DIFFERENT TECTONO-THERMAL EVOLUTIONARY PATHS IN ECLOGITIC ROCKS FROM THE AXIAL ZONE OF THE VARISCAN CHAIN IN SARDINIA (ITALY) COMPARED WITH THE LIGURIAN ALPS

In the Ulten Zone (Upper Austroalpine), small bodies of mantle peridotites are incorporated within high-grade basement rocks (gneisses and migmatites) which represent remnants of lower crust subducted and reequilibrated at eclogite- facies conditions during the Variscan orogenic cycle. The Ulten peridotites record a complex metamorphic and deformative evolution, which is testified by the transition from coarse-grained protogranular spinel-bearing peridotites, to fine-grained garnet and amphibole (Ca-hornblende) -bearing peridotites with porphyroclastic to mosaic granoblastic textures. Thermometric estimates on the coarse-type spinel lherzolites have yielded high temperatures of equilibration, in the range 1100-1300°C (Obata and Morten, 1987). In the porphyroclastic peridotites, the metamorphic recrystallization to (garnet + amphibole)-facies conditions is evidenced by the development of: i) garnet coronas around spinel, ii) fine-grained granoblastic aggregates made by olivine + garnet + Ca-hornblende + new pyroxenes, iii) garnet and Ca-hornblende exsolutions within primary spinel-facies clino- and ortho-pyroxenes. The P-T conditions of the high-pressure eclogitic recrystallization which produced the spinel- to garnet-facies transition have been recently estimated to 850°C and 27 kbar (Nimis and Morten, 1999). The peculiar thermobarometric reequilibration recorded by the Ulten peridotites has been interpreted as the result of a wedge to slab evolution (Nimis and Morten, 1999; Godard et al., 1996). In this scenario, the spinel peridotites represent portions of a mantle wedge which were incorporated (by convection ) in a downgoing slab of cold continental crust, and were then subducted together with the slab to depths of about 90 km. Entrainment in the cold slab and subduction caused the reequilibration of the peridotites at 850°C and 27 kbar. The metamorphic transition from spinel- to garnetbearing assemblage occurred therefore in a dynamic regime, and was accompanied by significant input of metasomatic fluids, as testified by the crystallization of abundant amphibole in the garnet-bearing high-pressure assemblage. Petrologic investigations on the host gneissic basement rocks have evidenced that they also experienced high-pressure recrystallization, which was accompanied by in-situ partial melting and migmatization (Godard et al., 1996). The particular geodynamic evolution of the Ulten peridotites thus offer the unique opportunity to investigate the effects of crustal-derived metasomatism on mantle rocks involved in a subducting environment. Previous whole-rock chemical and isotopic investigations on the Ulten peridotites have evidenced that the fine-type garnet-facies ultramafics are enriched in LREE, K, Sr and that the alkalis enrichment is positively correlated with the 87Sr/86Sr ratios. In this study, we present the results of detailed in situ investigations (performed by the ion microprobe operating at CSCC, Pavia, Italy) on the trace element chemistry of the main mineral phases (clinopyroxene, amphibole and garnets) from seven selected samples representative of the various stages of the tectonometamorphic evolution recorded by the peridotites. Major aims have been to investigate the geochemical signature of fluids responsible of the amphibole crystallization, and provide further constraints on the nature of the metasomatic processes. The data obtained are potentially usefull to characterize, by direct evidence, the chemical changes induced in mantle rocks by crustal metasomatism. The coarse-type spinel peridotites, which are relics of the “pre-subduction”, mantle-wedge equilibration stage, display modest metasomatic effects. In these samples, modal metasomatism is only recorded by the incipient crystallization of amphibole as rims around clinopyroxene. Clinopyroxenes have almost flat REE spectra (CeN/SmN = 0.76-0.87) at 4-8 x C1 values, or display concave shape with selective LREE enrichment (CeN/SmN = 2.50-4.50, SmN/YbN = 0.53-0.97). The REE concentrations of amphiboles are very similar to those of clinopyroxenes. Both amphiboles and clinopyroxenes in sample MK5D, a coarse-type garnet-bearing peridotite, exhibit a convex-upward REE pattern characterized by LREE and HREE depletion (CeN/SmN = 0.17-0.19; SmN/YbN = 3.07-6.13). Their low HREE abundances are due to the equilibration with garnet which, as expected, show severely fractionated patterns (CeN/YbN < 0.001; HREE at about 20-30 x C1). Amphiboles, in the coarse-type rocks, also show low Sr (18-35 ppm) and K (171-964 ppm) abundances. The most evident metasomatic effects are recorded by the eclogite-facies recrystallized fine-type peridotites. In these rocks, modal metasomatism is documented by abundant crystallization of amphibole (Ca-hornblende, Mg values: 90- 92) in equilibrium with garnet. In some samples, the gnt+cpx+amph+opx+ol assemblage is replaced by amph+opx+ol assemblages, this feature indicating progressive degrees of hydration. Amphiboles display significant LREE enrichment (CeN/YbN = 3.90-11.50; LREE in the range 20-50 x C1) and high Sr (150-250 ppm), K (1910- 7280 ppm) and Ba (280-800 ppm) contents. By contrast, they have relatively low concentrations in HFSE (e.g., Zr = 14-25 ppm, Y = 6.7-16 ppm, Ti = 1150-2500 ppm, Nb = 2-7 ppm). The geochemical signature recorded by amphibole in the fine-type peridotites, i.e. the strong enrichment in LILE relative to HFSE, is a peculiar feature of crustal-derived metasomatic agents. The lack of evidence of major element modifications in mantle minerals (e.g. Mg-value decrese, crystallization of orthopyroxene around olivine) strongly suggest that the metasomatic agent was an hydrous fluid rather than a silica-rich melt. Moreover, experimental studies have demonstrated that aqueous fluids preferentially partition elements like alkalies, Ba, Sr and Pb, whereas they have scarce affinity for HFSE. The results of our study therefore indicate that the chemical modifications occurred in the Ulten peridotites during the high-pressure reequilibration were most likely produced by the input of hydrous, LILE-enriched, fluids, which caused crystallization of abundant amphibole. Such H2O-rich fluids could represent the residual fluids left after the crystallization of leucosomes, starting from water-undersaturated melts produced during migmatization of the host gneisses.

