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1.
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   

2.
Structural overprinting relationships indicate that two discrete terranes, Mt. Stafford and Weldon, occur in the Anmatjira Range, northern Arunta Inlier, central Australia. In the Mt. Stafford terrane, early recumbent structures associated with D1a,1b deformation are restricted to areas of granulite facies metamorphism and are overprinted by upright, km-scale folds F1c), which extend into areas of lower metamorphic grade. Structural relationships are simple in the low—grade rocks, but complex and variable in higher grade equivalents. The three deformation events in the Mt. Stafford terrane constitute the first tectonic cycle (D1-D2) deformation in the Weldon terrane comprises the second tectonic cycle. The earliest foliation (S2a) was largely obliterated by the dominant reclined to recumbent mylonitic foliation (S2b), produced during progressive non-coaxial deformation, with local sheath folds and W- to SW-directed thrusts. Locally, (D2d) tectonites have been rotated by N—S-trending, upright (F2c) folds, but the regional upright fold event (F2d), also evident in the adjacent Reynolds Range, rotated earlier surfaces into shallow-plunging, NW—SE-trending folds that dominate the regional outcrop pattern.The terranes can be separated on structural, metamorphic and isotopic criteria. A high-strain D2 mylonite zone, produced during W- to SW-directed thrusting, separates the Weldon and Mt. Stafford terranes. 1820 Ma megacrystic granites in the Mt. Stafford terrane intruded high-grade metamorphic rocks that had undergone D1a and D1b deformation, but in turn were deformed by S1c, which provides a minimum age limit for the first structural—metamorphic event. 1760 Ma charnockites in the Weldon terrane were emplaced post-D2a, and metamorphosed under granulite facies conditions during D2b, constraining the second tectonic cycle to this period.Each terrane is associated with low-P, high-T metamorphism, characterized by anticlockwise P—T—t paths, with the thermal peaks occurring before or very early in the tectonic cycle. These relations are not compatible with continental-style collision, nor with extensional tectonics as the deformation was compressional. The preferred model involves thickening of previously thinned lithosphere, at a stage significantly after (>50 Ma) the early extensional event. Compression was driven by external forces such as plate convergence, but deformation was largely confined to and around composite granitoid sheets in the mid-crust. The sheets comprise up to 80% of the terranes and induced low-P, high-T metamorphism, including migmatization, thereby markedly reducing the yield strength and accelerating deformation of the country rocks. Mid-crustal ductile shearing and reclined to recumbent folding resulted, followed by upright folding that extended beyond the thermal anomaly. Thus, thermal softening induced by heat-focusing is capable of generating discrete structural terranes characterized by subhorizontal ductile shear in the mid-crust, localized around large granitoid intrusions.  相似文献   

3.
Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D1. The main mylonitization event was during the D2 deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D3 folding and related mylonitization. We documented the systematic changes in the orientations of D2 linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.  相似文献   

4.
Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M 1 phengite.K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M 1+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon.K-Ar and Rb-Sr ages on 2M 1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.  相似文献   

5.
Chronology of Sanbagawa metamorphism   总被引：5，自引：0，他引：5  
By collating age data based on the fossil age of the protoliths, radiometric dating of the metamorphic minerals, and sedimentary records of erosion at the earth's surface, the history of the Sanbagawa metamorphism can be summarized as follows. (1) The pre-metamorphic sedimentary rocks (Carboniferous-Jurassic + Early Cretaceous?) became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction-related tectono-sedimentary processes, probably in Early Cretaceous time (c., 130-120 Ma). (2) The subducted protoliths underwent progressive metamorphism reaching a maximum depth of c. 30 km in late Early Cretaceous time (c. 116 ± 10 Ma). (3) The high-P/T metamorphic rocks began to rise toward the surface (during the interval 110-50 Ma) with minimum estimates for the average cooling rate around 9-12°C/Ma and an average uplift rate around 0.4-0.5 mm/year. (4) Finally, at some stage after reaching the erosional surface, the high-P/T metamorphic rocks were covered unconformably by the middle Eocene (c. 50-42 Ma) Kuma Group. On the basis of the present chronological summary of the Sanbagawa metamorphism, the areal extent of the Sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction-accretion complex of the next tectonic belt to the south, the Northern Chichibu belt.  相似文献   

6.
A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K‐feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late‐Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K‐feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and aH2O of 0.75–1, was followed either during post‐contact metamorphism cooling or, more likely, during the early‐Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early‐Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high‐pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite‐ and muscovite‐bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.  相似文献   

