E. Mposkos

Diamond, former coesite and supersilicic garnet in metasedimentary rocks from the Greek Rhodope: a new ultrahigh-pressure metamorphic province established

Abstract We report here the first discovery of ultrahigh-pressure (UHP) indicator minerals and textures from the Rhodope Metamorphic Province (RMP), northern Greece. In particular, we document the exsolution of quartz, rutile and apatite in sodic garnet from metapelites (garnet–biotite–kyanite gneisses), which attests to the presence of a Si–Ti–Na–P-rich precursor garnet phase. Similar textures in garnet have been reported in the literature only once before for eclogites from the Su Lu UHP metamorphic province, China. We also document the presence of microdiamonds and multicrystalline quartz pseudomorphs after coesite, included in garnet from both eclogites and metapelites. We then argue that these rocks had once been transported to depths exceeding 220 km, well into the upper mantle, thus establishing the RMP as another important UHP metamorphic belt in the world.

Multiple generations of extensional detachments in the Rhodope Mountains (northern Greece): evidence of episodic exhumation of high-pressure rocks

Abstract An integrated structural and petrological study shows that exhumation of high-P rocks of the Rhodope occurred in several pulses. The structurally uppermost Kimi Complex recording an Early Cretaceous high-P metamorphism was exhumed between about 65 and >42 Ma. The Sidironero, Kardamos and Kechros Complexes that record Early Tertiary high-P metamorphism (at least 19 kbar at 700 °C) were exhumed at >42–30 Ma. Exhumation occurred with isothermal decompression. Strain episodes depict thrusting of medium/high grade above lower grade high-P rocks, syn-thrusting extension and post-thrusting extension. Polyphase extension created several generations of detachment zones, which, in sum, excise about 20 km of material within the crustal profile. This reduction in crustal thickness is consistent with a reduction of the present crustal thickness from more than 40 km to less than 30 km in the eastern Rhodope. We mapped the post-thrusting Xanthi low-angle detachment system over 100 km, from its break up zone above the Sidironero Complex (Central Rhodope) into the eastern Rhodope. This detachment shows an overall ESE-dip with a ramp and flat geometry cutting across the earlier thrust structures. The Kimi Complex is the hanging wall of all syn- and post-thrusting extensional systems. On top of the Kimi Complex, marine basins were formed from the Lutetian (c. 48–43 Ma) through the Oligocene, during extension. Successively, at c. 26 to 8 Ma, the Thasos/Pangeon metamorphic core complexes were exhumed. In these times representing the early stages of Aegean back-arc extension, the Strymon and Thasos detachment systems caused crustal thinning in the western Rhodope. Renewed heating of the lithosphere associated with magmatism and exhumation of hot middle crust from beneath the Sidironero Complex occurred. We focus on the geometry, timing and kinematics of extension and contraction structures related to the >42–30 Ma interval and how these exhumed high-P rocks. We interpret high-P rocks exhumed in this interval as a window of the Apulian plate beneath the earlier (in the Cretaceous) accreted Kimi Complex.

Evolution of the eclogites in the Rhodope Zone of northern Greece

Abstract The polymetamorphic complex of Rhodope in the southern Balkan Peninsula has been affected by three successive metamorphic episodes. These are (1) a high-pressure, (2) a Barrow-type and (3) a greenschist-facies, retrograde metamorphic episode. Based on the different metamorphic grades of the Barrovian metamorphism, the Rhodope Zone can be subdivided into two tectonic units, a lower and an upper one. Commonly amphibolitised eclogites of probable Mesozoic age occur in both units (a) in the shape of lenses, (b) as concordant interbeds with other rock types such as gneisses and marbles and (c) as metagabbroic dykes. The high-pressure mineral assemblage is: garnet-omphacite-quartz-Ca-amphibole-clinozoisite/zoisite-rutile±kyanite. In eclogites of gabbroic origin, igneous relics, such as igneous plagioclase, augite and ilmenite are sometimes preserved. PT-conditions of 14 kbar and 550°–600°C were determined for the high-pressure metamorphism in the lower tectonic unit. Symplectites of clinopyroxene-plagioclase, Ca-amphibole-plagioclase, biotite-plagioclase and spinel-plagioclase are common and are attributed to post-eclogitic reactions during uplift. Changes in mineral composition of high-pressure phases such as garnet, Ca-amphibole and Na-pyroxene reflect a re-equilibration tendency during unloading.

Raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh-pressure metamorphic rocks from diamonds in sample preparation materials

Abstract Natural diamond from three ultrahigh-pressure metamorphic (UHPM) terranes (Erzgebirge Massif, Germany; Kokchetav Massif, Northern Kazakhstan; Rhodope Metamorphic Province, Greece) and synthetic diamond from cutting and polishing materials (paste, spray, saw blade) were studied by means of optical microscopy and Raman microspectroscopy, to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond. Several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones [i.e., diamond as residual particles (contaminants) from the cutting and polishing material] such as the diamond size, the presence of inclusions, coatings, or coexistent phases and two diamond Raman band parameters, i.e., the Raman shift and the full-width at half maximum height (FWHM).