The study of late and post-orogenic sedimentary basins is a powerful tool to understand uplift, exhumation and erosion of an orogen. In the Ligurian Alps, high-pressure ophiolitic rocks are directly overlain by the sediments of the Tertiary Piedmontese Basin (TPB); the conglomerates at the bottom of the TPB succession contain clasts of metaophiolites and metasediments which display deformation structures acquired at peak metamorphic conditions ranging from eclogite- to blueschist- facies. The deformation events prevalently recorded by the highpressure rocks of the Ligurian Alps are either related to subduction and peak metamorphism, or to the late-stage greenschist collision. The main structures which accomplish the early exhumation of such high-pressure terrains are only locally recorded and poorly explored. Further information on these early tectonic events can be gathered through the study of the clasts in the conglomerates. In this paper we present a textural and petrologic outline and some thermobarometric estimates of the main types of high-pressure clasts sampled and their comparison with the high pressure equivalents presently exposed in the Ligurian Alps. In particular, some clasts displaying a Na-amphibole + white mica + epidote + sphene blueschist foliation superimposed to an eclogitic garnet + Na-clinopyroxene + rutile tectonitic assemblage have been studied in detail. These kind of rocks record a pressure-temperature path implying cooling during exhumation, whereas the high-pressure bedrocks are characterized by either isothermal decompression, or initial heating and subsequent cooling. We conclude that the blueschist foliation of these clasts most likely developed along shear zones and/or contacts among different slices, formed during tectonic coupling of warm, uprising eclogite units, with cooler blueschist slices during the early stages of exhumation.

This paper investigates Al-silicate-bearing migmatite from NE Sardinia by using the P-T pseudosection approach with the aim to determine the P-T conditions of partial melting and those of melt crystallization. P-T pseudosections were calculated in the NCKFMASH system within the P-T range 500-800°C, 0.1-1.5 GPa by using the average compositions of metapelitic greywacke, average mesosome and average trondhjemitic leucosome, respectively. The P-T pseudosections calculated for the average metapelitic greywacke and for the average mesosome, contoured for melt volume %, Si/Al and Na/K molar ratios in melt point to P–T conditions  700-740°C, 1.1-1.3 GPa which are indicative of partial melting. The P-T pseudosection calculated for the average composition of trondhjemitic leucosomes, contoured for kyanite and biotite modal content and for XMg ratio in biotite indicates P-T conditions of 660-730°C, 0.75-0.90 GPa for the crystallization of the melt. The comparison between the Na/K and Si/Al ratios in leucosomes, and the same ratios modeled for the anatectic melt by an haplogranitic melt model is thus a powerful tool for the reconstruction of P-T conditions of partial melting also in pelitic rocks, provided that leucosomes represent pure melts and are not contaminated by restitic phases or feldspar cumulates.

