Fluid entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P–T–d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by (L  S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO₂ inclusions by preferential loss of H₂O. Those inclusions armoured within garnet retained their primary 3-phase H₂O–CO₂ compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H₂O leakage along dislocations and planar lattice defects which results in the predominance of CO₂ inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO₂-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear.This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P–T conditions, are a welcome supplement for the reconstruction of a rather detailed P–T–d path.     

Über den Eisengehalt von Alumosilikaten aus Quarzknauern

Zusammenfassung Der Eisengehalt von Andalusiten und Disthenen aus Quarzknauern im Gebiet des Ötztaler—Stubaier Altkristallins wurde regional untersucht. Der Gehalt and Gesamteisen in primären Disthen liegt zwischen 0,17 und 0,22 Gew.% Fe₂O₃, während primäre Andalusite zwischen 0,26 und 1,93 Gew.% Fe₂O₃ schwanken. Dieser Unterschied in den Andalusiten wird auf ein wechselndes Angebot von Fe₂O₃ aus dem Nachbargestein der Knauern zurückgeführt.Ein Zusammenhang zwischen dem Eisengehalt der Andalusite und den verschiedenen Umwandlungsstadien Andalusit—Disthen konnte nicht nachgewiesen werden.

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On the iron content of alumosilicates in quartz nodules

Summary A determination of the regional distribution of the ironcontent of andalusites and kyanites in quartz-segregations of the Oetztal—Stubai crystalline complex gave values of 0.17–0.22% for the total iron in primary kyanite and 0.26–1.93% in andalusite. This difference is probably due to a variation in the Fe₂O₃-content in the country rocks of the quartz veins. No relationship between the iron-content in andalusite and the different stages of transformation from andalusite to kyanite could be established.

     

Dolomitic marbles from the ultrahigh-pressure metamorphic Kimi complex in Rhodope, N.E. Greece

Summary Dolomitic marbles from the Organi and Pandrosos areas of the ultrahigh-pressure (UHP) metamorphic Kimi complex in East Rhodope, N.E. Greece have the mineral assemblage: Cal + Dol + Ol + Phl ± Di ± Hbl ± Spl ± Ti–Chu + retrograde Srp and Chl. Several generations of calcite and dolomite with variable composition and texture represent different stages of the P–T evolution: The first stage is represented by matrix dolomite ( = 0.48) and relic domains of homogenous composition in matrix calcite ( = 0.11–0.13); the second stage is evident from precipitation of lath-shaped and vermicular dolomite in matrix calcite. The third stage is represented by veinlets of almost pure CaCO₃ and domainal replacement of prior calcite by nearly pure CaCO₃ + Ca-rich dolomite ( = 0.34–0.43). Matrix dolomite adjacent to CaCO₃ veinlets also becomes Ca-rich ( = 0.42). In fact, Ca-rich dolomites with in the range of 0.40–0.34 are reported for the first time from metamorphic marbles. Coexisting Ca-rich dolomite and Mg-poor calcite cannot be explained by the calcite-dolomite miscibility gap. This assemblage rather suggests that Mg-poor calcite was aragonite originally, which formed together with Ca-rich dolomite according to the reaction Mg–Cal → Arg + Dol (1) at ultrahigh pressures and temperatures above at least 850 °C, when dolomite becomes disordered and incorporates more Ca than coexisting aragonite does in terms of Mg. The simplest explanation of these observations probably is to suggest two metamorphic events: The first one represented by relic matrix carbonates at relatively low to moderate pressures and temperatures of ca. 750 °C, and the second one limited by the minimum temperatures for dolomite disorder (ca. 850 °C) and in the aragonite + dolomite stability field, i.e. at a minimum pressure of 3 GPa and, if the presence of diamond-bearing metapelites nearby is considered, at conditions of at least 850 °C and 4.3 GPa in the diamond stability field. As there is hardly any back-reaction of Ca-rich dolomite + Mg-poor calcite to Mg-rich calcite, peak temperatures remained below the reaction (1) and the exhumation path probably crossed the aragonite-calcite transition at much lower than peak temperature. Cooling and decompression must have both occurred extremely fast in order for the μm-sized Ca-rich dolomite textures to be preserved. An alternative explanation of the formation of “UHP”-textures and compositions is by a fluid influx that not only caused serpentinisation and chloritisation of silicates but also Mg-leaching from carbonates, particularly from Mg-rich calcite and its fine grained dolomite-precipitates, thus transforming them into Mg-poor calcite + Ca-rich dolomite.     

