Kubler Index Research Articles

Szenes anyagok maximális átalakulási hőmérsékletének meghatározása Raman-spektroszkópiai mérésekkel: standardizálás és esettanulmány az Aggtelek–Rudabányai-hegységből

A mély diagenetikus – kisfokú metamorf üledékes kőzetek csúcshőmérsékletének meghatározását illit (Küblerindex) és klorit „kristályossági”, valamint vitrinit reflexiós értékek alapján nagy bizonytalanság terheli. A hőmérséklet pontosabb meghatározására az elmúlt időszakban egyre elterjedtebbé vált a szenes anyag szemcsék mérésén alapuló Raman-spektroszkópia. Jelen munka célja, hogy a szenes anyag Raman-spektroszkópia (RSCM) egyik módszerének alapjairól, valamint hazai alkalmazási lehetőségeiről adjon részletes, magyar nyelvű összefoglalást. A bemutatott módszer alkalmazásához elengedhetetlen egy ismert metamorf csúcshőmérsékletű referencia kőzetsorozat elemzése, amely az ismeretlen minták metamorf csúcshőmérsékletének meghatározásához szükséges kalibrációs görbe alapjául szolgál. Ez a referencia kőzetsorozat teszi lehetővé az egyes laboratóriumokban született eredmények összehasonlíthatóságát is. Munkánkban ismertetjük az ELTE TTK KKIC Raman-laboratóriumában mért referencia kőzetsorozat eredményeit, valamint az ez alapján létrehozott kalibrációs görbét. A módszer alkalmazhatóságát az Aggtelek–Rudabányai-hegység Telekesoldali Formációjának példáján mutatjuk be. Az öt mintán (négy fúrás, egy felszíni) elvégzett mérések alapján az összletre meghatározott metamorf csúcshőmérséklet 273 ± 10 °C-nak adódott, amely adat jelentősen pontosabb a korábban rendelkezésre álló illit- és klorit kristályossági, valamint vitrinit reflexiós értékek alapján becsült metamorf csúcshőmérsékleti értékeknél. A módszer kitűnően kiegészíti, valamint részben helyettesítheti a szénhidrogén-, illetve szén geológiában alkalmazott vitrinit reflexió méréseket.

The zone of incipient 40Ar* loss-monitoring 40Ar* degassing behavior in a contact metamorphic setting

The ⁴⁰Ar* retentivity behavior was studied in the contact metamorphic aureole of the Torres del Paine intrusion (51°S, Chile) and provides constraints on the closure temperature in contact metamorphic settings. The temperature conditions of the regional anchizonal to epizonal metamorphosed pelitic host rock and the contact metamorphic pelite were recorded by illite crystallinities (Kubler Index) and by Raman spectroscopy on carbonaceous matter. K/Ar age determinations were carried out on illite fine fractions (<0.2 and <2 µm). Differing reaction progresses are observed between organic matter and illite. Organic matter equilibrated with the ambient temperatures, whereas illite is lacking behind the contact metamorphic conditions evident by the absence or limited recrystallization progress. We propose two models to explain the degassing of ⁴⁰Ar* in the absence of recrystallization considering surface loss and volume diffusion as two main pathways. Incipient ⁴⁰Ar* loss appears at temperatures of about 248°C in the <0.2 µm and at 260°C in the <2 µm fraction. The high temperature end of ⁴⁰Ar* retentivity is derived by extrapolation yielding temperatures of 330°C for the <0.2 µm fraction and 345°C for the <2 µm fraction. Thus, the Zone of incipient ⁴⁰Ar* loss, called ZIAL, is constrained to between 248°C and 330°C for the <0.2 µm fraction and to between 260 and 345°C for the <2 µm fraction. The extrapolated values for the upper ZIAL limit are in good agreement with calculated bulk closure temperature values of 348°C (<2 µm) and 286°C (<0.2 µm) assuming ca. 10 kyrs of heating during the emplacement of the Torres del Paine intrusion.

Clay minerals and isotopes of Pleistocene lacustrine sediments from the western Qaidam Basin, NE Tibetan Plateau