Nitrogen-bearing fluids, brines and carbonate liquids in Variscan migmatites of the Tatra Mountains, Western Carpathians - heritage of high-pressure metamorphism

High- to very-high-grade migmatitic basement rocks of the Wilson Hills area in northwestern Oates Land (Antarctica) form part of a low-pressure high-temperature belt located at the western inboard side of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from four very-high grade migmatites (migmatitic gneisses to diatexites) and zircon from two undeformed granitic dykes from a central granulite-facies zone of the basement complex were dated by the SHRIMP U-Pb method in order to constrain the timing of metamorphic and related igneous processes and to identify possible age inheritance. Monazite from two migmatites yielded within error identical ages of 499 +/- 10 Ma and 493 +/- 9 Ma. Coexisting zircon gave ages of 500 +/- 4 Ma and 484 +/- 5 Ma for a metatexite (two age populations) and 475 +/- 4 Ma for a diatexite. Zircon populations from a migmatitic gneiss and a posttectonic granitic dyke yielded well-defined ages of 488 +/- 6 Ma and 482 +/- 4 Ma, respectively. There is only minor evidence of age inheritance in zircons of these four samples. Zircon from two other samples (metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U ages. While there is a component similar in age and in low Th/U ratio to those of the other samples, inherited components with ages up to c. 3 Ga predominate. In the metatexite, a major detrital contribution from 545 - 680 Ma old source rocks can be identified. The new age data support the model that granulite- to high-amphibolite-facies metamorphism and related igneous processes in basement rocks of northwestern Oates Land were confined to a relatively short period of time of Late Cambrian to early Ordovican age. An age of approximately 500 Ma is estimated for the Ross-orogenic granulite-facies metamorphism from consistent ages of monazite from two migmatites and of the older zircon age population in one metatexite. The variably younger zircon ages are interpreted to reflect mineral formation in the course of the post-granulite-facies metamorphic evolution, which led to a widespread high-amphibolite-facies retrogression and in part late-stage formation of ms+bi assemblages in the basement rocks and which lasted until about 465 Ma. The presence of inherited zircon components of latest Neoproterozoic to Cambrian age indicates that the high- to very-grade migmatitic basement in northwestern Oates Land originated from clastic series of Cambrian age and, therefore, may well represent the deeper-crustal equivalent of lower-grade metasedimentary series of the Wilson Terrane.

The Monviso metamorphic ophiolite, one of the best preserved relics of oceanic crust in the Western Alps, was formed during the opening of the Mesozoic Western Alpine Tethys and underwent metamorphism to eclogitic conditions during Alpine subduction. The Monviso ophiolite encompasses the whole lithological spectrum of the Piedmont-Liguria ophiolite rocks, with a basal unit of serpentinized peridotite in tectonic contact with the overlying metagabbros, eclogites and pillowed metabasalts. Slivers of serpentinized peridotite hosting banded eclogites and metagabbros divide these units from the overlying Forciolline Unit. The latter (formerly called Costa Ticino Series) is an overturned sequence of gabbros with pods of cumulate troctolite and lenses of serpentinized peridotite, overlain by massive and pillow metabasalts. A unit of massive metabasalts tops the tectonic stack. A body of jadeite-quartz bearing metaplagiogranite has been recently found in the Basal Serpentinite Unit near Verne, northwest of Sampeyre, Val Varaita. Zircon crystals recovered from the Verne metaplagiogranite have large domains with typical magmatic zoning with broad oscillatory bands. They have Th/U ratios in the range 0.3-0.7, as commonly observed in magmatic zircon. In situ ion microprobe (SHRIMP) U-Pb dating of the magmatic domains yielded a mean age of 152±2 Ma, which is interpreted as the crystallization age of the Monviso plagiogranite. Unzoned domains that crosscut magmatic zircon yielded younger, apparent ages which are most likely due to Pb loss during Alpine metamorphism. In conjunction with previous works on ophiolites from the Western Alps, Northern Apennines, and Alpine Corsica, the new data from Monviso suggest that the plutonic activity in the Piedmont-Liguria domain of the western Tethys may have lasted only 15 to 20 Ma, between ca. 170 and ca. 150 Ma. As shown by Radiolarian biostratigraphy, this is approximately the same time span encompassed by the extrusion of tholeiite basalts which cap both the gabbro plutons and their peridotite country rocks. The new data indicate that the plutonic activity recorded at Monviso was coeval with basalt extrusion and deep-sea sediment deposition in some Liguriantype ophiolite bodies of the Cottian Alps. This suggests that the oceanic crust preserved in the Monviso ophiolite may have formed later, and in a more central position of the basin, than the oceanic crust preserved in such Ligurian-type ophiolite bodies.

