Magmatic and metamorphic evolution of the Shotur Kuh metamorphic complex (Central Iran)

Metamorphic basement rocks, that are exposed beneath the very low-grade to unmetamorphosed Upper Jurassic-Eocene formations north of the Torud fault zone within the Great Kavir Block, were investigated to elucidate the origin of their protoliths and the pressure and temperature conditions of metamorphism. The basement, previously assumed as a pre-Cambrian metamorphic complex, is mostly formed by amphibolite-facies orthogneisses (tonalite, granodiorite, and granite) with amphibolites and small amounts of metasediment-micaschists. Major- and trace-element geochemistry in combination with U–Pb age dating of zircon showed that the protoliths formed during Late Neoproterozoic continental arc magmatism that has also been identified in other tectonic blocks of Central Iran. In addition to quartz, feldspar(s), micas in orthogneisses, and amphibole + plagioclase in amphibolite, all rocks may contain garnet that shows prograde zoning. Kyanite was found only in some Al-rich amphibolite together with gedrite. The PT conditions of the rocks, based on conventional geothermobarometry and the pseudosection method, show a medium-pressure amphibolite-facies metamorphism. Ar–Ar age dating of muscovite reveals that this metamorphism occurred in the Middle Jurassic (166 Ma) and related to the closure of the Neotethyan basin.     

TECTONIC EVOLUTION OF THE WESTERN KUNLUN AND KARAKORAM MOUNTAINS—SOME NEW OBSERVATIONS FROM A MULTI-DISCIPLINARY GEOSCIENTIFIC TRANSECT (MGT) IN NW TIBET

TECTONIC EVOLUTION OF THE WESTERN KUNLUN AND KARAKORAM MOUNTAINS—SOME NEW OBSERVATIONS FROM A MULTI-DISCIPLINARY GEOSCIENTIFIC TRANSECT (MGT) IN NW TIBET     

Tectonic evolution of the Aravalli orogen (NW India): an inverted Proterozoic rift basin?

The Aravalli mountain range (AMR) in the northwestern part of the Indian Peninsula consists of two main Proterozoic metasedimentary and metaigneous sequences, the Aravalli and Delhi Supergroups, respectively, which rest over the Archaean gneissic basement. A synthesis and reinterpretation of the available geological, geochronological and geophysical data, including results of own field work and geophysical interpretations pertaining to the AMR, indicate its origin as an inverted basin: rifting into granitoid basement began ca. 2.5; Ga ago with Aravalli passive rifting (ca. 2.5–2.0 Ga) and Delhi active rifting (ca. 1.9–1.6 Ga). Associated mafic igneous rocks show both continental and oceanic tholeiitic geochemistry and are comparable with Phanerozoic, rift-related magmatic products. Available data showed no conclusive evidence for oceanic lithoshere and island-arc/active margin magmatic activity in the AMR. Subsequent inversion and orogeny (Delhi orogeny, ca. 1.5-1.4 Ga) lead to complex deformation and metamorphism. Only in the western and central zones has the basement been involved in this mid-Proterozoic (Delhi) deformation, whereas it is unaffected in the eastern part, except for local shear zones mainly along the basement/cover interface. The grade of metamorphism increases from the greenschist facies in the east to the amphibolite facies in the west with local HP assemblages. These latter are explained by rapid burial and exhumation of thin and cool continental lithosphere. Subsequently, during a final, mild phase of inversion, the Vindhyan basins consisting mainly of sandstones, limestones and shales, flanking the AMR formed which are comparable to foreland basins. The tectonic evolution of the AMR is therefore interpreted as an example of a major inverted continental rift and of a Proterozoic intra-continental orogen.     

