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1.
Abstract The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north-west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo-Hercynian metamorphic episode (c. 374 Ma) of high-P, low- to intermediate-T. The mineral assemblage of the high-P episode is preserved as a very thin Si= S1 foliation included in albite porphyroblasts, being composed of: albite + garnet-I + white mica-1 + chlorite-1 + epidote + quartz + rutile ± ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 ± 10 °C and 14.7 ± 0.7 kbar (probably minimum pressure). The later evolution (syn-D2) of the schists defines a decompressive and slightly prograde P-T path which reached its thermal peak at c. 525 ± 10 °C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet-II, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation. The estimated P-T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character. The general metamorphic evolution of the Santiago Schists, with the development of high-P assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high-P, low- to intermediate-T circum-Pacific belts.  相似文献   

2.
Structural, metamorphic and geochronological studies of the Chewore Inliers of the Zambezi Belt within the Karoo age Zambezi Rift, allow recognition of a protracted multi-stage evolution, from the Mesoproterozoic to culminating in the Early. Palaeozoic Pan-African Orogeny. Tectono metamorphic events recognised in the Chewore Inliers occur throughout the Zambezi Belt and alternative models for the history of the Zambezi Belt are presented.Four terranes are recognised in the Chewore Inliers, and contacts between them are observed or inferred to be ductile thrusts, along which juxtaposition of the terranes occurred late in the Pan-African metamorphic cycle (M2, at 526 Ma). The oldest portion of the inliers is a metamorphosed sequence of mafic and ultramafic gneisses with an age of 1393 Ma. These constitute what is tentatively called the Ophiolite Terrane, together with closely associated high-P/moderate T schists possibly represents a suture. The other three terranes (Granulite, Zambezi and Quartzite Terranes) experienced a common history of tectonothermal events but show variable degrees of reworking during the latest tectono metamorphic event (M2). Concordant granitic orthogneisses were emplaced at 1087 Ma into supracrustal sequences. No Pan-African supracrustals are recognised in the Chewore Inliers, which are wholly basement gneisses and quartzites that have been reworked during successive orogenies including the Pan-African Orogeny.A high-T/low-P metamorphic event (M1 of possibly 1068–1071 Ma age, with a minimum age of 943 Ma, was responsible for totally recrystallizing the Granulite Terrane during south to north tectonic transport. M1 mineral parageneses are only preserved as inclusion phases and overgrown fabrics in the other terranes. These other terranes were pervasively recrystallised at high-P/moderate T conditions accompanying a clockwise P-T path related to northeast over southwest tectonic transport and crustal over-thickening during the Pan-African metamorphic cycle (M2) at approximately 526 Ma. Reworking of the Granulite Terrane during M2 was minor, leaving M1 fabrics and mineral assemblages preserved with little recrystallization. M2 orogenesis culminated in the juxtaposition of the terranes, rapid uplift through the thermal peak and eventual slow cooling accompanying a multitude of post-tectonic intrusions; pegmatites at 480 Ma, the Chewore Ultramafic Complex and dolerite dykes. The 830 Ma tectonothermal event involving pervasive syn-tectonic granitic orthogneisses in the south Zambezi Belt is not recognised in the Chewore Inliers, suggesting a localised, possibly extensional, regime restricted to the southern part of the Zambezi Belt at 830 Ma.  相似文献   

3.
Three successive metamorphic stages M1, M2 and M3 have been distinguished in polymetamorphic granulite facies quartz-feldspathic gneisses from the Seiland Igneous Province, Caledonides of northern Norway. An early period of contact metamorphism (M1; 750–950°C, ca. 5 kbar) was followed by cooling, accompanied by strong shearing and recrystallization at intermediate-P granulite facies conditions (M2; 700–750°C, 5–6kbar). High-P granulite facies (M3; ca. 700°C, 7–8 kbar) is related to recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. On the basis of composition, fluid inclusions in cordierite, quartz and garnet can be divided into three major types: (1) CO2 inclusions; (2) mixed CO2–N2 inclusions; (3) N2 inclusions. Fluid chronology and mineral assemblages suggest that the earliest inclusions consist of pure CO2 and were trapped at the M1 contact metamorphic episode. A carbonic fluid was also present during the intermediate-P granulite facies M2 metamorphism. The CO2-rich inclusions in M2 garnet can be divided into two generations, an early lower-density and a late higher-density, with isochores crosscutting the P-T box of M2 and M3, respectively. The nitrogen-rich fluids were introduced at a late stage in the fluid evolution during the high-P M3 event. The mixed CO2–N2 inclusions, with density characteristics compatible with M3 conditions, are probably produced from intersection between pre-existing pure CO2 inclusions and N2 fluids introduced during M3. The fluid inclusion data agree with the P-T evolution established from mineral assemblages and mineral chemistry.  相似文献   

