Composition Of Muscovite Research Articles

Petrology, geochemistry and zircon U–Pb–Lu–Hf isotopes of Paleoproterozoic granite gneiss from Bomdila in the western Arunachal Himalaya, NE India

Abstract Felsic magmatic bodies are exposed widely in the Bomdila region of the western Arunachal Himalaya, NE India. The litho-units of this region are primarily composed of two-mica (muscovite–biotite (ms–bt)) granite gneiss, referred to herein as the Bomdila granite gneiss (BGGn), and the metasediments of the Bomdila Group forming an integral part of the NE Lesser Himalayan thrust sheet. Phase petrology, whole-rock elemental geochemistry and zircon U–Pb–Lu–Hf isotopes of the BGGn have been investigated in order to decipher the origin and timing of felsic magmatism and its implications for understanding the pre-Himalayan tectonic environment. Modally, the BGGn can be classified into monzogranite, syenogranite and quartz-rich granitoids. The composition of muscovite (Ti = 0.03–0.07, average Na = 0.07 and Al IV = 2.50–2.90 apfu), biotite (FeO t /MgO = 3.1–4.6, 2Al⇌3Fe and Mg⇌Fe substitutions, and the presence of siderophyllite) and tourmaline (Fe/Fe + Mg = 0.56–0.96, Ca < 0.17 apfu) implies their primary magmatic nature crystallized typically in a peraluminous (S-type) felsic parental melt. This is further supported by the presence of ms–bt, whole-rock molar Al 2 O 3 /CaO + Na 2 O + K 2 O (A/CNK = 1.03–1.64) and normative corundum. Whole-rock multi-cationic parameters indicate a syn-collisional tectonic environment. However, the content of Rb (average 300 ppm) and high-field strength elements (HFSEs) in the BGGn indicates syn- to post-collisional tectonic settings. The BGGn parental melt was most likely to have been generated by dehydration melting of metasedimentary sources at middle–upper crustal depths. Geochemical modelling constrains the evolution of the parental melt of the BGGn by a moderate degree of fractional differentiation ( F = 0.45) involving a biotite–plagioclase–K-feldspar–muscovite–titanite–apatite (bt–pl–Kfs–ms–ttn–ap) assemblage. Laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS)-analysed zircons from the BGGn yield a weighted mean 207 Pb/ 206 Pb age of 1752 ± 23 Ma as the crystallization age for the zircons in the BGGn melt, which falls well within the period of magmatism formed during the Columbia supercontinent accretionary orogen. The observed negative ε Hf (t) values (−1.67 to −7.99) and three-stage Hf-model ages (2818, 2586–2424 and 2393–2250 Ma) of zircons, strongly point to the involvement of ancient continental crust and source heterogeneity (Neoarchean–Paleoproterozoic continental crust) in the generation of the BGGn melt. The reworked ancient crustal components would have once been part of the northern Indian lithosphere, as indicated by the observed 207 Pb/ 206 Pb concordant ages (2436, 2136, 2013 Ma) of the inherited zircons.

Age constraints on the deformation style of the South Tibetan Detachment System in Garhwal Himalaya

In the Garhwal Himalaya, the Malari granite, a small pluton, intrudes the upper portion of the Greater Himalayan Sequence (GHS) and is deformed in the South Tibetan Detachment System (STDS) shearing. In the present work, we undertook a multidisciplinary approach involving microstructural, chemical and geochronological analyses of the samples in order to constrain the timing of shearing along the STDS. Microstructural observations of samples suggest a switch in deformation features with structural position from the structurally upper samples to the structurally lower one. X-ray maps and electron microprobe analyses reveal a quite uniform chemical composition of muscovite in the three selected samples. 40Ar/39Ar muscovite ages constrain the waning stages of ductile deformation: in rocks ductilely deformed by the STDS activity, they record recrystallization at 16.0-16.5 Ma; the age of static muscovite grown after sillimanite at 14.3 Ma post-dates cessation of movement on the STDS and pre-dates the main activity of the Main Central Thrust in the Western Himalaya.

The origin of the sediment-hosted Kanmantoo Cu-Au deposit, South Australia: Mineralogical considerations

Multiply deformed sediments of the Cambrian Kanmantoo Group, which were metamorphosed to the amphibolite facies, host numerous Cu-Au, Fe-S, and Pb-Zn-Ag-(Cu-Au) deposits, of which the largest Cu-Au deposit is Kanmantoo (34.5 Mt @ 0.6% Cu and 0.1 g/t Au). Mineralization at Kanmantoo is characterized by discordant and pipe-like orebodies (Kavanagh and Emily Star) along with mineralization locally concordant to bedding (Nugent) that is spatially related to meta-exhalative rocks. Previous studies have suggested a syn-sedimentary origin for the Pb-Zn-Ag-(Cu-Au) and Fe-S deposits, whereas the Kanmantoo deposit remains controversial, with syn-sedimentary, metamorphogenic, and post-peak metamorphic models having being applied. The stratiform nature of some parts of the Nugent orebody, and disseminated sulfides locally concordant to bedding along with the recognition of a zone of chalcopyrite-magnetite-rich rocks at the syn-sedimentary Wheal Ellen Pb-Zn-Ag-(Cu-Au) deposit, which shows a metallic mineral assemblage almost identical to the most common assemblage at Kanmantoo, supports a genetic link between the Pb-Zn-Ag-(Cu-Au) deposits and Cu-Au mineralization, and is consistent with a metamorphosed syn-sedimentary model for Cu-Au mineralization. The discordant nature of most orebodies at Kanmantoo is the result of the mineralization having formed as stockwork zones in sub-seafloor pipes. Varying degrees of remobilization were subsequently associated with syn- to post-peak metamorphism. Major and trace element compositions, coupled with principal component analyses, show that the compositions of garnet, biotite, staurolite, chlorite, muscovite, and magnetite in metamorphosed altered rocks spatially associated with sulfide mineralization at Kanmantoo can be distinguished from those, where present, in metamorphosed country rocks (i.e., metapsammites and metapelites). Garnet, chlorite, biotite, and muscovite in quartz garnetite within quartz mica schist associated with the Nugent orebody are elevated in Mn (up to 19.5 wt% MnO – garnet, 2,825 ppm – chlorite, 3,206 ppm – biotite, and 108 ppm – muscovite) and Zn (up to 170 ppm – garnet, 1,602 ppm – chlorite, 1,592 ppm – biotite, and 108 ppm – muscovite) relative to samples in other orebodies and in unmineralized rocks elsewhere in the Kanmantoo Group. Such enrichments in these elements mimic similar enrichments in the same minerals in metamorphosed altered rocks associated with Pb-Zn-Ag-(Cu-Au) deposits in the Kanmantoo Group. Biotite in metamorphosed altered rocks at Kanmantoo contains elevated concentrations of Pb (up to 110 ppm), and, in general, Zn (up to 841 ppm), whereas muscovite is also elevated in Pb, Zn, and Cu (up to 272 ppm Pb, 78 ppm Zn, and 173 ppm Cu). Staurolite in these same rocks contains up to 1.6 wt% ZnO (with one outlier that contains 3.2 wt% ZnO) and is considerably more enriched in Zn than in unaltered country rocks (∼0.1 wt% Zn). The trace element enrichments in silicates studied here constitute a potential pathfinder to metamorphosed Cu-Au mineralization in the Kanmantoo Group and emphasize the geochemical and genetic links between the Pb-Zn-Ag-(Cu-Au) and Cu-Au deposits.

Magmatic and meteoric fluid flow in the Bitterroot extensional detachment shear zone (MT, USA) from ductile to brittle conditions

The Bitterroot shear zone developed as a rolling-hinge detachment system where a syntectonic granodiorite in the footwall was progressive exhumed beneath a detachment shear zone, providing a record of deformation and fluid-rock interaction during progressive exhumation and cooling. The shear zone displays a high strain gradient over ∼1km of structural section from the relatively undeformed footwall, where the granodiorite contains a magmatic foliation and lineation, through a mylonite sequence that culminates upward in the fine interlayering of ultramylonite layers and the development of a breccia zone. We measured the stable isotope composition of quartz, muscovite, biotite, chlorite, and epidote across the shear zone and estimated equilibrium temperatures using oxygen isotope thermometry based on mineral pairs. We also measured the hydrogen isotope ratios of hydrous minerals and of quartz fluid inclusions. The main results are: (1) The relatively undeformed footwall granodiorite interacted with magmatic fluids at 500–600°C; (2) the mylonitic fabric defined by muscovite, biotite, and chlorite developed between 500 and 300°C and interacted with a fluid system that was connected to the Earth’s surface (meteoric fluids), as indicated by the low δD values of hydrous mineral phases, including muscovite; (3) the fluxes of surface fluids were not sufficient to shift the δ18O values of muscovite significantly, but were sufficient to control the δ18O composition of biotite and chlorite during deformation-induced recrystallization and chloritization, and (4) the isotopic composition of fluid inclusions in quartz as well as the δD values of late quartz veins track the mixing of fluid sources between the magmatic and meteoric reservoirs. The distribution of stable isotope compositions in the various tectonites of the granodioritic Bitterroot shear zone, from ductile to brittle, provides a rich spatial and temporal record of the interaction between deformation and fluid flow in a crustal-scale detachment.

