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1.
 右江盆地三叠纪槽盆相浊流沉积岩系遭受过区域极低级变质作用。依据地质观察和伊利石结晶度、绿泥石-云母堆垛集合体、标志性粘土矿物及白云母(伊利石)b0参数测定资料,阐述了泥质岩石的成岩变质作用经浅层(近)变质作用到浅变质作用的转换特征。变质温度区间为150-350℃,低压类型,具高地温梯度(40-43℃/km).变质级及亚带总体与地层时代及岩层在地层柱中的位置有耦合关系,而与区域变形强度无关。最后指出区域极低级变质作用是印支-燕山构造旋回早期及区域变形前的地质事件,属于地壳伸展构造背景下右江边缘型盆地内部的埋藏型变质作用。  相似文献   

2.
The Sanbagawa metamorphic belt of southwest Japan is one of the type localities of subduction‐related high‐P metamorphism. However, variable pressure–temperature (P–T) paths and metabasic assemblages have been reported for eclogite units in the region, leading to uncertainty about the subduction zone paleo‐thermal structure and associated tectonometamorphic conditions. To analyse this variation, phase equilibria modelling was applied to the three main high‐P metabasic rock types documented in the region – glaucophane eclogite, barroisite eclogite and garnet blueschist – with modelling performed over a range of P, T, bulk rock H2O and bulk rock ferric iron conditions using thermocalc . All samples are calculated to share a common steep prograde P–T path to similar peak conditions of ～16–20 kbar and 560–610 °C. The results establish that regional assemblage variation is systematic, with the alternation in peak amphibole phase due to peak conditions overlapping the glaucophane–barroisite solvus, and bulk composition effects stabilizing blueschist v. eclogite facies assemblages at similar P–T conditions. Furthermore, the results reveal that a steep prograde P–T path is common to all eclogite units in the Sanbagawa belt, indicating that metamorphic conditions were consistent along strike. All localities are compatible with predictions made by a ridge approach model, which attributes eclogite facies metamorphism and exhumation of the Sanbagawa belt to the approach of a spreading ridge.  相似文献   

3.
The early Palaeozoic South Qilian–North Qaidam orogenic belt in northwestern China records a nearly complete history of early‐stage long‐lived oceanic subduction–accretion followed by late‐stage continental collision. Most previous studies have focused on low dT/dP metamorphism (HP–UHP) in this belt whereas the paired high dT/dP belt in the hinterland has received little attention. In this contribution, phase equilibrium modelling is combined with zircon petrochronology to determine the P–T–t evolution of granulites in the North Wulan gneiss complex in the high dT/dP hinterland of the South Qilian–North Qaidam orogen. Granulites record a clockwise P–T path with near‐peak temperatures of ~800–900°C at 5.5–7 kbar. Peak metamorphism was followed by high‐T decompression. Zircon petrochronology reveals protracted zircon growth from c. 474 to 446 Ma during the high‐T portion of the P–T path. High dT/dP metamorphism in the North Wulan gneiss complex was likely the result of heat transfer from the underlying hot asthenosphere and minor coeval magmatism in an arc–back‐arc system during slab retreat and roll‐back of the South Qilian oceanic plate. Broadly contemporaneous but slightly younger HP–UHP metamorphism in the foreland of the South Qilian–North Qaidam orogenic belt indicates that the region records an early Palaeozoic paired metamorphic belt. This early Palaeozoic paired metamorphic belt provides a detailed example of dual thermal regimes in a modern‐style orogenic system that can be applied to understanding the time‐scales and P–T conditions of high dT/dP metamorphism that accompany subduction in Phanerozoic and Precambrian orogenic belts.  相似文献   

