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Abstract Syn-metamorphic re-imbrication of the internal part of thrust belts can result in distinct pressure–temperature–time–deformation ( P–T–t–d ) pathways for different structural–metamorphic domains. In the early Proterozoic Cape Smith Thrust Belt (Canada), an external (piggyback-sequence thrusting) domain is characterized by thermal peak metamorphism occurring after deformation. In contrast, thermal peak metamorphism in an internal domain occurred during re-imbrication by out-of-sequence thrusting. The interactions of tectonic and thermal processes have been studied using three methods: (i) qualitative evaluation of the timing between mineral growth and deformation; (ii) analytical P–T paths from growth-zoned garnet porphyroblasts; and (iii) numerical modelling of vertical heat conduction. Derived P–T–t–d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading. In contrast, paths for the internal domain indicate that the out-of-sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain. This is attributed to thickening by out-of-sequence thrusting and possibly to extensional faulting at (now eroded) higher structural levels. Observations on the timing of metamorphism, coupled with numerical modelling, suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out-of-sequence thrusts stack in the internal portion of the thrust belt. 相似文献
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Zoned Poikiloblastic Garnets: P-T Paths and Syn-Metamorphic Uplift through 30 km of Structural Depth, Wopmay Orogen, Canada 总被引：2，自引：0，他引：2
In the early Proterozoic Wopmay Orogen (Northwest Territories,Canada), the occurrence of garnet-biotite-sillimanite/kyanite-plagioclase-quartzassemblages in pelitic schists at a variety of obliquely exposedstructural levels enables the use of calibrated geothermometersand geobarometers through 30 km of composite structural relief.Direct derivation of multipoint P-T paths from single garnetsis attained from core-to-rim microprobe analyses of zoned poikiloblasticgarnets, which contain biotite, plagioclase, quartz, and lesscommonly Al2SiO5 inclusions. The documented garnet zoning andthe entrapment of the mineral inclusions is compatible withpartial-equilibrium growth models. The lack of significant diffusionre-equilibration in the garnet interiors is favored by samplerestriction to medium-grade schists and is attested by the preservationof normal-zoning profiles, the lack of garnet diffusion babesaround biotite inclusions, the matching composition trends ofgarnet-core to -rim plagioclase inclusions with those of zonedmatrix plagioclase grains, and the systematic variation of thederived P-T data. Only the garnet rims, which are characterizedby a reversal of compositional trends and by textural resorption,are interpreted to indicate local post-thermal-peak re-equilibration. The calculated P-T paths quantify segments of uplift trajectoriescorresponding to pressure drops of 2?5–1?5 kb from maximaof 9?3–5?0 kb depending on structural level. This is concurrentwith initial increases of 25–75?C to peak-temperatureconditions and is followed by variable drops in temperatureduring continued decompression. Individual paths are consistentwith modelled variations of metamorphic conditions as a functionof loading, uplift, and erosion in overthrust terrains. Consideredwith U-Pb zircon geochronological data the P-T paths, studiedas a set, indicate an average uplift rate that varies spatiallyfrom 1?5–2?7 mm y–1. This variation can be relatedto late cross folding of the orogenic internal zone, suggestingthat the syn-metamorphic uplift was structurally controlled.The distribution of peak-temperature conditions attained duringdecompression is independent of structural depth. This, andthe inverted metamorphism documented in the Wopmay Orogen, requirethat final variations in temperature result from thermal relaxationof isotherms in, and away from, a hot crustal allochthon composedin part of high-T rift-fill units and a syntectonic graniticbatholith. 相似文献
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MARC R. ST-ONGE 《Journal of Metamorphic Geology》1984,2(4):315-326
In north-central Wopmay Orogen, syntectonic low-P(Buchan-type) suites of mineral isograds outline regional metamorphic temperature culminations that are associated, at the higher structural levels, with emplacement of early Proterozoic plutons in the west part of a deformed and eastward transported continental margin prism. The mapped isograds mark the first occurrence of biotite, staurolite, andalusite, sillimanite, sillimanite-K feldspar and K feldspar-plagioclase-quartz ± muscovite (granitic) pods in metapelites, with increasing proximity to the plutons.
Microprobe analyses and field observations have resulted in the formulation of reactions for the 'ideal'pelitic system K2 O-Na2 O-FeO-MgO-Al2 O3 -SiO2 -H2 O-Al2 O3 -SiO2 -H2 O, to account for the various mineral assemblages of each metamorphic zone. A P-T petrogenetic grid showing erosion surface P-T curves for the northern Wopmay Orogen pelites, compiled on the basis of the mapped isograds and the inferred reaction(s) for each metamorphic zone, documents a variation in exposed metamorphic pressure ranging between 2 and 4 kbar.
