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1.
The Leverburgh Belt and South Harris Igneous Complex in South Harris (northwest Scotland) experienced high-pressure granulite facies metamorphism during the Palaeoproterozoic. The metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic, quartzofeldspathic and mafic gneisses, especially in pelitic gneisses from the Leverburgh Belt: (1) some coarse-grained garnet in the pelitic gneiss includes biotite and quartz in the inner core, sillimanite in the outer core, and is overgrown by kyanite at the rims; (2) garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims; (3) the AlVI/AlIV ratio of clinopyroxene from mafic gneiss increases from core to rim; (4) retrograde reaction coronas of cordierite and hercynite+cordierite are formed between garnet and kyanite, and orthopyroxene+cordierite and orthopyroxene+plagioclase reaction coronas develop between garnet and quartz; (5) a P–T path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet+sillimanite and garnet+sillimanite+hercynite with increasing temperature; and (6) in sheared and foliated rocks, hydrous minerals such as biotite, muscovite and hornblende form a foliation, modifying pre-existing textures. The inferred metamorphic history of the Leverburgh Belt is divided into four stages, as follows: (M1) prograde metamorphism with increasing temperature; (M2) prograde metamorphism with increasing pressure; (M3) retrograde decompressional metamorphism with decreasing pressure and temperature; and (M4) retrograde metamorphism accompanied by shearing. Peak P–T conditions of the M2 stage are 800±30 °C, 13–14 kbar. Pressure increasing from M1 to M2 suggests thrusting of continental crust over the South Harris belt during continent–continent collision. The inferred P–T path and tectonic history of the South Harris belt are different from those of the Lewisian of the mainland.  相似文献   

2.
The Winding Stair Gap in the Central Blue Ridge province exposes granulite facies schists, gneisses, granofelses and migmatites characterized by the mineral assemblages: garnet–biotite–sillimanite–plagioclase–quartz, garnet–hornblende–biotite–plagioclase–quartz ± orthopyroxene ± clinopyroxene and orthopyroxene–biotite–quartz. Multiple textural populations of biotite, kyanite and sillimanite in pelitic schists support a polymetamorphic history characterized by an early clockwise P–T path in which dehydration melting of muscovite took place in the stability field of kyanite. Continued heating led to dehydration melting of biotite until peak conditions of 850 ± 30 °C, 9 ± 1 kbar were reached. After equilibrating at peak temperatures, the rocks underwent a stage of near isobaric cooling during which hydrous melt ± K‐feldspar were replaced by muscovite, and garnet by sillimanite + biotite + plagioclase. Most monazite crystals from a pelitic schist display patchy zoning for Th, Y and U, with some matrix crystals having as many as five compositional zones. A few monazite inclusions in garnet, as well as Y‐rich cores of some monazite matrix crystals, yield the oldest dates of c. 500 Ma, whereas a few homogeneous matrix monazites that grew in the main foliation plane yield dates of 370–330 Ma. Culling and analysis of individual spot dates for eight monazite grains yields three age populations of 509 ± 14 Ma, 438 ± 5 Ma and 360 ± 5 Ma. These data suggest that peak‐temperature metamorphism and partial melting in the central Blue Ridge occurred during the Salinic or Taconic orogeny. Following near isobaric cooling, a second weaker thermal pulse possibly related to intrusion of nearby igneous bodies resulted in growth of monazite c. 360 Ma, coinciding with the Neoacadian orogeny.  相似文献   

