Journal
LITHOS
Volume 314, Issue -, Pages 165-186Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.lithos.2018.05.030
Keywords
Eastern Alps; Pyroxenite dyke P-T path; Zircon U-Pb SIMS age; Lu/Hf isotopes; Rock and mineral chemistry
Categories
Funding
- Slovak Research and Development Agency [APVV-15-0050]
- China Ministry of Science and Technology [2016YFE0203000]
- VEGA Agency [1/0079/15]
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Websterite and garnet orthopyroxenite and clinopyroxenite dykes crosscutting harzburgite occur in the hanging wall of a Cretaceous eclogitized continental fragment of the Upper Austroalpine Unit in the Eastern Alps. The dyke textures reflect pressure-temperature changes from the mantle, subduction channel and an accretionary wedge. A clinopyroxenite dyke, dated at 252 +/- 2 Ma on zircon by U-Pb SIMS, provided epsilon(Hf)(t) zircon values of 16-8 (ay. 11 from 18 spots) suggesting a depleted mantle source. The rock modal mineral composition and the mineral and whole-rock chemistry revealed that the pyroxenite dykes' petrogenesis is most likely related to sub-crustal cumulate-type melt derivates, less evolved for websterites and highly evolved for clino- and orthopyroxenites. The porphyroclastic clinopyroxenel (10-13 mol% Ca-Ts), orthopyroxene1, spinel1 and amphibole1 are inferred remnants after magmatic phases. They contain Ti-spinel and ilmenite exsolutions which indicate cooling of the dykes in harzburgite in a Spl stability field (D0-1 stage). Garnet exsolutions in Cpx1, Opx1 and Amp1, and Cpx exsolutions in Amp1 document continuous cooling and/or pressure increase (D0-2 stage). The Grt coronas around Spl1, Opx1, Cpx1 and Amp1, and the granoblastic matrix of Prp-Grs-rich Grt (or Cr-Spl in websterites), low-Al Cpx, low-Al Opx, Fe-enriched olivine, zoisite, kyanite, rutile and pargasite record a subduction channel compression (D1 stage). While the formation of Cpx-Grt or Amp-Grt symplectites replacing the Cpx1 and Amp1 rims suggests the HP-HT decompression (D2-1 exhumation stage). The Grt (or Cr-Spl) replacement by Spl, and Opx1, Cpx1 and Prg replacement by Ol-Spl, Opx-Spl or Prg-Spl symplectites document continuous HT decompression (D2-2 exhumation stage). The MP/MT-LT conditions were estimated from tremolite, Grs-rich Grt, Sr-epidote, clinozoisite, Cl-apatite, titanite, Ce-allanite, chlorite, serpentine and talc assemblage (D3 stage). Anticlockwise P-T path was reconstructed for the D0 to D1 stages followed by almost isothermal exhumation (D2) and emplacement in the Cretaceous accretionary wedge (D3). The pseudosection P-T calculation from a clinopyroxenite dyke yielded 850-825 degrees C and 25-28 kbar (D1). The whole-rock and mineral trace element data of relic Cpx1 and Amp1, Grt(1-3), Ep-Zo, and younger zircon ages due to Pb-loss, reflect the impact of metamorphic fluids during the D1 to D3 evolutionary stages. (C) 2018 Elsevier B.V. All rights reserved.
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