Kinetic control of staurolite–Al2SiO5 mineral assemblages: Implications for Barrovian and Buchan metamorphism

The distribution and textural features of staurolite–Al₂SiO₅ mineral assemblages do not agree with predictions of current equilibrium phase diagrams. In contrast to abundant examples of Barrovian staurolite–kyanite–sillimanite sequences and Buchan‐type staurolite–andalusite–sillimanite sequences, there are few examples of staurolite–sillimanite sequences with neither kyanite nor andalusite anywhere in the sequence, despite the wide (~2.5 kbar) pressure interval in which they are predicted. Textural features of staurolite–kyanite or staurolite–andalusite mineral assemblages commonly imply no reaction relationship between the two minerals, at odds with the predicted first development (in a prograde sense) of kyanite or andalusite at the expense of staurolite in current phase diagrams. In a number of prograde sequences, the incoming of staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is coincident or nearly so, rather than kyanite or andalusite developing upgrade of a significant staurolite zone as predicted. The width of zones of coexisting staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is much wider than predicted in equilibrium phase diagrams, and staurolite commonly persists upgrade until its demise in the sillimanite zone. We argue that disequilibrium processes provide the best explanation for these mismatches. We suggest that kyanite (or andalusite) may develop independently and approximately contemporaneously with staurolite by metastable chlorite‐consuming reactions that occur at lower P–T conditions than the thermodynamically predicted staurolite‐to‐kyanite/andalusite reaction, a process that involves only modest overstepping (<15°C) of the stable chlorite‐to‐staurolite reaction and which is favoured, in the case of kyanite, by advantageous nucleation kinetics. If so, the pressure difference between Barrovian kyanite‐bearing sequences and Buchan andalusite‐bearing sequences could be ~1 kbar or less, in better agreement with the natural record. The unusual width of coexistence of staurolite and Al₂SiO₅ minerals, in particular kyanite and andalusite, can be accounted for by a combination of lack of thermodynamic driving force for conversion of staurolite to kyanite or andalusite, sluggish dissolution of staurolite, and possibly the absence of a fluid phase to catalyse reaction. This study represents an example of how kinetic controls on metamorphic mineral assemblage development have to be considered in regional as well as contact metamorphism.     

Petrogenesis of andalusite–kyanite–sillimanite veins and host rocks, Sanandaj-Sirjan metamorphic belt, Hamadan, Iran

Quartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al₂SiO₅ polymorphs, and, more rarely, three coexisting Al₂SiO₅ polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al₂SiO₅ polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe₂O₃ relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al₂SiO₅, kyanite‐quartz veins crosscut rocks that lack Al₂SiO₅ (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al₂SiO₅ triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.     

Transition of metamorphic series from the Kyanite- to andalusite-types in the Altai orogen, Xinjiang, China: Evidence from petrography and calculated KMnFMASH and KFMASH phase relations

Metamorphic zones in the Chinese Altai orogen have previously been separated into the kyanite- and andalusite-types, the andalusite-type being spatially more extensive. The kyanite-type involves a zonal sequence of biotite, garnet, staurolite, kyanite, sillimanite and, locally, garnet–cordierite zones. The andalusite-type zonal sequence is similar: it includes biotite, garnet and staurolite zones at lower-T conditions and sillimanite and garnet–cordierite zones at higher-T conditions, but additionally contains staurolite–andalusite and andalusite–sillimanite zones at intermediate-T conditions. As relic kyanite-bearing assemblages commonly persist in the staurolite–andalusite, andalusite–sillimanite and sillimanite zones, it is not clear that the distinction is valid. On the basis of a reevaluation of phase relations modelled in KMnFMASH and KFMASH pseudosections, kyanite and andalusite-bearing rocks of the Chinese Altai orogen record, respectively, the typical burial and exhumation history of the terrane. Mineral assemblages distributed through the various zones reflect a mix of portions of the ambient P–T array and the effects of evolving P–T conditions. The comparatively low-T biotite, garnet and staurolite zones mostly preserve kyanite-type peak assemblages that only experienced minor changes during exhumation. Rocks in the comparatively high-T sillimanite and garnet–cordierite zones are dominated by mineral assemblages of a transitional sillimanite type, having formed by the extensive modification of earlier higher pressure assemblages during exhumation. Only rocks in the intermediate-T kyanite and probably some lower sillimanite zones were clearly recrystallized by late stage andalusite metamorphism, producing the staurolite–andalusite and andalusite–sillimanite zones. This andalusite metamorphism could not reach an equilibrium state because of limited fluid availability.     

