Transition from eclogite to amphibolite-facies metamorphism in the Austroalpine Ulten Zone

Summary The Ulten Zone of the Austroalpine crystalline basement south-west of Meran (Italy) contains metapelitic schists and granoblastic paragneisses, leucocratic orthogneisses, migmatites (in both gneiss-lithologies), metabasites and ultramafic lenses. Metamorphic textures of the metapelitic schists and granoblastic paragneisses indicate two different metamorphic events, characterized by two mineral assemblages, which differ in mineral chemistry: (1) an eclogite facies mineral assemblage (M1) comprising Grt-Ky I-Bt. Ms-Kfs-PI-Qtz-Rt, and (2) an amphibolite facies mineral assemblage (M2) comprising Grt-KyII-Bt-Ms-PI-Qtz-Ilm±St. For the M1 event, pressures of at least 15kbar and temperatures of about 700°±50°C can be estimated. The later amphibolite facies overprint occurred at pressures of 6 to 8kbar and about 600°±50°C. The M1 and M2 assemblages belong to a continuous clockwise metamorphic evolution during the Variscan orogeny. Evidence for Alpine metamorphism can only be detected by sericite rims around kyanite and reset biotite ages. The migmatites, which contribute about 15–30vol.% of all rocks in the investigated area, were formed on the prograde path during the M1 event. Dissolution of H₂O in the melted part of the migmatites resulted in a CO₂dominated fluid, which was trapped in primary kyanite (M1) fluid inclusions. Secondary H₂O-rich fluid inclusions are found in quartz grains and may represent the fluid which enabled a pervasive equilibration during M2.

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Übergang von eklogit-zu amphibolitfazieller Matamorphose in der austroalpinen Ultenzone

Zusammenfassung Die Ulten Zone, ein Teil des ostalpinen kristallinen basements, südwestlich von Meran, wird aus Metapeliten and granoblastischen Paragneisen, leukokraten Orthogneisen, Migmatiten (in beiden Lithologien), Metabasiten and ultramafischen Linsen aufgebaut. Metamorphe Texturen der Metapelite und granoblastischen Paragneise lassen auf zwei verschiedene metamorphe Ereignisse schließen, die durch unterschiedliche Mineral-chemismen und Paragenesen charakterisiert sind: (1) eine eklogitfazielle Paragenese (M1), bestehend aus Grt-KyI-Bt-Ms-Kfs-P1-Qtz-Rt und (2) eine amphibolitfazielle Paragenese (M2), bestehend aus Grt-KyII-Bt-Ms-P1-Qtz-Ilm±St. Für M1 konnten Minimaldrucke von 15kbar und Temperaturen von 700°±50°C abgeleitet werden. Die spätere amphibolitfazielle Überprägung fand bei 6 bis 8kbar und 600°±50°C statt. M1 und M2 gehören einer kontinuierlichen Metamorphoseentwicklung während der variszischen Orogenese an.Die Migmatite, ungefähr 15–30vol.% der Gesteine im untersuchten Gebiet, wurden am prograden Pfad während des M1 Ereignisses gebildet. Aufgrund der höheren Löslichkeit von H₂0 in der Schmelze, blieb ein CO₂, reiches Fluid zurück, das im primären Kyanit (M1) eingeschlossen wurde. Wässrige Flüssigkeitseinschlüsse können in Quarzkörnern gefunden werden. Dieses Fluid ist wahrscheinlich für die Reequilibrierung zu amphibolitfaziellen Bedingungen verantwortlich.

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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     

Pumpellyite-actinolite and greenschist facies metamorphism in lesvos island (Greece)

Summary The southern part of Lesvos island consists of a Late Palaeozoic-Triassic marble-phyllite series with intercalations of mafic metavolcanics which exhibit mineralogies of the pumpellyite-actinolite and greenschist facies. Pumpellyite is developed best in the metabasalts of the south-eastern part and towards the northwestern part of the terrain it gives way to actinolite and/or epidote bearing assemblages. Local variations in CO₂ are suggested to explain the extensive distribution of chlorite-calcite instead of the Ca–Al-silicate bearing assemblages diagnostic of subgreenschist facies metamorphism.The mineral assemblages observed and the comparison of the compositions of co-existing phases with the compositions of minerals from other low-grade metamorphic areas, indicate metamorphism at temperatures in the order of 270–360°C and pressures little lower than 5 kb.

