The high-pressure granulites of the Bacariza Formation: an earlier stage in the exhumation of other eclogites in the Cabo Ortegal Complex (Hecynian belt, NW Spain)

The inner zone of the Sardinia Variscan segment consists of two metamorphic complexes: I) A polymetamorphic Migmatite Complex, with migmatites showing polyphase anatectic processes, in the presence of kyanite or sillimanite. The Migmatite complex preserved decametric lenses of eclogite relicts (eclogites A) affected by high T, high- to intermediate P recrystallization under granulite facies conditions The decompressional garnet + Ca-clinopyroxene + amphibole ± orthopyroxene-bearing assemblages developed in granoblastic textures generally in no stress conditions. In most cases, only symplectite textures provide evidence for the eclogitic event. II) A medium grade, mostly metapelitic complex consisting of Grt, Ky, Stau-bearing micaschists and paragneisses includes quartzites and garnet-bearing amphibolite boudins with N-MORB chemical affinity. Relicts of eclogite assemblages were locally found in the metabasite (eclogites B). In eclogites A, the geothermobarometric parameters yield temperatures in the range 690°-760°C for minimum pressure A1.3 GPa. Pyroxene compositions accord with temperatures in excess of 700°C. In eclogites B, the thermometric calibrations provide temperatures in the range 610°-700°C for pressures 1.3-1.5 GPa, based on the jadeite content. The temperatures are consistent with the biotite+muscovite+garnet+kyanite+staurolite assemblage in the host paragneisses, and with lack of anatectic processes. The age of 457±2 Ma, obtained by U/Pb dating on one sample of Type A eclogite is interpreted as a minimum estimate for the magmatism of the eclogite protolith. A second zircon population defined an age of 403±4 Ma interpreted as dating the zircon crystallization during the high-grade event. The relationships between Types A and B eclogites, and their bearing on the regional framework (Sardinia, Ligurian Alps) are discussed.

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.

There is a typical assemblage of garnet + kyanite + microperthite + quartz + rutile in high-pressure (HP) felsic granulite of Qinling complex in Songshugou area. East Qinling. The HP granulite was formed at 800 -900℃ and 1.3 -1.6GPa and has experienced two stages of retrograde metamorphism at 600- 650 ℃, 0.8-1.0GPa and 500-600℃, 0.3-0.6GPa, forming two retrograde metamorphic assemblages of margarite + plagiodase (PlI)+quartz and sillimanite + biotite + plagioclase(PlII) + microdine+quartz, respectively. They construct a two-stage clockwise P-T path which shows down-pressure cooling in both early and late stage.

New evidence for ultrahigh-pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet peridotites of Pohorje Mts. in Slovenia. In this area, an eo- Alpine UHPM has been recently documented in the eclogites (Janak et al. 2004). These eclogites are closely associated with metaultrabasites - predominantly serpentinised dunite and harzburgite with garnet peridotite remnants. The country rocks of eclogites and metaultrabasites are amphibolites, orthogneisses, paragneisses and micaschists. All these rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. At least four stages of recrystallization have been identified in the garnet peridotite based on an analysis of reaction textures and mineral compositions. Stage I is a high-temperature protolith assemblage of olivine + orthopyroxene + clinopyroxene + Cr-spinel. Aluminous pyroxenes occur as inclusions in garnet, chromian spinel is preserved in the matrix. Stage II – an ultrahigh-pressure stage is defined by matrix assemblage garnet + olivine + orthopyroxene + clinopyroxene + Cr-spinel. Garnet contains up to 67 mol% of pyrope, olivine has 90 mol% of forsterite, orthopyroxene is low in Al2O3 (~0.8 wt%) and spinel has a Cr* ~ 50. Stage III – a decompression stage is manifested by formation of kelyphitic rims of high-Al orthopyroxene, aluminous spinel and pargasitic hornblende replacing garnet. Due to retrogression, garnet shows a decrease in MgO. Stage IV – is represented by formation of tremolitic amphibole, chlorite, serpentine and talc. P-T estimates based on geothermobarometric calculatios a) Fe-Mg exchange between garnet, olivine and orthopyroxene thermometers, b) the Al-in-orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at ~820-900oC and 3-3.5 GPa. This is consistent with previous estimation of very high P-T conditions in metaultrabasites by Hinterlechner-Ravnik et al. (1991) and the associated eclogites (Janak et al. 2004). These results suggest that the mantle fragment (garnet peridotite) and the crustal fragment (eclogite) in the Pohorje Mts. both experienced a common UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of a continental slab which incorporated peridotites from an overlying mantle wedge.