4.
The Cordilleran orogen in south-eastern Alaska includes 14 distinct metamorphic belts that make up three major metamorphic complexes, from east to west: the Coast plutonic–metamorphic complex in the Coast Mountains; the Glacier Bay–Chichagof plutonic–metamorphic complex in the central part of the Alexander Archipelago; and the Chugach plutonic–metamorphic complex in the northern outer islands. Each of these complexes is related to a major subduction event. The metamorphic history of the Coast plutonic–metamorphic complex is lengthy and is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Stikine terrane to the east. The metamorphic history of the Glacier Bay–Chichagof plutonic–metamorphic complex is relatively simple and is related to the roots of a Late Jurassic to late Early Cretaceous island arc. The metamorphic history of the Chugach plutonic–metamorphic complex is complicated and developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes. The Coast plutonic–metamorphic complex records both dynamothermal and regional contact metamorphic events related to widespread plutonism within several juxtaposed terranes. Widespread moderate-P/T dynamothermal metamorphism affected most of this complex during the early Late Cretaceous, and local high-P/T metamorphism affected some parts during the middle Late Cretaceous. These events were contemporaneous with low- to moderate-P, high-T metamorphism elsewhere in the complex. Finally, widespread high-P–T conditions affected most of the western part of the complex in a culminating late Late Cretaceous event. The eastern part of the complex contains an older, pre-Late Triassic metamorphic belt that has been locally overprinted by a widespread middle Tertiary thermal event. The Glacier Bay–Chichagof plutonic–metamorphic complex records dominantly regional contact-metamorphic events that affected rocks of the Alexander and Wrangellia terranes. Widespread low-P, high-T assemblages occur adjacent to regionally extensive foliated granitic, dioritic and gabbroic rocks. Two closely related plutonic events are recognized, one of Late Jurassic age and another of late Early and early Late Cretaceous age; the associated metamorphic events are indistinguishable. A small Late Devonian or Early Mississippian dynamothermal belt occurs just north-east of the complex. Two older low-grade regional metamorphic belts on strike with the complex to the south are related to a Cambrian to Ordovician orogeny and to a widespread Middle Silurian to Early Devonian orogeny. The Chugach plutonic–metamorphic complex records a widespread late Late Cretaceous low- to medium/high-P, moderate- T metamorphic event and a local transitional or superposed early Tertiary low-P, high-T regional metamorphic event associated with mesozonal granitic intrusions that affected regionally deformed and metamorphosed rocks of the Chugach terrane. The Chugach complex also includes a post-Late Triassic to pre-Late Jurassic belt with uncertain relations to the younger belts.  相似文献   

5.
Garnet‐bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi‐akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk‐rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P–T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell‐shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P–T conditions using core and rim compositions of orthopyroxene are 1.5–2.4 GPa/700–800 °C and 2.9–3.8 GPa/700–810 °C, respectively, implying a high P/T gradient (> 3.1 GPa/100 °C) during prograde metamorphism. The presence of relatively low P–T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P–T path that shows increasing P/T with higher T, similar to P–T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab–wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP–UHP prograde metamorphism.  相似文献   

6.
北祁连早古生代大洋俯冲带以发育早古生代的蛇绿（混杂）岩带及高压低温变质带为特征。在清水沟高压低温混杂岩片中出露蛇纹岩以及榴辉岩、蓝片岩、红帘石硅质岩和多硅白云母石英片岩等典型高压低温变质岩石。本文对北祁连清水沟含红帘石的变硅质岩进行了详细的岩相学、矿物化学及地球化学分析,发现该岩石主要由石英、多硅白云母、红帘石、石榴子石、蓝闪石、单斜辉石以及砷硅锰矿、赤铁矿等矿物组成,结合矿物化学及围岩变质条件,推测该岩石可能也经历了高压低温榴辉岩相变质作用。全岩地球化学分析结果表明,北祁连清水沟含红帘石变硅质岩的原岩为远洋环境沉积的含泥硅质岩,由于热液活动的参与,使得Fe、Mn富集沉积,进而与大陆活动边缘或大陆岛弧物质被卷入到俯冲带中,共同经历了高压低温变质作用。变硅质岩中红帘石、砷硅锰矿以及石榴子石中大量的赤铁矿包裹体表明该岩石形成于高氧逸度条件,而石榴子石中的Fe3+从核部到边部的降低趋势,也表明俯冲变质过程中氧逸度的变化,这一过程释放的氧所形成的流体对于探究岩石圈地幔氧逸度变化、岛弧岩浆生成以及俯冲带氧循环等方面具有重要的意义。  相似文献   

7.
新疆阿尔泰阿祖拜地区发育两种类型区域变质作用,第一种为区域低温动力变质作用,第二种为区域动力热流变质作用.阿祖拜地区递增变质带是后者的典型代表之一,从花岗岩体外接触带依次发育矽线石-堇青带、红柱石-十字石带、黑云母-石榴石带,递增变质作用强度呈逐渐减弱趋势.结合变形变质作用特征和同位素年龄,可识别出3个变形变质阶段.早期阶段以区域低温动力变质作用为主,形成绿片岩相变质岩石组合;峰期阶段为区域动力热流变质作用,形成典型的递增变质带;晚期阶段则属退化变质作用.不同变质作用类型代表了不同大地构造环境,记录了造山带演化历史年和动力学过程.  相似文献   

