The El Teniente porphyry Cu–Mo deposit from a hydrothermal rutile perspective

Mineralogical, textural, and chemical analyses (EPMA and PIXE) of hydrothermal rutile in the El Teniente porphyry Cu–Mo deposit help to better constrain ore formation processes. Rutile formed from igneous Ti-rich phases (sphene, biotite, Ti-magnetite, and ilmenite) by re-equilibration and/or breakdown under hydrothermal conditions at temperatures ranging between 400°C and 700°C. Most rutile nucleate and grow at the original textural position of its Ti-rich igneous parent mineral phase. The distribution of Mo content in rutile indicates that low-temperature (∼400–550°C), Mo-poor rutile (5.4 ± 1.1 ppm) is dominantly in the Mo-rich mafic wallrocks (high-grade ore), while high-temperature (∼550-700°C), Mo-rich rutile (186 ± 20 ppm) is found in the Mo-poor felsic porphyries (low-grade ore). Rutile from late dacite ring dikes is a notable exception to this distribution pattern. The Sb content in rutile from the high-temperature potassic core of the deposit to its low-temperature propylitic fringe remains relatively constant (35 ± 3 ppm). Temperature and Mo content of the hydrothermal fluids in addition to Mo/Ti ratio, modal abundance and stability of Ti-rich parental phases are key factors constraining Mo content and provenance in high-temperature (≥550°C) rutile. The initial Mo content of parent mineral phases is controlled by melt composition and oxygen fugacity as well as timing and efficiency of fluid–melt separation. Enhanced reduction of SO₂-rich fluids and sulfide deposition in the Fe-rich mafic wallrocks influences the low-temperature (≤550°C) rutile chemistry. The data are consistent with a model of fluid circulation of hot (>550°C), oxidized (ƒO₂ ≥ NNO + 1.3), SO₂-rich and Mo-bearing fluids, likely exsolved from deeper crystallizing parts of the porphyry system and fluxed through the upper dacite porphyries and related structures, with metal deposition dominantly in the Fe-rich mafic wallrocks.     

Rutile geochemistry and thermometry of eclogites and associated garnet-mica schists in the Biga Peninsula,NW Turkey

In northwest Turkey, high-pressure metamorphic rocks occur as exotic blocks within the Çetmi mélange located on the south of the Biga Peninsula. Rutile chemistry and rutile thermometry obtained from the eclogite and associated garnet-mica schist in the Çetmi mélange indicate significant trace element behaviour of subducted oceanic crust and source-rock lithology of detrital rutiles. Cr and Nb contents in detrital rutile from garnet-mica schist vary from 355 to 1026 μg/g and 323 and 3319 μg/g, respectively. According to the Cr-Nb discrimination diagram, the results show that 85% of the detrital rutiles derived from metapelitic and 15% from metamafic rocks. Temperatures calculated for detrital rutiles and rutiles in eclogite range from 540 °C to 624 °C with an average of 586 °C and 611 °C to 659 °C with an average of 630 °C at P = 2.3 GPa, respectively. The calculated formation temperatures suggest that detrital rutiles are derived from amphibolite- and eclogite-facies metamorphic rocks. Amphibolite-facies rocks of the Kazdağ Massif could be the primary source rocks for the rutiles in the garnet-mica schist from the Çetmi mélange. Nb/Ta ratios of metapelitic and metamafic rutiles fall between 7–24 and 11–25, respectively. Nb/Ta characteristics in detrital rutiles may reflect a change in source-rock lithology. However, Nb/Ta ratios of rutiles in eclogite vary from 9 to 22. The rutile grains from eclogites are dominated by subchondritic Nb/Ta ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.     

Pb diffusion in rutile

Diffusion of Pb was measured in natural and synthetic rutile under dry, 1 atmosphere conditions, using mixtures of Pb titanate or Pb sulfide and TiO₂ as the sources of diffusant. Pb depth profiles were then measured with Rutherford Backscattering Spectrometry (RBS). Over the temperature range 700–1100 °C, the following Arrhenius relation was obtained for the synthetic rutile: D=3.9 × 10⁻¹⁰exp(−250 ± 12 kJ mol⁻¹/RT) m²s⁻¹. Results for diffusion in natural and synthetic rutile were quite similar, despite significant differences in trace element compositions. Mean closure temperatures calculated from the diffusion parameters are around 600 °C for rutile grains of ∼100 μm size. This is about 100 °C higher than rutile closure temperature determinations from past field-based studies, suggesting that rutile is more resistant to Pb loss through volume diffusion than previously thought. Received: 28 June 1999 / Accepted: 29 December 1999     

