bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metamorphism and tectonic evolution of the Lancang paired metamorphic belts, south‐western China
Authors: ZHANG RUYUAN , CONG BOLIN , MARUYAMA SHIGE , LIOU J. G. 
Published: May 2007

Abstract
The Lancang metamorphic terrane consists of an eastern low‐P/T belt and a western high‐P/T belt divided by a N–S‐trending fault. Protoliths of both units are mid–late Proterozoic basement and its cover. The low‐P/T belt includes the Permian Lincang batholith, related amphibolite facies rocks of the Damenglong and Chongshan groups, and Permo‐Triassic volcanic and volcaniclastic rocks. Most whole‐rock Rb–Sr isochron and U–Pb zircon ages of the Lincang batholith are in the range 290–279 and 254–212 Ma, respectively. Metamorphism of the low‐P/T belt reaches upper amphibolite with local granulite facies (735°C at 5 kbar), subsequently retrogressed at 450–500°C during post‐Triassic time. The high‐P/T rocks grade from west to east from blueschist through transitional blueschist/greenschist to epidote amphibolite facies. Estimated P–T conditions follow the high‐P intermediate facies series up to about 550–600°C, at which oligoclase is stable. The 40Ar/39Ar plateau age of sodic amphibole in blueschist is 279 Ma.The paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of south‐east Asia since the latest Precambrian. Tectonic juxtaposition of paired belts with contrasting P–T conditions, perhaps during collision of the Baoshan block with south‐east Asia, suggests that an intervening oceanic zone existed that has been removed. The Baoshan block is a microcontinent rifted from the northern periphery of Gondwana. Successive collision and amalgamation of microcontinents from either Gondwana or the Panthalassan ocean resulted in rapid southward continental growth of c. 500 km during the last 200 Ma. Hence, the Lancang region in south‐east Asia represents a suture zone between two contrasting microcontinents.
Prograde reactions and garnet zoning reversals in staurolite schist, British Columbia: significance for thermobarometric interpretations
Authors: WHITNEY D. L. , GHENT E. D. 
Published: May 2007

Abstract
An outcrop of staurolite‐bearing pelitic schist from the Solitude Range in the south‐western Rocky Mountains, British Columbia, was examined in order to determine the nature of prograde garnet‐ and staurolite‐producing reactions using information from garnet zoning and inclusion mineralogy. Although not present as a matrix phase, chloritoid is present as inclusions in garnet and is interpreted to have participated in the simultaneous growth of garnet and staurolite by a reaction such as chloritoid + quartz = garnet + staurolite + H2O.A garnet zoning trend reversal, which is most pronounced with respect to almandine and grossular components, is present in the outer core of garnets. The location of the zoning reversal corresponds to the outer limit of chloritoid inclusions in garnet. As there is no evidence for polymetamorphism, the zoning reversal is interpreted to indicate continued garnet growth by prograde reaction(s) during a single metamorphic event after the exhaustion of chloritoid as a matrix phase.Metamorphic conditions recorded by mineral rim compositions are 550–600° C at 6–7 kbar. Because there is no evidence for partial resorption of garnet during production of staurolite, we interpret these results to represent peak conditions.
Progressive metamorphism in a crustal‐scale shear zone: an example from the Léon region, north‐west Brittany, France
Authors: JONES K. A. 
Published: May 2007

ABSTRACTThis paper describes the progressive metamorphism and deformation of a series of metasediments, Le Conquet Schists and their higher grade equivalents, which occur as tectonically emplaced screens within a sequence of foliated gneisses, the Gneiss de Brest and Gneiss de Lesneven. The sequence exhibits a steep south to north increase in metamorphic grade from garnet‐staurolite schist to sillimanite gneiss and sillimanite‐K‐feldspar migmatite. The relationship of mineral growth to foliation development has been established for individual screens. At least five phases of deformation (D1‐D5) are preserved. Analysis of porphyroblast inclusion trails is used to demonstrate sequential mineral growth during the successive development of orthogonal foliations S1‐S4. Porphyroblasts continued to grow during the subsequent development of C‐S mylonite fabrics and extensional crenulation cleavages which are genetically related to a series of high‐strain zones (D5). Mineral assemblages, phase relations and mineral chemistry are consistent with porphyroblast growth being the result of continuous reactions. Microstructure‐porphyroblast relations are used to show that although mineral growth proceeded during continuous reactions, these only operated episodically. Phase relations, mineral chemistry and P‐T estimates are used to constrain P‐T trajectories and these are linked to the deformation histories within individual screens. A comparison between the resulting pressure‐temperature‐deformation paths is used to demonstrate that the metamorphic peak occurred progressively later and at successively lower pressures with increasing metamorphic grade. It is suggested that the early evolution of the belt is the result of crustal thickening by overthrusting. The subsequent history is one of progressive heating and unroofing of the higher grade rocks in a dextral strike‐slip transtensional shear zone.
Rotation of porphyroblasts in non‐coaxial deformation: insights from computer simulations
Authors: BJØRNERUD M. G. , ZHANG HUBAO 
Published: May 2007

