sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance, both Hf and Nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism.     

Petrological evolution of amphibolite shear zones, Cheyenne Belt, south-eastern Wyoming, USA

Abstract Metre-scale amphibolite boudins in the Cheyenne Belt of south-eastern Wyoming are cut and deformed by shear zones which preserve a full strain transition across 7 cm, from relatively undeformed amphibolite with a relict igneous texture to mylonitic amphibolite with an L-S tectonic fabric. The strain transition is marked by the progressive rotation of amphibole + plagioclase aggregates into parallelism with the shear-zone boundary. An increase in strain magnitude is indicated by development of the tectonic fabric and progressive reduction of amphibole and plagioclase grain size as a result of cataclasis. Bulk chemistry of five samples across a single strain transition shows no significant or systematic variation in major element chemistry except for a minor loss of SiO₂, which indicates that the shear zone was a system essentially closed to non-volatile components during metamorphism and deformation. Amphibolites throughout the shear zone consist of amphibole and plagioclase with only minor amounts of quartz, chlorite, epidote, titanite and ilmenite. Within the relatively undeformed amphibolite, amphibole and plagioclase have wide compositional ranges in single thin sections. Amphibole compositions vary from actinolitic hornblende to magnesio-hornblende with increases in Al, Fe, Na and K contents and decreases in Si and Mg that can be modelled as progress along tschermakite, edenite and FeMg_-1 exchange vectors from tremolite. Plagioclase ranges from An₆₀ in cores to An₃₀ within grain-boundary domains. With increasing strain magnitude, local variation of amphibole composition decreases as amphibole becomes predominantly magnesio-hornblende. Plagioclase composition range also decreases, although grain-boundary domains still have higher albite content. These petrological data indicate that shear-zone metamorphism was controlled by the magnitude of strain during synmetamorphic deformation. SEM and microprobe imaging indicate that chemical reactions occurred by a dissolution and reprecipitation process during or after cataclastic deformation. This suggests that grain-boundary formation was an important process in the petrological evolution of the shear zone, possibly by providing zones for fluid ingress to facilitate metamorphic reactions. These results highlight the necessity for conducting detailed microstructural evaluation of rocks in order to interpret petrological, isotopic and geochronological data.     

Progressive deformation across a ductile shear zone: an example from the Singhbhum Shear Zone,eastern India

Structural investigations in the Precambrian Singhbhum Shear Zone of eastern India document an intimate relationship between micro- to meso-scale structures and the deformation history. Shear zone rocks are characterized by composite foliation, a well-developed stretching lineation, folds, shear planes, and quartz veins. These structures reflect thrusting of the Proterozoic north Singhbhum hanging wall block over the Archaean south Singhbhum footwall block. Microstructural analysis of multiple foliation and mylonitic rocks within the shear zone helps to define its progressive evolution. During progressive deformation, overprinting of microstructures resulted in incomplete transposition or complete erasing of previously formed structures and mineral assemblages, allowing room for new dynamic equilibrium structures to form. The dominant deformation mechanism was dissolution–recrystallization, with locally important fluid circulation responsible for transformation of the quartzo-feldspathic mass into phyllonite, and quartzites and schists into mylonite. Textural features suggest that the bulk deformation was non-coaxial, evolving from dominant pure shear in the early stage followed by simple shear in a single progressive strain history of the Singhbhum Shear Zone.     

韧性剪切带的变形分域现象——以辽东的丹东韧性剪切带为例

丹东韧性剪切带的实例表明:韧性剪切带的持续变形包含了共轴与非共轴两种应变线路或状态,由于分布的不均匀性,导致变形分域现象,形成平面的变形岩石分区。持续变形过程中,又存在应变线路的转换和叠加。微构造是确定变形体制的主要依据。野外观测与应变分析证实,变形分域存在于不同尺度,在总体剪切(非共轴)变形条件下,初始糜棱岩往往表现以共轴变形占优势,而糜棱岩和超糜棱岩则以非共轴变形为主。持续变形导致从共轴向非共轴转变,最后形成后者的主体地位。     

Deformation mechanisms in the frontal Lesser Himalayan Duplex in Sikkim Himalaya,India

