The molecular structure of chloritoid: A mid-infrared and near-infrared spectroscopic study

The mineral chloritoid collected from the argillite in the bottom of Yaopo Formation of Western Beijing was characterized by mid-infrared (MIR) and near-infrared (NIR) spectroscopy. The MIR spectra showed all fundamental vibrations including the hydroxyl units, basic aluminosilicate framework and the influence of iron on the chloritoid structure. The NIR spectrum of the chloritoid showed combination (�� + ��)OH bands with the fundamental stretching (��) and bending (��) vibrations. Based on the chemical component data and the analysis result from the MIR and NIR spectra, the crystal structure of chloritoid from western hills of Beijing, China, can be illustrated. Therefore, the application of the technique across the entire infrared region is expected to become more routine and extend its usefulness, and the reproducibility of measurement and richness of qualitative information should be simultaneously considered for proper selection of a spectroscopic method for the unit cell structural analysis.

High Resoulution Electron Microscopy of Chloritoid Minerals From Different Geological Melieu

Structural defects of three chloritoid minerals from distinet geologic melieu have been investigated by high resolution electron microscopy. X-ray powder and electron diffraction patterns indicate that the chloritoid from one geological source (A) is2M 1+2M2 monoclinic variant while those from another geological source (B) are 2M 2 monoclinic variants. In a typical one-dimensional lattice image of a crystal from sourceA, the 2M 2 matrix is broken by insertion of triclinic inter-growths. Another crystal with the 2M 2 matrix showed single, triple, quadruple and quintuple layers displaying an unusually high degree of disorder. Lattice images of 2M 2 monoclinic variants from sourceB yielded more homogeneous micrographs. The important finding from the present studies is that the chloritoid from sourceA is a severely disordered low-temperature intermediate phase in the conversion of the triclinic chloritoid to the high-temperature ordered monoclinic variants of sourceB. Severely disordered chloritoids, marking the beginning of low grade metamorphism, are generated as intermediates between the state of complete disordered arrangement towards the end of low grade metamorphism within the narrow stability range of 400�����500��C.

Triassic eclogite from northern Vietnam: inferences and geological significance

The first finding of low-temperature eclogites from the Indochina region is reported. The eclogites occur along the Song Ma Suture zone in northern Vietnam, which is widely regarded as the boundary between the South China and Indochina cratons. The major lithology of the area is pelitic schist that contains garnet and phengite with or without biotite, chloritoid, staurolite and kyanite, and which encloses blocks and lenses of eclogite and amphibolite. The eclogites commonly consist of garnet, omphacite, phengite, rutile, quartz and/or epidote with secondary barroisite. Omphacite is commonly surrounded by a symplectite of Na-poor omphacite and Na-rich plagioclase. In highly retrograded domains, diopside + tremolite + plagioclase symplectites replace the primary phases. Estimated peak-pressure metamorphic conditions based on isochemical phase diagrams for the eclogites are 2.1-2.2 GPa and 600-620 degrees C, even though thermobarometric results yield higher pressure and temperature conditions (2.6-2.8 GPa and 620-680 degrees C). The eclogites underwent a clockwise P-T trajectory with a post-peak-pressure increase of temperature to a maximum of > 750 degrees C at 1.7 GPa and a subsequent cooling during decompression to 650 degrees C and 1.3 GPa, which was followed by additional cooling before close-to-isothermal decompression to similar to 530 degrees C at 0.5 GPa. The surrounding pelitic schist (garnet-chloritoid-phengite) records similar metamorphic conditions (580-600 degrees C at 1.9-2.3 GPa) and a monazite chemical age of 243 +/- 4 Ma. A few monazite inclusions within garnet and the cores of some zoned monazite in garnet-phengite schist record an older thermal event (424 +/- 15 Ma). The present results indicate that the Indochina craton was deeply (> 70 km) subducted beneath the South China craton in the Triassic. The Silurian cores of monazite grains may relate to an older non-collisional event in the Indochina craton.

