首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 285 毫秒
1.
The Vincent thrust of the San Gabriel Mountains, southern California, separates eugeoclinal Pelona Schist from overlying Precambrian to Mesozoic igneous and metamorphic rocks of North American continental affinity. The thrust is generally considered to be synmetamorphic because of similarity in structural orientations and mineral assemblages between the Pelona Schist and mylonites at the base of the upper plate. In this study, compositions of calcic amphibole and plagioclase in the upper plate and structurally high Pelona Schist were compared to further test this interpretation. Amphibole in the schist is mostly actinolite to actinolitic hornblende with high Na/Al ratio, indicating relatively high-P/low-T metamorphism. Individual grains are zoned, with concentrations of both Na and Al decreasing from cores to rims. Premylonitic amphibole in the upper plate is hornblende, tschermakite and pargasite with compositions indicative of low- or medium-P metamorphism. During mylonitization, this amphibole was replaced by actinolite to actinolitic hornblende with a similar range of Na and Al as amphibole rims in the Pelona Schist, but with slightly lower Na/Al ratio. This is consistent with the decrease of Na/Al up-section previously noted within the Pelona Schist of this area, and is considered to be the result of an inverted thermal gradient during thrusting. Convergence of composition between schist and upper plate also occurs for K and Ti contents of amphibole and An content of plagioclase. These features provide strong evidence that mylonitization of the upper plate is closely related in space and time to metamorphism of the Pelona Schist and therefore that the Vincent thrust is a remnant of the primary fault along which the Pelona Schist and correlative units were subducted beneath North America. Nonetheless, very fine-scale differences in amphibole composition between the schist and upper plate may indicate that metamorphic re-equilibration could not quite keep pace with movement on the fault.  相似文献   

2.
The formation of paragonite at the transition from the low-grade to the medium-grade matamorphism and its breakdown in the presence of quartz in the upper medium grade in common metapelites is investigated.The microprobe work on the white micas from the low and medium-grade rocks yields compositional differences in respect to the celadonite substitutions and the paragonite content. The low-grade white micas are phengites having Si[4] 6.25 to 6.44 and Altot 4.89 to 5.20. The paragonite component in solid solution in the phengites ranges from 11 to 17 mole %. In the transition from the low-grade to the medium-grade metamorphism, concomitant with the breakdown of chlorite, the phengites change to muscovites having Si[4] 6.07 to 6.16 and Altot 5.36 to 5.56. At the same time, the amount of paragonite in solid solution increases up to 22±2 mole % and paragonite makes its first appearance as a separate mineral. The increase of the percentage of paragonite in solid solution in the muscovites is due to the drastical modal decrease of muscovite in the course of the breakdown of chlorite. The formation of paragonite is readily explained by the muscovite-paragonite solvus. Paragonite forms thin lamellae (1–20 m) interlayered with muscovite lamellae (1–40 m). The average composition is Pg88.5Ms7Mar4.5. Paragonite occurs together with staurolite+biotite, kyanite+biotite, cordierite +biotite, and andalusite+biotite. In the presence of quartz, it breaks down in the lower part of the andalusite zone to andalusite and albite-rich plagioclase. At the same time, the amount of paragonite in solid solution in the muscovites decrease to 11–15 mole %. The basal spacings d(002) of the phengites and muscovites investigated show a clear dependence on the Na+ content and the celadonite substitutions.  相似文献   

3.
The Proterozoic basins of India adjoining the Eastern Ghats Granulite Belt (EGGB) in eastern and southern India contain both Mesproterozoic and Neoproterozoic successions. The intracratonic set-up and contractional deformation fo the Neoproterozoc successions in the Paland sub-basin in the northeastern part of Cuddapah basin and similar crustal shortening in contemporaneous successions lying west of the EGGB and Nellore Schist Belt (NSB) are considered in relation to the proposed geodynamic evolution of the the Rodinia and Gondwana supercontinents. Tectonic shortening in the Palnad sub-basin (northeast Cuddapah), partitioned into top-to-westnorthwest thrust shear, flexural folds and cleavage development under overall E-W contraction, suggests foreland style continental shortening within an intracratonic set-up. A thrust sheet containing the Nallamalai rocks and overlying the Kurnool rocks in the northeastern part of Palnad sub-basin exhibits early tight to isoclinal folds and slaty (phylllitic) cleavage, which can be correlated with early Mesoproterozoic deformation structures in the nothern Nallamalai Fold Belt (NFB). NNE-SSW trending folds and cleavage affect the Kurnool Group and overprint earlier structures in the thrust sheet. Thrusting of the Nallamalai rocks and the later structures may have been related to convergence of the Eastern Ghats terrane and the East-Dharwar-Bastar craton during Early Neoproterozoic (Greenvillian) and/or later rejuvenation related to Pan-African amalgamation of East and West Gondwana.  相似文献   