8.
Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.  相似文献   

9.
A structural, metamorphic and geochronological study of the Staré Město belt implies the existence of two distinct metamorphic events of similar peak P–T conditions (700–800 °C, 8–10 kbar) during the Cambro‐Ordovician and the Carboniferous tectonometamorphic events. The hypothesis of two distinct periods of metamorphism was suggested on the basis of structural discordance between an undoubtedly Carboniferous granodiorite sill intrusion and earlier Cambro‐Ordovician fabrics of a banded amphibolite complex. The analysis of crystal size distribution (CSD) shows high nucleation density (N0) and low average growth rate (Gt) for Carboniferous mylonitic metagabbros and mylonitic granodiorites. The parameter N0 decreases whereas the quantity Gt increases towards higher temperatures progressively approaching the values obtained from the Cambro‐Ordovician banded amphibolite complex. The spatial distribution of amphibole and plagioclase shows intense mechanical mixing for lower‐temperature mylonitic metagabbros. In high‐temperature mylonites a strong aggregate distribution is developed. Cambro‐Ordovician amphibolites unaffected by Carboniferous deformation show a regular to anticlustered spatial distribution resulting from heterogeneous nucleation of individual phases. This pattern, together with CSD, was subsequently modified by the grain growth and textural equilibration controlled by diffusive mass transfer during Carboniferous metamorphism. The differences between the observed textures of the amphibolites are interpreted to be a consequence of the different durations of the Carboniferous and Cambro‐Ordovician thermal events.  相似文献   

10.
Magnetic fabric and rock-magnetic studies have been carried out for the non-fossiliferous, low- to medium-grade metasedimentary rocks in the southwestern part of the Okcheon metamorphic belt (OMB). Two major metamorphic events in the study area were previously recognized: regional metamorphism (M1) in the late Carboniferous to early Permian and contact metamorphism (M2) due to the intrusion of granite in the middle Jurassic. The metamorphic grade of the study area increases from the biotite zone in southeast through the garnet zone to the staurolite zone towards the northwest. Magnetic fabrics of the study area are generally well defined and can be characterized according to the metamorphic zones. Magnetic foliation is the dominant magnetic fabric in the biotite zone, while magnetic lineation prevails in both garnet and staurolite zones. We interpret the metamorphism-related deformation history of the study area based on magnetic fabrics, magnetic mineralogy and previously reported metamorphic evolution as follows. Penetrative NW-dipping cleavage, represented by magnetic foliation, was formed in the study area by prevailing NW–SE shortening event during the M1 regional metamorphism in the late Carboniferous–early Permian. This shortening event is interpreted to be associated with the collisional event between the North and South China blocks. Cleavages dipping steeply to the southeast in the staurolite zone are attributed to the pressure exerted from the intrusion of Jurassic granite in the northwestern area.  相似文献   

11.
One‐dimensional thermal (1DT) modelling of an Acadian (Devonian) tectonothermal regime in southern Vermont, USA, used measured metamorphic pressures and temperatures and estimated metamorphic cooling ages based on published thermobarometric and geochronological studies to constrain thermal and tectonic input parameters. The area modelled lies within the Vermont Sequence of the Acadian orogen and includes: (i) a western domain containing garnet‐grade pre‐Silurian metasedimentary and metavolcanic rocks from the eastern flank of an Acadian composite dome structure (Rayponda–Sadawga Dome); and (ii) an eastern domain containing similar, but staurolite‐ or kyanite‐grade, rocks from the western flank of a second dome structure (Athens Dome), approximately 10 km farther east. Using reasonable input parameters based on regional geological, petrological and geochronological constraints, the thermal modelling produced plausible P–T paths, and temperature–time (T –t) and pressure–time (P–t) curves. Information extracted from P–T –t modelling includes values of maximum temperature and pressure on the P–T paths, pressure at maximum temperature, predicted Ar closure ages for hornblende, muscovite and K‐feldspar, and integrated exhumation and cooling rates for segments of the cooling history. The results from thermal modelling are consistent with independently obtained pressure, temperature and Ar cooling age data on regional metamorphism in southern Vermont. Modelling results provide some important bounding limits on the physical conditions during regional metamorphism, and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as much as 2.5 kbar lower than the actual maximum pressure experienced by rocks along various particle paths. In addition, differences in peak metamorphic grade (garnet‐grade versus staurolite‐grade or kyanite‐grade) and peak temperature for rocks initially loaded to similar crustal depths, differences in calculated exhumation rates, and differences in 40Ar/39Ar closure ages are likely to have been consequences of variations in the duration of isobaric heating (or ‘crustal residence periods’) and tectonic unroofing rates. Modelling results are consistent with a regional structural model that suggests west to east younging of specific Acadian deformational events, and therefore diachroneity of attainment of peak metamorphic conditions and subsequent 40Ar/39Ar closure during cooling. Modelling is consistent with the proposition that regional variations in timing and peak conditions of metamorphism are the result of the variable depths to which rocks were loaded by an eastward‐thickening thrust‐nappe pile rooted to the east (New Hampshire Sequence), as well as by diachronous structural processes within the lower plate rocks of the Vermont Sequence.  相似文献   