This paper reports the major, minor and trace element abundances and Nd isotope compositions of bulk rock samples of eclogites from the Stak Valley in northwestern Himalaya and discusses their protolith. Major element compositions confirm the basaltic nature of the protolith. Trace element abundances normalized to the primitive mantle show almost flat patterns at ten times the primitive mantle values with slightly high concentrations of Th and light rare earth elements. These patterns are similar to those of enriched MORBs. Neodymium isotope compositions are chondritic and the values of eNd lie between those of MORB and old Indian continental crust. Immobile trace element contents of Stak eclogites are similar to those of the Permian Panjal Traps and significantly different from those of the Deccan Traps erupted at 73–66 Ma in the northwestern Indian plate, suggesting that the Panjal Traps is the protolith of the Stak eclogites. The protoliths of ultra-high pressure eclogites in the Kaghan and Tso Morari massifs in northwestern Himalaya are also interpreted to be the Panjal Traps. The results of this study suggest that a large Permian igneous province developed at the northwestern margin of the Indian plate before the collision with the Eurasian continent.

Mantle heterogeneity is very important for magma genesis within the upper mantle. Melting of the heterogeneous mantle consisting of peridotite and mafic (or pyroxenite/ eclogite) rocks will yield voluminous and compositionally diverse magmas upon a melting process because of selective fusion of the mafic layers (e.g., Yasuda et al., 1994; Hauri, 1996; Takahashi et al., 1998). Melting experiments using heterogeneous mixtures of peridotite and MORB as a starting material, for example, have just started to examine this problem (Yaxley and Green, 1998; Kogiso et al., 1998). Mantle heterogeneity demonstrated by layered structure of peridotites and mafic (or pyroxenite/eclogite) rocks is conspicuous in many orogenic lherzolite massifs. We need to know how and when the inhomogeneous mantle materials have been produced. The Horoman Peridotite Complex, Hokkaido, northern Japan, has two specific features; one is the presence of symplectite, possibly of garnet origin, and the other is a symmetric layered structure characterized by an arrangement of cumulus peridotite and mafic rock in the middle of a series of residual peridotite with increasing melt component outward. The symmetrical layered structure repeats several times with intervals of several meters to a few hundred meters in the complex (Niida, 1984; Obata and Nagahara, 1987; Takahashi, 1992). Symplectite-bearing rocks in the Horoman complex are divided into three types based on petrography. The first, and the most abundant, is a kind of mantle restite, that is clinopyroxene- rich spinel lherzolite and plagioclase lherzolite. The second is a pyroxenite alternating with cumulus peridotites. The third is pyroxenites, which occur as thin layers in the mantle restite. Mineral assemblages and chemical compositions of the symplectites suggest that they were generally formed by the decompression reaction between pyrope-rich garnet and olivine. The presence of symplectite in cumulus peridotite and pyroxenite suggests the garnet was involved in the formation of cumulates at about 2 GPa or more. The mafic rocks in the Horoman complex have been divided into several types. One of these mafic layers, which is called Type II layer of Takazawa et al. (in press) or GB II of Niida (1984) and Shiotani and Niida (1984), restrictedly occurs in the cumulus peridotite which is located in the middle of residual peridotite with the symmetric layered structure. Some textural characteristics of the Type II mafic rocks are similar to those of the symplectite-bearing pyroxenites in the cumulus peridotite. The Type II mafic rocks have the same origin as symplectite-bearing pyroxenites, that is the subsolidus breakdown product of garnet-bearing pyroxenites of high-pressure origin to the gabbroic rock at lowerpressure conditions. On the other hand, their geochemical signatures indicated that the Type II mafic rocks were originally formed at lower-pressure conditions (Shiotani and Niida, 1997; Takazawa et al., in press). Textural characteristics of a corundum-bearing Type II mafic rock (Morishita and Kodera, 1998) show that corundum was not stable at the latest P-T conditions of the Horoman complex and require that it had experienced heating and/or decompression. A possible P-T history for the Type II mafic rock is as follows. (1) Type II mafic rock was formed as a cumulate at lower-pressure conditions from the melts responsible for the formation of the cumulus peridotite. (2) The protolith of Type II mafic rocks had been metamorphosed to garnetbearing pyroxenite at high P-T conditions during compression due to subduction or convection within the mantle. (3) The complex ascended from the garnet stability field to the plagioclase peridotite stability filed as a diapir. The Type II mafic rocks as a member of the diapir were formed from garnet-bearing pyroxenite through symplectite-bearing rock due to breakdown of garnet and corundum at low pressures. The Type II mafic rocks have a complex P-T trajectory after it was formed as a member of the layered structure. We favor the possibility that the symmetrical layered structure in the Horoman complex have been repeated by deformation processes (Toramaru, 1997), not by the melting process along multiple parallel cracks (Takahashi, 1992). The melting process, however, had an important role in formation of a stratified lithological unit composed of cumulate rock, residual peridotite and primitive peridotite.