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Pumpellyit-Aktinolith und Grünschiefer-Fazies-Metamorphose auf der Insel Lesbos (Griechenland)

Zusammenfassung Der südliche Teil der Insel Lesbos besteht aus einer Serie von jungpaläozoischen-triassischen Schichten von Marmor und Phylliten mit Einlagerungen von basischen Metavulkaniten, deren Mineralogie beweist, daß sie zu den Pumpellyit-Aktinolith- und Grünschiefern-Fazien gehören.Der Pumpellyit entwickelt sich besser in den Metabasalten des südöstlichen Teils. Gegen Nordwesten zu wird er durch Paragenesen ersetzt, welche Aktinolith oder Aktinolith mit Epidot oder nur Epidot aufweisen. Örtliche Unterschiede im CO₂-Wert erklären, wie angenommen wird, die verbreitete Verteilung der Chlorit-Calzit-Paragenese anstelle der für Sub-Grünschiefer-Fazies charakteristischen Ca–Al-Silikat-Paragenesen.Die Mineralparagenesen, die beobachtet wurden, und der Vergleich der Zusammensetzung der koexistierenden Phasen mit der Zusammensetzung der Mineralien aus anderen Gebieten mit niedrigem Metamorphose-Grad zeigt, daß die Metamorphose hier bei Temperaturen von 270°C bis 360°C und bei einem Druck von etwas weniger als 5 kb stattgefunden hat.

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Complex chemical zoning in eclogite facies garnet reaction rims: the role of grain boundary diffusion

In metapelites of the Saualpe complex (Eastern Alps) continuous 10 µm to 20 µm wide garnet reaction rims formed along biotite-plagioclase and biotite-perthite interfaces. The pre-existing mineral assemblages are remnants of low pressure high temperature metamorphism of Permian age. The garnet reaction rims grew during the Cretaceous eclogite facies overprint. Reaction rim growth involved transfer of Fe and Mg components from the garnet-biotite interface to the garnet-feldspar interface and transfer of the Ca component in the opposite direction. The garnets show complex, asymmetrical chemical zoning, which reflects the relative contributions of short circuit diffusion along grain boundaries within the polycrystalline garnet reaction rims and volume diffusion through the grain interiors on bulk mass transfer. It is demonstrated by numerical modelling that the spacing of the grain boundaries, i.e. the grain size of the garnet in the reaction rim is a first order control on its internal chemical zoning.     

Phase relations of pyroxene and amphibole in greenstone,blueschist and eclogite of the Franciscan Complex,California

The phase relations of pyroxenes, amphiboles and associated minerals in metamorphic rocks of the Franciscan Complex can be graphically depicted on a ternary diagram which has at its apices the metamorphic clinopyroxene end members, viz NaAl-NaFe³⁺-Ca(Fe²⁺, Mg). Phases are plotted by projection from a constant subassemblage of minerals. This analysis allows interpretation of the effects of pressure, temperature, bulk rock composition and fluid composition on stability of minerals within the Franciscan.Pyroxenes in meta-igneous rocks and metagraywackes have a limited compositional range and fall into two groups: the omphacites, with 50±5% diopside +hedenbergite component; and the jadeitic pyroxenes with 10±5% diopside+hedenbergite. Pyroxenes intermediate between these two groups are unstable relative to assemblages containing Na-amphibole+other minerals.Coexisting pyroxenes and amphiboles in eclogites and associated coarse blueschists comprise equilibrium assemblages, and the proportion of pyroxene to amphibole is a function of rock composition. Eclogites are stable at higher temperature than regionally developed fine-grained greenstones and blueschists in the Franciscan, and at higher pressure than amphibolites. X _H2O ^fluid is not an important factor in the stability of Franciscan eclogite relative to amphibolite.     