Geothermobarometry and development of inverted metamorphism in the Darjeeling-Sikkim region of the eastern Himalayan

ABSTRACT The Darjeeling-Sikkim region provides a classic example of inverted Himalayan metamorphism. The different parageneses of pelitic rocks containing chlorite, biotite, garnet, staurolite, kyanite, sillimanite, plagioclase and K-feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones. Microprobe data of coexisting minerals show that X_Mg varies in the order: garnet < staurolite < biotite < chlorite. White mica is a solid solution between muscovite and phengite. Garnet is mostly almandine-rich and shows normal growth zoning in the lower part of the Main Central Thrust (MCT) zone, and reverse zoning in the upper part of the zone. Chemographical relations and inferred reactions for different zones are portrayed in AFM space. In the low-grade zones oriented chlorites and micas and rolled garnets grew syntectonically, and were succeeded by cross-cutting chlorites and micas and garnet rims. In the upper zones sillimanite, kyanite and staurolite crystallized during a static inter-kinematic phase. P-T contitions of metamorphism, estimated through different models of geothermobarometry, are estimated to have been 580°c for the garnet zone to a maximum of 770°c for the sillimanite zone. The preferred values of pressure range from 5.0 kbar to 7.7 kbar. Models to explain the inverted metamorphism include overthrusting of a hot high Himalayan slab along a c. 5 km wide ductile MCT zone and the syn- or post-metamorphic folding of isograds.     

Polymetamorphic evolution of pelitic schists and evidence for Permian low-pressure metamorphism in the Vepor Unit, West Carpathians

Phase equilibrium modelling and monazite microprobe dating were used to characterize the polymetamorphic evolution of metapelites from the northern part of the Vepor Unit, West Carpathians. Three generations of garnet and associated metamorphic assemblages found in these rocks correspond to three distinct metamorphic events related to the Variscan orogeny, a Permian phase of crustal extension and the Alpine orogeny. Variscan staurolite‐bearing and Alpine chloritoid‐bearing assemblages record medium‐temperature and medium‐pressure regional metamorphisms reaching 540–570 °C/5–7.5 kbar and 530–550 °C/5–6.5 kbar respectively. The Permian metamorphic assemblage involves garnet, andalusite, sillimanite, biotite, muscovite, plagioclase and corundum and locally forms silica‐undersaturated andalusite‐biotite‐spinel coronas around older staurolite. The transition from andalusite to sillimanite indicates a prograde low‐pressure and medium‐temperature metamorphism characterized by temperature increase from 500 to 650 °C at ～3 kbar. As accessory monazite is abundant in the rocks, an attempt was made to derive its age of formation by means of electron microprobe‐based Th‐U‐Pb chemical dating. Despite the polymetamorphic nature of the metapelites, the monazite yielded uniform Permian ages. Microstructures confirm that monazite was formed in relation to the low‐pressure and medium‐temperature paragenesis, and the weighted average ages obtained for two different samples are 278 ± 5 and 275 ± 12 Ma respectively. The virtual lack of Variscan and Alpine monazite populations points to interesting aspects concerning the growth systematics of monazite in metamorphic rocks.     

Petrology and Evolution of an Archaean Metamorphic Aureole in the Slave Craton, Canada

