Chromium-rich lawsonite in high-Cr eclogites from the Făgăras Massif (South Carpathians)

Lawsonite is a relatively rare phase in natural rocks, because of its thermal decomposition during exhumation, and Cr-bearing lawsonite being restricted to only a few occurrences worldwide. Here we report Cr-lawsonite in eclogites hosted in a medium-grade metamorphic complex. Several high-Cr eclogite lenses occur in the Topolog Complex (Făgăras Massif) of dominantly gneissic-amphibolitic composition. High Cr contents are the result of emerald-green mm-sized nodules containing Cr-rich minerals, embedded in a gray-green matrix of kyanite, clinopyroxene, garnet, amphibole, zoisite, and rutile. Garnets occur as porphyroblasts or in coronas around clinopyroxene aggregates probably replacing former magmatic pyroxene. Relict gabbroic textures (sometimes pegmatoid) and whole rock geochemistry indicate a gabbroic cumulate origin. The REE pattern, displaying a slight positive Eu anomaly and a tea spoon-shaped LREE depletion is also indicative of a cumulate origin, as also noted by Pe Piper & Piper (2002) for the Othrys gabbro (as well as others in the Vourinos and Pindos ophiolitic suites) with the same unusual REE-pattern. The emerald-green Cr-rich nodules are unevenly distributed in the rock, and always enclosed in Cr-rich clinopyroxenes (up to 5.46% Cr2O3) which may exhibit Cr-diffusion haloes towards normal Cr-free matrix pyroxene. The nodules consist of diablastic chromite, rutile and Cr-rich kyanite of up to 15.67 wt% Cr2O3, Cr-bearing epidote, to which Cr-rich staurolite (up to 10.45% Cr2O3; XMg up to 0.68) and Cr-rich lawsonite (up to 9.17% Cr2O3) may exceptionally associate. Cr concentrations in kyanite and lawsonite are, to our knowledge, the highest reported so far. Cr-lawsonite was identified in a single sample, as small single phase inclusions armoured in Cr-kyanite. Equilibrium PT-conditions of 2.6 GPa and 610o C were derived from the garnet-mantled clinopyroxene aggregates using multi-equilibria calculation with the PTGIBBS routine of Brandelik & Massonne (2004). These conditions fall still inside the stability field of lawsonite, below the thermal breakdown of this phase (Pawley, 1994), and consistent with its metastable persistence as armoured relics, during a near-isobaric late stage heating episode recorded in eclogites from the same structural setting (Săbău & Negulescu, 2006). Acknowledgements Financial support by grant PN-II-ID-PCE-2011-3-0030 Romanian Executive Unit for Financing Higher Education, Research, Development and Innovation. Prof. Hans-Joachim Masonne kindly provided access to the analytical facilities at Institut für Mineralogie und Kristallchemie in Stuttgart. References Brandelik, A., Massonne, H.-J. (2004). PTGIBBS - an Excel™ Visual Basic program for computing and visualizing thermodynamic functions and equilibria of rock-forming minerals. Computers and Geosciences 30, 909-923. Pawley, A. (1994) The pressure and temperature stability limits of lawsonite: Implication for H2O recycling in subduction zones. Contrib. Mineral. Petrol. 118, 99-108. Săbău, G., Negulescu, E. (2006) Metamorphic forming and evolution conditions of the eclogites in the Topolog Complex, Făgăras Mountains. Romanian Journal of Petrology 78, 39-46.