四川丹巴地区中低压变质作用及P-T轨迹

四川丹巴地区地处华北板块、扬子板块和羌塘板块的汇合处,这里集中了中压型的巴罗变质带和低压型的巴肯变质带,该区中低压变质作用的研究对于探讨松潘一甘孜造山带的形成过程与地壳演化过程具有重要的理论意义和大陆动力学意义。本文通过地质温压力计估算了研究区的峰期变质温度和压力,从微区的角度分析了各变质带的P—T轨迹和地热梯度,结果表明,丹巴地区巴罗型变质带和巴肯型变质带都具有顺时针的P-T轨迹,但是巴肯型变质带的地热梯度高于巴罗带：,结合研究区的变质带的分布,变质矿物共生组合及变质反应,本文认为丹巴地区的确并存巴罗带和巴肯带,两种地热梯度反映了构造条件上的差异。     

Eoalpine tectonics of the Eastern Alps: implications from the evolution of monometamorphic Austroalpine units (Schneeberg and Radenthein Complex)

Monometamorphic metasediments of Paleozoic or Mesozoic age constituting Schneeberg and Radenthein Complex experienced coherent deformation and metamorphism during Late Cretaceous times. Both complexes are part of the Eoalpine high-pressure wedge that formed an intracontinental suture and occur between the polymetamorphosed Ötztal–Bundschuh nappe system on top and the Texel–Millstatt Complex below. During Eoalpine orogeny Schneeberg and Radenthein Complexes were south-dipping and they experienced a common tectonometamorphic history from ca. 115 Ma onwards until unroofing of the Tauern Window in Miocene times. This evolution is subdivided into four distinct tectonometamorphic phases. Deformation stage D1 is characterized by WNW-directed shearing at high temperature conditions (550–600°C) and related to the initial exhumation of the high-pressure wedge. D2 and D3 are largely coaxial and evolved during high- to medium-temperature conditions (ca. 450 to ≥550°C). These stages are related to advanced exhumation and associated with large-scale folding of the high-pressure wedge including the Ötztal-Bundschuh nappe system above and the Texel–Millstatt Complex below. For the area west of the Tauern Window, F2/F3 fold interference results in the formation of large-scale sheath-folds in the frontal part of the nappe stack (formerly called “Schlingentektonik” by previous authors). Earlier thrusts were reactivated during Late Cretaceous normal faulting at the base of the Ötztal–Bundschuh nappe system and its cover. Deformation stage D4 is of Oligo-Miocene age and accounted for tilting of individual basement blocks along large-scale strike-slip shear zones. This tilting phase resulted from indentation of the Southern Alps accompanied by the formation of the Tauern Window.     

P-T paths and tectonic evolution of shear zones separating high-grade terrains from cratons: examples from Kola Peninsula (Russia) and Limpopo Region (South Africa)

Summary ?The petrology and P-T evolution of mica schists from two regional scale tectonic (shear) zones that separate high grade terrains (“mobile belts”) from cratons are described. These are the 2.4–1.9 Ga Tanaelv Belt, a suture zone that separates the Lapland granulite complex from the Karelian craton (Kola Peninsula–Fennoscandia), and the 2.69 Ga Hout River Shear Zone that separates the > 2.9 Ga Kaapvaal craton from the 2.69 Ga South Marginal Zone of the Limpopo high-grade terrain (South Africa). Two metamorphic zones are identified in strongly deformed mica schists from the 1.9 Ga Korva Tundra Group of the Tanaelv belt: (1) a chlorite-staurolite zone tectonically overlaying gneisses of the Karelian craton, and (2) a kyanite-biotite zone tectonically underlying garnet amphibolites of the Tanaelv Belt, which are in tectonic contact with the Lapland granulite complex. The prograde reaction Chl+St+Ms ↠ Ky+Bt+Qtz+H₂O clearly defines a boundary between zones (1) and (2). Rotated garnet porphyroblasts from zone (1) contain numerous inclusions (Otz, Chl, Ms), and show clear Mg/Fe chemical zoning, suggesting garnet growth during prograde metamorphism. The metamorphic peak, T = 650°C and P = 7.5 kbar, is recorded in the kyanite-biotite zone and characterized by unzoned snowball garnet. In many samples of mica schists euhedral garnet porphyroblasts of the retrograde stage are completely devoid of mineral inclusions while N _Mg decreases from core to rim, indicating a decrease in P-T from 650°C, 7.5 kbar to 530°C, 5 kbar. The Hout River Shear Zone (South Africa) shows metamorphic zonation from greenschists through epidote amphibolites to garnet amphibolites. Rare strongly deformed mica schists (Chl+Grt+Pl+Ms+Bt+Qtz) occur as thin layers among epidote-amphibolites only. Garnet porphyroblasts in the schists are similar to that of the Tanaelv Belt recording a prograde P-T path with peak conditions of T = 600°C and P∼ 5.5 kbar. The retrograde stage is documented by the continuous reaction Prp+2Ms+Phl ↠ 6Qtz+3East recording a minimum T = 520°C and P ∼ 3.3 kbar. Similar narrow clock-wise P-T loops recorded in mica schists from both studied shear zones suggest similarities in the geodynamic history of both shear zones under consideration.