The metamorphic evolution of the Muxia-Finisterre region (Galice,NW Spain) during the Hercynian Orogenesis

Mineral chemistry and crystal growth-deformation relationships in metapelites, orthogneises and metabasic rocks from the Muxia-Finisterre region show that this area has been affected by plurifacial regional metamorphism during the Hercynian Orogenesis. Paragenetic evolution seems to be related to a change of the metamorphic gradient towards relatively lower P_s conditions in the mesozone, while this character is less evident in catazonal rocks. In both cases there was a continuous increase in T and decrease in {ie657-02}.The metamorphic climax was reached during the second metamorphic episode (M₂). This event took place during and even after the second deformation phase (P₂). The latter gave rise to the main foliation observed in studied rocks (S₂) which almost practically erased signs of previous schistosity. Relationships between metamorphic evolution and different geotectonic models are discussed.

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Zusammenfassung Mineralchemismus und die Beziehungen Kristallwachstum-Deformation in Metapeliten, Orthogneisen und metabasischen Gesteinen aus der Muxia-Finisterra-Region, zeigen eine Beeinflussung dieses Gebietes durch eine plurifazielle Regionalmetamorphose während der herzynischen Orogenese.Die paragenetische Entwicklung scheint mit einer Veränderung des metamorphen Gradienten in Richtung niedrigerer P_s-Bedingungen in der Mesozone einherzugehen, während dies in katazonalen Gesteinen weniger deutlich ist.In beiden Fällen gab es eine kontinuierliche Zunahme der Temperatur (T) bzw. Abnahme des {ie657-01}.Der Höhepunkt der Metamorphose wurde während der zweiten metamorphen Episode (M₂) erreicht. Dieses Ereignis fand statt während und sogar nach der zweiten Deformationsphase (P₂). Letzteres verursachte die Hauptschieferung (S₂) in den untersuchten Gesteinen, die die Merkmale einer älteren Schieferung nahezu ausgelöscht hat.Beziehungen zwischen metamorpher Entwicklung und verschiedener geotektonischer Modelle werden diskutiert.

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Résumé Les compositions chimiques des minéraux ainsi que leur relation croissance-déformation dans les métapélites, ortho-gneiss et métabasites de la région Muxia-Finisterre, suggèrent que cette secteur a subi un métamorphisme régional plurifacial lors de l'Orogenèse Hercynienne. L'évolution des paragenèses semble être liée à un changement du gradient métamorphique vers des conditions de P_s relativement plus basses dans la mésozone, ceci étant moins évident dans les roches catazonales. Dans les deux cas il y a eu une augmentation continue de la T et une diminution de la {ie657-03}. Le paroxisme du métamorphisme a été atteint lors du deuxième épisode métamorphique (M₂). Cet événement a lieu généralement pendant ou légèrement après la deuxième phase de déformation (P₂). Celle-ci donne la foliation principale de la roche (S₂), laquelle a presque entièrement effacé les marques d'une schistosité pré-existante. Des relations entre l'évolution métamorphique et différents modèles géotectoniques sont discutées.

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Tectonic evolution of high-grade metamorphic terranes in central Vietnam: Constraints from large-scale monazite geochronology

