Interpreting prograde-growth histories of Al2SiO5 triple-point rocks using oxygen-isotope thermometry: an example from the Truchas Mountains, USA

Oxygen‐isotope compositions of kyanite, andalusite, prismatic sillimanite and fibrolite from the Proterozoic terrane in the Truchas Mountains, New Mexico differ from one another, suggesting that these minerals did not grow in equilibrium at the Al₂SiO₅ (AS) polymorph‐invariant point as previously suggested. Instead, oxygen‐isotope temperature estimates indicate that growth of kyanite, andalusite and prismatic sillimanite occurred at c. 575, 615 and 640 °C respectively. Temperature estimates reported in this paper are interpreted as those of growth for the different AS polymorphs, which are not necessarily the same as peak metamorphic temperatures for this terrane. Two distinct temperature estimates of c. 580 °C and c. 700 °C are calculated for most fibrolite samples, with two samples yielding clear evidence of quartz‐fibrolite oxygen‐isotope disequilibrium. These data indicate that locally, and potentially regionally, oxygen‐isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation. Pressures of mineral growth that were extrapolated from oxygen‐isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during M1 at 5 kbar, and that andalusite, prismatic sillimanite and a second generation of fibrolite grew during M2 at 3.5 kbar. M1 and M2 therefore represent two distinct metamorphic events that occurred at different crustal levels. The ability of the AS polymorphs to retain δ¹⁸O values of crystallization make these minerals ideal to model prograde‐growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure–temperature histories of multiple metamorphic events.     

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.     

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.     

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.     

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     

Disequilibrium in the Ross of Mull Contact Metamorphic Aureole, Scotland: a Consequence of Polymetamorphism

The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone II–III boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium     

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.     

Oxygen isotope study of quartz-Al2SiO5 pairs from the Mica Creek area, British Columbia: implications for the recovery of peak metamorphic temperatures

Oxygen isotope analyses of quartz-Al₂SiO₅ pairs have been made for samples from the Mica Creek area, British Columbia. We have analysed quartz–kyanite nodules and quartz–kyanite and quartz–sillimanite in multiphase pelitic rocks from the staurolite–kyanite, kyanite, and sillimanite zones. Apparent temperatures calculated from oxygen isotopic fractionation range from 555 °C (staurolite–kyanite zone) to 695 °C (sillimanite zone). Temperatures from the quartz–kyanite nodules range from 630 to 675 °C. Some of the nodules show isotopic disequilibrium. Most of the results confirm predictions that bimineralic rocks will yield an estimate of peak metamorphic temperatures, when the less abundant mineral (an aluminium silicate) is the slower oxygen diffuser. Using cooling rates of 10–100 °C Ma^?1 for the multiphase rocks, measured crystal sizes and modes, the Fast Grain Boundary diffusion model with ‘wet’ diffusion data (P_H2O?1.0 kbar) yields predicted apparent temperatures which are generally lower than the measured apparent temperatures. The agreement is improved if slower diffusion coefficients are used. This suggests that f (H₂O) during cooling was lower than that of the hydrothermal experiments and thus that there was little interaction with aqueous fluids of internal or external origin to modify the isotopic compositions. The measured apparent isotopic temperatures and apparent garnet–biotite Fe–Mg exchange temperatures show very poor agreement for the sillimanite zone samples, with the garnet–biotite Fe–Mg exchange temperatures generally higher than the oxygen isotope temperatures. Compared with the other calibrations that we tested the measured apparent temperatures using the Sharp calibration show the best agreeement with recently published P–T grids, although some variability in agreement is expected due to variable f (H₂O) during cooling.     

Evolution of a kyanite-bearing belt within a HT-LP orogen: the case of NW Variscan Iberia

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high-grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich, black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite– chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al-rich and Al-poorer compositions from the belt. Chloritoid zoning in Al-rich rocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chlorite thermometry in Al-poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid– chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite-consuming reaction across a KFMASH field, leading eventually to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded by cordierite–biotite in Al-poor pelites. Garnet-bearing MnKFMASH assemblages in Al-poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite-bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.     

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.     

P–T–t evolution of granulite facies metamorphism and partial melting in the Winding Stair Gap,Central Blue Ridge,North Carolina,USA

