Relative strength of the three major phases of fertile continental lithospheric mantle during deformation

Abstract:

The Dinaride peridotite massifs in the eastern Mediterranean are a part of the Alpine - Himalayan suture zone of the former Tethys ocean that separated Gondwana and Eurasia during the Mesozoic. In contrast to the more extensively studied Alpine and western Mediterranean massifs the western-most Dinarides are comparatively fertile, containing 10 - 15 % clinopyroxene. Rare earth element patterns of cpx from spinel peridotites from the Krivaja-Konjuh massif, one of the largest in the Dinarides, show modest depletion without refertilization. The mantle portion of the massif consists predominantly of spinel peridotite, with plagioclase peridotite occurring localized, stratigraphically beneath troctolites, gabbros and minor basalt. The spinel peridotites throughout the massif show variable degrees of deformation at relatively low (lithospheric) temperatures, with cpx thermometry yielding temperatures predominantly around 900oC. The least deformed samples indicate pre-deformation grain sizes near 1 cm, the most highly deformed samples are mylonytic/ultramylonitic with grain sizes around 50 micron. The massif may therefore represent a record of continental break-up, with deformation occurring at modest temperatures in a four phase lherzolite. The range of deformation grades in different parts of the massif enables examination of progressive grain size reduction due to recrystallization. The relative strength of the three major phases olivine, opx and cpx can be inferred from grain shapes and degree of recrystallization of each phase. Opx is typically the strongest phase, and in some cases rotates as rigid clasts. Cpx is in some samples elongated to aspect ratios > 20:1, in others completely recrystallized, similar to olivine. In some samples opx is stretched and begins to recrystallize, while some olivine grains remain porphyroclastic. Detailed microstructural observations together with thermometry will help illuminate deformation processes that may lead to strain localization in a fertile peridotite in the absence of melt or significant water contents. In particular the role of phase mixing will be examined.