The Late Cretaceous sedimentary melanges from the External Liguride Units of Northern Apennines include large slide-blocks of subcontinental mantle peridotites, MORbasalts and lower and upper continental crust rocks. The slide-block association has been interpreted as representative of a continent-ocean transition between the Internal Liguride oceanic domain (Late Jurassic Western Tethys) and the thinned continental margin of the Adria plate (Marroni et al., 1998). The slide-blocks of lower continental crust consist of mafic and felsic granulites, which locally preserve primary contacts. The mafic granulites commonly display a metamorphic layering, but undeformed rocks preserving a gabbroic fabric are locally found. Undeformed mafic granulites are mostly represented by spinel-bearing gabbronorites, usually containing significant amounts of either olivine or Fe-Ti-oxides. Olivine- and Fe-Ti oxide-bearing rocks locally show spinel-pyroxene symplectites and garnet coronas, respectively. The felsic granulites are mainly quartzo- feldspathic rocks consisting of mesoperthitic to perthitic feldspar, quartz and garnet. The gabbroic protoliths of the granulites were emplaced at about 290 Ma at deep crustal levels, where they underwent slow cooling and recrystallisation under granulite-facies conditions (P = 0.7-0.8 GPa, T = 800-900°C). They were exhumed to upper levels, in association with the felsic granulites, in late Triassic-middle Jurassic times. The gabbro-derived granulites can be recognized as cumulus rocks with negligible amounts of residual trapped liquid, on the basis of low SiO2/Al2O3 ratios and overall low contents of incompatible trace elements. The Mg# value ranges from 80 to 52, and point to negative correlations with TiO2 and MnO, thus indicating a tholeiitic differentiation trend. Most gabbro-derived granulites have slightly LREEenriched patterns showing decreasing Eu positive anomaly with increasing total REE abundances. Chondrite normalization of incompatible trace elements reveals spikes at Ba and Sr, and a slight Zr depletion. The quartzo-feldspathic granulites have LREE enriched patterns, with nearly flat HREE and no or slightly positive Eu anomaly; Ba is abruptly enriched relative to REE, whereas Nb and Ti are depleted. The gabbro-derived granulites show a wide range in Sr and Nd isotopic compositions. The Sr isotopic ratio recalculated at 290 Ma varies between 0.7031 and 0.7077, and the initial eNd ranges between +6.8 and -4.5. Two samples of quartzo-feldspathic granulite yield age-corrected Sr isotopic ratios of 0.7107 and 0.7109, and eNd of -8.0 and -5.7. As a whole, the Nd and Sr isotopic data at 290 Ma form a hyperbolic array, in which the olivine-bearing gabbronorites have the highest eNd values and the lowest Sr isotopic ratios. Clinopyroxenes have been analyzed for trace elements by ion microprobe. Clinopyroxene from olivine-bearing gabbronorites shows peculiar compositions that indicate a metamorphic origin through olivine-plagioclase reaction, i.e. the igneous protoliths of the olivine-bearing gabbronorites were most likely troctolite-type cumulates. Clinopyroxenes from Fe-Ti oxide bearing gabbronorites show igneous geochemical trends, thus suggesting that these rocks contained clinopyroxene as original igneous phase. Petrography, bulk-rock and mineral composition indicate that the gabbro-derived granulites can be related to a fractional crystallization process, with early separation of olivine and plagioclase, followed by the replacement of olivine by pyroxene at the liquidus. Trace element modelization of the parental liquid compositions applied to the olivine-bearing rocks yields LREE- and LILE-enriched liquids, with absence of negative Nb anomaly, similar to plume-type MOR-basalts and continental tholeiites. However, a P-MORB origin seems to contrast with the initial Nd and Sr isotopic compositions, which are close to depleted mantle values at the time of emplacement. AFC modelization was successfully applied to obtain the isotopic compositions of the most contaminated samples, starting from the trace element and isotopic compositions of the parental liquids of the olivine gabbronorites and assuming a crustal contaminant with low Sr/Nd and isotopic composition comparable to that of the quartzo-feldspathic granulites. AFC calculations also indicate that the parental liquids of the olivine-bearing gabbronorites cannot be ascribed to N-MORB primary liquids. The primary mantle magma was necessarily characterized by moderate LILE enrichment, although an increase in LILE concentrations could have been enhanced by a small crustal contribution. The LILE enrichment in the parental liquids of the gabbro-derived granulites may be explained with a low degree partial melting of a rather fertile lithospheric mantle source. Alternatively, the primary liquids of the gabbro-derived granulites were related to a mantle source enriched in LILE as a result of the Variscan subduction event.