O Grupo Canastra em sua ��rea-tipo, regi��o de Tapira, sudoeste do estado de Minas Gerais

This work aimed at describing the Neoproterozoic evolution of a Southern Bras��lia Fold Belt segment, in Tapira area (southwest of Minas Gerais state, Brazil), using detailed geologic mapping. This area, the Canastra Group type-area, has showed great tectonic and stratigraphic complexities unlike the simplicity suggested in previous works. From recognizing the main tectonic discontinuities, it was possible to subdivide the area into some domains. In the west domain, they were individualized in tectonic sheet I, marked by pelitic rocks and pelitic-graphite rocks with psammitic intercalations, and II, pelitic rocks with psammitic and mafic-ultramafic intercalations overlapped by gneisses. In the east domain, a group of three tectonic sheets was defined, in which, in the two lower tectonic sheets, pelitic and pelitic-graphite rocks with psammitic rock intercalations prevailed, which is different in metamorphic conditions. The lower tectonic sheet is marked by mineralogical associations with muscovite + chlorite + quartz �� graphite �� albite, without biotite; however, the superior one is with muscovite + quartz + garnet �� chlorite �� biotite �� chloritoid �� graphite �� albite. In the upper tectonic sheet, pelitic rocks with local contributions of psammitic and ultramafics rocks occur. In the south domain, psammitic rocks basically occur with contributions of pelitics and rudaceous rocks, where the preservation of textures and sedimentary structures is common. Rocks of the several domains are interpreted as part of a passive continental margin basin, located in the western margin of the S��o Francisco paleocontinent. Thus, the south domain rocks would represent the facies of proximal platform; rocks of the lower and middle tectonic sheets (east domain) and of the tectonic sheet I (west domain) are of facies distal platform; and the ones from the upper tectonic sheet (east domain) and tectonic sheet II (west domain) were acknowledged as deposited in an environment of continental shelf and/or oceanic seafoor.

Evolu����o geol��gica da faixa Bras��lia na regi��o de Tapira, Sudoeste de Minas Gerais

Funda����o de Amparo �� Pesquisa do Estado de S��o Paulo (FAPESP)

Eocene nappe tectonics and late Alpine extension in the Central Anatolide belt, western Turkey