3.
浙西南八都杂岩早中生代泥质麻粒岩变质作用及构造意义   总被引：1，自引：0，他引：1  
遂昌-大柘泥质麻粒岩出露于华夏地块东北部的浙西南八都杂岩中,该岩石保留了典型的减压反应结构.但其变质演化特点、变质作用时代及构造意义目前尚不明确.通过系统的岩相学、矿物化学和同位素年代学分析,结果表明遂昌-大柘泥质麻粒岩记录了4个阶段的变质矿物组合,其中早期进变质阶段M1的矿物组合为石榴石+黑云母+石英;压力峰期变质阶段M2的矿物组合为石榴石+铝绿泥石+金红石+蓝晶石+刚玉+黑云母+石英±十字石,该矿物组合可能预示着岩石曾经历了超高压变质作用过程;峰期变质阶段M3的矿物组合为石榴石+黑云母+夕线石+石英±钾长石±斜长石±钛铁矿;峰后近等温降压M4-1阶段的矿物组合为石榴石+黑云母+夕线石+堇青石+石英+钛铁矿±尖晶石±斜长石±钾长石;M4-2阶段的矿物组合为石榴石+堇青石+夕线石+斜长石+黑云母+石英±钾长石.相平衡模拟结合传统地质温压计限定其峰期变质阶段的温压条件为T=780~810 ℃、P=8.0~9.2 kbar;峰期后近等温降压的M4-1阶段的温压条件为T=780~860 ℃和P=5.7~6.0 kbar,M4-2阶段的温压条件为T=~700 ℃和P=~4.4 kbar,具有典型的顺时针近等温减压型P-T轨迹特征.LA-ICP-MS U-Pb定年结果表明其麻粒岩相变质作用时代为233.5~238.9 Ma.变质作用历史说明浙西南地体可能卷入了古特提斯洋域内印支-华南-华北板块之间的俯冲-碰撞过程,并经历了早中生代的麻粒岩相变质作用后快速折返至地表.   相似文献   

4.
The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages(M1) are mineral inclusions(biotite + plagioclase + quartz ± sillimanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages(M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals(plagioclase + quartz ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages(M3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C at 5.5–6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression(ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.  相似文献   

5.
Ailaoshan orogenic belt located at the northeastern margin of the Indochina block, southeastern Tibet, was formed by subduction and collision between the Indochina and South China blocks in Triassic and slip shearing resulted from the extrusion of the Indochina block in Cenozoic. The high‐pressure pelitic granulite is located at the southeastern margin of the Ailaoshan metamorphic belt, occurs as a slice of about 500～700m in thickness, consists of garnet, sillimanite, feldspar, biotite and quartz with accessory of kyanite, sapphirine, spinel, rutile, ilmenite, zircon and apatite. The petrography and mineral chemistry show that the high‐pressure pelitic granulite had suffered three stages of metamorphism: 1) the prograde metamorphism recorded by the mineral assemblage of garnet, kyanite, feldspar, biotite and rutile; 2) the peak metamorphism shown by the mineral assemblage of garnet, sillimanite, sapphirine, ternary feldspar, K‐feldspar, plagioclase, biotite, spinel, quartz, rutile and zircon mantle; 3) the retrograde metamorphism recorded by the mineral assemblage of biotite, muscovite, plagioclase, quartz and zircon rim. Zircon SHRIMP U‐Pb dating indicates that the protolith of the pelite granulite was deposited before 336 Ma, the prograde to peak metamorphism occurred at P‐T conditions of ≥10.4 kbar at 850～919 °C in 235 Ma, and the retrograde metamorphism occurred at the P‐T condition of 3.5～3.9 kbar at 572～576 °C until to 33 Ma. They are consistent with the times of Indochina separated from Gondwanaland during late Paleozoic, the amalgamation of the south China and Indochina blocks during the Triassic, and the sinistral slip‐shearing since the Early Cenozoic respectively. It is inferred that that the sedimentary rock was subducted to the lower continental crust (30 km) and suffered granulite‐facies metamorphism due to the collision during Indosinian, then exhumed quickly to middle‐upper crust (10–12km) and superimposed retrograde metamorphism since the Cenozoic.  相似文献   

6.
The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ～530 °C and 4.6 kbar during the N–S‐shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ～530 °C and 4.6 kbar to ～630 °C and 6.2 kbar. The overprinting of garnet‐, staurolite‐ and kyanite‐bearing mineral assemblages by low‐pressure andalusite and cordierite assemblages implies ～4‐kbar decompression during Early Devonian exhumation of the Greenvale Province.  相似文献   

7.
The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre-Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.
(a) A medium- to low- P granulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K-feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.
(b) A post-granulite re-equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.
(c) An eo-Alpine eclogite stage giving rise to the crystallization of high- P and low- T assemblages.
In an effort to quantify this evolution, independent well-calibrated thermobarometers were applied to basic and acid rocks. For the granulite event, P-T estimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution, P-T was 4–5 kbar and 600° C. For the eo-Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.
The inferred P-T path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre-Alpine granulite and post-granulite retrograde evolution to the Permo-Jurassic extensional regime. The complex granulite-eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.  相似文献   

