Discrete ultrahigh‐pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation

Journal of Metamorphic Geology - Tập 23 Số 1 - Trang 45-61 - 2005
D. B. Root1,2, Bradley R. Hacker2, Phillip B. Gans2, Mihai N. Ducea3, Elizabeth A. Eide4, J. L. Mosenfelder5
1Smithsonian Institution
2University of California at Santa Barbara
3Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
4Geological Survey of Norway
5Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

Tóm tắt

AbstractNew eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/39Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. 40Ar/39Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr−1), perhaps because of continued tectonic unroofing.

Từ khóa


Tài liệu tham khảo

10.1016/S0040-1951(97)00277-1

10.1144/gsjgs.155.2.0297

10.1016/S0040-1951(98)00281-9

10.1007/978-94-009-2549-6_18

10.1144/GSL.SP.1999.154.01.09

10.1046/j.1525-1314.2003.00467.x

10.1016/S0024-4937(03)00014-8

10.1093/petrology/41.10.1471

10.1016/0040-1951(92)90133-Q

10.1016/0040-1951(94)90048-5

10.1016/0024-4937(80)90051-1

10.1016/S0024-4937(99)00090-0

10.1029/2001GC000184

10.1086/314351

Gjelsvik T., 1951, Oversikt over bergartene på Sunnmøre og tilgrensende deler av Nordfjord, Norges Geologiske Undersøkelse, 179, 1

10.1029/JB095iB04p04895

10.1016/S0024-4937(03)00092-6

Hacker B.R., 2004, Formation and exhumation of the Western Gneiss Region ultrahigh‐pressure terrane: geochronology, structural geology, and petrology, Geological Society of America Abstracts with Program, 36, 533

Hernes I., 1956, The Surnadal syncline, central Norway, Norsk Geologisk Tidsskrift, 36, 25

10.1111/j.1525-1314.1998.00140.x

10.1130/0091-7613(2000)28<1127:RNTRIF>2.0.CO;2

Krabbendam M., 1997, Late‐Caledonian structures, differential retrogression and structural position of (ultra)high‐pressure rocks in the Nordfjord–Stadlandet area, Western Gneiss Region, Norges Geologiske Undersøkelse Bulletin, 432, 127

10.1007/BF00374722

Lutro Ø., 1990, Berggrunnskart 1119 IV

Lux D.R., 1985, K/Ar ages from the Basal Gneiss Region, Stadlandet area, Western Norway, Norsk Geologisk Tidsskrift, 65, 277

10.1016/0024-4937(86)90026-5

10.1007/BF00372836

10.1007/978-94-015-9050-1_2

10.1111/j.1525-1314.1998.00157.x

10.1016/S0024-4937(03)00098-7

10.1016/S0009-2541(97)00159-9

Rickard M.J., 1985, The Caledonide Orogen–Scandinavia and Related Areas, 485

Robinson P., 1995, Extension of Trollheimen tectono‐stratigraphic sequence in deep synclines near Molde and Brattvåg, Western Gneiss Region, southern Norway, Norsk Geologisk Tidsskrift, 75, 181

10.1016/j.epsl.2004.10.019

10.1016/0012-821X(80)90065-5

Schouenborg B.E., 1988, U/Pb–zircon datings of Caledonian cover rocks and cover‐basement contacts, northern Vestranden, central Norway, Norsk Geologisk Tidsskrift, 68, 75

10.1029/RG018i001p00269

10.1038/310641a0

Spear F.S., 1989, Program gibbs: a generalized Gibbs method algorithm, American Mineralogist, 74, 942

10.1029/2002TC001401

10.2138/am-2000-11-1207

10.2138/am-2000-11-1208

10.1144/gsjgs.145.3.0413

Tucker R.D., 1990, Mid‐Proterozoic Laurentia–Baltica, 149

Tveten E., 1998, Geologisk kart over Noreg, berggrunskart

Tveten E., 1998, Geologisk kart over Noreg, berggrunskart

10.1130/0091-7613(1997)025<0927:NEFCIE>2.3.CO;2

10.1111/j.1525-1314.2004.00541.x

Walsh E.O., Time constraints on the exhumation of (ultra)high‐pressure rocks across the Western Gneiss Region, Norway, Tectonophysics

10.1016/0168-9622(91)90010-T

10.1046/j.0263-4929.2000.00303.x

10.1016/S0012-821X(68)80059-7

Young D., 2003, Exhumation of UHP rocks in western Norway: the role of the Nordfjord–Sogn Detachment Zone, Geological Society of America Abstracts with Programs, 35, 28

Thông tin
Thông tin xuất bản

Journal of Metamorphic Geology

Tập 23 Số 1

45-61

Thông tin tác giả