7.
The metasedimentary rocks of the area around Mangpu constitute a portion of the hinge zone of the northern limb of the major synform of Lower Darjeeling Himalaya. The rocks display evidences of multiple deformation and at least three major phases of deformation have been recognized. The time relations between the phases of deformation (D1, D2, D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syn-tectonic lowgrade (greenschist facies) which may be designated (M1, early). This was followed by regional static metamorphism (M1, late) in the post-tectonic phase between D1 and D2 deformations (upper green schist and amphibolite facies). This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations (lower greenschist facies). Within the study area four isograds have been delineated by the first appearance of index minerals in the pelitic schists and gneiss which display Barrovian type of metamorphism.  相似文献   

8.
Four polymetamorphic complexes in the vicinity of regional faults in the Trans-Angarian region of the Yenisey Ridge were studied to determine their metamorphic evolution and to elucidate distinctive features of the regional geodynamic processes. Based on our geological and petrological studies using geothermobarometry and P–T path calculations, we show that a Neoproterozoic medium-pressure metamorphism of the kyanite-sillimanite type at c. 850 Ma overprinted regionally metamorphosed low-pressure andalusite-bearing rocks. A positive correlation between rock ages and P–T estimates for the kyanite-sillimanite metamorphism provides evidence for regional structural and tectonic heterogeneity. The medium-pressure recrystallization was characterized by (1) localized distribution of metamorphic zones in the area directly underlying thrust faults with a measured thickness of 2.5–8 km; (2) syntectonic formation of kyanite-bearing mineral assemblages related to thrusting; (3) gradual increase in metamorphic pressure towards the thrust faults associated with a low metamorphic field gradient (from 1–7 to 12°C/km); and (4) equally steep burial P–T paths recorded for the highest grade rocks. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence of near-isothermal loading in accordance with the transient emplacement of thrust sheets. The proposed model for tectono-metamorphic evolution of the study areas due to crustal thickening at high thrusting rates and subsequent rapid exhumation explains these tectonic features. Data analysis allowed us to consider the medium-pressure kyanite-bearing metapelites as a product of collisional metamorphism, reflecting unidirectional thrusting of Siberian cratonal blocks onto Yenisey Ridge along regional deep faults (Angara, Mayakon, and Chapa areas) and by opposite movements in the zone of secondary splay faults (Garevka area).  相似文献   

9.
Ion microprobe dating of zircon and monazite from high-grade gneisses has been used to (1) determine the timing of metamorphism in the Western Province of New Zealand, and (2) constrain the age of the protoliths from which the metamorphic rocks were derived. The Western Province comprises Westland, where mainly upper crustal rocks are exposed, and Fiordland, where middle to lower crustal levels crop out. In Westland, the oldest recognisable metamorphic event occurred at 360–370 Ma, penecontemporaneously with intrusion of the mid-Palaeozoic Karamea Batholith (c. 375 Ma). Metamorphism took place under low-pressure/high-temperature conditions, resulting in upper-amphibolite sillimanite-grade metamorphism of Lower Palaeozoic pelites (Greenland Group). Orthogneisses of younger (Cretaceous) age formed during emplacement of the Rahu Suite granite intrusives (c. 110 Ma) and were derived from protoliths including Cretaceous Separation Point suite and Devonian Karamea suite granites. In Fiordland, high-grade paragneisses with Greenland Group zircon age patterns were metamorphosed (M1) to sillimanite grade at 360 Ma. Concomitant with crustal thickening and further granite emplacement, M1 mineral assemblages were overprinted by higher-pressure kyanite-grade metamorphism (M2) at 330 Ma. It remains unclear whether the M2 event in Fiordland was primarily due to tectonic burial, as suggested by regional recumbent isoclinal folding, or whether it was due to magmatic loading, in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous Westland region. Metamorphism in Fiordland accompanied and outlasted emplacement of the Western Fiordland Orthogneiss (WFO) at 110–125 Ma. The WFO equilibrated under granulite facies conditions, whereas cover rocks underwent more limited recrystallization except for high-strain shear zones where conditions of lower to middle amphibolite facies were met. The juxtaposition of Palaeozoic kyanite-grade rocks against Cretaceous WFO granulites resulted from late Mesozoic extensional deformation and development of metamorphic core complexes in the Western Province.  相似文献   