8.
Petrology of high-pressure granulites from the eastern Himalayan syntaxis   总被引：36，自引：0，他引：36  
The eastern Himalayan syntaxis, situated at the eastern terminus of the Himalayas, is the least-known segment of the Himalayas. Recent research in this area has revealed that the syntaxis consists of the Gangdise, the Yarlung Zangbo, and the Himalayan units, each of which is bounded by faults. The Himalayan unit, the northernmost exposed part of the Indian plate, mainly contains amphibolite facies rocks, marked by the assemblages staurolite+kyanite+plagioclase+biotite+muscovite±sillimanite and garnet+amphibole+plagioclase, in the south; to the north, low- to medium-pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet+sillimanite+K-feldspar+plagioclase or antiperthite+biotite+quartz±spinel±cordierite±orthopyroxene in gneisses, and anorthite+diopside±wollastonite and plagioclase+diopside+quartz+phlogopite+calcite in marbles. Within this unit, the Namula thrust system is a series of moderately north-dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks. High-pressure granulites occur as relics within these granulite facies rocks and contain garnet–kyanite granulite and garnet clinopyroxenite. The peak assemblage of the garnet–kyanite granulite includes garnet (core part)+kyanite+ternary feldspar+quartz+rutile. Sillimanite+garnet (rim part)+K-feldspar+ oligoclase+ilmenite+biotite and spinel+albite+biotite or spinel+cordierite±orthopyroxene, which are coronas around sillimanite and garnet, are retrograde products of this peak assemblage. Another peak assemblage includes very-high-Ca garnet (CaO 32–34 wt%, Alm10±Grs>80) and diopside (CaO 22–24 wt%), scapolite, meionite, quartz, and accessory Al-bearing titanite (Al2O3 4–4.5 wt%). The diopside has kink bands. Partial or complete breakdown of Ca-rich garnet during post-peak metamorphism produced pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of hedenbergite and anorthite. Thermobarometric estimates in combination with reaction textures, mineral compositions, and recent experimental studies indicate that these peak assemblages were formed at P=c. 1.7–1.8 GPa, T =c. 890 °C, and the retrograde assemblages experienced near-isothermal decompression to P=0.5±0.1 GPa, T =850±50 °C. The whole-rock compositions indicate that marble and pelite are plausible candidates for the protoliths. These facts suggest the following (1) sedimentary rocks were transported to upper-mantle depths and equilibrated at those conditions to form these high-pressure granulites, which were then emplaced into the crust quickly. During the rapid exhumation of these rocks, the earlier high-pressure assemblages were overprinted by the later low- to medium-pressure assemblages, that is, the high-pressure granulite belt formed in the syntaxis. (2) The Namula thrust system is an important tectonic boundary in the syntaxis, or even in the Higher Himalaya more generally.  相似文献   

9.
Interpretations based on quantitative phase diagrams in the system CaO–Na2O–K2O–TiO2–MnO–FeO–MgO–Al2O3–SiO2–H2O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).  相似文献   

10.
吉南新太古代泥质片麻岩出露于龙岗岩群四道砬子河岩组中,本文通过系统的岩相学、矿物化学、激光拉曼和锆石U Pb同位素年代学等分析,研究其变质演化特点、变质作用时代及构造意义.结果表明:龙岗岩群四道砬子河岩组泥质片麻岩记录了３个变质演化阶段,其中峰前期进变质阶段(M１)的矿物组合为石榴石＋黑云母＋斜长石＋石英＋白云母;峰期变质阶段(M２)的矿物组合为石榴石＋矽线石＋钾长石＋黑云母＋斜长石＋石英,达到麻粒岩相;峰后期退变质阶段(M３)以矽线石转变为蓝晶石为特征标 志,矿物组合为蓝晶石＋石榴石＋黑云母＋斜长石＋石英＋白云母.变质矿物地质温压计限定其变质作用,峰前期(M１)T 为４６８~５１５ ℃,p 为(３．８~４．３)×１０５ kPa;峰期(M２)T 为７０３~７６０ ℃,p 为(６．６~７．１)×１０５kPa;峰后期(M３)T 为５５２~５９１℃,p 为(５．５~６．０)×１０５kPa;具有典型的近等压冷却型逆时针变质作用p T 演化轨迹特征,可能是在地幔柱与岩石圈相互作用的环境中变质作用与大量的幔源岩浆底侵作用有关.LA ICP MS锆石U Pb定年结果显示麻粒岩相变质作用时代为２４９５~２４４２Ma,属于新太古代变质热事件产物.  相似文献   

