Generation of granites after amphibolites

This paper presents the results of a comprehensive experimental study of the formation of granitoid melts at the expense of olivine-normative amphibolites. It was shown that trondhjemite-tonalite and granite-granodiorite melts can be generated by incongruent melting reactions at pressures of 5–25 kbar at T = 800–1000°C. The compositions of coexisting phases and phase reactions were investigated in detail. It was found that interaction between these hydrous melts and the overlying peridotite material results in the metasomatic alteration of peridotites and formation of andesite melts. The granitization of amphibolite was explored. Infiltration granitization was experimentally reproduced for the first time at T = 750°C and P _f = 5 kbar. Fluid percolation through amphibolite produced a column of feldspathized and debasified rocks and granite melt completely replacing amphibolite in the proximal zone. Another extreme type of granitization occurring in amphibolite at the contact with granite melt was investigated at T = 800–950°C and P _f = 7 kbar. The diffusion of silica and alkalis resulted in the metasomatic alteration of amphibolite and formation of granitic droplets and lenses with the development of migmatite-like zones, which significantly differ in composition and structure from the zones of infiltration granitization. All the models addressed in this paper (derivation of granitoid series, interaction of granitoid melts with peridotites, and infiltration and diffusion granitization) provide insight into the mechanism of formation of many natural objects.     

Experimental Study of REEs,Ba, and Sr Interphase Partitioning in Fluid–Magmatic Silicate Systems at T = 1250°C and P = 2 kbar

The results of study of the alkaline silicate melt–hydrous saline (carbonate, sulfate, and fluoride) fluid system, as well as partitioning of ore metals (Ba, Sr, and REEs) between coexisting phases at T = 1250°C and P = 2 kbar are reported. It is shown that aqueous solutions with the compositions studied cannot be effective concentrators and transporters of ore elements such as REEs, Sr, and Ba. The sulfate melt accumulates Sr and Ba in the alkaline silicate melt–hydrous sulfate fluid system, which provides evidence for the efficiency of sulfate ore extraction of Sr and Ba from the melt. The results obtained support the important oregenerating role of dense saline phases formed upon the development of liquid heterogeneity in fluid–magmatic systems.

     

Petrology and P–T evolution of garnet peridotites from central Sulawesi, Indonesia

Alpine‐type orogenic garnet‐bearing peridotites, associated with quartzo‐feldspathic gneisses of a 140–115 Ma high‐pressure/ultra‐high‐pressure metamorphic (HP‐UHPM) terrane, occur in two regions of the Indonesian island of Sulawesi. Both exposures are located within NW–SE‐trending strike–slip fault zones. Garnet lherzolite occurs as <10 m wide fault slices juxtaposed against Miocene granite in the left‐lateral Palu‐Koro (P‐K) fault valley, and as 10–30 m wide, fault‐bounded outcrops juxtaposed against gabbros and peridotites of the East Sulawesi ophiolite within the right‐lateral Ampana fault in the Bongka river (BR) valley. Six evolutionary stages of recrystallization can be recognized in the peridotites from both localities. Stage I, the precursor spinel lherzolite assemblage, is characterized by Ol+Cpx+Opx±Prg‐Amp ± Spl±Rt±Phl, as inclusions within garnet cores. Stage II, the main garnet lherzolite assemblage, consists of coarse‐grained Ol+Opx+Cpx+Grt; whereas finer‐grained, neoblastic Ol+Opx+Grt+Cpx±Spl±Prg‐Amp±Phl constitutes stage III. Stages IV and V are manifest as kelyphites of fibrous Opx+Cpx+Spl in inner coronas, and Opx+Spl+Prg‐Amp±Ep in outer coronas around garnet, respectively. The final (greenschist facies) retrogressive stage VI is accompanied by recrystallization of Serp+Chl±Mag±Tr±Ni sulphides±Tlc±Cal. P–T conditions of the hydrated precursor spinel lherzolite stage I were probably about 750 °C at 15–20 kbar. P–T determinations of the peak stage IIc (from core compositions) display considerable variation for samples derived from different outcrops, with clustering at 26–38 kbar, 1025–1210 °C (P‐K & BR); 19–21 kbar, 1070–1090 °C (P‐K), and 40–48 kbar, 1205–1290 °C (BR). Stage IIr (derived from rim compositions) generally records decompression of around 4–12 kbar accompanied by cooling of 50–240 °C from the IIc peak stage. Stage III, which post‐dates a phase of ductile deformation, yielded 22±2 kbar at 750±25 °C (P‐K) and 16±2 kbar at 730±40 °C (BR). The granulite–amphibolite–greenschist decompression sequence reflects uplift to upper crustal levels from conditions of 647–862 °C at P=15 kbar (stage IV), through 580–635 °C at P=10–12 kbar (stage V) to 350–400 °C at P=4–7 kbar (stage VI), respectively, and is identical to the sequence recorded in associated granulite, gneiss and eclogite. Sulawesi garnet peridotites are interpreted to represent minor components of the extensive HP‐UHP (peak P >28 kbar, peak T of c. 760 °C) metamorphic basement terrane, which was recrystallized and uplifted in a N‐dipping continental collision zone at the southern Sundaland margin in the mid‐Cretaceous. The low‐T , low‐P and metasomatized spinel lherzolite precursor to the garnet lherzolite probably represents mantle wedge rocks that were dragged down parallel to the slab–wedge interface in a subduction/collision zone by induced corner flow. Ductile tectonic incorporation into the underthrust continental crust from various depths along the interface probably occurred during the exhumation stage, and the garnet peridotites were subsequently uplifted within the HP‐UHPM nappe, suffering a similar decompression history to that experienced by the regional schists and gneisses. Final exhumation from upper crustal levels was clearly facilitated by entrainment in Neogene granitic plutons, and/or Oligocene trans‐tension in deep‐seated strike–slip fault zones.     

