bluuebunny/crossref_metadata_2025_split huggingface dataset:
Migmatite structures in the Central Gneiss Complex, Boca de Quadra, Alaska
Authors: McLELLAN E.L. 
Published: May 2007

AbstractMigmatite structures in the Coast Plutonic‐Metamorphic Complex are well exposed in the inlet of Boca de Quadra, southeast Alaska. Two types of anatectic migmatites are present.Patchmigmatites formed byin situmelting and subsequent crystallization of melt.Diktyoniticmigmatites comprise a discontinuous veined network of leucocratic material, in which leucosomes enclose boudins of host rock. The margins of these boudins show the development of both melanosomes and shear band fabrics.Strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20‐27%. This volume decrease is attributed to melt extraction into the adjacent fracture‐filling leucosomes. Thus, diktyonitic migmatites formed by shear‐induced segregation of partial melt, whereas in patch migmatites the lack of shear stresses inhibited melt segregation. The variable structural style of anatectic migmatites in Boca de Quadra is not related to host‐rock composition, but may be due to differences in the amount of differential stress during migmatization. These in turn may be controlled by host‐rock strength and/or diachroneity of migmatization and deformation.Determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels.
Contrasting sapphirine parageneses from Wilson Lake, Labrador and their tectonic implications
Authors: CURRIE K.L. , GITTINS J. 
Published: May 2007

AbstractRegionally distributed pelitic granulites in the Wilson Lake region contain the assemblage sapphirine + hypersthene + sillimanite + quartz. Geochronology and geobarometry suggest it developed in early Proterozoic rocks at temperatures approaching 900°C and pressures above 10 kbar. Vein‐like metasomatized rocks around a suite of mafic to ultramafic intrusions, emplaced near the peak of metamorphism about 1700 Ma ago, contain sapphirine, but these assemblages developed at temperatures near 750°C and pressures of 4.5 kbar. Both types of assemblage occur as relics in amphibolite‐grade (biotite–sillimanite) migmatites.P–Tdeterminations indicate rapid isothermal uplift of 20 km accompanied by mafic intrusion and hydration. The metamorphic history and tectonic setting suggest exposure of deep continental crust by thrusting during continental collision, followed by essentially isothermal decompression.
Polyphase metamorphism and the development of the Main Central Thrust
Authors: STÄUBLI A. 
Published: May 2007

ABSTRACT Along a cross‐section through the Lesser and Higher Himalayan units at the Kishtwar window area (north‐west India), a polyphase, Barrovian‐type metamorphism has been delineated in relation to the development of the Main Central Thrust (MCT). In the metapelitic mineral assemblages, three metamorphic phases have been distinguished:

conditions up to amphibolite grade at moderate to high pressures (alm + rut + ilm + kya + qtz) characterize the M1 phase;
pressure release and/or temperature increase as a result of movement along the MCT and the formation of gneiss domes in the Higher Himalaya, as expressed by oriented (N70°‐100° E) fibrolite, defines the M2 phase; and,
finally during uplift of the Kishtwar window area, a retrogressive M3 phase is characterized by the assemblage quartz‐muscovite‐chlorite.

Both optically zoned and single‐stage garnets have been examined with the electron microprobe to determine their element partitioning. Normal zoning has been found in samples below the MCT in the Lesser Himalaya, indicating prograde growth during the M2 phase, whereas tectonically above, in the Higher Himalaya unit, the garnets reveal double‐stage growth with a complex zoning pattern due to reaction‐partitioning during M1 and M2 and reverse‐zoning at their rims during the retrogressive M3 phase. Geothermometry on metapelites along a cross‐section through the MCT zone and the Higher Himalaya imply distinct readjustments of garnet‐biotite exchange equilibria and indicate isothermal conditions (500‐600° C) throughout the section during the M3 retrogression. Pressure calculations (gro‐an‐kya‐qtz and alm‐rut‐ilm‐kya‐qtz) suggest a decrease in pressure towards the top of the section (6‐7.5 to 4.5‐5 kbar), as corroborated by fibrolite replacing kyanite. The spatially inverse metamorphism exposed within the Lesser Himalaya of the Kishtwar window is regarded as a product of polyphase metamorphism combined with ongoing thrusting and shearing and is reflected by condensed M2 isograds around the Kishtwar window.
The dynamics of the suture between the Kohistan island arc and the Indian plate in the Himalaya of Pakistan
Authors: CHAMBERLAIN C. P. , ZEITLER P. K. , JAN M. Q. 
Published: May 2007

