bluuebunny/crossref_metadata_2025_split huggingface dataset:
The (Na–Ca)amphibole–albite–chlorite–epidote–quartz geothermobarometer in the system S–A–F–M–C–N–H2O. 2. Applications to metabasic rocks in different metamorphic settings
Authors: TRIBOULET C. , THIÉBLEMONT D. , AUDREN C. 
Published: May 2007

ABSTRACTIn order to illustrate different applications of the amphibole‐albite‐chlorite‐epidote‐quartz geothermobarometer, pressure‐temperature‐time (P–T–t) ± space (P–T–t–s) ± deformation (P–T–t–d) paths have been established from literature data. They are discussed as a function of the chemical, equilibrium and microstructural data available in each case, and compared with the conclusions already established by other methods. It is clear that it is necessary to know the relative chronology of the events (directions of zoning of minerals in successive microstructural positions) to establish precise P–T paths; this enables reconstruction of complex geodynamic histories. From this point of view, it is necessary to analyse the maximum possible number of minerals in a few well‐chosen metabasic rocks showing different generations of blastesis. The rocks should belong to different tectonic units to obtain the best overall picture of a metamorphic complex.
Archaean granulite facies metamorphism of the Lewisian of Tiree, Inner Hebrides, north‐west Scotland
Authors: CARTWRIGHT I. 
Published: May 2007

The Lewisian of Tiree, north‐west Scotland, underwent granulite facies metamorphism prior to 2.4 Ga. The temperatures and pressures estimated from garnet–clinopyroxene, garnet–orthopyroxene, hornblende–plagioclase and garnet–biotite geothermometers and clinopyroxene–plagioclase–garnet–quartz and orthopyroxene–plagioclase–garnet–quartz geobarometers are 810 ± 50° C and 10.5 ± 1.5 kbar. The imprecision of pressure estimates stems largely from uncertainties in garnet activity models. Calculations of blocking temperatures for Fe–Mg interdiffusion in clinopyroxene and garnet suggest that these temperatures and pressures represent only slightly reset peak‐metamorphic conditions.Down‐temperature re‐equilibration resulted in chemical zoning over the outer 50–100 μm of the mafic minerals. P–T paths calculated from this mineralogical zoning suggest nearly isobaric cooling. However, the growth of late sillimanite in metapelites requires that the retrograde P–T path had a significant decompression component, suggesting that the mineralogical zonation does not define the retrograde P–T path. The discrepancy between the P–T path calculated from mineralogical zonation and that implied by mineral reactions probably results from the net‐transfer geobarometry reactions closing at higher temperatures than the exchange geothermometers.The Tiree rocks have a similar history to the mainland Scourian complex. Granulite facies metamorphism accompanied by partial melting occurred prior to the intrusion of the Scourie dykes at c. 2.4 Ga, and the rocks underwent retrogression both prior to and after dyke emplacement. However, peak metamorphic temperatures and pressures on Tiree were lower than those recorded in the Scourian complex, and the Tiree rocks may have been at a different crustal level at that time.
Variable Variscan thermal rejuvenation in the St Malo region, Cadomian Orogen, France: evidence from 40Ar/39Ar mineral ages
Authors: DALLMEYER R. DAVID , BROWN MICHAEL , D'LEMOS RICHARD S. , STRACHAN ROBIN A. 
Published: May 2007

Abstract
The St Malo region in north‐west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. 40Ar/39Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting.Eleven muscovite concentrates and one whole‐rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record 40Ar/39Ar plateau ages of c. 527 and 521 Ma. An internally discordant 40Ar/39Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain‐Deux Evailles‐Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north‐western sector of the St Malo region exhibit internally discordant 40Ar/39Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south‐eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole‐rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan‐Bobital Granite Complex, yields a 40Ar/39Ar plateau age of c. 307 Ma.The 40Ar/39Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.
High‐pressure eclogites in northern Jiangsu – southern Shandong province, eastern China
Authors: ENAMI MASAKI , ZANG QUIJA , YIN YUJUN 
Published: May 2007

Abstract
Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino‐Korean and Yangtze cratons in central and eastern China. These eclogites usually have high‐P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high‐P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO2 phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO2 phase, and are divided into prograde eclogites and mantle‐derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle‐derived eclogites have high MgO/(FeO + Fe2O3) and Cr2O3 compositions in bulk rock and minerals, and consist mainly of pyrope‐rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular‐rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile.Pargasitic and taramitic amphiboles, calcic plagioclase (An68), epidote, zoisite, K‐feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near‐isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high‐P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near‐isothermal decompression during the retrograde stage. The conditions at which symplectites re‐equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle‐derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.
An empirical estimate of the diffusion rate of oxygen in diopside
Authors: SHARP Z. D. , JENKIN G. R. T. 
Published: May 2007