Several metamorphic complexes in Southeast Asia have been interpreted as Precambrian basement, characterized by amphibolite to granulite facies metamorphism. In this paper, we re-evaluate the timing of this thermal event based on the large-scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central Vietnam. Most of the samples in this study are from metamorphic rocks (n = 38) and granitoids (n = 11) in the Kontum Massif. Gneisses (n = 6) and granitoids (n = 5) from the Hai Van Migmatite Complex and the Truong Son Belt, located to the north of the massif, were also studied. Two distinct thermal episodes (245–230 Ma and 460–430 Ma) affected Kontum Massif gneisses, while a single dominant event at 240–220 Ma is recorded in the gneisses from the Hai Van Complex and the Truong Son Belt. Monazites from granitoids commonly yield an age of 240–220 Ma. Mesoproterozoic ages (1530–1340 Ma) were obtained only from monazite cores that are surrounded by c. 440 Ma overgrowths. Thermobarometric results, combined with concentrations of Y₂O₃, Ce₂O₃, and heavy rare earth elements in monazite, and recently reported pressure–temperature paths suggest that Triassic ages correspond to retrograde metamorphism following decompression from high- to medium-pressure/temperature conditions. Ordovician–Silurian ages reflect low-pressure/temperature metamorphism accompanied by isobaric heating during prograde metamorphism. Some samples were affected by both metamorphic events. We conclude that high-grade metamorphism observed in so-called Precambrian basement terranes in central Vietnam occurred during both the Permian–Triassic and the Ordovician–Silurian, while peraluminous granitoid magmatism is Triassic. Additionally, our preliminary analyses for U–Pb zircon age and whole-rock chemistry of granitic gneisses from the Truong Song Belt suggests the presence of the Ordovician–Silurian volcanic arc magmatism in the region. Based on the pressure–temperature–time–protolith evolutions, metamorphic rocks from central Vietnam provide a continuous record of subduction–accretion–collision tectonics between the South China and Indochina blocks: in the Ordovician–Silurian, the region was characterized by active continental margin tectonics, followed by continental collision during the Late Permian to Early Triassic and subsequent exhumation during the Late Triassic. The results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other Southeast Asian terranes should be verified to achieve a better understanding of the Precambrian to Early Mesozoic tectonic history in Asia.     

Metamorphic conditions and structural evolution of the Kesebir-Kardamos dome: Rhodope metamorphic complex (Greece-Bulgaria)

International Journal of Earth Sciences - The synmetamorphic nappe system of the Rhodope Metamorphic Complex has been deformed into dome-and-basin structures attributed to syn- to post-convergent...     

Geochemistry of eclogites of the Tso Morari complex,Ladakh, NW Himalayas:Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.     

Tectonic evolution of kid metamorphic complex and the recognition of Najd fault system in South East Sinai,Egypt

International Journal of Earth Sciences - A low-to medium-grade metamorphic belt of a volcano-sedimentary succession occurs in the eastern side of South Sinai as a part of the northernmost...     

Geomorphological evolution of the Tilcara alluvial fan (Jujuy Province, NW Argentina): Tectonic implications and palaeoenvironmental considerations

The development and evolution of the Tilcara alluvial fan, in the Quebrada de Humahuaca (Andean Eastern Cordillera, NW Argentina), has been analysed by using geomorphological mapping techniques, sedimentological characterisation of the deposits and OSL chronological methods. It is a complex segmented alluvial fan made up of five evolutionary stages (units Qf1, Qf2, Qf3, Qf4 and Qf5) developed under arid climatic environments as well as compressive tectonic conditions. Segmentation processes, including aggradation/entrenchment cycles and changes in the location of the depositional lobe, are mainly controlled by climatic and/or tectonic changes as well as channel piracy processes in the drainage system. Alluvial fan deposits include debris flows, sheet flows and braided channel facies associated with high water discharge events in an arid environment. The best mean OSL age estimated for stage Qf2 is 84.5 ± 7 ka BP. In addition, a thrust fault affecting these deposits has been recognized and, as a consequence, the compressive tectonics must date from the Upper Pleistocene in this area of the Andean Eastern Cordillera.     

雪峰山早中生代构造演化:构造学和年代学分析木

雪峰山主体地处湖南省境内,位于华南板块的中心区域,是一条典型的陆内造山带.通过详细的野外地质观察,我们将其分为3个构造单元:西部外区,主要以大型箱状褶皱为主;中部区,与西部区以主逆冲断层相分隔,劈理发育呈扇状,是雪峰山构造带的核心区域,也是变质级别最深、变形最强的区域;东部区,变形集中在脆韧性区域之上,以极性北西构造为...     