Understanding deformation mechanisms in Himalayan rocks is a challenging proposition due to the complex nature of the deformed rocks and their genesis. Crustal deformation in the Himalayan thrust belt typically occurs in elastico-frictional (EF) or quasi-plastic (QP) regimes at depths controlled mainly by regional strain-rate and geothermal gradient. However, material property, grain-size and their progressive changes during deformation are also important controlling factors. We present evidence of EF deformation from Gondwana rocks developed during the emplacement of one of the frontal horses (Jorthang horse) in the Lesser Himalayan Duplex (LHD) structure associated with Lesser Himalayan rocks in the footwall of the Ramgarh thrust in the Rangit window near Jorthang in the Sikkim Himalaya. The rocks in the horse exhibit systematic changes in microand meso-structures from an undeformed protolith to cataclasite suggesting that it was emplaced under elastico-frictional conditions. Meso- to micro-scale shear fractures are seen developed in Gondwana sandstone and slate while intercalated fine-grained shale-coal-carbonates are deformed by cataclastic flow suggesting that material property and grain-size have played an important role in the deformation of the Jorthang horse. In contrast, the hanging wall schists and quartzites of the Ramgarh thrust exhibit quasi-plastic deformation structures. This suggests that the Jorthang horse was emplaced under shallower crustal conditions than the antiformally folded Ramgarh thrust sheet even though the Ramgarh sheet presently overlies the Jorthang horse.     

Application of kinematic vorticity and gold mineralization for the wall rock alterations of shear zone at Dungash gold mining,Central Eastern Desert,Egypt

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in wall rocks alterations of shear zone at Dungash gold mine. Furthermore, it shows the relationship between the gold mineralization and deformation and also detects the orientation of rigid objects during progressive deformation. The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Neoproterozoic basement rocks. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. The kinematic vorticity number for the metavolcanic and metasedimentary samples in the Dungash area range from 0.80 to 0.92, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. Furthermore, we conclude that disseminated mineralization, chloritization, carbonatization and silicification of the wall rocks are associated with fluids migrating along shearing, fracturing and foliation of the metamorphosed wall rocks.     

Sm-Nd isotope and trace element evidence for heterogeneous igneous protoliths of Variscan mafic blueschists in the East Krkonoše Complex (West Sudetes,NE Bohemian Massif,Czech Republic)

Granitoid rocks of the southern Menderes Massif, SW Turkey include widespread possibly Ediacaran high-grade granitic orthogneisses and younger (Tertiary) sheets, sills and/or dikes of variably deformed tourmaline-bearing leucogranites. The latter are confined to the immediate footwall of the regional-scale ductile southern Menderes shear zone. Although both sets of granitoid rocks are essentially calc-alkaline and peraluminous, the syn- to post-collisional tourmaline-bearing leucogranites are chemically distinguishable from both the granitoid orthogneisses and from two sets of mostly sodic siliceous dyke rocks. The leucogranites were generated by partial melting induced by shear heating during the waning stages of the Eocene main Menderes metamorphism and associated top-to-the-NNE thrusting along the southern Menderes ductile shear zone, which transported schists northwards over the granitoid orthogneisses of the core Menderes complex. Upward migration and emplacement of leucogranitic melt weakened formerly sheared rocks, so that when thrust-related deformation ceased it facilitated rapid crustal extension along the shear zone. The emplacement of leucogranites, in turn, promoted the reactivation of the southern Menderes shear zone as a top-to-the-SSW extensional feature. Continued extensional deformation affected the leucogranites which became parallel to the shear-zone foliation; local S-C fabrics were also generated. The additional occurrence of less or almost undeformed leucogranites suggests that the latest stages of extension might have induced adiabatic decompressional melting. Hence the leucogranite melt generation and emplacement in the southern Menderes Massif occurred in pulses. Both compressional and extensional processes played key roles in melt generation, emplacement, deformation and exhumation of the massif.

A clear distinction may also be made between the composition of granite-hosted tourmalines and those from metasedimentary schists. Tourmalines from a pebble of uncertain provenance in the Gökçay metaconglomerate plotted with schist-hosted tourmalines, suggesting that it was unlikely to be derived from granitoid gneiss. This crucial piece of evidence suggests that the presence of a major (Pan-African) unconformity at the so-called “core (orthogneiss)-cover (schist)” boundary in the southern Menderes Massif is unnecessary.     

