Importance of fracturing during retro‐metamorphism of eclogites

Journal of Metamorphic Geology - Tập 17 Số 6 - Trang 637-652 - 1999
Straume1, Austrheim1
11 Mineralogisk‐Geologisk Museum, Sarsgt. 1, N‐0562‐Oslo, Norway (e‐mail: [email protected])

Tóm tắt

Presented textural and petrological data show that the deep to intermediate continental crust may fracture and that microfractures are the locus of fluid and mass transfer necessary for retrograde metamorphism. Kyanite eclogites from Ulsteinvik, Norway, underwent partial retrogression to granulite and amphibolite facies assemblages during near‐isothermal exhumation from depths equivalent to more than 2.0 GPa at temperatures of 700–800 °C. Plagioclase‐bearing assemblages, rich in hydrous phases, formed along margins of eclogite lenses and along mesoscopic fracture systems. Hydrated zones are from 1–50 cm thick, with adjacent wall‐rock eclogite replaced by symplectites. At a low degree of reaction, the secondary minerals in the wall‐rock are found along intra‐ and intergranular microfractures (typically 50–100 μm wide). Minerals filling the microfractures include orthopyroxene–plagioclase–spinel in garnet; plagioclase–sapphirine, plagioclase–corundum and plagioclase–spinel in kyanite; and diopside–plagioclase in omphacite. The microfractures are often arranged en echelon and are connected through microfaults. Releasing bends filled with amphibole and spinel form along microfaults in garnet. The faulting and fracturing caused localized chemical change in garnet: the damage zones close to faults are enriched in FeO and MnO with steep compositional gradients (8 wt% FeO over <20 μm). These FeO‐ and MnO‐enriched zones form wedge‐like structures around the tip of the faults (horsetail structures) and rose‐ or flame‐like structures at sticking points along faults. They may represent examples of stress‐induced chemical transport during fracture propagation. The change from dry to amphibole‐bearing assemblages at the tip of the fracture, and fractures ending in splays of fluid inclusions trails, reflect the involvement of a fluid phase during fracture propagation. This suggests that the ‘dry’ granulite facies retrogression was also driven by fluid infiltration and that metamorphism at depth in collision zones may not be controlled by pressure and temperature alone.

Từ khóa


Tài liệu tham khảo

10.1029/RG013i002p00383

10.1111/j.1365-3121.1991.tb00148.x

AtkinsonB. K.&MeredithP. G.1987The theory of subcritical crack growth with applications to minerals and rocks. In:Fracture Mechanics of Rocks(ed. Atkinson B. K.) pp. 111166Academic Press London

10.1016/0012-821X(87)90158-0

10.1111/j.1365-3121.1991.tb00184.x

10.1126/science.265.5168.82

10.1093/petrology/11.2.337

Brady J. B., 1983, Intergranular diffusion in metamorphic rocks., American Journal of Science, 283, 181

10.1111/j.1525-1314.1995.tb00237.x

10.1127/ejm/1/3/0455

Chopin C., 1997, Granulite‐facies overprint in ultrahigh‐pressure rocks (Dora Maira Massif): evidence for low temperature granulites?, Terra Nova, 9

10.1111/j.1525-1314.1993.tb00151.x

CuthbertS. J.&CarswellD. A.1990Formation and exhumation of medium‐temperature eclogites in the Scandinavian Caledonides. In:Eclogite Facies Rocks(ed. Carswell D. A.) pp.180203Blackie Glasgow

Deichmann N., 1987, Focal depth of earthquakes in northern Switzerland., Annales Geophysicae, 5, 395

Dewey J. F., 1993, Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites. In:, Magmatic Processes and Plate Tectonics, 76, 325

Dunn S. R., 1989, Retrograded eclogites in the Western Gneiss Region, Norway, and thermal evolution of a portion of the Scandinavian Caledonides., Lithos, 22, 229

10.1007/BF00371878

10.1007/BF00321220

10.1016/0016-7037(78)90292-2

10.1130/0091-7613(1981)9<419:AMFPSS>2.0.CO;2

10.1007/BF00371609

GriffinW. L. AustrheimH. BrastadK. BryhniI. KrillA. G. KroghE. J. MørkM. B. E. QualeH. TørudbakkenB.1985High‐pressure metamorphism in the Scandinavian Caledonides. In:The Caledonide Orogen – Scandinavia and Related Areas(eds Gee D. G. & Sturt B. A.) pp. 783801Wiley New York

Hames W. E., 1993, Fluid‐assisted modification of garnet composition along rims, cracks, and mineral inclusion boundaries in samples of amphibolite facies schists., American Mineralogist, 78, 338

10.1007/BF01187140

10.1093/petrology/25.3.665

10.1007/BF00371156

10.1007/BF00518032

10.1016/0024-4937(87)90017-X

10.1016/0040-1951(93)90326-F

10.1038/267017a0

KroghE. J.&CarswellD. A.1995HP and UHP eclogites and garnet peridotites in the Scandinavain Caledonides. In:Ultrahigh Pressure Metamorphism(eds Coleman R. G. & Wang X.) pp.244298Cambridge University Press Cambridge

10.1016/S0040-1951(96)00288-0

10.1007/BF00373761

Mysen B. O., 1973, Pyroxene stoichiometry and the breakdown of omphacite., American Mineralogist, 58, 60

10.1007/BF00372836

10.1016/S0191-8141(96)80023-X

10.1007/BF00698322

10.1016/0012-821X(79)90023-2

10.1180/minmag.1986.050.357.05

RubieD. C.1990Role of kinetics in the formation and preservation of eclogites. In:Eclogite Facies Rocks(ed. Carswell D. A.) pp.111140Blackie Glasgow

10.1144/gsjgs.154.3.0437

SchmittH. H.1963Petrology and structure of the Eiksunddal eclogite complex Hareidlandet Sunnmøre.Unpublished PhD Thesis Harvard University.

10.1038/310641a0

SmithD. C.1988A review of the peculiar mineralogy of the ‘Norwegian‐eclogite province’ with crystal‐chemical petrological geochemical and geodynamical notes and an extensive bibliography. In:Eclogites and Eclogite Facies Rocks(ed. Smith D. L.) pp.1206Elsevier Amsterdam

StraumeÅ. K.1997Retrograde metamorfose av eklogitter fra Haddal‐Ulsteinvik Sunnmøre.Unpublished MS Thesis University of Oslo.

10.1126/science.259.5102.1729

10.1038/331061a0

10.1007/BF00372216

10.1007/BF00372002

10.1130/0091-7613(1996)024<0147:GAOSDR>2.3.CO;2

10.1029/97JB01587

Thông tin
Thông tin xuất bản

Journal of Metamorphic Geology

Tập 17 Số 6

637-652

Thông tin tác giả