Abstract Since Singer (1984) reviewed clay minerals as they relate to paleoclimates, clay minerals have been used to reconstruct paleoclimates and environments. However, the paleoclimate information recorded by the paleo-clays in sediments will be overprinted or changed if the clay minerals undergo diagenetic transformations or post-depositional diagenetic changes, which are quite common. However, numerous studies still use clay minerals to reconstruct paleoclimates and paleo-environments. Uplift of the Tibetan Plateau since the Late Miocene has significantly impacted the sediments of the Qaidam Basin. However, due to the scarcity of fully dated sediment records, we know little about whether or not and/or how these clay minerals responded to this uplift. In order to understand their response to the uplift, in this study, we present the results of high-resolution profiling of the clay minerals from a 938.5-m borehole core (SG-1) from the Qaidam Basin (NE Tibetan Plateau). These clay minerals are mainly composed of illite, mixed layers of illite/smectite (I/Sm) and chlorite, with minor kaolinite and smectite. The clay minerals may have undergone early diagenesis. The δ18O-δD values of the interlayer water and the clay minerals imply the following. (a) The primary amounts of illite and chlorite were lower than those observed, while the primary amounts of smectite and kaolinite were higher than those observed; the amounts of clay minerals should be careful to be used to reconstruct paleo-environment singly; the changes in the illite “crystallinity” (Kubler Index) and chlorite “crystallinity” (Arkai Index) were related to changes in salinity and temperature. (b) Chemical weathering from 2.77 to 0.1 Ma was stronger than that at present. (c) The δ18O and δD of the interlayer water ranged from −11.8 to 82.95‰ and from −114.3 to 165.5‰, respectively, which was related to pore water and/or lake water undergoing strong evaporation. The isotopic composition of the interlayer water reflects variations in the pore water/lake water or of the reacting solutions, and records changes in environment. (d) The clay minerals recorded climate changes and uplift in the core, with a dramatic shift at 1.2 Ma and two different environment stages at 2.77–1.2 Ma and 1.2–0.1 Ma.

Tectonothermal analysis of a major unit of the Cantabrian Zone: the Ponga unit (Variscan belt, NW Spain)

The paleothermometric evolution of the Ponga unit at the core of the Cantabrian Zone, the arcuate foreland fold and thrust belt of the Variscan orogen in NW Spain, is analyzed using the conodont Color Alteration Index and the Kubler Index of illite. The results indicate that the distribution of the thermal maturity patterns is independent of the stratigraphy and the structure of the unit. These include the occurrence of dominant diagenetic values and very low-to-low-grade metamorphism in the southern area. The main thermal event is interpreted to be related to hydrothermal fluids ascending through late-Variscan extensional faults. The anomalous high heat flow in the southern sector of the Ponga unit is part of a previously identified large band in the Cantabrian Zone, where the common occurrence of low-grade metamorphic rocks, hydrothermal dolostones, and ore mineralizations is related to an extensional orogenic episode that started near the Carboniferous–Permian boundary, soon after the closure of the Asturian arc.

Clay mineralogy and its palaeoclimatic significance in the Luochuan loess-palaeosols over ∼1.3 Ma, Shaanxi, northwestern China

To understand climate changes recorded in the Luochuan loess-palaeosols, Shaanxi province, northwestern China, clay mineralogy was studied using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM) methods. XRD results show that clay mineral compositions in the Luochuan loess-palaeosols are dominantly illite, with minor chlorite, kaolinite, smectite, and illite-smectite mixed-layer clays (I/S). Illite is the most abundant species in the sediments, with a content of 61%–83%. The content of chlorite ranges from 5%–22%, and the content of kaolinite ranges from 5%–19%. Smectite (or I/S) occurs discontinuously along the loess profile, with a content of 0–8%. The Kubler index of illite (IC) ranges from 0.255°–0.491°, and the illite chemical index (ICI) ranges from 0.294–0.394. The CIA values of the loesspalaeosols are 61.9–69.02, and the R3+/(R3+ + R2+ + M+) values are 0.508–0.589. HRTEM observations show that transformation of illite to illite-smectite has occurred in both the loess and palaeosol, suggesting that the Luochuan loess-palaeosols have experienced a certain degree of chemical weathering. The Luochuan loess-palaeosols have the same clay mineral assemblage along the profile. However, the relative contents of clay mineral species, CIA, ICI, and IC values fluctuate frequently along the profile, and all these parameters display a similar trend. Moreover, climate changes suggested by the clay index are consistent with variations in the deep-sea δ18O records and the magnetic susceptibility value, and thus, climate changes in the Luochuan region have been controlled by global climate change.

Timing and thermal evolution of fold-and-thrust belt formation in the Ultima Esperanza District, 51°S Chile: Constraints from K-Ar dating and illite characterization