Petrology and thermobarometry of eclogite rocks in the Krasnaya Guba dike field,belomorian mobile belt

A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all, pressure decreased, the SiO₂ activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes, which were restricted to zones of intense deformations, at variable SiO₂ activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels.     

Fluid transfer of gold,palladium, and rare earth elements and genesis of ore occurrences in the Subpolar Urals

To estimate the behavior of Au, Pd, REE, and Y in magmatic and postmagmatic processes, a series of experimental studies on the solubility of noble metals and REE in magma, magmatic fluid, and hydrothermal solutions has been performed in wide temperature and pressure ranges (300–400°C, 860–1350°C; 1–14 kbar). The coefficients of Au and Pd partitioning (D ^F/L) between fluid and tholeiitic melt have been determined. Depending on P, T, and the composition of the system, they vary from 1 to 11 for Au and 0.02 to 1 for Pd. The phase solubility technique was used to determine Au and Pd solubility in hydrothermal fluid. The effects of temperature, composition, and fluid acidity on Au and Pd solubility have been estimated. The high solubility of these metals in aqueous chloride solutions has been established for both Au (28–803 mg/kg at T = 300°C, 305–1123 mg/kg at T = 350°C, and 330–1400 mg/kg at T = 400°C) and Pd (40–126 mg/kg at T = 300°C, 62–152 mg/kg at T = 350°C, and 20–210 mg/kg at T = 400°C). The coefficients of REE and Y partitioning (D ^F/L) between fluid and tholeiitic or alkaline melts have been determined. They vary from 0.00n to 2 depending on P, T, and fluid composition. The experimental data on Au and Pd solubility in solutions and magmatic fluids and the wide variation of REE D ^F/L between fluid and melt show that magmatic and hydrothermal fluids are efficient agents of Au, Pd, and REE transfer and fractionation. The obtained experimental data were used for elucidating sources of fluids and their role in the genesis of Au-Pd-REE occurrences in the Subpolar Urals.     

Modeling of geochemical processes that occurred during the formation of the Schlema deposit, Erzgebirge. I. Synmineralization processes in the Aue granites

The geochemical events that occurred within the outlines of the Schlema ore field during the formation of vein bodies cannot be understood without considering the processes that occurred synchronously in the footwall zone, in the Aue granite cupola, from where mineral-forming thermal solutions ascended into the deposit. It is known that the upper part of the cupola experienced hydrothermal alterations, which had a minor influence on the appearance of the rocks, but caused a number of important geochemical consequences: an increase in the degree of iron and sulfur oxidation in grantie, the replacement of accessory sulfides by oxygenbearing salts, etc. These changes were analyzed using computer models for a decrease in T and P of the granitewater system from 400°C (3.0–1.5 kbar) to 140°C (2.0–0.2 kbar). The role of these changes in the fate of disseminated ore elements of granite and in the extraction of ore elements from the changing rock into the equilibrium pore solution was evaluated.     