太行山南段自立庄韧性剪切带变形特征

太行山南段临城自立庄地区古元古界甘陶河群中低级变质岩中发育一条左行逆冲型韧性剪切带。自立庄韧性剪切带出露长约10 km,宽约1 km,走向NNE,往西缓倾,在EW方向上由若干条强变形带与其间的弱变形域或岩块组成,平面上呈现平行式的组合特征。该韧性剪切带内发育糜棱岩、拉伸线理和皱纹线理、不对称褶皱、石香肠构造和构造透镜体、S-C面理和旋转碎斑等宏观和微观构造。S-C面理、旋转碎斑、不对称褶皱等宏微观变形特征一致表明自立庄韧性剪切带上盘由西往东逆冲的运动学性质。在对韧性剪切带宏观、微观构造特征研究基础上,结合区域资料,认为自立庄韧性剪切带的形成与华北克拉通古元古代末期西部陆块与东部陆块的EW向碰撞拼合有关,是18.5 Ga吕梁运动的产物。自立庄韧性剪切带的厘定为太行山南段古元古代构造演化提供了基础资料。     

Metamorphic history of the South Tibetan Detachment System,Mt. Everest region,revealed by RSCM thermometry and phase equilibria modelling

This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U–Pb dating of titanite to delineate the metamorphic history of a well‐exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of ～340 °C. Temperatures increase down‐section, reaching ～650 °C at the base of the shear zone, defining an apparent metamorphic field gradient of ～310 °C km^?1 across the entire structure. U–Th–Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1‐km‐thick zone of distributed top‐down‐to‐the‐northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo‐isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up‐section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.     

哀牢山-红河剪切带左行走滑作用起始时间约束

位于哀牢山-红河剪切带NW延伸方向上的点苍山变质杂岩体遭受强烈的左行走滑剪切变形、变质作用改造,岩石中保存了典型的高温矿物组合以及由它们构成的宏观和微观高温变形构造特征,其中糜棱岩中具有极其发育的长石矿物拉伸线理而形成典型的L与LS型构造岩是其一个明显的特征。本文对点苍山地区高温糜棱岩主要矿物开展了显微构造与矿物变形、变形机制及组构分析,并对于遭受高温糜棱岩化改造的一个花岗质岩体开展了SHRIMP锆石U-Pb定年分析。结果表明岩石中长石、角闪石、石英等主要矿物具有典型的达角闪岩相条件下的高温晶质塑性变形和动态生长特征,它们也为走滑剪切变形活动提供了充分的微观构造证据。对于点苍山高温糜棱岩化改造的眼球状或似斑状二长花岗岩的显微构造分析结果表明,这套花岗质岩石从走滑剪切前期岩浆的侵位之后经历了早期强烈的岩浆期后交代作用—亚岩浆流动—高温固态塑性剪切变形的递进演化过程。由此可见,岩浆的上升与就位受左行走滑剪切作用的制约,岩体又遭受了强烈剪切变形改造。同时对这套构造前期就位花岗质岩石中的锆石进行定年分析,获得33.88±0.32Ma的岩浆结晶年龄,为此,我们有充分的理由认为,在点苍山地区哀牢山-红河剪切带左行走滑剪切作用的起始时间至少应该为早渐新世30.88±0.32Ma。     

Role of chemical processes on shear zone formation: an example from the Grimsel metagranodiorite (Aar massif,Central Alps)

Alpine deformation in the Grimsel granodiorite (Aar massif, Central Alps) at greenschist facies conditions (6.5 ± 1 kbar for 450°C ± 25°C) is characterized by the development of a network of centimetre to decametre localized shear zones that surround lenses of undeformed granodiorite. Localization of deformation is assumed to be the result of a first stage of extreme localization on brittle precursors (nucleation stage) followed by a transition to ductile deformation and lateral propagation into the weakly deformed granodiorite (widening stage). A paradox of this model is that the development of the ductile shear zone is accompanied by the crystallization of large amounts of phyllosilicates (white mica and chlorite) that maintains a weak rheology in the localized shear zone relative to the host rock so that deformation is localized and prevents shear zone widening. We suggest that chemical processes, and more particularly, the metamorphic reactions and metasomatism occurring during re‐equilibration of the metastable magmatic assemblage induced shear zone widening at these P–T–X conditions. These processes (reactions and mass transfer) were driven by the chemical potential gradients that developed between the thermodynamically metastable magmatic assemblage at the edge of the shear zone and the stable white mica and chlorite rich ultramylonite formed during the first stage of shear zone due to localized fluid infiltration metasomatism. P–T and chemical potential projections and sections show that the process of equilibration of the wall rocks (μ–μ path) occurs via the reactions: kf + cz + ab + bio + MgO + H₂O = mu + q + CaO + Na₂O and cz + ab + bio + MgO + H₂O = chl + mu + q + CaO + Na₂O. Computed phase diagram and mass balance calculations predict that these reactions induce relative losses of CaO and Na₂O of ～100% and ～40% respectively, coupled with hydration and a gain of ～140% for MgO. Intermediate rocks within the strain gradient (ultramylonite, mylonite and orthogneiss) reflect various degrees of re‐equilibration and metasomatism. The softening reaction involved may have reduced the strength at the edge of the shear zone and therefore promoted shear zone widening. Chemical potential phase diagram sections also indicate that the re‐equilibration process has a strong influence on equilibrium mineral compositions. For instance, the decrease in Si‐content of phengite from 3.29 to 3.14 p.f.u, when white mica is in equilibrium with the chlorite‐bearing assemblage, may be misinterpreted as the result of decompression during shear zone development while it is due only to syn‐deformation metasomatism at the peak metamorphic condition. The results of this study suggest that it is critical to consider chemical processes in the formation of shear zones particularly when deformation affects metastable assemblages and mass transfer are involved.     