4.
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5.
This paper describes the progressive metamorphism and deformation of a series of metasediments, Le Conquet Schists and their higher grade equivalents, which occur as tectonically emplaced screens within a sequence of foliated gneisses, the Gneiss de Brest and Gneiss de Lesneven. The sequence exhibits a steep south to north increase in metamorphic grade from garnet-staurolite schist to sillimanite gneiss and sillimanite-K-feldspar migmatite. The relationship of mineral growth to foliation development has been established for individual screens. At least five phases of deformation (D1-D5) are preserved. Analysis of porphyroblast inclusion trails is used to demonstrate sequential mineral growth during the successive development of orthogonal foliations S1-S4. Porphyroblasts continued to grow during the subsequent development of C-S mylonite fabrics and extensional crenulation cleavages which are genetically related to a series of high-strain zones (D5). Mineral assemblages, phase relations and mineral chemistry are consistent with porphyroblast growth being the result of continuous reactions. Microstructure-porphyroblast relations are used to show that although mineral growth proceeded during continuous reactions, these only operated episodically. Phase relations, mineral chemistry and P-T estimates are used to constrain P-T trajectories and these are linked to the deformation histories within individual screens. A comparison between the resulting pressure-temperature-deformation paths is used to demonstrate that the metamorphic peak occurred progressively later and at successively lower pressures with increasing metamorphic grade. It is suggested that the early evolution of the belt is the result of crustal thickening by overthrusting. The subsequent history is one of progressive heating and unroofing of the higher grade rocks in a dextral strike-slip transtensional shear zone.  相似文献   

6.
Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the T min of the older P-T trajectory corresponded to T max of the younger trajectory, and often T max–T min > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating.  相似文献   

7.
Cordierite-orthopyroxene migmatitic gneisses exposed in Achankovil unit of the Kerala Khondalite Belt, southern India show evidences of melting, melt extraction and in-situ crystallization of melt under granulite-facies conditions. The sequential mineral assemblages garnet + biotite + orthopyroxene + plagioclase + quartz (± melt) in the mesosomes and garnet + biotite + orthopyroxene + cordierite + plagioclase + K-feldspar + quartz + melt in the melanosomes makes the Achankovil cordierite-orthopyroxene migmatitic gneisses a good example of anatectic rocks, where substantial melt fractions remained in-situ during decompression and cooling. Therefore, the rocks provide an opportunity to investigate deep crustal processes and record of rheological (thermal and mechanical) reequilibration prevailed during the final stages of orogeny. The significance of cordierite formation and its possible relationship with melt formation are investigated applying theoretical calculations in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (MnNCKFMASH) system. Results of numerical modelling of the mineral assemblages in pressure-temperature-composition (P-T-X) pseudosections using Perple_X infer that the sequence of reactions involving formation of cordierite-orthopyroxene-melt assemblage is consistent with an isothermal decompression (with a pressure drop of >1.5 kbars) at high temperatures (>800 °C), forming leucosomes. Biotite dehydration melting reactions, occurring above 4.5 kbars constrain prograde arm of the P-T trajectory and is interpreted as a product of crustal thickening, which was followed by rapid decompression. The final stage of exhumation is characterized by rehydration of cordierites in the melanosome by melt-solid interactions at exceptionally low-pressure (??3.2 kbars) conditions. The high-temperature isothermal decompression inferred from the mineral reactions and P-T-X pseudosections constitute a clockwise P-T path for the exhumation of the lower crust. This clockwise P-T path is consistent with the common tectonic model accepted for the genesis of granulite-facies migmatites during crustal thickening and later unroofing, accompanied with arc-continent collision. Our conclusions indicate low-P metamorphism and anatexis can be traced to convergent setting, where melt buoyancy considerably decreases density of the lithosphere and modifies rheology leading to rapid exhumation of the lower crust. Therefore, the crustal evolution in the Kerala Khondalite Belt is correlated with two stage processes: (i) thickening of the crust in relation to a continental-arc setting, followed by (ii) exhumation along a high-temperature stable geotherm with sufficient pressure release associated with syn- to post-convergence transpression and transtension.  相似文献   

