Formation Mechanism and Exhumation Processes for HP-UHP Metamorphic Rocks in Dabie Mountains     

HouMingjin TangJiafu 《中国地质大学学报(英文版)》2004,15(2):193-205

The high, ultrahigh pressure metamorphic rocks, widely distributed in Dabie Mountains,were described in terms of the geological setting, the marks of the petrology and the mineralogy of the ultrahigh pressure (UHP) metamorphic rocks. According to the estimated uplifting and denudation of the Dabie Mountains, and to the thermodynamics theory, were assessed the depth and pressure (high pressure autoclave) of the formation setting of the UI-IP metamorphic rocks. Based on all the information mentioned above, a new explanation is derived from the mechanism of formation and the processes of exhumation of the UI-IP metamorphic rocks.  相似文献   

Metamorphism and deformation of blueschist belts and their tectonic implications, North Qilian Mountains, China   总被引：25，自引：1，他引：25  

WU HANQUAN  FENG YIMIN  SONG SUGUANG 《Journal of Metamorphic Geology》1993,11(4):523-536

Abstract Two blueschist belts in the North Qilian Mountains occur in Middle Cambrian and Lower Ordovician strata and strike N30–35°W for about 500 km along the Caledonian fold belt on the south-west margin of the Sino-Korean plate. The styles of metamorphism and deformation are quite different in the two belts. The Middle Cambrian to Ordovician rocks in the high-grade belt are mainly blueschists and C-type eclogites in which six phases of lower and upper crustal deformation have been recognized. The rocks contain glaucophane, phengite, epidote, clinozoisite, chlorite, garnet, stilpnomelane, piedmontite, albite, titanite and quartz. The estimated P–T conditions of eclogites are 340 ± 10°C, 8 ± 1 kbar and, of blueschist, >380°C, 6–7 kbar. The Ordovician rocks in the low-grade belt are characterized by ductile to brittle deformation in the middle to upper crust. The low-grade blueschists contain glaucophane, lawsonite, pumpellyite, aragonite, albite and chlorite. The estimated P–T conditions are 150–250°C and 4–7 kbar.
K–Ar and ³⁹Ar/⁴⁰Ar geochronology on glaucophane and phengite from the high-grade blueschist belt suggest two stages of metamorphism at 460–440 and 400–380 Ma, which may represent the times of subduction and orogeny. The subduction metamorphism of the northern low-grade blueschist belt took place approximately at the end of the Ordovician.  相似文献   

滇西云龙漕涧地区高压片麻岩的发现及厘定     

黄亮  田素梅  张虎  熊波  王晓林  浦涛  杨仕潘  方雄 《岩石矿物学杂志》2021,40(4):747-756

笔者在滇西云龙县漕涧新发现了一套以黑云斜长片麻岩为主的变泥质岩石,其呈构造透镜体状产出,出露面积极小,中高级变质矿物发育,与围岩特征明显不同。岩相学研究表明,片麻岩保留有明显的俯冲-折返的矿物学记录,矿物世代具顺时针变质演化特征,其中还残存有蓝晶石+铝直闪石的共生矿物组合,对应的变质压力达1.0GPa以上。通过GBP平均温压计法计算得到岩石的变质温压范围为566～623℃和0.36～0.53 GPa;此外,片麻岩中还有少量的多硅白云母矿物,根据相平衡模拟p-T视剖面图及其硅原子数最大值为3.23的特点,推断其属蓝片岩类。综合研究认为,黑云斜长片麻岩属于高压变质岩(峰期变质压力最小约为1.60 GPa),为昌宁-孟连结合带的北段寻找高压变质岩提供了新的信息,并表明昌宁-孟连结合带为一条连续的高压-超高压变质带。  相似文献   

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

R. A. Cliff  C. E. Bond  R. W. H. Butler  J. E. Dixon 《Journal of Metamorphic Geology》2017,35(2):197-211

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

牡丹江地区黑龙江杂岩的变质变形特征   总被引：4，自引：0，他引：4  

王跃  张兴洲  宋海峰  张春艳  熊小松 《吉林大学学报(地球科学版)》2009,39(6):1066-1072

黑龙江牡丹江地区出露一套具有洋壳性质的构造混杂岩,根据特征矿物组合及岩相学特点,将这套杂岩自北向南划分为3个变质变形相带：蓝片岩带、黑硬绿泥石带和黑云母带,带与带之间被强烈的韧性变形带所分隔,表明彼此间为构造接触,而非连续的变质相带,说明了牡丹江地区的黑龙江杂岩不是正常的变质地层单元。3个相带的出露与自北向南的逆冲推覆事件有关。根据黑龙江杂岩变质变形特征和其年代学资料,将本区构造演化划分为3个阶段: (1)305~296 Ma之前的陆间洋洋壳俯冲及闭合阶段;(2)170.26~154 Ma佳木斯和兴凯地块之间的陆陆碰撞和后期作为整体统一受西太平洋构造域影响的阶段;(3)154 Ma之后敦密断裂左行走滑并对黑龙江杂岩进行改造的阶段。  相似文献   

Blueschist-facies metamorphism during Paleozoic orogeny in southwestern Japan: Phengite K–Ar ages of blueschist-facies tectonic blocks in a serpentinite melange beneath early Paleozoic Oeyama ophiolite     

