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1.
The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote-pumpellyite-actinolite zone and the epidote-actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote-actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote-actinolite zone of the Sambagawa metamorphic belt.
The sharp change of the ductile strain magnitude also takes place at the epidote-actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300-400°C. and is independent of pressure of metamorphism.  相似文献   

2.
Many high-temperature–low-pressure (high- T –low- P ) metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating–cooling cycle. Such terranes are not readily explained as the conductive response to crustal thickening since the resulting Moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric-scale strains must greatly outlast the deformation. Consequently, high- T –low- P metamorphism may be generated during crustal thickening only when significant heat is advected within the crust, as for example may occur during the segregation of granitic melts. We show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially, for example, by removing the mantle part of the lithosphere. A simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust, c, and the lithosphere, 1 respectively, allows the potential energy of the evolving orogen to be readily evaluated. Using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high- T –low- P metamorphism during crustal thickening (with c1) is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations (fc= f1) for the same initial lithospheric configuration.  相似文献   

3.
Abstract Five whole-rock 40Ar/39Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined 40Ar/39Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north ( c. 110 Ma) to south ( c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.  相似文献   

4.
Regional variation in the P–T path of the Sambagawa metamorphic rocks, central Shikoku, Japan has been inferred from compositional zoning of metamorphic amphibole. Rocks constituting the northern part (Saruta River area) exhibit a hairpin type P–T path, where winchite/actinolite grew at the prograde stage, the peak metamorphism was recorded by the growth of barroisite to hornblende and sodic amphibole to winchite/actinolite grew at the retrograde stage. In the southern part (Asemi River area), rocks exhibit a clockwise type P–T path, where barroisite to hornblende core is rimmed by winchite to actinolite. The difference in P–T path could suggest a faster exhumation rate (i.e. more rapid decompression) in the southern than in the northern part. On the other hand, physical conditions of deformation during the exhumation stage have been independently inferred from microstructures in deformed quartz. Recrystallized quartz grains in rocks from the low‐grade (chlorite and garnet) zones are much more stretched in the southern part (aspect ratio ≥ 4.0) than in the northern part (aspect ratio< 4.0), indicating a higher strain rate in the former than in the latter. These facts may indicate that the exhumation and strain rates are correlated (i.e. the exhumation rate increases with increasing the strain rate). The difference in the exhumation rate inferred from amphibole zoning between the northern and southern parts could be explained by an extensional model involving normal faulting, where the lower plate can be exhumed faster than the upper plate due to the displacement along the fault. Furthermore, the model may explain the positive correlation between the exhumation and strain rates, because the lower plate tended to support more stress than the upper plate.  相似文献   

5.
A number of microstructural features indicate a difference in the dominant deformation mechanism between the higher temperature Ryoke and the lower temperature Sambagawa and Shimanto metamorphic belts of Japan. The microstructures of metacherts containing deformed radiolaria are divided into two types: in both the Sambagawa and Shimanto belts the quartz grains are tabular while in the Ryoke belt they are equiaxed. TEM studies of these metacherts revealed that the tabular grains contain abundant subboundaries consisting of large numbers of network dislocations and bowe-out dislocations, while the equiaxed grains contain no subboundaries and have low densities of dislocations which are not bowed-out. There is a corresponding difference in the textures (lattice preferred orientation of quartz): the Ryoke metacherts display randomly distributed c-axes of quartz, while the Sambagawa and Shimanto metacherts show conspicuous crossed girdle patterns with some asymmetry. There is a third difference between these regions: in the metacherts of the Ryoke metamorphic belt, the strain magnitudes determined from deformed radiolaria increase with increasing volume fraction of mica in the same metamorphic P and T conditions, while in the Sambagawa and the Shimanto metamorphic cherts the strain magnitudes decrease with increasing the mica fraction.These microstructures, textures, and rheological behaviours of quartz-mica rocks suggest a change of deformation mechanism between the lower temperature Sambagawa and Shimanto, and the higher temperature Ryoke metamorphic belts. Since random fabrics of c-axes of quartz are inconsistent with lattice rotation due to dislocation glide, the Ryoke metacherts may have deformed by pressure-solution.  相似文献   