Strukturgeologische Untersuchungen belegen, da�� die Anatoliden der Westt��rkei im Eoz��n durch die Plazierung der Kykladischen Blauschiefereinheit entlang einer durchbrechenden ��berschiebung auf die Menderes-Decken unter gr��nschieferfaziellen Metamorphosebedingungen entstanden.Die kykladischen Blauschiefer in der Westt��rkei enthalten Relikte eines prograden alpinen Gef��ges (DA1), welches hochruckmetamorph von Disthen und Chloritoid poikiloblastisch ��berwachsen wurde. Dieses Mineralstadium dauerte noch w��hrend des Beginns des nachfolgenden Deformationsereignisses (DA2) an, welches durch NE-gerichtete Scherung und Dekompression charakterisiert ist. Die nachfolgende Deformation (DA3) war das erste Ereignis, das beide Einheiten, sowohl die kykladische Blauschifereinheit als auch die Menderes-Decken, gemeinsam erfa��te. Der ��berschiebungskontakt zwischen der kykladischen Blauschiefereinheit und den Menderes-Decken ist eine DA3-Scherzone: die ��Cycladic-Menderes Thrust�� (CMT). Entlang der CMT-��berschiebungsbahn wurden die kykladischen Blauschiefer gegen veschiedene Einheiten der MN plaziert. Die CMT steigt nach S zum strukturell Hangenden hin an und kann daher als eine durchbrechende ��berschiebung entlang einer nach S ansteigenden Rampe betrachtet werden. In den kykladischen Blauschiefern ��berpr��gen DA3-Strukturen, die im Zusammenhang mit der CMT stehen, hochdruckmentamorphe Gef��ge.In den Menderes-Decken, dem Liegenden der CMT, wird DA3 durch regional vebreitete Gef��geelemente dokumentiert, die im Zusammenhang mit S-gerichteten Schersinnindikatoren stehen. DA3-Gef��ge haben die Decken intern deformiert und bilden jene Scherzonen, welche die Decken untereinander abgrenzen. In der ��ine-Decke k��nnen granitische Gesteine in Orthogneise und Metagranite unterteilt werden. Die Deformationsgeschichte dieser Gesteine dokumentiert zwei Ereignisse. Ein fr��hes amphibolitfazielles Ereignis erfa��te nur die Orthogneise, in denen vorwiegend NE-SW orientierte Lineare und NE-gerichtete Schersinnindikatoren entstanden. Die j��ngeren Metagranite wurden sowohl durch vereinzelte DA3-Scherzonen, als auch in einer gro��ma��st��blichen DA3-Scherzone am S��drand des ��ine-Massivs deformiert. In DA3-Scherzonen sind die Lineare N-S orientiert und die zugeh��rigen Schersinnindikatoren zeigen S-gerichtete Scherung unter gr��nschieferfaziellen Bedingungen an. Diese gr��nschieferfaziellen Scherzonen ��berpr��gen die amphibolitfaziellen Gef��ge in den Orthogneisen. Magmatische Zirkone aus einem Metagranit, der einen Orthogneiss mit Top-NE Gef��gen durchschl��gt, ergaben ein 207Pb/206Pb-Alter von 547,2��1,0 Ma. Dies deutet darauf hin, da�� DPA proterozoischen Alters ist. Dies wird auch durch die Tatsache gest��tzt, da�� triassische Granite in der ��ine- und der Bozdag-Decke keine DPA-Gef��ge zeigen. Die j��ngeren Top-S-Gef��ge sind wahrscheinlich zur gleichen Zeit entstanden wie die ��ltesten Gef��ge der Bayindir-Decke.Das Fehlen von Hochdruck-Gef��gen im Liegenden der CMT impliziert eine Exhumierung der kykladischen Blauschiefer von mehr ca. 35 km, bevor diese im Eoz��n auf die Menderes-Decken aufgeschoben wurden. Die substantiellen Unterschiede bez��glich in der tektonometamorphen Geschichte der kykladischen Blauschiefer und der Menderes-Decken widersprechen der Modellvorstellung eines lateral kontinuierlichen Orogeng��rtels, nach der die Menderes-Decken als ��stliche Fortsezung der kykladischen Blauschiefer angesehen werden.Die Analyse spr��der sp��talpiner Deformationsstrukturen und das regionale Muster mit Hilfe von Spaltspurdatierung modellierter Abk��hlalter deuten darauf hin, da�� die Struktur des Eoz��nen Deckenstapels durch mioz��ne bis rezente Kernkomplex-Bildung stark modifiziert wurde. Eine gro��ma��st��bliche Muldenstruktur im zentralen Teil der Anatoliden hat sich als Folge zweier symmetrisch angeordneter Detachment-Systeme von initial steilen zu heute flachen Orientierungen im Einflu��breich von ��Rolling Hinges�� gebildet. Die Detachment-St��rungen begrenzen den ��Central Menderes metamorphic core complex�� (CMCC). Das Muster der Apatit-Spaltspuralter belegt, da�� die Bildung des CMCC im Mioz��n begann. Durch die R��ck-Deformierung von parallel zur Foliation konstruierten Linien gleicher Abk��hlalter kann gezeigt werden, da�� die Aufw��lbung im Liegenden der Detachments zur Entstehung der Muldenstruktur f��hrte. Das hohe topographische Relief im Bereich des CMCC ist eine Folge der Detachment-St��rungen, was darauf hindeutet da�� der obere Mantel in den Proze�� mit einbezogen gewesen ist.