K/Ar dating on llite in Upper Cretaceous low-grade metamorphic pelites in the Torres del Paine area was used to set new time constraints on the development of the Patagonian retroarc fold-and-thrust belt (FTB) caused by the subduction of the Antarctic Plate beneath the South American Plate. The combined use of illite crystallinity (Kubler Index), polytype quantification and K/Ar dating of illite fractions (<0.2, <2 and 2-6 µm) allowed to distinguish four distinct periods of illite growth based on their K/Ar ages and degree of regional metamorphism: (1) early Cenomanian (98 Ma) illite crystallization, (2) widespread early Campanian (ca. 80 Ma) diagenetic illite growth under anchizonal metamorphic conditions, (3) a significant period of illite formation in the early Paleocene (ca. 60 Ma), and (4) a late stage of illite growth in the early Eocene (55-46 Ma) under epizonal conditions. The earliest indication for the emergent FTB formation in the hinterland is documented in a metapelitic clast (14-9, <2 µm) within the Upper Cretaceous Cerro Toro conglomerate which yields a K/Ar cooling age of 98.3±1.2 Ma and an epizonal KI value of 0.24 ∆°2Θ. After a certain period of geological quietness an interval of major thrusting and uplift occurred between ca. 60 and 46 Ma. The east dipping Rio Nutria and Rio Rincon thrusts record the onset of thrust and fold activity which can be placed close to 60 Ma. They also mark the frontal thrust towards the less deformed Magallanes foreland basin. In the western part of the internal domain, widespread fault and thrust activity of the frontal wedge and associated thermal overprint continued and is recorded until 46 Ma by K/Ar illite cooling ages. The flexural subsidence that is driven by the thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and the rapid increase of sedimentation rate along the monoclinal belt. No Miocene thrusting nor uplift event has been recorded by K/Ar illite dating in the study area.

Outlining the thermal posdepositional evolution of the Ordovician successions of northwestern Argentina by clay mineral analysis, chlorite geothermometry and Kübler index.

The thermal post-depositional evolution of metapelitic and metavolcanic rocks of the upper Cambrian-Ordovician succession in the Central Andes of northwestern Argentina, was estimated through X ray diffraction of clay mineral analysis, Kubler Index (KI), SEM-EDS study of selected samples and chlorite geothermometry. The study area comprise five representative regions in Puna and Cordillera Oriental, namely at Sierra de Santa Victoria (Nazareno and Santa Victoria river areas), Cordon de los Siete Hermanos (Yavi), Sierra de Rinconada and Sierra de Cochinoca-Escaya. The paleotemperatures obtained with the chlorite geothermometer, in coincidence with KI values, show an E-W trend from diagenesis/low anchizone in the eastern flank of Sierra de Santa Victoria, to high anchizone/epizone in the Puna, with intermediate values in the western flank of Sierra de Santa Victoria. This trend, in conjunction with the development of slaty cleavage in the Ordovician rocks from Yavi, Sierra de Cochinoca-Escaya and western localities coincide with the limits proposed for the “Ocloyic deformation belt”. The common occurrence of kaolinite in the slates and the metavolcanic rocks of Sierra de Cochinoca-Escaya, coupled with the substitution of chlorite by interstratified lower-temperature phases in most of these rocks and the occurrence of jarosite in a metadacite, indicate presence of hydrothermal fluids with high H+/cations ratios, producing acid type alteration (Utada, 1980), at temperatures between 100 and ~300 °C. The hydrothermal alteration should have been subsequent to the maximum burial and the attainment of epizonal metamorphism, and was very probably related with the posthumous activity of the Ordovician volcanic arc. The widespread occurrence of retrograde diagenesis products (smectite, kaolinite, as well as interstratified Chl/Sm and Chl/Vrm phases) in the anchizonal/epizonal metapelites of Sierra de Cochinoca-Escaya and Sierra de Rinconada constitute, up to our knowledge, the first report of an extensive hydrothermal activity affecting not only the metavolcanic rocks but also the Lower Ordovician sediments of northern Puna.

DIAGENETIC HISTORY OF THE ROCK UNITS OF BOZKIR UNIT CONTROLLED BY THE TRIASSIC RIFTING, BOZKIR-KONYA

The Bozkir Unit representing the northern edge of the Taurus Belt comprises, from bottom to top, three distinct structural entities; the Upper Triassic pre-rift (Korualan Group), the Upper Triassic-Upper Cretaceous syn-rift (Huglu Group) and the Jurassic-Cretaceous Boyali Tepe Group as to their structural settings. The Korualan Group is represented by the alternations of carbonate (limestone, dolomitic limestone, dolomite) with radiolarite and chert intercalations and clastic rocks (sandstone, siltstone, mudstone, shale). The Huglu Group is made up of volcanic (basalt, andesite) and pyroclastic (tuffaceous sandstone) rocks including radiolarite, limestone and clastic rock (sandstone, siltstone, shale) intercalations. The Boyali Tepe Group is completely made of carbonate rocks. The carbonate-silisiclastic-volcanogenic rocks of the Bozkir Unit contain carbonate (calcite, dolomite), quartz, feldspar (plagioclase, anorthoclase), phyllosilicate (illite, chlorite, mixed-layered illite-chlorite / I-C, chlorite-vermiculite / C-V, chlorite-smectite / C-S, rare smectite), augite, hematite, analcime and heulandite in order of abundance. On the basis of illite Kubler Index data; Korualan and Huglu Group reflect low grade diagenetic, high grade diagenetic and high grade diagenetic-anchizonal characteristics, respectively. The illite/micas of the pre-rift units and units related to the rifting have muscovitic, and phengitic and seladonitic compositions, respectively. The distributions of chondrite-normalized trace and rare earth element ( REE ) contents in illites present similar trends for Korualan ve Huglu groups, but the quantities of these elements slightly increase in the Huglu Group. d 18 O–dD isotopic compositions of water forming the illite minerals are different than that of sea water and are found to be between the Eastern Mediterranean Meteoric Water (EMMW) and magmatic water compositions. It also shows that temperature of the water forming illite minerals varies from low to high values. The findings from the rocks of Bozkir Unit suggest that pre- and syn-rift units have different mineralogical-petrographic and geochemical properties. The younger units within the rift due to extension and crustal thinning related to rifting must have been exposed in higher diagenetic conditions by more burial and heat with respect to older units on the edges.