ABSTRACTThe intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9 ± 0.3‰ vs. SMOW) over the entire measured size range (0.3–3.2 mm diameter). The δ18O values of the cores of calcite grains are 23.0‰. Temperature estimates based on the Δ18O(calcite‐diopside) are 800d̀C, in agreement with the highest previous thermometric estimates for these rocks.The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates (D800d̀C parallel to the c‐axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 times 10‐20cm2/s is two orders of magnitude faster than from the FRM; at Q=500kJ/mol, the D800d̀C estimate for both methods is c. 5.6 times 10‐20 cm2/s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct.The δ18O(SMOW) and δ13C(PDB) values of the calcite, measured in situ with a CO2 laser, are 22.9 ± 0.3, 0.1±0.3‰ in the grain cores, 22.1 ±0.3, 0.2 ±0.1‰ at the grain boundaries and 21.7 ±0.4, ‐0.6±0.1‰ abutting diopside grains. The δ18O and δ13δC values measured conventionally are: crystal cores, 22.96, ‐0.95‰; abutting diopside grains, 22.38, ‐0.93‰; bulk, 22.79, ‐0.95%. Use of the bulk δ18O(calcite) values for thermometry yields unreasonably high temperatures. The lower δ18O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a result of a late retrograde fluid infiltration.
A calculated petrogenetic grid for the system K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O, with particular reference to contact‐metamorphosed pelites
Authors: XU GUOWEI , WILL T. M. , POWELL R. 
Published: May 2007

ABSTRACTA 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.
Phase relations in andalusite‐sillimanite type Fe‐rich metapelites: Tono contact metamorphic aureole, northeast Japan
Authors: OKUYAMA‐KUSUNOSE Y. 
Published: May 2007

ABSTRACTMineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2‐3 kbar. The bulk rock compositions of metapelites are aluminous with A′ values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low‐grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of Al2SiO5 polymorphs is limited in typical pelitic rocks.The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5–0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K‐feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite‐sillimanite type aureoles. Rocks with FeO/(FeO + MgO) > 0.8 progressively develop chloritoid‐bearing assemblages from Bt‐Chl‐Cld, And‐Bt‐Cld, to And‐Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low‐temperature side of the formation of cordierite by the terminal equilibrium of chlorite. A P‐T model depicting the relative stability of chloritoid and staurolite at low‐ and medium‐pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, chloritoid, garnet and staurolite.
Sm‐Nd isotopic evidence on the age of eclogitization in the Zermatt‐Saas ophiolite
Authors: BOWTELL S. A. , CLIFF R. A. , BARNICOAT A. C. 
Published: May 2007

ABSTRACTRocks within the Zermatt‐Saas ophiolite of the western Alps have undergone eclogite facies metamorphism during subduction prior to the Alpine collision. The metamorphic history of these rocks is well defined, with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite. Subsequent greenschist alteration occurs adjacent to faults, veins and metasediments. Away from such sources of water, retrogression is very limited. Sm‐Nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 ± 18 Ma. The large uncertainty is due to the presence of very small amounts of Nd‐rich epidote present as inclusions within garnet. As the closure temperature of garnet to Sm & Nd is thought to be >600d̀C, resetting due to post‐high‐pressure diffusion is thought to be insignificant. Given the fine‐grained protolith to the sample analysed, and its extensive deformation under eclogite facies conditions, incomplete homogenization of pre‐metamorphic isotopic variations is also considered unlikely to be responsible for the young age. A Tertiary age of eclogitization means that models of early Alpine evolution based on the cessation of high‐pressure metamorphism in the Cretaceous need to be revised.
Incipient metamorphism and deformation in the Variscides of SW Dyfed, Wales: first steps towards isotopic equilibrium
Authors: SLATER D. J. , YARDLEY B. W. D. , SPIRO B. , KNIPE R. J. 
Published: May 2007

ABSTRACTIncipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew.Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200‐310d̀ C (slightly higher in the Marloes‐Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6‐13 km.Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes‐Musselwick Thrust Sheet are distinctly lighter (δ18Oveins=+14 to +18% and δ18Omatrix=+11 to +14%) than those from other thrust sheets (δ18O =+17 to +20% and +14 to +17%, respectively).We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large‐scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here.
Fluid inclusion evidence for an anticlockwise metamorphic P‐T path in central Massachusetts
Authors: WINSLOW D. M. , BODNAR R. J. , TRACY R. J. 
Published: May 2007