8.
The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.  相似文献   

9.
A high-grade metamorphic complex is exposed in Filchnerfjella (6–8°E), central Dronning Maud Land. The metamorphic evolution of the complex has been recovered through a study of textural relationships, conventional geothermobarometry and pseudosection modelling. Relicts of an early, high-P assemblage are preserved within low-strain mafic pods. Subsequent granulite facies metamorphism resulted in formation of orthopyroxene in rocks of mafic, intermediate to felsic compositions, whereas spinel + quartz were part of the peak assemblage in pelitic gneisses. Peak conditions were attained at temperatures between 850–885 °C and 0.55–0.70 GPa. Reaction textures, including the replacement of amphibole and garnet by symplectites of orthopyroxene + plagioclase and partial replacement of garnet + sillimanite + spinel bearing assemblages by cordierite, indicate that the granulite facies metamorphism was accompanied and followed by decompression. The observed assemblages define a clock-wise P-T path including near-isothermal decompression. During decompression, localized melting led to formation of post-kinematic cordierite-melt assemblages, whereas mafic rocks contain melt patches with euhedral orthopyroxene. The granulite facies metamorphism, decompression and partial crustal melting occurred during the Cambrian Pan-African tectonothermal event.  相似文献   

10.
S. P. Korikovsky 《Petrology》2009,17(4):315-330
The paper is devoted to mineral equilibria occurring during the transformation of medium-pressure metabasites into eclogites at geothermal gradients of 11–19°C/km; it presents materials on the evolution of the composition and types of prograde zoning in garnet and clinopyroxene, significance and distinctive features of armored inclusions of earlier metamorphic stages in garnets and the possibility of their application in calculating the P-T path of eclogitization. Results of the analysis of mineral assemblages, data on mineral zoning, and evolution of phase equilibria are used to outline depth subfacies of crustal eclogites and describe in detail equilibria in basic and acid rocks of each subfacies. Conditions under which orthopyroxene is stable in the eclogite facies are analyzed within the temperature range of 630–700°C in correlation with the bulk composition of the eclogites. Published materials on high-pressure mineral transformations in eclogite-facies acid gneisses are analyzed.  相似文献   

11.
The gneisses of the Makuti Group in north-west Zimbabwe are characterized by complex geometries that resulted from intense non-coaxial deformation in a crustal scale high-strain zone that accommodated extensional deformation along the axis of the Zambezi Belt at c. 800 Ma. Within low-strain domains in the Makuti gneisses, undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages, which record a part of the metamorphic history that predates Pan-African events. Eclogitic rocks can be subdivided into: (1) corona-textured metagabbros that preserve igneous textures, and (2) garnet–omphacite rocks in which primary textures are destroyed. The lenses of eclogitic rocks are enveloped in a mantle of garnet–clinopyroxene–hornblende gneiss, which is a common rock type in the Makuti gneisses. The eclogites preserve multi-staged, domainal, symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression–heating. In the metagabbros, the original clinopyroxene, plagioclase and olivine domains acted separately during the peak of metamorphism, with plagioclase being replaced by garnet and kyanite, and olivine being replaced by orthopyroxene and possibly omphacite. The peak assemblage was overprinted by: (1) the multi-mineralic corona assemblage pargasite–orthopyroxene–spinel–plagioclase replacing garnet–kyanite–clinopyroxene (possibly at c. 19 kbar, 760±25 °C); (2) orthopyroxene–pargasite–plagioclase–scapolite coronas replacing orthopyroxene (15±1.5 kbar, 750±50 °C); and (3) moats of orthopyroxene–plagioclase replacing garnet (10±1 kbar, 760±50 °C). The garnet–omphacite rocks record similar peak conditions (15±1.1 kbar, 760±60 °C). Garnet–clinopyroxene–hornblende–plagioclase gneisses envelop the eclogites and record matrix conditions of 11±1.5 kbar at 730±50 °C using assemblages that are oriented in the regional fabric. These rocks are characterized by decompression-heating textures, reflecting temperature increases during exhumation of the Makuti gneisses. The eclogite facies rocks formed during a collisional event prior to 850 Ma. Their formation could be related to a suture zone that developed along the axis of the Zambezi Belt during the formation of Rodinia (between 1400 and 850 Ma). The main deformation-metamorphism in the Makuti gneisses occurred around 800 Ma and involved extension and exhumation of the high-P rocks (break-up of Rodinia), which experienced a high-T metamorphic overprint. Around 550–500 Ma, a collisional event associated with the formation of Gondwana resulted in renewed burial and metamorphic recrystallization of the Makuti gneisses.  相似文献   