Tectonothermal evolution of the northeastern Cantabrian zone (Spain)

A tectonothermal study of the northeastern sector of the Cantabrian zone (Ponga-Cuera and Picos de Europa units) using the conodont color alteration index (CAI) and Kubler index (KI) methods shows a variation from diagenetic to anchizonal conditions. The latter are illustrated in geological maps and cross sections. The greater part of the studied area has CAI values lower than 2, caused by two long periods of burial separated by a compressive interval (corresponding to the Variscan deformation), in which thrusts were the dominant structures. Tectonic superimposition was balanced by intense and fast erosion and had little effect on the CAI values. In contrast, a high thermal gradient was produced in the southern outcrops that led to recognize the transition between diagenetic and anchizonal conditions through CAI values near the basal thrust of the Picos de Europa unit. The diagenesis/anchizone boundary appears a little further south through the determination of the Kubler index in phyllosilicate minerals (KI = 0.42). CAI isograds cut the Variscan structures as a consequence of a thermal episode that occurred near the Carboniferous–Permian boundary at the beginning of an extensional regime. The thermal source was located further south, in the adjacent Pisuerga-Carrion unit. Alterations in the CAI values, as well as dolomitization and ore deposits, locally resulted from Permian–Mesozoic hydrothermal activity. Microtextural analysis of the conodonts allowed us to relate several types of apatite overgrowth to diagenetic conditions and recrystallization to anchizonal ones, whereas dissolution was common during hydrothermalism. Alpine deformation hardly produced any changes in the previous thermal pattern, but was responsible of the northwards tilting of the structure and CAI isograds.

Pyrophyllite formation in the thermal aureole of a hydrothermal system in the Lower Saxony Basin, Germany

A combined clay mineralogical, fluid inclusion, and K-Ar study of Upper Jurassic metasediments at the Gehn (Lower Saxony Basin, Germany) provides evidence for a transient hydrothermal event during Upper Cretaceous basin inversion centered on a prominent gravimetric anomaly. Kaolinite and smectite in Oxfordian pelitic parent rocks that cap a deltaic sandstone unit were locally transformed into pyrophyllite, 2M1 illite, R3 illite–smectite, chlorite, and berthierine at the Ueffeln quarry. The pyrophyllite-bearing metapelites lack bedding-parallel preferred orientation of sheet silicates and experienced peak temperatures of about 260–270°C consistent with microthermometric data on quartz veins in the underlying silicified sandstones. The presence of expandable layers in illite–smectite and high Kubler Index values indicate that the thermal event was rather short-lived. K-Ar dating of the <0.2 μm fraction of the pyrophyllite-bearing Ueffeln metapelite yields a maximum illitization age of 117 ± 2 Ma. Lower trapping temperatures of aqueous fluid inclusions in quartz veins and the absence of pyrophyllite in metapelites of the Frettberg quarry in a distance of about 2.5 km from the Ueffeln quarry infer maximum paleotemperatures of only 220°C. The highly localized thermal anomaly at Ueffeln suggests fault-controlled fluid migration and heat transfer that provided a thermal aureole for pyrophyllite formation in the metapelites rather than metamorphism due to deep burial. A pH neutral hydrothermal fluid that formed by devolatilization reactions or less likely by mixing of meteoric and marine waters that interacted at depth with shales is indicated by the low salinity (3–5 wt. % NaCl equiv.) of aqueous inclusions, their coexistence with methane–carbon dioxide-dominated gas inclusions as well as carbon, hydrogen, and oxygen isotope data. The upwelling zone of hydrothermal fluids and the thermal maximum is centered on a gravimetric anomaly interpreted as an igneous intrusion (‘Bramsche Massif’) providing the heat source for the intrabasinal hydrothermal system.

