bluuebunny/crossref_metadata_2025_split huggingface dataset:
Thermobarometry and granite genesis: the Hercynian low‐P, high‐T Velay anatectic dome (French Massif Central)
Authors: MONTEL J. M. , MARIGNAC C. , BARBEY P. , PICHAVANT M. 
Published: May 2007

The Velay dome (French Massif Central) offers a quasi‐continuous section across an anatectic domain comprising low‐ to high‐grade schists, gneisses and granites. Two main tectonometamorphic events, and their related generation of granitic material, were recognized in addition to a major Barrrovian tangential event (D2) attributed to intracontinental collision tectonics: (i) a medium‐ to low‐P, high‐T event (D3) which gave rise to migmatites and syntectonic monzonitic granites and granodiorites, and (ii) a widespread melting event (D4) which led to the generation of migmatities, the Velay granite and post‐anatectic granites.Thermobarometry on samples collected from both the metamorphic envelope and the granitic core distinguishes two distinct geotherms: (i) a first, associated with the D3 event, characterized by P > 5 kbar, T≤ 750° C and water‐present melting (biotite remains stable) which led to large‐scale migmatization but minor amount of granites; (ii) a second, associated with the D4 event and characterized by vapour‐absent melting (P= 4–5 kbar, T= 760–850° C) which gave rise to the Velay granites and late‐migmatitic granites. The temperature increase during the D4 event is attributed to the intrusion of hot mafic magmas within the crust.The time‐integrated features of the different granitic rocks in the Velay dome can be directly related to aH2O in the source region and illustrate the progressive dehydration of a middle to lower crustal segment over 60 Ma.
P–T–X relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria
Authors: BERTRAND P. , OUZEGANE KH. , KIENAST J. R. 
Published: May 2007

The Al–Mg‐rich granulites from the In Ouzzal craton, Algeria, show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the FeO–MgO–Al2O3–SiO2 system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a high‐P stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel‐bearing assemblages; (2) a high‐T stage characterized by the development of Sa–Qz‐bearing assemblages; and (3) a later stage, in which garnet‐bearing assemblages are replaced by more or less fine symplectites involving cordierite.During the course of early mineral reactions, the distribution coefficient, Kd, between the various ferromagnesian phases decreased significantly whereas Al2O3 in pyroxene increased concomitantly. These observations, when combined with topological constraints, clearly indicate that the high‐P stage 1 was accompanied by a significant rise in temperature (estimated at 150 ± 50° C) under near isobaric conditions, in agreement with the reaction textures. By stage 2, pressure and temperature were extreme as evidenced by the low Kd value between orthopyroxene and garnet (Kd= 2.06–1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [100 Mg/(Mg + Fe) = 60.6]. Mineral thermometry based on Fe–Mg exchange between garnet and pyroxene and on Al‐solubility in pyroxene gives temperatures close to 970 ± 70° C at 10 ± 1.5 kbar. These results are in agreement with the development of Sa–Qz assemblages on a local scale.Late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar. Except for local re‐equilibration of Mg and Fe at grain boundaries, there is no evidence for further reactions below 700° C.We interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c. 2 Ga. Because of the lack of evidence for further uplift after the thermal relaxation which occurred at c. 6 kbar, it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation.
Charnockitic alteration: evidence for CO2infiltration in granulite facies metamorphism
Authors: NEWTON R. C. 
Published: May 2007

Charnockitic alteration (arrested orthopyroxene formation in biotite‐ and amphibole‐bearing rocks) occurs in high‐grade terranes of all ages. Three criteria are used to show that this alteration was produced in many locations by a migrating fluid phase: (i) diffuseness of the alteration—the alteration zones are often quite unlike discrete migmatitic veins; (ii) relation to deformation—most occurrences show alteration closely associated with warping of foliation or dilation cracks; (iii) open‐system alteration—whilst some occurrences represent nearly isochemical alteration, slight changes in bulk composition, often loss of mafic constituents and gain of Na and Si, are evident in detailed mass‐balance analysis. Y and sometimes Rb are characteristically depleted. Partial melting sometimes accompanied volatile infiltration, as evidenced by more discrete veins and euhedral orthopyroxene. It is quite unlikely, however, that open‐system alteration was produced by escape of viscous quartzo‐feldspathic melts. Pervasive migration of low‐Tlamprophyric (mafic–alkaline, CO2‐charged) interstitial liquids is a possibility by virtue of their extreme fluidity, but CO2infiltration was needed to generate these liquids. Vapour‐deficient dehydration melting is another feasible mechanism of orthopyroxene formation which may have operated in conjunction with CO2infiltration.Characteristic development of charnockitic alteration in some prograde amphibolite to granulite facies transitions, as in the Dharwar Craton of South India, suggests that the alteration is a fundamental feature of the granulite facies metamorphism, implying active and causal participation of migrating fluids. In other high‐grade terranes like the Adirondack Mountains of New York, this kind of alteration is rare, and fluid action does not seem to have been important in the metamorphism.A vapour phase participating in charnockitic metamorphism was necessarily one of relatively low H2O, therefore presumably rich in CO2. Consideration of possible large CO2sources leads to the conclusion that emanations from volatile‐rich basalts emplaced in the lower crust are the most probable source of charnockitizing fluids. The ultimate source would therefore be enriched subcontinental lithosphere or asthenosphere. The Rb‐depleted pyroxene gneiss (charnockitic) terranes may be characteristic of zones of large‐scale transcurrent or oblique‐motion faults which tap such great depths.
Tertiary deep crustal ultrametamorphism in the Hidaka metamorphic belt, northern Japan
Authors: OSANAI Y. , OWADA M. , KAWASAKI T. 
Published: May 2007