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Zusammenfassung ?P-T Pfade und tektonische Entwicklung von Scherzonen, die hochgradige Terranes von Kratonen trennen: Zwei Beispiele von der Halbinsel Kola (Russland) und der Limpopo-Region (Südafrika) Die Petrologie und P-T Entwicklung von Glimmerschiefern aus zwei regionalen tektonischen Scherzonen, die hochgradige Terranes (“mobile belts”) von Kratonen trennen, werden beschrieben. Diese sind der 2.4−1.9 Ga Tanaev Belt, eine Suturzone, die die Lappland Granulite vom karelischen Pluton (Halbinsel Kola - Fennoskandien) trennt, sowie die 2.69 Ga Hout River Shear Zone, die den > 2.9 Ga Kaapvaal Kraton von der 2.69 Ga South Marginal Zone des hochgradigen Limpopo Terranes (Südafrika) trennt. Zwei metamorphe Zonen sind in stark deformierten Glimmerschiefern der 1.9 Ga Korva Tundra Group zu unterscheiden: (1) eine Chlorit-Staurolith-Zone, die den Gneisen des karelischen Kratons auflagert, und (2) eine Kyanit-Biotit-Zone, die die Granatamphibolite des Tanaev Belt unterlagert und in tektonischem Kontakt mit dem Lappland Granulitkomplex steht. Die prograde Reaktion Chl+St+Ms ↠ Ky+Bt+Qtz+H₂O trennt die beiden Zonen. Rotierte Granatporphyroblasten aus der Zone (1) enthalten zahlreiche Einschlüsse (Qtz, Chl, Ms) und zeigen eine Mg/Fe Zonierung, die Granatwachstum w?hrend des prograden Metamorphosestadiums nahelegen. Der Metamorphoseh?hepunkt (650°C, 7.5 kbar) wurde in der Kyanit-Biotit-Zone erreicht und ist durch nicht zonierte Schneeballgranate charakterisiert. In vielen Glimmerschieferproben sind die euhedralen Granatporphyroblasten des retrograden Stadiums vollkommen einschlu?frei und N _Mg nimmt vom Kern zum Rand hin ab. Das zeigt eine Abnahme der P-T Bedingungen von 650°C, 7.5 kbar auf 530°C, 5 kbar an. Die Hout River Shear Zone in Südafrika zeigt eine metamorphe Zonierung von Grünschiefern, über Epidotamphibolite zu Granatamphiboliten. Selten kommen stark deformierte Glimmerschiefer (Chl+Grt+Pl+Ms+Bt+Qtz) als dünne Lagen zwischen den Epidotamphiboliten vor. Die Granatporphyroblasten sind ?hnlich wie die aus dem Tanaev Belt und belegen eine prograde P-T Entwicklung mit Peak-Bedingungen von 600°C und ≈ 5.5 kbar. Das retrograde Stadium ist durch die kontinuierliche Reaktion Prp+2Ms+Phl ↠ 6Qtz+3East mit minimal 530°C und ≈ 3.3 kbar dokumentiert. Die sehr ?hnlichen P-T Pfade der Glimmerschiefer belegen ?hnlichkeiten in der geodynamischen Geschichte der beiden bearbeiteten Scherzonen.

