Pressure–temperature evolution of eclogites from the Kechros complex in the Eastern Rhodope (NE Greece)

The Rhodope Domain in NE Greece consists of different tectonometamorphic complexes involved in the Alpine collisional history between the Eurasian and African plates. In the Kechros Complex, which is the lowermost tectonic unit in the East Rhodope, a lense of kyanite eclogite occurs within orthogneiss and common eclogites are found between serpentinized peridotite and underlying pelitic gneisses. In kyanite eclogite, the high-pressure (HP) mineral assemblage is Grt?+?Omp (Jd_35–55)?+?Ky?+?Ph?+?Qz?+?Rt?+?(indirectly inferred Tlc?+?Law); a Na-rich tremolite and zoisite formed at or near peak metamorphic conditions. In common eclogites, the HP mineral assemblage is Grt?+?Omp (Jd29–41)?+?Rt and, with less certainty, Amp (Gln-rich?+?Brs?+?Wnc?+?Hbl)?±?Czo. The inclusions in garnet are glaucophane, actinolite, barroisite, hornblende, omphacite, clinozoisite, titanite, rutile and rarely paragonite and albite. In kyanite eclogite, peak P–T conditions are constrained at 2.2?GPa and 615°C using garnet–omphacite–phengite geothermobarometry and very similar values of 585?±?32°C and 2.17?±?0.11?GPa with the average P–T method, by which conditions of formation could also be narrowed down for the common eclogite (619?±?53°C and 1.69?±?0.17?GPa) and for a retrogressed eclogite (534?±?36°C and 0.77?±?0.11?GPa). Ages for the HP metamorphism in the Kechros Complex are not yet available. A Rb–Sr white mica age of 37?Ma from orthogneiss records a stage of the exhumation. The HP event may be coeval with the Eocene HP metamorphism (49–55?Ma) recorded in the Nestos Shear Zone in Central Rhodope and in the Attic-Cycladic crystalline belt, where it is interpreted as the result of subduction and final closure of the Axios/Vardar ocean and subsequent subduction of the Apulian continental crust (a promontory of the Africa continent) under the southern margin of the European continent in the late Cretaceous and early Tertiary.     

Mineral chemistry of the Tissint meteorite: Indications of two‐stage crystallization in a closed system

The Tissint meteorite is a geochemically depleted, olivine‐phyric shergottite. Olivine megacrysts contain 300–600 μm cores with uniform Mg# (~80 ± 1) followed by concentric zones of Fe‐enrichment toward the rims. We applied a number of tests to distinguish the relationship of these megacrysts to the host rock. Major and trace element compositions of the Mg‐rich core in olivine are in equilibrium with the bulk rock, within uncertainty, and rare earth element abundances of melt inclusions in Mg‐rich olivines reported in the literature are similar to those of the bulk rock. Moreover, the P Kα intensity maps of two large olivine grains show no resorption between the uniform core and the rim. Taken together, these lines of evidence suggest the olivine megacrysts are phenocrysts. Among depleted olivine‐phyric shergottites, Tissint is the first one that acts mostly as a closed system with olivine megacrysts being the phenocrysts. The texture and mineral chemistry of Tissint indicate a crystallization sequence of: olivine (Mg# 80 ± 1) → olivine (Mg# 76) + chromite → olivine (Mg# 74) + Ti‐chromite → olivine (Mg# 74–63) + pyroxene (Mg# 76–65) + Cr‐ulvöspinel → olivine (Mg# 63–35) + pyroxene (Mg# 65–60) + plagioclase, followed by late‐stage ilmenite and phosphate. The crystallization of the Tissint meteorite likely occurred in two stages: uniform olivine cores likely crystallized under equilibrium conditions; and a fractional crystallization sequence that formed the rest of the rock. The two‐stage crystallization without crystal settling is simulated using MELTS and the Tissint bulk composition, and can broadly reproduce the crystallization sequence and mineral chemistry measured in the Tissint samples. The transition between equilibrium and fractional crystallization is associated with a dramatic increase in cooling rate and might have been driven by an acceleration in the ascent rate or by encounter with a steep thermal gradient in the Martian crust.