Combined thermobarometry and geochronology of peraluminous metapelites from the Karakoram metamorphic complex,North Pakistan; New insight into the tectonothermal evolution of the Baltoro and Hunza Valley regions

Petrographic analysis of peraluminous metapelites from two separate regions of the Karakoram metamorphic complex, North Pakistan, has produced new insights into the P–T–t evolution of the deep crust along the south Asian margin before and after the India‐Asia collision. Average P–T estimates and pseudosection construction in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (MnNCKFMASHTO) system using THERMOCALC have provided prograde and peak metamorphic conditions and U–Pb geochronology of metamorphic monazite has provided age constraints. Two new events in the tectonothermal evolution of the Hunza Valley have been documented; an andalusite‐grade contact metamorphic event at 105.5 ± 0.8 Ma, at unknown P–T conditions, associated with the widespread subduction‐related granite magmatism before the India‐Asia collision, and a kyanite‐grade overprint of sillimanite‐grade rocks with peak P–T conditions of ～7.8 kbar, 645 °C at 28.2 ± 0.8 Ma associated with the ongoing India‐Asia collision. A kyanite‐grade event observed in the Baltoro region with similar peak P–T conditions (～7.4–8.0 kbar, ～640–660 °C) is interpreted to have occurred sometime after 21.8 ± 0.6 Ma, however, previous studies have suggested that this event commenced in the Baltoro as early as c. 28 Ma. A calculated prograde P–T path for this kyanite‐grade event in the Baltoro indicates that garnet first nucleated on an initially high geothermal gradient (～30 °C km^?1) and grew during a significant increase in pressure of ～2.6 kbar over a temperature increase of ～100 °C. This event is thought to represent evidence for conductive heating of the middle crust during early stages of intrusion and lateral migration of the Baltoro batholith, with thermal conditions comparable with tectonic models of magmatic over‐accretion.     

The role of amphiboles in the metamorphic evolution of the UHP rocks: a case study from the Tso Morari Complex, northwest Himalayas

Amphiboles represent a crucial phase of the ultra-high-pressure (UHP) metamorphic rocks as their solid solution behavior reflects both bulk compositional and P–T changes. Three different types of amphibole have been reported from the UHP metamafic rocks of the Tso Morari Crystalline Complex, NW Himalayas: Na-rich (glaucophane); Na–Ca-rich (barroisite, taramite, winchite) and Ca-rich (tremolite, magnesio-hornblende, pargasite). The Na-amphibole is presented as a core of the zoned amphibole with Na–Ca-rich rim; Na–Ca-amphibole is presented as inclusion in garnets as well as in matrix, and Ca-amphibole is generally found in the matrix. The Na–Ca-amphibole is observed at two different stages of metamorphism. The first is pre-UHP, and the second is post-garnet–omphacite assemblage though with a significant difference in composition. The pressure–temperature estimations of the formation of these two sets of Na–Ca-amphiboles corroborate their textural associations. Ca-rich amphiboles are generally present in the matrix either as symplectite with plagioclase or as a pseudomorph after garnet along with other secondary minerals like chlorite and biotite. Two different types of zoning have been observed in the amphibole grains: (1) core is Na-rich followed by Na–Ca rim and (2) core of Na–Ca-amphibole is followed by Ca-rich rim. The pre-UHP (or the prograde P–T path) and post-UHP stages (or the retrograde P–T path) of Tso Morari eclogites are defined by characteristic amphibole compositions, viz. Na/Na–Ca-amphibole, Na–Ca-amphibole and Ca-amphibole and thus indicate their utility in inferring crustal evolution of this UHP terrain.     

Petrology and Alpine metamorphic evolution of the metabasic and ultrabasic rocks in the Berisal crystalline complex (Switzerland)

Relict diopsides have been found in meta-ultrabasic rocks from the Berisal crystalline complex in the Pennine nappe system of the Alps. They show optical and chemical zonation. The idiomorphic cores are enriched in chromium. During the Eo-Alpine high-pressure metamorphic event the originally magmatic clinopyroxene suffered substantial replacement of Ca by Na. New diopside, poor in chromium grew around the borders of the idiomorphic Cr-rich cores. During the subsequent main stage of Alpine metamorphism the diopside was mainly altered into tremolite. Some of the diopsides survived but alteration induced exsolution of Na out of the rims, creating the observed zoning in the pyroxenes. Field and microscopic investigations of the metabasic rocks support the existence of an Eo-Alpine high-pressure metamorphic event. The amphibolites reflect a typical retrograde evolutionary stage between “eclogite” amphibolite and garnet amphibolite with strongly corroded kelyphitic garnet and the symplectitic, radially fibrous growth of amphibole, plagioclase, epidote and quartz. However, the surprisingly low pyrope and high almandine contents in the garnets are not typically representing a high-pressure metamorphic event.     