12.
麻粒岩的研究进展与方法   总被引：2，自引：0，他引：2  
近年来,有关麻粒岩的研究取得了长足进展,本文讨论了4个相关问题:(1)麻粒岩的大地构造环境与P-T轨迹。麻粒岩可以形成于4种大地构造环境中:(a)碰撞造山带以形成高压麻粒岩为特征,为中压相系,包括曾位于地壳浅部的岩石经历构造埋深达到变质峰期后再折返的过程,为顺时针型P-T轨迹;也包括曾经历洋壳或陆壳俯冲形成的高压-超高压榴辉岩相岩石折返叠加变质形成的麻粒岩,P-T轨迹以减压为主。(b)地壳伸展区以形成低压麻粒岩为特征,并可达到超高温条件,其P-T轨迹为减压加热至温度峰期,随后发生等压或降压冷却。(c)岛弧或陆缘岩浆增生区的下地壳多为高压麻粒岩相,其中侵入的辉长岩首先经历等压冷却,然后再经历升温升压进变质过程。(d)太古宙克拉通麻粒岩相表壳岩呈皮筏状分布于TTG片麻岩内部,多达到超高温条件,发育逆时针型P-T轨迹,受太古宙特殊的垂直构造体制控制。(2)麻粒岩的进变质过程与流体行为。按照流体行为,麻粒岩的进变质过程分为3种型式:(a)流体饱和进变质过程,指岩石在饱水固相线之前达到流体饱和,随后发生饱水固相线熔融与含水矿物的脱水熔融,以及阶段性熔体丢失,导致岩石中水含量降低,缺流体固相线温度升高;在峰期之后的降温过程中,发生熔融反应的逆反应,或结晶反应,形成含水矿物,结晶反应终止于缺流体固相线。(b)流体不饱和或缺流体进变质过程,指岩石在进变质过程中会处于流体缺失状态,不会发生变质反应,岩石中原来的矿物组合以亚稳定状态保留至缺流体固相线后,才开始变质演化,因此经常形成一些不平衡结构。(c)流体过饱和进变质过程,指有过量水参与的熔融反应过程,也称为水化熔融,与熔体注入或局部汇聚有关;水化熔融过程中会更多地消耗斜长石、石英及辉石等无水矿物,导致残余物中富集角闪石和黑云母等含水矿物。(3)确定麻粒岩P-T条件的视剖面图方法。利用视剖面图方法分析麻粒岩的变质条件时,首先需要通过岩相学观察区分出峰期组合和最终组合;然后通过计算T-M(H2O)图解确定最终组合的含水量;最后利用所确定的水含量计算P-T视剖面图。利用P-T视剖面图分析麻粒岩的峰期变质条件时,首先找到峰期矿物组合在视剖面图上的稳定域,然后再结合有价值的矿物成分等值线确定P-T条件。特别需要注意的是,岩相学观察确定的峰期组合和最终组合都可能受局部结构域控制,与滞留熔体的不均匀分布或原地分凝有关,此时不能简单地用全岩成分模拟其相平衡关系。(4)相平衡模拟时需要选择有效的全岩成分。当选择实测全岩成分进行相平衡模拟时,首先需要检验其有效性,即检验实测全岩成分是否能够代表薄片中所观察到的相平衡关系。方法是计算有效全岩成分,并与实测全岩成分进行对比。对于成分不均匀的变质岩石,需要处理局部结构域的成分。分如下3种情况:(a)宏观尺度的结构域,可以分别取样;(b)微观尺度的结构域,需要在显微薄片中进行图像分析,针对不同结构域分别进行相平衡模拟;(c)由叠加或退变质形成的结构域,需要确定相应的变质反应,通过对反应配平,确定有效全岩成分。此外,文中还介绍了计算岩石中的水含量、O含量和各种矿物相含量的方法与注意事项。  相似文献   

13.
J.V. Owen  J.D. Greenough 《Lithos》1994,33(4):225-239
Migmatitic granitic gneiss and associated garnetiferous granite at Grand Lake are tectonically interleaved with high-pressure ( 9 kbar) metapelites that contain Barrovian (Ky-St-Grt-Bt-Ms-Rt) mineral assemblages. The migmatites contain metabasites that are compositionally similar to the latest Proterozoic Long Range dykes, suggesting that the gneisses correlate to Grenvillian inliers in western Newfoundland.