The configuration of a new bathograd, based on the invariant model reaction sillimanite + K feldspar + plagioclase + biotite + quartz + vapor ± muscovite + liquid and interpolated across three metamorphic suites, is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite-grossular-quartz-Al2 SiO5 geobarometry. The positive correlation between the configuration of the bathograd and the structural and pressure culmination points to the pressure-dependence of anatectic-granitic-pod mineral associations. 相似文献
Microprobe analyses and field observations have resulted in the formulation of reactions for the 'ideal'pelitic system K
The configuration of a new bathograd, based on the invariant model reaction sillimanite + K feldspar + plagioclase + biotite + quartz + vapor ± muscovite + liquid and interpolated across three metamorphic suites, is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite-grossular-quartz-Al
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Two Pleistocene channel fills located in two different geological settings, on Guadeloupe in the lesser Antilles and along the Coppermine River in the Northwest Territories of Canada, have different clast compositions, scale, and origin of fragmentation but have similar depositional characteristics. Massive bedding or absence of structures caused by traction, sharp non-erosive contacts, reverse coarse-tail grading, matrix-supported clasts together indicate a debris-flow mechanism in which mixtures of fine and coarse sediments moved downslope by laminar flow. Field evidence suggests subaqueous deposition for both deposits. Both bed thickness (BTh)/maximum grain size (MGS) ratios and correlations appear characteristic of subaqueous masses capable of flowing on slopes less than 1°. The two channel fills have random BTh and MGS variations, they do not thin and fine up-section. It is suggested that fills originating from laminar mass transport could normally have random BTh and MGS up-section variations. In flysch and volcaniclastic sequences, where coarse sediments interpreted as laminar flow deposits are common, up-section BTh and MGS variations may result from processes related to source, slope, and flow characteristics rather than from the environment in which the sediments accumulate (channel). 相似文献
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Abstract The metamorphic history of the Archaean Superior Province crystalline basement in the Palaeoproterozoic Ungava Orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite-granulite transition. Two distinct metamorphic suites, separated in age by nearly one billion years, are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses. The older suite comprises c. 2.7-Ga granulite facies assemblages (orthopyroxene-clinopyroxene-hornblende-plagioclase-ilmenite ± biotite ± quartz) that record moderate pressures (±5 kbar) and high temperatures (±800° C). A younger, c. 1.8-Ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal décollement of the overlying Proterozoic Cape Smith Thrust Belt. Deformation/mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re-equilibration of the Archaean granulites occurred during thrust belt deformation. The transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet-hornblende-quartz ° Cummingtonite coronas between plagioclase and orthopyroxene-clinopyroxene, as well as titanite coronas on ilmenite. Multi-equilibrium thermobarometry on the coronitic assemblages documents re-equilibration of the granulitic gneiss to 7.7 kbar at 644° C in the south and 9.8 kbar at 700° C in the north. The variably deformed, amphibolite facies domain sandwiched between the coronitic garnet zone and the basal décollement is marked by significant metasomatic changes in major element concentrations within tonalite. These changes are compatible with equilibrium flow of an aqueous-chloride fluid down a temperature gradient. The source of fluids for basement hydration/metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt, with fluid flow probably focused along the basal décollement. 相似文献
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Mineral Corona Formation During High-P Retrogression of Granulitic Rocks, Ungava Orogen, Canada 总被引：1，自引：0，他引：1
Crystalline basement exposed in tectonic windows within theUngava Orogen records a polycyclic Archean granulite-faciesto Paleoproterozoic amphibolite-facies history. Amphibolite-faciesassemblages comprise garnet coronas around plagioclase, clinopyroxeneor cummingtonite coronas on orthopyroxene, hornblende coronason clinopyroxeneorthopyroxene, sodic rims on calcic plagioclase,and/or titanite coronas on ilmenite. Petrographic observationsand model reactions suggest that growth of coronitic garnetis closely associated with amphibolitization of twopyroxenegneisses. Calcic plagioclase constitutes a key reactant in allgarnet-producing reactions and possibly acted as a rate-controllingphase. Multi-equilibrium thermobarometric calculations showgood convergence of possible equilibria in the amphibolite-faciesrocks, indicating that coronitic textures need not imply completechemical disequilibrium. P—T determinations for the amphibolite-faciesgneisses beneath the thrust belt of Ungava Orogen are in therange 7.7–9.8 kbar and 585–723C. These values areconsistent with prograde determinations from pelitic schistswithin the thrust belt. Estimates of water activity clusterinto two populations. High aH2O values are obtained for highlystrained basement rocks adjacent to the thrust belt whereaslow aH2O values are derived for orthogneiss samples which showno thrust-related fabrics and are distal to the thrust belt. KEY WORDS: corona; high-P retrogression; multi-equilibrium thermo-barometry
*Corresponding author. Telephone: (613) 995-4935. Fax: (613) 995-9273. Internet: mstonge{at}cc2smtp.emr.ca 相似文献
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