Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier
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Keywords:
Anticline
Sillimanite
Orogeny
Isograd
Andalusite
The distribution of pyrite and pyrrhotite is documented within an andalusite-sillimanite type (high-temperature, low-pressure) metasedimentary succession exposed in the Hyland River region of southeastern Yukon, Canada. The following metamorphic zones are recognized: chlorite, biotite, cordierite/staurolite (porphyroblast-in), andalusite, sillimanite, and K-feldspar + sillimanite. Pyrite occurs in the chlorite zone through the biotite zone, while pyrrhotite occurs from the chlorite zone to K-feldspar + sillimanite zone. The pyrite-pyrrhotite transition, therefore, occupies an interval in the chlorite and lower biotite zones that is terminated upgrade by a pyrite-out isograd in the upper part of the biotite zone or lowest grade part of the cordierite/staurolite zone. Pressure and temperature conditions of the rocks were estimated from phase equilibrium modelling and from Raman spectroscopy of carbonaceous material (RSCM) thermometry. Modelling indicates pressures of 3.7-4.1 kbar with temperatures of ~425 °C at the biotite isograd, 560-570 °C for chlorite-out/porphyroblast-in, ~575 °C for andalusite-in, 575-600 °C for the sillimanite isograd, and 645-660 °C at the K-feldspar + sillimanite isograd. RSCM temperatures are greater than or equal to 420 °C in the Chl zone, 500 °C at the Bt isograd, 525-550 °C for porphyroblast-in isograd, ~550 °C at the And isograd, and 580 °C at the Sil isograd. These results suggest the pyrite-pyrrhotite transition occurs from less than or equal to 420°C to ~560 °C. Thermodynamic modelling shows 0.6 wt. % H2O is released during metamorphism over the ~140 °C interval of the pyrite-pyrrhotite transition. The gradual release of fluid in the biotite zone is interpreted to have broadened the pyrite-pyrrhotite transition compared to other studies that predict a small interval of vigorous fluid release associated with volumetric chlorite consumption. Samples from the pyrite-pyrrhotite transition zone contain lower whole rock and pyrite Au values than samples from unmetamorphosed/lower rocks, suggesting that Au was removed from the rock at conditions below the pyrite-pyrrhotite transition (<420 °C). The chlorite zone and higher-grade metamorphic rocks of the Hyland River area do not appear to be a plausible source region for orogenic gold.
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ABSTRACT In 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 K D [= X ] values close to unity; the low Fe 2 O 3 contents are compatible with reducing conditions implied by the ubiquity of graphite. Because K D → 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.
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Andalusite
Sillimanite
Isograd
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Pelite
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Andalusite
Sillimanite
Staurolite
Isograd
Geothermobarometry
Muscovite
Hornblende
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Sillimanite
Isograd
Andalusite
Equilibrium fractionation
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Abstract A high‐temperature ( T ) metamorphic complex occurs in the Omuta district, northern Kyushu, Japan. Three metamorphic zones are defined based on pelitic mineral assemblage, i.e. chlorite–biotite zone, muscovite–andalusite zone and sillimanite–K‐feldspar zone with ascending metamorphic grade from north to south. Two isograds trend approximately east–west, which is oblique to the boundary between the metamorphic complex and the Tamana Granodiorite located on the southeast. The metamorphic condition of two pelitic rocks that occur in the muscovite–andalusite zone and sillimanite–K‐feldspar zone are estimated as 510 ±30 °C, 300 ±60 MPa and 720 ±30 °C, 620 ±60 MPa, respectively. Thermodynamic consideration reveals that use of the same geothermobarometer enables precise determination of the difference in pressure between the samples as 320 ±10 MPa. This indicates that the pelitic samples were metamorphosed at different depth by 11–12 km that is significantly larger than the geographic distance of 6.8 km between the sample localities. This also suggests that crustal thinning took place after the high‐ T metamorphism. The high‐ T metamorphic complex is, therefore, not of static contact metamorphism but of dynamic regional metamorphism. The present result combined with petrological and chronological similarities implies that this complex suffered the regional Ryoke metamorphism.
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Muscovite
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The Orrs Island-Harpswell Neck area on the southern Maine coast experienced one major low-pressure metamorphic event, which was synchronous wilh D2 deformation. Metamorphic grade increases from southwest to northeast toward plutons outside the area. Isograds in pelitic rocks divide the area into garnet, staurolite, staurolite-andalusite, staurolite-sillimanite, and sillimanite zones. At the sillimanite isograd, andalusite began to be replaced by coarse-grained muscovite to form prograde pseudomorphs, while sillimanite formed in the matrix. At slightly higher grade, staurolite was replaced by muscovite pseudomorphs. and additional sillimanite formed.
