bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluid inclusion constraints on the uplift history of the metamorphosed massive sulphide deposits at Ducktown, Tennessee
Authors: HALL D. L. , BODNAR R. J. , CRAIG J. R.
Published: May 2007

Abstract
Standard petrographic, microthermometric and Raman spectroscopic analyses of fluid inclusions from the metamorphosed massive sulphide deposits at Ducktown, Tennessee, indicate that fluids with a wide range of compositions in the C–O–H–N–S–salt system were involved in the syn‐ to post‐metamorphic history of these deposits. Primary fluid inclusions from peak metamorphic clinopyroxene contain low‐salinity, H2O–CH4 fluids and calcite, quartz and pyrrhotite daughter crystals. Many of these inclusions exhibit morphologies resembling those produced in laboratory experiments in which confining pressures significantly exceed the internal pressures of the inclusions. Secondary inclusions in metamorphic quartz from veins, pods, and host matrix record a complex uplift history involving a variety of fluids in the C–O–H–N–salt system. Early fluids were generated by local devolatilization reactions while later fluids were derived externally.

Isochores calculated for secondary inclusions in addition to the chronology of trapping and morphological features of primary and secondary fluid inclusions suggest an uplift path which was concave toward the temperature axis over the P–T range 6–3 kbar and 550–225° C. Immiscible H2O–CH4–N2–NaCl fluids were trapped under lithostatic to hydrostatic pressure conditions at 3–0.5 kbar and 215 ± 20° C. Entrapment occurred during Alleghanian thrusting, and the fluids may have been derived by tectonically driven expulsion of pore fluids and thermal maturation of organic material in lower‐plate sedimentary rocks which are thought to underlie the deposits. Episodic fracturing and concomitant pressure decreases in upper‐plate rocks, which host the ore bodies, would have allowed these fluids to move upward and become immiscible. Post‐Alleghanian uplift appears to have been temperature‐convex.

Uplift rates of 0.10–0.05 mm year−1 from middle Ordovician to middle Silurian – late Devonian, and 0.07–0.12 mm year−1 from middle Silurian – late Devonian to late Permian are suggested by our uplift path and available geochronological data.
Tectonometamorphic evolution of the Sebadani eclogitic metagabbro and the Sambagawa schists, central Shikoku, Japan: 40Ar/39Ar mineral age constraints
Authors: DALLMEYER R. D. , TAKASU A.
Published: May 2007

Abstract
The Sambagawa metamorphic belt exposed in central Shikoku records a high‐P–T metamorphic event. It is represented by the Oboke nappe and structurally overlying, internally imbricated, Besshi nappe complex. These major structural units are in ductile thrust contact. A melange is developed along a ductile internal tectonic contact within the Besshi nappe complex. Tectonic emplacement of a high‐T enclave (Sebadani eclogite) in the melange zone resulted in the development of a contact metamorphic aureole within the host Sambagawa rocks.

36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende from the Sambagawa basic schists which surround the Sebadani enclave are 83.4 ± 0.3 Ma (within contact aureole) and 83.6 ± 0.5 Ma (outside aureole). 40Ar/39Ar plateau ages recorded by muscovite from the same samples are 87.9 ± 0.3 and 89.3 ± 0.4 Ma. Amphibole from the amphibolite within the Sebadani enclave records isotope correlation ages of 93.7 ± 1.1 and 96.5 ± 0.7 Ma (massive interior) and 84.6 ± 1.2 Ma (marginal shear zone). Amphibole within the massive amphibolite is significantly higher in XMg than that within the host Sambagawa basic schists. The older ages recorded by amphibole within the Sebadani enclave are interpreted to date cooling through somewhat higher closure temperatures than which characterize the more Fe‐rich amphibole in surrounding schists. The younger amphibole age recorded within the marginal shear zone probably indicates that crystallization of amphibole continued until cooling through the relatively lower amphibole closure temperatures.

These results, together with the previously published 40Ar/39Ar ages of the Sambagawa schists, suggest: (i) metamorphic culmination occurred in the Besshi nappe complex at c. 100–90 Ma; (ii) at c. 95 Ma the Besshi nappe complex was internally imbricated and tectonic enclaves were emplaced; (iii) at c. 85 Ma, the composite Besshi nappe was rapidly exhumed and tectonically emplaced over the Oboke nappe (which attained peak metamorphic conditions at c. 75 Ma); (iv) the Besshi and Oboke nappe complexes were further exhumed as a coherent tectonic unit and unconformably overlain by the Eocene Kuma Group at c. 50 Ma.
Shear sense
Authors: BELL T. H. , JOHNSON S. E.
Published: May 2007

Investigation of microstructural relationships in major movement zones in metamorphic rocks, where the sense of displacement is known from regional geological relationships, indicates numerous problems with current concepts of shear‐sense criteria and their application. The direction of apparent shearing commonly conflicts from one criterion to another (e.g. from the symmetry of quartz c‐axis orientation diagrams to the asymmetry of extensional crenulation cleavages). This implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous.