In situ U-Pb geochronology was carried out on amphibolites and siliciclastic metasediments of the Kinzigite Formation exposed in the northernmost sector of the Ivrea-Verbano Zone (Finero area). The aim is to shed light on the tectono-metamorphic evolution of this intermediate-lower crustal section and its bearing with the evolution of the southern and better known sectors of the IVZ. Based on field observation and petrography a metamorphic gradient gently increasing from amphibolite to upper amphibolite facies (from SE to NW) characterizes the whole metamorphic sequence. Metapelites consist mainly of biotite, quartz, plagioclase, garnet, and sillimanite; muscovite progressively disappears as K-feldspar appears and becomes abundant. Amphibolites are made of green-brown hornblende and plagioclase and may contain clinopyroxene defining thin layers together with plagioclase and titanite. Both metapelites and amphibolites show mylonitic deformation which is more intense towards NW, i.e. towards the lower structural levels. The mylonitic deformation strongly affected the lower crustal metabasic rocks of External Gabbro unit (Finero Mafic Complex). Zircon, monazite and titanite U-Pb geochronology was carried out with laser ablation (LA)-ICP-MS on amphibolites, migmatitic paragneiss and mafic granulites occurring as slivers of the Kinzigite Formation in the External Gabbro unit of the Finero Mafic Complex. The multi-chronological approach allowed recognizing three discrete tectono-metamorphic events, at Permian, Triassic and Jurassic. Zircon and monazite yielded Permian ages suggesting (re)crystallization during an high temperature event characterized by both metamorphism and magmatism. Titanite dating provided Triassic and Jurassic ages that were interpreted as U-Pb resetting ages. A Triassic perturbation of the U-Pb system was also recorded by zircon and monazite as rare domains. The tectono-metamorphic reconstruction of the evolution of the northernmost IVZ, as revealed by the new geochronological data, is only partially in agreement with the temperature-time evolutions depicted for the southern sectors of the IVZ. Permian ages indicating magmatism and high temperature metamorphism are common throughout the IVZ, as well as the Jurassic ages related to local thermal pulses and tectonic activity. Conversely, the occurrence of well-constrained Triassic ages is in fact peculiar of the Finero area. Two possible explanations may account for this Triassic event: Triassic ages are possibly related to the thermal effect and fluid circulation during the emplacement of the External Gabbro unit; or alternatively, they are the response to the ductile deformation largely recognized in the whole area. This study is a further evidence of the necessity of approaching crystalline basement with multiple geochronometers in order to unravel the complete tectono-metamorphic evolution.