12.
Prograde P–T paths recorded by the chemistry of minerals of subduction‐related metamorphic rocks allow inference of tectonic processes at convergent margins. This paper elucidates the changing P–T conditions during garnet growth in pelitic schists of the Sambagawa metamorphic belt, which is a subduction related metamorphic belt in the south‐western part of Japan. Three types of chemical zoning patterns were observed in garnet: Ca‐rich normal zoning, Ca‐poor normal zoning and intrasectoral zoning. Petrological studies indicate that normally‐zoned garnet grains grew keeping surface chemical equilibrium with the matrix, in the stable mineral assemblage of garnet + muscovite + chlorite + plagioclase + paragonite + epidote + quartz ± biotite. Pressure and temperature histories were inversely calculated from the normally‐zoned garnet in this assemblage, applying the differential thermodynamic method (Gibbs' method) with the latest available thermodynamic data set for minerals. The deduced P–T paths indicate slight increase of temperature with increasing pressure throughout garnet growth, having an average dP/dT of 0.4–0.5 GPa/100 °C. Garnet started growing at around 470 °C and 0.6 GPa to achieve the thermal and baric peak condition near the rim (520 °C, 0.9 GPa). The high‐temperature condition at relatively low pressure (for subduction related metamorphism) suggests that heating occurred before or simultaneously with subduction.  相似文献   

13.
The Altınekin Complex in south central Turkey forms part of the south‐easterly extension of the Tavşanlı Zone, a Cretaceous subduction complex formed during the closure of the Neo‐Tethys ocean. The protoliths of metamorphic rocks within the Altınekin Complex include peridotites, chromitites, basalts, ferruginous cherts and flysch‐facies impure carbonate sediments. Structurally, the complex consists of a stack of thrust slices, with massive ophiolite tectonically overlying a Cretaceous sediment‐hosted ophiolitic mélange, in turn overlying a sequence of Mesozoic sediments. Rocks within the two lower structural units have undergone blueschist–facies metamorphism. Petrographic, mineral–chemical and thermobarometric studies were undertaken on selected samples of metasedimentary and metabasic rock in order to establish the time relations of deformation and metamorphism and to constrain metamorphic conditions. Microstructures record two phases of plastic deformation, one predating the metamorphic peak, and one postdating it. Estimated peak metamorphic pressures mostly fall in the range 9–11 kbar, corresponding to burial depths of 31–38 km, equivalent to the base of a continental crust of normal thickness. Best‐fit peak metamorphic temperatures range from 375 to 450°C. Metamorphic fluids had high H2O:CO2 ratios. Peak metamorphic temperature/depth ratios (T/d values) were low (c. 10–14°C/km), consistent with metamorphism in a subduction zone. Lawsonite‐bearing rocks in the southern part of the ophiolitic mélange record lower peak temperatures and T/d values than epidote blueschists elsewhere in the unit, hinting that the latter may consist of two or more thrust slices with different metamorphic histories. Differences in peak metamorphic conditions also exist between the ophiolitic mélange and the underlying metasediments. Rocks of the Altınekin Complex were subducted to much shallower depths, and experienced higher geothermal gradients, than those of the NW Tavşanlı Zone, possibly indicating dramatic lateral variation in subduction style. Retrograde P–T paths in the Altınekin Complex were strongly decompressive, resulting in localized overprinting of epidote blueschists by greenschist–facies assemblages, and of lawsonite blueschists by pumpellyite–facies assemblages. The observation that the second deformation was associated with decompression is consistent with, but not proof of, exhumation by a process that involved deformation of the hanging‐wall wedge, such as gravitational spreading, corner flow or buoyancy‐driven shallowing of the subduction zone. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