Petrology and mineral chemistry of peraluminous Marziyan granites, Sanandaj-Sirjan metamorphic belt (NW Iran)

AbstractThe Marziyan granites are located in the north of Azna and crop out in the Sanandaj-Sirjan metamorphic belt. These rocks contain minerals such as quartz, K-feldspars, plagioclase, biotite, muscovite, garnet, tourmaline and minor sillimanite. The mineral chemistry of biotite indicates Fe-rich (siderophyllite), low TiO2, high Al2O3, and low MgO nature, suggesting considerable Al concentration in the source magma. These biotites crystallized from peraluminous S-type granite magma belonging to the ilmenite series. The white mica is rich in alumina and has muscovite composition. The peraluminous nature of these rocks is manifested by their remarkably high SiO2, Al2O3and high molar A/CNK (> 1.1) ratio. The latter feature is reflected by the presence of garnet and muscovite. All field observations, petrography, mineral chemistry and petrology evidence indicate a peraluminous, S-type nature of the Marziyan granitic rocks that formed by partial melting of metapelite rocks in the mid to upper crust possibly under vapour-absent conditions. These rocks display geochemical characteristics that span the medium to high-K and calc-alkaline nature and profound chemical features typical of syn-collisional magmatism during collision of the Afro-Arabian continental plate and the Central Iranian microplate.

Geochemical evolution of micas and Sn-, Nb-, Ta- mineralization associated with the rare metal pegmatite in Angwan Doka, central Nigeria

The pegmatites in Angwan Doka, north central Nigeria are genetically related to the basement granites formed during the Pan-African orogeny, 550–530 Ma ago. They occur as sharply discordant dykes in the granitic and metasedimentary basement rocks. The pegmatite population comprises of mineralogically simple and complexly zoned types that are characterized by LCT (Li, Cs and Ta) geochemical signature. The host granitoids range in composition from hornblende, titanite-bearing to biotite–muscovite granodiorites. Analysis of geochemical data of whole rock and muscovite from the different zones reveals compositional variations and evolution across the pegmatite body from border zone to the lepidolite-quartz core zone. Fractionation of Rb, Cs, Sr, Li, F, B, Be Sn, Zn, Ta, Nb and Mn which increases from host granitoids, through the border zone to the central core, with decrease in Fe, Mg, Ti, Ba content, is typical and marks the magmatic crystallization trend of the pegmatites. Other distinctive attribute of the pegmatites is occurrence of cassiterite believed to have formed as a consequence of greisenization, albitization and late-stage metasomatism, which led to enrichment in Sn (up to 886 ppm) in the intermediate zone. Chemical composition of muscovite from the different zones of the pegmatite reveals high concentration of primary magmatic columbite-Fe (ferrocolumbite and ferrotantalite) in the border zone and tantalite-Mn (manganocolumbite and manganotantalite) in the core zone. Ta predominates (352 ppm) in the most evolved lepidolite (Li- and F-rich) zone while Nb was enriched (up to 714 ppm) in the border zone. These geochemical features are ascribed to undercooling of the melt and crystallization in boundary layers accompanied with increased accumulation of incompatible and fluxing components. With increasing fractionation, Nb/Ta and Fe/Mn ratio decreased and is accompanied with increase in Rb, Cs, Li, F and Be typical of crystallization from magmatic process. The sequence of zonation, origin and formation of the different pegmatite zones can be explained by a single path of fractional crystallization.

Quartz-sericite and argillic alterations at the Peschanka Cu-Mo-Au deposit, Chukchi Peninsula, Russia

The porphyry Peschanka copper-molybdenum-gold deposit and the Nakhodka ore field located in the Baimka ore trend on the western Chukchi Peninsula are spatially related to monzonitic rocks of the Early Cretaceous Egdykgych Complex. Two types of quartz-sericite metasomatic rocks (QSR) have been identified at both the deposits and the ore field: (I) chlorite-quartz-muscovite rock with bornite and chalcopyrite (porphyry type) and (II) tourmaline-quartz-carbonate-muscovite ± phengite rock accompanied by veins with base-metal mineralization (subepithermal or transitional type), as well as carbonate-quartz-illite rock (argillic alteration) accompanied by veins with precious metal mineralization (epithermal type). The QSR I chlorite evolves from chamosite to clinochlore, which is caused by increasing H2S activity in mineralizing fluid and precipitation of sulfide minerals. The QSR I clinochlore is significantly depleted in silica as compared with that from the rocks affected by argillic alteration. The chemical composition of muscovite from both quartz-sericite alterations is similar. The QSR II carbonates evolve from calcite through dolomite to siderite, which results from the increasing activity of CO2 followed by the decreasing activity of H2S in mineralizing fluid. The Mn content in dolomite is similar to that in beresite (quartz-muscovite-carbonate-pyrite metasomatic rock) of the intrusion-related gold deposits. Illite from argillic alteration is depleted in Al as compared with that of postvolcanic epithermal Au-Ag deposits. However, carbonates from the discussed argillic alteration rhodochrosite and Mn-rich dolomite are similar to those from quartz-illite rock at postvolcanic epithermal Au-Ag deposits.

K-Extraction from Muscovite by the Isolated Fungi

The aim of this work was to isolate and identify the muscovite-solubilizing microorganisms, and then to exploit the mechanism of K-extraction by the isolated microorganisms. Ten strains of microorganisms were isolated from soil sample in K-bearing mineral area (Anhui province, China) using Aleksandov agar medium, which had different degrees of muscovite-solubilizing in the K-extraction experiments. After 15-day incubation, the K-extraction ability was evaluated through dissolved structural element of muscovite in aqueous medium (K, Al, Si) by Inductively Coupled Plasma. Four best muscovite-solubilizing microorganisms were identified by 18S rDNA analysis as Penicillium purpurogenum, Talaromyces radicus, Aspergillus fumigatus and Aureobasidium pullulans, respectively. Furthermore, organic acids in the medium were determined by high performance liquid chromatography, and the surface morphology and composition of original and leached muscovite were evaluated using scanning electron microscopy and energy dispersive spectroscopy. According to these analytical results, it was deducted that the interactions between fungi and muscovite, along with excreted organic acids and K-adsorption by the fungal hyphae contribute to muscovite weathering and K-release.

A unique sequential melting mechanism for the generation of anatectic granitic rocks from the Penafiel area, northern Portugal

Granitic rocks derived by sequential partial melting are very uncommon occurrences worldwide with only three known cases in Portugal (Tourém, Guarda–Sabugal and Penafiel) and one in Argentina (Achiras complex). The suite documented in this paper comprises two of the three main phases of a batholith in the Penafiel area. These Variscan peraluminous two-mica granite and granodiorites formed at 310 and 305Ma during and near the end of the Variscan D3 deformation and have the characteristics of S-type granites. None of the metamorphic country rocks have an adequate Sr isotopic composition to be a protolith. It is argued that progressive isotopic re-equilibration of crustal material, probably a metagreywacke for G1 and G3 with also some metapelitic contribution for G2, during granulization of the lower Variscan crust explains the difference in isotopic composition between outcropping granitic rocks and metamorphic country rocks. Biotite>muscovite granite G1 and biotite>muscovite granodiorite G3 have similar (87Sr/86Sr)i and εNdt values and were derived by sequential partial melting of the same source material. This is indicated by coherent trends of the whole rock major and trace element compositions, the compositions of feldspar, biotite, muscovite and tourmaline, the presence and type of xenocrystic mineral phases and xenoliths, and whole rock oxygen isotope compositions. This mechanism was the result of post-thickening Variscan extension and mantle upwelling. Another member of the same batholith is a biotite≈muscovite granodiorite (G2), with an intermediate age of 307Ma, but with geochemical characteristics suggesting that it corresponds to a chemically distinct pulse of magma, although likely stemming from a comparable, though more metapelitic, lower crustal source.

Relationship Between Composition and Spectral Feature of Muscovite

The aim of this study is the exploration of the intrinsic relationship between chemical composition and spectral features of rocks and minerals so as to detect mineral micro-information based on the hyperspectral remote sensing technology.Based on rock and mineral microscopic identification,the authors analyzed spectral features of muscovite from the rock sample by using TSG geological spectrum analysis software,and studied the chemical composition of muscovite by means of electron microprobe analysis.On such a basis,spectral variational features of muscovite were statistically analyzed.The results indicate that the diagnostic spectral wavelength position of muscovite moves towards the long wavelength with the reduction of AlⅥ cations.

Mineralogy and petrogenesis of pegmatites in the Spruce Pine District, North Carolina, USA

*Corresponding author Granitic rocks (mainly granodiorites) of the Spruce Pine Plutonic Suite occur as small plutons and numerous pegmatite bodies intruded into amphibolite-grade schists and gneisses of the Ashe Formation. Intrusion ages range from 377 to 404 Ma and are near the peak of amphibolite-facies metamorphism in the country rocks. The granodiorites have a mortar texture with coarse (~ 1 cm) grains of feldspar, quartz and muscovite enclosed in a finer grained matrix of the same phases. Large (10–30 cm) crystals of perthitic K-feldspar and plagioclase occur in the granodiorite. Pegmatites occur within the granodiorite and as isolated intrusions within the country rocks. Contacts between pegmatites and host granodiorites range from sharp, cross-cutting to completely gradational. Some of the plutons contain up to 50 % pegmatite. Pegmatite mineralogy is similar to the granodiorites; major amounts of feldspars and quartz, accessory amounts of muscovite, garnet, and epidote. Secondary minerals, formed by subsolidus recrystallization, include epidote, muscovite, quartz, and grossular garnet. A few of the pegmatites are more fractionated and contain tourmaline and beryl. Compositions of feldspars, muscovite, and garnet show some regional variation, but are generally similar for pegmatites and associated granodiorite. About half of the pegmatites are unzoned. Zoning in the pegmatites is simple with a quartz core, an intermediate zone of graphic granite, plagioclase and muscovite and an outer finer grained zone that locally shows mineral layering parallel to the contact. The abundance of pegmatitic textures relative to granitoid textures in the Spruce Pine Plutonic Suite is related to the depth of crystallization. Spruce Pine granitoids crystallized at depths of 20–30 km (0.7 to 1.1 GPa ) based on feldspar thermobarometry and experimentally determined melting relations. Skeletal crystals of quartz and feldspar along with plumose muscovite clusters are common in the Spruce Pine pegmatites. Water content of magmas at such high pressures can be high. The subtle transition from granitic to pegmatitic texture so common at Spruce Pine (and in other deep-seated systems) was probably related to the development of zones of water-enriched magma and delayed nucleation in the hydrous melts. Deformation (evidenced by mortar texture) of the Spruce Pine Plutonic Suite was related to later regional deformation.

Microstructural and metamorphic evolution of a high‐pressure granitic orthogneiss during continental subduction (Orlica–Śnieżnik dome, Bohemian Massif)

AbstractA microstructural and metamorphic study of a naturally deformed medium‐ to high‐pressure granitic orthogneiss (Orlica–Śnieżnik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction‐type apparent thermal gradient (∼10 °C km−1). The granitic orthogneisses develops three distinct microstructural types, as follows: type I – augen orthogneiss, type II – banded orthogneiss and type III – mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K‐feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase‐ and K‐feldspar‐rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K‐feldspar ribbons preserved in a fine‐grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K‐feldspar + muscovite + biotite + garnet + sphene ± ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a progradeP−Tpath ranging from <15 kbar and <700 °C (type I orthogneiss) to 19–20 kbar and >700 °C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constantP−Tconditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K‐feldspar whereas a grain boundary sliding‐dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding‐enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

Early Cambrian eclogites in SW Mongolia: evidence that the Palaeo‐Asian Ocean suture extends further east than expected