Geochemistry of amphibolitized eclogites and cross-cutting tonalitic–trondhjemitic dykes in the Metamorphic Kimi Complex in East Rhodope (N.E. Greece): implications for partial melting at the base of a thickened crust

In the ultra-high pressure Metamorphic Kimi Complex widespread tonalitic–trondhjemitic dykes, with an intrusion age ca. 65–63 Ma, cross-cut boudins and layers of amphibolitized eclogites. Geochemical investigation proclaims the tied genetic relationship of the amphibolitized eclogites and the associated tonalitic–trondhjemitic dykes. The major and trace element contents and rare earth element patterns of the amphibolitized eclogites indicate formation of their protoliths by fractional crystallization of tholeiitic magmas in a back-arc environment. The tonalites and trondhjemites are characterized by moderate to high Sr contents (>130 ppm), and low Y (<8.2 ppm) and heavy rare earth element contents (Yb content of 0.19–0.88 ppm). The chemical composition of the tonalitic and trondhjemitic dykes are best explained by partial melting of a tholeiitic source like the amphibolitized eclogites with residual garnet and amphibole, at the base of a thickened crust during Early Tertiary subduction/accretion at the southern margins of the European continent.     

Petrogenesis of Ultramafic Rocks from the Ultrahigh-pressure Metamorphic Kimi Complex in Eastern Rhodope (NE Greece)

Widespread bodies of garnet–spinel metaperidotites withpyroxenitic layers occur in the ultrahigh-pressure metamorphicKimi Complex. In this study we address the origin of such peridotite–pyroxeniteassociations in the context of polybaric melting regimes. Weconduct a detailed geochemical investigation of major and traceelement relations and compare them with a range of major elementmodelling scenarios. With increasing bulk-rock MgO content,the garnet–spinel metaperidotites exhibit decreasing CaO,Al₂O₃, TiO₂, and Na₂O along with increasing Ni and a graduallyincreasing Zr/Zr* anomaly, consistent with an origin as residuesafter variable degrees of melt extraction. The major elementmodelling further suggests a polybaric adiabatic decompressionmelting regime beginning at high to ultrahigh pressure, withan intermediate character between pure batch and fractionalmelting and a mean extent of melting of 9–11%. The pyroxenitesexhibit major element compositions that cannot be reproducedby experimental or calculated melts of peridotite. Moreover,the Kimi pyroxenites have highly variable Ni and Sc contentsand a wide range of Mg-number (0· 76–0·89), inconsistent with an origin as frozen melts or the productsof melt–peridotite interaction. However, both the majorelement systematics and the observed rare earth element patterns,with both convex and concave shapes, can be explained by anorigin as clinopyroxene-rich, high-pressure cumulates involvinggarnet and/or Cr-spinel. KEY WORDS: peridotite; pyroxenite; partial melting; UHP metamorphism; cumulate     

Formation of basins and mountain ranges in Attica (Greece): The role of Miocene to Recent low-angle normal detachment faults

