Ibergirhynchia contraria (F. A. Roemer, 1850), an Early Carboniferous seep-related rhynchonellide brachiopod from the Harz Mountains, Germany - A possible successor to Dzieduszyckia?

A new genus Ibergirhynchia, a member of the rhynchonellide superfamily Dimerelloidea, is described for the species Terebratula contraria Roemer, 1850, from Early Carboniferous deposits of the Harz Mountains, Germany. Ibergirhynchia contraria is from a monospecific brachiopod limestone that formed on top of the drowned Devonian Iberg Reef which persisted as a seamount during Famennian and Early Carboniferous times. Ibergirhynchia contraria is considered a cold seep-related brachiopod based on this locality. Such seep associations have been observed for Mesozoic representatives of the rhynchonellide superfamily Dimerelloidea. Ibergirhynchia is considered the first Paleozoic representative of the family Rhynchonellinidae. Ibergirhynchia resembles Dzieduszyckia externally and may be derived from this dimerelloid.

Brachiopods in Early Carboniferous

Changes in Mississippian global paleogeography derived from the reconfiguration of the continents, a reversal in ocean currents and global cooling. Although the tectonic and climatic changes are well-documented, their effects on the distribution of brachiopod fauna are poorly documented. Here we present systematic quantitative analyses on global paleobiogeography based on a global brachiopod database from the Mississippian (i.e., Tournaisian, Visean, and Serpukhovian). The dataset consists of 2123 species of 344 brachiopod genera from 1156 localities. Our results reveal that global provincialism was not evident during the Tournaisian and Visean Stages. Two realms, i.e., the Gondwanan and Paleoequatorial Realms, are recognized during the Tournaisian. The Paleoequatorial Realm dominates during the Visean Stage, whereas the Gondwanan Realm is not documented due to the absence of data points. In contrast to the early and middle Mississippian stages, faunal provincialism is greatly enhanced in the Serpukhovian Stage with Paleotethyan and North American realms easily distinguished. This indicates that the Rheic Ocean was closed before the Serpukhovian due to the collision between Gondwana and Laurussia, that disrupted faunal interchange between the Paleotethys and North America. In addition, the paleolatitude-related thermal gradient was enhanced and the Boreal Realm was distinguished from the Paleotethyan Realm during the onset of the Late Palaeozoic Ice Age (LPIA) in the Serpukhovian. The paleolatitude diversity gradient pattern further shows a distinct shift of diversity center from the southern tropic zone in the Tournaisian and Visean to the northern tropic zone in the Serpukhovian.

Early Carboniferous synorogenic basins evolution of the Ossa-Morena and

The stratigraphic record of the Early Carboniferous in Iberia reveals that synorogenic deposition was important and occurred simultaneously in basins influenced by extension and contraction with gravitational instability. In NW Iberia (Galicia – Trás-os-Montes Zone) contraction was dominant and the deposition took place in a forebulge outwards from the nappe stacking front. Here, synorogenic deposits were strongly affected by folding and thrusting as they were imbricated and incorporated in the allochthonous pile. In a different way, in SW Iberia (Ossa-Morena Zone) synorogenic deposition was influenced by extension and happened simultaneously with the onset of significant magmatism.

Upper Devonian biostratigraphy of the Iberian Pyrite Belt, southwest Spain Part One: Miospores

The Iberian Pyrite Belt (IPB), which forms part of the Variscan orogenic massif, is renowned for the magnitude and extent of its massive sulfide mineralization. The stratigraphic record of the IPB consists of Upper Palaeozoic sedimentary and igneous rocks. In ascending order, these comprise the thick Phyllite-Quartzite Group attributed to the Middle and Upper Devonian and characterized by shales and quartzites with conglomeratic and carbonate intercalations towards the top; the appreciably thinner Volcano-Sedimentary Complex, a heterogeneous uppermost Devonian-Mississippian unit embodying diverse volcanic, subvolcanic, and sedimentary rocks that host the massive sulfide deposits; and the shaly and sandy, turbiditic Culm Group (Carboniferous). This entire succession was folded and faulted during the Asturian phase of the Variscan Orogeny that gave rise to a thin-skinned type structure. The present study constitutes a detailed blostratigraphic investigation of palynologically productive samples representative of the Phyllite-Quartzite Group and the basal (anoxic) portion of the Volcano-Sedimentary Complex. These were collected from surface and mine exposures variously located in the Spanish part of the IPB; out of 282 samples processed, 117 proved to be productive palynologically. The aim of this project is to provide comprehensive palynostratigraphic data applicable to precise dating and correlation of the IPB's stratigraphic succession (i.e., of the two sampled lithostratigraphic units), which has hitherto been investigated biostratigraphically on a relatively localized basis. The results are incorporated in two successive parts. The first of these, i. e., the present paper, focuses on the systematic analysis of the terrestrial (miospore) component of the palynological assemblages. The second part, devoted to the marine, organic-walled microphytoplankton (acritarchs and prasinophytes), will evaluate the stratigraphic significance of the IPB palynofloras and their application to elucidating the geological history of the region. In the systematic-descriptive section, which occupies the bulk of this paper, 55 species of trilete miospores are described and are allocated among 34 genera, two of which (Cristicavatispora and Epigruspora) are newly instituted herein. The majority of the species are either positively identifiable or closely affiliable with previously named species. The nine newly established species are as follows: Camptozonotriletes confertus, Indotriradites diversispinosus, Cristicavatispora dispersa (type species), Epigruspora regularis (type species), Ancyrospora? implicata, Endosporites tuberosus, Rugospora explicata, Spelaeotriletes plicatus, and Teichertospora iberica.