6.
A hydrothermally metamorphosed greenstone complex, capped by bedded cherts and banded iron formations (BIFs), is exposed in the Cleaverville area, Pilbara Craton, Western Australia. It has been interpreted as an accretionary complex characterized by both a duplex structure and an oceanic plate stratigraphy, and is shown to represent a 3.2 Ga upper oceanic crust. Three metamorphic zones are identified in the basaltic greenstones. The metamorphic grade increases from sub-greenschist facies (zones A and B) to greenschist facies (zone C) under low-pressure conditions. The boundaries between three mineral zones are subparallel to the bedding plane of overlying chert/BIF, and metamorphic temperature increases stratigraphically downward. The zones correspond to the thermal structure of ocean-floor metamorphism, at a mid-ocean ridge.
The uppermost greenstone in the study area is more pervasively altered and carbonatized than the modern upper oceanic crust. This indicates the enrichment of CO2 in the metamorphic fluid by which widespread formation of carbonate occurred, compared with a narrow stability region of Ca-Al silicates. It is, therefore, suggested that the Archean hydrothermal alteration played a more important role in fixation of CO2 than present-day ocean-ridge hydrothermal alteration, as an interaction between sea water and oceanic crust.  相似文献   

7.
High‐ to ultrahigh‐pressure (HP‐UHP) metamorphic rocks that resulted from deep continental subduction and subsequent exhumation in the Sulu orogenic belt, China, have experienced multiphase deformation and metamorphic overprint during its long journey to the mantle and return to the surface. HP‐UHP shear zones are strain‐localized weak zones on which the UHP slab is transported over long distances. HP‐UHP shear zones are well exposed along a 200‐km belt in the Sulu UHP metamorphic belt. The shear zones lie structurally below the UHP rocks and above the non‐UHP rocks, suggesting the early exhumation of the UHP rocks by thrusting. The large area distribution, HP‐UHP nature, high strain and structural association of the shear zones with the UHP rocks suggest that the shear zones are probably a regional detachment developed during the early stage of exhumation of the UHP rocks. Kinematic indicators suggest top‐to‐the N–NW motion of the UHP slab during the exhumation, which, combined with isotope signature in Mesozoic igneous rocks, leads us to the interpretation that the subduction polarity is the North China plate down to the south rather than the Yangtze plate down to the north in the Sulu region.  相似文献   

8.
The convex form of subduction-stage pressure–temperature ( P–T ) paths up to c. 2.0 GPa implies the Sambagawa high- P metamorphic belt, Japan, formed a few million years before ridge subduction. Additional compilation of P–T conditions for higher- P Sambagawa rocks ( c. 2.0–2.5 GPa) reveals that the thermal profile along the slab surface shows a remarkable high- T -ward warping at c. 2.0 GPa ( c. 65 km). Previous thermal models indicate that this warping corresponds to the onset of induced mantle flow towards the subducting slab. If a normal thickness continental crust of c. 30 km was present, this implies the hangingwall region between 30 and 65 km depth was occupied by serpentinized wedge mantle isolated from large-scale mantle flow. Subsequent arrival of the spreading ridge, reheating and dehydration of the serpentinized wedge probably supplied the water necessary for causing granitic magmatism in the Ryoke high- T metamorphic belt, which is paired with the Sambagawa belt.  相似文献   

9.
Shrimp U–Pb zircon dating of structurally constrained felsic orthogneiss samples in the western Musgrave Block has been used to delineate discrete magmatic and metamorphic events at c . 1300 and c . 1200  Ma. The dating of pre-D1 and post-D1 felsic orthogneiss constrains D1 to have occurred at 1312±16 to 1324±4  Ma. This is the first geochronological study to identify such a metamorphic and deformation event in the Musgrave Block. D1 was accompanied by a major magmatic event involving the emplacement of voluminous felsic orthogneiss between 1296 and 1324  Ma. Zircon overgrowths on numerous igneous zircon cores give a consistent age of c . 1200  Ma, reflecting zircon growth during a second high-grade metamorphic event (D2). This c . 1200  Ma metamorphic event was followed by the intrusion of a c . 1190  Ma megacrystic granite. The c . 1300 and c . 1200  Ma events in the Musgrave Block can be tentatively correlated with metamorphic events in the Albany-Fraser Orogen, and the Windmill Islands and Bunger Hills in east Antarctica. A major continuous Grenville-age orogenic belt joining these areas may have represented a plate boundary between the pre-Rodinian proto-Australian continent and proto-Antarctica during the formation of Rodinia in the Mesoproterozoic.  相似文献   

