Metabasite amphiboles of the Scottish Dalradian
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Actinolite
Dalradian
Amphibole
Hornblende
Abstract Metabasites in the Dalradian Argyll and Southern Highland Groups experienced multiphase deformation and coeval Barrovian-type prograde and retrograde metamorphism during the Caledonian Grampian orogeny. Metamorphic Ca-amphiboles crystallized with plagioclase,epidote,chlorite and quartz, and sometimes with garnet and clinopyroxene. The minor changes in metabasite mineral assemblages and systematic mineral-chemical variations in Ca-amphibole confirm the classical concept of increasing metamorphic grade in metapelitic Chlorite to Sillimanite zones. In the Chlorite zone, high-Ti brown amphibole enclosed by green Ca-amphibole is interpreted as a magmatic relic. In the Chlorite, Biotite,Garnet and Andalusite zones, Ca-amphibole displays zonation with actinolite in cores and magnesio-hornblende to tschermakite in rims. Poor amphibole zonations occur in the Kyanite and Sillimanite zones. Ca-amphibole zonations are best described in terms of IV Al, VI Al and Ti which semiquantitatively monitor temperature and pressure changes. Maximal IV Al in amphibole increase with metamorphic grade. Empirically calibrated amphibole equilibria enabled us to reconstruct coherent prograde P-T paths and maximal P and T from the mineral zones; with T max of 680±50°C in the Sillimanite zone and P max of 8±1.2 kbar in the Kyanite zone. Prograde P-T paths indicate that elevated geothermal gradients should be considered even at the beginning of the Grampian Barrovian metamorphism.
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Sillimanite
Actinolite
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Staurolite
Hornblende
Andalusite
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Amphibolites from the Manzano Mountains, New Mexico, include two chemically and microstructurally distinct amphibole populations. Petrographic and electron-microprobe studies show that early actinolite is overgrown and crosscut by younger foliation-forming hornblende. An older foliation defined by actinolite porphyroclasts (S 1 ) is discordant to the regionally extensive hornblende foliation (S 2 ). Microstructural relationships both in amphiboles and muscovite-chlorite schists indicate that S 1 and S 2 record two distinct episodes of metamorphism and deformation, rather than a single progressive event. 40 Ar/ 39 Ar geochronologic analyses on hornblende, actinolite, muscovite, and biotite constrain timing of the youngest metamorphic/deformational event. Most amphiboles yield complex 40 Ar/ 39 Ar age spectra, but two hornblende concentrates give preferred ages of 1410 ± 12 Ma and 1399.1 ± 5.4 Ma, and one actionlite has a preferred age of 1391.6 ± 5.0 Ma, suggesting cooling below ∼450°C at this time. These ages are interpreted to record the timing of near-peak metamorphism. Muscovite sampled over a 1.5-km vertical section of muscovite-chlorite schists shows an age discordance, with the structurally highest sample being ∼50 m.y. older than the structurally lowest sample. This age discordance is interpreted to sugest cooling from ∼300°C at 0.5°C/m.y. following the peak of ca. 1400-Ma metamorphism. Biotites from similar structural levels yield variable preferred ages, which range from 1402.6 ± 5.1 to 1267.8 ± 5.7 Ma and corroborate the slow cooling suggested by the muscovite results. Together, the thermochronologic, structural, and petrologic data support a model of regional deformation, metamorphism, and mineral growth at ca. 1450–1400 Ma. These data add to a growing body of evidence from the southwestern United States that ca. 1400-Ma plutonism was not anorogenic, but rather was contemporaneous both with metamorphism and deformation.
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ABSTRACT The prograde amphibole that coexists with chlorite, epidote, muscovite, albite, quartz and hematite in Sanbagawa schists was examined to investigate the relationship between the prograde P‐T paths of individual rocks and the metamorphic field gradient in the Sanbagawa metamorphic belt, central Shikoku. The amphibole changes from actinolite, through ferri‐winchite and crossite, to barroisite and hornblende with increasing grade along the metamorphic field gradient. However, the sequence of prograde amphibole compositions in each sample varies in different mineral zones. The general scheme can be summarized as: magnesioriebeckite‐riebeckite crossite in the upper chlorite zone of lower‐grade rocks; crossite or glaucophane barroisite in the garnet zone of medium‐grade rocks; and actinolite or winchite barroisite hornblende in the albite‐biotite zone of higher‐grade rocks. Changes of amphibole composition indicate that the prograde P‐T path recorded in the higher‐grade rocks was situated on the higher‐temperature side of that of the lower‐grade rocks and on the lower‐pressure side of the metamorphic field gradient. The systematic change of P‐T paths implies an increasing d P /d T during continuous subduction. These features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a non‐steady‐state geotherm.
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Microscopic and electronic microprobe studies carried out on the chlorite-amphibole schists and the metadiabases from the vicinity of Vrteska determined an actinolite, Mg-hornblende, biotite, epidote, chorite, albite, calcite, quartz, and ilmenite assemblage. The metamorphic mineral assemblage and structural-textural characteristics indicate that these rocks metamorphosed in conditions of regional dynamothermal metamorphism in a Barovian type series of facies which, according to the degree of metamorphism, corresponds to the transition from quartz-albite-epidote-biotite to quartz-albite-epidote-almnadine subfacies. The chemical composition of actinolite and hornblende indicate that the rocks under investigation metamorphosed in conditions of prograde metamorphism at T = 400-500°C and P = 2-5 kbar.