俯冲带深部流体特征——以西南天山变质俯冲杂岩为例

The subduction zone is an important site of the fluid activity and recycling of chemical elements. The fluid characteristic of deep subduction zones is a top scientific problem attracting the petrologists, geochemists and tectonists. In this dissertation, the characteristics of fluid activity within a deep subduction zone have been explored on the basis of the studies on the petrography, mineral chemistry, fluid inclusions, geochemistry and metamorphic P–T conditions of the omphacite-bearing high-pressure veins and related hosts from the low-temperature/high-pressure metamorphic belt in southwestern Tianshan, China. Multiple high-pressure veins are exposed in host eclogites and blueschists. The veins are composed predominantly of omphacite, garnet, quartz, and other minerals. Some veins contain cm-sized rutiles. In general, the vein can be divided into three types, the ‘in situ dehydration’ vein, the ‘external transport’ vein and the ‘composite’ vein. The omphacites within the veins and related host rocks contain lots of two-phase or three-phase primary fluid inclusions. The final melting temperature (Tfm) of fluid inclusions varies mainly from -0.6 to -4.3 °C, the homogeneous temperature (Th) varies from 185 to 251 °C, the salinity varies from 1.1 to 6.9 wt.% NaCl equivalent and the density varies from 0.81 to 0.9 g/cm3. The fluids were released under the conditions of T = 520–580°C and P = 15–19 kbar at blueschist facies to eclogite facies transition. The fluids include not only Li, Be, LILE, La, Pb-enriched and HFSE- and HREE-depleted aqueous fluids but also HFSE (Ti-Nb-Ta)-rich aqueous fluids. The complex composed of aluminosilicate polymers and F was the catalyst which had caused the Ti-Nb-Ta to be dissolved into the fluids. During the transport of the LILE-rich and HFSE- and HREE-poor fluids, they can exchange some chemical elements with country rocks and leach some trace elements in some extent. The rutile could be precipitated from the HFSE (Ti-Nb-Ta)-rich aqueous fluids when CO2 was added into the fluids. The host rocks could obtain some elements, such as Ca, Cs, Rb, Ba and Th, from the external fluids. The fluids with complex composition had been released within the deep subduction zone (>50 km) in Early Carboniferous during the subduction of the South Tianshan Ocean under the Yili–Central Tianshan Plate. The results obtained in this dissertation have made new progress compared with the published data (e.g. Tatsumi, 1989; Becker et al., 1999; Scambelluri and Philippot, 2001; Manning, 2004; Hermann et al., 2006; Spandler and Hermann, 2006).