10.
与剪切带有关金矿床成矿流体研究进展   总被引：2，自引：0，他引：2  
与剪切带有关的金矿床是一种重要类型的金矿床，矿床产于与俯冲作用有关的碰撞造山带中压性或压扭性的构造环境，赋矿围岩岩性复杂，且普遍遭受了低绿片岩相到麻粒岩相的变质作用，成矿时代从太古代到第三系。矿床强烈受剪切带控制，以富集Au、Ag、As、Sb、W、Bi、Te等为特征，围岩蚀变以富集CO 2、S、K、H 2 O、SiO 2为特征。成矿流体为低盐度（质量百分含量为 3％～10％的NaCl）的富含CO2（≥5mol％）近中性流体，δ 18 O H 2 O＝5‰～12‰，成矿流体来源有岩浆源、幔源、天水源、变质源及混合源等多种来源，对这类矿床进行深入研究对成矿理论及找矿勘探都具有重要意义。  相似文献   

11.
The George Sound Paragneiss (GSP) represents a rare Permo-Triassic unit in Fiordland that occurs as a km-scale pillar to gabbroic and dioritic gneiss of c . 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser-ablation U–Pb ages as young as c . 190 Ma and 176Hf/177Lu >0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S1 foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S2 foliation. S1 staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T  <   650 °C and P <  8 kbar. Contact aureole and S2 assemblages include Mg-rich, Ca-poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S2 kyanite-bearing matrix assemblages and Ca-enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post-S2 high- P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high- P anti-clockwise P–T history for the GSP involving: (i) mid- P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high- P and (iv) high- P cooling induced by tectonic juxtaposition of cooler country rock.  相似文献   

12.
The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa= 0.10–0.28)-bearing assemblages in pelitic schists, and represents the highest-grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH2O = Psolid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.
Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized'upper staurolite zone'as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:  相似文献   

13.
追溯和重塑超高压变质岩由100多千米地幔深度折返至上地壳及地表的过程,对理解会聚板块边缘及大陆碰撞带的运动学和动力学是极为重要的.主要依据构造学、岩石学、地球化学和可利用的地质年代学资料,结合区域多期变形分析,大别-苏鲁区超高压变质岩的折返过程至少可分解出4个大的阶段.块状榴辉岩记录了三叠纪(约250~230 Ma)大陆壳岩石的深俯冲/碰撞作用.超高压变质岩早期迅速折返发生于超高压峰期变质作用(P＞3.1~4.0 GPa,T≈800±50 ℃)之后,处于地幔深度和柯石英稳定域,相当于区域D2变形期阶段.分别与区域变形期D3、D4和D5对应的折返过程,以及后成合晶、冠状体等卸载不平衡结构发育和减压部分熔融作用2个中间性构造热事件,均发生在地壳层次. 网络状剪切带在折返过程的不同阶段和不同层次均有发育,标志着在超高压变质带内的变质和变形分解作用曾重复进行.着重指出,超高压变质岩的折返,主要是由大陆壳的深俯冲/碰撞和伸展作用控制的构造过程,且受到俯冲带内、带外诸多因素的约束,其中水流体就起关键作用.   相似文献   

14.
阿尔金南缘构造带西段新元古代-早古生代变质岩系为一套绿片岩相-低角闪岩相的中浅变质岩系,并形成绢云母-白云母带、黑云母带和石榴石-角闪石带3个递增变质带。通过对石榴石-角闪石带变质作用p-T轨迹的深入研究表明,其经历了3个阶段变质作用:即经历了俯冲碰撞深埋到30～35 km深度的中高压变质作用(M1),压力可达7.27×108～9.63×108 Pa;随后经历了快速抬升所导致的近等温降压变质过程(M2),压力降至4.1×108～5.62×108 Pa;晚期经历了降温降压退变质作用（M3）。整个过程为顺时针方向演化的p-T轨迹,其地质动力学过程属于较典型的板块俯冲碰撞→抬升模式,表明阿尔金南缘构造带的地质构造演化经历了俯冲碰撞及碰撞后的快速抬升过程。  相似文献   

15.
A sequence of regional metamorphic isograds indicating a range from prehnite-pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature- X CO2 and pressure-temperature (constant X CO2) diagrams were calculated to quantify the effects of CO2 in the metamorphic fluid on the stability of prehnite-pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X co2 <0.002. For X co2>0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H2O-rich fluid and a more CO2-rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO2 in the metamorphic fluid should be assessed when considering the pressure-temperature implications of mineral assemblages in low-grade metabasites.  相似文献   