AbstractNewly discovered eclogites, Early Cambrian carbonates and chloritoid‐bearing metapelites form the Tsakhir Uul accretionary wedge, which was thrust during the Early Cambrian over the Mesoproterozoic Dzabkhan‐Baydrag continent. The rock association of the wedge forms a tectonic window emerging through the hangingwall Khantaishir ophiolite unit, which preserves a typical Tethyan‐type ophiolitic sequence. The eclogites correspond geochemically to T‐MORB modified by fluid circulation. They are composed of garnet, omphacite, amphibole, rutile ±muscovite ±quartz ±epidote and exhibit well‐equilibrated matrix textures. Jadeite content of the omphacite reaches up to 45 mol.%, the Si content of muscovite is between 3.40 and 3.45 p.f.u., amphibole is winchite to barroisite, but reaches tschermakitic composition at some rims, and garnet composition is grs0.24–0.36, alm0.43–0.56, py0.05–0.18, sps0.00–0.18, . The peak assemblage, together with the composition of garnet rims, omphacite, amphibole and muscovite, correspond in a pseudosection to 20−22.5 kbar and 590−610 °C. The tschermakitic rim of amphibole is interpreted as partial reequilibration on decompression below 16 kbar and ∼600−630 °C. Two muscovite separates from the eclogite yielded an Ar–Ar plateau age of 543.1 ± 3.9 Ma (1σ) and a mean age of 547.9 ± 2.6 Ma (1σ), whereas muscovite from an interbedded garnet‐chloritoid micaschist yielded an Ar–Ar plateau age of 536.9 ± 2.7 Ma (1σ); these ages are interpreted as cooling ages. The P–T data, geochemistry of eclogites and cooling ages suggest an affinity between the Tsakhir Uul wedge and the Gorny Altai and the north Mongolian blueschist belt, which are believed typical for subduction of warm oceanic lithosphere and closure of small oceanic basins. Thus, the discovery of the Tsakhir Uul eclogites represents an important finding suggesting extension of the Early Cambrian subduction system of the Central Asian Orogenic Belt far to the east in a region where it was not expected.

Phosphate and sulfate-phosphate mineralization in sillimanite-bearing rocks at the Kyakhta deposit, western Transbaikal region

The phosphate and sulfate-phosphate minerals in the sillimanite-bearing rocks of the Kyakhta deposit are considered. The mineral assemblages of the high-Al rocks were formed during prograde and retrograde stages of metamorphism. The first stage is characterized by the formation of sillimanite, corundum, muscovite, quartz, rutile, titanohematite, magnetite, feldspar, biotite, lazulite, and wagnerite. The muscovite composition showed that sillimanite paragenesis was formed at temperatures above 510–600°C. According to oxygen isotope thermometry, the minimum metamorphic temperature for quartz and titanohematite is 690°C. Andalusite, diaspore, quartz, pyrophyllite, muscovite, and a wide range of phosphates and sulfate-phosphates crystallized during the retrograde stage. The decrease in temperature and increase in the water content led to the following sequence of mineral formation: Mg-Fe-Al-Ca-REE-rich phosphates (lazulite, scorzalite, augelite, apatite, and monazite) → Ca-Sr sulfate-phosphates (woodhouseite and svanbergite) → sulfate (barite) → Sr-Ca-Ba aluminophosphates (goyazite, crandallite, and gorceixite). The chemical compositions of phosphates and sulfate-phosphates minerals and their formation conditions are discussed.

Differentiation of peraluminous leucogranites “en route” to the surface

Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes in the source of melts or to secondary processes such as fractional crystallization. In the Armorican Massif, a Hercynian domain in western France, syntectonic leucogranites were emplaced along major shear zones, the so-called South Armorican Shear Zone. The Lizio and Questembert massifs are derived from a similar metasedimentary source ( ε Nd(T) between − 4 and − 6) and have similar magmatic volumes. They display, however, distinct depths of emplacement, with Questembert being emplaced ca. 5–10 km shallower than Lizio. The shallower emplacement of Questembert is recorded by contact metamorphic assemblages and the levels of intrusion in correlated sedimentary sequences. The Questembert magmas are more differentiated than the Lizio ones, as demonstrated by a lower amount of modal biotite, a higher SiO 2 content (72.8–74.2 wt.% vs. 71.7–73.3 wt.%) and a more pronounced peraluminous character. The high-SiO 2 Questembert magmas can be derived from the low-SiO 2 Lizio ones by 15–20 wt.% fractionation of the assemblage alkali feldspar + plagioclase + biotite. Fractionation of zircon and monazite, hosted in biotite, is indicated by a systematic decrease in Zr and Th + LREE contents. The magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks and mineral separates. The Questembert massif records evidence of a two stage fluid–rock interaction. A high-T stage corresponds to the magmatic fluid exsolution, and is recorded in the most evolved samples: appearance of tourmaline in the matrix, changes in the composition of muscovite, increase in whole rock Be, Sn, Rb and W contents, fractionation of K/Rb, Zr/Hf and Nb/Ta ratios, and fractionation of REE patterns. A second post-solidus alteration is indicated by oxygen isotope disequilibrium between quartz and feldspar in the Questembert massif and requires the circulation of low- δ 18O fluids, likely derived from the surface. We propose that the distinct petrological, mineralogical and geochemical evolutions of the Questembert and Lizio magmas are related to processes acting during the ascent of magmas through the continental crust. Questembert is more differentiated than Lizio because it covered a greater distance during its vertical migration, enhancing crystal fractionation from melts.