新疆东昆仑蛇绿岩岩石学、地球化学特征及其大地构造意义

The East Kunlun area of Xinjiang (briefly EKAX) is the western part of broadly speaking East Kunlun orogenic zone. The absence of geological data (especially ophiolites) on this area has constrained our recognition to its geology since many years. Fund by National 305 Item (96-915-06-03), this paper, by choosing the two ophiolite zones (Muztag and Southwestern Margin of Aqikekule Lake ophiolite zones) exposed at EKAX as the studied objects and by the analysis of thin section, electron probe, XRF, ICP-MS, SEM and Sm-Nd isotope, totally and sys ematically dealt with the field geological, petrological, minerological, petrochemical and geochemical characteristics (including trace, rare earth element and Sm-Nd isotope) and the tectonic setting indicated by them for each ophilite zone. Especially, this paper discussed the trace and rare earth element patterns for metamorphic peridotites, their implications and related them to the other components of ophiolite in order to totally disclose ophiolite origins. Besides, this paper also studied the petrological, geochemical and paleobiological characteristics for the cherts coexsisted with the Muztag ophiolite and the tectonic setting indicated by them. Based on these, the author discussed the tectonic evolution from Proterozoic to Permian for this area. For Muztag ophiolite, their field geological, petrological, minerological, petrochemical and geochemical characteristics show that: ① outcropped along the Muztag-Jingyuhu fault with west-to-east strike, the ophiolite is composed of such three components as metamorphic peridotites, cumulates and volcanic rocks; ② metamophic peridotites consist of such types as lherzolites, serpentinized lherzolites and serpentinites, only pyroxenites is seen of cumulates and volcanic rocks include basalts, basaltic andesites and andesites; ③ mineralogical data on this ophiolite suggest it formed in supra-subduction zone (SSZ)environment, and its mantle wedge is heterogeneous; ④ whole-rock TiO_2 and Al_2O_3 of metamorphic peridotites indicate their original environment with the MORB and SSZ characteristics; ⑤ metamorphic peridotites have depleted LREE and flat REE patterns and volcanic rocks have enriched LREE patterns; ⑥ trace element characteristics of metamorphic peridotites imply that they had undergone Nb and Ta enrichment event after partial melting; ⑦ trace element characteristics of volcanic rocks and their tectonic diagrams show they are formed in the spreading and developed island arc environment with back-arc basin, such as rifted island arc, which is supported by the ε_(Nd)(t) -2.11～+3.44. In summary, the above evidence implies that Muztag ophiolite is formed in SSZ environment, where heterogeneous mantle wedge was metasomatised by the silica-enriched melt from subducted sediments and/or oceanic crust, which makes the mantle wedge enriched again, and this enriched mantle wedge later partially melted to form the volcanic rocks. For Southwestern Margin of Aqikekule Lake ophiolite, their field geological, petrological, minerological, petrochemical and geochemical characteristics show that: ① it outcropped as tectonic slices along the near west-to-east strike Kunzhong fault and is composed of metamorphic perodotties, cumulates and volcanic rocks, in which, chromites are distributed in the upper part of metamorphic peridotites as pods, or in the lower part of cumulates as near-strata; ② metamorphic peridotites include serpentinites, chromite-bearing serpentinites, thlorite-epidote schists and chromitites, of which, chromitites have nodular and orbicular structure, and cumulates include pyroxenits, serpentinites, chromite-bearing serpentinites, chromites and metamorphically mafic rocks and only basalts are seen in volcanic rocks; ③ Cr# of chromites suggest that they formed in the SSZ and Al_2O_3 and TiO_2 of metamorphic peridotites also suggest SSZ environment; ④metamorphic peridotites have V type and enriched LREE patterns, cumulates have from strongly depleted LREE, flat REE to enriched LREE patterns with universally striking positive Eu anomalies and basalts show flat REE or slight enriched LREE patterns with no Eu anomalies; ⑤ trace element and Sm-Nd isotope characteristics of metamorphic peridotites imply their strikingly heterogeneous mantle character(ε_(Nd)(t)+4.39～+26.20) and later Nb, Ta fertilization; ⑥ trace element characteristics of basalts and their tectonic diagrams show they probably formed in the rifted island arc or back-arc basin enviromnent. In summary, the above evidence shows that this ophiolite formed in the SSZ environment and melts from subudcted plate are joined during its formation. Rare earth element, whole-rock and sedimentary characteristics of cherts with the Muztag ophiolite show that they formed in the continental margin environment with developed back-arc basin, and radiolarias in the cherts indicate that the upper age of Muztag ophiolite is early carboniferous. Based on the accreted wedge models of Professor Li Jiliang for Kunlunshan Mountain and combined with study on the two typical ophiolite profiles of EKAX, the author discussed the tectonic evolution of EKAX from Proterzoic to Permian.

鄂尔多斯盆地西缘中生代构造与地层分析及盆地演化研究

Through the detailed analyses of Mesozoic tectono-stratigraphy and basin formation dynamic mechanism and the styles of different units in the western margin of Ordos Basin(Abbreviated to "the western margin"), while some issues of the pre-Mesozoic in the western margin and central part of Ordos Basin also be discussed, the main views and conclusion as follows: 1. There are three types of depositional systems which are related with syndepositional tectonic actions and different tectonic prototype basins, including: alluvial fan systems, river system (braided river system and sinuosity river system), lacustrine-river delta system and fan delta system. They have complex constitutions of genetic facies. For the tectonic sequence VI, the fan sediments finning upper in the north-western margin and coarse upper in the south-western margin respectively. 2. In order to light the relationship between basin basement subsidence rate and sediment supply and the superposed styles, five categories of depositional systems tracts in different prototype basins were defined: aggrading and transgressive systems tracts during early subsidence stage, regressive and aggrading systems tracts during rapid subsidence, upper transgessive systems tracts during later subsidence stage. Different filling characteristics and related tectonic actions in different stages in Mesozoic period were discussed. 3. In order to determined the tectonic events of the provenance zones and provenance strata corresponding to basins sediments, according the clastics dispersal style and chemical analyses results of sediments in different areas, the provenance characteristics have been described. The collision stage between the "Mongolia block" and the north-China block may be the late permian; The sediments of Mesozoic strata in the north-western margin is mainly from the Alex blocks and north-Qilian Paleozoic orogeny, while the south-western margin from Qinling orogeny. The volcanic debris in the Yan'an Formation may be from the arc of the north margin of north-China block, although more study needed for the origin of the debris. The provenance of the Cretaceous may be from the early orogeny and the metamorphic basement of Longshan group. 4. The subsidence curve and subsidence rate and sedimentary rate in different units have been analyzed. For different prototype basin, the form of the subsidence curves are different. The subsidence of the basins are related with the orogeny of the basins.The beginning age of the foreland basin may be the middle Triassic. The change of basement subsidence show the migration of the foredeep and forebulge into the basin. The present appearance of the Ordos basin may be formed at the late stage of Cretaceous, not formed at the late Jurassic. 5. The structure mode of the west margin is very complex. Structure transfer in different fold-thrust units has been divided into three types: transfer faults, transition structures and intersected form. The theoretic explanations also have been given for the origin and the forming mechanism. The unique structure form of Hengshanpu is vergent west different from the east vergence of most thrust faults, the mechanism of which has been explained. 6. In Triassic period, the He1anshan basin is extensional basin while the Hengshanbu is "forland", and the possible mechanism of the seemingly incompatible structures has been explained. First time, the thesis integrate the Jurassic—early Cretaceous basins of west margin with the Hexi corridor basins and explain the unitive forming mechanism. The model thinks the lateral extrusion is the main mechanism of the Hexi corridor and west margin basins, meanwhile, the deep elements and basement characters of the basins. Also, for the first time, we determine the age of the basalt in Helanshan area as the Cretaceous period, the age matching with the forming of the Cretaceous basins and as the main factor of the coal metamorphism in the Helanshan area. 7. The Neoprotterozoic aulacogen is not the continuation of the Mesozoic aulacogen, while it is another new rift stage. In the Paleozoic, the Liupanshan—southern Helanshan area is part of the back-arc basins of north Qilian ocean. 8. The Helanshan "alacogen" is connected with the north margin of north China block, not end at the north of Zhouzishan area like "appendices". Also, I think the upper Devonian basin as the beginning stage of the extensional early Carboniferous basins, not as a part of the foreland basins of Silurian period, not the collision rift. 9. The controlling factor of the difference of the deformation styles of the north-west margin and the south-west margin is the difference of the basements and adjacent tectonic units of the two parts.