Geological evolution of the Monte Rosa

Eine detaillierte geochemische und geochronologische Studie an Gesteinen der Monte Rosa Decke (MR; Westalpen) wurde durchgef��hrt. Die MR wurde w��hrend der alpinen Orogenese zun��chst eklogitfaziell und nachfolgend gr��nschieferfaziell ��berpr��gt.Eine detaillierte U-Pb geochronologische Studie an Zirkonen und Monaziten des MR Granits ergab ein Permisches Intrusionsalter (270 �� 4 Ma). Der MR Granit geh��rt zu den post-variszischen magmatischen Einheiten, welche die Instabilit��t der variszischen kontinentalen Kruste andeuten. F��r die MR kann eine pal��ogeographische Position als Teil der 'Brian��onnais-Schwelle' angenommen werden.Innerhalb des MR Granits treten Talk-Kyanit-Chloritoid-Gesteine ('Wei��schiefer') auf. Diese stellen wesentliche Indikatoren f��r eine alpine Hochdruckmetamorphose in der MR dar. Massenbilanzberechnungen wurden durchgef��hrt, um den Massentransfer zu quantifizieren, welcher f��r die Bildung eines Wei��schiefers aus einem granitischen Protolith notwendig ist. Ein Modell f��r die Entwicklung der Wei��schiefer wurde entwickelt.Es wurde eine in-situ 40Ar/39Ar UV-Laser-Ablationsstudie an Hellglimmern der alpinen Mineralparagenese durchgef��hrt. Sie ergab eine heterogene Altersverteilung. Diese Alter k��nnen durch Glimmerrekristallisation unter relativ 'trockenen' hochdruckmetamorphen Bedingungen begleitet von partiellem Verlust von radiogenem Argon w��hrend der alpinen Metamorphose erkl��rt werden. Eine ��hnlich komplexe Entwicklung mit partieller Homogenisierung des Isotopensystems kann in der Strontium-Isotopie beobachtet werden. Diese Isotopenstudien liefern Hinweise auf das Schlie��verhalten von Isotopensystemen unter hochdruckmetamorphen Bedingungen.

A structural and metamorphic study of the southern Menderes Massif (Western Turkey)