Anatectic record and contrasting P–T paths of aluminous gneisses from the central Grenville Province

Anatectic aluminous gneisses, some derived from sedimentary rocks of broadly pelitic composition and others from hydrothermally altered felsic volcanic rocks, are exposed in the mid‐P and high‐P segments of the hinterland in the central Grenville Province. These gneisses consist dominantly of garnet, biotite, K‐feldspar, plagioclase and quartz, with sillimanite or kyanite, and display microstructural evidence of anatexis by fluid‐absent reactions consuming muscovite and/or biotite. Melt‐related microstructures, such as inter‐granular films and/or interstitial quartz or feldspar enclosing relict phases, are most abundant in the metasedimentary samples. Despite anatexis at granulite facies conditions, the hydrothermally altered rocks preserve earlier features attributed to the circulation of hydrothermal fluids, such as sillimanite seams, dismembered quartz veins and garnet‐rich aluminous nodules in a K‐feldspar‐dominated matrix. Microstructural and mineral chemical data, integrated with P–T pseudosections calculated with thermocalc for the metasedimentary rocks, permit qualitative constraints on the P–T paths. Data from a high‐P kyanite‐bearing sample are consistent with a steep prograde P–T path up to ~14.5 kbar and 860–900 °C, followed by decompression with minor cooling to the solidus at ~11 kbar and 870 °C. This pressure‐dominated P–T path is similar to those inferred in other parts of the high‐P segment in the central Grenville Province. In contrast, the P–T path predicted from a mid‐P sillimanite‐bearing paragneiss has a strong temperature gradient with P–T of ~9.5 kbar and 850 °C at the thermal peak, and a retrograde portion down to ~8 kbar and 820 °C. In a broad sense, these two contrasting P–T patterns are consistent with predictions of thermo‐mechanical modelling of large hot orogens in which P–T paths with strong pressure gradients exhume deeper rocks in the orogenic flanks, whereas P–T paths with strong temperature gradients in the orogenic core reflect protracted lateral transport of ductile crust beneath a plateau.     

On the formation conditions of porphyry Cu-Mo polymetallic deposits in the Xiaosigou district

The host mass consists of quartz diorite-porphyry (marginal facies), granodiorite porphry (intermediate facies) and granite porphry (central facies). The petrographical facies are asymmetrically spread. The formation temperature of the rock mass ranges from 890 to 800°C, the pressure from 330 to 380 bars and the depth from about 1.5 to 2 km. Mineralization is controlled by the N-E contact zone and adjacent fault structures. Mo-mineralization occurs in the granite porphyry with strong potash metasomatism. Cu-Mo-mineralization is distributed in the inner contact zone of the rock mass, and the wallrock is K-silicified granite porphyry. Cu-pyrite-mineralization is recognized in skarn and serpentinite. Pb-Zn ore veins occur in marbleized limestones. Rock and mineral analyses, fluid inclusion studies and highT-P experiments indicate that extensive precipitation of Cu-bearing pyrite took place atT = 290–250°C, andP = 330–380 bars and that of copper atT = 400–310°C andP = 330–380 bars. Precipitation of Pb and Zn was followed by the transformation of hydrothermal solutions from alkaline to intermediate with decreasing temperature.     

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone,Guatemala: insights from phase equilibria and Raman spectroscopy

Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P–T path that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐T of ～520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ～10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P–T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐T subduction zone.     

High-pressure eclogites in northern Jiangsu – southern Shandong province, eastern China

Abstract Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino-Korean and Yangtze cratons in central and eastern China. These eclogites usually have high-P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high-P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO₂ phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO₂ phase, and are divided into prograde eclogites and mantle-derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle-derived eclogites have high MgO/(FeO + Fe₂O₃) and Cr₂O₃ compositions in bulk rock and minerals, and consist mainly of pyrope-rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular-rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile. Pargasitic and taramitic amphiboles, calcic plagioclase (An₆₈), epidote, zoisite, K-feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near-isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high-P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near-isothermal decompression during the retrograde stage. The conditions at which symplectites re-equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle-derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.     