14.
Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.  相似文献   

15.
滇西澜沧变质带中199件白云母b_0值分析结果表明,b_0值均在0.9020nm以上。与澜沧变质带遭受蓝片岩相高压变质作用的事实相吻合。此外,白云母b_0值的统计规律显示,澜沧变质带经历了多期变质作用,一期为高压变质作用,另一期为中压变质作用,这也与已知的地质现象相符。由此可见,白云母b_0值可以很好地反映多期变质作用。  相似文献   

16.
During the Late Palaeozoic Variscan Orogeny, Cambro‐Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low‐pressure/high‐temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite‐muscovite, biotite, andalusite‐cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench‐dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P–T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P–T estimates show that rocks from the biotite‐in isograd reached peak‐metamorphic conditions of 2.5 kbar, 400 °C; rocks in the low‐grade part of the andalusite‐cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 °C; rocks located at the transition between andalusite‐cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 °C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 °C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 °C. A clockwise P–T trajectory is inferred using a combination of reaction microstructures with appropriate P–T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision.  相似文献   

17.
据３３３个白云母ｂ０值，结合区域地层和构造的研究成果，提出：（１）原称昌宁－孟连变质带解体为西盟－勐统变质带（加里东期中压变质带）和澜沧双变质带的高压亚带，两期高压变质作用分别与海西－印支期的洋壳消减和陆－陆碰撞有关；（２）石鼓变质带是三个逆冲岩片的叠置，各岩片内岩石的变质作用各具特色；（３）金沙江洋盆为地块间小洋盆，其消减及嗣后的弧－弧碰撞可能不产生高压变质作用。在此基础上探讨了白云母ｂ０值在造山带研究中的意义。  相似文献   

18.
华北原地台古元古代热力活动遗迹及其构造边界意义   总被引：4，自引：0，他引：4  
白瑾 《岩石学报》2000,16(1):39-48
热力活动遗迹是由区域变质相带表现出来的。热动力变质作用,除负荷压力外,侧向挤压剪切应力起了重要的作用,在造山带内,由同构造变质矿物组合方向性平行列构成的片理,就是显著的证据。当活动带由侧向缩短而造山时,两侧地块在惯性参照系控制下,受地块底部摩擦力的影响,使侧向挤压力逐渐变小;最后,当相向运动地地块发生碰撞时,由于惯性参照系的稳底破坏增大了侧向压力而产生低温高压变质相,显示出降温升压趋势。成为探索构  相似文献   

19.
《International Geology Review》2012,54(12):1055-1115
The Isua supracrustal belt (～3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of regional metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causing a complex mineral paragenesis throughout the area. For example, a high FeO content of metabasites expands the stability field of hornblende to both lower and higher grades. The compositional and mineralogical characteristics above also indicate that the Isua supracrustal belt underwent a single regional metamorphic event, involving minor contact metamorphism and mylonitization; however, weak ocean-floor metamorphism and low-grade regional metamorphism during accretion cannot be ruled out.

Metamorphic pressures and temperatures are estimated to be 5–7 kbar from garnet-hornblende-plagioclase-quartz geobarometry and 380–550°C from garnet-biotite geothermometry in Zones B to D. These P-T estimates indicate an intermediate P/T ratio metamorphic facies series. Geological investigations and chronological constraints of the Isua metamorphic belt indicate that the regional metamorphism was related to the subduction of Archean lithosphere, and records a geothermal gradient for the Archean subduction zone that is much higher than geotherms for Phanerozoic subduction zones. The high geothermal gradient may have resulted from the young age of subducted lithosphere and high potential temperature of the mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin, descending oceanic plate, creating many huge granitic (tonalite-trondhjemite-granodiorite [TTG]) batholiths. Slab melting changed the oceanic crust (density = 3.07) to denser garnet-bearing assemblages (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics.  相似文献   

20.
Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (XJd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low XCa bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low XCa bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.  相似文献   

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