K-white Mica Research Articles

The Arosa zone in Eastern Switzerland: oceanic, sedimentary burial, accretional and orogenic very low- to low grade patterns in a tectono-metamorphic mélange

In the area of Arosa–Davos–Klosters (Eastern Switzerland) the different tectonic elements of the Arosa zone mélange e.g. the Austroalpine fragments, the sedimentary cover of South Penninic ophiolite fragments, as well as the matrix (oceanic sediments and flysch rocks) show distinctively different metamorphic histories and also different climaxes (“peaks”) of Alpine metamorphism. This is shown by a wealth of Kübler-Index, vitrinite and bituminite reflectance measurements, and K-white mica b cell dimension data. At least six main metamorphic events can be recognized in the area of Arosa–Davos–Klosters: (1) A pre-orogenic event, typical for the Upper Austroalpine and for instance found in the sediments at the base of the Silvretta nappe but also in some tectonic fragments of the Arosa zone (Arosa zone mélange). (2) An epizonal oceanic metamorphism observed in the close vicinity of oceanic basement rocks units of the Arosa zone (South Penninic) is another pre-orogenic process. (3) A metamorphic overprint of the adjacent Lower Austroalpine nappes and structural fragments of the Lower Austroalpine in the Arosa zone. This metamorphic overprint is attributed to the orogenic metamorphic processes during the Late Cretaceous. (4) A thermal climax observed in the South Penninic sediments of the Arosa zone can be bracketed by the Austroalpine Late Cretaceous event (3) and the middle Tertiary event (5) in the Middle Penninic units and predates Oligocene extension of the “Turba phase”. (6) North of Klosters, in the northern part of our study area, the entire tectonic pile from the North Penninic flysches to the Upper Austroalpine is strongly influenced by a late Tertiary high-grade diagenetic to low-anchizone event. In the Arosa zone mélange an individual orogenic metamorphic event is evidenced and gives a chance to resolve diagenetic–metamorphic relations versus deformation. Six heating episodes in sedimentary rocks and seven deformation cycles can be distinguished. This is well explained by the propagation of the Alpine deformation front onto the foreland units. Flysches at the hanging wall of the mélange zone in the north of the study area (Walsertal zone) show data typical for low-grade diagenetic thermal conditions and are therefore sandwiched between higher metamorphic rock units and separated from theses units by a disconformity. The Arosa zone s.s., as defined in this paper, is characterised by metamorphic inversions in the hanging wall and at the footwall thrust, thus shows differences to the Walsertal zone in the north and to the Platta nappe in the south.

Diagenesis and very low grade metamorphism in a 7,012 m-deep well Hongcan 1, eastern Tibetan plateau

This study uses clay mineral assemblages, illite “crystallinity” (IC), chlorite “crystallinity” (CC), illite polytypes, the b cell-dimension of K-white mica, mineral geothermo-geobarometers and homogenization temperatures of fluid inclusions to investigate the transition from diagenesis to metamorphism in a 7 km thick Triassic flysch sequence in the well Hongcan 1, eastern Tibetan plateau. The 7,012.8 m deep borehole penetrated flysch of Upper to the Middle Triassic age and represents a unique chance to characterize low temperature metamorphic processes in an unusually thick sedimentary sequence developed on thickened continental crust. Mineral assemblage analysis reveals a burial metamorphic pattern with kaolinite and chlorite/smectite mix-layer phases present in the upper 1,500 m, and illite/smectite mixed-layer phases extending to a depth of 3,000 m. The metamorphic index mineral, graphite, was detected in sedimentary rock below 5,000 m using Raman spectroscopy. There exists a good correlation between IC and CC within the prograde burial sequence; with CC anchizonal boundaries of 0.242 and 0.314°2θ (upper and lower boundaries, respectively) corresponding to Kübler’s IC limits at 0.25 and 0.42°2θ. Illite polytypism also shows an increase in the 2M 1 polytype with increasing depth, with ca. 60 % 2M 1 abundance compared to the 1M type at the surface, to 100 % 2M 1 at the bottom of the borehole. Fluid inclusion analysis show HHC-rich bearing fluids correspond to the diagenetic zone, CH4-rich bearing fluids appear at transitional zone from diagenetic to low anchizone and H2O-rich bearing fluids mark the high anchizone to epizone. Based on chlorite chemical geothermometer, calcite–dolomite geothermo-barometers as well as homogenization temperature of fluid inclusions, a paleotemperature range of 118–348 °C is estimated for the well with a pressure facies of low to intermediate type.