塔里木盆地奥陶系不整合面岩溶作用特征与油气储层研究

Based on the principle and methods of carbonate sedimentology and reservoir geology, and guided by the theories of carbonate reservoir geology, the palaeokarst of Ordovician carbonate rocks in Tarim Basin has been comprehensively studied with multiple methods from different branches of geology. It is indicated that the features and distribution of palaeokarstification have developed in Ordovician carbonates. The controlling of karstification to Ordovician carbonate reservoirs has been discussed. Regional distribution of carbonate reservoirs controlled by karstification has been predicted within this basin. The main consents and conclusions of the this dissertation is as follows: Nine key indicators to the recognition of palaeokarst are proposed in terms of careful observation upon the well cores, lithological and geochemical analyses, and drilling and logging responses to the karst caves and fractures. The time and environment of cave filling are documented from careful research of lithofacies, mineralogy, and geochemistry of the physical and chemical fillings within karst caves. The caves in Ordovician carbonates were filled in Early Carboniferous in Lunnan area. The muddy filling in upper caves was deposited under subaerial fresh-water setting, while the muddy filling in lower caves was formed in the mixed water body of fresh-water and dominated sea water. Although most chemical fillings are suggested being precipated in the burial diagenetic environment after karstification but mineralogic and geochemical characteristics of some chemical fillings indicates they formed in meteoric environment during the karstification. It is obvious that the palaeokarst has been zoned in vertical profile. It can be divided into four units from top to bottom: surface karst, vadose karst, phreatic and tranquil flow zones. Between two types of limestone karst and dolostone karst are firstly differentiated in Tarim Basin, based on the comparison of features of each karst zone in limestone and dolostone regions. In Tabei area, the lowest depth of karstification is approximately 300 m below the Upper Ordovician unconformity interface, while the bottom depth of karstification in Tazhong area ranges commonly from 300 to 400 m, in rare cases may be up to 750 m below the upper Ordovician unconformity interface. In Lunnan and Tazhong areas, the palaeokarst morphology and the surface hydrosystem are firstly reconstructed based on the top of carboniferous "Shuangfeng limestone bed (Double-Peaks limestone)" as basal. According to the palaeomorphologic feature, karst topography can be divided into three units: karst upland, karst slope, and karst valley. Vadose zone was well developed in karst upland, and it can be found in a quite depth. Both vadose and phreatic zones were well developed in karst slope and upstream valley. In downstream valley, the karstification is not strong, the vadose and phreatic zones are thin in thickness. In Tazhong and Yingmaili areas, karstification is also developed in relict carbonate palaeo-hills which existed as isolated blocks admits clastic strata.

新疆萨吾尔金矿带构造-流体地球化学及成矿作用

The Sawuer gold belt is located in the transition belt between Siberian plate and Kazakhstan-Junggar plate. Based on the geological and geochemical studies on the Kuoerzhenkuola and Buerkesidai gold deposits, in Sawuer gold mineralization belt, the time-space structure of mineralization and mineralizing factor are studied, the metallogenic regularity is concluded in thistheses. The ore bodies have the regularity that orebody are of the extensive and compressive in the sallow and depth of volcanic apparatue, respectively, and the vertical extension of orebody is more intensive than the horizontal extension. The gold deposits were controlled by the fractures of volcanic apparatus and regional faults, and featured by the hydrothermal alteration and metasomatism type disseminated mineralization and filling type vein mineralization. By virtue of the geological and geochemical studies on the two deposits that the formation of the two deposits are significantly related to the volcanic activity, we propose new ideas about their origin: (1) the two deposits are located in the same strata, and share the same genesis. (2) both of two deposits are volcanogenic late-stage hydrothermal gold deposits. Based on mapping of volcanic lithofacies and structure for the first time, it is discovered that a volcanic apparatus existed in the study area. Volcanic-intrusive activity can be divided into three cycles and nine lithofacies. where the two deposits are hosted in the same volcanic cycle, in this case, the wall-rock should belong to the same strata. The 40Ar-39Ar age method is employed in this work to analyze the fluid inclusions of quartz in the ore bodies from Kuoerzhenkuola and Buerkesidai gold deposits. The results show that the main mineralization occurred in 332.05 + 2.02-332.59 + 0.5IMa and 335.53 + 0.32Ma~336.78 + 0.50Ma for Kuoerzhenkuola and Buerkesidai gold deposits respectively, indicating that the two deposits are formed almost at the same time, and the metallogenic epoch of the tow deposits are close to those of the hosting rocks formed by volcanic activity of Sawuer gold belt. This geochronological study supplies new evidence for determining the timing of gold mineralization, the geneses of gold deposits? and identifies that in Hercynian period, the Altai developed tectonic-magmatic-hydrothermal mineralization of Early Carboniferous period, except known two metallogenic mineralization periods including tectonic-magmatic-hydrothermal mineralization of Devonian period and Late Carboniferous-Permian period respectively. The study of fluid inclusions indicates that the ore-forming fluid is a type of NaCl-HbO fluid with medium-low temperature and low salinity, Au is transported by the type of auric-sulfur complex (Au (HS)2-), the ore is formed in reduction condition. Hydrogen and oxygen isotopes of fluid inclusions in the major mineralizating stage show that the solutions mainly originated from magmatic water and meteoric water. The fluid mixing and water-rock reaction cause the deposition of Au. The helium and argon isotope compositions of fluid inclusions hosted in pyrite have been measured from Kuoerzhenkuola and Buerkesidai gold deposits in Sawuer gold belt. The results show that the ore-forming fluids of two deposits possessed the same source and is a mixture of mantle- and partial meteoric water-derived fluid, and the reliability of He and Ar isotopic compositions in Hercynian period is discussed. Isotopic studies including H, O, He, C, S, Pb and Sr reveal the same result that the ore-forming fluids of two deposits possessed the same source: the water derived mainly from magmatic water, partially from meteoric water; the mineralizers and ore materials derived mainly from mantle beneath the island arc, and partially from crust. The ore-forming fluids of two deposits are a mixture of mantle-derived fluids being incorporated by crust-derived fluid, and shallow partial meteoric water. Based on these results, it is proposed that the geneses of the two gold deposits are the same, being volcanogenic late-stage hydrothermal gold deposits that the ore-forming fluids filled in fractures of volcanic apparatus and metasomatized the host rocks in the volcanic apparatus. It is the first time we carried out the geophysical exploration, that is, the EH-4 continuous electrical conductivity image system measurement, the results show that relative large-size mineralizing anomalies in underground have been discovered.lt can confirm the law and genesis of the deposits mentioned above, and change the two abandoned mines to current large-size potenial exploration target.