Relics of eclogite facies metamorphism in the Austroalpine basement,Hochgrössen (Speik complex), Austria

Summary Retrograde eclogites and serpentinites from the Hochgr?ssen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-floor basalts with affinities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py_15–19) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5–8 mol% Jd) + albite ± amphibole. Amphiboles are classified as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 °C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8–2.2 GPa at 700 °C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6–0.8 GPa at 590–640 °C. ⁴⁰Ar/³⁹Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of 397.3 ± 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole (∼500 °C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date. Received June 11, 2000; revised version accepted January 2, 2001     

Petrology of eclogites from north of Shahrekord, Sanandaj-Sirjan Zone, Iran

Summary Metabasic rocks were recently found within a ductile shear zone in the north of Shahrekord, being a part of the structural zone of Sanandaj-Sirjan, SW Iran. The rocks give evidence of a so far unrecognized eclogite facies metamorphic event and testify to high pressure metamorphism in the Sanandaj-Sirjan Zone, near the Main Zagros Reverse Fault, which is the assumed suture zone between the Arabian plate and the Iranian block. The eclogites occur as lenses or blocks within ortho- and paragneisses. The petrographic features and reaction textures display at least two main metamorphic stages: (1) a peak eclogite facies stage, and (2) a subsequent amphibolite facies stage. The eclogite facies metamorphism is indicated by omphacite + garnet + sodic-calcic amphiboles (barroisite, magnesiokatophorite and magnesiotaramite) + phengite + rutile + (clino-)zoisite + quartz ± dolomite. The garnets are mainly almandine-rich, which fits with the C-type eclogite classification. Calcic amphiboles (hornblende, tschermakite and pargasite) + plagioclase are secondary phases formed during the retrograde amphibolite-facies metamorphism. P-T estimates for the eclogite facies give pressures of 21–24 kbar and temperatures of 590–630 °C (geothermometry) and 470–520 °C (THERMOCALC), respectively. Geothermobarometry for the amphibolite-facies metamorphism yields 10–11 kbar and 650–700 °C. Author’s address: Ali Reza Davoudian, Department of Natural Resources, Shahrekord University, Shahrekord, Iran     

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with D _REE ^Cpx/Grt decreasing with decreasing D _Ca . This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE. Received: 11 February 1999 / Accepted: 31 January 2000     

Simulated garnet-clinopyroxene geothermometry of eclogites

The Fe-Mg thermometer widely used to infer the “equilibration” temperature of garnet-clinopyroxene assemblages of eclogites records the progressive blocking of diffusion-limited exchanges between coexisting mineral phases. It is argued that equilibrium is achieved through the fast grain boundary model in which Fe and Mg circulate in an interstitial medium fast enough for the rate-limiting step to be the volume diffusion in each mineral phase. A semi-analytic solution is found and the influence of the cooling history, grain size distribution, and rock composition on the temperature at which the Fe-Mg exchange between garnet and clinopyroxene is frozen in are quantitatively evaluated. In particular, the model simulates the temperatures that would be obtained from the concentration of Fe and Mg in the rim of adjacent garnet and clinopyroxene crystals such as those commonly obtained by electron probe. For eclogites, the simulations show that correct peak temperatures are retrieved as long as the temperature of the metamorphic climax does not exceed 650 °C. At higher climax temperatures, rim-rim temperatures underestimate the peak temperature and cluster around 650 °C. Fast cooling, however, strongly limits diffusive equilibration. In crustal-type eclogites, cooling rates of a few degrees per million years at 700 °C and of a few tens of degrees at 800 °C preserve the record of the peak temperature. It is shown that these results are largely independent of the chemical composition of the garnet and clinopyroxene. On the contrary, the mineral grain size and the respective proportions of garnet and clinopyroxene have a major effect on retrogressive diffusion. For eclogites formed at a temperature of 800 °C and under conditions of slow cooling, the record of the climax temperature is preserved by crystal rims as long as the volume of clinopyroxene is smaller than that of garnet and crystal size is at least millimetric. The choice of a particular set of experimental data for the diffusion coefficients is found to be immaterial. We suggest that the temperature of the metamorphic climax of eclogites is best estimated from the composition of clinopyroxene rims combined with that of the inner edge of the diffusion boundary layer fringing the coexisting garnet grains. Received: 15 January 1998 / Accepted: 6 November 1998     

