U-Pb dating of major and accessory minerals formed during metamorphism and deformation of metapelites
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Staurolite
Isochron dating
Sillimanite
Pegmatite
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A kyanite-sillimanite isograd is well-exposed in the Mica Creek area, British Columbia, and both pelitic and basic rocks occur within a few hundred meters of the isograd. This provides us with an opportunity to test geothermobarometry in pelitic and basic bulk compositions against an independent metamorphic equilibrium, kyanite-sillimanite. The kyanite-sillimanite isograd is modeled by a univariant P-T curve that passes through the following points (T °C, P kbar): 564/5; 610/6; 656/7; and 796/10. The quoted experimental uncertainty is 25 °C or about 350 J in ΔG. Samples that bracket the kyanite-sillimanite isograd in the Mica Creek area have the following mineral assemblages: quartzmuscovite- biotite-garnet-plagioclase-kyanite or sillimanite in pelitic rocks; and hornblende-plagioclase- clinopyroxene-garnet-quartz in basic rocks. Using the thermodynamic database of Berman (1988) and the INVEQ program of Gordon (1992), we have calculated metamorphic pressures and temperatures for 17 samples that bracket the kyanite-sillimanite isograd.
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Connemara pelites show progressive metamorphism from staurolite to upper sillimanite zones and possess low Mg/(Fe + Mg) values, typically 0.30 to 0.35 from about 100 analyses. As a consequence of their composition, many sillimanite zone pelites lack both muscovite and K-feldspar. Staurolite, garnet, biotite, muscovite, feldspars and iron ores have been microprobe analysed in 48 samples. Assemblages, textures and mineral compositions indicate that metamorphism followed a sequence of continuous and discontinuous reactions with systematic variations in mineral Mg/(Mg + Fe) as predicted by theory. Contrary to some common assumptions, most reaction takes place along divariant equilibria; univariant reactions are seldom reached because reactants such as chlorite or muscovite are first consumed along divariant curves. Pelite petrogenetic grids showing univariant curves can only indicate limits to natural assemblages; they typically do not show which reactions have actually taken place. Physical conditions of metamorphism have been calculated by a variety of means; temperatures range from 550° for the staurolite zone to 650° for the upper silimanite zone, with the first appearance of sillimanite near 580°. An early kyanite-staurolite metamorphism at pressures above about 5 kb was followed by a steepening of the thermal gradient leading to regional cordierite and andalusite. This was probably accompanied by uplift with pressures of around 4 kb for roeks near the sillimanite-in isograd.
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A complete Barrovian sequence ranging from unmetamorphosed shales to sillimanite–K‐feldspar zone metapelitic gneisses crops out in a region extending from the Hudson River in south‐eastern New York state, USA, to the high‐grade core of the Taconic range in western Connecticut. NNE‐trending subparallel biotite, garnet, staurolite, kyanite, sillimanite and sillimanite–K‐feldspar isograds have been identified, although the assignment of Barrovian zones in the high‐grade rocks is complicated by the appearance of fibrolitic sillimanite at the kyanite isograd. Thermobarometric results and reaction textures are used to characterize the metamorphic history of the sequence. Pressure–temperature estimates indicate maximum metamorphic conditions of 475 °C, c . 3–4 kbar in the garnet zone to >720 °C, c . 5–6 kbar in the highest grade rocks exposed. Some samples in the kyanite zone record anomalous (low) peak conditions because garnet composition has been modified by fluid‐assisted reactions. There is abundant petrographic and mineral chemical information indicating that the sequence (with the possible exception of the granulite facies zone) was infiltrated by a water‐rich fluid after garnet growth was nearly completed. The truncation of fluid inclusion trails in garnet by rim growth or recrystallization, however, indicates that metamorphic reactions involving garnet continued subsequent to initial infiltration. The presence of these textures in some zones of a well‐constrained Barrovian sequence allows determination of the timing of fluid infiltration relative to the P–T paths. Thermobarometric results obtained using garnet compositions at the boundary between fluid–inclusion‐rich and inclusion‐free regions of the garnet are interpreted to represent peak metamorphic conditions, whereas rim compositions record slightly lower pressures and temperatures. Assuming that garnet grew during a single metamorphic event, infiltration must have occurred at or slightly after the peak of metamorphism, i.e. 4–5 kbar and a temperature of c . 525–550 °C for staurolite and kyanite zone rocks.
