Petrology of the ultra-high pressure metamorphic Kimi complex in Rhodope (NE Greece). A new insight into the Alpine geodynamic evolution of Rhodope

Abstract

Structural, penological and geochronological work has revealed that the Rhodope metamorphic province is a synmetamorphic nappe-system of Alpine age. The Kimi complex representing the uppermost entity underwent UHP metamorphism in Lower Cretaceous (>119 Ma). Diamond inclusions in garnet porphyroblasts, exsolutions of quartz rods and rutile needles in garnet from Grt-Ky-Bt-gneisses constrain pressures >4 GPa (probably -7 GPa) and temperatures > 1000 °C, indicating subduction of continental crust into the asthenospheric mantle. The garnet-spinel peridotite of the Kimi area represents a segment of upwelling asthenosphere reequilibrated into the lithospheric mantle wedge at -2.5 GPa and 1235 °C. The spinel-garnet clinopyroxenites, associated with the peridotite, represent HP mantle cumulates crystallized from a melt at similar P-T conditions (i.e. P-2.4 GPa, T~1200°C). Decompression and cooling took place in the mantle wedge within the Cr-Spinel peridotite field up to -1.8 GPa and 900 °C. Subsequent isobaric cooling crossed the stability field of garnet peridotite. At this stage, the peridotite was tectonically emplaced into the educted underlying continental crust. Three stages of exhumation of the crustal assemblage occurred in the Kimi Complex. The first stage, from the maximum depth of -200- 220 Km to -60 Km (P-1.6 GPa, T-800 °C), is characterized by slow cooling rates, indicating rapid exhumation. The second stage, from -60 Km to -38 Km (P-1.05 GPa, T~640°C), is indicated by cooling at slow rates and is characterized by hydration and annealing reequilibration/recrystallization processes. The third stage of exhumation started between 73 and 65 Ma and is characterized by rapid uplift, continuous influx of water, intrusion of muscovite pegmatites at -20 Km depth, and finally by rapid cooling at shallow levels. The Kimi Complex reached the surface before 48-42 Ma.