Late Cenozoic transpressional ductile deformation north of the Nazca–South America–Antarctica triple junction

The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics of the southern Andes.Two east–west structural transects were mapped at Puyuhuapi and Aysén, immediately north of the Nazca–South America–Antarctica triple junction. At Puyuhuapi (44°S), north–south striking, high-angle contractional and strike-slip ductile shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysén (45–46°), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it.New ⁴⁰Ar–³⁹Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4±0.3 Ma, laser on muscovite–biotite aggregates, Aysén transect, 45°S) and on mylonitic plutonic rocks with contractional deformation (3.8±0.2 to 4.2±0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45°S) and at the Queulat shear zone at 44°S. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation.The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.     

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone,Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.     

PETROLOGY AND AGE OF THE KINNAR KAILAS GRANITE:EVIDENCES FOR AN ORDOVICIAN POST-OROGENIC EXTENSION IN THE HIGHER HIMALAYAN CRYSTALLINE, SUTLEJ, INDIA.

PETROLOGY AND AGE OF THE KINNAR KAILAS GRANITE:EVIDENCES FOR AN ORDOVICIAN POST-OROGENIC EXTENSION IN THE HIGHER HIMALAYAN CRYSTALLINE, SUTLEJ, INDIA     

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.     

滇西无量山地区的构造变形和变质作用

无量山构造带位于滇西兰坪-思茅盆地中段西侧北北西-南南东走向的复背斜。在复背斜中部发育北北西-南南东向的左旋走滑韧性剪切带。中部剪切带变质作用达绿帘角闪岩相的蓝晶石带高于绿片岩相黑云母带的围岩。用共生的黑云母-石榴石温压计计算得到中部剪切变质带西缘变质温度为600 ℃～650 ℃,压力为5.6 kba,东缘温度为550 ℃～600 ℃,压力为5 kba左右。结合同构造期石榴石变斑晶的环带成分和多硅白云母b₀值的分析,首次在滇西提出该剪切变质带是进变质的,以不均一的、非连续的变质作用为主要特征,与区域变质作用不同。无量山中部韧性剪切变质带的进变质作用与剪切带中岩石的变形强度有关。岩石变形愈强,变形能就愈大,随之转化成的热量就愈大,该热能参与到岩石变质作用中,提高了岩石的变质程度。这一思路有可能成为研究变形与变质作用的一条有效途径。     

江西大背坞地区同构造变斑晶研究

大背坞地区浅变质碎屑岩内的韧性剪切带中发育有同构造成因的菱铁矿变斑晶和黄铁矿变斑晶,它们是进变质剪切变形的产物和标志。作为同构造成因,这类变斑晶的发育与剪切变形及强度、变形变质矿物绢云母的含量和晶体大小关系密切,具有独特的晶形并保留基质片理等特征。根据变斑晶的旋转效应可以恢复大背坞韧性剪切带的成生演化历史:右行逆冲剪切→压扁变形→左行正断剪切.      

Sequence of Ductile Shear Zones in UHP Metamorphic Province within Dabie Massif, China

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Local disequilibrium of plagioclase in high-temperature shear zones of the Ivrea Zone, Italy

Abstract Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid-assisted grain-boundary migration recrystallization, as well as by solid-state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history.
The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation.