Growth of mica porphyroblasts under low-grade metamorphism – A Taiwanese case using in-situ 40Ar/39Ar laser microprobe dating
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Muscovite
Mylonite
Staurolite
Recrystallization (geology)
Phengite
Orogeny
Pressure solution
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The Xiaotian-Mozitan fault zone(XMF) is a very important fault in the northern part of the Dabie Mountain.The compositional analysis of muscovite in mylonites of XMF was carried out by means of an electronic microprobe.The Si atom of the matrix muscovite is more than 3.3(p.f.u) on average,which shows these muscovites are classic phengites.And the data show that the growing pressure of these phengites was 1.0 GPa on average as determined by Massonne's geobarometer and these phengites were formed at the depth of about 35 km.These results showed that the deformation temperature of mylonites is 600— 700 ℃ as revealed by the deformation characteristics of quartz and feldspar and the geothermal gradient is 16.0— 22.5 ℃/km.All of these suggest that the XMF recorded the information about HP-UHP rocks exhumation as a synorogenic exhumation boundary.Additionally,the porphyroclastic muscovite is similar to the matrix muscovite in Si atom,which indicates that 1.0 GPa is the minimum pressure during the formation of porphyroclastic muscovite,suggesting the protolith of the mylonites had possiblly experienced higher-degree metamorphism.
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The metamorphic evolution of metapelites from the eastern part of the Monte Rosa nappe and the Camughera–Moncucco unit, both situated in the upper Penninic units SW of the Simplon line, were investigated using microstructural relationships and equilibrium phase diagrams. The units under consideration experienced pre-Alpine amphibolite-facies conditions and underwent a complex metamorphic evolution during the Alpine orogeny. Peak pressures during an early Alpine high-pressure stage of 12·5–16 kbar were similar in the Monte Rosa nappe and Camughera–Moncucco unit. A pronounced thermal gradient is indicated during decompression leading to an amphibolite-facies overprint, as the decompression paths went through the chlorite, biotite and plagioclase stability fields in most of the Monte Rosa nappe, through the staurolite field in the easternmost Monte Rosa nappe and in the Camughera–Moncucco unit, and through the sillimanite field in the easternmost Camughera–Moncucco unit. In high-Al metapelites the initial formation of staurolite is related to continuous paragonite breakdown and associated formation of biotite. In the course of this reaction phengite becomes successively sodium enriched. In low-Al metapelites, in contrast, the initial staurolite formation occurs via the continuous breakdown of sodium-rich phengite. In both low- and high-Al metapelites the largest volume of staurolite is formed during the continuous breakdown of sodium-rich phengite below P–T conditions of about 9·5 kbar at 600–650°C. During this reaction phengite becomes successively potassium enriched as sodium from phengite is used to form the albite component in plagioclase. For 'normal' pelitic chemistries, phengite becomes Na enriched during decompression through the breakdown of paragonite along a near-isothermal decompression path. The Na content in phengite reaches its maximum when paragonite is entirely consumed. During further decompression the paragonite component in phengite decreases again because Na is preferentially incorporated into the albite component of plagioclase.
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<p>Pressure recorded in metamorphic rocks is typically assumed to represent a hydrostatic stress and thus depends linearly on depth. Recently, work in the Monte Rosa nappe in the western Alps has challenged this lithostatic assumption. Observable pressure differences of 0.8 &#177; 0.3 GPa between chloritoid, talc, and phengite-bearing lithologies (locally known as &#8216;whiteschists&#8217;) at ca. 2.2 - 2.5 GPa, and metagranite lithologies at 1.4 &#8211; 1.6 GPa have been recorded. These pressure variations, rather than being attributed to variable rock kinetics, partial retrogression, or tectonic mixing, have been interpreted to be mechanically induced. As part of this ongoing investigation, we will present work undertaken on newly discovered staurolite-chloritoid bearing metapelites belonging to the Monte Rosa basement, in order to constrain the peak pressure and temperature conditions during burial within the Alpine orogeny and the resulting tectono-metamorphic and geodynamic implications.</p><p>Metapelitic samples from the Monte Rosa basement show a rich polymetamorphic history from high-T Variscan garnet growth through to high-P Alpine equilibration and decompression. Extensive phase petrology calculations have been undertaken on staurolite + chloritoid + phengite + paragonite assemblages, as well as garnet + chlorite + phengite + paragonite assemblages, representing equilibration at peak Alpine conditions. Various mixing models were employed due to non-negligible amounts of ZnO recorded in staurolite (~5% wt% and ~1 a.p.f.u) and the lack of available solution models. These result in peak Alpine conditions of 1.6 &#177; 0.2 GPa and 580 &#177; 15 &#186;C. These findings confirm the presence of significant disparities in pressure of 0.6 &#177; 0.2 GPa within the coherent Monte Rosa nappe.</p><p>Vital for the reconstruction and tectonic history for the western Alps is the maximum burial depth of units involved. We argue that the maximum burial depth of the Monte Rosa unit was significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at > 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal or less than ca. 60 km, estimated from pressures of 1.4 - 1.6 GPs recorded frequently in metagranite and metapelitic lithologies. This depth is compatible with burial and exhumation within an orogenic wedge, rather than a complex exhumation mechanism such as within a weak and long subduction channel. Equally, the relatively slower exhumation rates from shallower crustal depths fit more reasonable tectonic velocities.</p>
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