In the Slave Craton of northern Canada, extensive areas weremetamorphosed in broad aureoles (typically ca. 10–15 kmwide) around granitie batholiths emplaced about 2575 m.y. ago.Meta-greywackes and meta-pelites from two areas traversing oneof these aureoles near Yellowknife have been studied. New petrographicdata are given and integrated with previously published mineralogicaldata to elucidate the metamorphic history of the area. Metasedimentsin the aureole contain the concentrically zoned succession ofindex minerals chlorite, biotite, cordierite, gedrite, andalusite,sillimanite. In addition, garnet, staurolite, and parageneticallylate andalusite occur more irregularly, and cummingtonite characterizessubordinate calcic rock-types. The chemistry of all these mineralsis given and their origins discussed. The aureole evolved by the development and decay of a thermaldome. This was a continuous process, but three recognizablemetamorphic phases can be correlated as follows with establisheddeformational phases. The cycle began with a deformation phase(D₁) unaccompanied by metamorphism. This evolved into D₂ whichwas accompanied by broad regional metamorphism M₂ (characterizedby the index succession chlorite, biotite, garnet, staurolite)as thermal doming began. With continued updoming of the isotherms,the third phase (D₃) produced only minor folding but causedmajor metamorphic recrystallization (M₃), culminating in theemplacement of granite at the core of the thermal dome. A concentriczonation of the metamorphic index minerals biotite, cordierite,gedrite, andalusite+sillimanite was superimposed on earlierassemblages. This M₃ phase occurred at lower pressure (2.5–3.5kb) than M₂ because of erosional unloading, but the temperatureswere more extreme, ranging up to about 700 °C. With deformationthen complete, the thermal dome decayed, and minor mineralogicalchanges occurred in this (M₄) decay phase. The region has sincebeen effectively stable.     

Sequential porphyroblast growth during deformation in a low-pressure metamorphic terrain, Orrs Island–Harpswell Neck, Maine

Poikiloblastic index minerals in pelitic rocks from the Orrs Island–Harpswell Neck area of coastal Maine contain inclusion textures that indicate sequential growth of progressively higher grade metamorphic minerals during development of a near-vertical crenulation foliation. The sequence of zones in the field is garnet, staurolite, staurolite–andalusite, staurolite–sillimanite and sillimanite. Inclusion fabrics characteristic of different stages in crenulation cleavage development indicate that index minerals nucleated and grew sequentially: biotite began to grow before deformation, garnet began to grow during early stages of crenulation cleavage development, staurolite grew during intermediate stages, and andalusite grew relatively late, when transposition of the foliation was nearly complete. Muscovite pseudomorphs and sillimanite were mainly post-kinematic. The fact that metamorphic index minerals grew sequentially in individual rocks in the same order in which they appear across the field area indicates that the high temperature part of the pressure–temperature path was similar to the metamorphic field gradient. Metamorphism in the Orrs Island–Harpswell Neck area is consistent with the magmatic heating model that has been proposed for western Maine. Sequential development of index minerals in pelitic rocks in the Orrs Island–Harpswell Neck area apparently resulted from sequential nucleation after substantial overstepping of mineral-forming reactions. Once nucleation of an index mineral had taken place, initial growth was rapid and poikiloblasts preserved inclusion trails characteristic of the prevailing stage of crenulation cleavage development. Because nucleation of sillimanite may have required more overstepping of the andalusite–sillimanite reaction than nucleation at dehydration reactions, determination of metamorphic conditions for rapidly heated rocks such as these by comparison with a petrogenetic grid is problematic. Garnet zoning patterns in these rocks should reflect the fact that growth of garnet interiors occurred early during metamorphism in equilibrium with a low-grade assemblage. Only garnet rims would be expected to record the subsequent pressure–temperature path.     

LPHT metamorphism in a late orogenic transpressional setting, Albera Massif, NE Iberia: implications for the geodynamic evolution of the Variscan Pyrenees

During the Late Palaeozoic Variscan Orogeny, Cambro‐Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low‐pressure/high‐temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite‐muscovite, biotite, andalusite‐cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench‐dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P–T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P–T estimates show that rocks from the biotite‐in isograd reached peak‐metamorphic conditions of 2.5 kbar, 400 °C; rocks in the low‐grade part of the andalusite‐cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 °C; rocks located at the transition between andalusite‐cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 °C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 °C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 °C. A clockwise P–T trajectory is inferred using a combination of reaction microstructures with appropriate P–T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision.     

Metamorphic and tectonic evolution of a structurally continuous blueschist‐to‐Barrovian terrane,Sivrihisar Massif,Turkey

Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn‐ to post‐subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist‐to‐Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod‐shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have ⁴⁰Ar/³⁹Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod‐shaped calcite has been progressively recrystallized into massive marble within a ～200‐m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ～1 km present‐day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite‐rich quartzite; in one location, all three are in the same rock. Andalusite pre‐dates Barrovian metamorphism, kyanite is both pre‐ and syn‐Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite‐in isograd are evidence for pre‐Barrovian subduction metamorphism preserved at the low‐T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn‐kinematic high‐T minerals defining the fabric of L‐tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.     