The petrogenesis of basic and ultrabasic alkaline rocks of western Makhtesh Ramon,Israel: geochemistry,melt and fluid inclusion study

The basic and ultrabasic alkaline rocks of western Makhtesh Ramon, Israel crop out in numerous lava flows and subvolcanic bodies. The rock suite is composed of tephrite, basanite, basanitic nephelinite, analcimite, olivine nephelinite, and melilite-olivine nephelinite and in many outcrops is represented by glass-bearing varieties. Melt and fluid inclusions have been studied in olivine, clinopyroxene, and plagioclase phenocrysts. The EP, SIMS and microthermometry methods were used for inclusion study. The geochemical data obtained on glasses of melt inclusions (major, REE, trace elements, volatiles) are compared with the data on whole-rock and groundmass glass compositions. The compositions of melt inclusions reflect the different stages of rock crystallization: the initial products of crystallization are similar to whole-rock compositions whereas final portions of melts are usually enriched in SiO₂, Al₂O₃, and alkalis, and depleted in mafic components. The data on contemporaneous melt and CO₂ inclusions were used for the evaluation of the P–T conditions of rock generation. The following parameters were obtained: tephrite: P = 6.3–7.7 kbar and T = 1,150–1,250°C; basanite: P = 6.6–9.2 kbar and T = 1,150–1,250°C; olivine and analcime-olivine nephelinite: P = 5.6–8.2 kbar and T = 1,150–1,250°C; melilite-olivine nephelinite: 4.0–5.4 kbar and T mainly between 1,150 and 1,200°C. Magma genesis was restricted to P–T conditions of spinel- and plagioclase-lherzolite fields. These data suggest the shallowest depth of magma genesis occurred in Makhtesh Ramon compared to other occurrences of Early Cretaceous magmatism at the Middle East. Differences in the degree of batch partial melting of the same source rocks best explain the diversity of the igneous suite in western Makhtesh Ramon.     

The first evidence of UHP metamorphism in the Polar Urals (Russia)

A new ultrahigh-pressure (UHP) metamorphic terrane, the Marun-Keu eclogite–peridotite–gneiss complex was established in the Polar Urals (Russia). Owing to the P–T parameters of formation of garnet peridotites obtained for the first time, it was established that they experienced UHP metamorphism with a peak in the diamond stability field (39 kbar, 830oC) due to subduction of rocks of the Marun-Keu complex to the mantle (a depth of more than 100 km). Relics of plagioclase peridotites preserved in central parts of large peridotite bodies and petrochemical data are evidence that Marun-Key garnet peridotites cannot be of mantle origin. A basic-ultrabasic intrusion, which formed in the upper crust, can be considered as their protolith. Later, rocks of the massif plunged to the mantle along the subduction zone and metamorphosed under high-P to ultrahigh-P conditions.

     

Multistage metamorphism of the Huangtuling granulite, Northern Dabie Orogen, eastern China: implications for the tectonometamorphic evolution of subducted lower continental crust

Granulites from Huangtuling in the North Dabie metamorphic core complex in eastern China preserve rare mineralogical and mineral chemical evidence for multistage metamorphism related to Palaeoproterozoic metamorphic processes, Triassic continental subduction‐collision and Cretaceous collapse of the Dabie Orogen. Six stages of metamorphism are resolved, based on detailed mineralogical and petrological studies: (I) amphibolite facies (6.3–7.0 kbar, 520–550 °C); (II) high‐pressure/high‐temperature granulite facies (12–15.5 kbar, 920–980 °C); (III) cooling and decompression (4.8–6.0 kbar, 630–700 °C); (IV) medium‐pressure granulite facies (7.7–9.0 kbar, 690–790 °C); (V) low‐pressure/high‐temperature granulite facies (4.0–4.7 kbar, 860–920 °C); (VI) retrograde greenschist facies overprint (1–2 kbar, 340–370 °C). The P–T history derived in this study and existing geochronological data indicate that the Huangtuling granulite records two cycles of orogenic crustal thickening events. The earlier three stages of metamorphism define a clockwise P–T path, implying crustal thickening and thinning events, possibly related to the assembly and breakup of the Columbia Supercontinent at c. 2000 Ma. Stage IV metamorphism indicates another crustal thickening event, which is attributed to Triassic subduction/collision between the Yangtze and Sino‐Korean Cratons. The dry lower crustal granulite persisted metastably during the Triassic subduction/collision because of the lack of hydrous fluid and deformation. Stage V metamorphism records the Cretaceous collapse of the Dabie Orogen, possibly due to asthenosphere upwelling or removal of the lithospheric mantle resulting in heating of the granulite and partial melting of the North Dabie metamorphic core complex. Comparison of the Huangtuling granulite in North Dabie and the high‐pressure–ultrahigh‐pressure metamorphic rocks in South Dabie indicates that the subducted upper (South Dabie) and lower (North Dabie) continental crusts underwent contrasting tectonometamorphic evolution during continental subduction‐collision and orogenic collapse.     