The Winding Stair Gap in the Central Blue Ridge province exposes granulite facies schists, gneisses, granofelses and migmatites characterized by the mineral assemblages: garnet–biotite–sillimanite–plagioclase–quartz, garnet–hornblende–biotite–plagioclase–quartz ± orthopyroxene ± clinopyroxene and orthopyroxene–biotite–quartz. Multiple textural populations of biotite, kyanite and sillimanite in pelitic schists support a polymetamorphic history characterized by an early clockwise P–T path in which dehydration melting of muscovite took place in the stability field of kyanite. Continued heating led to dehydration melting of biotite until peak conditions of 850 ± 30 °C, 9 ± 1 kbar were reached. After equilibrating at peak temperatures, the rocks underwent a stage of near isobaric cooling during which hydrous melt ± K‐feldspar were replaced by muscovite, and garnet by sillimanite + biotite + plagioclase. Most monazite crystals from a pelitic schist display patchy zoning for Th, Y and U, with some matrix crystals having as many as five compositional zones. A few monazite inclusions in garnet, as well as Y‐rich cores of some monazite matrix crystals, yield the oldest dates of c. 500 Ma, whereas a few homogeneous matrix monazites that grew in the main foliation plane yield dates of 370–330 Ma. Culling and analysis of individual spot dates for eight monazite grains yields three age populations of 509 ± 14 Ma, 438 ± 5 Ma and 360 ± 5 Ma. These data suggest that peak‐temperature metamorphism and partial melting in the central Blue Ridge occurred during the Salinic or Taconic orogeny. Following near isobaric cooling, a second weaker thermal pulse possibly related to intrusion of nearby igneous bodies resulted in growth of monazite c. 360 Ma, coinciding with the Neoacadian orogeny.     

Proterozoic anticlockwise P–T path of the Lewisian Complex of South Harris, Outer Hebrides, NW Scotland

The Leverburgh Belt and South Harris Igneous Complex in South Harris (northwest Scotland) experienced high-pressure granulite facies metamorphism during the Palaeoproterozoic. The metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic, quartzofeldspathic and mafic gneisses, especially in pelitic gneisses from the Leverburgh Belt: (1) some coarse-grained garnet in the pelitic gneiss includes biotite and quartz in the inner core, sillimanite in the outer core, and is overgrown by kyanite at the rims; (2) garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims; (3) the Al^VI/Al^IV ratio of clinopyroxene from mafic gneiss increases from core to rim; (4) retrograde reaction coronas of cordierite and hercynite+cordierite are formed between garnet and kyanite, and orthopyroxene+cordierite and orthopyroxene+plagioclase reaction coronas develop between garnet and quartz; (5) a P–T path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet+sillimanite and garnet+sillimanite+hercynite with increasing temperature; and (6) in sheared and foliated rocks, hydrous minerals such as biotite, muscovite and hornblende form a foliation, modifying pre-existing textures. The inferred metamorphic history of the Leverburgh Belt is divided into four stages, as follows: (M1) prograde metamorphism with increasing temperature; (M2) prograde metamorphism with increasing pressure; (M3) retrograde decompressional metamorphism with decreasing pressure and temperature; and (M4) retrograde metamorphism accompanied by shearing. Peak P–T conditions of the M2 stage are 800±30 °C, 13–14 kbar. Pressure increasing from M1 to M2 suggests thrusting of continental crust over the South Harris belt during continent–continent collision. The inferred P–T path and tectonic history of the South Harris belt are different from those of the Lewisian of the mainland.     

攀西地区低级和中级泥砂质片岩中的契尔马克替换

攀西中元古结晶片岩系遭受了前进区域变质作用。盐边和米易的砂屑岩和泥质岩分别可划出:绿泥石、黑云母、铁铝榴石、十字石和夕线石带以及绿泥石、黑云母-石榴子石、红柱石和夕线石带。在中、低级泥砂质岩石中,白云母的Na/(Na+K)比值随变质级增高。白云母、绿泥石和黑云母中的契尔马克替换范围大体上随变质级增高而降低。白云母和绿泥石之间契尔马克替换交换反应的分配系数,大致是白云母的绿鳞石含量的函数,并随变质温度升高而降低,在夕线石带,该分配系数变得很小。黑云母和白云母契尔马克替换交换反应的分配系数有类似的趋势。     

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K₂O content in muscovite, but in biotite it reflects an increase of not only the K₂O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.     

Granulites of the Kolpakovskaya Series in the sredinny range,Kamchatka: A myth or reality?

Geological, mineralogical, and geothermobarometric data testify that the regional metamorphism of the terrigenous protolith of the Kolpakovskaya Series, which composes the stratigraphic basement of the Kamchatka Median Crystalline Massif, corresponded to the kyanite mineral subfacies of the amphibolite facies at temperatures of 560–660°C and pressures of 5.9–6.9 kbar. This metamorphism predetermined wide kyanite development in high-Al garnet-biotite plagiogneisses. The younger granitization and migmatization of the plagiogneisses took place at a decrease in the pressure (depth), as follows from the textures of kyanite reaction replacement by andalusite in both the metamorphic rocks and the vein synmetamorphic granitoids and pegmatites. The temperature of the granitization and migmatization processes in the plagiogneisses was estimated at 620–650°C, and the pressure was evaluated at 1.9–3.0 kbar. Acid leaching that accompanied the granitization and migmatization processes resulted in the intense replacement of biotite by sillimanite (fibrolite) and, to a lesser degree, muscovite in the metamorphic and vein magmatic rocks. The highest temperature orthopyroxene-cordierite-biotite-orthoclase-plagioclase-quartz mineral assemblages were determined to have been formed in the Kolpakovskaya Series at a temperature of 830–840°C not by the regional metamorphism but in contact aureoles around gabbro-granitoid intrusions of the Lavkinskii intrusive complex of Oligocene-Miocene age in garnet-biotite and kyanite-garnet-biotite plagiogneisses of the amphibolite facies and cannot thus be regarded as evidence of an early granulite stage in the metamorphism of these rocks.     