Isograd reactions deduced from changes in AFM topology were located in pressure-temperature space on the basis of experimental and thermodynamic data. The resulting petrogenetic grid constrains the pressure during metamorphism to between 2.3 and 3.0 kbars (230 and 300 MPa). Estimated temperature ranges for each of the metamorphic zones are as follows: garnet zone, <500°C; staurolite zone, 500 to 530°C; stauroli te-andalusite zone, 530 to 570°C; staurolite-sillimanite zone, 570 to 590°C; and sillimanite zone, >590°C. Metamorphism in the Orrs Island-Harpswell Neck area is similar to M3 metamorphism in west-central Maine, because both events occurred at pressures where staurolite gave way to sillimanite at high grade, and both were related to plutons. Metamorphism in the study area differs from M3 in west-central Maine in diat it produced andalusite at intermediate grade.
RÉSUMÉ
Sur la cote méridionale du Maine, la région d'Orrs Island-Harpswell Neck subit un épisode majeur de métamorphisme à basse pression synchrone de la déformation D2. Le degré de métamorphisme augmente du sud-ouest au nord-est en direction des plutons hors de la région. Les isogrades des pélites définissent dans cette région des zones à grenat, staurotide, staurotide-andalousite, staurotide-sillimanite, et a sillimanite. A l'isograde de la sillimanite, l'andalousite commença, à être remplacée par une muscovite grossiere pour former des pseudomorphes pro grades, alors que lasillimanite se forma dans la malrice. A un degré un peu plus éleveé des pseudomorphes de muscovite remplacerent la staurotide et d'autre sillimanite apparût.
On situa dans l'espace pression-température les réactions d'isogrades déduiles des changements dans la topologie AFM à partir de données expérimentales et thermodynamiques. Le champ pétrogénéique résultant ceme la pression durant le métamorphisme entre 2.3 et 3.0 kbars (230 et 300 MPa). Voici l’e'tendue estimée des températures pour chaque zone métamorphique: zone à grenat <500°C, zone a staurotide 500 à 530°C, zone a staurolide-andalousite 530 à 570°C, zone à staurt'tide-sillimanite 570 à 590°C et zone a sillimanite 590°C. Le métamorphisme dans la région d'Orrs Island-Harpswell Neck rappelle le métamorphisme M3 du centre et de l’ouest du Maine en ce que l'un el l'autre épisodes prirent place à des press ions auxquelles la sillimanite se substitua à la staurotide a un haut degré et que tous deux sont lies à des plutons. Le métamorphisme dans la région étudiée différede M3 du centre étde l'ouestdu Maine par la production d'andalousite à un degré intermédiate.
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Muscovite
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【The Janggun mine area is occupied by the Proterzoic and the Paleozoic meta-pelites, which are intruded by the Jurassic Chunyang granite. The metamorphic terrain is divided into four zones of progressive metamorphism on the basis of mineral assemblages. The zones are chlorite zone, staurolite zone, andalusite zone, sillimanite zone ascending order. Boundary lines between the zones resemble outline of the Chunyang granite mass. Isograd reactions are chlorite+chloritoid+muscovite=staurolite+biotite+quartz+water, staurolite+chlorite+muscovite+quartz=andalusite+biotite+water, and staurolite+muscovite+quartz=andalusite+biotite+garnet+water between the chlorite zone and the staurolite zone, the staurolite zone and the andalusite zone, and the andalusite zone and the sillimanite zone, repectively. They are univariant reactions in KFMASH component system. Metamorphic conditions estimated from garnet-biotite geothermometers and phase equlibria are $530^{\circ}C$ and lower than 4 kb.】
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Abstract In north‐central Wopmay Orogen, syntectonic low‐P(Buchan‐type) suites of mineral isograds outline regional metamorphic temperature culminations that are associated, at the higher structural levels, with emplacement of early Proterozoic plutons in the west part of a deformed and eastward transported continental margin prism. The mapped isograds mark the first occurrence of biotite, staurolite, andalusite, sillimanite, sillimanite‐K feldspar and K feldspar‐plagioclase‐quartz ± muscovite (granitic) pods in metapelites, with increasing proximity to the plutons. Microprobe analyses and field observations have resulted in the formulation of reactions for the ‘ideal’pelitic system K 2 O‐Na 2 O‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ‐H 2 O‐Al 2 O 3 ‐SiO 2 ‐H 2 O, to account for the various mineral assemblages of each metamorphic zone. A P‐T petrogenetic grid showing erosion surface P‐T curves for the northern Wopmay Orogen pelites, compiled on the basis of the mapped isograds and the inferred reaction(s) for each metamorphic zone, documents a variation in exposed metamorphic pressure ranging between 2 and 4 kbar. The configuration of a new bathograd, based on the invariant model reaction sillimanite + K feldspar + plagioclase + biotite + quartz + vapor ± muscovite + liquid and interpolated across three metamorphic suites, is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite‐grossular‐quartz‐Al 2 SiO 5 geobarometry. The positive correlation between the configuration of the bathograd and the structural and pressure culmination points to the pressure‐dependence of anatectic‐granitic‐pod mineral associations.
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