Genesis of monazite and Y zoning in garnet from the Black Hills, South Dakota

The paragenesis of monazite in metapelitic rocks from the contact aureole of the Harney Peak Granite, Black Hills, South Dakota, was investigated using zoning patterns of monazite and garnet, electron microprobe dating of monazite, bulk-rock compositions, and major phase mineral equilibria. The area is characterized by low-pressure and high-temperature metamorphism with metamorphic zones ranging from garnet to sillimanite zones. Garnet porphyroblasts containing euhedral Y annuli are observed from the garnet to sillimanite zones. Although major phase mineral equilibria predict resorption of garnet at the staurolite isograd and regrowth at the andalusite isograd, textural and mass balance analyses suggest that the formation of the Y annuli is not related to the resorption-and-regrowth of garnet having formed instead during garnet growth in the garnet zone. Monazite grains in Black Hills pelites were divided into two generations on the basis of zoning patterns of Y and U: monazite 1 with low-Y and -U and monazite 2 with high-Y and -U. Monazite 1 occurs in the garnet zone and persists into the sillimanite zone as cores shielded by monazite 2 which starts to form in the andalusite zone. Pelites containing garnet porphyroblasts with Y annuli and monazite 1 with patchy Th zoning are more calcic than those with garnet with no Y annuli and monazite with concentric Th zoning. Monazite 1 is attributed to breakdown of allanite in the garnet zone, additionally giving rise to the Y annuli observed in garnet. Monazite 2 grows in the andalusite zone, probably at the expense of garnet and monazite 1 in the andalusite and sillimanite zones. The ages of the two different generations of monazite are within the precision of chemical dating of electron microprobe. The electron microprobe ages of all monazites from the Black Hills show a single ca. 1713 Ma population, close to the intrusion age of the Harney Peak Granite (1715 Ma). This study demonstrates that Y zoning in garnet and monazite are critical to the interpretation of monazite petrogenesis and therefore monazite ages.     

Stable isotope constraints on the Al2SiO5'triple-point' rocks from the Proterozoic Priest pluton contact aureole, New Mexico, USA

The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one‐dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite‐in isograd is coincident with the staurolite‐out isograd in pelitic schist, and K‐feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite‐bearing pelitic schist. Muscovite‐rich aluminous schist locally preserves the Al₂SiO₅ polymorph triple‐point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co‐nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K‐feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (<500 °C).     

Thermal evolution of the Tseel terrane,SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.     

Metasomatic albitites and related biotite-rich schists from a low-pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, north-eastern Australia: microstructures and P–T–d paths

Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high‐T/low‐P events extending from D1 to D5, with the metamorphic peak occurring late to post‐D3. Albitites are widespread, but are concentrated in five areas. They are typically fine‐ to medium‐grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn‐D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite‐rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na‐rich fluids.     

The PTst Evolution of Metamorphism and Development Mechanism of the Thermal - Structural - Gneiss Domes in the Chinese Altaides1

Abstract A series of thermal-structural-gneiss domes (briefly TSG domes) are developed in the Chinese Altaides. Sericite-chlorite zone, biotite zone, garnet zone, staurolite zone, kyanite (andalusite) zone, sillimanite - cordierite (sillimanite - garnet) zone, migmatite zone and migmatic granite - gneiss field are developed from the low-grade metamorphic area to the centre of the TSG domes. The succession of the formation and evolution of the progressive metamorphic zone, migmatite zone and migmatic granite-gneiss corresponds to the spatial sequence from the outer part to the centre of the TSG domes. The peak temperature of the metamorphism and granitization increases progressively from 400 °C to 800 °C while the pressure decreases progressively from 1.05 GPa to 0.10 GPa from the biotite zone to the migmatic granite-gneiss field. The metamorphism of the orogenic belt may be described by the pressure-temperature-space-time model (PTst). In the main episode of orogeny, the deep heat flow and structural flow upsurged along a series of the centres of the regional thermodynamic anomalies, giving rise to the progressive metamorphism, granitization, and the differential uplift and the formation of TSG domes.     