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Received January 29, 1999;/revised version accepted August 10, 1999     

P-T grids for silica-undersaturated granulites in the systems MAS (n+ 4) and FMAS (n+ 3)-tools for the derivation of P-T paths of metamorphism

Abstract Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al₂O₃-SiO₂ (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low P _H2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7-10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe-Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high-temperature assemblages and by Al-rich cores in orthopyroxene. Application of the FMAS grid to some well-documented granulite occurrences introduces important constraints on their P-T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P-T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and _XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low-temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.     

Influence of ferric iron on phase equilibria in greenschist facies assemblages: the hematite‐rich metasedimentary rocks from the Monti Pisani (Northern Apennines)

We have investigated the effects of different Fe₂O₃ bulk contents on the calculated phase equilibria of low‐T/intermediate‐P metasedimentary rocks. Thermodynamic modelling within the MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (MnNKFMASHTO) chemical system of chloritoid‐bearing hematite‐rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe₂O₃ is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P–XFe₂O₃ diagrams, obtaining equilibration conditions of 475 °C and 9–10 kbar related to a post‐compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock‐forming silicates that often yield important thermobaric information. In Fe₂O₃‐rich compositions, garnet‐ and carpholite‐in curves shift towards higher temperatures with respect to the Fe₂O₃‐free systems. The presence of a ferric‐iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P–T–XFe₂O₃ phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.     

Metapelites of the Kanskiy Granulite Complex (Eastern Siberia): Kinked P-T Paths and Geodynamic Model

The Southern Yenisey Range (Eastern Siberia) consists of thegranulite-facies Kanskiy complex bordered by the lower-gradeYeniseyskiy and Yukseevskiy complexes. Samples of metapeliteof the Kanskiy complex typically show characteristic garnet-formingreaction textures and near-isobaric cooling P–T paths.An important new result of this study concerns the differencein shape of the P–T paths from different parts of theKanskiy granulite complex: metapelites collected 8 km from theboundary with the Yeniseyskiy complex followed a linear pathwith dP/dT 0·006 kbar/°C; metapelites collected3 km from this boundary reveal a kinked P–T path withan interval of burial cooling (dP/dT –0·006 kbar/°C).The difference in the shape of the P–T paths is supportedby the chemical zoning of garnet studied in the second groupof samples. A mechanism of buoyant exhumation of granulite issuggested by comparison with the results of numerical modelling,which indicate that such a diversity of P–T paths mayresult from a transient disturbance of the thermal structureby rapid differential movement of material from different crustallevels. To arrive at a correct tectonic interpretation, thewhole assemblage of interrelated P–T paths of metamorphicrocks collected from different localities within the same complexmust be studied. KEY WORDS: crustal diapirism; exhumation; granulites; numerical modelling; P–T path     

Amphibolites from Panama: anticlockwise P-T paths from a Pre-upper Cretaceous metamorphic basement in Isthmian Central America

Rocks from the metamorphic basement of the Azuero and Sona peninsulas, Panama, consist of schistose amphibolites and minor amounts of metasediment. In the Sona peninsula, strongly zoned amphiboles indicate that the amphibolites followed a progressive anticlockwsie P-T path prograde from low T /low P to medium T /high P , and are retrograded into the greenschist facies. In contrast, the amphibolites of the Azuero peninsula are affected by a low to medium T /low P metamorphism.
The metamorphic events of the Sona amphibolites occurred prior to the intra-Senonian tectonic phase which affects the Mesozoic formations along the Pacific coast of Costa Rica and Panama. The regional significance of such a basement in Isthmian Central America is discussed.     

Anticlockwise P-T Path of Granulites from the Monte Castelo Gabbro (Ordenes Complex, NW Spain)

The study of mafic and aluminous granulites from the Monte CasteloGabbro (Órdenes Complex, NW Spain) reveals an anticlockwiseP–T path that we interpret as related to the tectonothermalactivity in a magmatic arc, probably an island arc. The P–Tpath was obtained after a detailed study of the textural relationshipsand mineral assemblage succession in the aluminous granulites,and comparing these with an appropriate petrogenetic grid. Additionalthermobarometry was also performed. The granulites are highlyheterogeneous, with distinct compositional domains that mayalternate even at thin-section scale. Garnets are generallyidiomorphic to subidiomorphic, and in certain domains of thealuminous granulites they show overgrowths forming xenomorphiccoronas around a more or less idiomorphic core. Both types ofgarnets show significant Ca enrichment at the crystal rims,which, together with the other mineralogical and textural characteristics,is compatible with a pressure increase with low T variation.P–T estimations indicate a peak of T > 800°C andP     