PART I: Phase relationships have been determined for the dehydration-melting of a (powdered and solid) calcic, low-K, olivine tholeiitic amphibolite (hornblende 70%, plagioclase 30%), in runs at 10 kbar, 750 to 1000°C,fO2 ~= Ni-NiO, and for 1 to 21 days. Hornblende is involved in a sliding reaction: hornblende + anorthitic plagioclase -> clinopyroxene + liquid + aluminous hornblende + calcic hornblende + orthopyroxene + garnet. The liquid fraction varies from <1% at 750°C to ~47% at 1000°C, with the big increase occurring above 875°C. Liquids are tonalitic but have very high Al2O3 contents (18-21 wt.%). At high liquid fractions (~0.5), liquids are high-alumina basaltic. Liquids become more sodic with increasing temperature, but the compositional trends reverse direction, and liquids become more calcic above 975°C, where garnet is unstable. The water contents of liquids range from over 7 wt.% at low liquid fractions to 2 wt.% at high liquid fractions. In the solid amphibolite runs, liquid interconnectivity may be attained at 875°C with only 2 vol.% liquid and dihedral angles less than 60°. The removal of water-rich tonalitic liquids from a substantially melted amphibolitic source could help generate a relatively dry mafic granulite terrane, with densities up to 3.5 gm/cm3. Delamination of this dense lower crust is possible. PART II: A cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment (the Owens Mountain area) of the western Foothills terrane (Sierra Nevada, California). The swarm was mylonitized and transposed during emplacement, from 155 to 148 m.y. (U-Pb zircon data), at an estimated depth of 10 km. Steeply SE-plunging fold axes and S-fold geometries indicate a left-lateral sense of shear. The Late Jurassic Nevadan orogeny is a manifestation of dramatic changes in magnitude and direction of North American motion. The Cordilleran dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed during this period of change, at the J2 (~150 m.y.) apparent polar wander cusp, and during subsequent, rapid northwestward acceleration of North America. EXTENDED ABSTRACT (PART I): Phase relationships and morphologies and reaction kinetics have been determined for the dehydration-melting of a natural amphibolite (mode: hornblende 70%, plagioclase 30%) with no added water, at 10 kbar and 750 to 1000°C, and for durations of l to 21 days, using both finely-powdered and solid starting materials. The amphibolite composition is equivalent to a calcic, low-K, olivine tholeiite. Experimental conditions simulated the dehydration-melting of deep mafic continental crust and hot, subducted oceanic crust. Experiments were conducted in unbuffered Au capsules at oxygen fugacities probably just above the Ni-NiO buffer. Hornblende is involved in the following sliding reaction (the order of the product phases represents the order of appearance with increasing temperature): hornblende + anorthitic plagioclase -> clinopyroxene + liquid + aluminous hornblende + calcic hornblende + orthopyroxene + garnet. The liquid fraction ranges from <1% at 750°C to ~47% at 1000°C, with most of the increase occurring above 875°C. The liquids are generally tonalitic but have very high Al2O3 contents (18-21 wt.%). At high liquid fractions (~0.5), the liquids have a composition of high-alumina basalt. Fractionation of the plagioclase would be necessary to reduce both the CaO and Al2O3 contents of the liquids to calc-alkaline compositions. The liquid compositions become more sodic with increasing temperature, but the compositional trends reverse direction, and the liquid compositions become more calcic above 975°C, as garnet disappears. The water contents of the liquids range from over 7 wt.% at low liquid fractions to 2 wt.% at high liquid fractions. The high-temperature mineral assemblage that coexists with the liquid is clinopyroxene, orthopyroxene and plagioclase ± garnet ± aluminous hornblende. Thus, dehydration-melting of the amphibolite can reproduce natural granulite and garnet pyroxenite mineral assemblages. The removal of the water-rich tonalitic liquids from a substantially melted amphibolitic source would help generate a relatively dry granulite terrane. The stability of garnet plays a major role in determining the REE composition of the liquids. Garnet modes from these runs are consistent with REE patterns of Archean tonalites. Delamination of the garnet clinopyroxenite restite is possible due to the very high densities (up to 3.5 gm/cm3) of these assemblages after liquid segregation. Garnet phenocrysts show syn-growth compositional zoning. The total alumina in hornblende geobarometer appears to work for this mafic mineral assemblage. The solid amphibolite runs indicate that anisotropic crystal structures and rock texture control liquid morphology and distribution during dehydration-melting. The shapes of most liquid pockets are crystallographically-controlled, with many corners having angles greater than 60°. Few crystal/liquid triple-junctions develop the interfacial energy-controlled dihedral angles ([theta]), which form in experiments using finely-ground powders of minerals with poor cleavage. Liquid interconnectivity probably is attained at 875°C with only 2 vol.% liquid, indicating that dihedral angles less than 60° may not be necessary to achieve interconnectivity in partially melted metamorphic rocks. The surfaces between elongated grains in lineated rocks can become pathways for the migration of liquid or the diffusion of components. EXTENDED ABSTRACT (PART II): The geology, petrology and geochronology (U-Pb zircon) of a southern fragment of the western Foothills terrane has been studied (the Owens Mountain area of the western Sierra Nevada foothills, northeast of Fresno, California). A previously unrecognized dike swarm/shear zone is identified within the steeply dipping, Callovian to Kimmeridgian metavolcanic and metasedimentary strata. The dike swarm consists predominantly of cogenetic tonalitic and mafic dikes and tonalitic tabular bodies. Mutually cross-cutting relationships indicate that the tonalitic and mafic dikes also were coeval. Some of the tonalitic dikes range up to ~100 m in thickness, and individual dikes of both tonalitic and basaltic composition can be followed for up to 3 km. The dike swarm is sheeted in places, comprising almost 100% of some outcrops. Textures and fabrics within the dike swarm range from partially recrystallized igneous to strongly deformed S and L metamorphic tectonites, implying that dike emplacement occurred during ductile deformation. Hot subsolidus mylonitization has transposed layering parallel to foliation and has greatly thinned many of the dikes to centimeter to meter thicknesses. Layering and parallel foliation dip subvertically and strike NNW-SSE. Post-tectonic annealing has destroyed most microscopic shear indicators, but macroscopic intrafolial folds are common and have steeply SE-plunging fold axes and S-fold geometries that indicate a left-lateral sense of shear. The geochronological data on the tonalite dikes reveal that emplacement and crystallization of the coeval tonalitic and mafic magmas at Owens Mountain occurred over an 8 m.y. period, from 155 to 148 Ma, at an estimated depth of 10 km. Thus the beginning of intrusion occurred within 5 m.y. of deposition of the metavolcanic and metasedimentary strata into which the dikes were emplaced. A correlation between age and degree of deformation and recrystallization of the tonalites implies syntectonic dike emplacement. Undeformed granitic dikes that cut the strata are younger than 124 Ma. The regional tectonics of the Owens Mountain and other Cordilleran dike swarms can be related in a broad dynamic sense to the absolute motion of North America by using the apparent polar wander (APW) analysis of May and Butler (1986). The Late Jurassic Nevadan orogeny is the manifestation of the drastic changes in magnitude and direction of North American motion (from ~45 km/m.y. to the NNE to ~200 km/m.y. to the NW; May and Butler, 1986). The Late Jurassic dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed at the J2 (~150 Ma) APW cusp and during subsequent, rapid northwestward acceleration of North America.