Coexisting chloritoid-staurolite from the Sillimanite (fibrolite) zone,Sini, district Singhbhum,India

Microprobe analyses of the minerals from an unusual chloritoid-staurolite-garnet (+ muscovite + quartz + ilmenite) assemblage from the sillimanite (fibrolite) zone of Sini, India are presented and the petrological significance of the paragenesis is discussed. The X Mg in the different minerals from the chloritoid-staurolite-bearing rock varies in the order, muscovite > chlorite > chloritoid > staurolite > garnet > ilmenite, and from the associated sillimanite-bearing schists: muscovite > biotite > staurolite > garnet rim > garnet core > ilmenite. A graphical representation of the mineral compositions in an AFM projection displays a consistent topology if the effects of non-AFM components such as Zn in the staurolite and Mn in the garnet are taken into account. Petrographic and mineralogical data are consistent with a prograde formation of the chloritoid-staurolite-garnet assemblage. It is suggested that the paragenesis has been formed at similar PT conditions to the associated sillimanite (fibrolite)-staurolite-garnet-mica schists. These conditions are estimated to be 600–625°C/6±0.5 Kb.     

A calculated petrogenetic grid for the system K2O-FeO-MgO-Al2O3-SiO2-H2O, with particular reference to contact-metamorphosed pelites

A petrogenetic grid is presented for the system KFMASH (K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O), including biotite, muscovite, K-feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H₂O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz-bearing and quartz-absent equilibria are shown. Quartz-absent equilibria are particularly relevant at high- T and low- P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg-1 and (Fe, Mg)SiAl_-2 exchange vectors are broadly compatible with observations on natural rocks, particularly when non-KFMASH components are taken into account.     

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.     

Cenozoic tectonics and landform evolution of the coast and adjacent highlands of southeast New South Wales

The Upper Precambrian and Lower Palaeozoic Rocks in the Mt Lofty Ranges, South Australia, have been subjected to at least three phases of folding. The first involved the formation of inclined folds and less common reclined folds. These structures are overprinted by usually upright, moderately tight, second and third generation folds which may show a well developed axial plane crenulation cleavage.

The metamorphism commenced prior to the appearance of penetrative structures and continued in many areas until after the third phase of deformation. It appears to have had its greatest effect during the static period following the first phase of folding.

Mineral assemblages of the pelitic rocks indicate that the metamorphism is of the low pressure‐intermediate type and that there are at least four progressive zones of metamorphism, namely, chlorite, biotite, andalusite‐staurolite, and sillimanite. Cordierite occurs in the sillimanite zone and kyanite is sporadically distributed in the andalusite‐staurolite zone. In the Angaston‐Springton region separate andalusite and staurolite zone boundaries may be delineated which cross as they are traced towards Angaston. This relationship is considered to be due to higher pressures operating during metamorphism in the latter area.

The maximum pressure and temperature reached in the metamorphism of these rocks are discussed in the light of recent experimental data.     

Textural relations,P-T path,polymetamorphism and also geodynamic significance of metamorphic rocks of the Aligudarz-Khonsar region,Sanandaj-Sirjan zone,Iran

The metamorphic rocks of the Aligudarz-Khonsar region can be divided into nine groups: slate, phyllite, sericite schist, biotite-muscovite schist, garnet schist, garnet-staurolite schist, staurolite schist, mylonitic granite, and marble. In this metamorphic region, four phases of metamorphism can be identified (dynamothermal, thermal, dynamic and retrograde metamorphism) and there are three deformation phases (D1, D2 and D3). Paleozoic pelagic shales experienced prograde metamorphism and polymetamorphism from the greenschist to amphibolite facies along the kyanite geotherm. The metapelites show prograde dynamothermal metamorphism from the greenschist to amphibolite facies. Maximum degree of dynamothermal metamorphism is seen in the Nughan bridge area. Also development of the mylonitic granites in the Nughan bridge area shows that dynamic metamorphism in this area was more intense than in other parts of the AligudarzKhonsar metapelitic zone. The chemical zoning of garnets shows three stages of growth and syn-tectonic formation. With ongoing metamorphism, staurolite appeared, and the rocks reached amphibolite facies, but the degree of metamorphism did not increase past the kyanite zone. Thus, metamorphism of the pelitic sediments occurred at the greenschist to amphibolite facies (kyanite zone). Thermodynamic studies of these rocks indicate that the metapelites in the Aligudarz-Khonsar region formed at 490–550°C and 0.47–5.6 kbar.