Zr-in-rutile thermometry of eclogites from the Karakaya Complex in NW Turkey: Implications for rutile growth during subduction zone metamorphism

Eclogites occur as a tectonic slice within a metabasite-phyllite-marble unit of the Karakaya Complex in northwest Turkey. The high-pressure mineral assemblage in eclogite is mainly composed of garnet + omphacite + glaucophane + epidote + quartz. Trace element characteristics of rutile and Zr-in-rutile temperatures were determined for eclogites from the Karakaya Complex. Core-rim analyses of rutile grains yield remarkable trace element zoning with lower contents of Zr, Nb and Ta in the core than in the rim. The variations in Zr, Nb and Ta can be ascribed to growth zoning rather than diffusion effects. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents, which could be ascribed to the effect of metamorphic dehydration in subduction zones on rutile Nb/Ta differentiation. The rutile grains from eclogites in the Karakaya Complex are dominated by subchondritic Nb/Ta and Zr/Hf ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.The Zr contents of all rutile grains range between 81 and 160 ppm with an average of 123 ppm. The Zr-in-rutile thermometry yields temperatures of 559–604 °C with an average temperature of 585 °C for eclogites from the Karakaya Complex. This average temperature suggests growth temperature of rutile before peak pressure during the subduction. However, some rutile grains have higher Zr contents in the outermost rims compared to the core. Zr-in-rutile temperatures of the rims are about 20 °C higher than those of the cores. This suggests that the outermost rims would have grown from a distinct fluid at higher temperatures than that of the cores. Moreover, Zr contents and calculated temperatures in both inclusion rutile and matrix rutile from eclogites are identical, which suggests that eclogites within the Karakaya Complex belong to the same tectonic slice and underwent similar metamorphic evolution.     

Unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite + kyanite in an eclogite from the Erzgebirge

In the Erzgebirge Crystalline Complex, eclogites occur in three different high pressure (HP) units (1, 2 and 3) recording contrasting pressure (P)–temperature (T) conditions. Eclogites from HP-unit 1 experienced peak metamorphic conditions in the coesite stability field at about 33 kbar/850 °C. Commonly, these eclogites from HP-unit 1 are all very similar, with an eclogitic peak assemblage of omphacite–garnet–coesite–K-feldspar, rarely accompanied by kyanite, and omphacites systematically deviating from a stoichiometric composition. In contrast, an eclogite recently found near Blumenau, is mineralogically and geochemically different from the typical eclogites of HP-unit 1. This unusual eclogite reveals the eclogitic equilibrium assemblage omphacite–garnet–coesite–phengite–phlogopite–kyanite, and yields metamorphic peak conditions of 870 °C and >29 kbar. There is clear textural evidence of the formation of phlogopite and kyanite under partial consumption of phengite and garnet. Moreover, the omphacite is stoichiometric and contains abundant exsolution lamellae, the thickest of which were identified as quartz by the electron microprobe. The finer lamellae were studied by transmission electron microscopy (TEM). Oligoclase was identified as an exsolution phase. Other lamellae proved to consist of K-white mica, also interpreted as exsolution. Prior to exsolution, the omphacite composition must have been cation-deficient, as that of the other, common HP-unit 1 eclogites. These non-stoichiometric compositions are ascribed to partial substitution by the Ca-Eskola pyroxene component, which calculates to an average of 8 mol% for omphacite in HP-unit 1 eclogites. According to experiments, this substitution becomes significant at P > 30 kbar. Exsolution of K-white mica may indicate hydroxyl defects in the original omphacite, also favoured by high pressure. Oligoclase and K-white mica exsolution from Ca-Eskola-rich clinopyroxene has not previously been reported. The omphacite has a disordered C2/c structure; and in just one case very small (a few tens of nanometres) antiphase domains, resulting from the C2/c to P2/n transformation, are present. These features may indicate a brief thermal history and rapid tectonic processes. Received: 4 January 1999 / Accepted: 20 April 2000     

Factors controlling copper solubility and chalcopyrite deposition in the Sungun porphyry copper deposit, Iran