Prograde and retrograde metamorphic fabrics – a key for understanding burial and exhumation in orogens (Bohemian Massif)

In the Orlica–?nie?nik Dome (NE Bohemian massif), alternating belts of orthogneiss with high‐pressure rocks and belts of mid‐crustal metasedimentary–metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE–SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat‐lying foliation. The metamorphic evolution of the mid‐crustal metasedimentary rocks involved successive crystallization of chlorite–muscovite–ilmenite–plagioclase–garnet, followed by staurolite‐bearing and then kyanite‐bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow‐dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat‐lying fabric. With reference to a P–T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X_Fe are compatible with a P–T path from 3.5–5 kbar/490–520 °C to peak conditions of 6–7 kbar/～630 °C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at ～4 kbar/～630 °C and to andalusite–cordierite stability at 2–3 kbar indicating exhumation from 25 km to around 9–12 km. Subsequent exhumation to ～6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary–metavolcanic rocks, and gneissic and high‐pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp‐like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high‐pressure rocks in lobe‐shaped and volumetrically more important anticlines. Converging P–T–D paths for the metasedimentary rocks and the adjacent high‐pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow‐dipping fabric affects both the metasedimentary–metavolcanic rocks and the gneissic and high‐pressure rocks, and indicates that the ～15‐km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.     

Petrology of thermally metamorphosed pelitic rocks in the Champawat Area,Kumaun Himalaya

In the Champawat area, Kumaun Himalaya, greenschist facies regionally metamorphosed rocksviz chlorite-phyllite and schist have been subjected to thermal metamorphism due to emplacement of batholithic granite/granodiorite body. As a consequence, biotite, garnet, andalusite, fibrolite, sillimanite and perthite minerals have formed in the contact rocks. The conspicuous absence of cordierite and staurolite reported from such aureole rocks is due to higher FeO/MgO ratio of the bulk rock composition in the former while the absence of staurolite is due to low Al₂O₃/FeO+MgO ratio in the schists. AFM diagram demonstrates that in muscovite-bearing schist, the bulk composition of chlorite- and cordierite-bearing rocks are restricted to low FeO/MgO side and thus the restricted occurrence of former and the absence of latter in the contact rocks of the area. This is further evident from the common occurrence of almandine-rich garnet in the rocks.     

Amphibolites with staurolite and other aluminous minerals: calculated mineral equilibria in NCFMASH

Amphibolite facies mafic rocks that consist mainly of hornblende, plagioclase and quartz may also contain combinations of chlorite, garnet, epidote, and, more unusually, staurolite, kyanite, sillimanite, cordierite and orthoamphiboles. Such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites. Because of the high variance of most of the assemblages, the phase relationships in amphibolites depend on rock composition, in addition to pressure, temperature and fluid composition. The mineral equilibria in the Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCFMASH) model system demonstrate that aluminium content is critical in controlling the occurrence of assemblages involving hornblende with aluminous minerals such as sillimanite, kyanite, staurolite and cordierite. Except in aluminous compositions, these assemblages are restricted to higher pressures. The iron to magnesium ratio (X_Fe), and to a lesser extent, sodium to calcium ratio, have important roles in determining which (if any) of the aluminous minerals occur under particular pressure–temperature conditions. Where aluminous minerals occur in amphibolites, the P–T–X dependence of their phase relationships is remarkably similar to that in metapelitic rocks. The mineral assemblages of Fe‐rich amphibolites are typically dominated by garnet‐ and staurolite‐bearing assemblages, whereas their more Mg‐rich counterparts contain chlorite and cordierite. Assemblages involving staurolite–hornblende can occur over a wide range of pressures (4–10 kbar) at temperatures of 560–650 °C; however, except in the more aluminous, iron‐rich compositions, they occupy a narrow pressure–temperature window. Thus, although their occurrence in ‘typical’ amphibolites may be indicative of relatively high pressure metamorphism, in more aluminous compositions their interpretation is less straightforward.     