The Main Zone of the Hidaka metamorphic belt is an imbricate stack of crustal material derived from an island arc in which a sequence of units with increasing metamorphic grade from low to high structural levels is exposed. The basal part of the metamorphic sequence underwent granulite facies metamorphism with peak P–T conditions of 7kbar, 870°C. In this zone pelitic granulite includes leucosomes which consist mainly of orthopyroxene‐plagioclase‐quartz.To test whether the leucosome was derived by partial melting of the surrounding pelite, melting experiments of the pelitic granulite were carried out for water‐saturated and dry systems at 7 kbar and 850°C. The chemical composition of the leucosome produced during these runs shows a peraluminous S‐type tonalitic affinity and is located very close to the tie‐line between the average melts produced in water‐saturated systems and the average composition of the residual orthopyroxene + plagioclase. This therefore suggests that the lecosome in pelitic granulite was formed by incipient anatexis at close to the highest P–T condition of the Main Zone.The age of the crustal anatexis is determined by the Rb‐Sr whole rock isochron method for garnet‐cordierite‐biotite gneiss (host rock), garnet‐orthopyroxene‐cordierite gneiss (restite) and S‐type tonalite (melt). This gives an age of 56.0 Ma with an initial 87Sr/86Sr ratio of 0.705711. The S‐type tonalite magmas that form large intrusive masses in the Main Zone were probably generated by crustal anatexis in deeper parts of the crust at the same time (late Palaeocene).
Calcite–graphite isotope thermometry: a test for polymetamorphism in marble, Tudor gabbro aureole, Ontario, Canada
Authors: DUNN S. R. , VALLEY J. W. 
Published: May 2007

ABSTRACTGraphitization and coarsening of organic material in carbonate‐bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory, pressure‐independent geothermometer. Comparison of observed isotopic fractionations between calcite and graphite (δ13CCal–Gr) with independent petrological thermometers provides the following empirical calibration over the range 400–800°C: δ13CCal–Gr= 5.81 times 106×T–2(K) ‐ 2.61. This system has its greatest potential in marbles where calcite + graphite is a common assemblage and other geothermometers are often unavailable. The temperature dependency of this empirical calibration differs from theoretical calibrations; reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications.Both calcite‐graphite isotopic thermometry and calcite‐dolomite solvus thermometry are applied to marble adjacent to the Tudor gabbro in the Grenville Province of Ontario, Canada. The marble has undergone two metamorphic episodes, early contact metamorphism and later regional metamorphism. Values of δ13CCal–Gr decrease regularly from c. 8‰ in samples over 2 km from the pluton to values of 3–4‰ within 200 m of the contact. These samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer, and indicate temperatures of 450–500° C away from the intrusion and 700–750°C near the gabbro. This thermal profile around the gabbro is consistent with conductive heat flow models. In contrast, the distribution of Mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c. 500°C, even at the contact.Graphite textures are important for interpreting the results of the calcite–graphite thermometer. Coarsening of graphite approaching the Tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion. In contrast to isotopic exchange during prograde metamorphism, which is facilitated by graphitization, retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally. Resistance of the calcite–graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems.
The metamorphic history of the Vredefort dome at approximately 2 Ga as revealed by coesite‐stishovite‐bearing pseudotachylites
Authors: MARTINI J. E. J. 
Published: May 2007