Cenozoic deep crust in the Pamir

Multiple high-grade crystalline domes across the Pamir contain Barrovian facies-series metapelites with peak metamorphic assemblages of garnet + kyanite ± staurolite + biotite + oligoclase ± K-white mica. Thermobarometry yields pressures of 6.5–8.2 kbar and temperatures of 600–650 °C for the Kurgovat dome in the northwestern Pamir, 9.4 kbar and 588 °C for the west-central Yazgulom dome, 9.1–11.7 kbar and 700–800 °C for the east-central Muskol dome, and 6.5–14.6 kbar and 700–800 °C for the giant Shakhdara dome in the southwestern Pamir. These new data indicate exhumation of the Pamir crystalline domes from crustal depths of ~ 30–40 km. New titanite, monazite and zircon geochronology, in conjunction with published ages, illustrate that this metamorphism is Oligocene–Miocene in all but the Kurgovat dome (where it is Triassic). If the Pamir had a pre-collisional crustal thickness less than 30 km and if the India–Asia convergence within the Pamir is less than 600 km, the current 70 km-thick crust could have been created by plane strain with no net gain or loss of material. Alternatively, if the pre-collisional crustal thickness was greater than 30 km or India–Asia convergence within the Pamir is more than 600 km, significant loss of continental crust must have occurred by subhorizontal extrusion, erosion, or recycling into the mantle. Crustal recycling is the most likely, based on deep seismicity and Miocene deep crustal xenoliths.

Very low-grade metamorphism of Rheno-Hercynian allochthons (Variscides, Germany): facts and tectonic consequences

The Horre Belt in the SE part of the Rhenish Massif has been regarded as autochthonous because of the apparent lack of metamorphic inversion. We have performed a K-white mica study using powder X-ray diffraction derived Kubler Index (KI) and K-white mica b cell dimension, and the spectrally derived wavelength of the 2,200 nm absorption feature (“2200wvl”) of supposedly allochthonous units and the underlying par-autochthon. KI reveals a slight, but significant inversion of metamorphic grade, which shows that the Horre Belt and the Hessische Schieferserie adjacent to the South were emplaced by thrusting after they had acquired peak metamorphism (M 1) in deeper parts of the orogenic wedge. Only in the westernmost part of the study area, the par-autochthon and the allochthon show the same grade of anchizonal metamorphism. This observation, together with K-white mica b cell dimensions <9.000 A and 2200wvl <2,204 nm, suggests a second, low-pressure metamorphism M 2 subsequent to the emplacement of the allochthon. This is supported by an anomaly of published conodont alteration index data. Our results expand the original width of the Rheno-Hercynian passive margin to at least 580, more probably to 670 or even 750 km. Palinspastic restoration also reveals that the Givetian to Early Carboniferous volcanic belt of the Lahn/Dill area is not only younger than the Emsian MORB-type lavas of the Rheno-Hercynian ocean, but was also separated, from the latter, by a largely non-volcanic belt of at least c. 220 km width. Apparently, the Givetian to Early Carboniferous volcanic episodes were unrelated to the Rheno-Hercynian drift stage.

Diachronous evolution of the alpine continental subduction wedge: Evidence from P–T estimates in the Briançonnais Zone houillère (France – Western Alps)