Contribution to the geology of Thailand and implications for the geodynamic evolution of Southeast Asia

Abstract The purpose of this study is to unravel the geodynamic evolution of Thailand and, from that, to extend the interpretation to the rest of Southeast Asia. The methodology was based in a first time on fieldwork in Northern Thailand and Southernmost Myanmar, using a multidisciplinary approach, and then on the compilation and re-interpretation, in a plate tectonics point of view, of existing data about the whole Southeast Asia. The main results concern the Nan-Uttaradit suture, the Chiang Mai Volcanic Belt and the proposition of a new location for the Palaeotethys suture. This led to the establishment of a new plate tectonic model for the geodynamic evolution of Southeast Asia, implying the existence new terranes (Orang Laut and the redefinition of Shan-Thai) and the role of the Palaeopacific Ocean in the tectonic development of the area. The model proposed here considers the Palaeotethys suture as located along the Tertiary Mae Yuam Fault, which represents the divide between the Cimmerian Sibumasu terrane and the Indochina-derived Shan-Thai block. The term Shan-Thai, previously used to define the Cimmerian area (when the Palaeotethys suture was thought to represented by the Nan-Uttaradit suture), was redefined here by keeping its geographical location within the Shan States of Myanmar and Central-Northern Thailand, but attributing it an East Asian Origin. Its detachment from Indochina was the result of the Early Permian opening of the Nan basin. The Nan basin closed during the Middle Triassic, before the deposition of Carnian-Norian molasse. The modalities of the closure of the basin imply a first phase of Middle Permian obduction, followed by final eastwards subduction. The Chiang Mai Volcanic Belt consists of scattered basaltic rocks erupted at least during the Viséan in an extensional continental intraplate setting, on the Shan-Thai part of the Indochina block. The Viséan age was established by the dating of limestone stratigraphically overlying the basalts. In several localities of the East Asian Continent, coeval extensional features occur, possibly implying one or more Early Carboniferous extensional events at a regional scale. These events occurred either due to the presence of a mantle plume or to the roll-back of the Palaeopacific Ocean, subducting beneath Indochina and South China, or both. The Palaeopacific Ocean is responsible, during the Early Permian, for the opening of the Song Ma and Poko back-arcs (Vietnam) with the consequent detachment of the Orang Laut Terranes (Eastern Vietnam, West Sumatra, Kalimantan, Palawan, Taiwan). The Late Triassic/Early Jurassic closure of the Eastern Palaeotethys is considered as having taken place by subduction beneath its southern margin (Gondwana), due to the absence of Late Palaeozoic arc magmatism on its northern (Indochinese) margin and the presence of volcanism on the Cimmerian blocks (Mergui, Lhasa). Résumé Le but de cette étude est d'éclaircir l'évolution géodynamique de la Thaïlande et, à partir de cela, d'étendre l'interprétation au reste de l'Asie du Sud-Est. La méthodologie utilisée est basée dans un premier temps sur du travail de terrain en Thaïlande du nord et dans l'extrême sud du Myanmar, en se basant sur une approche pluridisciplinaire. Dans un deuxième temps, la compilation et la réinterprétation de données préexistantes sur l'Asie du Sud-est la été faite, dans une optique basée sur la tectonique des plaques. Les principaux résultats de ce travail concernent la suture de Nan-Uttaradit, la « Chiang Mai Volcanic Belt» et la proposition d'une nouvelle localité pour la suture de la Paléotethys. Ceci a conduit à l'établissement d'un nouveau modèle pour l'évolution géodynamique de l'Asie du Sud-est, impliquant l'existence de nouveaux terranes (Orang Laut et Shan-Thai redéfini) et le rôle joué par le Paléopacifique dans le développement tectonique de la région. Le modèle présenté ici considère que la suture de la Paléotethys est située le long de la faille Tertiaire de Mae Yuam, qui représente la séparation entre le terrain Cimmérien de Sibumasu et le bloc de Shan-Thai, d'origine Indochinoise. Le terme Shan-Thai, anciennement utilise pour définir le bloc Cimmérien (quand la suture de la Paléotethys était considérée être représentée par la suture de Nan-Uttaradit), a été redéfini ici en maintenant sa localisation géographique dans les états Shan du Myanmar et la Thaïlande nord-centrale, mais en lui attribuant une origine Est Asiatique. Son détachement de l'Indochine est le résultat de l'ouverture du basin de Nan au Permien Inférieur. Le basin de Nan s'est fermé pendant le Trias Moyen, avant le dépôt de molasse Carnienne-Norienne. Les modalités de fermeture du basin invoquent une première phase d'obduction au Permien Moyen, suivie par une subduction finale vers l'est. La "Chiang Mai Volcanic Belt" consiste en des basaltes éparpillés qui ont mis en place au moins pendant le Viséen dans un contexte extensif intraplaque continental sur la partie de l'Indochine correspondant au bloc de Shan-Thai. L'âge Viséen a été établi sur la base de la datation de calcaires qui surmontent stratigraphiquement les basaltes. Dans plusieurs