ABSTRACTThe Vredefort dome (2.0 Ga) represents the central uplift of a very large impact structure. This uplift exposed a nearly complete cross‐section through the continental crust in the region, which is 25–30 km thick. Two metamorphic events took place at about the same time as the impact. The first event, so‐called static metamorphism, is pre‐impact and produced lithologies varying from low‐grade shale to high‐grade hornfels. It resembles contact metamorphism by its lack of schistosity, but is more regional as it extends over a large area and is not associated with large intrusions.The second event, the post‐shock metamorphism, is responsible for the recrystallization of the shock features. The investigation of this event has been focused on the degree of alteration of the coesite‐stishovite‐bearing pseudotachylite veins that formed during the transit of the shock wave. These high‐pressure silica polymorphs are only present in the upper part of the stratigraphic sequence; downward they have been converted to fibrous quartz. At the highest grade, the fibrous quartz is in turn replaced by triple‐junctioned mosaic quartz. The post‐shock metamorphism was generated by the heat of the rock before shock, plus the heat released by the shock wave. The isograds, plotted on a map, can be translated into depth of burial and therefore provide valuable information regarding the geological setting immediately before impact. At the time of impact, the rocks were relatively cool and the static metamorphism had ceased with several tens of millions of years separating the two metamorphic events. The static metamorphism was probably caused by continental crustal extension in a stress‐free environment and the lack of deformation is probably due to rapid uplift during the later stages of the impact event.
Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south‐eastern San Gabriel Mountains, southern California
Authors: BARTH A. P. , MAY D. J. 
Published: May 2007

ABSTRACTMid‐Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south‐eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc‐silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic‐intermediate magmatism and high‐grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near‐isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo‐chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma‐1. The geotectonic evolution of the Cucamonga granulites is similar to mid‐Cretaceous high‐P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.
Experimental study of the stability of sudoite and magnesiocarpholite and calculation of a new petrogenetic grid for the system FeO–MgO–Al2O3–SiO2–H2O
Authors: VIDAL O. , GOFFÉ B. , THEYE T. 
Published: May 2007

ABSTRACTThe solid‐solid reaction magnesiocarpholite = sudoite + quartz has been bracketed between 350 and 500°C, 6.3 and 7.8 kbar. Because it is impossible to synthesize end‐member sudoite, all experiments were carried out using natural minerals as starting materials. Although mineral compositions were very close to those of the end‐members, the effect of the fluorine content in carpholite was significant. Particularly in those experiments where sudoite grows at the expense of carpholite, electron microprobe analysis of the run products shows that a more stable F‐rich carpholite crystallizes too, and consumes the fluorine released in solution by the breakdown of the original carpholite.Our experimental results are combined, through a thermodynamic analysis, with a previous data set and with previous experimental data concerning the relative stability of chlorite, talc and magnesiocarpholite with excess of quartz and water as a function of P–T and AlAl(SiMg)‐1 substitutions in phyllosilicates. This allows us to constrain the feasible thermodynamic parameters (H°f, sud; S° sud) and (H°f,car; S°car) for the Mg end‐members. Using the partition coefficients calculated from natural parageneses, we have computed a petrogenetic grid for the system FeO–MgO–Al2O3–SiO2–H2O. It demonstrates that parageneses involving sudoite and carpholite can be used as indicators of P–T conditions, up to 600° C, 8 kbar for sudoite, and at higher pressure for carpholite.
Stable isotopic and petrological constraints on scapolitization of the Whitestone meta‐anorthosite, Grenville Province, Ontario*
Authors: MOECHER D. P. , ESSENE E. J. , VALLEY J. W. 
Published: May 2007

The Whitestone Anorthosite (WSA), located in the Central Gneiss Belt of the south‐western Grenville Province, Ontario, exhibits a nearly concentric metamorphic envelope characterized by an increase in modal scapolite, hornblende, epidote and garnet, developed around a core of granulite facies clinopyroxene ± orthopyroxene ± garnet meta‐anorthosite. Scapolite‐ and hornblende‐bearing assemblages develop mainly at the expense of plagioclase and pyroxene within the envelope.Stable isotopic and petrological data for scapolite‐bearing mineral assemblages within meta‐anorthosite constrain the source of carbon responsible for CO3‐scapolite formation and the extent of fluid/rock interaction between the anorthosite and adjacent lithologies. Stable isotopic data indicate increasing δ18O and δ13C from core to margin of the meta‐anorthosite and for samples from the southern extension of the WSA, where it is ductilely deformed within the Parry Sound Shear Zone (PSSZ). The average δ18OSMOW value (whole rock) for the WSA core is 6.9‰, increasing to 11.5‰ where the WSA is in tectonic contact with marble breccia. The average δ13CPBD value of scapolite in meta‐anorthosite from the centre of the WSA is ‐3.4‰, increasing to ‐0.5‰ at the eastern (marble) contact. Average values of δ13C for scapolite and whole‐rock δ18O for samples from the shear zone are ‐1.0 and 8.0‰, respectively. Marbles have average δ18O and δ13C values of 19.2 and ‐0.4‰, respectively.The sulphate content of texturally primary scapolite decreases from the core of the WSA (XSO4= 0.48) to the eastern contact (≤0.05). Texturally late scapolite after plagioclase and garnet tends to be CO3‐rich relative to texturally primary scapolite, and some scapolite grains show zoning in the anion site with CO3‐enriched rims. Scapolite composition may vary at any scale from a single grain to outcrop.The pattern of isotopic enrichment in 13C and 18O preserved in the eastern margin of the WSA is consistent with marble as the major source of fluid contributing to the formation of the metamorphic envelope. The decrease in XSO4 and increase in XCO3 in scapolite toward the margin of the WSA indicate that the volatile content was reset by, or developed from, a CO2‐bearing fluid. Assuming derivation of fluid from marble, minimum fluid/rock values at the margin of the WSA range from 0.03 for the least enriched, to 0.30 for the most isotopically enriched samples. Although marble is not found in immediate contact with samples of sheared meta‐anorthosite from the PSSZ, a marble source is also consistent with the C and O isotope composition and anion chemistry of scapolite within these samples.
Granulite facies metamorphism, palaeo‐isotherms and disturbance of the U‐Pb systematics of zircon in anorogenic plutonic rocks from the Adirondack Highlands
Authors: CHIARENZELLI J. R. , McLELLAND J. M. 
Published: May 2007