This paper deals with the metamorphic evolution of the Bacariza Fm that outcrops in the two uppermost structural units of the Cabo Ortegal Complex (NW Iberian Massif). This formation includes ultramafic and mafic granulites, garnet amphibolites and garnet trondhjemitic gneisses. Although mineral associations characteristic of high pressure granulites predominate in the least retrogressed of these rocks, the presence of relic kyanite along with the fact that plagioclase only appears in symplectitic textures resulting from de-jadeitization of pyroxenes point to an earlier eclogite facies metamorphism. Thermobarometric estimations indicate higher P-T conditions for the rocks in the uppermost structural unit.

The Kwoiek Area of British Columbia contains a pendant or screen of metamorphosed sedimentary and volcanic rocks almost entirely surrounded by a portion of the Coast Range Batholith, and intruded by several dozen stocks. The major metamorphic effects were produced by the quartz diorite batholithic rocks, with minor and later effects by the quartz diorite stocks. The sequence of important metamorphic reactions in the metasedimentary and metavolcanic rocks, ranging in grade from chlorite to sillimanite, is: 1. chlorite + carbonate + muscovite → epidote + biotite 2. chlorite + carbonate → actinolite + epidote 3. chlorite + muscovite → garnet + biotite 4. chlorite + epidote → garnet + hornblende 5. chlorite + muscovite → garnet + staurolite + biotite 6. chlorite + muscovite → aluminum silicate + biotite 7. muscovite + staurolite → garnet + aluminum silicate + biotite 8. staurolite → garnet + aluminum silicate Continuous reactions, occurring between reactions 5 and 7, are: A. chlorite + (high Ti) biotite + Al2O3 (from plagioclase?)→ garnet + staurolite + (low Ti) biotite + O2 B. muscovite (phengitic) → garnet + staurolite +muscovite (less phengitic) + O2 (?) Detailed electron microprobe work on garnet, staurolite, biotite, and chlorite shows that: (1) The garnet porphyroblasts are zoned according to a depletion model, called the Rayleigh depletion model, which assumes equilibrium between the edge of a growing garnet and the minerals which are unzoned, notably biotite, chlorite, and muscovite, but which assumes disequilibrium within the garnet. (2) The staurolite porphyroblasts are also zoned, and from their zoning patterns reactions A, B, and 5 are documented. Progressive reduction of iron with increasing grade of metamorphism is also inferred from the staurolite zoning patterns. (3) During a late period of falling temperature garnet continued to grow and the biotite and chlorite reequilibrated. The biotite, chlorite, and garnet edge compositions can vary from point to point in a given thin section, indicating that the volume of equilibrium at the final stage of metamorphism was only a few cubic microns. (4) The horizon within the garnet that grew at maximum temperature can be identified. The Mg/Fe ratio of this horizon, if the garnet composition is a limiting composition in the Al2O3 - K2O - FeO - MgO tetrahedron, increases systematically with increasing metamorphic grade. Biotite and chlorite compositions also show a general increase in Mg/Fe ratio with increasing metamorphic grade, but staurolite appears to show the reverse effect. (5) The Mg/Fe ratio at the maximum temperature horizon of the garnet porphyroblasts is a function of its Mn content as evidenced from the study of five garnet-bearing rocks, collected from one outcrop area, with the same assemblage but with differing proportions of minerals. An important implication of zoned minerals is that the effective composition of a system in a phase lies on the join between the homogeneous minerals (if there are two) and not within three-or- four-phase fields when a zoned mineral, such as garnet or staurolite, is present in the assemblage. Study of the three aluminum silicates found in the Kwoiek Area showed that a constant pressure change in polymorphs from andalusite to kyanite to sillimanite took place with increasing temperature. This transition series is best explained by the metastable formation of andalusite. Photographic materials on pages 15, 121, 160, 162, and 164 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

