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Título

Pressure-Temperature trajectory confirms ultrahigh-pressure eclogite in the South Carpathians

Texto do resumo

Subduction zones are surfaces along which rocks from the shallow parts of the crust are transported to greater depths and transformed into high-grade metamorphic rocks (e.g., eclogites). Tracking the thermobaric evolution (i.e., pressure-temperature [P-T] paths) undergone by these rocks is pivotal for understanding global geodynamics. The South Carpathians, built during the Alpine Orogeny, stand out as a host to a wide variety of eclogites, likely formed during different pre-Alpine orogenic cycles. However, the origin and evolution of these rocks still lack detailed study, particularly regarding the use of modern approaches that help us unravel more information on the evolution of metamorphic rocks. In this work, we focus on reconstructing the P-T path of a phengite-kyanite eclogite from the easternmost part of the South Carpathians (Leaota Massif) using petrography, mineral chemical analysis combined with X-ray compositional maps processed in XMapTools, whole-rock chemical analysis, and phase diagram modeling with Theriak-Domino. Our goal is to understand the subduction geodynamics that preserved this rock with a remarkable metamorphic record. The phengite-kyanite eclogite contains almandine- and pyrope-rich garnet, high-jadeite (Jd) clinopyroxene (XJd up to ~0.85, jadeite), high-Si phengite (up to ~3.5 c.p.f.u.), kyanite, quartz and rutile as peak metamorphic paragenesis. These are followed by the formation of amphibole coronas around garnet, paragonite and lower-Si phengite (down to ~3.25 c.p.f.u.) coronas and aggregates after kyanite, lower-XJd clinopyroxene (~0.55, omphacite) and amphibole + albite symplectites after jadeite-omphacite during the retrograde path. We have identified a distinct chemical zoning in garnet with relatively Xalmandine- and Xspessartine-rich cores, Xpyrope-rich inner mantle, Xgrossular-rich outer mantle, and Xpyrope-rich rims. Inclusions of glaucophane, paragonite and jadeite-omphacite in the garnet core indicate initial garnet growth under blueschist facies conditions. The whole-rock geochemical data indicate trace element signatures similar to the average continental crust, except for slight differences regarding depletion in Cs, Rb, Ba, Sr, and P, and slight enrichment in Dy, Y, Yb and Lu. The absence of glaucophane and paragonite in the garnet mantle and rim indicates their breakdown due to dehydration reactions during the prograde path, leading to the formation of jadeite, kyanite, and XPrp-rich garnet. The presence of blueschist facies parageneses in garnet cores and their consumption during garnet growth point to prograde metamorphic evolution along a subduction setting with low thermobaric ratio (T/P), up to eclogite facies conditions. The phase diagram yields minimum peak P-T conditions around 29 kbar and 620 °C, indicative of ultrahigh-pressure metamorphism (UHPM) at depths around 110 km, followed by a retrograde path of near-isothermal decompression, down to ~16 kbar and 555 °C. The geochemical data suggests a continental protolith, with minor disturbances likely caused by high-pressure metamorphism or later metasomatism. The record of UHPM in this rock is likely due to the more buoyant nature of its protolith compared to other eclogites from the same area. The ages of subduction and exhumation are still being determined, but textural evidence and the inferred P-T path indicate a rapid subduction setting, followed by relatively fast exhumation driven by both buoyancy and tectonics.

Palavras Chave

Extreme metamorphism; geodynamics; XMapTools; phase diagram modelling; Theriak-Domino