ABSTRACTAcadian (Late Silurian to Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low‐P, high‐T (Buchan‐type) metamorphism followed by thickening at high temperature (>650d̀ C) and then by cooling to 100‐200d̀ C below peak recorded temperatures before eventual unroofing. Mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, replacement of low‐P cordierite‐bearing assemblages by high‐P garnet‐bearing assemblages, and recrystallization of mylonites associated with late shear zones to form lower‐T and higher‐P assemblages. Peak conditions in the highest grade rocks were 685‐780d̀ C and 5‐6 kbar; the cooling path passed through 550d̀ C at about 6.5 kbar.The well‐constrained P‐T path documented from geological and mineralogical evidence for the CMT offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long‐lived metamorphic event spanning a broad range of P‐T conditions. Fluid inclusion data from the CMT document a range of fluid compositions (CO2‐rich, mixed CO2‐N2‐rich, N2‐rich and H2O‐rich) and densities during metamorphism. Densities of CO2 fluid inclusions range from 0.20 to 1.03 g cm‐3. Medium‐density CO2 fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650‐700d̀ C at pressures below 5 kbar. Other CO2 fluid inclusions from matrix quartz yield isochores which pass through 700d̀ C and 5.2 kbar. The highest density inclusions associated with rocks containing the late high‐P assemblages have isochores which pass below the estimated P‐T conditions for recrystallization of the mylonite. Fluid inclusion evidence suggests an early low‐P heating event, followed by thickening at high temperature, and then by nearly isobaric cooling to about 500d̀ C with later decompression. This interpretation is also consistent with previously published petrological models and supports an anticlockwise P‐T path for the CMT of south‐central Massachusetts.
Metapelitic granulites from Jetty Peninsula, east Antarctica: formation during a single event or by polymetamorphism?
Authors: HAND M. , SCRIMGEOUR I. , POWELL R. , STÜWE K. , WILSON C. J. L. 
Published: May 2007

ABSTRACTGranulite facies metasedimentary gneiss exposed on Jetty Peninsula, east Antarctica, contains assemblages involving garnet‐sillimanite‐biotite‐cordierite‐spinel‐ilmenite‐rutile and garnet‐orthopyroxene‐cordierite‐biotite, as well as quartz and K‐feldspar. Peak assemblages involve garnet + sillimanite + ilmenite (±rutile) and garnet + orthopyroxene. P‐T calculations suggest formation conditions of approximately 800d̀ C at 7‐7.5 kbar. Cooling from peak conditions is suggested by biotite + garnet (±sillimanite) overprinting some peak assemblages. A subsequent increase in temperature is inferred from the formation of cordierite + garnet + biotite + ilmenite, garnet + sillimanite + cordierite + ilmenite and cordierite + orthopyroxene assemblages during D2. In slightly zincian bulk compositions, hercynitic spinel + cordierite + sillimanite constitutes the peak D2 assemblage. Average pressure calculations indicate peak pressures of 5.9 ±0.4 kbar at 700d̀ C for the cordierite‐bearing D2 assemblages. Available radiometric data suggest that peak metamorphism occurred at c. 1000 Ma and D2 occurred after 940 ± 20 Ma. The following two possibilities exist for the metamorphic evolution. (1) The formation of the lower pressure cordierite‐bearing assemblages is associated with a separate metamorphic event (M2), unrelated to the peak assemblage (M1), and the lower pressure assemblages have no relevance in terms of a single tectonothermal event. (2) The cordierite‐bearing assemblages formed during a progression from peak conditions. In this case, the lower pressure assemblages reflect a broadly decompressional metamorphic evolution, during which temperatures fluctuated. Comparison with P‐T paths from granulites of similar age in adjacent areas suggests that the second possibility should be preferred. The cooling interval between peak conditions and the development of cordierite‐bearing coronas and symplectites suggests affinities with isobarically cooled granulites of similar age immediately to the west, and the low‐P/high‐T post‐peak conditions are similar to the later stages of decompressional paths recognized in much of east Antarctica.
Rb‐Sr and U‐Pb isotope studies on migmatites from the Schwarzwald (Germany): constraints on isotopic resetting during Variscan high‐temperature metamorphism
Authors: KALT A. , GRAUERT B. , BAUMANN A. 
Published: May 2007

ABSTRACTMineral and isotope studies were undertaken on migmatites from the Schwarzwald, Moldanubian zone of the Variscan belt. The aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting. Textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre‐scale migmatite profiles, respectively, suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation (deformation‐enhanced mass transport) rather than by partial melting (anatexis). The results of Rb‐Sr thin‐slab dating on these profiles indicate that Sr isotopes were not completely reset during migmatization. No true isochron ages, but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating 87Sr/86Sr ratios back to various stages in their evolution. The coincidence of these Rb‐Sr data with U‐Pb ages of monazites from migmatites and non‐migmatitic gneisses shows that gneisses and migmatites were formed during the same high‐temperature event in the Carboniferous (330‐335 Ma). The observation that high‐temperature metamorphism failed to equilibrate Sr isotopes on the centimetre‐scale imposes limitations on the use of conventional whole‐rock isochron techniques in dating migmatites.