ABSTRACTPorphyroblast inclusion trails have the potential to provide critical information about tectonometamorphic events. Recently, however, traditional interpretations of inclusion trails have been called into question by the suggestions that porphyroblasts do not rotate during non‐coaxial deformation and that apparent spiral inclusion trails can be generated in coaxial deformation. We present a new computer model that simulates inclusion trail development. Model results suggest: (1) that the extent of porphyroblast rotation is controlled by conditions at the porphyroblast‐matrix boundary; (2) that curved inclusion trails may develop in unrotated porphyroblasts; (3) that classic “snowball” inclusion trails are most simply explained by rotational growth histories; and (4) that some of the observations used to support the view that porphyroblasts do not rotate (e.g. weakly sigmoidal inclusion trails, apparent truncations of inclusion trails) can be accounted for by variations in the growth rate of rotating porphyroblasts.
Fluid migration and vein formation during deformation and greenschist facies metamorphism at Ormiston Gorge, central Australia
Authors: CARTWRIGHT I. , POWER W. L. , OLIVER N. H. S. , VALENTA R. K. , MCLATCHIE G. S. 
Published: May 2007

ABSTRACTDuring the Alice Springs Orogeny, deformation at Ormiston Gorge, central Australia, occurred under lower‐ to middle‐greenschist facies conditions. Dolomites of the Bitter Springs Formation and quartzites. metagreywackes, and metapelites of the Heavitree Quartzite contain abundant early‐, syn‐, and post‐tectonic veins. However, though vein densities locally approach 15%, the distribution of veins and the oxygen isotope geochemistry of wallrocks and veins suggest that fluid movement was on a local scale. The Heavitree Quartzite contains quartz veins that, even along the main thrust plane, have similar δ18O values (13.5–16.9%o) to those of their wallrocks (13.6–16.9%o), with Δ18O(vein‐wallrock) values of ‐0.6 to 0.4%o. In contrast, the Bitter Springs Formation contains predominantly dolomite veins that have δ18O values of 23.4 to 27.7%o. These differences are observed even at the boundary between the Heavitree and Bitter Springs rocks, implying that significant fluid exchange between these rocks has not occurred, or that fluid flow was channelled through areas outside those sampled for this study. By contrast with the Heavitree Quartzite, δ18O values of wallrocks in individual samples of the Bitter Springs Formation are significantly higher (23.3–29.1%o) than those of the veins, with δ18O(vein‐wallrock) values up to ‐4%o (average of ‐2.1%o). These systematic differences in δ18O values most likely result from oxygen isotope fractionation caused by fluid immiscibility or disequilibrium dissolution. Smaller differences in δ13C values between some dolomite veins and wallrocks [δ13C(vein‐wallrock) up to ‐1.9%o, average of ‐0.5%o] are also explained by these processes. This study indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units, without involving large volumes of externally derived fluids.
Zoning patterns in orthopyroxene and garnet in granulites: implications for geothermometry
Authors: PATTISON D. R. M. , BÉGIN N. J. 
Published: May 2007

ABSTRACTCompositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and Al in orthopyroxene had been quenched‐in. The extent of late Fe‐Mg exchange was controlled by neighbouring minerals, with negligible Fe‐Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe‐Mg minerals. Cores of grains in contact with exchangeable Fe‐Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high‐temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe‐Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe‐Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120d̀ C; (2) Fe‐Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperature‐sensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150d̀ C higher than maximum Fe‐Mg exchange temperatures, even in grains with flat Fe/(Fe ‐ Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than Fe‐Mg exchange thermometry.
Microstructural timing of the Rosebery massive sulphides, Tasmania: evidence for a metamorphic origin through mobilization of disseminated base metals
Authors: AERDEN D. G. A. M. 
Published: May 2007