Mineralogy of metamorphosed manganese deposits of the South Urals

A mineralogical investigation of metamorphosed manganese rocks was carried out at ore deposits related to the Devonian volcanic complexes of the Magnitogorsk paleovolcanic belt of the South Urals. The mineralogical appearance of these rocks is determined by three consecutively formed groups of mineral assemblages: (1) assemblages occupying the main volume of orebodies and formed during low-grade regional metamorphism (T = 200−250°C, P = 2–3 kbar); (2) assemblages of segregated and metasomatic veinlets that fill the systems of late tectonic fractures; and (3) assemblages of near-surface supergene minerals. Sixty-one minerals have been identified in orebodies and crosscutting hydrothermal veinlets. The major minerals are quartz, hematite, hausmannite, braunite, tephroite, andradite, epidote, rhodonite, caryopilite, calcite, and rhodochrosite. The mineral assemblages of metamorphosed manganese rocks (metamanganolites) are characterized. Chemical compositions of braunite, epidote-group minerals, piemontite, pyroxenes, rhodonite, pyroxmangite, and winchite are considered. The bibliography on geology and mineralogy of the South Ural manganese deposits is given.     

P–T evolution of the Teslin suture zone and Cassiar tectonites, Yukon, Canada: evidence for A- and B-type subduction

Mineral composition and quantitative thermobarometric studies indicate that the Teslin-Taylor Mountain and Nisutlin terranes within the Teslin suture zone (TSZ), Yukon, record widespread high-P/T metamorphic conditions consistent with subduction zone dynamothermal metamorphism. The highest P–T conditions (575–750° C and 9–17 kbar) are preserved in tectonites formed during normal dip-slip ductile shear. Dextral strike-slip tectonites record lower P–T conditions (400–550° C and 5–8 kbar), and tectonites which show reverse shear have peak temperatures of c. 420° C and a minimum peak pressure of 3 kbar. Dynamothermal metamorphism took place in a west-dipping B-type subduction zone outboard of western North America in Permo-Triassic time. TSZ tectonites were underplated against the hangingwall plate of the subduction zone. Following subduction of the ocean basin which separated North America from the hangingwall plate, TSZ tectonites were overthrust eastward as a coherent structural package as a result of A-type subduction of Cassiar strata in early Jurassic time. (Par)autochthonous Cassiar tectonites, which comprised the leading edge of the western North American margin, record prograde moderate-P, high-T metamorphism (550–750° C and 7–13 kbar) synchronous with top-to-the-east ductile shear. Metamorphism occurred as a result of subduction of the North American margin into the TSZ subduction zone in early Jurassic time. Following metamorphism Cassiar tectonites cooled slowly from 500 to 300° C during the period middle Jurassic to middle Cretaceous. TSZ and Cassiar tectonites were deformed during changing P–T conditions. Data from each of these tectonite packages indicate that grain-scale strain partitioning may have allowed local recrystallization of individual minerals by the addition of mechanical energy. The composition of the new grains reflects the P–T conditions under which that particular grain was deformed.     

Physicochemical computer model of the formation of the granitoid-associated uranium mineralization

The extraction and transfer of uranium was modeled in the “granite-water” system at 800–100°C and 700–3000 bar. It was shown that the scale of ore accumulation depends on T,P parameters of source and amount of hydrothermal fluid that infiltrated through ore precipitation zone. The modeling results give further arguments that granitoid-associated uranium mineralization was derived from high temperature source.     

Prograde–retrograde P–T–t–deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps)

A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet-bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path. Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D₁–D₂, which produced sheath folds as well as the main foliation S₂. Spessartine-rich first-generation garnets, situated in microlithons enclosed by S₂, record the onset of shearing under increasing high-pressure–low-temperature conditions (7 kbar/380°C). Geothermobarometry on second-generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting P_max of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high-pressure–high-temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions. Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D₃ and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D₄ (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension. Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line.     