The study of continental subduction processes requires detailed Pressure Temperature (P–T) paths to understand the kinematic of burial and exhumation of continental units. In the French Western Alps, the Briançonnais zone is a remnant of the continental subduction wedge. P–T conditions have been estimated in its most internal parts, but there is a lack of data in the western part, known as the “Zone houillère”. This Briançonnais Zone houillère is classically divided into two sub-units: the upper and lower Houiller units. This study focuses on both of these in the Clarée valley, north of Briançon. In this low-grade metamorphic terrain, estimation of P–T history is complicated because there are few adapted methods and these rocks have a poor metamorphic mineralogical content, including detrital metamorphic minerals inherited from their hercynian history. Therefore, to acquire accurate P–T estimates a multi-method approach is required, involving qualitative and quantitative Raman Study of Carbonaceous Material (RSCM), chemical analysis from quantified X-ray maps and thermodynamic modelling of chlorites and K-white micas. Such multi-approach P–T estimates on a sandstone sample allow distinguishing hercynian peak metamorphic conditions of 371±26°C and 3.5±1.4kbar and alpine peak metamorphic conditions of 275±23°C and 5.9±1.7kbar. These results are consistent with our RSCM and Tmax estimates. Raman study conducted on organic-rich schist samples shows an eastward increase of the alpine Tmax in the upper Houiller unit, from 280 to 300°C across the Briançonnais Zone houillère. In contrast, carbonaceous material included in detrital grains of muscovite in the sandstone exhibits higher temperatures. This hercynian Tmax is estimated using thermodynamic modelling at 376±50°C. According to these results and previous work in more internal parts of the Briançonnais zone, a geodynamic reconstruction is proposed, which is characterized by a diachronous evolution of the Briançonnais zone involved in alpine continental subduction at different times. The geothermal gradient in the Briançonnais zone changes from 8°C/km during early continental subduction, to 40°C/km during the collisional event at about 35–30Ma. The intermediate gradient of 15°C/km estimated in the Briançonnais Zone houillère suggests that this unit was buried later, than the more internal Briançonnais units, after 40Ma.

Tectonic Implications of Early Paleozoic Metamorphism in the Anakie Inlier, Central Queensland, Australia

Well-defined metamorphic zones are developed in pelitic and psammitic rocks of the Late Neoproterozoic to Cambrian Anakie Metamorphic Group of the Anakie Inlier, central Queensland. They are defined by the incoming of biotite, garnet, and andalusite, with or without staurolite. Mineral assemblages indicate that low pressure–high temperature metamorphism is associated with D1, medium pressure–high temperature metamorphism with D2, and retrograde, low pressure–low temperature metamorphism with D3. A mean b cell parameter of 9.035 obtained from K-white micas in the lowest-grade rocks suggests upper intermediate pressure conditions during D2. The timing of the growth of the index minerals indicates that isotherms retreated to the southwest during D2. Phase diagram calculations for both Al-saturated and Al-undersaturated metapelites containing D1 associations indicate pressure-temperature (P-T) conditions of 0.4 GPa and 560°C. These changed to 0.64–0.79 GPa and 580°–640°C as a result of crustal thickening. The samples thus record a history of heating (D1), followed by near-isothermal compression (D2). This P-T pathway shows that contraction rather than extension, as suggested by some authors, occurred during D2. The contractional event is suggested to have taken place during the Delamerian Orogeny as a result of convergence and collisional processes along the former passive margin of Gondwana.

Short-wavelength infrared spectroscopy: A new petrological tool in low-grade to very low-grade pelites

The illite spectral maturity (ISM) method uses short-wavelength infrared reflectance spectroscopy (SWIR) to measure K-white mica (KWM) physicochemistry within very low-grade metamorphic pelites. The three ISM measures used in this study parameterize KWM absorption features at 1900 nm and 2200 nm in terms of area, depth, and asymmetry. Through comparison with the powder X-ray diffraction (XRD)−derived Kubler index, we demonstrate that ISM differentiates anchizonal and epizonal from diagenetic domains in very low-grade pelites. The wavelength (wvl) of the 2200 nm absorption feature (2200wvl) provides a measure of the celadonite substitution in KWM. It shows a linear correlation (R 2 = 0.85) with the KWM b cell dimension (as determined by powder XRD), and can be used to differentiate the metamorphic pressure facies and related metamorphic thermal gradients in pelites of greenschist facies and anchizonal metamorphic grade. The boundaries between low/medium-pressure facies and medium/high-pressure facies series can be defined at 2204 and 2220 nm, respectively. In addition to their use as laboratory-based techniques, both ISM and 2200wvl show potential for remote sensing studies.

Testing the validity of organic and inorganic thermal indicators in different tectonic settings from continental subduction to collision: the case history of the Calabria–Lucania border (southern Apennines, Italy)

Abstract: This paper investigates the applicability of organic and inorganic thermal parameters in a range of thermal conditions, from diagenesis to epizone, and in HP–LT settings within the framework of continental subduction and exhumation in the southern Apennines, where independent thermobarometric constraints are available. Two juxtaposed tectonic units, the HP–LT Lungro–Verbicaro Unit (in the hanging wall) and the non-metamorphic Pollino–Ciagola Unit (in the footwall) have been analysed. For the Lungro–Verbicaro Unit, clay mineral thermal parameters and vitrinite reflectance underestimate metamorphic conditions (constrained by petrology), suggesting that, in relatively HP units, these indicators are ‘retarded' and strongly depend on P – T evolution through time. On the other hand, the K-white mica b cell dimension provides a robust estimate of peak pressure. Fluid inclusion textures indicate a fast isothermal decompression during exhumation. Two-phase inclusions from early stage syntectonic veins were trapped after isothermal exhumation at a pressure of 0.3 GPa. For the Pollino–Ciagola Unit, the correlation of analysed parameters effectively unravels the thermal evolution, with maximum palaeotemperatures of c . 150 °C. Apatite fission tracks provide details of the final stages of exhumation, indicating that both units were coherently exhumed in the Late Miocene, following the tectonic emplacement of the Lungro–Verbicaro Unit on top of the Pollino–Ciagola Unit. Supplementary material: X-ray diffraction data (whole-rock composition, quantitative analysis of the &lt;2 μm grain-size fraction, mica polytypes, illite content in mixed layer illite–smectite (I–S), Kübler and Árkai data, and b cell dimension) are available at http://www.geolsoc.org.uk/SUP18416 .

A hydrothermal clay mineral assemblage at the Late Proterozoic unconformity in the Buenos Aires Complex – La Tinta Formation, Barker area, Tandilia Ranges (Argentina)

AbstractA general alteration pattern of two transitional clay mineral assemblages was determined by petrography and X-ray diffractometry studies at the Tandilia Late Proterozoic unconformity zone, around the Barker locality: (1) K-white mica + chlorite + calcite ± anatase-rutile ± secondary quartz (farther from the unconformity) and (2) pyrophyllite + K-white mica + Ti-rich hematite ± aluminium phosphate-sulphate minerals ± tourmaline ± anatase-rutile (closer to the unconformity). The local occurrence of Na in K-white micas and K-Na in pyrophyllite is described for the first time. Possible interlayering with intermediate K-Na mica and paragonite is indicated by detailed X-ray diffraction analyses. A negative Eu anomaly (Eu/Eu* = 0.24), strongly positive Eu anomaly (Eu/Eu* =11.7) and positive Ce anomaly characterized rapid changes from an unaltered basement to the most altered basement and sedimentary rocks at the unconformity. Aluminium phosphate-sulphate minerals dominate patterns of LREE. A model of hydrothermal alteration comparable to that of unconformity-related uranium deposits is presented.

Diagenesis to metamorphism transition in an episutural basin: the late Paleozoic St. Mary’s Basin, Nova Scotia, Canada

The Late Devonian – Early Carboniferous St. Mary’s Basin in the Canadian Appalachians consists of Horton Group fluviatile and lacustrine clastic rocks. The basin occurs along the boundary between the Avalon and Meguma terranes and developed during coeval dextral shear along that boundary. X-ray diffraction reveals that the rocks contain ubiquitous quartz, K-white mica, and albite; illite–smectite mixed layers and chlorite are very common and Na–K mica, kaolinite, chlorite–smectite mixed layers, K-feldspar, berthierine, and rutile occur in some samples. Crystal-chemical parameters of white mica indicate the pressure and temperature of mineral growth and discriminate between diagenetic, anchizone, and low-grade metamorphic processes. Kübler index values measured in the 5 Å peak and the presence of chlorite–mica stacks are indicative of high-anchizone–epizone grades, with a crystallinity (crystal size and number of defects) that increases towards the Chedabucto Fault, which defines the northern margin of the basin. Kübler index values measured in the 10 Å peak indicate that a late fluid-rich event could have produced the observed illite–smectite mixed layers. The overall clay-mineral content and the b-cell dimension of the K-white micas are typical of postdepositional evolution in extensional sedimentary basins with high heat flow (&gt;35 °C/km). Taken together, our data record two superposed events related to deformation along the basin margins and coeval regional fluid flow, in which retrograde reactions at temperature T &lt; 200 °C were superimposed on a pre-existing prograde assemblage typical of high-anchizone – lower greenschist-facies conditions (T &gt; 300 °C). Regional syntheses indicate that this fluid flow may have occurred during episodes of Late Carboniferous dextral shear along the Avalon–Meguma terrane boundary.

Preliminary results on degree of thermal alteration recorded in the eastern part of Mt. Papuk, Slavonia, Croatia

In order to improve present knowledge of metamorphic conditions to which rocks of the Mt. Papuk have been subjected, selected samples belonging to the Radlovac metamorphic complex as well as overlying sedimentary rocks and parts of Psunj metamorphic complex beneath it were studied. Rocks from Mt. Papuk were investigated, in order to determine thermal conditions, by Kubler index (illite “crystallinity”) and Arkai index (chlorite “crystallinity”) while b0-parameter of K-white mica was used for pressure conditions estimation. Treatment with dimethyl-sulfoxide (DMSO) was used in order to extend application of chlorite “crystallinity” measurements to kaolinite containing samples. Obtained results are: T=250-300°C and P=2-3 kb. Similar T data recorded on various lithologies imply the existence of unknown post lower Triassic thermal event (Alpine very low to low grade metamorphism) affecting different complexes on Mt. Papuk. New data presented and discussed in this paper will set fundament for further research and interpretation of tectono-metamorphic history of the studied area and its correlation with other similar European metamorphic complexes.

Alpine metamorphism of organic matter in metasedimentary rocks from Mt. Medvednica (Croatia)

The diagenetic to low-temperature metamorphic alteration of Mt. Medvednica (Internal Dinarides, Croatia) has been studied by vitrinite reflectance, ordering data of organic matter determined by X-ray powder diffraction and Raman spectroscopy of extracted carbonaceous material.In metapelites of the Medvednica Metamorphic Complex (MMC), maximum vitrinite reflectance varies between 6.9 and 9.8%. By X-ray powder diffraction, the finely dispersed carbonaceous material is classified as d1-graphite. Based on the “Raman Spectroscopy of Carbonaceous Material Thermometer”, peak metamorphic temperatures of ca. 410 °C are estimated. The degree of organic maturation and the estimated peak temperature correlate fairly well with the Cretaceous (120–80 Ma) high-temperature anchizonal to epizonal metamorphism of the unit, determined by illite Kübler index, chlorite “crystallinity” and K-white mica – chlorite thermobarometry.In the tectonically higher Jurassic Ophiolitic Mélange and in the unconformably overlying Cretaceous–Paleocene Sequence, random vitrinite reflectance between ca. 0.7 and 2.2% suggest a stratigraphical trend of the data and therefore a common burial history. In the Jurassic Ophiolitic Mélange and the Cretaceous–Paleocene Sequence, the synmetamorphic carbonaceous material is classified as d3-graphite. In both units, organic thermometers provide peak temperatures of 100–240 °C. These estimates are in accordance with phyllosilicate reaction progress indicators, showing no systematic variation in the stratigraphic succession.

Polyphase low-grade metamorphism of the Ingleton Group, northern England, UK: a case study of metamorphic inversion in a mudrock succession