Extremely rare veinlets and reaction textures composed of symplectites of olivine (similar to Fo(43-55)) + plagioclase +/- spinel +/- ilmenite, associated with more common pyroxene + plagioclase and amphibole + plagioclase varieties, are preserved within eclogites and garnet pyroxenites in the Moldanubian Zone of the Bohemian Massif. Thermodynamic modelling integrated with conventional geothermometry conducted on an eclogite reveals that the symplectite-forming stage occurred at high T (similar to 850 degrees C) and low P (< 6 and > 2 center dot 5 kbar). The development of the different symplectite types reflects reactions that took place in micro-scale domains. The breakdown of high-P garnet controlled the formation of olivine-bearing and amphibole + plagioclase symplectites, whereas breakdown of high-P omphacite led to formation of pyroxene + plagioclase symplectites. In addition, post-eclogite facies but pre-symplectite stage porphyroblastic amphibole and phlogopite were also replaced by olivine-bearing symplectites. Material transfer calculations and thermodynamic modelling indicate that the formation of different symplectite types was linked despite their different bulk compositions. For example, the olivine-bearing symplectites gained Fe +/- Mg, whereas adjacent amphibole + plagioclase and pyroxene + plagioclase symplectites show losses in Fe and Mg; Al, Si and Ca were also variably exchanged. The olivine-bearing symplectites were particularly sensitive to Na despite the small concentration of this element. In eclogites where Na was readily available, the plagioclase composition in the olivine-bearing symplectites shifted from pure anorthite to bytownite, with the less calcic feldspar partitioning Si and inhibiting the formation of orthopyroxene. This regional high-T, low-P granulite-facies symplectite overprint may have been caused by advective heat loss from rapidly exhumed high-T, high-P granulitic bodies (Gfohl Unit) that were emplaced into and over the middle crust (Monotonous and Varied Series) during Carboniferous continent-continent collision.

Symplectite, defined as plagioclase + Ca-pyroxene (±amphibole) intergrowths after omphacite, and kelyphite, defined as amphibole + plagioclase coronas around garnet, are common features of retrogressed eclogites. These textures are related to exhumation under (ultra) high pressure towards the surface, but the estimation of the pressure–temperature (<i>P</i>–<i>T</i>) of symplectite formation is difficult because of the narrowness of pyroxene and plagioclase lamellas, and the compositional variability of the phases. Retrogressed eclogites from Norwegian localities with different eclogite peak conditions have been chosen to investigate the formation of symplectite and associated kelyphite. Thermobarometry calculations show that symplectite crystallizes as soon as the rocks enter the stability field of plagioclase and continues crystallizing until they have reached amphibolite facies. Symplectite yields a pressure range from 18 to 10 kbar, and a temperature range from 700 to 550°C. Amphibole found in the symplectite assemblage crystallizes later, at lower pressures and temperatures (10–4 kbar, 680–420°C). Kelyphite is always associated with well-developed symplectite, when the former omphacite is totally transformed into symplectite. These features likely testify to the influence of an external fluid during retrogression. Samples with limited symplectite and no kelyphite are likely retrogressed with an internal fluid.