10.
The Paleoproterozoic Liaohe assemblage and associated Liaoji granitoids represent the youngest basement in the Eastern Block of the North China Craton. Various structural elements and metamorphic reaction relations indicate that the Liaohe assemblage has experienced three distinct deformational events (D1 to D3) and four episodes of metamorphism (M1 to M4). The earliest greenschist facies event (M1) is recognized in undeformed or weakly deformed domains wrapped by the S1 schistosity, suggesting that M1 occurred before D1. The D1 deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F1), an associated penetrative axial planar schistosity (S1), a mineral stretching lineation (L1) and regional-scale ductile shear zones. Concurrent with D1 was M2 metamorphism, which occurred before D2 and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F1 folds and other sense-of-shear indicators, together with the radial distribution of the L1 lineation surrounding the Liaoji granitoids, imply that D1 represents an extensional event. The D2 deformation produced open to tight F2 folds of varying scales, S2 axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW–SE compression. Following D2 was M3 metamorphism that led to the formation of sillimanite and cordierite in low-pressure type rocks and kyanite in medium-pressure rocks. The last deformational event (D3) formed NW-WNW-trending folds (F3), axial planar kink bands, spaced cleavages (S3), and strike–slip and thrust faults, which deflect the earlier D1 and D2 structures. D3 occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M4).The Liaohe assemblage and associated Liaoji granitoids are considered to have formed in a Paleoproterozoic rift, the late spreading of which led to the occurrence of the early extensional deformation (D1) and the M1 and M2 metamorphism, and the final closing of which was associated with the D2 and D3 phases of deformation and M3 and M4 metamorphism.  相似文献   

11.
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   

12.
An integrated geological study of the tectono-metamorphic evolution of the metamorphic complex of Beloretzk (MCB) which is part of the eastern Bashkirian mega-anticlinorium (BMA), SW Urals, Russia shows that the main lithological units are Neoproterozoic (Riphean and Vendian age) siliciclastic to carbonate successions. Granitic, syenitic and mafic intrusions together with subaerial equivalents comprise the Neo- and Mesoproterozoic magmatic rocks. The metamorphic grade ranges from diagenetic and very low grade in the western BMA to high-grade in the MCB. The N–S trending Zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern BMA. Structural data, Pb/Pb-single zircon ages, 40Ar/39Ar cooling ages and the provenance signature of Riphean and Vendian siliciclastic rocks in the western BMA give evidence of Mesoproterozoic (Grenvillian) rifting, deformation and eclogite-facies metamorphism in the MCB and a Neoproterozoic (Cadomian) orogenic event in the SW Urals. Three pre-Ordovician deformation phases can be identified in the MCB. The first SSE-vergent, isoclinal folding phase (D1) is younger than the intrusion of mafic dykes (Pb/Pb-single zircon: 1350 Ma) and older than the eclogite-facies metamorphism. High P/low T eclogite-facies metamorphism is bracketed by D1 and the intrusion of the Achmerovo granite (Pb/Pb-single zircon: ≤970 Ma). An extensional, sinistral, top-down-to-NW directed shearing (D2) is correlated with the first exhumation of the MCB. E-vergent folding and thrusting (D3) occurred at retrograde greenschist-facies metamorphic conditions. The tremolite 40Ar/39Ar cooling age (718±5 Ma) of amphibolitic eclogite and muscovite 40Ar/39Ar cooling ages (about 550 Ma) of mica schists indicate that a maximum temperature of 500±50 °C was not reached during the Neoproterozoic orogeny. The style and timing of the Neoproterozoic orogeny show similarities to the Cadomian-aged Timan Range NW of the Polar Urals. Geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the Zuratkul fault, suggest that the MCB is exotic with respect to the SE-margin of the East European Platform. Thus, the MCB is named the ‘Beloretzk Terrane’. Recognition of the ‘Beloretzk Terrane’ and the Neoproterozoic orogeny at the eastern margin of Baltica has important implications for Neoproterozoic plate reconstruction and suggests that the eastern margin of Baltica might have lain close to the Avalonian–Cadomian belt.  相似文献   

13.
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   

14.
Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.  相似文献   

15.
First-order factors controlling the textural and chemical evolution of metamorphic rocks are bulk composition and pressure–temperature–time (P–T–t) path. Although it is common to assume that major element bulk composition does not change during regional metamorphism, rocks with reaction textures such as corona structures record evidence for major changes in effective bulk composition (EBC) and therefore provide significant insight into the scale, pathways, and mechanisms of element transport during metamorphism. Quantifying changes in EBC is essential for petrologic applications such as calculation of phase diagrams (pseudosections). The progressive growth of complex corona structures on garnet and Al2SiO5 porphyroblasts in orthoamphibole-cordierite gneiss Thor-Odin dome (British Columbia, Canada) reduced the EBC volume of the rock during metamorphism and therefore had a dramatic effect on the evolution of the stable mineral assemblage. These rocks contain a chemical and textural record of metamorphic reactions and preserve 3D networks (reaction pathways) connecting corona structures. These coronal networks record long (>cm) length scales of localized element transport during metamorphism. P–T, T–X, and P–X pseudosections are used to investigate the control of effective bulk composition on phase assemblage evolution. Despite textural complexity and evidence for disequilibrium, mineral assemblages and compositions were successfully modeled and peak metamorphic conditions estimated at 750°C and 9 kbar. These results illustrate how textural and chemical changes during metamorphism can be evaluated using an integrated petrographic and pseudosection approach, highlight the importance of effective bulk composition choice for application of phase equilibria methods in metamorphic rocks, and show how corona structures can be used to understand the scale of compositional change and element transport during metamorphism.  相似文献   