A new approach to interpreting shear sense, involving the use of strain fields, resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear‐sense histories extending back before the last shearing event for ‘any foliated metamorphic rock’. It also provides a powerful tool for determining the structural/metamorphic path that a rock has followed within an orogen. For determination of the shear sense on the last foliation developed in a rock, this approach uses geometries developed around competent heterogeneities such as quartz pebbles, pegmatite pods, veins, porphyroclasts, porphyroblasts and breccia clasts. A shear‐sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows.
P–T evolution of the Teslin suture zone and Cassiar tectonites, Yukon, Canada: evidence for A‐ and B‐type subduction
Authors: HANSEN V. L.
Published: May 2007

Mineral composition and quantitative thermobarometric studies indicate that the Teslin‐Taylor Mountain and Nisutlin terranes within the Teslin suture zone (TSZ), Yukon, record widespread high‐P/T metamorphic conditions consistent with subduction zone dynamothermal metamorphism. The highest P–T conditions (575–750° C and 9–17 kbar) are preserved in tectonites formed during normal dip‐slip ductile shear. Dextral strike‐slip tectonites record lower P–T conditions (400–550° C and 5–8 kbar), and tectonites which show reverse shear have peak temperatures of c. 420° C and a minimum peak pressure of 3 kbar. Dynamothermal metamorphism took place in a west‐dipping B‐type subduction zone outboard of western North America in Permo‐Triassic time. TSZ tectonites were underplated against the hangingwall plate of the subduction zone. Following subduction of the ocean basin which separated North America from the hangingwall plate, TSZ tectonites were overthrust eastward as a coherent structural package as a result of A‐type subduction of Cassiar strata in early Jurassic time.

(Par)autochthonous Cassiar tectonites, which comprised the leading edge of the western North American margin, record prograde moderate‐P, high‐T metamorphism (550–750° C and 7–13 kbar) synchronous with top‐to‐the‐east ductile shear. Metamorphism occurred as a result of subduction of the North American margin into the TSZ subduction zone in early Jurassic time. Following metamorphism Cassiar tectonites cooled slowly from 500 to 300° C during the period middle Jurassic to middle Cretaceous.

TSZ and Cassiar tectonites were deformed during changing P–T conditions. Data from each of these tectonite packages indicate that grain‐scale strain partitioning may have allowed local recrystallization of individual minerals by the addition of mechanical energy. The composition of the new grains reflects the P–T conditions under which that particular grain was deformed.
Segregation of leucogranite microplutons during syn‐anatectic deformation: an example from the Taylor Valley, Antarctica
Authors: ALLIBONE A. H. , NORRIS R. J.
Published: May 2007

A suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn‐anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre‐scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre‐thick stromatic leucosomes. Vein‐ and dyke‐like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome‐to‐rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite ‘microplutons’ up to 50 m across beneath refractory ‘cap‐rock’ layers.

Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranite‐bearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10–50‐m‐wide ‘microplutons’ of xenolith‐rich leucogranite.

Lack of vapour‐absent melting and consequent low melt‐to‐rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome ‘microplutons’ did not exceed 50 m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic‐scale granitoids.
Rotated staurolite porphyroblasts in the Littleton Schist at Bolton, Connecticut, USA
Authors: BUSA M. D. , GRAY N. H.
Published: May 2007

Staurolite porphyroblasts, 1.5–8cm in length and 0.3–2cm in width, in the Littleton Schist at Bolton, Connecticut, contain curved quartz inclusion trails which document synkinematic rotations of at least 135°. The orientations of long axes of these staurolite crystals define a weak preferred orientation in a plane approximately parallel to the external foliation. Serial sections of four differently orientated crystals and U‐stage measurements of the orientations of their inclusion trails demonstrate that the inflection hinge line and the statistical ‘symmetry axis’ characterizing the foliation within a porphyroblast are unrelated to the orientations of external crenulations and are, in all cases, parallel to the long axis of the porphyroblast. The cumulative rotation reflected in the curvature of the inclusion trails is a maximum in a c‐axis section through the initial core of a crystal. The amount of rotation about the c‐axis decreases linearly along the length of the crystal away from the nucleation site.

The sense and amount of rotation recorded by a porphyroblast is related to its orientation. A tightly constrained transition from clockwise to anticlockwise rotation defines a slip direction that coincides with the preferred orientation of the staurolite c‐axes. The total rotation reflected by the inclusion trails increases as a function of the angle between the c‐axes of the staurolite crystals and the slip direction.

Initially random staurolite porphyroblasts rotated during growth, as a consequence of laminar shear in the surrounding viscous matrix. This interpretation is quantitatively consistent with: the staurolite preferred orientation; its coincidence with the apparent slip direction; the correlation between both the sense and the amount of rotation and the orientation of the long axis of the porphyroblast; and the twisted conical shape of the family of surfaces defined by the inclusion trails.
Origin of CO2‐rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA
Authors: WHITNEY D. L.
Published: May 2007

CO2–CH4 fluid inclusions are present in anatectic layer‐parallel leucosomes from graphite‐bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water‐rich fluid during high‐temperature metamorphism and migmatization.

CO2‐rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta‐igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO2‐rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post‐entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO2‐rich fluid is attributed to water‐saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al2SiO5+ water‐rich fluid = garnet + melt + CO2–CH4. The presence of CO2‐rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis.

Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high‐temperature metamorphism. The infiltrating fluid was a water‐rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.
Pressure–temperature evolution of lawsonite‐bearing eclogites, Pinchi Lake, British Columbia
Authors: GHENT E. D. , STOUT M. Z. , ERDMER P.
Published: May 2007

Abstract
Blueschists and retrogressed eclogites are located along the Pinchi Fault Zone (near 54°30′N and 124°W) in central British Columbia. The Pinchi Fault separates rocks of contrasting geological histories, and the blueschists and eclogites occur with ultramafic rocks as fault‐bounded blocks. The retrogressed eclogites occur as tectonic blocks, now in glacial debris, a few metres across; blueschists occur in coherent kilometre‐sized tracts. Eclogites contain garnet–omphacite–rutile–quartz, with glaucophane, lawsonite and titanite. Some of the lawsonite appears to be stable with omphacite and garnet. Tectonic blocks of eclogite from two different localities have recorded different P–T histories. In one tectonic block, P–T estimates for garnet inclusions in clinopyroxene and lawsonite (c. 565° C, >13.1 kbar) suggest that garnet and omphacite initially equilibrated outside the stability field of lawsonite. A decrease in temperature, as recorded in garnet rims and matrix clinopyroxene, resulted in crystallization of lawsonite and other retrogressive minerals. Later crystallization of stilpnomelane (locally pseudomorphing garnet), howieite, winchite and actinolite was triggered by an influx of fluid under P–T conditions outside the stability field of garnet. Lawsonite appears to have been stable, suggesting a minimum pressure of about 3 kbar. In the second tectonic block, clinopyroxene inclusions in garnet suggest temperatures near 350° C (P > 10 kbar) and garnet rims equilibrated with matrix clinopyroxene suggest temperatures near 450° C at pressures above c. 12 kbar.
P‐T‐t‐deformation paths recorded by kinzigites during diapirism in the western Variscan belt (Golfe du Morbihan, southern Brittany, France)
Authors: AUDREN C. , TRIBOULET C.
Published: May 2007

Abstract
The high‐grade metamorphic rocks of southern Brittany underwent a complex tectonic evolution under various P‐T conditions (high‐P, high‐T), related to stacking of nappes during Palaeozoic continentcontinent collision.

The east to west thrusting observed in the whole belt is strongly perturbed by vertical movements attributed to the ascent of anatectic granites in the high‐T area. The field reconstruction of subvertical, closed elliptical structures in gneisses and migmatites, associated with the subhorizontal, doubly radial pattern of stretching lineation in the mica schists, suggests the existence of an elliptical diapiric body buried at depth beneath the present erosion level.

Deformation is associated with a complex P‐T evolution partly recorded in aluminous gneisses (kinzigites, e.g. morbihanites). A chronology of successive episodes of mineral growth at different compositions is established by detailed studies of the mineral‐microstructure relationships in X‐Z sections, using the deformation‐partitioning concept (low‐ and high‐strain zones).

Several thermometric and barometric calibrations are applied to mineral pairs either in contact or not in contact but in equivalent microstructiiral positions with respect to the deformation history. This methodology provides a continuous microstructural control of P‐T variations through time and leads to three P‐T‐t‐d paths constructed from numerous successive P‐T estimations. Path 1 is a clockwise retrograde path preserved in low‐strain zones, which records general exhumation movements after crustal thickening. Paths 2 and 3 are clockwise prograde/retrograde paths from high‐strain zones; they are interpreted and discussed in the light of models of crustal anatexis and upward movement of magma (diapirism). Deformation and P‐T effects induced by diapirism can be distinguished from the general deformation‐metamorphic history of a belt, and would seem to be produced during a late stage of its history.