4.
大别-苏鲁造山带不同岩片(块)经历了不同的褶皱变形.榴辉岩块(或透镜体)和硬玉石英岩片经历了高压-超高压背景下的两幕褶皱变形之后,在区域性第一幕变形期间主要发生透镜化为主,后期与围岩共同经历紧闭同斜第二幕褶皱.而其它岩片主要经历了现今野外可见的区域性三幕褶皱,其中区域性第一幕褶皱为片内残留褶皱,在斜长角闪岩透镜体中多见,宏观规律不明.区域性第二幕褶皱在露头尺度多见,轴面为折劈理,局部强烈置换成片理化带(复合片理或第二期片理),恢复第三幕褶皱改造作用后,揭示出各种岩片中的各级尺度的第二幕褶皱都为轴面北西倾南东倒、轴迹走向为NNE向的紧闭不对称褶皱,不对称性一致反映其指向与各种岩片向南东的逆冲运动有关.第三幕褶皱为以片理或折劈理为变形面的宽缓褶皱,轴迹走向NWW,枢纽向西倾伏.韧性剪切带为非透入性构造,分早晚两期,早期为韧性逆冲,新县穹隆以南,运动学标志指示向北逆冲,错切第二幕褶皱,结合新县穹隆北部向南的逆冲特征,反映这些韧性逆冲断层多数为第二幕大型褶皱翼部的次级逆冲断层;晚期为韧性滑脱带,其发育局限于几个岩性差异较大的接触带,带内伸展型折劈理发育,并对挤压构造样式有重要的改造作用.华北克拉通东部地块是华北克拉通的重要组成,其盖层古生界和三叠系在印支运动期间经历了一幕宽缓褶皱作用,其轴迹方向主体也为NWW向.这一褶皱构造明显在变形时间、变形样式和展布方向上都和大别-苏鲁造山带中的第三幕褶皱非常一致,说明它们具有动力学上的必然联系.同时,研究表明在华北克拉通东部地块中没有经历大别-苏鲁造山带中区域性第一、第二幕褶皱变形的记录,故本文认为印支期这两幕变形主要发生在华北板块东南缘的边界上,并没有波及到板内,而且从东向西高压-超高压岩石剥露具有穿时性.只有当华北板块和华南板块在第二幕变形之后构成了统一块体后,第三幕变形才波及华北板内.  相似文献   

5.
The Phyllite-Quartzite (PQ) Nappe constitutes an external, allochthonous complex of the Hellenides on the island of Crete and shows a polyphase structural history. A first phase of deformation (F 1) produced recumbent isoclinal folds, a penetrative schistosity, and boudinage under high-P/low-T metamorphic conditions. Mylonite formation at the top of the PQ Nappe, below the overriding Tripolitza Nappe, further boudinage, and schistosity (S 2) represent a late tectono-metamorphic episode. Post-metamorphic small folds (F 3), lineations, and a crenulation cleavage were formed synchronously with transport of the PQ Nappe. A last phase (F 4) developed small folds, a fracture/crenulation cleavage, and large-scale folds after nappe movement. It is suggested that high-P/low-T metamorphism in the PQ rocks originated during subduction. Nappe transport of the higher, unmetamorphosed units, which were thrust over the PQ Nappe, began under waning metamorphic conditions. Subsequent transport of the PQ Nappe itself also occurred after the completion of metamorphism and after the formation of the mylonite at its top.  相似文献   