16.
The Luning–Fencemaker fold-thrust belt (LFTB) of central Nevada reflects major Mesozoic shortening in the western US Cordillera, and involved contractional deformation in Triassic and lower Jurassic back-arc basinal strata. Structural analyses in the Santa Rosa Range, in the northern LFTB, provide new insight into the evolution of this belt. Four phases of deformation are recognized in the Santa Rosa Range. D1 involved tight to isoclinal folding, cleavage development under low-grade metamorphic conditions, and reverse faulting. This deformation phase reflects NW–SE shortening of 55–70% in the Early and/or Middle Jurassic. D2 structures are similar in orientation to D1 but involved much less overall strain and are well developed only to the southeast. D2 appears to be related to thrusting along the eastern margin of the LFTB in the Middle and/or Late Jurassic. D3 deformation reflects very minor shortening (<5%) in a subvertical direction, and is tentatively interpreted to reflect stresses generated during initial intrusion of mid-Cretaceous plutons in the area. D4 deformation demonstrably occurred synchronously with emplacement of Cretaceous granitoids dated at 102 Ma (U–Pb zircon) based on syntectonic relations between D4 structures and thermal metamorphism associated with intrusion, and an upgrade in D4 strain in the thermally softened metamorphic aureoles of the intrusions. This last phase of deformation reflects minor regional NE–SW shortening, coupled with localized strain associated with pluton emplacement.Formation of the LFTB structural province was accomplished during the D1 and D2 phases of deformation, and most shortening occurred during the D1 event. This Jurassic deformation led to structural closure of the back-arc basin by top-to-the-SE tectonic transport and development of a largely ductile fold-thrust belt. Subsequent deformation (D3 and D4) is >50 m.y. younger and unrelated to development of the LFTB. The younger deformation reflects a combination of minor regional shortening, interpreted to be related to the Cretaceous Sevier orogeny, plus localized shortening related to emplacement of Cretaceous intrusions.  相似文献   

17.
The Variscan metamorphism in the Pyrenees is dominantly of the low‐pressure–high‐temperature (LP‐HT) type. The relics of an earlier, Barrovian‐type metamorphism that could be related to orogenic crustal thickening are unclear and insufficiently constrained. A microstructural and petrological study of micaschists underlying an Ordovician augen orthogneiss in the core of the Canigou massif (Eastern Pyrenees, France) reveals the presence of two syntectonic metamorphic stages characterized by the crystallization of staurolite (M1) and andalusite (M2), respectively. Garnet is stable during the two metamorphic stages with a period of resorption between M1 and M2. The metamorphic assemblages M1 and M2 record similar peak temperatures of 580°C at different pressure conditions of 5.5 and 3 kbar, respectively. Using chemical zoning of garnet and calculated P–T pseudosections, a prograde P–T path is constrained with a peak pressure at ~6.5 kbar and 550°C. This P–T path, syntectonic with respect to the first foliation S1, corresponds to a cold gradient (of ~9°C/km), suggestive of crustal thickening. Resorption of garnet between M1 and M2 can be interpreted either in terms of a simple clockwise P–T path or a polymetamorphic two‐stage evolution. We argue in favour of the latter, where the medium‐pressure (Barrovian) metamorphism is followed by a period of significant erosion and crustal thinning leading to decompression and cooling. Subsequent advection of heat, probably from the mantle, leads to a new increase in temperature, coeval with the development of the main regional fabric S2. LA‐ICP‐MS U–Th–Pb dating of monazite yields a well‐defined date at c. 300 Ma. Petrological evidence indicates that monazite crystallization took place close to the M1 peak pressure conditions. However, the similarity between this age and that of the extensive magmatic event well documented in the eastern Pyrenees suggests that it probably corresponds to the age of monazite recrystallization during the M2 LP‐HT event.  相似文献   

18.
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   

19.
Data collected in the Port Wells gold mining district, Alaska, indicate several stages in the structural history of the district. The first stage was the accretion and associated deformation of the Valdez group flysch sequence at the end of the Cretaceous. The deformation of the semilithified rocks included two folding phases forming isoclinal NE-SW-striking and SE-vergent folds during a D1 phase, and minor open warps in NW-SE direction during a D2 phase. Intrusion of early Oligocene (36 Ma) calc-alkaline granitoids followed deformation and was terminated by the emplacement of aplitic dikes. The major fracturing processes in both the granitoids and the country rocks occurred subsequently, probably during the uplift of the Chugach mountains in the late Tertiary. Several generations of epigenetic gold-bearing quartz veins were emplaced along the fractures at a later stage. Due to the significant time gap between peak metamorphism and mineralization, the metamorphic secretion model proposed for the vein formation is reconsidered.  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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