In seismically active regions, active low-angle detachment faults are probably more frequent as is commonly thought and may play an important but still underestimated role in the evolution of landforms and basins. We investigate the tectonically active region of Attica (Greece) in the Aegean back arc as a model region to show how basins and mountain ranges commonly thought to be formed by movements on high-angle normal faults in fact reflect the surface expression of displacements on yet undetected, deep-seated, active low-angle normal detachment faults. Inferences are made based on an integrated study of Attica linking the petrology of clastic sediments with geomorphology and structures, and including few new palynological data. From the Miocene to Recent, three sets of normal detachment fault systems were successively active. Shear zones of the 1st (Early Miocene) stage emplaced rocks of the Attic Cycladic high-P metamorphic belt (AC-HP-belt) from depth corresponding to greeschist facies conditions in the brittle, upper crust. In the 2nd stage the WNW dipping Attica low-angle normal detachment fault system between the AC-HP-belt and the un- or weakly metamorphosed rocks of the sub-Pelagonian Zone (SPZ) was active. Clastic sedimentation started in the Late Miocene, during the 2nd stage. Late Miocene and Early Pliocene clastic sediments reveal that during the 2nd stage many areas that presently expose the AC-HP-belt were still covered by the overlying SPZ. Also, now uplifted areas such as the Parnitha mountain range that currently undergo strong erosion were then the sites of sedimentary sinks. The 3rd stage (Late Pliocene through Recent) is associated with dramatic changes in the morphology and recurring steepening of the relief. Reversal of the Parnitha area from the site of deposition into the site of erosion is associated with deposition of coarse conglomerates to the SE of the Parnitha Mt. and S of the Penteli Mt. Sediments of the 3rd stage reflect activity of the here newly described, SSE-dipping Penteli—Athens low-angle detachment fault (PADF) system formed at a high angle to the Attica detachment fault. The outcome of this study is that the present-day geomorphology is to a high degree related to the operation of the PADF system. Steep fault bounding the Athens and Mesogea basins as well as the mountain ranges (Parnitha, Penteli, Hymittos mounts) belongs to its breakaway zone or root into the PADF. Ongoing tectonic movements related to this fault system were responsible for the 1999 Athens (Mw = 6.0) earthquake. We particularly discuss how the PADF may continue into greater depth, the translation magnitude, and how the PADF fits into the wider kinematic framework of the Aegean region.     

Tracing high-pressure metamorphism in marbles: Phase relations in high-grade aluminous calcite–dolomite marbles from the Greek Rhodope massif in the system CaO–MgO–Al2O3–SiO2–CO2 and indications of prior aragonite

Four different types of parageneses of the minerals calcite, dolomite, diopside, forsterite, spinel, amphibole (pargasite), (Ti–)clinohumite and phlogopite were observed in calcite–dolomite marbles collected in the Kimi-Complex of the Rhodope Metamorphic Province (RMP). The presence of former aragonite can be inferred from carbonate inclusions, which, in combination with an analysis of phase relations in the simplified system CaO–MgO–Al₂O₃–SiO₂–CO₂ (CMAS–CO₂) show that the mineral assemblages preserved in these marbles most likely equilibrated at the aragonite–calcite transition, slightly below the coesite stability field, at ca. 720 °C, 25 kbar and a_CO2 ~ 0.01. The thermodynamic model predicts that no matter what activity of CO₂, garnet has to be present in aluminous calcite–dolomite-marble at UHP conditions.     

Continental rift and oceanic protoliths of mafic–ultramafic rocks from the Kechros Complex,NE Rhodope (Greece): implications from petrography,major and trace-element systematics,and MELTS modeling

The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO₂ content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO₂ loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.     

Diffusion rings (sphaeroids) in bauxite

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.     

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

Summary The 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 (Si_max = 7 atoms p.f.u.) + biotite and in the high alumina metapelites: garnet + chloritoid + chlorite + phengite (Si_max. = 6.85 atoms p.f u.) + paragonite + quartz. Pressures between 14 and 15.5 kbar, for T_min = 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.

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Hochdruck-Metamorphose in Gneisen und pelitischen Schiefern der östlichen RhodopeZone, Nord-Griechenland

Zusammenfassung Die 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 (Si_max = 7 atoms p.f.u.) + Biotit und in den Aluminium-reichen Metapeliten: Granat + Chloritoid + Chlorit + Phengit (Si_max = 6.85 atoms p.f.u.) + Paragonit + Quarz. Drucke zwischen 14 and 15.5 kbar für T_min = 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.