Phase relationships in Buchan facies series pelitic assemblages: calculations with application to andalusite-staurolite parageneses in the Mount Lofty Ranges,South Australia

Low-pressure, medium- to high-temperature (Buchan-type) regional metamorphism of pelitic rocks in the Mount Lofty Ranges, South Australia, is defined by the development of biotite, staurolite-andalusite, fibrolite, prismatic sillimanite and migmatite zones. K-feldspar makes its first appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovite, concentrating particularly on well-developed andalusitestaurolite parageneses. In general, the spatial distribution and mineral chemical variation of these assemblages accord with the predictions of petrogenetic grids and P-T and T-X _Fe pseudo-sections constructed from the internally consistent data set of Holland and Powell (1990) in the system KFMASH, assuming a(H₂O) 1, although analysed white mica compositions are systematically more aluminous than predicted. Importantly, the stability ranges of most critical assemblages predicted by these grids and pseudo-sections coincide closely with P-T estimates calculated using the data set of Holland and Powell (1990) from the Mount Lofty Ranges assemblages. With the exception of Mn in garnet and Zn in one staurolite-cordierite-muscovite assemblage non-KFMASH components do not significantly appear to have affected the stability ranges of the observed assemblages. An apparent local reversal in isograd zonation in which andalusite first appears down-grade of staurolite suggests a metamorphic field gradient concave towards the temperature axis and, together with evidence for essentially isobaric heating of individual rocks, is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels.Mineral abbreviations used in figures als Al₂SiO₅ phase - bi biotite - chl chlorite - ky kyanite - ph phengite - sill sillimanite - and andalusite - cd cordieritc - gt garnet - mu muscovite - q quartz - st staurolite     

The variscan geology of the Pyrenees

The main outlines of the geology of the Variscan part of the Pyrenees are discussed. Rocks involved in this cycle are high-grade basement gneisses, Palaeozoic sediments and their metamorphic equivalents, late intrusive granodiorites and early, pre-Variscan granites. The main features of the stratigraphy of the Palaeozoic are given.Structures fall into two domains: a low-grade suprastructure, essentially with steep folds and cleavages, and a high-grade infrastructure with dominantly low-dipping foliations. An important phase of early, pre-cleavage folding occurs in low-grade rocks mainly along the southern border of the Axial zone. In high-grade rocks most structures and the metamorphism postdate the main cleavage phase in low-grade rocks. The influence of the Alpine orogeny on the Variscan structures consists mainly of faults, steep, reverse faults in the northern, and south-directed thrusts in the southern part of the Pyrenees. Metamorphism took place under high geothermal gradients and low pressures, as indicated by the abundant occurrence of andalusite and cordierite     

Multi-stage pseudomorphic replacement of garnet during polymetamorphism: 1. Microstructures and their interpretation

Metapelites from the southern aureole of the Vedrette di Ries tonalite (eastern Alps) were variably overprinted by contact and earlier regional metamorphic events during pre-Alpine and Alpine metamorphic cycles. In these rocks, starting from a primary garnet mica-schist (garnet stage), a complex sequence of transformations, affecting the site of the garnet, has been recognized. In the outermost part of the aureole, the primary garnet sites are occupied by nodules of kyanite (kyanite stage). Closer to the tonalite, kyanite is replaced by staurolite (staurolite stage), which in turn is pseudomorphed by muscovite (muscovite stage). The aggregates of kyanite do not overgrow garnet directly; they post-date a stage (fibrolite stage) represented by the pseudomorphic alteration of garnet into fibrolitic sillimanite plus biotite. A further sericite stage is likely to have occurred between the fibrolite and kyanite stages. Preservation of the sub-spherical garnet shape during all these transformations and persistence of mineralogical and textural relicts from earlier stages were favoured by the very low strain experienced by the rocks since the garnet stage. The textural sequence is in agreement with the metamorphic history of this part of the Austroalpine basement of the Eastern Alps: the garnet and fibrolite stages, and the coeval main foliation of the samples, are referred to the high-grade Hercynian metamorphism; the kyanite stage to the Eo-Alpine metamorphism; the staurolite and muscovite stages to the Oligocene contact metamorphism. It is suggested that kyanite growth as microgranular aggregates took place in polymetamorphic rocks where static, high- P /low- T  metamorphism overprinted high- T  assemblages that contained sillimanite or andalusite.