Tectono-metamorphic evolution of the Neyriz metamorphic complex, Quri-Kor-e-Sefid area (Sanandaj-Sirjan Zone, SW Iran)

The Neyriz region includes outcrops of metamorphic rocks that are thrust over the Neotethyan ophiolites. These rocks are affected by a major deformational event, the result of which includes a shearing polyphase foliation present in gneissic core domes, overprinted by a crenulation cleavage. These fundamental structures developed contemporaneously with a medium-pressure metamorphism which is characterized by the syn-kinematic crystallization of kyanite and the beginning of anatexis, followed by the development of retrometamorphic mineral parageneses. The major deformation phase in the area occurred during the Early-Cimmerian orogeny in the Late Triassic. Following the orogeny, the gneiss domes started to rise into the upper levels of the crust. From the geodynamic point of view, after the Mid-Permian the studied area was situated at southern passive margin of the Iranian plate; the central Iranian microcontinent at that time was separated by the Neotethys ocean from the Gondwanian supercontinent. After the Late Triassic the region became an active margin associated with an accretionary prism. The margin was finally involved in an orogenic wedge after the closure of the Neotethyan oceanic basin in the Late Mesozoic. Closure of the basin resulted in a major thrusting of the metamorphic rocks of the southern Iranian margin over the Neotethyan ophiolites.     

Alpine metamorphic evolution of Ligurian Alps (North-West Italy): chemography and petrological constraints inferred from metamorphic climax assemblages

The up-to-date petrological and microtextural information on the Ligurian Alps indicates that the metamorphic rocks from the oceanic lithosphere and the paleo-European continental margin underwent an alpine-type metamorphic evolution characterized by low dT/dP gradients. In particular, rocks from the Ligurian-Piedmontese oceanic lithosphere underwent an alpine metamorphism typical of alpine-type blueschist rocks. The distribution of the alpine metamorphic facies in paleo-European continental margin is closely related to the structural position of the different tectonic units. The prograde evolution frequently preserves paragenetic and textural relics of the earlier parageneses. If relics of the earlier parageneses are preserved, the rock exhibits continuous prograde reactions confirmed by strong compositional zoning of the metamorphic minerals. Therefore, these reactions lead to chemical and microtextural equilibrium relations, between the minerals, in limited domains of the rocks (microtextural sites). The main compositional aspect of coronitic textures is the mineral zoning, particularly when the minerals of the coronas are the consequence of a wide range of solid solutions. In such cases, the reacting minerals are armored and the kinetics are lowered. The prograde metamorphic evolution, which involved the rocks from the oceanic lithosphere and the paleo-European continental margin, is quite consistent with a subduction-type geodynamic process in different ages during alpine times.Mineral Abbreviations Ab albite - Acm acmite - Alm almandine - Amp amphibole - Ant antigorite - Act actinolite - Bt biotite - Ced celadonite - Chl chlorite - Chlrm chloromelanite - Chltd chloritoide - Cpx clinopyroxene - Di diopside - Ep epidote - Fo forsterite - Gl glaucophane - Gnt garnet - Gro grossular - Hed hedenbergite - Hor hornblende - Jd jadeite - Kfld K-feldspar - Lau laumontite - Lw lawsonite - Ma margarite - Mu muscovite - Omp omphacite - Pa paragonite - Ph phengite - Pl plagioclase - Py pyrope - Pyr pirophyllite - Prh prehnite - Pump pumpellyite - Qz quartz - Rieb riebeckite - Rut rutile - Tchu titanclinohumite - Tc talc - Tr tremolite - Zeo zeolites - Zo zoisite - Wm white mica     