Discovery and study of high-pressure basic granulites in Songshugou area of Shangnan, East Qinling

The Limpopo Belt of southern Africa is generally believed to represent the root of a late Archaean continental collision zone, and has been used to demonstrate the validity of the uniformitarian concept in tectonics Large scale tectonic models have been applied in spite of the fact that large portions of the belt yet await the most basic investigations. Here we report the first detailed field and petrographic study of the northernmost part of the Limpopo Belt, the Northern Marginal Zone sensu stricto (NMZ s.s) and conclude, on the basis of relative age relation, nature of PT evolution and deformation, that none of the current models can correctly explain the evolution of the study area. The evolution of NMZ s.s, is complex and includes four metamorphic stages, two major plutonic episodes and at least three deformation events. The oldest rocks, mafic granulites, record all four stages of metamorphic mineral growth. The first two stages predate deposition of sediments and the intrusion of voluminous enderbite and charnockite between 2.72 and 2.62 Ga. The bulk of our observations relate to the late Archaean (stage 3) granulite facies event. Abundant reaction textures are preserved in mafic granulite, metasediments, metamorphosed charnockite and enderbite and allow us to qualitatively reconstruct the PT evolution. Prograde heating occurred in the sillimanite stability held. During peak temperature conditions vapour-absent melting is observed in most felsic lithologies,Temperatures between 800 and 850 degrees C at pressures as low as 0.4-0.5 GPa are indicated by various mineral assemblages. The thermal peak was followed by an increase in pressure. Typical reactions of anti-clockwise PT evolution, like breakdown of cordierite + spinel to sapphirine and orthopyroxene + plagioclase to garnet + quartz, are frequently found. Maximum pressure is constraint to ca. 0.85 GPa by the complete absence of garnet in mafic granulites. Initial cooling was rapid, and is indicated by the back reaction of melt + orthopyroxene to biotite + quartz symplectites preserved in migmatites. This metamorphic event was accompanied by intrusion of porphyritic charnockite and granite, and by coeval compressional deformation. The observed evolution, especially the combination of an anti-clockwise PT loop and compressional tectonics, requires a strong, transient heat-source affecting the base of the crust. Neither the thermal evolution nor the relative timing is correctly predicted by existing collision models. The NMZ s.s. granulites were finally exhumed in a separate event along upper greenschist-facies thrusts, in response to a transpressive orogeny affecting the units further south at 2.0 Ga. In spite of potential ambiguities inherent to a qualitative approach, our observations show that petrography and field work, if used in conjunction with dating of a few key age relations and structural interpretation, are a pre-requisite to the erection of realistic tectonic models. The example of the NMZ s.s. may encourage geoscientists with limited access to analytical facilities to reassess the geological evolution of terrains which lack basic description.