The Sungun porphyry copper deposit is hosted in a Diorite/granodioritic to quartz-monzonitic stock that intruded Eocene volcanosedimentary and Cretaceous carbonate rocks. Copper mineralization is associated mainly with potassic alteration and to a lesser extent with sericitic alteration. Based on previously published fluid inclusion and isotopic data by Hezarkhani and Williams-Jones most of the copper is interpreted to have deposited during the waning stages of orthomagmatic hydrothermal activity at temperatures of 400 to 300 °C. These data also indicate that the hydrothermal system involved meteoric waters, and boiled extensively. In this work, thermodynamic data are used to delineate the stability fields of alteration and ore assemblages as a function of fS₂, fO₂ and pH. The solubility of chalcopyrite was evaluated in this range of conditions using recently published experimental data. During early potassic alteration (>450 °C), Copper solubility is calculated to have been >50 000 ppm, whereas the copper content of the initial fluid responsible for ore deposition is estimated, from fluid inclusion data, to have been 1200–3800 ppm. This indicates that initially the fluid was highly undersaturated with respect to chalcopyrite, which agrees with the observation that veins formed at T > 400 °C contain molybdenite but rarely chalcopyrite. Copper solubility drops rapidly with decreasing temperature, and at 400 °C is approximately 1000 ppm, within the range estimated from fluid inclusion data, whereas at 350 °C it is only 25 ppm. These calculations are consistent with observations that the bulk of the chalcopyrite deposited at Sungun is hosted by veins formed at temperatures of 360 ± 60 °C. Other factors that, in principle, may reduce chalcopyrite solubility are increases in pH, and decreases in fO₂ and aCl⁻. Our analysis shows, however, that most of the change in pH occurred at high temperature when chalcopyrite was grossly undersaturated in the fluid, and that the direction of change in fO₂ increased chalcopyrite solubility. We propose that the Sungun deposit formed mainly in response to the sharp temperature decrease that accompanied boiling, and partly as a result of the additional heat loss and decrease in aCl⁻, which occurred as a result of mixing of acidic Cu-bearing magmatic waters with cooler meteoric waters of lower salinity. Received: 8 July 1998 / Accepted: 8 April 1999     

超高压榴辉岩金红石中高场强元素变化的控制因素及其地球动力学意义

利用LA-ICP-MS对CCSD-MH超高压榴辉岩中金红石进行了详细的原位微区微量元素组成分析.金红石中高场强元素Nb和Ta含量主要受全岩Nb、Ta和TiO₂含量控制, Zr、Hf含量比较稳定基本不受全岩含量影响.粒间金红石中, 同一颗粒金红石核部Zr含量系统高于边部, 而边部则出现了明显的Pb和Sr富集特征.CCSD-MH榴辉岩中金红石与全岩的Nb/Ta比值呈现明显的不一致性.全岩Nb/Ta比值明显低于金红石且与全岩TiO₂含量负相关, 而金红石的Nb/Ta比值与全岩Nb、Ta含量和Nb/Ta比值没有明显的相关关系.金红石和全岩之间非完全耦合的Nb/Ta组成表明, 金红石并非形成于原岩的结晶过程中而是在超高压变质作用过程中形成, 尽管金红石是榴辉岩中Nb、Ta含量的主要载体矿物, 但金红石的Nb/Ta比值并不一定能完全代表全岩的特征, 而与全岩Nb、Ta和TiO₂的含量有关.粒间金红石核部Zr含量所记录的温度与粒径之间具有明显的正相关性, 反映金红石中的Zr在其形成后没有封闭.粒间金红石所表现出的明显的边部富集Pb和Sr的特征, 反映了后期流体活动对金红石组成的影响.这些研究结果为金红石中Zr在高温下的扩散作用和后期流体活动的影响提供了重要证据, 这可能是利用金红石Zr含量地质温度计计算的苏鲁-大别榴辉岩变质温度(598~827℃) 偏低的主要原因.      

Zr-in-rutile thermometry in blueschists from Sifnos, Greece

Zr-in-rutile thermometry on samples of blueschist from Sifnos, Greece, yields temperatures that reflect progressive crystallization of rutile from ca. 445 to 505°C with an analytical precision of + 18/−27 and ± 10°C using the electron microprobe and ± 1.5–3.5°C using the ion microprobe. Individual grains are generally homogeneous within analytical uncertainty. Different grains within a single sample record temperature differences as large 55°, although in most samples the range of temperatures is on the order of 25°. In several samples, Zr-in-rutile temperatures from grains within garnet are lower than temperatures from matrix grains, reflecting growth of rutile with increasing temperature of metamorphism. Although the specific rutile-producing reactions have not been identified, it is inferred that rutile grows from either continuous reaction involving the breakdown of lower grade phases (possibly ilmenite), or from pseudomorph reactions involving the breakdown of relic igneous precursors at blueschist-facies conditions. No systematic variation in rutile temperatures was observed across the blueschist belt of northern Sifnos, consistent with the belt having behaved as a coherent block during subduction.     