Single-crystal X-ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from western Maine

A crystal-chemical study of thirteen biotite (twelve of 1 M polytype and one of 2 M 1 polytype) and four muscovite samples was made. The biotite coexists with the muscovite. Samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, in order of increasing metamorphic grade: the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ), and the K-feldspar + Sillimanite Zone (K + SZ). The muscovite compositions cluster near ideal muscovite and display a small celadonite substitution and a small, but variable, paragonite substitution. The biotite composition has values of VIMg2+/VI(Mg2+ + Fe2+) ranging from 0.26 to 0.54 and significant octahedral Al content (0.48 ≤ VIAl ≤ 0.72 apfu in metamorphic biotite samples, 0.51 ≤ VIAl ≤ 0.67 in those from granites). In trioctahedral micas from western Maine and especially in those with graphite, there are a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation-basal oxygen atom distances and a decrease in tetrahedral flattening angle τ, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle α is related to crystallization temperature. In particular, α decreases with a temperature increase, and α is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters ( T and f O2) on mica crystal structure, and not just chemical composition.

Stable isotope paleoaltimetry of Eocene core complexes in the North American Cordillera

The hydrogen isotope composition of Eocene muscovite in mylonitic quartzite from the Kettle and Shuswap metamorphic core complexes (Washington and British Columbia) permits estimates of paleoaltimetry of the North American Cordillera at the onset of post‐collisional lithospheric extension. Coupled oxygen, hydrogen, and 40Ar/39Ar isotope data indicate that meteoric water penetrated to significant depths during normal faulting along the Columbia River Detachment bounding both Kettle and Shuswap metamorphic core complexes. Synkinematic muscovite attains δDmuscovite values as low as −135‰ and −157‰, respectively, consistent with δDwater values of −115 ± 5‰ and −135 ± 5‰. In context with stable isotope data from Eocene sedimentary basins of continental North America, these data constrain the isotopic composition of precipitation from which paleoelevation estimates can be made. Stable isotope paleoaltimetry indicates that during detachment formation (49–47 Ma), orogen‐parallel variations in topography existed, and mean elevations decreased from ≥4000 m at the latitude of the Shuswap core complex to ≥3000 m in the Kettle core complex. In addition, these data indicate a net decrease in mean surface elevation by ∼1000 m since the Eocene. Our results for the Kettle core complex are consistent with paleoelevation estimates based on fossil floral physiognomy in the Eocene Republic basin (Washington), indicating that high elevations characterized not only the frontal escarpment near the crustal‐scale detachment but continued into the Okanogan highlands further west. Collectively, the stable isotope, geochronological, and paleofloral data support tectonic models of high Eocene surface elevations contemporaneous with magmatism and lithospheric extension.

The uranium mineralization potential of the Paleoproterozoic Sioux Basin and its relationship to other basins in the southern Lake Superior region