In the metapelites, the Barrovian porphyroblastic assemblage overprints microfolds and is itself overprinted by greenschist-facies assemblages in thrust-related high-strain zones. Evidence that the quartzofeldspathic rocks also experienced Barrovian metamorphism is provided by low Ti/>Al amphiboles in metabasites in the gneiss, and, despite the migmatitic character of these rocks, the presence of growth-zoned garnets with bell-shaped compositional profiles (e.g., rimward increase in XPrp and decrease in XSps) similar to those exhibited by garnet in the metapelite. The range of garnet-biotite paleotemperatures for the quartzofeldspathic rocks is only marginally higher (Tmax=530–660°C) than that determined for the metapelites (Tmax=500–615°C). There are no systematic differences in Grt-Bt temperatures between (proto) mylonitic rocks and nearby precursors.

These results indicate that the quartzofeldspathic rocks and metapelites share a common, post-migmatization (of the gneisses) metamorphic history; they may well have originally been linked by a basement/cover relationship. Mineralogical evidence for the earlier, higher grade metamorphic history of the migmatites has virtually been eradicated even outside the high-strain zones. In this area, there is no clear relation between qualitatively-estimated superimposed strain and the degree of resetting of the Grt-Bt geothermometer.  相似文献   


14.
假蓝宝石是Mg-Al质麻粒岩中一种特殊的高温矿物,对超高温变质作用的研究有重要的意义。本文通过对全球66个超高温麻粒岩中47个含假蓝宝石麻粒岩地区的文献调研,总结了几种最常见的含假蓝宝石矿物组合产出的结构位置和变质反应关系,以及假蓝宝石的矿物化学特征。假蓝宝石的化学成分一般位于7∶9∶3端元左右,X_(Mg)大于0. 7,XFe_(3+)变化范围很宽,为0～0. 7。含假蓝宝石矿物组合的形成和演化指示了岩石经历的P-T轨迹。岩石中保留的假蓝宝石取代尖晶石、Grt/Opx+Sil取代Spr+Qz组合,以及随后的Spr+Crd±Opx后成合晶取代Grt/Opx+Sil组合的结构,一般可能指示了逆时针P-T轨迹中冷却和随后减压的部分;岩石中Grt/Opx+Sil/Ky或富Mg十字石反应形成Spr+Qz组合的结构可能指示了顺时针P-T轨迹中减压升温的部分。超高温变质岩不同的P-T轨迹暗示着它们的成因机制并不单一,前者可能是幔源基性岩浆底侵或增生作用的结果,后者可能与长期的热造山作用相关。  相似文献   

15.
Available petrological, structural and geochronological data suggest that metamorphism and deformation of greenstone sequences and the evolution of intrusive granitoids in the Eastern Goldfields Province, Yilgarn Block, were related to a widespread and integrated tectonic event in the time interval 2700-2600 m.y.Polyphase deformation of the greenstone sequences involved the superimposition of a series of upright folds and related subvertical foliations on earlier macroscopic recumbent folds. Metamorphism was imposed rapidly on these previously deformed but relatively unaltered greenstone sequences, synchronously with a third phase of deformation. Static-style metamorphic recrystallization at very low to medium grades occurred over most of the province, but contemporaneous high grade recrystallization of dynamic style was restricted to elongate narrow zones which were also the sites of synkinematic granitoid diapirism. These zones commonly mark the present margins of greenstone belts.The extensive areas between greenstone belts are dominated by outcrops of post-kinematic granitoids whose abundance may be overestimated because of the limited exposure. Their emplacement caused only minor contact metamorphic overprinting on the pre-existing metamorphic patterns. Also present are banded gneisses interpreted as modified basement to the greenstone sequences. These gneisses are enclosed in post-kinematic granitoid batholiths or occur as remnants in synkinematic diapirs within the dynamic domains. All major granitoid groups, including gneisses, are geochemically similar and show parallel but limited variations. Both field and chemical evidence points to the gneisses being parental to intrusive granitoids derived by both anatectic and solid-state processes.The data provide important constraints on any model for greenstone belt evolution. Our preferred model involves a widespread disturbance resembling the kind currently referred to as a “mantle plume”, which initially led to extrusion of mafic and ultramafic magmas via tensional fractures in a sialic crust, then subsequently caused their deformation and metamorphism and generated the intrusive granitoids by widespread reactivation of the basement. The dynamic metamorphic domains may reflect pre-greenstone crustal lineaments that controlled the initial vulcanism. The evolution of Archean greenstone terrains proposed here appears distinct from that of subsequent Proterozoic and Phanerozoic tectonic belts.  相似文献   