The present microstructural‐petrological approach to defining successive mineral equilibria in relation to progressive deformation steps provides a far more accurate evaluation of the metamorphic evolution than is possible by ‘standard’thermobarometry.
Regional and local patterns of low‐grade metamorphism in the North Shore Volcanic Group, Minnesota, USA
Authors: SCHMIDT S. Th.
Published: May 2007

Abstract
The North Shore Volcanic Group in northern Minnesota is part of the Middle Proterozoic Keweenawan sequence, one of the largest plateau lava provinces in the world. The primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment. The subaerial lava flows, mainly amygdaloidal olivine tholeiites and tholeiites, have undergone low‐grade metamorphism from zeolite to lower greenschist facies. On the basis of alteration phases replacing the primary magmatic minerals, infilling amygdales and veins, and replacing secondary minerals, the following zones have been distinguished: (1) thomsonite‐scolecite‐smectite, (2) heulandite‐stilbite‐smectite, (3) laumontitechlorite‐albite, (4) laumontite‐chlorite‐albite ± prehnite ± pumpellyite and (5) epidote‐chlorite‐albite ± actinolite zone.

In addition to the overall zonation based on mineral parageneses, zonations in the composition of the Ab content of the newly formed albite replacing primary Ca‐rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows. Mafic phyllosilicates in the upper part of the sequence (mainly smectites and mixed‐layer smectite/chlorites) display high Si and Ca + Na + K contents, whereas in the lower part of the sequence the amounts of Si and Ca + Na + K are markedly lower (mainly chlorites and mixed‐layer chlorite/smectites). Similar zonations are observed within the individual flows. The albite content of the newly formed plagioclase is highest, and the Si and Ca + Na + K content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior.

The above features suggest that the overall pattern is one of burial‐type metamorphism associated with extension in the rift setting. In detail, the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts. This suggests that at the first stages of alteration (lowest grade) the patterns of fluid flow were important effects in controlling the assemblages. At greater burial depth, assemblages are more homogeneous, perhaps representative of a more even and pervasive flow pattern.

Using the observed assemblages at face value to define grade and/or facies, different conditions would be assigned within the different morphological flow portions. Thus at low‐grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions.
Metamorphism and deformation of blueschist belts and their tectonic implications, North Qilian Mountains, China
Authors: HANQUAN WU , YIMIN FENG , SUGUANG SONG
Published: May 2007

Abstract
Two blueschist belts in the North Qilian Mountains occur in Middle Cambrian and Lower Ordovician strata and strike N30–35°W for about 500 km along the Caledonian fold belt on the south‐west margin of the Sino‐Korean plate. The styles of metamorphism and deformation are quite different in the two belts. The Middle Cambrian to Ordovician rocks in the high‐grade belt are mainly blueschists and C‐type eclogites in which six phases of lower and upper crustal deformation have been recognized. The rocks contain glaucophane, phengite, epidote, clinozoisite, chlorite, garnet, stilpnomelane, piedmontite, albite, titanite and quartz. The estimated P–T conditions of eclogites are 340 ± 10°C, 8 ± 1 kbar and, of blueschist, >380°C, 6–7 kbar. The Ordovician rocks in the low‐grade belt are characterized by ductile to brittle deformation in the middle to upper crust. The low‐grade blueschists contain glaucophane, lawsonite, pumpellyite, aragonite, albite and chlorite. The estimated P–T conditions are 150–250°C and 4–7 kbar.

K–Ar and 39Ar/40Ar geochronology on glaucophane and phengite from the high‐grade blueschist belt suggest two stages of metamorphism at 460–440 and 400–380 Ma, which may represent the times of subduction and orogeny. The subduction metamorphism of the northern low‐grade blueschist belt took place approximately at the end of the Ordovician.
Metamorphism and tectonic evolution of the Shangdan fault zone, Shaanxi, China
Authors: NENGGAO HU , JIAXI YANG , SANYUAN AN , JIANMIN HU
Published: May 2007

Abstract
The Shangdan fault in the Qinling Orogenic Belt of China is an important boundary between the Caledonian North Qinling Fold Belt and the Hercynian South Qinling Fold Belt. In the Danfeng area, the fault zone strikes WNW–ESE and comprises four strongly deformed zones and three weakly deformed domains parallel to each other. The fault zone has a complex history of multiple deformation and each domain has a different tectonic style that was formed at different stages of the deformation.