Plagioclase crystal size distribution (CSD) has been investigated in a quartz-diorite body, in the leucosome of migmatites and in the melanosome of un-melted contact metamorphic rocks from Gennargentu Complex (Sardinia, Italy). During the crystallization of the dioritic magma, a variety of competing kinetic processes determine the evolution of the igneous microstructure, but the relative contribution of each process remains elusive. Our approach was aimed to study the plagioclase crystallisation from a liquid (quartz-diorites and migmatite leucosomes), comparing it to a crystallisation at subsolidus conditions. CSD indicates that plagioclase in the quartz-diorite nucleated and grew in a cooling system at a constant cooling rate, producing straight-line CSD in a diagram of ln of population density vs. size range. The plagioclase crystallisation continued until the latent heat was available and the temperature was high enough to allow the plagioclase growing. This can occur only when a crystal is held at temperature close to its liquidus for a long period of time. Under these conditions, the plagioclase nucleation rate is zero, but growth rate is high for crystal larger than the critical size. This does not necessarily mean that the temperature was held constant, just that the undercooling remained small (Ostwald ripening process). The aggregated small crystals, due to their high surface energy per unit volume, to minimise energy in the system dissolved and“fed” the growth of larger crystals. This process occurs because small grains have a higher surface energy per unit volume than do larger grains. The crystallisation temperature (~900 °C, 100 MPa) allows the formation of plagioclase as liquidus phase. From CSD measurements we calculated the different cooling ages for the different sample types.

Stretching for > 2000 km between the Sino-Korean craton in the north and the Yangtze craton in the South, the Qinling-Tongbai-Hong’an-Xinxian-Dabie-Sulu-Imjingang orogen is the centrepiece of Chinese geology. From north to south, it comprises the Kuanping, the Erlangping, the Qinling unit Liuling, the Douling and the Wudang, i.e. tectono-metamorphic units with complex evolutions. In Cambrian times, deep subduction of the Qinling microcontinent below an intra-oceanic Erlangping arc created ultra-high pressure metamorphic eclogites and gneisses. The coesite-eclogite facies stage was constrained at 550°C and 3.1 GPa. During uplift, a quartz-eclogite facies recrystallization occured at 2.0-2.3 GPa and ~660 °C. Further uplift was characterized by nearly isothermal decompression and a penetrative overprint at 630-640 °C and 1.1-1.5 GPa. Ar/Ar phengite and U/Pb titanite ages of ~470 Ma highlight exhumation into the crust together with cooling in the Middle Ordovician. The southern margin of the Qinling microcontinent faced a north-trending subduction zone and a magmatic arc set up that was active from the Middle Cambrian till the Early Devonian (~110 Ma). Due to the high heat flow, the central-southern part of the Qinling unit underwent ultra-metamorphism at peak metamorphic conditions of 680-775 °C at 0.5-0.75 GPa. Metamorphics of the Liuling indicate an at least a two-stage burial-exhumation history during Late Carboniferous-Permian. Thermodynamic modelling of zoned garnet reveals a first clockwise Barrovian metamorphism, which took place under medium pressure amphibolite facies conditions (560-590 °C at 0.4-0.6≈GPa) and was followed by a second stage of high pressure amphibolite facies metamorphism (590 °C at 0.9 GPa). Lithology and geochronology classify the Liuling as a Devonian to Early Carboniferous forearc basin, which received its metamorphic overprint at 250-320 Ma. The Douling forms a basement wedge intercalated between the Liuling and the Wudang. Medium to high-grade metamorphic conditions (560-710 °C at 0.8-1.2 GPa) likely reflect a Neoproterozoic event. A thorough LT/HP metamorphic overprint (280-340 °C at 0.5-0.9 GPa) of probable Triassic age affected the Douling Complex as well as its cover units. Similar HP/LT metamorphic conditions (~300 °C at 0.4-0.9 GPa) are recorded in blueschists of the neighbouring northern Wudang Complex. In the centre of the Wudang Complex, HP/MT metamorphic conditions (500-550 °C at 1.0-1.2 GPa) recorded by garnet gneisses and amphibolites are followed by a HP/LT overprint of 300 °C and 0.6-0.7 GPa. Thermobarometry and geochronology indicate that the metamorphism in the Wudang Complex occurred due to subduction of the Yangtze craton underneath the amalgamated Qinling-Erlangping-Sino-Korean continent in the Triassic, which is also true for the Douling complex and its cover. In the western part of the orogen, along a north-south profile, petrological invesigations reveal amphibolite facies PT conditions of 590 °C at 0.6 GPa in the northern part and upper amphibolite facies conditions of 760 °C at ~0.7 GPa along with the migmatisation of felsic gneisses in the centre of the profile. Further south, Theria_g modeling applied to garnet from a garnet-staurolite gneiss point to a rather fast prograde clockwise evolution with pronounced heating along with minor burial and nearly isothermal exhumation. The southermost sample gives evidence for medium to upper greenschist-facies conditions. The age of this metamorphic overprint is constrained by Th/Pb monazite ages of 205.8 ± 2.8 Ma and 194.1 ± 2.1 Ma as well as Ar/Ar ages of 192-207 Ma while Ar/Ar ages from adjacent magmatics give a somewhat larger timespan of 186-227 Ma as the younger ages originate from pegmatites and reflect late magmatic activities. Published and new U/Pb zircon arges highlight Triassic – Early Jurassic magmatism spanning ~50 Ma with a major cluster at ~225-205 Ma. Alltogether, these findings highlight a Triassic tectono-metamorphic event in the middle to lower crust of the western Qinling orogen which was strongly influenced by the intensive syn- and post-tectonic magmatic activity. Triassic-Early Jurassic Ar/Ar ages from the Mianlue mylonite zone reflect a Mesozoic metamorphic overprint. However, new and recently published U/Pb zircon ages from mafic rocks prove a Neoproterozoic origen of ophiolite blocks. Thus, there are no indications for the existence of a Late Palaeozoic Myanlue ocean and the hypothesis of the existence of a “Mianlue suture” stretching all through the northern Yangtze craton is falsified.