Introduction: The Lapland Granulite Belt is placed on the Kandalaksha region (Kola Peninsula, Russia). The rocks of this Belt are composed mainly of amphibolites and granulites. Materials and methods: The research were focused on the garnets from the amphibolite and granulite rocks of Lapland Granulite Belt. The petrological methods like polarizing microscopy (PM), SEM-EDS, XRD for powdered samples and single crystal diffraction were used together with IR and Mossbauer spectroscopy and REE analysis by ion–microprobe. Results: It was found that the garnets from studied amphibolite and granulite rocks could be classified to pyralspite group without hydrogarnets components, so they were formed in high metamorphic facies. Conclusions: The joint geological observations and results of the performed experiments suggest that the garnets were subject of a blastesy, i.e. there were formed in long lasting metamorphic processes of low dynamics, except of those garnets from tectonic zones, found in the vicinity of mineral veins.

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.

O Gnaisse alcalino de Serra do Matola - São Jõao Del Rei

Discovery of pelitic high-pressure granulite from Manjinggou of the Huai’an Complex, North China Craton: Metamorphic P–T evolution and geological implications

The Sharyzhalgay block, located in the south-western Baikal region, is an exposure of the Precambrian basement at the margin of the Siberian Platform. It chiefly consists of granitoids (80%) that host relic granulite facies rocks (20%). Metamorphism under granulite facies conditions took place about 2.4-2.5 Ga ago. Petrological and geochemical data on the granulites suggest protoliths of predominantly MORB-like basalt (~70%) and minor sedimentary rocks. Granulites also include ultramafic rocks. Relics of protogranular texture, high XMg in rocks and rock-forming minerals, occurrence of depleted harzburgites, mantle compositional trend of spinels, REE patterns, and thermobarometry data allow us to interpret the provenance of Saramta peridotites as mantle material depleted by partial melting in the stability field of spinel. Peridotites are cut by dikes of layered wehrlite, spinel websterite and garnet websterite. Spinel websterites have a chemistry and mineralogy of Cr-Di series. Their origin inferred to be the result of mineral segregations from percolating melt within the upper mantle. Garnet websterites differ from Al-augite pyroxenites by lower contents of TiO2 and Al2O3, and higher SiO2. Incongruent melting of Cr-Di spinel websterites on the scheme Cpx+ Opx+ Sp ® Mg-richer Ol+ Cr-richer Sp+L produced melts and restite wehrlites. Later on the melt crystallized in situ into garnet websterites with magmatic textures. Regional granulite-facies metamorphism at temperatures of 700-800°C and a pressure of 5-6 kb produced exsolution lamellae of spinel in pyroxene, kelyphite Sp+Opx+Pl rims on garnet, and zoning of orthopyroxene in garnet websterite. Tectonitic restite harzburgites with ultramafic dikes are correlative with the mantle parts of ophiolite sections. All these facts allow us to interpret relic granulite-facies rocks as fragments of Precambrian ophiolites.

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.