Influência da pressão e temperatura na cristalinidade da illita em sequências Proterozoicas: Norte do Distrito Federal e Goiás, Brasil

As sequências Proterozoicas, ao Norte no Distrito Federal e na região de Bezerra e Vila Boa (Goiás), são constituídas por calcários e dolomitos, pelitos e arenitos correspondentes aos Grupos Bambuí e Paranoá. No Distrito Federal, tais unidades são sobrepostas tectonicamente por rochas metassedimentares de fácies xisto-verde do Grupo Canastra. Entre os constituintes minerais dos termos pelíticos de todas estas unidades, predominam os filossilicatos, particularmente a illita e a muscovita, identificados por difração de Raios X, com o auxílio de decomposição da reflexão em d(001). O Índice de Kübler (Índice de Cristalinidade da Illita) complementa a interpretação da diagênese local e da possível influência de carga tectônica sobre diagênese/metamorfismo, resultante da sobreposição por falha de empurrão das rochas do Grupo Canastra sobre aquelas dos grupos Paranoá e Bambuí, na região do Distrito Federal. Os resultados indicam condições de epizona para as amostras analisadas do Grupo Canastra e anquizona para aquelas dos Grupos Bambuí e Paranoá, sendo que os valores do Índice de Kübler são menores nas rochas expostas ao norte do Distrito Federal do que na região de Bezerra, em Vila Boa. Além disso, os índices das rochas do Grupo Bambuí são inferiores aos do Paranoá em toda a região estudada, indicando que a influência da carga tectônica ocorre de forma gradativa, partindo do ponto de maior carga.

Evidence of the diagenetic history of sediment composition in Precambrian-early Paleozoic rocks: a systematic study from the Southeast Anatolian Autochthon, Mardin (Derik-Kızıltepe), Turkey

The mineralogical properties of the Precambrian-early Paleozoic series have been investigated in the Southeast Anatolian Autochthon (SEAA) and are represented by generally clayey siliciclastic and rarer volcanogenic rocks. These include basalt, andesite, and volcanic sandstones of the Precambrian period; siliceous sandstone, dolomite, shale, and siltstone of the Cambrian period; and sandy dolomite, shale, siltstone, sandstone, and limestone with red Fe nodules of the Lower-Upper Ordovician period. The rock-forming minerals determined in order of abundance are feldspar, pyroxene, quartz, and olivine in the Precambrian unit; quartz, moganite, calcite, dolomite, feldspar, and phyllosilicates (illite, chlorite, mixed-layered illite-smectite/I-S, smectite) in the Cambrian units; and phyllosilicates (kaolinite, illite, chlorite, I-S, mixed-layered chlorite-vermiculite/C-V, smectite), quartz, feldspar, calcite, dolomite, moganite, and goethite in the Lower-Upper Ordovician units. Short prismatic and partly radiating moganites discovered in the early Paleozoic sediments in Turkey are present between euhedral and coarse-grained quartz and feldspar in the sandstones. Thin platy illites have undergone a transition that introduces a fibrous/thread-like mixed-layer I-S through their edges. The rhombohedral dolomites have dissolution traces associated with siliceous balls, long thin filament-shaped illites, and acicular-radial chlorites. Illites usually show coarse and tiny plates parallel to each other and partly radiating flakes, and chamosite-type chlorites form thick plates in the siliciclastics. Kaolinites consist of typical pseudohexagonal sheets with parallel or accordion-like booklets in the siltstones. The Kubler index indicates low- to high-grade diagenesis to low-grade diagenesis, signifying that the location partially differs from the other units of the SEAA and Taurus Belt. In addition, Paleozoic rocks reflect no maturation characteristics with respect to the petroleum system.

Evidence of multi-stage faulting by clay mineral analysis: Example in a normal fault zone affecting arkosic sandstones (Annot sandstones)