CCSD主孔榴辉岩中金红石微量元素特征：LA-ICPMS分析及其意义

对中国大陆科学钻探（CCSD）主孔200~1005m范围内8件榴辉岩样品的金红石进行了LA-ICPMS原位微区微量元素分析,结合前人已发表的全岩和金红石分析数据,研究结果发现：在不同类型榴辉岩中,金红石的微量元素与其全岩成分具有不同的相关关系。金红石中的Nb和Ta元素含量不同程度地受控于全岩Nb和Ta含量。在高钛和低镁钛榴辉岩中,金红石的Cr与全岩Cr/TiO₂正相关;在富镁榴辉岩中,金红石的Cr含量受全岩MgO含量的控制;在高钛和富镁榴辉岩中,全岩成分明显影响着金红石的Zr含量,金红石Zr温度计可能不适用。低镁钛榴辉岩的金红石的平衡温度普遍低于榴辉岩峰期变质温度,可能是变质流体参与下的扩散作用和退变质作用所致;多数情况下,单个样品中大部分金红石颗粒的Zr含量是均匀的,金红石Zr温度计所给出的温度可能代表着退变质再平衡的温度;CCSD榴辉岩的全岩Nb/Ta比值普遍低于其中金红石的Nb/Ta比值,不支持金红石榴辉岩可能是地球上超球粒陨石Nb/Ta比值储库的观点。     

Constraining the prograde and retrograde P-T-t path of Eocene HP rocks by SHRIMP dating of different zircon domains: inferred rates of heating, burial, cooling and exhumation for central Rhodope, northern Greece

Ion microprobe (SHRIMP) dating was carried out on different zircon domains from metamorphic rocks of the HP-HT terrane of central Rhodope, northern Greece, to constrain the timing of prograde and retrograde stages within a single tectono-metamorphic cycle. A well determined P-T-t(relative) path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations. Ion microprobe work was assisted by cathodoluminescence (CL) images of the zircon crystals. The geochronological results revealed that Hercynian continental crust was subducted during the Eocene. Several stages of the Eocene tectono-metamorphic cycle – including both the prograde and retrograde parts of the P-T path above ca 300 °C, 0.3 GPa – were dated using zircons from the following rock types: (1) A deformed quartz vein probably formed at ca 300 °C, 0.3 GPa. Zircons in this vein precipitated from a hydrothermal fluid; they yielded an age of 45.3 ± 0.9 Ma which corresponds to the time of a low-T prograde stage of metamorphism. (2) In kyanite eclogites, zircons were entirely reset during eclogite-facies metamorphism. Resetting was very probably enhanced by the presence of fluids derived by H₂O liberating reactions close to the P-peak. They yielded an age of 42.2 ± 0.9 Ma. (3) Orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores, which yielded Hercynian ages of 294 ± 8 Ma (age of granitic protolith formation), whereas CL-bright, metamorphic rims yielded, like the eclogite zircons, ages of 42.0 ± 1.1 Ma. Therefore, both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 Ma. (4) In a leucosome of a migmatized orthogneiss, oscillatory zoned zircons yielded an age of 40.0 ± 1.0 Ma. At this time the rocks reached maximum temperatures during early decompression. (5) A late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36.1 ± 1.2 Ma. Thus, the whole tectono-metamorphic cycle above ca 300 °C, 0.3 GPa lasted from 45.3 ± 0.9 Ma to 36.1 ± 1.2 Ma, that is 9.2 Ma with an extreme error value of 2.1 Ma. Based on combined SHRIMP and petrological data, the average rates of heating and burial during subduction (above ca 300 °C, 0.3 GPa) are >94 °C/Ma and >15 mm/a, respectively. Rates of cooling and exhumation (also above 300 °C, 0.3 GPa) are calculated to be >128° C/Ma and >7.7 mm/a. The Eocene age of metamorphism in central Rhodope implies that the terrane of, at least, central Rhodope and the Cyclades very probably was part of the same continental crust. Received: 5 October 1998 / Accepted: 18 January 1999     

