High-pressure metamorphism in the Western Cordillera of North America   总被引：1，自引：0，他引：1  

D. L. WHITNEY 《Journal of Metamorphic Geology》1992,10(1):71-85

The Skagit Gneiss, a major component of the crystalline core of the North Cascades, was metamorphosed during a mid-Cretaceous(?) to early Tertiary high-P event driven by the collision of the Insular and Intermontane superterranes. Maximum pressures recorded by metapelitic rocks are 8–10 kbar at 650–725° C. High pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the Skagit Gneiss and adjacent Cascade River Schist. Mineral reactions continued during nearly isothermal decompression from 8–10 kbar to c. 3–5 kbar. Early high-P minerals (e.g. kyanite) are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass. Skeletal relics of kyanite are also present in the groundmass of lower-grade, staurolite-bearing schists that contain texturally later cordierite. This matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature. These results represent a revision of the metamorphic history of the Skagit Gneiss, which was formerly thought to have experienced only relatively low-P Barrovian metamorphism. Qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression. Geobarometric results for the Skagit Gneiss are interpreted as evidence for major crustal thickening in the North Cascades. Recognition that pressures of c. 9 kbar were attained supports a contractional model for North Cascades orogenesis and requires that tectonic syntheses account for the burial of the Skagit Gneiss protoliths to a depth of c. 25–30 km.  相似文献   

Crustal trajectory of sapphirine-bearing granulites from Ganguvarpatti, South India: evidence for an isothermal decompression path   总被引：7，自引：0，他引：7  

A. MOHAN  B. F. WINDLEY 《Journal of Metamorphic Geology》1993,11(6):867-878

Abstract Ganguvarpatti is part of a Precambrian terrane characterized by granulite facies rocks, including charnockites, mafic granulites, sapphirine-bearing granulites, leptynites and gneisses. A sequence of reactions deduced from the multiphase reaction textures provide information on the metamorphic history of this area, as they formed in response to decompression during uplift. Geothermobarometry and constraints from reaction textures define a segment of a P–T path traversed by the granulites of Ganguvarpatti. Near-peak metamorphic conditions of c. 800°C and 8 kbar were succeeded by a symplectitic stage at a significantly lower pressure ( c. 700°C and 4.5 kbar), documenting a nearly isothermal decompression P–T path and rapid uplift ( c. 12 km) followed by cooling. The presence of many fluid inclusions of extremely low density in the charnockites is consistent with a nearly isothermal uplift path. Attainment of a maximum pressure of c. 8 kbar indicates c. 27 km depth of burial during metamorphism. This would imply a total crustal thickness of c. 65–70 km at 2.6–2.5 Ga. Such a profound crustal thickness and a clockwise decompressive P–T path is interpreted as a consequence of tectonic thickening of crust, accomplished by collision tectonics of the southern granulite terrane against the Dharwar craton along the Palghat–Cauvery shear zone via northward subduction.  相似文献   

High-pressure–low-temperature metamorphism and deformation in the Bündnerschiefer of the Engadine window: implications for the regional evolution of the eastern Central Alps     

R. BOUSQUET  R. OBERHÄNSLI  B. GOFFE  L. JOLIVET  & O. VIDAL 《Journal of Metamorphic Geology》1998,16(5):657-674

A study of the metamorphic and tectonic evolution of the Bündnerschiefer of the Engadine window shows that the individual nappes have been thinned by a large amount and that extension was active during and soon after nappe stacking.
Based on contrasting P–T  histories the Penninic Bündnerschiefer can be divided in two major units bounded by a horizontal contact. The lower (Mundin) unit shows typical high- P /low- T  parageneses in metapelites (Mg-carpholite) and in metabasites (glaucophane); metamorphic conditions are estimated around 12  kbar, 375  °C. The upper (Arina) unit contains no specific high- P minerals; metamorphic conditions are estimated around 7  kbar, 325  °C. A minimum pressure gap of 5  kbar is thus observed. The contact between the two units is marked by a mappable normal shear zone with top-to-the-north-west sense of shear. Near the shear zone, fresh carpholite fibres trend parallel to the regional stretching lineation, implying that the detachment is an early structure active from the depth of stability of the carpholite and persisting during subsequent exhumation. The good preservation of carpholite and the absence of retrograde chloritoid below the shear zone show that exhumation occurred along a cooling path, whereas the deeper units are exhumed along an isothermal path. Exhumation probably occurred during convergence and further nappe stacking during the earlier Eocene. These results suggest that pre-collisional tectonic thinning of the Penninic oceanic units may be more widespread and significant than generally recognized.
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Ultrahigh temperature granulite metamorphism (1050  °C, 12  kbar) and decompression in garnet (Mg70)–orthopyroxene–sillimanite gneisses from the Rauer Group, East Antarctica     

S. L. HARLEY 《Journal of Metamorphic Geology》1998,16(4):541-562

Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to X_Mg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al₂O₃ to rims with up to 10.6 wt% Al₂O₃ adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al₂O₃–SiO₂ and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet X_Mg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.  相似文献   

Metamorphic reactions related to decompression and synkinematic intrusion of leucogranite, High Himalayan Crystallines, Bhutan'pa     

C. DAVIDSON  D.E. GRUJIC†  L. S. HOLLISTER  & S. M. SCHMID 《Journal of Metamorphic Geology》1997,15(5):593-612

The High Himalayan Crystallines (HHC) of Bhutan were penetratively deformed, intruded by leucogranite and metamorphosed during the collision of the Indian and Asian plates. Metamorphic reaction textures in the HHC show that it experienced decompression while maintaining a laterally heterogeneous, and locally inverted, internal temperature range of c. 600–750  °C. This thermal structure was produced by thrusting hot, migmatitic rocks over lower-grade rocks within the HHC and by the advection of heat from the intrusion of leucogranite dykes and sills during decompression. A variable velocity field within the HHC during exhumation and extrusion between India and Tibet caused the inversion of top to the south sense of shear present throughout most of the HHC to top down to the north shear near its top.  相似文献   

P-T  path determinations in the Tormes Gneissic Dome, NW Iberian Massif, Spain     

J. ESCUDER VIRUETE  A. INDARES  & R. ARENAS 《Journal of Metamorphic Geology》1997,15(5):645-663

The Tormes Gneissic Dome (TGD, NW sector of the Iberian Massif, Spain) is a high-grade metamorphic complex affected by a major episode of extensional deformation (D2). The syn-D2 P–T  path of the Lower Unit of the TGD was deduced from the analysis of reaction textures related to superimposed fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations. It comprises an initial phase of decompression, determined using the tweequ thermobarometric technique, from 6.4–8.1 kbar at 735–750 °C (upper structural levels) and 7.2 kbar at 770 °C (lower structural levels) to 3.3–3.9 kbar and 645–680 °C. This evolution is consistent with the observed sequence of melting reactions and the generation of garnet- and cordierite-bearing anatectic granitoids. The later part of the syn-D2 P–T  path consisted of almost isobaric cooling associated with the thermal re-equilibration of the unit in the new structural position. This segment of the P–T  path is recorded by assemblages with And +Bt+Ms and Ms+ Chl +Ab related to the later mylonitic S2 fabrics, which indicate retrogression to low-amphibolite and greenschist facies conditions.  相似文献