Alpine metamorphism in the south-east Tauern Window, Austria: 1. P–T variations in space and time

Abstract The Pennine rocks exposed in the south-east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid-Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are: (a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar; (b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar; (c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar; (d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basement-cover interface in the mid-Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro-Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro-Alpine’nappe) was itself buried beneath an ‘Upper Austro-Alpine’nappe or nappe-pile which was 7.4 km thick at that time. The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south-east Tauern experienced blueschist-facies conditions (T<450°C, P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist-facies imprint post-dated the main phase of tectonic thickening (D¹_A) and was thus a direct consequence of continental collision. Combined oxygen-isotope and fluid-inclusion studies on late-stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C. Early Alpine, late Alpine and vein-formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south-east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early- to mid-Tertiary (D²_A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary. The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one-dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro-Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth.     

Reconstruction of the metamorphic <Emphasis Type="Italic">P–T</Emphasis> path from the garnet zoning in aluminous schists from the Tsogt Block,Mongolian Altai

The paper presents original authors’ data on aluminous schists in the Tsogt tectonic plate in the Southern Altai Metamorphic Belt. The nappe includes a medium-temperature/medium-pressure zonal metamorphic complex, whose metamorphic grade varies from the greenschist to epidote-amphibolite facies. The garnet and garnet–staurolite schists contain three garnet generations of different composition and morphology. The P–T metamorphic parameters estimated by mineralogical geothermometers and geobarometers and by numerical modeling with the PERPLEX 668 software provide evidence of two successive metamorphic episodes: high-gradient (of the andalusite–sillimanite type, geothermal gradient approximately 40–50°/km) and low-gradient (kyanite–sillimanite type, geothermal gradient approximately 27°/km). The P-T parameters of the older episode are T = 545–575°C and P = 3.1–3.7 kbar. Metamorphism during the younger episode was zonal, and its peak parameters were T = 560–565°C, P = 6.4–7.2 kbar for the garnet zone and T = 585–615°C, P = 7.1–7.8 kbar for the staurolite zone. The metamorphism evolved according to a clockwise P–T path: the pressure increased during the first episode at a practically constant temperature, and then during the second episode, the temperature increased at a nearly constant pressure. Such trends are typical of metamorphism related to collisional tectonic settings and may be explained by crustal thickening due to overthrusting. The regional crustal thickening reached at least 15–18 km.     

High‐T and low‐P metamorphism in the Xilingol Complex of central Inner Mongolia,China: An indicator of extension in a previous orogeny

The Xilingol Complex comprises biotite gneisses and amphibolite interlayers with extensive migmatization. Four representative samples were documented and found to record either two or three metamorphic stages. Phase modelling using thermocalc suggests that the observed assemblages represent the final stages that underwent cooling from temperature peaks, and are consistent with a fluid‐absent solidus in P–T pseudosections. Their P–T conditions are further constrained to be 5–6 kbar/680–725°C and 4–5 kbar/650–680°C for two garnet‐bearing gneiss samples, 4–5 kbar/660–730°C for a cordierite‐bearing gneiss sample, and 4–5 kbar/680–710°C for an amphibolite sample based on mineral composition isopleths, involving measured Mg content in biotite, anorthite in plagioclase, grossular and pyrope in garnet and Ti content in amphibole. The peak temperature conditions recovered are 760–790°C or >760°C at 5–6 kbar based on the composition isopleths of plagioclase, biotite, garnet and especially the comparison of melt contents between the calculated and observed. A pre‐peak heating process with slight decompression can be suggested for some samples on the basis of the core–rim increase in the plagioclase anorthite, and the stability of ilmenite. Zircon U–Pb dating using the LA‐ICP‐MS method provides systemic constraints on the metamorphic ages of the Xilingol Complex to be 348–305 Ma, interpreted to represent the post‐peak cooling stages. Moreover, metagabbroic dykes that intruded into the Xilingol Complex yield 317 ± 3 Ma from magmatic zircon, and are considered to have played a significant role for heat advection triggering the high‐T and low‐P metamorphism. Thus, the clockwise P–T paths involving pre‐peak heating, peak and post‐peak cooling recovered for the Xilingol Complex are consistent with an extensional setting in the Carboniferous that developed on a previous orogen in response to addition of mantle‐derived materials probably together with upwelling of the asthenospheric mantle.