16.
High-pressure metamorphism in the Western Cordillera of North America   总被引：1，自引：0，他引：1  
The Skagit Gneiss, a major component of the crystalline core of the North Cascades, was metamorphosed during a mid-Cretaceous(?) to early Tertiary high-P event driven by the collision of the Insular and Intermontane superterranes. Maximum pressures recorded by metapelitic rocks are 8–10 kbar at 650–725° C. High pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the Skagit Gneiss and adjacent Cascade River Schist. Mineral reactions continued during nearly isothermal decompression from 8–10 kbar to c. 3–5 kbar. Early high-P minerals (e.g. kyanite) are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass. Skeletal relics of kyanite are also present in the groundmass of lower-grade, staurolite-bearing schists that contain texturally later cordierite. This matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature. These results represent a revision of the metamorphic history of the Skagit Gneiss, which was formerly thought to have experienced only relatively low-P Barrovian metamorphism. Qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression. Geobarometric results for the Skagit Gneiss are interpreted as evidence for major crustal thickening in the North Cascades. Recognition that pressures of c. 9 kbar were attained supports a contractional model for North Cascades orogenesis and requires that tectonic syntheses account for the burial of the Skagit Gneiss protoliths to a depth of c. 25–30 km.  相似文献   

17.
Deformation partitioning is identified as the fingerprint of late Palaeozoic continental subduction that affected various lithologies whose field relationship, thermobarometric and petrofabric features are closely related. Different modes of deformation partitioning can be identified within medium temperature, high‐P eclogite lenses, between them and the host gneisses, and within the latter. Development of foliations and lineations with a coherent attitude in all these rocks and their related structural petrology demonstrate that eclogite enclosures and their country rocks underwent a common, pervasive deformational event. The published P–T stability fields of the eclogite phases that define the microscopic fabric are used to define the metamorphic conditions prevailing during the deformation event and relate it to the subduction process. The mineral equilibria of the gneisses (ortho‐ and paragneisses) fail to record the full range of those P–T conditions, but the field relationships show that eclogites were originally basic dykes emplaced in acid igneous rocks and demonstrate that the eclogites and gneisses shared a common tectonometamorphic evolution. Deformation partitioning within the latter occurred at variable scales and involved (1) meso macroscale preservation of virtually undeformed metagranite bodies, surrounded by (2) pervasively foliated and lineated gneisses, and (3) the simultaneous microscale operation in the latter of ductile and brittle–ductile mechanisms at conditions above 500°C and below 1.5 GPa. A subduction channel tectonic setting is proposed to explain the subduction of upper to mid‐crustal igneous rocks and exhumation subsequent to high‐P metamorphism. Its currently accessible dimensions, and its organization into several lithotectonic units mapped as nappes support tectonic amalgamation of units several km3 in volume. Maximum burial in the subduction channel likely reached depths shallower than the lithostatic pressure implied by geobarometric calculations, possibly conditioned by a sudden pressure drop during the initial retrogression stages accompanying exhumation.  相似文献   

18.
Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW-directed reverse shearing. The NE-trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright-to-steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3). Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast-matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K-feldspar, and the development of an increasing volume of anatectic migmatite in south-eastern sectors of the area record syn- to late-D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P-T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east. Titanite from an upper amphibolite facies calc-silicate gneiss yields U-Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. 40Ar/39Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south-east to north-west associated with displacement on major shear zones. 40Ar/39Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation. The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.  相似文献   

19.
Abstract Low-pressure/high-temperature (low-P/high-T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure-temperature-time-deformation (P-T-t-D) path, similar to those of some other low-P/high-T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite-andalusite schists, cordierite-K-feldspar gneisses and andalusite-K-feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P-T-t-D path involving rapid or even moderate decompression immediately post-dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed. Determination of the deformational-metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational-metamorphic history in the Cooma Complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined. Sequential porphyroblast growth and foliation development in the cordierite-andalusite schists is examined for situations involving rotation and non-rotation of porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of porphyroblast growths does not differ from the non-rotational situation. Thus, whether or not porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P-T-t paths inferred from the relative timing of porphyroblast growths, remain the same, for the deformational histories evaluated.  相似文献   

20.
A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all, pressure decreased, the SiO2 activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes, which were restricted to zones of intense deformations, at variable SiO2 activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels.  相似文献   

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