The Variegated Formations (VF) of the eastern Rhodope Mountains (SE Bulgaria) form part of the pre-Alpine basement of the region. They are composed of alternating sediments and igneous rocks with a high-grade metamorphic overprint. Numerous ophiolitic slivers are associated with the VF and include metamorphosed peridotites, ultramafic cumulates, and amphibolitized eclogites. The dismembered ophiolites usually form the base of the VF successions. The metasedimentary rock types contain terrigenous materials (metapsammites and quartzites) that frequently alternate with metapelites and marbles. The nature of this sedimentary package, along with the field relations and sedimentary features, reflects its flysch character. The metaigneous rocks of the VF occur either as layers interbedded with the metasediments, or as intrusive bodies that intersect the ultramafic rocks. The principal mineral phases in the metabasites are amphibole + plagioclase + quartz + epidote ± garnet ± chlorite. We calculate temperatures of 630°C to 520°C at 6-2 kbar pressures, indicating moderate amphibolite facies metamorphism. Major rock-forming minerals (amphibole, plagioclase, and garnet) exhibit zoning typical of retrograde P-T conditions. When plotted on tectonic-setting discrimination diagrams, the metabasic rocks of the VF fall mainly in the fields of modern boninites and arc tholeiites. They show low Ti and Zr contents and key elemental ratios of CaO/TiO2, Al2O3/TiO2, Ti/Zr, Ti/Y and Zr/Y, all transitional between island arc tholeiites and boninites. Chondrite-normalized REE patterns reveal the existence of two different trends: U-shaped REE patterns (for the majority of samples) and LREE depleted patterns. Regardless of the existence of these two trends, the [La/Sm]N ratios of the metabasites perfectly coincide with the same ratios for many Cenozoic boninite series. The flysch character of the sedimentary sequences, as well as the clear supra-subduction zone affinities of the igneous rocks, indicates that the VF formed in an oceanic island-arc setting. The boninitic affinities of the meta-igneous rocks indicate possible origin in an immature arc. The character of the VF and its association with the dismembered ophiolite slivers shows the presence of a suture zone. The East Rhodope suture zone distinguishes the VF from the rocks structurally below it, which consist of orthogneisses typical of continental crust. Existing U-Pb zircon data indicate that the orthogneisses are of Variscan age. New U-Pb zircon age data for the VF suggest Late Neoproterozoic ages for some protoliths. Based on regional correlations, the interpretation of the VF as a fossil accretionary prism can be useful for elucidating the structure of the whole Rhodope composite terrane, and for tracing the suture itself to the Central and Western parts of the Massif.