AbstractA series of boreholes in Horton Quarry, northwest Yorkshire (Horton-in-Ribblesdale Inlier) penetrated mudstones and slates belonging to the Austwick Formation (Windermere Supergroup) overlying laminated mudstones of the Ingleton Group. Illite (IC) and chlorite (ChC) crystallinity measurements indicate a metamorphic inversion between the two groups of mudrocks. The Windermere Supergroup mudrocks are mostly in the high anchizone or epizone, whereas the Ingletonian samples are lower grade in terms of IC, and are mostly deep diagenetic zone or low anchizone. Hence younger strata at higher grades rest on older strata at lower grades, creating a metamorphic inversion. Ingletonian slates exposed at Pecca Falls on the River Twiss show epizonal and anchizonal IC values, and greywacke samples from Ingleton Quarry contain pumpellyite. This suggests that grade in the Ingletonian may increase to the NW from the Horton to Ingleton inliers. K-white mica b cell dimensions show further differences between the Ingleton Group and the Windermere Supergroup. The Ingletonian samples are characterized by low b cell values (8.989–9.035, mean 9.007 Å), whereas the Windermere Supergroup has higher values in the range 9.022–9.034, mean 9.027 Å. The Windermere Supergroup values are similar to those recorded from the Windermere Supergroup of the southern Lake District, and Lower Palaeozoic rocks from the Scottish Southern Uplands, and are consistent with metamorphism in a low heat flow, convergent geotectonic setting. The Ingletonian b cell values suggest metamorphism in a higher heat flow setting, most likely an extensional basin. The metamorphic inversion at Horton and differences in K-white mica b cell dimensions suggest that the Ingleton Group and Windermere Supergroup strata evolved in different geotectonic settings and record two separate metamorphic events. The discovery of the metamorphic inversion at Horton provides further evidence in favour of an Ordovician rather than Neoproterozoic depositional age for the controversial Ingleton Group.

Polyphase metamorphism in the eastern Carnic Alps (N Italy–S Austria): clay minerals and conodont Colour Alteration Index evidence

Thermal evolution of the Palaeozoic–Triassic sequences of the Carnic Alps has been characterized by b cell dimension and Kubler Index (illite “crystallinity”) of K-white micas (KI), Arkai Index (AI) of chlorites, clay mineral assemblages and conodont Colour Alteration Index (CAI). Data indicate at least two metamorphic events, Variscan and Alpine. In the older event high anchizonal conditions predominated although epizonal conditions were reached over wide areas. It was characterized by low-intermediate pressure facies. The thermal peak was mainly due to an extensional regime during the Bashkirian. A younger thermal overprint generated by Alpine orogeny was of lower grade, reaching high diagenetic–anchizonal conditions characterized by high-pressure facies. Inverted metamorphic patterns are associated with middle to late Miocene thrusting. Hydrothermal alteration in the northern part of the region can be linked with emplacement of Oligocene plutons and high heat flow along the Periadratic lineament.

THE METASILICICLASTIC-CARBONATE SEQUENCE OF THE ACQUADOLCE UNIT (EASTERN ELBA ISLAND): NEW PETROGRAPHIC DATA AND PALEOGEOGRAPHIC INTERPRETATION

The Acquadolce Unit (upper part of Trevisan’s Complex II) consists of a phyllitic-quartzitic succession with calcschist and metagraywacke intercalations and a serpentinite wedge at the top. This unit was generally correlated to the Dogger-Eocene metasediments of the Tuscan Paleozoic-Jurassic succession of the Apuan Alps but, more recently, has been attributed to the Ligurian or Piedmontese Domain (e.g. the “Schistes Lustres” of the Gorgona Island). In this paper we present a detailed petrographic study of both the Acquadolce Unit and the Metasedimentary Unit of the Gorgona Island. In particular, we found significant mineralogical analogies between the lithotypes and tectono-metamorphic imprint of the two successions. The hypothesis that the Acquadolce Unit can be ascribed to a “Schistes Lustres”-type succession is also supported by the relatively high-pressure (probably >8kbar) estimated for the D1 deformation phase of the Acquadolce Unit from the K-white mica composition. The Acquadolce Unit is attributed to the metamorphic Ligurian-Piedmontese successions, allowing a redefinition of the main geological boundaries in the Tuscan Archipelago.

