| 1. |
- Berry, R.F., et al.
(författare)
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Chemical U-Th-Pb monazite dating of the Cambrian Tyennan Orogeny
- 2007
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Ingår i: Australian Journal of Earth Sciences. - : Informa UK Limited. - 0812-0099 .- 1440-0952. ; 54:5, s. 757-771
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Tidskriftsartikel (refereegranskat)abstract
- Chemical U – Th – Pb dating of monazite from 12 schists throughout western and central Tasmania define a peak metamorphic age of ca 510 Ma. This age is very close to the age of arc – continent collision and ophiolite emplacement, implying very rapid uplift and cooling. To the south, along the western margin of the South Tasman Rise, metamorphism occurred later at 495 Ma, which correlates with a late stage of the Ross Orogeny, Antarctica. The Tyennan Orogeny in Tasmania has a three-stage history similar in age to the Delamerian Orogeny in South Australia. However, the Tyennan Orogeny only produced metamorphic rocks during the early stage associated with ophiolite obduction at 515 – 505 Ma. The intense compressional event recognised in the Delamerian and Ross Orogeny at 500 – 495 Ma is correlated with a mild basin inversion in Tasmania, and no metamorphism on mainland Tasmania has been recognised associated with this event. The western margin of the South Tasman Rise is a fragment of the Ross Orogen and does not correlate directly with Tasmania.
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| 2. |
- Chmielowski, Reia, et al.
(författare)
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Duplex structure and Paleocene displacement of the Toyuk thrust zone near the Dalton Highway, north-central Brooks Range
- 2000
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Ingår i: Short Notes on Alaska Geology 1999. - : Alaska Department of Natural Resources. ; , s. 11-32
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Bokkapitel (refereegranskat)abstract
- The Toyuk thrust zone is a regional tectonic boundary in the central Brooks Range of northern Alaska that has been interpreted previously as a single large-displacement fault. Just west of the Dalton Highway, the thrust zone is defined by several north-vergent imbricate thrust faults. These faults have cut detachment folds formed in the competent Kanayut Conglomerate between detachments in the underlying Hunt Fork Shale and overlying Kayak Shale, thereby forming a duplex. The exposed structural geometry suggests two endmember models of duplex geometry. In model one, each linking thrust has roughly the same small displacement and the roof thrust is overlain by a normal stratigraphic succession. In model two, the uppermost and hindmost fault has greater displacement than the other faults and so forms a duplex roof above which the stratigraphic section is duplicated. Discrimination between these models is not possible because the duplex roof has been eroded in the field area.Apatite fission-track analyses of samples collected within the Toyuk thrust zone record two distinct episodes of cooling interpreted to represent unroofing in response to tectonic deformation. The initial episode occurred at ~100 Ma, while the later episode occurred at ~60 Ma. These two cooling/denudation events have been documented previously in the region, but the samples from the thrust zone are the first to document displacement on a specific fault during the ~60 Ma event so far south of the front of the central Brooks Range. The results suggest that at least part of the growth of the Toyuk thrust zone accommodated structural thickening within the orogenic wedge coeval with deformation at the range front.
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| 3. |
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| 4. |
- Palmieri, R., et al.
(författare)
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Petrology of the eclogites from western Tasmania : Insights into the Cambro-Ordovician evolution of the paleo-Pacific margin of Gondwana
- 2009
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 109:3-4, s. 223-239
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Tidskriftsartikel (refereegranskat)abstract
- Eclogite facies rocks along the Paleozoic active margin of Gondwana are rare. They are limited to isolated segments of Northern Victoria Land (Antarctica), western Tasmania, and southeastern Australia. New petrological data for mafic rocks and their host garnet-kyanite schists from the Franklin Metamorphic Complex (western Tasmania) permit reconstruction of six main stages of mineral growth for the eclogite. Stages I and II occurred at greenschist/ amphibolite-facies conditions (ca. 500–600 °C; 0.55–0.7 GPa for stage II) before attaining high-pressure conditions (at≈600–650 °C; N1.5GPa for stage III). The following stages, IV and V, record the decompression from high-pressure conditions to amphibolite-facies (ca. 500–600 °C; 0.4–1.0 GPa). Finally, stage VI represents the late greenschist-facies retrogression. However, the pelitic schist surrounding the eclogite records only the medium pressure amphibolite- facies stage. The P–T evolution over time outlines a clockwise path that is quite steep in both the prograde and retrograde segments. The latter shows a nearly isothermal decompression between the eclogite and the high-pressure amphibolite-facies stage IV, which was achieved at deep crustal levels (≈30 km), and a final decrease in both pressure and temperature from deep/intermediate to shallow crustal levels, with a typical cooling-unloading path. The lack of a complete re-equilibration during the different stages and the high dP/dT for both the prograde and retrograde paths are indicative of a rapid burial and initially rapid exhumation. The similarity of the mafic whole-rock chemical composition, including N, T to E-MORB and of the peak metamorphic age (≈500Ma) between the Tasmanian eclogites and the UHP rocks from Northern Victoria Land, supports the idea that they formed in the context of the same contractional event. However, the different P–T conditions and dP/dT point to different tectono-metamorphic settings for the two sectors of the paleo-Pacific margin of Gondwana during the Ross/Tyennan orogeny.