P-T conditions, paragenetic studies and the relation between mineral growth, deformation and - when possible- isograd minerals have been used to describe the type of metamorphism involved within lower units of the southern Menderes Massif of the Anatolide Belt in western Turkey. The study areas mainly consist of Proterozoic orthogneiss and surrounding schists of presumed Paleozoic age. Both units are seen as nappes in the southern study area, the ��ine and the Selimiye nappe, on the whole corresponding to Proterozoic orthogneiss and surrounding schists, respectively. The ��ine and Selimiye nappes are part of a complex geological structure within the core series of the Menderes Massif. Their emplacement under lower greenschist facies conditions, would result from closure of the northern Neo-Thethys branch during the Eocene. These two nappes are separated by a major tectonic structure, the Selimiye shear zone, which records top-to-the-S shearing under greenschist facies conditions. Amphibolite to upper amphibolite facies metamorphism is widely developed within the metasedimentary rocks of the ��ine nappe whereas no metamorphism exceeding lower amphibolite facies has been observed in the Selimiye nappe. In the southern margin of the ��ine Massif, around Selimiye and Millas villages, detailed sampling has been undertaken in order to map mineral isograds within the Selimiye nappe and to specify P-T conditions in this area. The data collected in this area reveals a global prograde normal erosion field gradient from south to north and toward the orthogneiss. The mineralogical parageneses and P-T estimates are correlated with Barrovian-type metamorphism. A jump of P-T conditions across the Selimiye shear zone has been identified and estimated c. 2 kbar and 100 ��C which evidences the presence of amphibolite facies metasedimentary rocks near the orthogneiss. Metasedimentary rocks from the overlying Selimiye nappe have maximum P-T conditions of c. 4-5 kbar and c. 525 ��C near the base of the nappe. Metasedimentary rocks from the ��ine nappe underneath the Selimiye shear zone record maximum P-T conditions of about 7 kbar and >550 ��C. Kinematic indicators in both nappes consistently show a top-S shear sense. Metamorphic grade in the Selimiye nappe decreases structurally upwards as indicated by mineral isograds defining the garnet-chlorite zone at the base, the chloritoid-biotite zone and the biotite-chlorite zone at the top of the nappe. The mineral isograds in the Selimiye nappe run parallel to the regional SR foliation. 40Ar/39Ar mica ages indicate an Eocene age of metamorphism in the Selimiye nappe and underneath the ��ine nappe in this area. Metasedimentary rocks of the ��ine nappe 20-30 km north of the Selimiye shear zone record maximum P-T conditions of 8-11 kbar and 600-650 ��C. Kinematic indicators show mainly top-N shear sense associated with prograde amphibolite facies metamorphism. An age of about 550 Ma could be indicated for amphibolite facies metamorphism and associated top-N shear in the orthogneiss and metasedimentary rocks of the ��ine nappe. However, there is no evidence for polymetamorphism in the 6 metasedimentary rocks of the ��ine nappe, making tectonic interpretations about late Neoproterozoic to Cambrian and Tertiary metamorphic events speculative. In the western margin of the ��ine Massif metamorphic mineral parageneses and pressure��� temperature conditions lead to similar conclusion regarding the erosion field gradient, prograde normal toward the orthogneiss. The contact between orthogneiss and surrounding metasedimentary rocks is mylonitic and syn-metamorphism. P-T estimates are those already observed within the Selimiye nappe and correlated with lower amphibolite facies parageneses. Finally additional data in the eastern part and a general paragenetic study within the Menderes Massif lower units, the ��ine and the Selimiye nappes, strongly suggest a single Barrovian-type metamorphism predating Eocene emplacement of the high pressure���low temperature Lycean and Cycladic blueschist nappes. Metamorphic mineral parageneses and pressure���temperature conditions do not support the recently proposed model of high pressure���low temperature metamorphic overprinting, which implies burial of the lower units of the Menderes Massif up to depth of 30 km, as a result of closure of the Neo-Tethys. According to the geochronological problem outlined during this thesis, there are two possible schemes: either Barrovian-type metamorphism is Proterozoic in age and part of the sediments from Selimiye nappe (lower amphibolite facies) has to be proterozoic of age too, or Barrovian-type metamorphism in Eocene of age. In the first case the structure observed now in the core series would correspond to simple exhumation of Proterozoic basement. In the latter case a possible correlation with closure of Neo-Tethys (sensu stricto, southern branch) is envisaged.

A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O. Applications to P-T sections and geothermometry

We present a new thermodynamic activity-composition model for di-trioctahedral chlorite in the system FeO���MgO���Al2O3���SiO2���H2O that is based on the Holland���Powell internally consistent thermodynamic data set. The model is formulated in terms of four linearly independent end-members, which are amesite, clinochlore, daphnite and sudoite. These account for the most important crystal-chemical substitutions in chlorite, the Fe���Mg, Tschermak and di-trioctahedral substitution. The ideal part of end-member activities is modeled with a mixing-on-site formalism, and non-ideality is described by a macroscopic symmetric (regular) formalism. The symmetric interaction parameters were calibrated using a set of 271 published chlorite analyses for which robust independent temperature estimates are available. In addition, adjustment of the standard state thermodynamic properties of sudoite was required to accurately reproduce experimental brackets involving sudoite. This new model was tested by calculating representative P���T sections for metasediments at low temperatures (<400 ��C), in particular sudoite and chlorite bearing metapelites from Crete. Comparison between the calculated mineral assemblages and field data shows that the new model is able to predict the coexistence of chlorite and sudoite at low metamorphic temperatures. The predicted lower limit of the chloritoid stability field is also in better agreement with petrological observations. For practical applications to metamorphic and hydrothermal environments, two new semi-empirical chlorite geothermometers named Chl(1) and Chl(2) were calibrated based on the chlorite + quartz + water equilibrium (2 clinochlore + 3 sudoite = 4 amesite + 4 H2O + 7 quartz). The Chl(1) thermometer requires knowledge of the (Fe3+/��Fe) ratio in chlorite and predicts correct temperatures for a range of redox conditions. The Chl(2) geothermometer which assumes that all iron in chlorite is ferrous has been applied to partially recrystallized detrital chlorite from the Zone houill��re in the French Western Alps.