Fault affecting silicoclastic sediments are commonly enriched in clay minerals. Clays are sensitive to fluid–rock interactions and deformation mechanisms; in this paper, they are used as proxy for fault activity and behavior. The present study focuses on clay mineral assemblages from the Point Vert normal fault zone located in the Annot sandstones, a Priabonian-Rupelian turbidite succession of the Alpine foredeep in SE France. In this area, the Annot sandstones were buried around 6–8 km below the front of Alpine nappes soon after their deposition and exhumed during the middle-late Miocene. The fault affects arkosic sandstone beds alternating with pelitic layers, and displays throw of about thirty meters. The fault core zone comprises intensely foliated sandstones bounding a corridor of gouge about 20 cm thick. The foliated sandstones display clay concentration along S–C structures characterized by dissolution of K-feldspar and their replacement by mica, associated with quartz pressure solution, intense microfracturation and quartz vein precipitation. The gouge is formed by a clayey matrix containing fragments of foliated sandstones and pelites. However, a detailed petrographical investigation suggests complex polyphase deformation processes. Optical and SEM observations show that the clay minerals fraction of all studied rocks (pelites and sandstones from the damage and core zones of the fault) is dominated by white micas and chlorite. These minerals have two different origins: detrital and newly-formed. Detrital micas are identified by their larger shape and their chemical composition with a lower Fe–Mg content than the newly-formed white micas. In the foliated sandstones, newly-formed white micas are concentrated along S–C structures or replace K-feldspar. Both types of newly formed micas display the same chemical composition confirmed microstructural observations suggesting that they formed in the same conditions. They have the following structural formulas: Na0.05 K0.86 (Al 1.77 Fe0.08 Mg0.15) (Si3.22 Al0.78) O10 (OH)2. They are enriched in Fe and Mg compared to the detrital micas. Newly-formed chlorites are associated with micas along the shear planes. According to microprobe analyses, they present the following structural formula: (Al1,48 Fe2,50 Mg1,84) (Si2,82 Al1,18) O10 (OH)8. All these data suggest that these clay minerals are synkinematic and registered the fault activity. In the gouge samples, illite and chlorite are the major clay minerals; smectite is locally present in some samples.In the foliated sandstones, Kubler Index (KI) ((001) XRD peak width at half height) data and thermodynamic calculations from synkinematic chlorite chemistry suggest that the main fault deformation occurred under temperatures around 220 °C (diagenesis to anchizone boundary). KI measured on pelites and sandstones from the hanging and footwall, display similar values coherent with the maximal burial temperature of the Annot sandstones in this area. The gouge samples have a higher KI index, which could be explained by a reactivation of the fault at lower temperatures during the exhumation of the Annot sandstones formation.

Metamorphic evolution of the Karakaya Complex in northern Turkey based on phyllosilicate mineralogy

The Triassic Karakaya Complex (KC) of the Sakarya Composite Terrane in northern Turkey is traditionally subdivided into two units. The Lower Karakaya Complex (LKC) consists of a tectonic melange with blocks of metabasic rocks, metacarbonates, meta-arenites and metapelites that have been affected by high pressure/low temperature metamorphism. It is followed by a low pressure/low temperature metamorphic overprint; the latter is the only metamorphic event in some tectonic slivers of the LKC. The Upper Karakaya Complex (UKC) units are primarily composed of diagenetic to low-grade metamorphic rocks, comprising Late Permian and Triassic cherts and blocks of OIB-type volcanic rocks interfingering with Anisian limestones. LKC slide-blocks of variable sizes are frequently observed within the UKC. Phyllosilicates of LKC and UKC were examined for their abundance, crystallinity, polytype and b cell dimension. Trioctahedral chamositic chlorites have IIb polytype and phengites 2 M1 polytypes in the LKC units and 2 M1 + 1 M + 1Md polytypes of phengitic dioctahedral illites in the UKC units. Kubler index data correspond to the low anchizone and epizone for the LKC units, and to the high diagenesis-low anchizone, and in part to the epizone for the UKC units. The b values of illites are consistent with a high-pressure facies series for the LKC, but only intermediate-pressure facies for the UKC. According to textural features, mineral paragenesis, clay transformations, index minerals, and b values, the lower-middle parts of the LKC represent an accretional tectonic setting, whereas the UKC units reflect pressure temperature conditions of an extensional basin affected by high heat flow.

Clay mineralogy, chemistry, and diagenesis of Late Devonian K-bentonite occurrences in northwestern Turkey

Thin beds of tephra (K-bentonites) formed by the diagenesis of volcanic ash are exposed within the limestone-dolomitic limestone successions of the Yilanli formation at Zonguldak and Bartin in northwestern Turkey. They were deposited on the Middle Devonian-Lower Carboniferous shallow carbonate platform of the Zonguldak terrane. In this study, K-bentonite samples collected from Gavurpinari and Yilanli Burnu limestone quarries are investigated in order to reveal their mineralogical and geochemical characteristics and diagenetic evolution. Illite is the major clay mineral in the studied K-bentonites. Additionally, kaolinite and mixed-layer illite-smectite are identified in some samples. The nonclay minerals calcite, dolomite, quartz, gypsum, feldspar, pyrite, and zircon are also found. Crystal-chemical characteristics (Kubler index, d060 values, and polytypes of illites) from two different sampling locations do not show significant variations. Kubler index values for the Yilanli Burnu and Gavurpinari sampling locations, 0.47-0.93 (average: 0.71 $\Delta$°2$\theta$) and 0.69-0.77 (average: 0.72 $\Delta$°2$\theta$), respectively, indicate that illites were affected by high-grade diagenetic conditions. The swelling (or smectite) component (~5%), crystallite size (N = 10-20 nm), and polytype (2M1 > 1Md) data of illites support the same conditions. Illite d060 values of 1.491-1.503 A correspond to a range of octahedral Mg+Fe values of 0.27-0.51 atoms per formula, indicating a composition between end-member muscovite and phengite unit. Trace and rare earth element-based chemical classification of the K-bentonite samples revealed that composition of original volcanic ash is basaltic. Illitization took place by fixation of K from volcanic minerals and ash, and diffusion of elements (Mg+Fe) into and out of the beds during diagenesis. Mineralogical-chemical data point out that these K-bentonites evolved in high-grade diagenetic conditions (approximately 100-150 °C) from the products of volcanic eruptions of disputed sources and distances during the Late Devonian time.