Tracing the high pressure stage in the polymetamorphic Texel Complex (Austroalpine basement unit, Eastern Alps): P–T–t–d constraints

Summary Integration of new mineral chemical, geochronological and structural data from the Texel Complex yielded information on (re)crystallization and deformation processes in metapelites, eclogites and tonalitic orthogneisses during eclogite facies metamorphism. Maximum P–T conditions reached 1.2 to 1.4 GPa and 540–620 °C in the Upper Cretaceous. In tonalitic orthogneisses and metapelites, substantial garnet growth took place prior to eclogite facies metamorphism and Sm–Nd data indicate the presence of pre-Cretaceous mineral relics. In contrast, complex garnet-growth and -resorption processes are inferred for eclogites, which produced characteristic atoll microstructures and occurred close to the pressure peak of a single, coherent high pressure event. Garnet Sm–Nd data indicate eclogite facies crystallization at 85 ± 5 Ma. While eclogites retained information on the maximum burial stage, matrix phases in metapelites and orthogneisses were intensely recrystallized during the amphibolite facies metamorphic decompression. All the meso- and macro-scale deformation structures formed during the high pressure event and subsequent exhumation. The major mylonitic foliation is represented by the high pressure phases but was refolded during amphibolite facies exhumation. A biotite-whole-rock Rb–Sr age of 70–80 Ma indicates that cooling below about 300 °C occurred in the Upper Cretaceous. Supplementary material to this paper is available in electronic form at Appendix available as electronic supplementary material     

Supercooled diopside melt: confirmation of temperature-dependent expansivity using container-based dilatometry

The expansivity of supercooled diopside liquid has been determined using techniques of container-based dilatometry. Two thermal strategies have been employed, one in which the sample is brought to volumetric equilibrium by long-duration dwells at low temperatures (817 °C) and one in which scanning dilatometry of the sample has been performed at somewhat higher temperatures (890–913 °C). The results of both experiments yield a supercooled liquid expansivity for diopside liquid in the temperature range of 817–913 °C of 84.4 ± 2.8 × 10⁻⁴ cm³/mol K. The expansivity is 65% higher than that obtained for diopside melt obtained at superliquidus temperatures using the double bob Archimedean method. Combined fitting of the new low temperature, volume–temperature data from the present study and the superliquidus data from the literature has been performed. The combined fit yields the following equations for the volume–temperature relationship of diopside liquid (T=temperature in °C):

Cation ordering of orthopyroxenes from the Skaergaard Intrusion: implications for the subsolidus cooling rates and permeabilities

 We have determined the quenched cation ordering states of five orthopyroxene crystals collected from the marginal border group and the lower zone a and b of the Skaergaard intrusion, and modeled these data to retrieve their closure temperatures (T _c) of Fe–Mg ordering and cooling rates. According to existing thermal models for the Skaergaard pluton, conductive cooling dominated the high and low temperature regimes, which were separated by an intermediate temperature regime in which the cooling was controlled primarily by convective fluid circulation. The cooling rates retrieved from the quenched cation ordering states of the orthopyroxene crystals strictly apply to temperatures around the closure temperatures of the ordering states, ∼340–400^° C, which fall at the transition from convective to the lower temperature conductive cooling. The cooling rates obtained from the cation ordering states of orthopyroxene vary from ∼1 to 270 K/ka. These results agree well with a thermal model calculated using an assumed average permeability of 10^-12 cm² for the pluton, but not completely with a model calculated on the basis of an average permeability of 10^-13 cm², although both values produced shifts of δ¹⁸O that are comparable to those observed in the pluton. Received: 27 February 1995/Accepted: 20 July 1995     