Valle d���Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly of a rifted continental margin

The spectrum characteristic of the EMC ranges from eclogites (containing omphacite and/or jadeite, garnet, phengite, glaucophane, zoisite, chloritoid, rutile) to phengite schists, calcschists, and marbles, as well as a variety of orthogneisses. Despite the intense polyphase deformation and HP-metamorphic recrystallization, it is possible in some locations to recognize pre-Alpine characteristics in some of the protoliths. For instance, two types of felsic orthogneiss can be distinguished in the Aosta Valley, one derived from Permian granitoids (with local preservation of intrusive contacts, magmatic inclusions, leucocratic veins and other magmatic structures; Stop 3), the other derived from pre-Variscan leuco-monzogranite, such as the building stone mined at the ���Argentera��� quarry near Settimo Vittone / Montestrutto (Stop 2; so-called ���Verde Argento��� contains jadeite, phengite, K-feldspar, quartz). Polycyclic and more rarely monocyclic metasediments contain evidence of a complex Alpine PTDt-evolution, locally including relics of their prograde history from blueschist, one or more stages at eclogite facies. Recent petrochronological studies have dated this HP-evolution of the Sesia Zone in some detail. In the area visited, clear evidence of HP-cycling has been identified in one km-size tectonic slice (Stop 1), but not in adjacent parts of the EMC, indicating ���yo-yo tectonics���. Partial retrogression and attendant ductile to brittle deformation of the HP-rocks is evident in one of the outcrops (Stop 4). Apart from the four localities in the Sesia Zone, a final outcrop introduces HP-rocks of the adjacent Piemonte oceanic unit, specifically calc-schists and ophiolite members of the ���Zermatt-Saas��� zone. The hilltop outcrop (Stop 5) displays foliated antigorite schist with peridotite relics (clinopyroxene, spinel) containing lenses derived from doleritic dykes. These fine-grained metarodingites and the folded veins containing Mg-chlorite and titanoclinohumite within serpentinite once again indicate equilibration under low-temperature eclogite facies conditions. However, these units reached that HP stage more than 20 Ma after the youngest eclogite facies imprint recognized in the Sesia Zone. Despite nearly half a century of intense study in the Sesia Zone, the complex assembly of its HP-terranes and their relation to more external parts of the Western Alps remains incompletely understood. This field guide merely introduces a few of the classic outcrops and discusses some of the critical evidence they contain, but it could not incorporate details on each stage of the evolution recognized so far.

Heavy mineral content of ODP Leg 107 holes

Analyses of rock clasts and of heavy minerals in upper Miocene coarse detrital units drilled along the East Sardinia passive-type continental margin (Sites 654, 653, 652, and 656) reveal that the stretched basement contains quite complex rock suites. Taking also into account previous sampling data, in moving from west to east across the margin, the nature of the basement changes drastically. To the west there are mostly Hercynian basement rocks with their cover, referable to the alpine foreland of the Corsica-Sardinia block. To the east, along the lower margin, where crustal thinning is quite severe, the basement contains rock suites referable to a pre-upper Tortonian orogenized zone with units constituting parts of the Alpine and Apenninic chains (presumably with thickened continental crust prior to stretching). Largest thinning and ocean forming occurred then, in a rather short time, mostly at the expense of unstable crust just thickened by orogenetic/tectogenetic processes.