11.
The staurolite–biotite–garnet–cordierite–andalusite–plagioclase–muscovite–quartz metapelitic mineral assemblage has been frequently interpreted in the literature as a result of superimposition of various metamorphic events, for example, in polymetamorphic sequences. The assemblage was identified in schists from the Ancasti metamorphic complex (Sierras Pampeanas of Argentina) where previous authors have favoured the polymetamorphic genetic interpretation. A pseudosection in the MnNCKFMASH system for the analysed XRF bulk composition predicts the stability of the sub‐assemblage staurolite–biotite–garnet–plagioclase–muscovite–quartz, and the compositional isopleths also agree with measured mineral compositions. Nevertheless, the XRF pseudosection does not predict any field with staurolite, andalusite and cordierite being stable together. As a result of more detailed modelling making use of the effective bulk composition concept, our interpretation is that the staurolite–biotite–garnet–plagioclase–muscovite–quartz sub‐assemblage was present at peak metamorphic conditions, 590 °C and 5.2 kbar, but that andalusite and cordierite grew later along a continuous P–T path. These minerals are not in mutual contact and are observed in separate microstructural domains with different proportions of staurolite. These domains are explained as a result of local reaction equilibrium subsystems developed during decompression and influenced by the previous peak crystal size and local modal distribution of staurolite porphyroblasts that have remained metastable. Thus, andalusite and cordierite grew synchronously, although in separate microdomains, and represent the decompression stage at 565 °C and 3.5 kbar.  相似文献   

12.
Abstract The central sector of Mühlig-Hofmannfjellet (3°E/71°S) in western Dronning Maud Land (East Antarctic shield) is dominated by large intrusive bodies of predominantly orthopyroxene-bearing quartz syenites (charnockites). Metasedimentary rocks are rare; however, two distinct areas with banded gneiss–marble–quartzite sequences of sedimentary origin were found during the Norwegian Antarctic Research Expedition NARE 1989/90. Cordierite-bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage: cordierite + garnet + biotite + K-feldspar + plagioclase + quartz ± sillimanite ± spinel. Thermobarometry indicates equilibration conditions of about 650°C and 4 kbar. Associated orthopyroxene–garnet granulites, on the other hand, revealed pressures of about 8 kbar and temperatures of 750°C. The earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess K-feldspar combined with interaction with an H2O-rich fluid phase, probably released by the cooling intrusives. These two features allowed the original high-grade K-feldspar + garnet assemblages to recrystallize as cordierite–biotite–sillimanite gneisses, completely re-equilibrating them. Phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in composition between the lithologies.  相似文献   

13.
The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone II–III boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium  相似文献   

14.
The Precambrian of Madagascar appears to be a part of the Mozambiquian mobile belt. Some authors consider the southern part of the island to be constituted, from east to west, by more and more recent formations, i.e., the ultrametamorphic ‘Androyan’ sequence with an Archean reworked crust as the oldest one, first followed by the ‘Graphite’ sequence, then by the ‘Vohibory’ sequence. The cordierite and garnet banded gneisses and leptynites are widely distributed in the Androyan sequence. These gneisses result from an incipient partial melting of paragneisses. Garnet—plagioclase, garnet—cordierite and garnet—biotite geothermo- and geobarometers allow the estimation of the conditions of this migmatisation (T > 700°C, PT = 5?5.5 kbar, PH2O = 0.3?0.4 PT). Some cordierite and garnet leptynites represent the leucosomes of this anatectic event. Since previous experiments showed that the seven-phase assemblage (quartz + plagioclase + K feldspar + garnet + biotite + sillimanite + cordierite) only occurs in a limited range of pressure and temperature (T = 690?710°C, PT = 4?5.7 kbar, PH2O = 0.2?0.5 PT), we can consider these gneisses as very precise thermobarometric reference levels in southern Madagascar. This migmatic event may be contemporary with the Pan-African orogenesis.  相似文献   