Tatsuki Tsujimori  & Tetsumaru Itaya 《Island Arc》1999,8(2):190-205

Blueschist-bearing Osayama serpentinite melange develops beneath a peridotite body of the Oeyama ophiolite which occupies the highest position structurally in the central Chugoku Mountains. The blueschist-facies tectonic blocks within the serpentinite melange are divided into the lawsonite–pumpellyite grade, lower epidote grade and higher epidote grade by the mineral assemblages of basic schists. The higher epidote-grade block is a garnet–glaucophane schist including eclogite-facies relic minerals and retrogressive lawsonite–pumpellyite-grade minerals. Gabbroic blocks derived from the Oeyama ophiolite are also enclosed as tectonic blocks in the serpentinite matrix and have experienced a blueschist metamorphism together with the other blueschist blocks. The mineralogic and paragenetic features of the Osayama blueschists are compatible with a hypothesis that they were derived from a coherent blueschist-facies metamorphic sequence, formed in a subduction zone with a low geothermal gradient (~ 10°C/km). Phengite K–Ar ages of 16 pelitic and one basic schists yield 289–327 Ma and concentrate around 320 Ma regardless of protolith and metamorphic grade, suggesting quick exhumation of the schists at ca 320 Ma. These petrologic and geochronologic features suggest that the Osayama blueschists comprise a low-grade portion of the Carboniferous Renge metamorphic belt. The Osayama blueschists indicate that the 'cold' subduction type (Franciscan type) metamorphism to reach eclogite-facies and subsequent quick exhumation took place in the northwestern Pacific margin in Carboniferous time, like some other circum-Pacific orogenic belts (western USA and eastern Australia), where such subduction metamorphism already started as early as the Ordovician.  相似文献   

Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction     

Nanfei Cheng  David M. Jenkins 《Journal of Metamorphic Geology》2021,39(1):39-56

The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane+paragonite to jadeite+pyrope+quartz+H₂O over the range of 2.2–3.5 GPa and 650–820°C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane+paragonite was observed at the intersectional stability field of both paragonite and jadeite+quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O. The second set of experiments involved unreversed experiments using glaucophane+zoisite ±quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780°C. These produced omphacite+paragonite/kyanite+H₂O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium–calcium amphiboles such as winchite (?(CaNa)(Mg₄Al)Si₈O₂₂(OH)₂) and barroisite (?(CaNa)(Mg₃Al₂)(AlSi₇)O₂₂(OH)₂). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane+zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H₂O. It also indicates that sodium–calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850°C involving a high‐Fe (Fe#=Fe_total/(Fe_total+Mg)≈0.36) natural glaucophane, synthetic paragonite and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet+omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane+zoisite is instrumental in controlling dehydration processes at the blueschist–eclogite transition during warm subduction.  相似文献   

Subduction and accumulation of lawsonite eclogite and garnet blueschist in eastern Australia     

Renée Tamblyn  Martin Hand  David Kelsey  Robert Anczkiewicz  David Och 《Journal of Metamorphic Geology》2020,38(2):157-182

Lawsonite eclogite and garnet blueschist occur as metre-scale blocks within serpentinite mélange in the southern New England Orogen (SNEO) in eastern Australia. These high-P fragments are the products of early Palaeozoic subduction of the palaeo-Pacific plate beneath East Gondwana. Lu–Hf, Sm–Nd, and U–Pb geochronological data from Port Macquarie show that eclogite mineral assemblages formed between c. 500 and 470 Ma ago and became mixed together within a serpentinite-filled subduction channel. Age data and P–T modelling indicate lawsonite eclogite formed at ~2.7 GPa and 590°C at c. 490 Ma, whereas peak garnet in blueschist formed at ~2.0 GPa and 550°C at c. 470 Ma. The post-peak evolution of lawsonite eclogite was associated with the preservation of pristine lawsonite-bearing assemblages and the formation of glaucophane. By contrast, the garnet blueschist was derived from a precursor garnet–omphacite assemblage. The geochronological data from these different aged high-P assemblages indicate the high-P rocks were formed during subduction on the margin of cratonic Australia during the Cambro-Ordovician. The rocks however now reside in the Devonian–Carboniferous southern SNEO, which forms the youngest and most outboard of the eastern Gondwanan Australian orogenic belts. Geodynamic modelling suggests that over the time-scales that subduction products accumulated, the high-P rocks migrated large distances (~>1,000 km) during slab retreat. Consequently, high-P rocks that are trapped in subduction channels may also migrate large distances prior to exhumation, potentially becoming incorporated into younger orogenic belts whose evolution is not directly related to the formation of the exhumed high-P rocks.  相似文献   

Exhumation of eclogite and blueschist (Cyclades,Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling          下载免费PDF全文

Valentin Laurent  Pierre Lanari  Inès Naïr  Romain Augier  Abdeltif Lahfid  Laurent Jolivet 《Journal of Metamorphic Geology》2018,36(6):769-798

High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.  相似文献   

Blueschist‐facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high‐pressure assemblages     

A. LÓPEZ‐CARMONA  P. PITRA  J. ABATI 《Journal of Metamorphic Geology》2013,31(3):263-280

The Malpica–Tui Unit (Galicia, NW Spain) records eclogite‐ and blueschist‐facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using thermocalc shows that the Upper Sheet of this unit, the Ceán Schists, recorded a three‐stage metamorphic evolution involving (i) Early subduction‐related medium‐pressure/low‐temperature metamorphism (M₁) constrained at ～350–380 °C, 12–14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Ceán Schists. (ii) Subduction‐related blueschist facies prograde metamorphism (M₂) going from ～19 kbar, 420 °C to 21 kbar, 460 °C in the LM, and from 16 kbar 430 °C to 21–22 kbar, 520 °C in the structurally upper metapelites (UM). (iii) Exhumation‐related metamorphism (M₃) is characterized by a decompression to 8–10 kbar, 470–490 °C in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P–T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluid‐undersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P–T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe₂O₃ may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P–T–X(Fe₂O₃) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P–T conditions.  相似文献