The Paleoproterozoic Sioux Basin and the Baraboo, Barron, Flambeau, McCaslin, and Waterloo quartzites in northcentral USA are all supermature quartz arenites that are among the world's oldest redbeds. These quartzites were deposited on a stable craton at ca. 1750 Ma, following exhumation and uplift of Penokean crust and major orogenic activity related to the amalgamation of Archean microcontinents at ca. 2.0–1.8 Ga to form Laurentia. 40Ar/ 39Ar ages of 1615–1543 Ma from diagenetic muscovite in the Sioux Basin provide the first geochronological evidence for Mazatzal-related deformation and low-grade metamorphism of the quartzites during subsequent convergence on the southern margin of Laurentia. One muscovite sample from the Sioux Basin has a 40Ar/ 39Ar age of 1465 Ma that likely corresponds with fluid and thermal resetting due to felsic magmatism in the region and is similar to 40Ar/ 39Ar ages for muscovite from the Baraboo and Waterloo quartzites. Thus, the Sioux Basin and other quartzites in the region have experienced a similar tectonic and fluid history. The Sioux Basin has similar sedimentologic characteristics as other Paleoproterozoic basins in North America, including the uranium-rich Athabasca Basin, Canada, that is host to some of the world's largest and highest-grade unconformity-type uranium deposits. We compared key characteristics, such as the stratigraphy, character of diagenetic fluids, paleoaquifer characteristics, and composition of leachable Pb in the sandstones from the Sioux Basin with the Athabasca Basin in order to evaluate the fluid history of the basin and the potential of the Sioux Basin to host unconformity-type uranium mineralization. Based on the hydrogen and oxygen isotopic composition of muscovite (M1) that formed during diagenesis at 150–200 °C, the pore fluids that were present during peak diagenesis in the Sioux Basin had δ 18O and δD values of −0.5 to +5.2‰ and −32 to −46‰, respectively, similar to those of the fluids associated with uranium mineralization and alteration in the Athabasca Basin. However, fluid flow in the Sioux Basin was likely restricted to relatively thin and discontinous proximal fluvial facies that could have been diagenetic paleoaquifers, in contrast to the extensive paleoaquifers in the Athabasca Basin that promoted large-scale diagenetic reactions and alteration that led to the formation of unconformity-type uranium mineralization. In addition, the non-radiogenic isotopic composition of leachable Pb in the Sioux Basin sandstones indicates that the majority of the Pb is from a normal crustal source, inconsistent with mobilization of radiogenic Pb from a uranium-rich source, such as a uranium deposit. Although the Sioux Basin shares similar general characteristics to the Athabasca Basin, the results of this study suggest that it has a lower potential than the Athabasca Basin to host significant high-grade unconformity-type deposits.

Microstructures and interlayering in pyrophyllite from the Coastal Range of central Chile: evidence of a disequilibrium assemblage

AbstractPyrophyllite from a Triassic sedimentary formation from the Coastal Range of Chile has been investigated by transmission/analytical electron microscopy (TEM/AEM). The mineral assemblage includes pyrophyllite, muscovite, paragonite, a kaolin mineral, boehmite, rutile and hematite. The textures indicate that the protolith was a volcanoclastic rock. Petrographic evidence, chemistry, and the mineral assemblage suggest the intense leaching of the parent rock by a weathering process, before the metamorphic episode, to create the protolith for the pyrophyllite. Pyrophyllite always grows from the kaolin mineral, and both phases show close orientation relationships. The presence of parallel intergrowths of pyrophyllite and muscovite indicate that muscovite also grew from the kaolin mineral. Nevertheless, the composition of muscovite suggests that this phase must also form from another precursor, probably Al smectite. The AEM data and textural relationships between pyrophyllite and muscovite reveal the presence of two generations of muscovite and suggest that Na-rich muscovite recrystallized into a Na-free muscovite and paragonite.

Recrystallization or cooling ages: in situ UV-laser 40 Ar/ 39 Ar geochronology of muscovite in mylonitic rocks

The intra-grain 40 Ar/ 39 Ar age distributions of muscovite from greenschist-facies mylonite zones are shown to be sensors of metamorphic history. Textures, compositions and in situ UV-laser ablation 40 Ar/ 39 Ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo- or recrystallization ages. Deformed muscovite porphyroclasts and recrystallized shear band muscovite from the Pogallo Shear Zone (Ivrea Zone, southern Alps) reveal variable intra-grain 40 Ar/ 39 Ar ages with internal age variations of more than 65 Ma. The wide range of 40 Ar/ 39 Ar ages within compositionally homogeneous grains is consistent with diffusion-dominated argon loss controlled by observable intra-grain microstructures. In contrast, intra-grain 40 Ar/ 39 Ar ages of muscovite fish from Proterozoic mylonite of the Porsgrunn–Kristiansand Shear Zone (Southern Norway) display only minor age dispersion, and age variations are correlated with changes in muscovite composition. The sympathetic age–composition correlations in different muscovite microstructures directly relate to sequential neo- and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the Porsgrunn–Kristiansand Shear Zone.