ABSTRACTMicrostructural timing relationships indicate that the Rosebery massive sulphide ore, western Tasmania, Australia, formed by metasomatic replacement of ‘sericite’ schist during a Devonian deformation event (D3). This interpretation is contrary to earlier volcanogenic‐based interpretations, but accords with the discordant position and inferred structurally controlled emplacement of the orebody. The main timing criteria are: overprinting of S3 by the late ore minerals, replacement textures in undeformed mineral parageneses, and a D3 structural control from the microscopic to the macroscopic scales. The consistent observation of these criteria in the orebody and the complete lack of pre‐D3 ore argue against in situ dissolution of a primary orebody and local redeposition of sulphides by replacement.D3 deformation at Rosebery is inferred to have been characterized by heterogeneous cleavage‐parallel extension, which resulted in localized microfracturing and selective replacement of zones of maximum strain rate. Continuous shifts in the strain‐rate distribution pattern during progressive mineralization led to the compositional ore banding.Published Pb‐isotope data point towards a Cambrian source rock for the orebody. This suggests a metamorphogenic origin by regional‐scale dissolution of dispersed volcanogenic metals, focused solution transfer and concentrated redeposition in a structural trap.
Synmetamorphic veining: origin of andalusite‐bearing veins in the Vedrette di Ries contact aureole, Eastern Alps, Italy
Authors: CESARE B. 
Published: May 2007

ABSTRACTDiscordant andalusite‐biotite‐quartz‐bearing veins occur in the contact aureole of the Vedrette di Ries pluton (Italian Eastern Alps), never outside the area of contact metamorphic andalusite development. Andalusite veins are found only within andalusite‐bearing hornfelses, and vein biotite occurs wherever host‐rock garnet is partially replaced by biotite. Veins formed during contact metamorphism, synchronously with the crystallization of andalusite and biotite within host rocks. Their pegmatitic structure and their orientation suggest that vein parageneses crystallized within fluid‐filled cavities that opened by hydraulic fracturing.A mechanism of synmetamorphic veining is proposed to explain rock failure and subsequent mineral deposition within veins. During hydrofracturing induced by dehydration reactions in response to heating in the aureole, fissures were immediately filled with locally derived fluids. The lack of large‐scale flux, together with high fluid pressures required by hydrofracturing, suggest fluid in the cavities was a virtually stagnant, passive medium, and that mass‐transport toward fractures was driven by intergranular diffusion. Because temperature and Pf values within veins are similar to those in the host rock, vein assemblages are interpreted as the stable, high‐T side of reactions taking place within pelitic schists, at the time when fractures opened. Once nucleation of product phases occurred, chemical components released by dissolution of reactant minerals were driven to precipitation sites by chemical potential gradients. Since nucleation was favoured at the strained grains of vein walls, andalusite and biotite simultaneously grew in vein and host rock.The proposed genetic model contrasts with generally adopted metasomatic mechanisms for the genesis of Al2SiO5‐bearing veins, in not requiring large fluid/rock ratios or a highly ‘aggressive’ fluid composition. The mechanism of synmetamorphic veining may be particularly useful in the interpretation of vein occurrences in medium‐ and deep‐crustal rocks which have undergone extensive devolatilization.
Interpretation of porphyroblast inclusion trails: limitations imposed by growth kinetics and strain rates
Authors: BARKER A. J. 
Published: May 2007

ABSTRACTPorphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock. However, there remains considerable controversy over whether porphyroblasts rotate during bulk non‐coaxial deformation.With reference to an area of the Scandinavian Caledonides and utilizing existing data from theoretical and experimental modelling, this study demonstrates that both ‘straight’ and ‘S‐shaped’ inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to Newtonian simple shear. At crustal strain rates of 10‐14 s‐1 and porphyroblast growth times of 0.1–1.0 Ma, it is shown that a maximum of 5d̀‐9d̀ angular rotation would occur during growth. At faster strain rates of 10‐12 s‐1 (e.g. those in a shear zone) porphyroblast angular rotations of 90d̀ are shown to occur in 0.1–0.25 Ma (i.e. times comparable with or faster than porphyroblastesis). In view of this, ‘S‐shaped’ inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain, whereas ‘straight’ inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates.
The effects of ferromagnesian components on the paragonite‐muscovite solvus: a semiquantitative analysis based on chemical data for natural paragonite‐muscovite pairs
Authors: GUIDOTTI C. V. , SASSI F. P. , SASSI R. , BLENCOE J. G. 
Published: May 2007