ABSTRACT The pressure‐temperature and temperature‐time paths derived for rocks in the Kohistan arc and adjacent Nanga Parbat‐Haramosh massif record the dynamics of the collision between the island arc and the Indian plate. Studies of P‐T‐t paths show that the Kohistan arc was thrust over the Nanga Parbat‐Haramosh massif at least 25 Ma ago, but not more than 30–35 Ma ago. Rocks in the Kohistan arc followed decreasing pressure paths, with the early metamorphism beginning at high pressures (9.5 kbar) and later metamorphism occurring at 8.0 kbar. In contrast, rocks in the Nanga Parbat‐Haramosh massif (Indian plate) experienced increasing pressure and temperature paths. Prior to thrusting, the massif was at low pressures (4.0 kbar) and low temperatures (450°c). Later, the pressure and temperature increased to 8 kbar and 580°c. The authors interpret the convergence (to approximately the same pressure and temperature) of the P‐T paths in the two terranes as being the result of thrusting and thermal equilibration between the thrust sheets. 40Ar/39Ar cooling ages of hornblendes and other geochronological data suggest that the time of peak metamorphism and hence the completion of thickening was approximately 30–35 Ma ago.Temperature‐time paths show that after thrusting, during the period 25–10 Ma, the Kohistan arc and Nanga Parbat‐Haramosh massif were uplifted at similar rates (0.5 km Ma). However, in the past 10 Ma the Nanga Parbat‐Haramosh massif has been uplifted more rapidly than the adjacent Kohistan arc. Rapid uplift has been accommodated by late faults along the edge of the massif.
The role of deformation partitioning in the deformation and recrystallization of plagioclase and K‐feldspar in the Woodroffe Thrust mylonite zone, central Australia
Authors: BELL T. H. , JOHNSON S. E. 
Published: May 2007

ABSTRACT In the Woodroffe Thrust mylonite zone, central Australia, recrystallization in plagioclase and K‐feldspar involved subgrain rotation, assisted by grain‐boundary or kink band boundary bulging, without contribution from a change in the chemical composition from host grains to new grains. The size of subgrains and new grains changes across the mylonite zone, apparently as a function of the strain rate and the H2O content of the rock.The partitioning of deformation into zones of progressive shearing and progressive shortening controls the sites of recovery and recrystallization in feldspar during mylonitization. The size of feldspar porphyroclasts in well developed mylonites is governed by the scale of deformation partitioning reached in the earlier stages of mylonitization, before the formation of a large proportion of fine‐grained matrix that can accommodate the progressive shearing component of the deformation.Recrystallization occurs in microcline, apparently without involving a translation to a monoclinic structure, as microcline‐twinned new grains are common adjacent to microcline‐twinned host grains. K‐feldspar triclinicity values calculated from XRD traces increase from the margins to the interior of the mylonite zone, in conjunction with deformation intensity. K‐feldspar host grains locally have cores of orthoclase or untwinned microcline, surrounded by mantles of twinned microcline, suggesting a relationship between the presence of microcline twinning and the degree of K‐feldspar triclinicity.
Dissolution, solution transfer, diffusion versus fluid flow and volume loss during deformation/metamorphism
Authors: BELL T. H. , CUFF C. 
Published: May 2007