6.
The Teloloapan volcanic arc in SW Mexico represents the easternmost unit of the Guerrero Terrane. It is overthrust by the Arcelia volcanic unit and is thrust over the Guerrero–Morelos carbonate platform. These major structures result from two closely related tectonic events: first, an eastward verging, ductile deformation (D1) characterized by an axial-plane schistosity (S1) supporting an E–W trending mineral stretching lineation (L1) and associated with synschistose isoclinal, curvilinear folds (F1). Numerous kinematic indicators such as asymmetrical pressure-shadows, porphyroclast systems, and micro-shear bands (S–C structures) indicate a top-to-the-east shear along L1. This first deformation was followed by another ductile event (D2) that produced a crenulation cleavage (S2) associated with westward overturned folds (F2), hence showing that the vergence of D2 is opposite to that of D1. Regionally, both D1 and D2 deformations have been identified east and west of the Teloloapan unit, in the Arcelia volcanic rocks as well as in the Mexcala flysch of Late Cretaceous age overlying the Guerrero–Morelos platform. This implies that all three units were deformed and thrust simultaneously, during the Late Cretaceous or Paleocene, prior to the deposition of the overlying, undeformed Eocene red beds of the Balsas group.  相似文献   

7.
In the Kolar Schist Belt well-preserved small-scale diastrophic structures suggest four phases of folding (F1 — F4). The near coaxial F1 andF 2folds are both isoclinal with long-drawn out limbs and sharp hinges. The axial planes of bothF 1andF 2folds are subvertical with N-S strikes; these control the linear outcrop pattern of the Schist belt. The later folds (F 3and F4) are important in small-to-intermediate scales only and are accommodation structures formed during the relaxation period of the early folding episodes. Mesoscopic shear zones, post-F2 but pre-F3 in age, are present in all the rock types in this area. The F1 and F2 folds and the mesoscopic shear zones were formed during a continuous E-W subhorizontal compression. Available geochemical and isotopic data show that the Kolar Schist Belt with ensimatic setting is bounded by two granitic terrains of contrasting evolutionary histories. This, together with E-W subhorizontal compression over a protracted period of time, strengthens the recent suggestions that the Kolar Schist Belt represents a suture. This belt then marks the site of a continent-continent collision event of late Archaean-early Proterozoic age.  相似文献   

8.
Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.  相似文献   

9.
《China Geology》2019,2(4):478-492
The Narooma-Batemans Bay (NBB) area along the southeast coast of Australia is a part of the eastern zone of the Early Paleozoic Lachlan Orogen. In the NBB, a set of rock association consisting of turbidites, siliceous rock, basic lava, and argillaceous melange zone is mainly developed. According to systematic field geological survey, the deformation of 3 stages (D1, D2, and D3) was identified in the NBB. At stage D1, with the original bedding S0 in a nearly east-west trending as the deformation plane, tight folds, isoclinal folds, and other structures formed in the NBB accompanied by structural transposition. As a result, crenulation cleavage developed along the axial plane of the folds and schistosity S1 formed. At stage D2, with north-south-trending schistosity S1 as the deformation plane, a large number of asymmetrical folds and rotated porphyroclasts formed owing to thrusting and shear. At stage D3, left-lateral strike-slip occurred along the main north-south-trending schistosity. Based on the analysis of the characteristics of tectonic deformation in the NBB and summary of previous research results, it is determined that the early-stage (D1) deformation is related to Ordovician Macquarie arc-continent collision and the deformation at stages D2 and D3 is the result of the westward subduction of Paleo-Pacific Plate. That is, it is not the continuous westward subduction of the Paleo-Pacific Plate that constitutes the evolution model of the NBB as previously considered.  相似文献   

10.
研究地区的地层属太古宙迁西群底部上川组。本文涉及其中一部分约4平方公里的面积。岩石都已高度变质,属麻粒岩相的区域紫苏辉石带。由于缺乏顶、底及面向(facing)标志,无法确定其确切层序。  相似文献   

11.
In the Upper Pennine nappe complex of the Simplon—Pennine Alps (Switzerland and Italy), at least three phases of major post-nappe folding (in places associated with thrusting) can be distinguished. These are superimposed on an earlier-formed, partly chaotic, complex of tectonic units, including the Bernhard and Monte Rosa continental flakes and the Zermatt—Saas and Antrona ophiolite complexes. The earliest post-nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas. Later, two successive phases of back-folding led to the present overall structure. Both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition. This has been demonstrated by systematically mapping the major axial traces over the whole region. Successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the Bernhard and Monte Rosa nappes.  相似文献   