15.
Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton (NCC). The Huai'an terrane, located within the northern segment of the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional belt in the central NCC expose mafic and pelitic granulites as well as TTG (tonalite-trondhjemite-granodiorite) gneisses. Here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M1) is recorded by relict biotite and the compositional profile of Xca (grt) isopleths. The Pmax (M2) is distinguished by the Xca (grt) isopleths, which corresponds to the kyanite stable area with an inclusion mineral assemblage of Grt-c–(Ky)-Qz-Rt-Kfs-liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 °C. However, kyanite is absent in thin sections suggesting its consumption during later stages. The Tmax metamorphism (M3) is characterized by the assemblage: Grt-m-Qz-Pl-Rt-Kfs-Sil-liq in the garnet mantle and also reflected in the compositional profile. Two-feldspar geothermometry yields a P-T range of 940 °C–950 °C and 9.5–10.5 kbar, indicating ultra-high temperature (UHT) metamorphic overprinting. The subsequent retrograde metamorphic stage (M4) is characterized by Grt-r-Bt-Sil-Kfs-Pl-Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770 °C–840 °C and 6.5–8 kbar. The pelitic granulites show a clockwise path, with P-T estimates higher than those in estimated in previous studies using conventional techniques.LA-ICP-MS U–Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1 Ma and 1873.3 ± 9.9 Ma. We suggest that the protoliths of the Manjinggou HP-UHT granulites were deep subducted where they experienced HP metamorphism associated with the collision of the Ordos and Yinshan blocks at ca. 1.97 Ga. Subsequently, the UHT metamorphic overprint occurred during the assembly of the unified Western and Eastern Blocks of the NCC along the TNCO at ca. 1.87 Ga.  相似文献   

16.
Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H2O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H2O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.  相似文献   

17.
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   

18.
The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.  相似文献   

19.
The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.  相似文献   

20.
Representative diamond-bearing gneisses and dolomitic marble, eclogite and Ti-clinohumite-bearing garnet peridotite from Unit I at Kumdy Kol and whiteschist from Unit II at Kulet, eastern Kokchetav Massif, northern Kazakhstan, were studied. Diamond-bearing gneisses contain variable assemblages, including Grt+Bt+Qtz±Pl±Kfs±Zo±Chl±Tur±Cal and minor Ap, Rt and Zrn; abundant inclusions of diamond, graphite+chlorite (or calcite), phengite, clinopyroxene, K-feldspar, biotite, rutile, titanite, calcite and zircon occur in garnet. Diamond-bearing dolomitic marbles consist of Dol+Di±Grt+Phl; inclusions of diamond, dolomite±graphite, biotite, and clinopyroxene were identified in garnet. Whiteschists carry the assemblage Ky+Tlc+Grt+Rt; garnet shows compositional zoning, and contains abundant inclusions of talc, kyanite and rutile with minor phlogopite, chlorite, margarite and zoisite. Inclusions and zoning patterns of garnet delineate the prograde P–T path. Inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss. Other ultrahigh-pressure (UHP) indicators include Na-bearing garnet (up to 0.14 wt% Na2O) with omphacitic Cpx in eclogite, occurrence of high-K diopside (up to 1.56 wt% K2O) and phlogopite in diamond-bearing dolomitic marble, and Cr-bearing kyanite in whiteschist. These UHP rocks exhibit at least three stages of metamorphic recrystallization. The Fe-Mg partitioning between clinopyroxene and garnet yields a peak temperature of 800–1000 °C at P >40 kbar for diamond-bearing rocks, and about 740–780 °C at >28–35 kbar for eclogite, whiteschist and Ti-bearing garnet peridotite. The formation of symplectitic plagioclase+amphibole after clinopyroxene, and replacement of garnet by biotite, amphibole, or plagioclase mark retrograde amphibolite facies recrystallization at 650–680 °C and pressure less than about 10 kbar. The exsolution of calcite from dolomite, and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c. 420 °C and 2–3 kbar. A clockwise P–T path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to UHP subduction-zone metamorphism within the diamond stability field. Tectonic mixing may have occurred prior to UHP metamorphism at mantle depths. During subsequent exhumation and juxtaposition of many other tectonic units, intense deformation chaotically mixed and mylonitized these lithotectonic assemblages.  相似文献   

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