Hydrothermal synthesis of pumpellyite–okhotskite series minerals

Summary ?Hydrothermal experiments to synthesize pumpellyite group minerals of the pumpellyite–okhotskite series and to investigate their stability have been carried out at 200, 300 and 400 MPa P _fluid and 250–500 °C by using cold-seal pressure vessels and solid buffers of MnO₂–Mn₂O₃, Cu₂O–CuO and Cu₂O–Cu buffer assemblages. Okhotskite and pumpellyite rich in the okhotskite component crystallized from an oxide mixture starting material of Ca₄MgMn³⁺ ₃Al₂Si₆O_24.5-oxide+excess H₂O at P _fluid of 200, 300 and 400 MPa and temperatures of 300 and 400 °C. However, a single phase of okhotskite was not produced, and associated piemontite, hausmannite, wollastonite, clinopyroxene, corundum, braunite–neltnerite solid solution and alleghanyite also formed. Mn-pumpellyite of the okhotskite–pumpellyite join occurs as aggregates of needle crystals, rounded grains or flaky crystals. Chemical compositions are variable and range from pumpellyite-(Mn²⁺) to okhotskite: 31–36 SiO₂, 13–21 Al₂O₃, 12–25 total Mn₂O₃, 0.6–4 MgO and 20–24 wt.% CaO. Reconnaissance experiments using a starting material of synthetic Ca₂Mn³⁺Al₂Si₃O₁₂(OH)-piemontite at 300 MPa and temperatures of 250, 300, 400 and 500 °C indicate that Mn-rich pumpellyite can crystallize from piemontite at lower temperatures than the stability field of piemontite. The Mn-rich pumpellyite was accompanied by garnet, wollastonite and alleghanyite. The chemical compositions of the Mn-pumpellyites are 32–36 SiO₂, 18–27 Al₂O₃, 8–18 total Mn₂O₃ and 20–23 wt.% CaO. This study shows that the stability fields of piemontite, piemontite+Mn-pumpellyite, and Mn-pumpellyite range in this order with decreasing temperature under high fO₂ conditions. The maximum stability temperature of Mn-rich pumpellyite lies between 400 and 500 °C at 200–400 MPa in high fO₂ conditions. Received March 3, 2000; revised version accepted December 28, 2001     

Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany

Metamorphic textures in medium-grade (~500–550°C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (~480°C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from ~500 to 630°C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (~530°C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure.     

Time calibration of a P–T path from a Variscan high-temperature low-pressure metamorphic complex (Bayerische Wald, Germany), and the detection of inherited monazite

A temperature–time path was constructed for high-temperature low-pressure (HT–LP) migmatites of the Bayerische Wald, internal zone of the Variscan belt, Germany. The migmatites are characterised by prograde biotite dehydration melting, peak metamorphic conditions of approximately 850 °C and 0.5–0.7 GPa and retrograde melt crystallisation at 800 °C. The time-calibration of the pressure–temperature path is based on U–Pb dating of single zircon and monazite grains and titanite separates, on ⁴⁰Ar/³⁹Ar ages obtained by incremental heating experiments on hornblende separates, single grains of biotite and K-feldspar, and on ⁴⁰Ar/³⁹Ar spot fusion ages of biotite determined in situ from sample sections. Additionally, crude estimates of the duration of peak metamorphism were derived from garnet zoning patterns, suggesting that peak temperatures of 850 °C cannot have prevailed much longer than 2.5 Ma. The temperature–time paths obtained for two areas approximately 30 km apart do not differ from each other considerably. U–Pb zircon ages reflect crystallisation from melt at 850–800 °C at 323 Ma (southeastern area) and 326 Ma (northwestern area). The U–Pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance. The preservation of inherited monazite and the presence of excess ²⁰⁶Pb resulting from the incorporation of excess ²³⁰Th in monazite formed during HT–LP metamorphism suggest that monazite ages in the migmatites of the Bayerische Wald reflect crystallisation from melt at 850–800 °C and persistence of older grains at these temperatures during a comparatively short thermal peak. The U–Pb ages of titanite (321 Ma) and ⁴⁰Ar/³⁹Ar ages of hornblende (322–316 Ma) and biotite (313–309 Ma) reflect cooling through the respective closure temperatures of approximately 700, 570–500 and 345–310 °C published in the literature. Most of the feldspars' ages (305–296 Ma) probably record cooling below 150–300 °C, while two grains most likely have higher closure temperatures. The temperature–time paths are characterised by a short thermal peak, by moderate average cooling rates and by a decrease in cooling rates from 100 °C/my at temperatures between 850–800 and 700 °C to 11–16 °C/my at temperatures down to 345–310 °C. Further cooling to feldspar closure for Ar was probably even slower. The lack of decompressional features, the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating, rapid uplift and extension due to lithospheric delamination as proposed elsewhere. Instead, the high peak temperatures at comparatively shallow crustal levels along with the short thermal peak require external advective heating by hot mafic or ultramafic material. Received: 7 July 1999 / Accepted: 28 October 1999