The rocks exposed in the weakly deformed domains belong to the Qinling, Danfeng and Liuling Groups. In this paper, the mineral chemistry and mineral assemblages are used to infer the metamorphic conditions and the P–T paths of these units. The metamorphic units in and near the fault zone have different metamorphic conditions and histories that are correlated with the tectonic evolution of the fault zone. Caledonian–Hercynian uplift and southward thrusting of the Proterozoic Qinling Group, over the Danfeng and the Liuling Groups, produced the main metamorphic and tectonic features of the fault zone. Folding of both the Liuling Group and the thrust faults during the Hercynian–Indosinian was accompanied by northward thrusting.
Unravelling the spirals: a serial thin‐section study and three‐dimensional computer‐aided reconstruction of spiral‐shaped inclusion trails in garnet porphyroblasts
Authors: JOHNSON S. E.
Published: May 2007

Abstract
Seventy‐seven spatially orientated, serial thin sections cut from a single rock reveal changes in the geometry of spiral‐shaped inclusion trails (SSITs) in garnet porphyroblasts. The observed SSITs are doubly curved, non‐cylindrical surfaces, with total inclusion‐trail curvature decreasing systematically from the cores to the rims of porphyroblasts. The three‐dimensional geometry of the SSITs, reconstructed with the aid of computer graphics, shows that the orientations of spiral axes defined by the SSITs are not related in any expected nor predictable way to the main foliation in the matrix. This suggests continued deformation after or during the latest stages of porphyroblast growth, which has important implications for the use of SSITs as shear‐sense indicators. Whether the formation of SSITs involves significant porphyroblast rotation with respect to a geographically fixed reference frame cannot be determined from the available data.
Testing models for the development of spiral‐shaped inclusion trails in garnet porphyroblasts: to rotate or not to rotate, that is the question
Authors: JOHNSON S. E.
Published: May 2007

Abstract
The formation of spiral‐shaped inclusion trails (SSITs) is problematical, and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing. One model argues for porphyroblast rotation, with respect to a geographically fixed reference frame, whereas the other argues for no such porphyroblast rotation, but instead rotation of the matrix foliation around the porphyroblast. Thus, porphyroblasts with SSITs cannot be used as shear‐sense indicators until it is conclusively determined which model best explains them.

Any successful model must explain features associated with SSITs, including: (1) foliation truncation zones, (2) smoothly curving SSITs, (3) millipede microstructure, (4) total inclusion‐trail curvature in median sections, (5) porphyroblasts with SSITs that have grown together, (6) evidence for relative porphyroblast displacements, (7) shear‐sense indicators inside and outside porphyroblasts; (8) crenulations associated with porphyroblasts and (9) geometries in sections subparallel to spiral axes (axes of rotation). A detailed study of these features suggests that most, if not all, can be explained by both the rotational and non‐rotational models, in spite of these models involving diametrically opposed movement senses. Therefore, geometrical analysis of individual porphyroblast microstructures may not determine which model best explains SSITs until the kinematics required to form these microstructures are better understood, in particular the sense of shear along a developing crenulation cleavage. Specific tests for determining the shear sense along crenulation cleavages are proposed, and results of such tests may conclusively resolve the debate over how SSITs form.
Polymetamorphism and isobaric cooling of pelitic schists from the Annex sulphide deposit, Gordonia Belt, South Africa
Authors: HUMPHREYS H. C. , SCHEEPERS D. J.
Published: May 2007

Abstract
Muscovite‐poor pelitic schists in the wallrocks of the Proterozoic Annex sulphide deposit, near Prieska, South Africa, contain peak metamorphic assemblages including Crd + Bt + Sil, St + Sil + Bt, Crd + St + Bt and, rarely, Ky + St ° Crd. All rocks include oligoclase, quartz and commonly Fe–Mn garnet, with or without muscovite. Peak assemblages, assigned to M2 regional metamorphism in the Gordonia Belt (Namaqua Province), are syn‐ to post‐kinematic with respect to the main S2 fabric although larger staurolite grains contain S1 inclusion trails. Garnet–biotite thermometry, utilizing corrections for Fe3+, Mn, AlVI and Ti, yields peak temperatures of 571–624°C at pressures of 4.5–6.0 kbar. Consideration of the sympathetic variation of XMn in garnet with XMg in biotite and the preserved zoning patterns in prograde garnets, together with the inferred prograde transition from kyanite to sillimanite, indicates that heating occurred during mild decompression to the M2 metamorphic peak. Sillimanite and cordierite grew last in the prograde sequence, possibly related to a pulse of thermal metamorphism (M3) that is found along the margin of the Keimoes Suite batholith to the north.

Retrograde assemblages, including Ms + Ky + Chl + Qtz (after Crd + Bt), Ky + Ms (after Sil) and Chl + Ms (after St) indicate a period of isobaric cooling (M4a) terminated by rehydration in the kyanite stability field at about 500°C.