From intra-oceanic subduction to arc accretion and arc-continent collision: Insights from the structural evolution of the Río San Juan metamorphic complex,northern Hispaniola

The Río San Juan metamorphic complex exposes a segment of a high-pressure subduction-accretionary complex built during Caribbean island arc-North America continental margin convergence. It is composed of accreted arc- and oceanic-derived metaigneous rocks, serpentinized peridotites and minor metasediments forming a structural pile. Combined detailed mapping, structural and metamorphic analysis, and geochronology show that the deformation can be divided into five main events (D1–D5). An early subduction-related D1 deformation and M1 metamorphism produced greenschist (mafic rocks of the Gaspar Hernández peridotite-tectonite), blueschist and eclogite (metamafic blocks in the Jagua Clara mélange), high-P epidote-amphibolite and eclogite (Cuaba unit), and lower blueschist and greenschist-facies conditions (Morrito unit). This was followed by M2 decompression and cooling in the blueschist, greenschist and low-P amphibolite-facies conditions. The shape of the retrograde P-T path, the age of the exhumation-related D2 structures, and the tectonic significance of D2 deformation are different in each structural unit. Published U–Pb and ⁴⁰Ar/³⁹Ar plateau ages and T-t/P-t estimations reveal diachronic Turonian-Coniacian to Maastrichtian retrograde M2 metamorphism in the different structural units of the complex, during a consistent D2 top-to-the-NE/ENE tectonic transport. Regionally, a similar top-to-the-ENE tectonic transport also took place in the metasedimentary nappes of the Samaná complex during the Eocene to earliest Miocene. This kinematic compatibility indicates a general northeastward progradation of deformation in the northern Caribbean convergent margin, as the successive tectonic incorporation of arc, oceanic and continental-derived terrains to the developing Caribbean subduction-accretionary complex took place. D3–D5 deformations are discontinuous and much less penetrative, recording the evolution from ductile to brittle conditions of deformation in the complex. The D3 event substantially modified the nappe-stack and produced open folds with amplitudes up to kilometer-scale. The Late Paleocene-Eocene D4 structures are ductile to ductile–brittle thrusts and inverse shear bands. D5 is a Tertiary, entirely brittle deformation that had considerable influence in the geometry of the whole complex. From the Miocene to the Present, it has been cut and laterally displaced by a D5 sinistral strike-slip fault system associated with the Septentrional fault zone.     

Tectonic evolution of the North Qinling Orogen from subduction to collision and exhumation: Evidence from zircons in metamorphic rocks of the Qinling Group

The North Qinling Block (NQB) is an important segment of the Qinling Orogen in Central China. Here we report the results from SIMS geochronology and oxygen isotopes, as well as LA-MC-ICPMS Hf isotopic analyses on zircon grains from a suite of metamorphic rocks (felsic gneisses, garnet plagioclase amphibolites, and retrograde eclogite dikes) in the Qinling Group of the NQB. The age data show that these rocks underwent at least two episodes of metamorphism with the peak at 483–501 Ma, followed by 454–470 Ma retrograde metamorphism. These results are generally coeval with the periods of 500–480 Ma for peak metamorphism and 460–420 Ma for retrograde metamorphism previously obtained from the HP/UHP metamorphic rocks of the NQB. During the prograde and retrograde metamorphism, widespread fluid and melt circulation within the block has been identified from the geochemical features of the metamorphic zircons. The fluids that circulated in the felsic gneisses and retrograde eclogite dikes originated from the dehydration of altered oceanic basalts as inferred from the exceedingly low Th/U ratios, positive ε_Hf(t) (> 5) and extremely δ¹⁸O (10.01–13.91‰) values in metamorphic zircons. In contrast, the melt involved in the formation of garnet plagioclase amphibolites appears to have been derived from continental sediments interlayered with the oceanic basalts since zircons crystallized during the peak and retrograde metamorphism show typical magmatic features with high U and Th contents and Th/U ratios and enriched Hf (ε_Hf(t) = − 5.42 to − 0.18) and oxygen isotope composition (δ¹⁸O around 8‰). Geochronological and geochemical features of the magmatic cores of the clear core-rim textured zircons demonstrate that the protoliths of the gneisses were intermediate-acid volcanic rocks erupted before Neoproterozoic (800 Ma), which is further supported by the intrusion of basaltic magma of asthenospheric origin as represented by protoliths of retrograde eclogite dikes, with the oldest magmatic zircon formed at 789 Ma. The protoliths of garnet plagioclase amphibolites appear to be altered oceanic basalts but had been significantly affected by the melt during the metamorphism. Combined with the previous studies, the Qinling Group experienced overall subduction in the Early Paleozoic. The NQB as represented by the Qinling Group was most likely a discrete micro-block in the Neoproterozoic, and underwent deep subduction in the Cambrian (483–501 Ma) and exhumation in Ordovician (454–470 Ma). We propose that the NQB preserves a complete cycle of tectonic evolution of an orogen from an oceanic basin spreading, and micro-continent formation to deep subduction and exhumation.     