localités du continent Est Asiatique, des preuves d'extension plus ou moins contemporaines ont été retrouvées, ce qui implique l'existence d'une ou plusieurs phases d'extension au Carbonifère Inférieur a une échelle régionale. Ces événements sont attribués soit à la présence d'un plume mantellique, ou au rollback du Paléopacifique, qui subductait sous l'Indochine et la Chine Sud, soit les deux. Pendant le Permien inférieur, le Paléopacifique est responsable pour l'ouverture des basins d'arrière arc de Song Ma et Poko (Vietnam), induisant le détachement des Orang Laut Terranes (Est Vietnam, Ouest Sumatra, Kalimantan, Palawan, Taiwan). La fermeture de la Paléotethys Orientale au Trias Supérieur/Jurassique Inférieur est considérée avoir eu lieu par subduction sous sa marge méridionale (Gondwana), à cause de l'absence de magmatisme d'arc sur sa marge nord (Indochinoise) et de la présence de volcanisme sur les blocs Cimmériens de Lhassa et Sibumasu (Mergui). Résumé large public L'histoire géologique de l'Asie du Sud-est depuis environ 430 millions d'années a été déterminée par les collisions successives de plusieurs continents les uns avec les autres. Il y a environ 430 millions d'années, au Silurien, un grand continent appelé Gondwana, a commencé à se «déchirer» sous l'effet des contraintes tectoniques qui le tiraient. Cette extension a provoqué la rupture du continent et l'ouverture d'un grand océan, appelé Paléotethys, éloignant les deux parties désormais séparées. C'est ainsi que le continent Est Asiatique, composé d'une partie de la Chine actuelle, de la Thaïlande, du Myanmar, de Sumatra, du Vietnam et de Bornéo a été entraîné avec le bord (marge) nord de la Paléotethys, qui s'ouvrait petit à petit. Durant le Carbonifère Supérieur, il y a environ 300 millions d'années, le sud du Gondwana subissait une glaciation, comme en témoigne le dépôt de sédiments glaciaires dans les couches de cet âge. Au même moment le continent Est Asiatique se trouvait à des latitudes tropicales ou équatoriales, ce qui permettait le dépôt de calcaires contenant différents fossiles de foraminifères d'eau chaude et de coraux. Durant le Permien Inférieur, il y a environ 295 millions d'années, la Paléotethys Orientale, qui était un relativement vieil océan avec une croûte froide et lourde, se refermait. La croûte océanique a commencé à s'enfoncer, au sud, sous le Gondwana. C'est ce que l'on appelle la subduction. Ainsi, le Gondwana s'est retrouvé en position de plaque supérieure, par rapport à la Paléotethys qui, elle, était en plaque inférieure. La plaque inférieure en subductant a commencé à reculer. Comme elle ne pouvait pas se désolidariser de la plaque supérieure, en reculant elle l'a tirée. C'est le phénomène du «roll-back ». Cette traction a eu pour effet de déchirer une nouvelle fois le Gondwana, ce qui a résulté en la création d'un nouvel Océan, la Neotethys. Cet Océan en s'ouvrant a déplacé une longue bande continentale que l'on appelle les blocs Cimmériens. La Paléotethys était donc en train de se fermer, la Neotethys de s'ouvrir, et entre deux les blocs Cimmériens se rapprochaient du Continent Est Asiatique. Pendant ce temps, le continent Est Asiatique était aussi soumis à des tensions tectoniques. L'Océan Paléopacifique, à l'est de celui-ci, était aussi en train de subducter. Cette subduction, par roll-back, a déchiré le continent en détachant une ligne de microcontinents appelés ici « Orang Laut Terranes », séparés du continent par deux océans d'arrière arc : Song Ma et Poko. Ceux-ci sont composés de Taiwan, Palawan, Bornéo ouest, Vietnam oriental, et la partie occidentale de Sumatra. Un autre Océan s'est ouvert pratiquement au même moment dans le continent Est Asiatique : l'Océan de Nan qui, en s'ouvrant, a détaché un microcontinent appelé Shan-Thai. La fermeture de l'Océan de Nan, il y a environ 230 millions d'années a resolidarisé Shan-Thai et le continent Est Asiatique et la trace de cet événement est aujourd'hui enregistrée dans la suture (la cicatrice de l'Océan) de Nan-Uttaradit. La cause de l'ouverture de l'Océan de Nan peut soit être due à la subduction du Paléopacifique, soit aux fait que la subduction de la Paléotethys tirait le continent Est Asiatique par le phénomène du « slab-pull », soit aux deux. La subduction du Paléopacifique avait déjà crée de l'extension dans le continent Est Asiatique durant le Carbonifère Inférieur (il y a environ 340-350 millions d'années) en créant des bassins et du volcanisme, aujourd'hui enregistré en différents endroits du continent, dont la ceinture volcanique de Chiang Mai, étudiée ici. A la fin du Trias, la Paléotethys se refermait complètement, et le bloc Cimmérien de Sibumasu entrait en collision avec le continent Est Asiatique. Comme c'est souvent le cas avec les grands océans, il n'y a pas de suture proprement dite, avec des fragments de croûte océanique, pour témoigner de cet évènement. Celui-ci est visible grâce à la différence entre les sédiments du Carbonifère Supérieur et du Permieñ Inférieur de chaque domaine : dans le domaine Cimmérien ils sont de type glaciaire alors que dans le continent Est Asiatique ils témoignent d'un climat tropical. Les océans de Song Ma et Poko se sont aussi refermés au Trias, mais eux ont laissé des sutures visibles