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| 5. |
- Rao, C.V. Dharma, et al.
(författare)
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New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh-temperature sapphirine granulites
- 2011
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Ingår i: Geological Journal. - : Wiley. - 0072-1050 .- 1099-1034. ; 46:2-3, s. 240-262
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Tidskriftsartikel (refereegranskat)abstract
- The sapphirine granulites from Gangaraju Madugula, Eastern Ghats Belt (EGB), India preserve a rich variety of mineral assemblages and unique isolated and composite mineral inclusions within garnet that provide robust evidence for extreme crustal metamorphism at ultrahigh temperature (UHT) conditions (>900°C). Diagnostic UHT assemblages in these rocks include sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz. The stability of spinel + quartz, sapphirine + quartz and orthopyroxene + sillimanite + quartz assemblages provides evidence for temperatures exceeding 960°C at moderate pressures. The mineral association of garnet–orthopyroxene is indicative of a subsequent high P–UHT metamorphic event as indicated by the high alumina contents of orthopyroxene (>10 wt% Al2O3) coexisting with garnet. Peak P–T conditions of ∼970°C and 9.5 kbars are calculated from conventional garnet–orthopyroxene geothermobarometry. Calculated isochemical sections constructed in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) for the sapphirine granulites and garnet–orthopyroxene granulites adequately predict phase relationships that are consistent with those observed in the rocks. An evaluation of the assemblages and textures and P–T estimates indicate a three-stage evolution of the sapphirine granulites and associated garnet–orthopyroxene granulites: (1) inclusion assemblages with sapphirine, spinel and quartz on the low-pressure prograde path (M1 stage); (2) a peak UHT assemblage of porphyroblastic garnet–orthopyroxene (M2 stage) and (3) a retrogression that resulted in orthopyroxene–cordierite intergrowths and biotite rims on Grt (M3 stage). The sapphirine granulites and associated garnet–orthopyroxene granulites indicate that they grew during the prograde and retrograde stage. The thermo-barometric estimates from mineral compositions and the complimentary isochemical sections approached from bulk rock compositions allow tighter constraints to be placed on the P–T evolution of this sector of the EGB.
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| 6. |
- Rao, C.V. Dharma, et al.
(författare)
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Sapphirine granulites from Panasapattu, Eastern Ghats belt, India : Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin
- 2011
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 3:1, s. 9-31
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Tidskriftsartikel (refereegranskat)abstract
- We report equilibrium sapphirine + quartz assemblage in biotite–orthopyroxene–garnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature (UHT) metamorphism at 1030–1050 °C and 10 kbar in this region. The development of migmatitic texture, stabilization of the garnet–orthopyroxene–plagioclase–K-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a Bt–Sill–Qtz assemblage, and constrain the prograde stage of the P–T path. The core domains of orthopyroxene porphyroblasts have up to wt(Al2O3) 9.6%, which suggest that the temperatures reached up to 1150 °C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garnet–orthopyroxene thermobarometery, which yields a P–T range of 1012–960 °C and 9.4 kbar. The P–T phase topologies computed using isochemical sections calculated in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These P–T results underestimate ‘peak’ conditions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at P < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the P–T path followed a near isobaric cooling stage to T < 900 °C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subduction–accretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subduction–collision tectonics in a Proterozoic convergent plate margin.
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