12.
The Arthur Lineament of northwestern Tasmania is a Cambrian (510 ± 10 Ma) high‐strain metamorphic belt. In the south it is composed of metasedimentary and mafic meta‐igneous lithologies of the ‘eastern’ Ahrberg Group, Bowry Formation and a high‐strain part of the Oonah Formation. Regionally, the lineament separates the Rocky Cape Group correlates and ‘western’ Ahrberg Group to its west from the relatively low‐strain parts of the Oonah Formation, and the correlated Burnie Formation, to its east. Early folding and thrusting caused emplacement of the allochthonous Bowry Formation, which is interpreted to occur as a fault‐bound slice, towards the eastern margin of the parautochthonous ‘eastern’ Ahrberg Group metasediments. The early stages of formation of the Arthur Lineament involved two folding events. The first deformation (CaD1) produced a schistose axial‐planar fabric and isoclinal folds synchronous with thrusting. The second deformation (CaD2) produced a coarser schistosity and tight to isoclinal folds. South‐plunging, north‐south stretching lineations, top to the south shear sense indicators, and south‐verging, downward‐facing folds in the Arthur Lineament suggest south‐directed transport. CaF1 and CaF2 were rotated to a north‐south trend in zones of high strain during the CaD2 event. CaD3, later in the Cambrian, folded the earlier foliations in the Arthur Lineament and produced west‐dipping steep thrusts, creating the linear expression of the structure.  相似文献   

13.
Seventy muscovites from schists in the Sanbagawa terrain in central Shikoku were dated by the K-Ar method. The muscovite ages are consistently older with increasing metamorphic grade. Within the same zone the ages are significantly younger in schists which have been more severely deformed. These K-Ar age variations could be due to systematic argon depletion during deformation i.e., to the dynamic recrystallization of muscovites during ductile deformation that formed a large-scale recumbent fold during the uplift and cooling. Argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low-temperature deformation than the higher grade zone. The relationships between age and grain size in a pelitic schist suggest that coarse-grained muscovites lost more argon than the finegrained ones. There was no significant resetting of ages in the vicinity of major strike-slip faults, such as the Median Tecotonic Line or near thrust faults. The combination of geochronological and geological data constrains the cooling rate of the Sanbagawa schists to 9–12° C/Ma in the oligoclase-biotite zone in central Shikoku, Japan.  相似文献   

14.
Mixing properties for muscovite–celadonite–ferroceladonite solid solutions are derived from combining available experimental phase equilibrium data with tabulated thermodynamic data for mineral end‐members. When a partially ordered solution model is assumed, the enthalpy of mixing among the end‐members muscovite–celadonite–ferroceladonite is nearly ideal, although the Gibbs energies of muscovite–celadonite and muscovite–ferroceladonite solutions are asymmetric due to an asymmetry in the entropy of mixing. Thermodynamic consistency is achieved for data on phengite compositions inassemblages with (a) pyrope+kyanite+quartz/coesite (b) almandine+kyanite+quartz/coesite (c)talc+kyanite+quartz/coesite and (d) garnet–phengite pairs equilibrated both experimentally at high temperatures and natural pairs from low‐grade schists. The muscovite–paragonite solvus has been reanalysed using the asymmetric van Laar model, and the effects of the phengite substitution into muscovite have been quantitatively addressed in order to complete the simple thermodynamic mixing model for the solid solution among the mica end‐members. Results are applied to a natural pyrope–coesite–phengite–talc rock from the Western Alps, and to investigate the conditions under which biotite‐bearing mica schists transform to whiteschist‐like biotite‐absent assemblages for average pelite bulk compositions.  相似文献   

15.
STRUCTURAL EVOLUTION OF THE KULU-RAMPUR AND LARJI WINDOW ZONES, WESTERN HIMALAYA, INDIA  相似文献   

16.
Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI1, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H2O that took place in psammitic rocks, and the second, BI2, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI1, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.  相似文献   