On the origin of cleavage in the Central Pyrenees: Structural and paleo-thermal study

Abstract The present work provides a combined structural and paleothermal (fluid inclusions, illite content in mixed layers illite–smectite, Kubler index and vitrinite reflectance) investigation of the Alpine cleavage domain affecting the western South Pyrenean Zone (Central Pyrenees). Integration of structural and paleothermal data allows us (i) to determine the position of the cleavage front in the sedimentary sequence and (ii) to define mechanisms and temperature conditions for cleavage development. Cleavage in the eastern part of the study area is approximately limited to the forelimb of the Gavarnie hanging wall anticline, but to the West it extends progressively southwards, affecting younger turbiditic units. At regional scale, the cleavage front is oblique to the strike of bedding and the overall trend of macrostructures. Pressure-solution is the main mechanism for cleavage development, and a strong thermal control on the position of the cleavage front is inferred. The transition between sedimentary and tectonic, compressional fabric is marked by vitrinite reflectance values of 1.12–1.14%, Kubler index values between 0.71 and 0.74°Δ2θ and the transition between R1 and R3 mixed layers illite–smectite. A lower thermal boundary of 140–160 °C is suggested for incipient cleavage development in the shaly beds of the turbiditic sequence, as well as an upper boundary constrained by a maximum measured value of about 215 °C.

Tectonothermal evolution of the Triassic flysch in the Songpan–Garzê orogen, eastern Tibetan plateau

Abstract Low temperature metamorphic indicators were used to investigate the tectonothermal evolution of the Triassic flysch in the Songpan–Garze orogen, eastern margin of the Tibetan plateau. Iso-thermal zones mapped with illite crystallinity (Kubler index, KI) describe a complex pattern of the diagenetic, anchimetamorphic and epimetamorphic zones. From this pattern, a general increase in grade toward the Longmenshan fault belt is observed, and across the Longmenshan fault belt, greenschist facies and anchizonal rocks on its NW side are juxtaposed to diagenetic rocks in the Sichuan basin on its SE side. This juxtaposition is marked by KI jumps of 0.23°Δ2θ in the SW portion of the Longmenshan fault through 0.39°Δ2θ in the Middle, and reaching 0.71°Δ2θ in the NE of the Longmenshan fault thus describing a clockwise rotation of the upper part of the Longmenshan fault after metamorphism. Across the Xianshuihe fault, the truncated KI zones within the Triassic flysch suggest a total offset of roughly 50–60 km due to post-metamorphic sinistral strike-slip. Peak metamorphic conditions are estimated at 365–431 °C with a pressure facies of low to intermediate type (3 kbar). Compression at the end of the Triassic was induced by the interaction of the South China, North China and North Tibetan blocks, which caused the closure of the Paleo-Tethys Ocean and led to the folding of the Triassic flysch within the Songpan–Garze basin. Very low to low grade metamorphism was caused by crustal thickening due to an accretionary wedge setting, which led to an increase in the thickness of the Triassic flysch to > 10 km, and resulted in a temperature increase in those rocks due to burial. Finally, the Indo-Asia collision caused the formation of the Longmenshan and the Xianshuihe strike-slip faults in the early Tertiary and disturbed the distribution of the metamorphic zones.

Short-wavelength infrared spectroscopy of chlorite can be used to determine very low metamorphic grades

This study investigates the use of short-wavelength infrared reflectance spectroscopy for measuring chlorite physicochemical information of value for determination of very low metamorphic grades. The method is termed chlorite spectral index (CSI) and assessed through quantitative comparison with XRD-derived Arkai and Kubler indices (AI and KI, respectively) in a set of 41 pelitic samples. The four CSI measures investigated measure the depth (D) of the absorption features at 1900, 2250 and/or 2350 nm, and are defined as reflectance 2250D/1900D index (CSI(H 2 O)), reflectance (2350D + 2250D × 1.7)/1900D index (CSI(H 2 O) sum ), reflectance 2250D index (CSI(2250)) and reflectance 2350D + 2250D × 1.7 index (CSI sum ). The results demonstrate the applicability of the method: all CSI measures show a good correlation with the AI, and respective correlation factors range between R 2 = 0.840 (for the CSI(H 2 O)) and R 2 = 0.616 (for the CSI sum ). The CSI measures also show a good correlation with the KI with R 2 = 0.704 (for CSI sum ) to 0.769 (CSI(2250)). By applying the correlation equations to define the metamorphic zone boundaries for each of the CSI measures, 64.3–78.6 % of the samples match the metamorphic grade indicated by AI for the epizone, 47.1–70.6 % for the anchizone and 70–80 % for the diagenetic zone. All CSI measures show a good reproducibility, with an intra sample variance ranging between 2.31 % CSI(H 2 O) sum and 3.34 % (CSI(2250)). CSI is not restricted to pelitic rocks and marls, but can also be applied in very low-grade metabasites. In addition to the existing laboratory method, CSI has application potential in remote sensing applications.