16.
The two-dimensional distribution patterns of garnet grains incontact metamorphic rocks and the Sambagawa high P/T type metamorphicrocks were studied by means of Fry's map in order to investigatethe development of segregation during synmetamorphic shear flow.The distribution of garnet in the undeformed hornfelses is nearlyrandom, indicating that the nucleation sites are approximatelyrandom. The distribution patterns of garnet in the highly deformedSambagawa pelitic schists, however, can be divided into twotypes: one is strongly random, and another is strongly clustered.The random distribution occurs in the relatively lower grademetamorphic rocks, whereas the clustered one is found in thehigher grade zones. This segregation of garnet comes from thekinematic collision of garnet grains in shear flow and the apparentattractive force operating between garnet grains due to an overgrowthmechanism.  相似文献   

17.
Calcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle-ductile transition, which has been raised to 5-6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H2O + NaCl-CO2 mixtures at 200-350 C. Bulk fluid composition is 15-20 mol% CO2 and <4.3 total mol CH4+ N2+ Ar/100mol H2O. Water hydrogen isotopic ratio δDH2O in the fissure veins spans -29 to -68‰, δ18OH2O -0.7 to 8.5‰, and bulk carbon isotopic ratio δ13C ranges from -3.7 to -11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.  相似文献   

18.
Porphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock. However, there remains considerable controversy over whether porphyroblasts rotate during bulk non-coaxial deformation.
With reference to an area of the Scandinavian Caledonides and utilizing existing data from theoretical and experimental modelling, this study demonstrates that both 'straight' and 'S-shaped' inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to Newtonian simple shear. At crustal strain rates of 10-14 s-1 and porphyroblast growth times of 0.1–1.0 Ma, it is shown that a maximum of 5-9 angular rotation would occur during growth. At faster strain rates of 10-12 s-1 (e.g. those in a shear zone) porphyroblast angular rotations of 90 are shown to occur in 0.1–0.25 Ma (i.e. times comparable with or faster than porphyroblastesis). In view of this, 'S-shaped' inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain, whereas 'straight' inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates.  相似文献   

19.
哀牢山—红河构造带哀牢山段可划分为东部高级变质带和西部低级变质带。构造分析表明:该构造带由3个不同变形域组成,可能代表其经历的3期左行走滑。第1期走滑发生在整个高级变质带,为拉张性左行走滑,形成角闪岩相L型构造岩。第2期走滑形成高级变质带中的高应变带,变形体制接近简单剪切,形成绿片岩相L-S型糜棱岩。第3期主要发生在低级变质带,为挤压性走滑,形成左行逆冲构造格局,并形成低绿片岩相千糜岩。地质年代学数据证明,3期左行走滑的形成时代分别是:距今58~56Ma、27~22Ma和13~12Ma±。哀牢山—红河构造带第1期左行走滑可能对应于印度与欧亚大陆距今60Ma左右的初始碰撞;第2期变形与青藏高原最强的挤压隆升期一致;第3期事件可能代表距今16~13Ma开始的青藏高原物质进一步东挤。哀牢山—红河构造带的3期主要左行走滑均发生在新生代印度与欧亚大陆的汇聚过程中。  相似文献   

20.
Abstract The Lancang metamorphic terrane consists of an eastern low- P/T belt and a western high- P/T belt divided by a N–S-trending fault. Protoliths of both units are mid–late Proterozoic basement and its cover. The low- P/T belt includes the Permian Lincang batholith, related amphibolite facies rocks of the Damenglong and Chongshan groups, and Permo-Triassic volcanic and volcaniclastic rocks. Most whole-rock Rb–Sr isochron and U–Pb zircon ages of the Lincang batholith are in the range 290–279 and 254–212 Ma, respectively. Metamorphism of the low- P/T belt reaches upper amphibolite with local granulite facies (735°C at 5 kbar), subsequently retrogressed at 450–500°C during post-Triassic time. The high- P/T rocks grade from west to east from blueschist through transitional blueschist/greenschist to epidote amphibolite facies. Estimated P–T conditions follow the high- P intermediate facies series up to about 550–600°C, at which oligoclase is stable. The 40Ar/39Ar plateau age of sodic amphibole in blueschist is 279 Ma.
The paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of south-east Asia since the latest Precambrian. Tectonic juxtaposition of paired belts with contrasting P–T conditions, perhaps during collision of the Baoshan block with south-east Asia, suggests that an intervening oceanic zone existed that has been removed. The Baoshan block is a microcontinent rifted from the northern periphery of Gondwana. Successive collision and amalgamation of microcontinents from either Gondwana or the Panthalassan ocean resulted in rapid southward continental growth of c. 500 km during the last 200 Ma. Hence, the Lancang region in south-east Asia represents a suture zone between two contrasting microcontinents.  相似文献   

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