ABSTRACTChemical data for 139 natural paragonite‐muscovite (Pg‐Ms) pairs illustrate the effects of ferromagnesian components on theP‐T‐Xtopology of the Pg‐Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of theP‐Tconditions of formation; evidence for close approach to equilibrium at peak metamorphic conditions; exclusion of pairs in which paragonite contains more than 5 mol% margarite; and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic Pg‐Ms pairs. Graphical analysis reveals considerable scatter in the data; nevertheless, it is evident that the muscovite limb of the solvus shifts markedly toward end‐member muscovite with increasing pressure from approximately 7 kbar to 21 kbar. This shift is attributed to a pressure‐induced increase of the ferromagnesian content of muscovite, which increases the size of the XII alkali site ‐ to the effect that K is more readily accommodated than Na. The data also suggest that the paragonite limb of the solvus migrates slightly toward end‐member paragonite with increasing pressure. Broadening of the Pg‐Ms solvus with increasing pressure reflects increasingly nonideal Na‐K mixing as the phengite content of muscovite increases. Due to the wide scatter of data for Pg‐phengitic‐Ms pairs, it is concluded that, at the present time, Pg‐Ms solvus thermometry is only viable for quasibinary Pg‐Ms pairs.
Metamorphic P‐T paths from calcic pelitic schists from the Strafford Dome, Vermont, USA
Authors: MENARD T. , SPEAR F. S. 
Published: May 2007

ABSTRACTMetamorphism of the Gile Mountain Formation and Waits River Formation in the Strafford Dome and Townshend‐Brownington Syncline in east‐central Vermont records two nappe‐style events, D1 and D2, followed by doming. D1 formed a muscovite + biotite ± ilmenite schistosity subparallel to compositional layering, SO, and was followed by heating to garnet grade. The temperature and pressure at the end of D1 are estimated to be c. 450d̀ C and 6‐8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1‐2 kbar, calculated from compositions of garnet, which have inclusions trails with progressive crenulation and rotation of the S1 fabric. Similar P‐T paths are computed for most of the area, suggesting that the later schistosity developed during emplacement of a regional nappe 3‐6 km thick. There is a general lack of D3 (dome‐stage) microstructures.Near the Strafford‐Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasing XAn overgrew S2 in calcic pelites, reflecting post‐D2 heating to a maximum of 550‐600d̀ C. Metamorphic pressures at the end of D2 are fairly constant on the west side of the dome, indicating minor dome‐stage uplift. In contrast, pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome. The observed pattern of isotherms and isobars is mainly the result of post‐metamorphic, differential uplift and unroofing.Finally, a minor, retrograde metamorphism produced the assemblage albite + epidote + K‐feldspar + muscovite + chlorite, with grade increasing east toward the Connecticut River.
Stepping stones and pitfalls in the determination of an anticlockwise P‐T‐t‐deformation path: the low‐P, high‐T Cooma Complex, Australia
Authors: JOHNSON S. E. , VERNON R. H. 
Published: May 2007

Abstract
Low‐pressure/high‐temperature (low‐P/high‐T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure‐temperature‐time‐deformation (P‐T‐t‐D) path, similar to those of some other low‐P/high‐T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite‐andalusite schists, cordierite‐K‐feldspar gneisses and andalusite‐K‐feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P‐T‐t‐D path involving rapid or even moderate decompression immediately post‐dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed.Determination of the deformational‐metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational‐metamorphic history in the Cooma Complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined.Sequential porphyroblast growth and foliation development in the cordierite‐andalusite schists is examined for situations involving rotation and non‐rotation of porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of porphyroblast growths does not differ from the non‐rotational situation. Thus, whether or not porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P‐T‐t paths inferred from the relative timing of porphyroblast growths, remain the same, for the deformational histories evaluated.