The pioneer work of Bernard Kübler and Martin Frey in very low-grade metamorphic terranes: paleo-geothermal potential of variation in Kübler-Index/organic matter reflectance correlations. A review

Low-temperature metamorphic petrology occupies the P–T field between sedimentary and metamorphic petrology. Two important pillars of low-temperature metamorphism are coal petrology and clay mineralogy. When low temperature petrology was established bridging a hiatus between the two classical geological disciplines of sedimentary geology and metamorphic petrology, geologists faced a need for the usage of different terminology tenets. Martin Frey and Bernard Kubler were two pioneers in low-grade metamorphic petrology. They focused their research on clarifying the relationships of clay mineralogy and organic petrology to metamorphic pressure (P) and temperature (T) conditions. The ultimate aim of M. Frey and B. Kubler was to establish a correlation between clay indices and organic parameters for different geodynamic setting and therefore for various pressure–temperature (P–T) conditions occurring in low grade metamorphic terranes. For this purpose, a special attention was addressed to the correlation between the Kubler-Index (KI) and vitrinite reflectance (VR). All these efforts are dedicated to estimate the P–T conditions and thus to gain insight into the geodynamic evolution of low-grade metamorphic terranes. B. Kubler and M. Frey honored here concentrated their studies to the Helvetic Central Alps area. The very low-grade Helvetic domain is therefore of basic interest of this paper. Ensuing the extensive compilation of data from the Helvetic domain, a reinterpretation of Kubler and Frey’s research is presented in the light of last decade’s scientific progress. A comprehensive dataset available enables to discriminate many factors influencing the Kubler-Index and organic-matter reflectance alongside to time, temperature and pressure. The correlation is restricted to the KI and organic matter reflectance (mostly VR) because most of the studies used both methods. Organic matter reflectance (OMR) includes data from vitrinite reflectance and bituminite reflectance measurements. Geodynamics has important control on the KI/VR (OMR) correlation. Tectonic units having a similar geodynamic evolution are featured by the comparable KI/OMR trends, related to the particular paleo-geothermal conditions. Obviously the KI/OMR correlations provide a mean to characterise geothermal gradients and metamorphic very-low-grade pressure–temperature conditions. In terranes where high deformations rates are reported, exceeding the high anchizone conditions, strain promotes the kinetic effects of temperature and pressure on the KI versus OMR ratio.

Fluid-driven low-grade metamorphism in polydeformed rocks of Avalonia (Arisaig Group, Nova Scotia, Canada)

The Lower Silurian—Lower Devonian Arisaig Group (Antigonish Highlands) in the Canadian Appalachians is a sequence of shallow marine strata deposited after the accretion of Avalonia to Baltica during the closure of the Iapetus Ocean. Deformation of the strata is widely attributed to the Devonian Acadian orogeny and produced shallowly plunging regional folds and a cleavage of varying penetrativity. Phyllosilicate minerals from the finest-grained rocks exhibit very low-grade (diagenetic-anchizone) metamorphic conditions. X-ray diffraction study reveals that the sampled rocks contain quartz, K-white mica, chlorite, and feldspars; illite–smectite and chlorite–smectite mixed-layers are common but Na–K mica and kaolinite occur only in some samples. The identification of illite–smectite mixed-layers in diagenetic samples, with Kubler Index >0.50 Δ°2θ and the highly heterogeneous b-cell dimension of the K-white micas are in agreement with the variable chemical composition of dioctahedral micas, which present low illitic substitution and variable phengitic content. The spatial variation in the above crystal-chemical parameters was plotted along a NW–SE composite cross section across the regional folds. No correlation was found between the metamorphic conditions and either the stratigraphic depth or the strain values measured by phyllosilicates orientation analyses, as a function of the penetrativity of the cleavage. However, the metamorphic grade generally increases towards the Hollow Fault, and is highest in samples located within a 1 km corridor from the fault surface. Incipient cleavage is observed in the anchizonal samples located in the vicinity of the Hollow Fault and in some of the diagenetic samples, indicating cleavage development under low temperatures (<200 oC). These relationships, together with regional syntheses, suggest low-grade metamorphism post-dated regional folding and was coeval with Late Carboniferous dextral movement along the Hollow Fault. Fluid circulation associated with movement along this major fault may be the driving mechanism for the increasing metamorphism towards it.