ABSTRACT Dissolution and solution transfer during deformation/metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components. Progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries.Progressive shortening is accommodated by the cores of most other minerals (including sulphides). These minerals develop strain, and hence dislocation density gradients, on their rims due to progressive shearing along grain boundaries. These gradients are particularly large when the mineral abuts phyllosilicate or graphite. The resulting chemical potential gradients between the core and rim drive dissolution, causing removal of the highly strained grain margins.Removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation. Interlayers and interfaces on boundaries lying at a low angle to the direction of shearing, and oriented relative to the sense of shear such that they can open, gape by small amounts. Water present in these interlayer spaces becomes destructured, considerably enhancing diffusion rates along the foliation.Penetrative volume loss, especially in deforming/metamorphosing pelitic rocks, is large at all metamorphic grades, increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies. Transference of material by fluid flow from deep to high levels in the earth's crust is precluded because thousands to tens of thousands of rock volumes of fluid are required, necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells, unless some extremely large‐scale form of fluid channelling is possible. Reassessment of diffusion mechanisms, and hence rates, during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid, indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required. The amount of fluid can be reduced sufficiently to allow large volume losses by a one‐way flow of fluid to the earth's surface, in deforming/metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure.Deformation partitioning‐controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation/metamorphism because matrix minerals, other than phyllosilicates and graphite, are preferentially removed. The large size of porphyroblasts, if present, tends to preserve them from dissolution. Hence, the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew, in rocks in which bedding is still well preserved.
Aluminum silicates in the Mount Raleigh pendant, British Columbia
Authors: KERRICK D. M. , WOODSWORTH G. J. 
Published: May 2007

ABSTRACTIn regionally metamorphosed pelites of the Mount Raleigh pendant, the fibrolite isograd occurs 5km downgrade from the sillimanite isograd. Fibrolite formed from the decomposition of biotite, a reaction that probably resulted from the late‐stage influx of acidic volatiles. In contrast, sillimanite formed by the direct,‘volume‐for‐volume’replacement of andalusite. Andalusite and sillimanite coexist in a 3 km‐wide zone above the sillimanite isograd. Electron probe analyses of these phases reveal low minor element contents and yield KD[=X] values close to unity; the low Fe2O3 contents are compatible with reducing conditions implied by the ubiquity of graphite. Because KD→ 1.0, the zone of coexisting andalusite + sillimanite cannot be attributed to multivariancy resulting from partitioning of minor elements between these phases. Rather, the metastable persistence of andalusite into the sillimanite P‐T stability field is suggested. The modal proportions of sillimanite versus andalusite imply that minimal (<5%) and alusitesillimanite reaction occurred in a zone 1.5km above the sillimanite isograd; in contrast, there was a marked increase in reaction progress immediately above this zone. With an estimated thermal gradient (in the plane of exposure) of approximately 20°C/km, the 1.5 km‐wide zone of nil reaction suggests that the andalusite‐sillimanite equilibrium boundary was overstepped by about 30 °C before significant reaction occurred. Inclusion‐rich areas in andalusite provided favourable sites for sillimanite nucleation; however, the growth of sillimanite may have been impeded by‘pinning’of sillimanite grain boundaries by inclusions.
The upper thermal stability of chlorite + quartz: an experimental study in the system MgO‐Al2O3‐SiO2‐H2O
Authors: MASSONNE H.‐J. 
Published: May 2007

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H2O, chlorite + quartz = talc + cordierite + H2O, and talc + kyanite + quartz = cordierite ± H2O by reversed runs in the system MgO‐Al2O3‐SiO2‐H2O (MASH). These reaction curves intersect at an invariant point (IP1) at PH2O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H2O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H2O at water pressures is clearly positive.The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg4.85Al1.15[Al1.15Si2.85O10](OH)8 over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg2.94Al0.06[Al0.06Si3.94O10](OH)2 at PH2O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H2O = 21 kbar.As a consequence of the new experimental data, the existing phase topologies of the MASH‐system and K2O‐MASH‐system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H2O = chlorite + cordierite are stable. For this reason, the development of medium‐ to high‐temperature metamorphic rocks compositionally approximating the MASH‐system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe2+ into magnesian chlorite extends its stability field in the presence of quartz by 10°‐15°C in comparison with pure Mg‐chlorite.
Tectonic implications from sapphirine‐bearing lithologies, south‐west Grenville Province, Canada
Authors: GRANT S. M. 
Published: May 2007

Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south‐west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association II) comes from meta‐eclogitic pods with foliated amphibolite rims. In each case the sapphirine‐bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite ± plagioclase and records pressures of around 14‐16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown.The host gneisses equilibrated at approximately 8 to 10 kbar and 700‐750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16‐1.0 Ga. It is unlikely that the higher pressures recorded in the meta‐eclogitic pods represent an earlier high‐pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south‐west Grenville Province.
Petrology and retrograde P‐T path in granulites of the Kanskaya formation, Yenisey range, Eastern Siberia
Authors: PERCHUK LEONID , GERYA TARAS , NOZHKIN ALEX 
Published: May 2007

The Kanskaya formation in the Yenisey range, Eastern Siberia is a newly studied example of retrogression of granulite facies rocks. The formation consists of two stratigraphical units: the lower Kuzeevskaya group and the upper Atamanovskaya group. Rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet, sillimanite and quartz, silimanite coronas around spinel and corundum, and garnet rims around plagioclase in metabasites, while plagioclase rims around garnet can be seen in associated metapelites. The paragenesis quartz + orthopyroxene + sillimanite is a feature of the Kuzeevskaya group. In many samples, chemical zoning of garnet and cordierite shows an increase in Mg from core to rim as well as the reverse.Biotite‐garnet‐cordierite‐sillimanite‐quartz as well as spinel±biotite‐garnet°Cordierite±sillimanite‐quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net‐transfer reactions (Perchuk & Lavrent'eva, 1983; Aranovich & Podlesskii, 1983; Gerya & Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yield P‐T paths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In a P‐T diagram these trajectories are subparallel lines, whose P‐T maxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model for P‐T evolution of the Kanskaya formation is proposed.
Lack of porphyroblast rotation in the Otago schists, New Zealand: implications for crenulation cleavage development, folding and deformation partitioning
Authors: JOHNSON S. E. 
Published: May 2007

Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non‐coaxial deformation that post‐dates their growth. Inclusion trails in most of the porphyroblasts are oriented near‐vertical and near‐horizontal, and the strike of near‐vertical inclusion trails is consistent over 3000 km2. Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non‐rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain‐boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.
The Narryer Gneiss Complex of the Yilgarn Block, Western Australia: a segment of Archaean lower crust uplifted during Proterozoic orogeny
Authors: MUHLING J. R. 
Published: May 2007

The Narryer Gneiss Complex of the Yilgarn Block is a key segment of the Western Australian Precambrian Shield. It is a regional granulite facies terrain comprised of predominantly quartzo‐feldspathic gneisses derived from granitic intrusions c. 3.6–3.4 Ga old. Granulite facies metamorphism occurred c. 3.3 Ga ago, and conditions of 750–850°C and 7–10 kbar are estimated for the Mukalo Creek Area (MCA) near Errabiddy in the north. The P–T path of the MCA has been derived from metamorphic assemblages in younger rocks that intruded the gneisses during at least three subsequent events, and this path is supported by reaction coronas in the older gneisses. There is no evidence for uplift immediately following peak metamorphism of the MCA, and a period of isobaric cooling is inferred from the pressures recorded in younger rocks. Pressures and temperatures estimated from metadolerites, which intruded the older gneisses during ‘granite–greenstone’tectonism at about 2.6 Ga and during early Proterozoic thrusting show that the Errabiddy area remained in the lower crust, although it was probably reheated during the younger events. Isothermal uplift to upper crustal levels occurred at c. 1.6 Ga ago, and was followed by further deformation and patchy retrogression of high‐grade assemblages. The effects of younger deformation, cooling and reheating can be discerned in the older gneisses, but as there has been no pervasive deformation or rehydration, the minerals and microstructures formed during early Archaean granulite facies metamorphism for the most part are retained.The MCA remained in the lower crust for about 1700 Ma following peak metamorphism and some event unrelated to the original metamorphism was required to exhume it. Uplift occurred during development of the Capricorn Orogen, when some 30–35 km were added to the crust beneath the Errabiddy area. The recognition of early Proterozoic thrusting, plus crustal thickening, suggests that the Capricorn Orogen is a belt of regional compression which resulted from convergence of the Yilgarn and Pilbara Cratons.