Temperature and age constraints on the metamorphism of the Tethyan Himalaya in Central Nepal: A multidisciplinary approach

Metasediments of Devonian to Triassic age of the Tethyan Himalaya (TH) from several areas in central and western Nepal, between western Dolpo and Marsyandi Valley, were sampled for thermo-metamorphic studies (illite and chlorite “crystallinity”, vitrinite reflectance, calcite–dolomite and chlorite-chloritoid geothermometers, K/Ar dating on illite-rich fractions and zircon fission track thermochronology). This paper reports, for the first time, the occurrence of chloritoid in the TH out of the range of granite contact aureoles. It also presents the first zircon fission track dating performed on TH metasediments. The peak temperatures of metamorphism have been around 250–300 °C, 320–350 °C, 330–370 °C and 400–450 °C in the western Dolpo, Hidden Valley, Manang and Marpha areas, respectively. In the Manang and Hidden Valley areas, illite K/Ar data are interpreted as ages of recrystallized K-white micas newly formed during metamorphism at around 25–30 Ma. In the Marpha area, illite K/Ar and zircon fission-track ages (12–15 Ma) are consistent representing cooling ages after metamorphic overprinting of higher grade than in the other areas. The joint investigation of the organic maturation and zircon FT chronology yields insight on the thermal calibration of zircon reset. The R max of 5.7% to ∼8.0% indicates a temperature range of ca. 315–325 °C which is in the partial annealing zone of the zircon FT thermochronometer. Our results might be explained by the presence of a thick thrust sheet once existing above the study area.

New age data on the low-temperature regional metamorphism of Mt. Medvednica (Croatia)

K-Ar age data of illite-K-white mica-rich, <2 µm grain size fraction samples were determined on various lithotypes of the Paleozoic-Mesozoic, very low- to low-grade metamorphic complex, the Jurassic ophiolitic melange and the Cretaceous-Paleocene sedimentary sequence of Mt. Medvednica, Croatia. K-Ar ages of the high-temperature anchizonal-epizonal Paleozoic-Mesozoic complex scatter around ca. 110 Ma for slates, phyllites and marbles with phyllite intercalations, while they are significantly younger (ca. 80 Ma) for metavolcanoclastic rocks that are devoid of detrital K-white mica. Beside the Cretaceous (Alpine) K-Ar radiometric age data, no evidence of a possibly older, Variscan metamorphic event was detected. In the Jurassic ophiolitic melange and the Cretaceous-Paleocene sequence the obtained mixed isotopic age data do not provide reliable estimates for the age of the diagenetic alterations. In the Paleozoic and Mesozoic formations from the Bukk, Szendro and Uppony Mts. (NE Hungary) and in the Paleozoic ...

Thermal evolution of the Lesser Himalaya, central Nepal: Insights from K-white micas compositional variation

The Lesser Himalayan low- to medium-grade metamorphic rocks in central Nepal are rich in K-white micas occurring as porphyroclasts and in matrix defining S 1 and S 2. Porphyroclasts are usually zoned with celadonite-poor cores and celadonite-rich rims. The cores are the relics of igneous or high grade metamorphic muscovites, and the rims were re-equilibrated or overgrown under lower T metamorphic conditions. The matrix K-white micas defining S 1, pre-dating the Main Central Thrust activity, are generally celadonite-rich. They show heterogeneous compositional zoning with celadonite-rich cores and celadonite-poor rims. They were recrystallized at lower T condition prior to the Main Central Thrust activity, most probably prior to the India–Asia collision (pre-Himalayan metamorphism). The matrix K-white micas along S 2, synchronous to the Main Central Thrust activity (Neohimalayan metamorphism), are relatively celadonite-poor and were recrystallized under relatively higher T condition. K-white micas defining S 1 also were partially re-equilibrated during the Neohimalayan metamorphism. The average compositions of recrystallized K-white micas defining both S 1 and S 2 become gradually poor in (Fe + Mg)- and Si-contents and rich in Al- and Ti-contents from south to north showing an increase of metamorphic grade from structurally lower to higher parts in the Lesser Himalaya. This shows that the metamorphism is inverted throughout the inner Lesser Himalaya. The tectono-metamorphic significance of the published K–Ar and 40Ar / 39Ar K-white micas ages from the Lesser Himalaya need re-evaluation in the context of observed intrasample compositional variation and zoning, and possible higher closure temperature (∼500 °C) for K–Ar system.