Coupling forward modelling of garnet growth with monazite geochronology: an application to the Rappold Complex (Austroalpine crystalline basement)

Results from forward modelling of garnet growth and U–Th–Pb chemical dating suggest three periods of metamorphism that affected metapelitic rocks of the Rappold Complex (Eastern European Alps). Garnet first grew during Barrovian-type metamorphism, possibly during the Carboniferous Variscan orogeny. The second period of metamorphism produced monazite and resulted in minor garnet growth in some samples. Variable garnet growth was controlled by changes to the effective bulk rock composition resulting from resorption of older garnet porphyroblasts. Monazite crystals have variable morphology, textures and composition, but all yield Permian ages (267 ± 12 to 274 ± 17 Ma). In samples in which there was Permian garnet growth, monazite forms isolated and randomly distributed grains. In other samples, monazite formed pseudomorphous clusters after allanite. This difference is attributed to higher transport rates of monazite-forming elements in samples which underwent dehydration reactions during renewed garnet growth. The third and final period of garnet growth took place during Eo-Alpine (Cretaceous) metamorphism. Garnet of this age displays a wart-like texture. This may reflect transport-limited growth, possibly as a result of repeated dehydration during polyphase metamorphism.     

Thermobarometry and P–T–t paths: the granulite to eclogite transition in lower crustal xenoliths from eastern Australia

‘Lower crustal’ suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites. Pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c. 12 kbar for exposed granulite terranes. Palaeogeotherms constructed from xenoliths for the lower crust beneath the Phanerozoic fold belts of eastern Australia (SEA) and beneath the eastern margin of the Australian craton (EMAC) indicate two distinct thermal regimes. The two geotherms have similar form, with the EMAC curve displaced c. 150°C to lower temperatures. Reaction microstructures show the partial re-equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures. Variations in mineral compositions and zoning are used to constrain further the history of several EMAC xenoliths to near-isobaric trajectories. Detailed graphical models are constructed to predict compositional changes for isobaric P–T paths (at 7, 14 & 21 kbar) to transform an SEA-type geotherm to a cratonic geotherm. The models show that for the assemblage grt + cpx ± ky + plag + qtz, the changes associated with falling temperature in X_gr, X_jd (increase) and X_an (decrease) will be greater at higher pressures. These results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths. The zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c. 12–22 kbar suggests isobaric cooling of a large crustal thickness (30–35 km). An isobaric cooling path is consistent with magma accretion models for the transition of a crust–mantle boundary from an SEA-type geotherm to a cratonic geotherm. The coexistence of granulite and eclogite over the depth range 35–75 km beneath the EMAC indicates that the granulite to eclogite transition in the lower crust is controlled by P–T conditions, bulk chemistry and kinetic factors. At shallower crustal levels, typified by exposed granulite terranes, isobaric cooling may not result in the transition to eclogite.     

Pressure-temperature paths from P-T pseudosections and zoned garnets: potential, limitations and examples from the Zanskar Himalaya, NW India