High-pressure metamorphism in gneisses and pelitic schists in the East Rhodope zone (N. Greece)

SummaryThe high-alumina metapelites and the orthogneisses of the lower tectonic unit of East Rhodope underwent high P/T metamorphism followed by partial reequilibration during decompression under epidote-amphibolite/amphibolite facies to greenschist facies conditions. The high P/T mineral paragenesis in the orthogneisses is: quartz + albite + microcline + phengite (Simax = 7 atoms p.f.u.) + biotite and in the high alumina metapelites: garnet + chloritoid + chlorite + phengite (Simax. = 6.85 atoms p.f u.) + paragonite + quartz. Pressures between 14 and 15.5 kbar, for Tmin = 550°C, are estimated for the high P/T metamorphism. During continuing uplift, staurolite + chlorite, staurolite + biotite and finally andalusite + Fe-ripidolite are grown at the expense of chloritoid in metapelites, while in the orthogneisses oligoclase, still coexisting with albite, is formed; in both rock types the Si content of white K-mica decreases considerably from almost pure phengite to pure muscovite.ZusammenfassungDie Aluminium-reichen Metapelite und die Orthogneise der unteren tektonischen Einheit der östlichen Rhodope-Zone wurde unter hohen Drucken und Temperaturen metamorphosiert. Darauf folgte eine teilweise Reequilibration unter Druck-Entlastung bei Bedingungen der Epidot-Amphibolit/Amphibolit bis Grünschiefer Fazies. Die Hoch-P/T Mineral-Assoziation in den Orthogneisen besteht aus: Quarz + Albit + Mikroklin + Phengit (Simax = 7 atoms p.f.u.) + Biotit und in den Aluminium-reichen Metapeliten: Granat + Chloritoid + Chlorit + Phengit (Simax = 6.85 atoms p.f.u.) + Paragonit + Quarz. Drucke zwischen 14 and 15.5 kbar für Tmin = 550°C wurden für die Hoch- P/T Metamorphose berechnet. Während andauernder Anhebung bildeten sich Staurolit + Chlorit, Staurolit + Biotit und schließlich Andalusit + Fe-Ripidolit auf Kosten von Chloritoid in den Metapeliten, während in den Orthogneisen Oligoklas der noch mit Albit koexistiert, gebildet wurde; in beiden Gesteinstypen nimmt der Si-Gehalt heller Kaliglimmer von fast reinem Phengit bis zu reinem Muskowit ab.

High-pressure/low-temperature metamorphism of basalts in Lavrion (Greece): implications for the preservation of peak metamorphic assemblages in blueschists and greenschists

The Upper Tectonic Unit of the Lavrion area is part of the Attic-Cycladic blueschist belt and was affected by high-pressure, low-temperature metamorphism. Blueschists and greenschists occur in the same outcrop and are believed to have experienced the same pressure-temperature (P-T) history which has been quantified using geothermobarometry and pseudosections for specific bulk-rock compositions. Calculated P-T conditions indicate minimum pressure of ∼ 0.9 GPa and temperature of ∼ 370 °C for the peak of metamorphism. The prograde and retrograde paths followed a very similar low geothermal gradient (10-12 °C/km) with cooling during decompression. Pseudosections show that both blueschists and greenschists can exist stably at the metamorphic peak, the dominant amphibole being a function of bulk composition: the blueschists, on average, have lower Mg# than the greenschists, which results in a larger P-T stability field of blue amphibole. A pseudosection analysis of the dehydration behaviour indicates that blueschists and some greenschists can preserve their peak assemblages (no dehydration along the retrograde path), whereas greenschist assemblages, in general, are rather prone to undergo dehydration and hence re-equilibration to lower P-T conditions during exhumation.

Single and multiphase inclusions in metapelitic garnets of the Rhodope Metamorphic Province, NE Greece.

Single and multiphase inclusions in garnet porphyroblasts from the diamond-bearing pelitic gneisses were studied by means of combined Raman Spectroscopy and Electron Scanning Microscopy (SEM/EDX). They are either randomly distributed or with preferred orientation within the garnet host and their dimensions vary from less than 5 up to 60 microm. In the single-phase inclusions quartz, rutile, kyanite and graphite dominate. Biotite, zircon, apatite, monazite and allanite are also common. Two types of multiphase inclusions were recognized, hydrous silicate (Type I) and silicate-carbonate (Type II) ones. The carbon-bearing multiphase inclusions predominantly consist of Mg-siderite+graphite+CO(2)+muscovite+quartz formed by a high density carboniferous fluid rich in Fe, Mg, Si and less Ca, Mn, Al and K trapped in the growing garnet in a prograde stage of metamorphism at high-pressure (HP) conditions. The carbon-free multiphase inclusions predominantly consist of biotite+quartz+rutile+/-kyanite+muscovite formed through decompression-dehydration/melting reactions of pre-existing phengite. Single and multiphase inclusions are characterized by polygonal to negative crystal shape formed by dissolution-reprecipitation mechanism between the garnet host and the inclusions during the long lasting cooling period (>100 Ma) of the Kimi Complex.