Clay mineral assemblages in British Lower Palaeozoic mudrocks

AbstractLower Palaeozoic rocks crop out extensively in Wales, the Lake District of northern England and the Southern Uplands of Scotland; they also form the subcrop concealed beneath the English Midlands and East Anglia. These mainly marine sedimentary rocks were deposited in basins created during plate tectonic assembly of the various terranes that amalgamated to form the British Isles, 400-600 Ma ago. Final amalgamation occurred during the late Lower Devonian Acadian Orogeny when the basins were uplifted and deformed, producing belts of cleaved, low-grade metasediments, so-called slate belts, with a predominantly Caledonian (NE-SW) trend. The clay mineralogy of mudrock lithologies - including mudstone, shale and slate - found in these belts is reviewed. Using X-ray diffraction data from the &lt;2 μm fractions of ~4500 mudrocks samples, clay mineral assemblages are summarized and discussed in terms of diagenetic and low-grade metamorphic reactions, and the metapelitic grade indicated by the Kübler index of illite crystallinity.Two sequences of clay mineral assemblages, or regional assemblages, are recognized. Regional Assemblage A is characterized by a greater diversity of clay minerals in assemblages from all metapelitic grades. It includes K-rich, intermediate Na/K and Na-rich white micas, chlorite and minor amounts of pyrophyllite. Corrensite, rectorite and pyrophyllite are found in the clay assemblages of contact or hydrothermally altered mudstones. K-white micas are aluminous and phengite-poor, with b cell dimensions in the range 8.98-9.02 Å. Regional Assemblage B has fewer clay minerals in assemblages from a range of metapelitic grades. Phengite-rich K-mica is characteristic whereas Na- micas are rare, and absent in most assemblages; chlorite is present and minor corrensite occurs in mudrocks with mafic-rich detritus. Minor amounts of kaolinite are sporadically present, but dickite and nacrite are rare; pyrophyllite and rectorite are generally absent. The b cell dimensions of K-white mica in Regional Assemblage B are in the range 9.02-9.06 Å. The two regional assemblages are found in contrasting geotectonic settings. Regional Assemblage A is characteristic of the extensional basin settings of Wales, the northern Lake District and the Isle of Man. These basins have a history of early burial metamorphism associated with extension, and syn-burial or post-burial intrusive and extrusive volcanic activity. Intermediate Na/K mica probably developed from hydrothermal fluids generated around submarine volcanic centres. Deep diagenetic and low anchizonal clay mineral in these basins may develop a bedding-parallel microfabric. Chlorite-mica stacks also occur in the extensional basins and the stacking planes represent another type of bedding-parallel microfabric. Both types of microfabric are non-tectonic and developed by burial during the extensional phase of basin evolution. Regional Assemblage B is developed in the plate-convergent settings of the Southern Uplands and the southern Lake District. In the accretionary complex of the Southern Uplands the processes of burial diagenesis, metamorphism and tectonism were synchronous events. In both plate- convergent basins, low temperatures and tectonic fabric-formation had an important role in clay mineral reactions, whereas hydrothermal fluids played no part in clay genesis.

P–T conditions of metapelites from metamorphic complexes in Aysen, Chile

Pressure and temperature metamorphic conditions were determined for K-white mica and chlorite in metapelites from the eastern Andean metamorphic complex (EAMC) and from the Chonos metamorphic complex (CMC) of southern Chile. The Kübler index (KI) of K-white micas was measured on a <2 μm fraction of 60 metapelites from both complexes. The KI-values obtained from the EAMC vary between 0.26 and 0.16 Δ°2 θ; in the CMC, their range is 0.27–0.17 Δ°2 θ, which indicates that the metamorphic temperatures reached by the metapelites are higher than 300 °C for both complexes. As confirmed by chlorite geothermometer, the temperatures are between 310 and 380 °C. Determination of the metamorphic pressure was carried out through the b-parameter of K-white micas and a phengite geobarometer. The b-parameter for the EAMC (9.033±0.10 Å) corresponds to a pressure of 4.0±1.2 kbar, and the phengite geobarometer yields pressures of approximately 2.7 kbar. For the CMC, 5.2±0.7 kbar (9.035±0.007 Å) is obtained by b-parameter and 3.8 kbar by phengite geobarometer. Although they differ in the pressure values, they are consistent, in that those of the EAMC are relatively lower than those of the CMC. The P–T metamorphic conditions are consistent with a subduction complex along the Gondwandan margin, of which the CMC and EAMC were parts.