Ultramafic cumulates of oceanic affinity in an intracontinental subduction zone: ultrahigh-pressure garnet peridotites from Pohorje (Eastern Alps, Slovenia)

Cees-Jan De Hoog, Marian Janak, Mirijam Vrabec, Keiko Hattori

Research output: Chapter in Book/Report/Conference proceedingChapter (peer-reviewed)

Abstract

Garnet peridotites from the Slovenska Bistrica Ultramafic Complex (SBUC) in the Pohorje Mts., Eastern Alps, are for the most part metamorphosed olivine-rich gabbros that crystallised at low pressures (<5 kbar) within depleted mantle of oceanic affinity. The metamorphic assemblage of the garnet peridotites consists of olivine (Fo 87-90), Mg-rich (Py 65-67) garnet and low-Al pyroxenes which recorded ultrahigh-pressure peak P-T conditions up to 4 GPa and 900°C during Eo-alpine intracontinental subduction in the Cretaceous. Remnants of magmatic minerals include exsolved pyroxenes, Ti-rich chromian spinel, ilmenite, apatite and possibly An-rich plagioclase. Relict igneous textures indicate plagioclase and olivine as cumulus minerals and clinopyroxene as an intercumulus phase. Two types of exsolved Cpx can be distinguished: Low-Ti (0.3-0.5 wt.% TiO2) Cpx has LREE-depleted trace-element patterns that strongly resemble Cpx from olivine-rich gabbros and troctolites from a mid-ocean ridge, whereas high-Ti Cpx (0.6-1.8 wt.% TiO2) have strongly negative Eu anomalies and less depleted LREE, indicative of crystallisation from more fractionated melts than low-Ti Cpx. A cumulate origin of the garnet peridotites is supported by low but fractionated whole rock PGE contents (0.1-0.2 ppb Ir, Pd/Ir=5-15) and trace-element patterns similar to those of associated Pohorje eclogites of predominantly MORB affinity. The shallow level of intrusion of the garnet peridotite protoliths indicates that the SBUC mantle rocks were (partially) exhumed at the time of magma emplacement, but their depleted composition argues against formation during continental rifting. Hence, the SBUC and garnet peridotites were probably emplaced within the continental crust either during Eo-alpine intracontinental subduction, in which case the SBUC may represent a remnant of subducted Meliata ocean floor, or before the Permian as an ophiolite complex.
Original languageEnglish
Title of host publicationUltrahigh-Pressure Metamorphism: 25 years since the discovery of coesite
EditorsLarissa Dobrzhinetskaya, Shah Faryad, Simon Wallis, Simon Cuthbert
Place of PublicationNew York
PublisherELSEVIER ACADEMIC PRESS INC
Pages399-439
ISBN (Print)978-0-12-385144-4
DOIs
Publication statusPublished - 2011

Keywords

  • garnet peridotite
  • ultramafic cumulate
  • olivine gabbro
  • continental subduction
  • ultrahigh-pressure metamorphism
  • Eastern Alps
  • Pohorje

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