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Metamorphic rocks in Bayah Complex included by Bayah mountain zone. Purpose of this research is to know characteristic of this rock with using petrography methods, that is, identify texture, structure, mineral contain, and metamorphic facies of the rocks. The result showed that kind of the rock is dominated by biotite schist, some actinolite schist, hornblend schist and chlorite schist. Almost all them have retrograde metamorphism, is characterized by biotite altered to chlorite and muscovite, hornblende to actinolite, and actinolite to chlorite. This proses probably associated by uplift processes in Eocene – Oligocene.
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Metabasites of the Lesser Himalaya along the Modi Khola valley in central Nepal are the supracrustal dikes and sills of basic igneous rocks emplaced in the clastic sediments and later metamorphosed together with the host rocks. They contain almost a constant metamorphic mineral assemblage of Ca-amphiboles+plagioclase+biotite+quartz±epidote±chlorite+(Fe-Ti oxides). Amphiboles occur in the form of porphyroblast and recrystallized matrix. The porphyroblasts are zoned with actinolitic cores and hornblende rims. The recrystallized matrix ranges in composition from actinolite in chlorite zone to hornblende in biotite and garnet zones. The cores of porphyroblasts are pre-kinematic and were probably formed prior to the Tertiary Himalayan orogeny. The rims of porphyroblasts and matrix amphiboles are syn-kynematic and were formed during the Upper Main Central Thrust activity in the Tertiary. The systematic changes in amphibole compositions as well as textural characteristics of metabasites are in good agreement with the classical concept of increasing metamorphic grade and intensity of deformation structurally upwards towards the Upper Main Central Thrust in the Lesser Himalaya. doi: 10.3126/bdg.v11i0.1428 Bulletin of the Department of Geology, Tribhuvan University, Kathmandu, Nepal, Vol. 11, 2008, pp. 5-12
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Abstract Regional variation in the P–T path of the Sambagawa metamorphic rocks, central Shikoku, Japan has been inferred from compositional zoning of metamorphic amphibole. Rocks constituting the northern part (Saruta River area) exhibit a hairpin type P–T path, where winchite/actinolite grew at the prograde stage, the peak metamorphism was recorded by the growth of barroisite to hornblende and sodic amphibole to winchite/actinolite grew at the retrograde stage. In the southern part (Asemi River area), rocks exhibit a clockwise type P–T path, where barroisite to hornblende core is rimmed by winchite to actinolite. The difference in P–T path could suggest a faster exhumation rate (i.e. more rapid decompression) in the southern than in the northern part. On the other hand, physical conditions of deformation during the exhumation stage have been independently inferred from microstructures in deformed quartz. Recrystallized quartz grains in rocks from the low‐grade (chlorite and garnet) zones are much more stretched in the southern part (aspect ratio ≥ 4.0) than in the northern part (aspect ratio< 4.0), indicating a higher strain rate in the former than in the latter. These facts may indicate that the exhumation and strain rates are correlated (i.e. the exhumation rate increases with increasing the strain rate). The difference in the exhumation rate inferred from amphibole zoning between the northern and southern parts could be explained by an extensional model involving normal faulting, where the lower plate can be exhumed faster than the upper plate due to the displacement along the fault. Furthermore, the model may explain the positive correlation between the exhumation and strain rates, because the lower plate tended to support more stress than the upper plate.
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Abstract Several bodies of moderately foliated and porphyroblastic metabasites crop out on the SE side of the metamorphic cover of the Karkonosze granite within metapelites of the Sowia Dolina area (West Sudetes, Saxothuringian zone). Depending on the microstructural setting of the Ca-amphiboles in the rocks, different mineral-chemical trends have been determined for Si, X Mg , Al VI , A [Na+K] which serve as semi-quantitative indicators of temperature and pressure changes. Porphyroblasts and prisms oblique to the main foliation in schistose metabasites show zoning from Mg-hornblende and actinolite to tschermakite, and then to Mg-hornblende (or actinolite). Matrix amphiboles and those in pressure shadows around some porphyroblasts have tschermakitic cores and actinolitic rims. Rarely, Ca-amphibole is accompanied in schists by late- to post-tectonic cummingtonite. Thermobarometric calculations involving empirically calibrated amphibole equilibria enable a reconstruction of P-T paths for individual rocks and the unravelling of the metamorphic evolution of the metabasites. Peak metamorphic temperatures of 615–640°C and pressures of 7.3–8.2 kbar were preceded by a variably preserved earlier stage (T = 370–550°C, P = 2.8–6.2 kbar). The final metamorphic episode took place at 450–550°C and 2.5–4.8 kbar and is recorded particularly in rocks close to the Karkonosze pluton. The metabasites shed new light on the history of metamorphism in the Sowia Dolina area. The first two stages of MP-MT metamorphism, coeval with Variscan deformation events (continental collision, burial and subsequent exhumation), took place under epidote-amphibolite then amphibolite facies conditions. The last stage partly concurred with the final stages of Variscan deformation and overlapped the onset of thermal activity associated with the Karkonosze granite. This metamorphic event is documented by metabasites (occasionally cummingtonite-bearing) outcropping close to the granite. Finally, a prehnitebearing assemblage reflects retrograde re-equilibration under greenschist/sub-greenschist facies conditions ( T <300–350°C, P <2.5–3 kbar), which might also be partly due to hydrothermal activity around the pluton.
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