A zircon study from the Rhodope metamorphic complex, N-Greece: Time record of a multistage evolution

Zircons from an eclogite and a diamond-bearing metapelite near the Kimi village (north-eastern Rhodope Metamorphic Complex, Greece) have been investigated by Micro Raman Spectroscopy, SEM, SHRIMP and LA-ICPMS to define their inclusion mineralogy, ages and trace element contents. In addition, the host rocks metamorphic evolution was reconstructed and linked to the zircon growth domains.

The eclogite contains relicts of a high pressure stage (ca. 700 °C and > 17.5 kbar) characterised by matrix omphacite with Jd_40–35. This assemblage was overprinted by a lower pressure, higher temperature metamorphic event (ca. 820 °C and 15.5–17.5 kbar), as indicated by the presence of clinopyroxene (Jd_35–20) and plagioclase. Biotite and pargasitic amphibole represent a later stage, probably related to an influx of fluids. Zircons separated from the eclogite contain magmatic relicts indicating Permian crystallization of a quartz-bearing gabbroic protolith. Inclusions diagnostic of the high temperature, post-eclogitic overprint are found in metamorphic zircon domain Z2 which ages spread over a long period (160 – 95 Ma). Based on zircon textures, zoning and chemistry, we suggest that the high-temperature peak occurred at or before ca. 160 Ma and the zircons were disturbed by a later event possibly at around 115 Ma. Small metamorphic zircon overgrowths with a different composition yield an age of 79 ± 3 Ma, which is related to a distinct amphibolite-facies metamorphic event.

The metapelitic host rock consists of a mesosome with garnet, mica and kyanite, and a quartz- and plagioclase-bearing leucosome, which formed at granulite-facies conditions. Based on previously reported micro-diamond inclusions in garnet, the mesosome is assumed to have experienced UHP conditions. Nevertheless, (U)HP mineral inclusions were not found in the zircons separated from the diamond-bearing metapelite. Inclusions of melt, kyanite and high-Ti biotite in a first metamorphic zircon domain suggest that zircon formation occurred during pervasive granulite-facies metamorphism. An age of 171 ± 1 Ma measured on this zircon domain constrains the high-temperature metamorphic event. A second, inclusion-free metamorphic domain yielded an age of 160 ± 1 Ma that is related to decompression and melt crystallization.

The similar age data obtained from the samples indicate that both rock types recorded a high-T metamorphic overprint at granulite-facies conditions at ca. 170 – 160 Ma. This age implies that any high pressure or even ultra-high pressure metamorphism in the Kimi Complex occurred before that time. Our findings define new constraints for the geodynamic evolution for the Alpine orogenic cycle within the northernmost Greek part of the Rhodope Metamorphic Complex. It is proposed that the rocks of the Kimi Complex belong to a suture zone squeezed between two continental blocks and result from a Paleo-ocean basin, which should be located further north of the Jurassic Vardar Ocean.     