The size difference between prograde (1–2‐mm) and retrograde (0.05–0.1‐mm) mineral grains indicates substantial undercooling below equilibrium positions of relevant retrograde reactions prior to rehydration, and explains why cordierite that grew during M2 is almost completely destroyed. Post‐M4a regrowth of staurolite and garnet (M4b) is spatially linked to sites of M4a rehydration. It reached temperatures of 510–530°C, remaining within the stability field of kyanite.

A best fit of the observed textural history to the Namaqua orogenic cycle involves collision and heating (M2/D2) followed by granite intrusion (M3), rifting (M4a) and renewed heating due to crustal loading during volcanism (M4b). The P–T path for the Annex region is consistent with those derived from elsewhere in the Gordonia Belt and, with modification, to that published already for the nearby Prieska Copper Mines.
The stable isotope signature of kilometre‐scale fracturedominated metamorphic fluid pathways, Mary Kathleen, Australia
Authors: OLIVER N. H. S. , CARTWRIGHT I. , WALL V. J. , GOLDING S. D.
Published: May 2007

Abstract
Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre‐scale fluid transport during amphibolite facies metamorphism. These 10‐ to 100‐m‐scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10‐km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δ13Ccalcite values of –2 to –7% and δ18Ocalcite values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δ18Owhole‐rock values of 3.5 to 7%. and unaltered banded calc‐silicate and marble wallrocks have δ13Ccalcite of –1.6 to –0.6%, and δ18Ocalcite of 18 to 21%. In the alteration zones adjacent to the pods, the δ18O values of both metadolerite and calc‐silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc‐silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100‐ to 500‐m‐scale isotopic exchange with the surrounding metadolerites and calc‐silicates does not explain the observation that the δ18O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc‐silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc‐silicates, or thermal fluctuations attending deformation and metamorphism. The overall L‐shaped trend of the data on plots of δ13C vs. δ18O is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc‐silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc‐silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism.
Tectonic evolution of Proterozoic rocks in the Cimarron Mountains, northern New Mexico, USA
Authors: GRAMBLING J. A. , DALLMEYER R. D.
Published: May 2007

Abstract
Portions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C.

The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low‐angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates a P–T path dominated by decompression and cooling. The low‐angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low‐grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right‐slip movement along the steeply dipping Fowler Pass shear zone.

An argon isotope‐correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a 40 Ar/39 Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope‐correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low‐grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post‐dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages of c. 1100 Ma. Similar ages are not obtained north‐east of the Fowler Pass shear zone, suggesting movement more recently than 1100 Ma.
Polyphase and anticlockwise P‐T evolution for Franciscan eclogites and blueschists from Jenner, California, USA
Authors: KROGH E. J. , OH C. W. , LIOU J. C.
Published: May 2007

High‐grade exotic blocks in the Franciscan Complex at Jenner, California, show evidence for polydeformation/metamorphism, with eight distinct stages. Two parallel sets of mineral assemblages [(E) eclogite, and (BS) laminated blueschist] representing different bulk chemistry were identified. Stage 1, recorded by parallel aligned inclusions (S1) of crossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central parts of zoned garnets, represents the epidote blueschist facies. The onset of a second stage (stage 2) is represented by a weak crenulation of S1 and growth of garnet. This stage develops a well‐defined S2 foliation of orientated barroisite + epidote + titanite (E), or subcalcic actinolite + epidote + titanite (BS) at c. 90d̀ to S1, with syntectonic growth of garnet, defining the (albite‐)epidote‐amphibolite facies. A third stage, with aligned inclusions of glaucophane + (subcalcic) actinolite + phengite parallel to S2 in the outermost rims of large garnet grains, is assigned to the transitional (albite‐)epidote‐amphibolite/(garnet‐bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix minerals in the least retrograded samples. In this stage the mineral assemblages garnet + omphacite + glaucophane + phengite (E) and garnet + winchite + phengite + epidote (BS) both represent the eclogite facies. Stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite/glaucophane + garnet + omphacite + epidote + phengite (E), and glaucophane + actinolite + epidote + phengite (BS), with the development of an S5 foliation subparallel to S2. Stage 6 represents a crenulation of S5, with the development of a well‐defined S6 crenulation cleavage wrapping around relics of the eclogite facies assemblages. This crenulation cleavage is further weakly crenulated during a D7 event. Post‐D7 (stage 8) is recorded by the growth of lawsonite + chlorite ± actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengite + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk chemistry.

The metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous, following an anticlockwise P‐Tpath: (1) epidote blueschist (garnet‐free) to (2) (albite‐)epidote‐amphibolite to (3) transitional epidote blueschist (garnet‐bearing)/(albite‐)epidote‐amphibolite to (4) eclogite to (5) epidote blueschist (garnet‐bearing) to (6‐7) epidote blueschist (garnet‐free) facies to (8) lawsonite + pumpellyite + aragonite‐bearing assemblages. This anticlockwise P‐T path may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre‐Franciscan metamorphism.
Structurally controlled fluid flow associated with breccia vein formation
Authors: VALENTA R. K. , CARTWRIGHT I. , OLIVER N. H. S.
Published: May 2007

A breccia vein sampled from a shear zone in greenschist facies metapelites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and faulting within the sample. Calcite vein‐fill is coarse grained and equigranular, suggesting precipitation in a fluid‐filled space. Partially folded veins suggest that veining occurred during folding and faulting. The breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying that this was a zone of high fluid pressure and nearly isostatic differential stress during folding and faulting. From these observations, it can be inferred that the breccia vein was a zone of high permeability and a likely fluid channel during deformation. This hypothesis was tested by stable isotope analysis of veins and host rocks. The calcite veins have δ13C values of ‐11.1 ± 0.1% and δ18O values of 6‐10%o, whereas the host metapelite has δ13C values of ‐10.62 and ‐10.11% and δ18O values of 14‐15%o. These values are consistent with an igneous‐derived, H2O‐dominated fluid that exchanged little oxygen with the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confirms that the fluid was externally derived, and that the breccia vein acted as a channel for large‐volume fluid flow within the shear zone.
Oxygen isotope geothermometers for metamorphic rocks
Authors: MATTHEWS A.
Published: May 2007

The Chicago mineral‐carbonate oxygen isotope fractionation curves have been combined with mineral‐water fractionation data for jadeite, zoisite and rutile and new data for grossular‐water to provide a set of self‐consistent mineral‐pair calibrations. The A coefficients in the equation 1000 In α=A× 106T‐2 of the new mineral‐pair fractionations are

Jadeite Zoisite Grossular Rutile

Quartz 1.69 2.00 3.03 5.02

Jadeite 0.31 1.34 3.33

Zoisite 1.03 3.02

Grossular 1.99

The isotopic fractionation properties of natural pyralspite garnet [(Ca, Fe, Mg, Mn)3Al2Si3O12] can be approximated by those of the grossular end‐member. Appropriate substitutions also yield coefficients for the solid‐solution minerals: sodic pyroxene and epidote, e.g.

A
quartz‐sodic pyroxene= 2.75 ‐ 1.06Xjd,

A
quartz‐epidote= 2.00 + 0.75Xps

where XJd and XPs are the mole fractions of the jadeite and pistacite components, respectively.

The new data set is particularly suitable for the geothermometry of metamorphic rocks. δ18O data from minerals of the high‐pressure metamorphic rocks of the Sesia Zone of Italy and Cyclades Complex of Greece yield well‐constrained mean temperatures of 572 and 478d̀ C, respectively. Type III blueschist metabasalts of the Franciscan Formation of California give mean quartz‐garnet temperatures of 354d̀ C.
A stable isotope study of the migmatitic rocks in the Ballachulish contact aureole, Scotland
Authors: LINKLATER C. M. , HARTE B. , FALLICK A. E.
Published: May 2007

The highest grade pelitic and semipelitic rocks of the Ballachulish aureole are dominantly potash feldspar + cordierite + biotite hornfelses with widely variable amounts of quartz, plagioclase, andalusite, sillimanite and corundum (together with accessory phases). On a microscopic scale these hornfelses show textural evidence of the presence of melt, whilst on a mesoscopic scale they contain a variety of leucosomes. Oxygen isotope studies have been carried out on both whole rocks and mineral separates in order to: (1) assess the sources of molten and volatile constituents and (2) determine the extents of isotopic homogenization and equilibration. Data from localities with both restricted and extensive evidence of leucosomes and melt development are compared, as well as one locality with petrographic evidence of melt incursion from the igneous complex.

The whole‐rock δ18O values of the leucosomes (10.5–14.9%.) are in general similar to the immediately adjacent mesosomes (9.9–14.5%.) which are typically cordierite‐ and feldspar‐rich hornfelses. Isotopic evidence is thus consistent with an in‐situ partial melt origin for the leucosomes, without the substantial addition of externally derived components. In the area of extensive melt development, the ‘chaotic zone’, it is possible there was addition of an H2O‐rich fluid phase (6‐13 wt%) from the igneous complex which resulted in a slight lowering of δ18O values by 0.5–1.0%.