Tectonothermal evolution and exhumation history of the Paleozoic Proto-Andean Gondwana margin crust: The Famatinian Belt in NW Argentina

We studied the P-T-t evolution of a mid-crustal igneous-metamorphic segment of the Famatinian Belt in the eastern sector of the Sierra de Velasco during its exhumation to the upper crust. Thermobarometric and geochronological methods combined with field observations permit us to distinguish three tectonic levels. The deepest Level I is represented by metasedimentary xenoliths and characterized by prograde isobaric heating at 20-25 km depth. Early/Middle Ordovician granites that contain xenoliths of Level I intruded in the shallower Level II. The latter is characterized by migmatization coeval with granitic intrusions and a retrograde isobaric cooling P-T path at 14-18 km depth. Level II was exhumed to the shallowest supracrustal Level III, where it was intruded by cordierite-bearing granites during the Middle/Late Ordovician and its host-rock was locally affected by high temperature-low pressure HT/LP metamorphism at 8-10 km depth. Level III was eventually intruded by Early Carboniferous granites after long-term slow exhumation to 6-7 km depth. Early/Middle Ordovician exhumation of Level II to Level III (Exhumation Period I,0.25-0.78 mm/yr) was faster than exhumation of Level III from the Middle/Late Ordovician to the Lower Carboniferous (Exhumation Period II, 0.01-0.09 mm/yr). Slow exhumation rates and the lack of regional evidence of tectonic exhumation suggest that erosion was the main exhumation mechanism of the Famatinian Belt. Widespread slow exhumation associated with crustal thickening under a HT regime suggests that the Famatinian Belt represents the middle crust of an ancient Altiplano-Puna-like orogen. This thermally weakened over-thickened Famatinian crust was slowly exhumed mainly by erosion during similar to 180 Myr. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.