Crystal Size Distributions (CSD) in Three Dimensions: Insights from the 3D Reconstruction of a Highly Porphyritic Rhyolite

Growth histories and residence times of crystals in magmaticsystems can be revealed by studying crystal sizes, size distributionsand shapes. In this contribution, serial sectioning has beenemployed on a sample of porphyritic rhyolite from a Permo-Carboniferouslaccolith from the Halle Volcanic Complex, Germany, to reconstructthe distribution of felsic phenocrysts in three dimensions inorder to determine their true shapes, sizes and three-dimensionalsize distributions. A model of all three phenocryst phases (quartz,plagioclase, K-feldspar) with 217 crystals, and a larger modelcontaining 1599 K-feldspar crystals was reconstructed in threedimensions. The first model revealed a non-touching frameworkof crystals in three dimensions, suggesting that individualcrystals grew freely in the melt prior to quenching of the texture.However, crystal shapes are complex and show large variationon a Zingg diagram (intermediate over long axis plotted againstshort over intermediate axis). They often do not resemble thecrystallographic shapes expected for phenocrysts growing unhinderedfrom a melt, indicating complex growth histories. In contrast,the three-dimensional size distribution is a simple straightline with a negative slope. Stereologically corrected size distributionsfrom individual sections compare well with stereologically correctedsize distributions obtained previously from the same sample.However, crystal size distribution (CSD) data from individualsections scatter considerably. It is shown that CSDs can berobustly reproduced with a sampling size of greater than 200crystals. The kind of shape assumed in stereological correctionof CSDs, however, has a large influence on the calculation andestimation of crystal residence times. KEY WORDS: 3D reconstruction; crystal shapes; CSD; porphyritic rhyolite; quantitative petrography     

Marble-hosted sulfide ores in the Angouran Zn-(Pb–Ag) deposit, NW Iran: interaction of sedimentary brines with a metamorphic core complex

The Angouran Zn-(Pb–Ag) deposit, Zanjan Province, NW Iran, is located within the central Sanandaj-Sirjan Zone of the Zagros orogenic belt. The deposit has proven and estimated resources of 4.7 Mt of sulfide ore at 27.7% Zn, 2.4% Pb, and 110 g/t Ag, and 14.6 Mt of oxidized carbonate ores at 22% Zn and 4.6% Pb. It is hosted by a metamorphic core complex that is unconformably overlain by a Neogene volcanic and evaporite-bearing marine to continental sedimentary sequence. The sulfide orebody, precursor to the significant nonsulfide ores, is located at the crest of an open anticline at the contact between Neoproterozoic to Cambrian footwall micaschists and hanging wall marbles. ⁴⁰Ar–³⁹Ar data on muscovite from mineralized and unaltered footwall micaschists suggest a rapid Mid-Miocene exhumation of the metamorphic basement (∼20 Ma) and yield an upper age constraint for mineralization. The fine-grained sulfide ore is massive, replacive, often brecciated, clearly postmetamorphic and dominated by Fe-poor sphalerite, with minor galena, pyrite, anhydrite, quartz, muscovite, dolomite, and rare calcite. Sphalerite contains Na–Ca–Cl brine inclusions (23–25 mass% total dissolved solids) with homogenization temperatures of 180–70°C. Fluid inclusion chemistry (Na–K–Li–Ca–Mg–Cl–Br), ore geochemistry, S, and Pb isotope data suggest that the Angouran sulfide ore formed by the interaction of modified, strongly evaporated Miocene seawater and the lithotypes of an exhumed metamorphic core complex. Minor contributions of metals from Miocene igneous rocks cannot be excluded. Mineralization occurred in a collisional intra-arc setting with high heat flow, probably during the transition from an extensional to a compressional regime. The Angouran deposit may represent a new type of low-temperature carbonate-hosted Zn–Pb ore that is distinct from Mississippi Valley type and sedimentary-exhalative deposits.Editorial handling: B. Lehmann