The extraction of P-T histories from metamorphic rocks provides a valuable dataset for the elucidation of the tectonic mechanisms for orogeny. While continued re-equilibration frequently obliterates early information, garnet zonation and inclusion assemblages can often surmount this problem. The task is more difficult in high variance assemblages or if inclusions are not preserved, but one approach is to use pseudosections that are specific to the bulk composition of a given rock. In the latter case, the compositions and abundances of all the minerals are fixed at a given P-T point such that, if the effective bulk composition is known, the garnet composition alone can be used to reconstruct the history. Here, we explore this approach using examples from the Zanskar Himalaya, NW India. Pseudosections have been calculated for four pelitic to semipelitic rocks from the Zanskar Himalaya and have been contoured for garnet composition. The calculations adequately model the mineral assemblages in the rocks and predict the presence of chlorite in the early assemblage where chlorite is found as inclusions within garnet. Moreover, the pseudosections successfully model the garnet core compositions, with all three independent compositional contours overlapping at a single pressure and temperature. This occurs at ∼550 °C and at pressures varying from 3–7 kbar for the four rocks studied. We have been less successful, however, at modelling garnet compositions beyond the cores because fractionation of the effective bulk composition is caused by garnet growth itself. However, in this case, a combination of the␣pseudosection and conventional thermobarometry using␣Fe-Ti inclusions and matrix phases allows us to reconstruct␣the entire P-T history. The resulting P-T paths record burial of 3–5 kbar without significant temperature increase followed by isobaric heating of 50–100 °C. This evolution is consistent with Himalayan collision in the early Tertiary but a combination of the P-T data presented here and published geochronological data suggests renewed thrusting south of the suture zone in the Oligocene. In addition, the data demonstrate that no extra heat source is required to cause melting of the Himalayan crust in the Miocene. While melting could have occurred both by dehydration during decompression or in the presence of a fluid, the lack of garnet resorption does suggest decompression was rapid and followed quickly by cooling. This scenario favours melting by decompression. Received: 17 July 1997 / Accepted: 6 April 1998     

南大别角闪岩相退变质与热液活动:朱家冲榴辉岩与闪石脉 Ar-Ar年代学研究

南大别山朱家冲榴辉岩中产有一条岩脉。电子探针分析结果表明,朱家冲榴辉岩中较大颗粒的白云母和岩脉中的白云母均为钠云母;而榴辉岩中细小颗粒的白云母则为多硅白云母;岩脉中的蓝绿色柱状矿物为闪石(amphibole)。采用激光阶段加热技术和真空阶段击碎技术,对朱家冲冷榴辉岩体和岩脉进行了Ar-Ar定年研究。选自榴辉岩和闪石脉的两个钠云母激光加热Ar-Ar年龄均约为200Ma,选自闪石脉的闪石真空击碎流体包裹体Ar-Ar年龄也约为200Ma,代表了热液活动钠云母化发生的时间;而提取流体包裹体后的闪石矿物粉末阶段加热Ar-Ar等时线年龄约为243Ma,则可能代表了朱家冲榴辉岩角闪岩相退变质作用发生的最小年龄。     

Rift-related inheritance in orogens: a case study from the Austroalpine nappes in Central Alps (SE-Switzerland and N-Italy)

The Austroalpine nappe systems in SE-Switzerland and N-Italy preserve remnants of the Adriatic rifted margin. Based on new maps and cross-sections, we suggest that the complex structure of the Campo, Grosina/Languard, and Bernina nappes is inherited largely from Jurassic rifting. We propose a classification of the Austroalpine domain into Upper, Middle and Lower Austroalpine nappes that is new because it is based primarily on the rift-related Jurassic structure and paleogeography of these nappes. Based on the Alpine structures and pre-Alpine, rift-related geometry of the Lower (Bernina) and Middle (Campo, Grosina/Languard) Austroalpine nappes, we restore these nappes to their original positions along the former margin, as a means of understanding the formation and emplacement of the nappes during initial reactivation of the Alpine Tethyan margin. The Campo and Grosina/Languard nappes can be interpreted as remnants of a former necking zone that comprised pre-rift upper and middle crust. These nappes were juxtaposed with the Mesozoic cover of the Bernina nappe during Jurassic rifting. We find evidence for low-angle detachment faults and extensional allochthons in the Bernina nappe similar to those previously described in the Err nappe and explain their role during subsequent reactivation. Our observations reveal a strong control of rift-related structures during the subsequent Alpine reactivation on all scales of the former distal margin. Two zones of intense deformation, referred to as the Albula-Zebru and Lunghin-Mortirolo movement zones, have been reactivated during Alpine deformation and cannot be described as simple monophase faults or shear zones. We propose a tectonic model for the Austroalpine nappe systems that link inherited, rift-related structures with present-day Alpine structures. In conclusion, we believe that apart from the direct regional implications, the results of this paper are of general interest in understanding the control of rift structures during reactivation of distal-rifted margins.     

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim ⁴⁰Ar/³⁹Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The ⁴⁰Ar/³⁹Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between ⁴⁰Ar/³⁹Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.