17.
The supracrustal rocks in the easternmost part of the Proterozoic fold belt of North Singhbhum, eastern India, are folded into a series of large upright folds with variable plunges. The regional schistosity is axial–planar to the folds. The folds were produced by a second phase of deformation (D2) and were preceded by D1 deformation, which gave rise to isoclinal folds (mapped outside the study area) and the locally preserved, bedding-parallel schistosity. A shearing deformation during D2 was responsible for the sheath-like geometry of a major fold. The axial planes were curved by D3 warping. The first metamorphic episode (M1) of low-pressure type produced andalusite porphyroblasts prior to, or in the early stage of, D1 deformation. The main metamorphism (M2), responsible for the formation of chloritoid, kyanite, garnet and staurolite porphyroblasts, was late- to post-D2 in occurrence. The Staurolite isograd separates two zonal assemblages recorded in the high-alumina and the low-alumina pelitic schists. Geothermobarometric calculations indicate the peak metamorphic temperature to be 550 °C at 5.5 kb. Fluid composition in the rocks before and during M2 metamorphism was buffered and fluid influx, if any, was not extensive enough to overcome the buffering capacity of the rocks. From M1 to M2, the P–T path is found to have a clockwise trajectory, that is consistent with a tectonic model involving initial asthenospheric upwelling and rifting, followed by compressional deformation leading to loading and heating.  相似文献   

18.
Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S1). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north–south shortening (D2), open to isoclinal south–southeast-oriented F2 folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D2 Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D2 history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.  相似文献   

19.
The relations between metamorphism and deformation of the crystalline rocks in the central part of the Serbomacedonian mass (mountain Vertiskos), are studied. It is found a syntectonic — amphibolitic metamorphic phase (Kr1) in reference to a folding phase (T1), characterized by almost vertical isoclinal shear-folds. This main metamorphism was followed by a syntectonic metamorphism of green schist facies (Kr2) in relation to a folding phase (T2), characterized by slightly inclined fractured folds, directied NNW-SSE to NNE-SSW. Finally, a very low grade metamorphic event (Diaphthorese, Kr3) influenced the whole rocks of the area. It can be regarded either as the final stage of the Kr2 phase or as a younger and distinct metamorphic event. The K/Ar and Rb/Sr datation of the white micas, biotites and hornblends showed that the Kr2 phase is of Cretaceous age and so the Kr1 phase must be older than Cretaceous.  相似文献   

20.
Late Palaeozoic deformation in the southern Appalachians is believed to be related to the collisional events that formed Pangaea. The Appalachian foreland fold and thrust belt in Alabama is a region of thin-skinned deformed Palaeozoic sedimentary rocks ranging in age from Early Cambrian to Late Carboniferous, bounded to the northwest by relatively undeformed rocks of the Appalachian Plateau and to the southeast by crystalline thrust sheets containing metasedimentary and metaigneous rocks ranging in age from late Precambrian to Early Devonian. A late Palaeozoic kinematic sequence derived for a part of this region indicates complex spatial and temporal relationships between folding, thrusting, and tectonic level of décollement. Earliest recognized (Carboniferous(?) or younger) compressional deformation in the foreland, observable within the southernmost thrust sheets in the foreland, is a set of large-scale, tight to isoclinal upright folds which preceded thrafing, and may represent the initial wave of compression in the foreland. Stage 2 involved emplacement of low-angle far-traveled thrust sheets which cut Lower Carboniferous rocks and cut progressively to lower tectonic levels to the southwest, terminating with arrival onto the foreland rocks of a low-grade crystalline nappe. Stage 3 involved redeformation of the stage 2 nappe pile by large-scale upright folds oriented approximately parallel to the former thrusts and believed to be related to ramping or imbrication from a deeper décollement in the foreland rocks below. Stage 4 involved renewed low-angle thrusting within the Piedmont rocks, emplacement of a high-grade metamorphic thrust sheet, and decapitation of stage 3 folds. Stage 5 is represented by large-scale cross-folding at a high angle to previous thrust boundaries and fold phases, and may be related to ramping or imbrication on deep décollements within the now mostly buried Ouachita orogen thrust belt to the southwest. Superposed upon these folds are stage 6 high-angle thrust faults with Appalachian trends representing the youngest (Late Carboniferous or younger, structures in the kinematic sequence.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号