Ti- and Zr-minerals in calcite-dolomite marbles from the ultrahigh-pressure Kimi Complex, Rhodope mountains, Greece: Implications for the P-T evolution based on reaction textures, petrogenetic grids, and geothermobarometry

Abstract Rutile, titanite, and zircon formed as relatively coarse-grained accessory minerals in several samples of high-grade calcite-dolomite marble with an early ultrahigh-pressure history. These minerals decomposed to a texturally complex set of secondary minerals during subsequent stages of retrograde metamorphism. The reactions involve several generations of geikielite-ilmenite as well as zirconolite [(Ca,Th,U)Zr(Ti,Fe,Nb,Ta)2O7], kassite/cafetite [CaTi2O4(OH)2/CaTi2O5·H2O], Ti-bearing humite group minerals, thorianite, and sometimes euxenite [(Ca,U,Th,REE)(Nb,Ta,Ti)2(O,OH)6]. Stable coexistence of zircon and olivine is observed and stably coexisting titanite with olivine and/or humite-group minerals is reported here for the first time outside of carbonatites, kimberlites, or lamprophyres. Petrogenetic grids constructed for Ti- and Zr-bearing olivine/antigorite-saturated calcite-dolomite marbles show that geikielite is stable at highest pressures, followed by titanite and rutile, and that baddeleyite + diopside replaces zircon + calcite to higher pressures. The observed reaction textures are consistent with an earlier derived P-T path for the Kimi Complex. They corroborate a period of heating during decompression from 25 to 20 kbar and ca. 800 °C, where the assemblage olivine-diopside-spinelrutile- zircon formed. This assemblage partially re-equilibrated during subsequent decompression and cooling, thus forming the observed reaction textures. Even though no memory of the UHP path is preserved in the accessory minerals, their reaction relationships turn out to be potentially very useful for geothermobarometry over a large range of metamorphic conditions.

The geological setting of base metal mineralisation in the Rhodope Region, Northern Greece

The Rhodope region of NE Greece is made up of a complex Lower Palaeozoic (?) basement sequence (metasediments, gneisses and amphibolites) unconformably overlain by Mesozoic and Tertiary formations. There is evidence for ophiolitic sequences in the structurally higher parts of the eastern basement and in the Mesozoic, Circum-Rhodope Belt. Metamorphic grades reach upper-amphibolite facies in the central Rhodope. Base metal mineralisation (Pb, Zn, Cu, Fe, Mn) occurs throughout the region in a variety of environments. Within the basement, sulphides are found associated with marbles (in the west) and amphibolite-serpentinites (in the east). Important base metal occurrences are found in the eastern Tertiary volcano-sedimentary basins, both as strata-bound, syngenetic sulphides and often as fault-controlled Pb-Zn deposits. All of the major deposits, both within the basement and cover sequences, appear to result from a major mineralisation event in the Tertiary. Evidence for this view comes from their homogeneous lead isotope values and the overall mineralisation style which appears to be controlled by large-scale fracture systems. Field and remotely sensed data provide the basis for a dynamic model in which extensional tectonics associated with arc magmatism, resulted in hydrothermal solutions migrating through well-defined fracture systems. Isotope data from the major sulphide occurrences support the model of large-scale homogenisation of crustal lead which was scavenged from the basement and cover sequences. The metals were subsequently dumped in chemically and physically favourable environments such as marble horizons or into actively depositing basins. Although the region is dominated by Pb-Zn occurrences, there is evidence from the Cu sulphides within the amphibolite-serpentinite sequences that local control did play an important role. Pb isotope data leave no doubt, however, that the mineralising solutions circulated through very large rock volumes outside of the mafic sequences. Overall, the study illustrates the potential of an exploration programme which uses remotely sensed data in combination with well-constrained ground studies.

Diffusion rings (sphaeroids) in bauxite

Abstract Diffusion rings (sphaeroids) in bauxites from Klisoura (Dunionia), Helicon, Greece, are described and are studied mineralogically by X-ray diffraction and ore-microscopically. These diffusion rings are also analysed by X-ray fluorescence spectroscopy. The diffusion rings have Fe-rich and relatively Fe-depleted zones. Element leaching and mobilization under weathering is proposed to be the mechanism responsible for the formation of these diffusion rings. A comparison with similar diffusion rings in granite also suggests element leaching as the process responsible for their formation.