Quartz mineral separates were used to assess the degree of local isotopic homogenization. In the extensively molten area (chaotic zone) there is extensive homogenization between rock layers (quartz δ18O usually within 1.0%), whilst in less molten areas δ18O quartz has a range of c. 3.0%. The greater homogenization in the chaotic zone is attributed to the increased degree of melting and infiltration of H2O‐rich fluid from the igneous complex.
Contribution of deviatoric stresses to metamorphic P‐T paths: an example appropriate to low‐P, high‐T metamorphism
Authors: STÜWE K. , SANDIFORD M.
Published: May 2007

P‐T paths for a simple situation appropriate to many low‐P, high‐T (LPHT) terranes in which metamorphism and deformation are localized by advection of heat in magmas, has been modelled assuming a medium with a power‐law rheology with an inverse exponential dependence of stress and temperature and capable of sustaining deviatoric stress, τ, in the order of 100 MPa at 400d̀ C and strain rates of up to 10‐13s‐1. Numerical simulations and analytical approximations for P‐T histories appropriate for simple convergent deformation histories show that the destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic temperature peak. Moreover, for a specified strain rate and temperature evolution, P‐T paths may vary from clockwise to anticlockwise merely as a function of vertical distance from the heat source. Inasmuch as mounting independent evidence suggests that the crust can support deviatoric stresses of up to about 100 MPa at temperatures of 400‐500d̀ C, and that the shear strength of the crust is strongly temperature‐dependent in the range 400‐800d̀ C, these results suggest that caution should be taken in the tectonic interpretation of P‐T paths involving decompression of the order of 100 MPa or less in LPHT terranes. The results illustrate a plausible mechanism for the close spatial association of both clockwise and anticlockwise P‐T paths documented in some LPHT terranes.
Very low‐grade metamorphism of the Taveyanne greywacke, Glarus Alps, Switzerland
Authors: RAHN M. , MULLIS J. , ERDELBROCK K. , FREY M.
Published: May 2007

Detailed textural and chemical data for mineral assemblages on a regional scale are presented for the metaandesitic Eocene‐Oligocene Taveyanne greywacke of the Glarus Alps, Eastern Switzerland. Presented data indicate an increase of metamorphic grade from zeolite facies to prehnite‐pumpellyite and pumpellyite‐actinolite facies. Low‐grade outcrops contain laumontite, minor corrensite and pumpellyite (assemblage type 1), whereas outcrops of higher metamorphic grade contain prehnite and two populations of pumpellyite (type 2), prehnite—pumpellyite‐(Al)—white mica (type 3), a single outcrop shows pumpellyite‐actinolite facies (type 4). From the zeolite to prehnite‐pumpellyite/pumpellyite‐actinolite facies there are indications for an increase of the chemical equilibrium domain size for the critical paragenesis from a single detrital grain ≤1 mm) in type 1, to a few millimetres in type 2, and to a whole thin section in type 3. MetamorphicP‐Tconditions were determined by a combination of chlorite thermometry, fluid inclusion and vitrinite reflectance data. Peak temperatures range from 170‐190d̀ C for zeolite facies to 270‐310d̀ C for prehnite‐pumpellyite and pumpellyite‐actinolite facies. For the higher temperature range, pressures of 2‐3 kbar are derived indicating a geothermal gradient of 24‐32d̀ C km‐1. The well‐constrained temperature estimations derived for the assemblages provide a useful test of the different empirical calibrations of chlorite thermometers recently proposed. The best correspondence to the temperatures determined here is for the Cathelineau calibration. In addition, in the lower grade samples differences in textures and calculated temperatures provide a mean to distinguish between detrital and newly formed chlorites.
Chemographic analysis of assemblages involving pyrophyllite, chloritoid, chlorite, kaolinite, kyanite, quartz: application to metapelites in the Witwatersrand goldfields, South Africa
Authors: ZHOU T. , DONG C. , PHILLIPS G. N.
Published: May 2007

The Witwatersrand goldfields contain abundant assemblages that include pyrophyllite, chloritoid, chlorite, kaolinite and/or kyanite, with quartz. A chemographic analysis of the system Fe(Mg)‐Al‐Si‐O‐H involving these minerals yields 22 potential phase diagrams. Using orientation criteria and thermodynamic calculations as further constraints, this list has been reduced to three possible diagrams. New thermodynamic data favour one of these in particular.

This chemographic analysis demonstrates that formation of chloritoid is not restricted to the breakdown reaction of kaolinite plus chlorite in the F(M)ASH system, as stated by previous studies, but could be from pyrophyllite + chlorite → chloritoid + quartz + H2O.

The metamorphic temperature variation between Witwatersrand goldfields exceeded 65d̀ C, based on chlorite and chloritoid compositions. The lower and upper pressure limits are constrained by the andalusite to kyanite, and the sudoite/chlorite to carpholite boundaries, i.e. 1.5–2.8, and 7 kbar, respectively. The widespread pyrophyllite, chlorite and Fe‐chloritoid in all the Witwatersrand goldfields, and the local occurrence of sudoite indicate a consistent low‐pressure environment in which Mg‐chloritoid would not be stable.