Abstract
In the Adirondack Highlands of New York State, the effect of granulite facies metamorphism on the physical and isotopic characteristics of zircon from anorogenic plutonic rocks has a distinct geographical pattern. The location of zircon populations which appear to have been altered describes a roughly circular area where metamorphic palaeotemperatures have been determined to be in excess of 750° C. Zircons from anorogenic plutonic rocks outside this area were undisturbed during metamorphism and yield well constrained ages.Granitic, charnockitic and mangeritic anorogenic plutonic rocks peripheral to the Marcy anorthosite massif have large, euhedral, prismatic zircons that display fine, internal, magmatic growth zonations and abundant, randomly orientated, mineral inclusions. Co‐genetic zircon fractions yield linear discordant arrays and well constrained upper intercepts of 1125–1157 Ma. Metamorphic zircon is limited to sporadically developed and volumetrically insignificant, clear, low‐U overgrowths or protuberances.In marked contrast, zircons from petrographically and geochemically identical rocks adjacent to, or within, the Marcy anorthosite massif are typically large, limpid, anhedral to subhedral crystals or crystal fragments lacking internal features except for tubular cavities and CO‐2‐rich inclusions. Co‐genetic zircon fractions yield nearly concordant, non‐linear clusters with 207Pb/206Pb minimum ages of 1073–1095 Ma. Metamorphic overgrowths cannot be readily identified by optical or cathodoluminscence techniques; however, many grains show complex and unusual external boundaries suggestive of post‐crystallization modification.These data indicate that temperatures as low as 750° C, in combination with other factors, may have been sufficient to facilitate recrystallization, and diffusion of radiogenic Pb from the zircon crystal structure, during the complex, protracted metamorphism of the Adirondack Highlands.
Metamorphism of a Late Jurassic volcano‐plutonic arc, northern California, USA
Authors: BEIERSDORFER R. E. 
Published: May 2007

Abstract
The metamorphic history of the Middle to Upper Jurassic volcanic and hypabyssal rocks exposed in the Klamath Mountains and Sierra Nevada of California is related, in part, to the rifting of a volcano‐plutonic arc. The Callovian to Kimmeridgian rocks exposed in the region consist of, from north‐west to south‐east, a back‐arc ophiolite, a rifted volcanic arc and a volcanic arc complex. All of these units have been metamorphosed and contain various combinations of the phases chlorite, amphibole, epidote, prehnite and pumpellyite. Projection of coexisting phases onto the composition plane MgO/(MgO + FeO) and Al2O3+ Fe2O3 ‐ 0.75 CaO ‐ Na2O through quartz, water, albite and epidote results in consistent mineralogical compatibilities within each region, but crossing tie‐lines between regions. This suggests that the volcanic and hypabyssal rocks from each region have equilibrated under different intensive conditions.The back‐arc ophiolite in the north has suffered subseafloor high‐T/P hydrothermal metamorphism with geothermal gradients on the order of 100° C km−1. The rifted volcanic arc has undergone synchronous burial, hydrothermal and contact metamorphism. Metamorphic field gradients in the region pass through the prehnite‐pumpellyite and greenschist facies suggesting geothermal gradients on the order of 30° C km−1. The southernmost volcanic arc complexes contain metavolcanics of the pumpellyiteactinolite and greenschist facies suggesting moderate‐ to high‐P/T metamorphism and geothermal gradients on the order of 20° C km−1.The apparent increase in rifting and calculated geothermal gradients from south‐east to north‐west suggest that the observed very low‐ and low‐grade metamorphism may be a response to enhanced thermal gradients during extension of the volcanic arc. This correlation between the extent of rifting and metamorphism is consistent with a model of diastathermal metamorphism of a propagating rift along the western margin of North America during the Late Jurassic. The plate tectonic setting may be analogous to the present‐day Andaman Sea region.