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Metamorphic reactions related to isograds derived from aluminum silicate-bearing pelitic schists were studied in an area of Grenville Province adjacent to Southern Province rocks near Sudbury, Ontario. Progressing from the northwest to southeast of the area, the meaningful hand-drawn isograds are: (1) sillimanite first occurrence, (2) the last occurrence of staurolite when associated with the entire assemblage, (3) K-feldspar first occurrence, (4) staurolite last occurrence as inclusions in garnet, (5) muscovite last occurrence, and (6) kyanite last occurrence. Whole-rock chemical analysis of 14 representative pelitic schist hand specimens in the area were collected and used to show that metamorphic factors, and not chemical differences, were responsible for the metamorphic isograds. The entire area lies thermally above the melting of rocks of granitic composition. Breakdown curves of the minerals related to the isograds have been used to imply a gradient of 670 °C to 750 °C and 6.3 to 7.3 kilobars, across the area, but the equations for these breakdowns are not entirely substantiated by the modal abundance and textural data.To a first approximation, the rocks may be considered homochemical, but many deviations (due partly to metasomatic change) from this exist. The ionic breakdown of kyanite to muscovite has been shown and an explanation as to why muscovite selectively replaces kyanite and not sillimanite is given. The breakdown of muscovite at the higher grades has been inferred to form K-feldspar, but not sillimanite. Near the kyanite isograd, textures showing the thermal breakdown of kyanite (left over after the partial ionic breakdown of the mineral) to sillimanite are shown. The rocks must have had at least K and possibly Fe added metasomatically to account for the textures shown. From generalized modal abundance surfaces (trend surface analysis), general equations representing the difference in modal abundance of minerals across various isograds were determined and from these, specific equations explaining the breakdown of a particular mineral at its isograd were derived. The most significant of these reactions is the first staurolite isograd, where it is inferred to breakdown in the following way, in the area studied:[Formula: see text]The dissolved Al and Si forms the fibrolite (sillimanite) lenses common in adjacent pelitic rocks
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Abstract The relations between Barrovian‐ and Buchan‐type metamorphic series in the Chinese Altai remain obscure, and hence a representative region of the central part of the Chinese Altai was investigated to address this issue, using combined microstructural, petrological and geochronological methods. In the region, Barrovian‐type garnet, staurolite, kyanite and sillimanite zones are locally overprinted by Buchan‐type andalusite‐ and cordierite‐bearing domains. Microstructural analysis shows that Barrovian garnet, staurolite and kyanite grew synchronously with the earliest regional metamorphic foliation S1 B . A sillimanite‐bearing assemblage locally overprinted the assemblage of the staurolite zone in a foliation parallel with S1 B , assigned as S1 M . The originally subhorizontal S1 B‐M foliation, metamorphic zones and mineral isograds were folded by F2 upright folds, leading to their inclination and juxtaposition to upper crustal levels. Subsequent D3 deformation affected heterogeneously all previous structures producing vertical high‐strain zones around low‐strain domains. The D3 high‐strain zones in the vicinity of Permian pegmatites are associated with Buchan‐type metamorphism and are characterized by syn‐D3 growth of andalusite and cordierite. Phase equilibria modelling of the staurolite/kyanite‐bearing assemblages suggests a prograde P–T path with an apparent thermal gradient of ~23°C/km associated with the S1 B fabric. The partial re‐equilibration occurred in the sillimanite stability at ~670°C, corresponding to an apparent thermal gradient of ~34°C/km in the S1 M fabric. Garnet to sillimanite metamorphic zones were subsequently exhumed without apparent re‐equilibration during the D2 event. The interpreted pressure – temperature ( P–T ) evolution of the Buchan‐type metamorphism on the basis of thermobarometry and phase equilibria modelling suggests significant heating processes, corresponding to apparent thermal gradient of 41°C/km or more. In situ U–Pb dating of monazite inclusions in staurolite revealed predominantly 280–260 Ma ages and minor older ages scattering between 350 and 290 Ma, interpreted as important monazite recrystallization during the D3 event. Monazite in andalusite and cordierite yielded only ages of 280–260 Ma, interpreted as dating the growth of these minerals. Lu–Hf garnet–whole rock isochron of a garnet–cordierite–chlorite schist gave an age of ca. 262 Ma, overlapping in time with the age of monazite in the cordierite. Combined with available regional data, the results suggest that the Barrovian‐type metamorphic cycle reflects a continued burial heating followed by decompression, probably connected with the Devonian suprasubduction tectonic switching between shortening and extension events. In contrast, the Permian Buchan‐type metamorphism documents an important heat input associated with regional NE–SW shortening, probably related to the Early Permian collision between the Chinese Altai and the southerly Junggar arc system.
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The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912 and the Barrovian metamorphic zones continue to be used as a framework for describing regional metamorphism. Pressure, temperature, and protolith composition are widely recognized as primary controls on index mineral distribution. Today, metamorphic fluid flow is also recognized as an important driver of metamorphic reactions. The aim of this study is to establish if and how metamorphic fluids control index mineral distribution during Barrovian metamorphism. We use samples from Barrow’s type locality in Glen Esk, SE Scottish Highlands, to study possible relationships between veining and index mineral distribution. In addition to petrographic and textural observations, we use whole-rock compositions, mineral compositions and oxygen isotope analyses. At low grade, in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. At higher grade, in the garnet and staurolite zones, index mineral abundance is shown to be higher adjacent to veins. These trends coincide with other mineralogical, chemical, and isotopic changes in the vein-proximal rock, indicative of fluid–rock interaction. Kyanite distribution is homogeneous in the kyanite zone. However, we show that this too relates to extensive fluid–rock interaction. Garnet-, staurolite-, and kyanite-bearing selvedges are common in the sillimanite zone. However, sillimanite distribution is unrelated to these selvedges, which supports models arguing that sillimanite formed during a separate metamorphic event. We infer fluid flow from high grade to low grade because the fluid was out of isotopic equilibrium with the lower grade rocks, but in equilibrium with the higher grade rocks. We conclude that fluid flow played a major role in the stabilization and distribution of Barrovian index minerals in Glen Esk, and that the importance of fluid flow was greater at higher metamorphic grades.
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Geothermobarometry
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