Data for: The geologic record of the exhumed root of the Central African Orogenic Belt in the Central Cameroon Domain (Mbé - Sassa-Mbersi region)

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The isotopic composition of zircon and garnet: A record of the metamorphic history of Naxos, Greece

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Oxygen isotopes, REE and U-Pb zircon formation

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Reconstructing paleoelevation in eroded orogens

Hydrogen isotope and Ar-40/Ar-39 geochronological data are presented from muscovite within a crustal-scale extensional detachment of the Shuswap Metamorphic Complex, North American Cordillera. The hydrogen isotope compositions (deltaD(ms)) of precisely dated muscovite attain values as low as -156parts per thousand in the detachment mylonite, whereas footwall quartzite has a deltaD(ms) value of -81parts per thousand. The very low deltaD(ms) values in the detachment are best explained by infiltration of meteoric water, with maximum deltaD values of -135parts per thousand +/- 3parts per thousand, during extensional unroofing of the orogen at 49.0-47.9 Ma. On the basis of the empirically determined relationship between elevation and isotopic composition of precipitation, the reconstructed early Eocene paleoelevations of the orogen are 4060 +/- 250 m to 4320 +/- 250 m, at least 1000 m higher than the highest present-day peaks. We propose that the isotopic composition of surface-derived waters in extensional detachments represents a newly recognized method to estimate maximum paleoelevations attained immediately preceding extensional orogenic collapse

Thermal maturation and exhumation of a middle orogenic crust in the Livradois area (French Massif central)

Upper Carboniferous heating and melting of the middle orogenic crust associated with the emplacement of syntectonic granitoids are documented in the Upper Gneissic Unit of the Livradois area (central part of the French Massif Central). Crustal melting post-dates peak metamorphism conditions (800-625°C, 10-8 kb) dated at 360 ± 4 Ma (U-Th-Pb on monazite). The P-T evolution of the metamorphic series indicates that Barrovian metamorphism was followed by a decompression (from 10 ± 1 kbar to 6 ± 1 kbar) associated with either a decrease in temperature in the southern part of the series or with an increase in temperature (of about 150°C) in the northern part of the series. This evolution records the first step of the exhumation of the series coeval with granitoids intrusion, of which the emplacements were dated at 315 ± 4 and 311 ± 18 Ma (U-Pb on zircon). The final stage of the exhumation is associated with an isobaric cooling of the whole series. Similarity of 40Ar/39Ar ages for biotite in the paragneisses (307-300 Ma) and K-feldspar in the granitoids (306-300 Ma) document rapid cooling for this stage. Moreover dextral reverse mylonites, at the border and the northern part of the metamorphic series indicate north-south compression coeval with the unroofing of the series. Youngest 40Ar-39Ar ages on K-feldspar (274.6 ± 5 Ma) combined with normal shearing in mylonites limiting the Carboniferous Brassac-les-Mines basin document the late Carboniferous-early Permian stage of extension coeval with the upwelling of the Velay granitic dome

The Role Of Crustal Anatexis And Mantle-Derived Magmas In The Genesis Of Synorogenic Hercynian Granites Of The Livradois Area, French Massif Central

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Understanding geological processes through modelling : a memorial volume honouring Evgenii Burov

The goal of this paper is to use the structural, metamorphic and geochronological record from the migmatitic core of the Naxos dome (Greece) and its associated subdomes to address the internal dynamics of a partially molten orogenic root. U-Pb ages from ca. 24 to 16 Ma and textures of zircon in the migmatites suggest successive dissolution and precipitation cycles with a period of 1 to 2 Ma, interpreted as the timescale of convective instabilities in a ca. 20 km thick partially molten layer. Dimensional analysis indicates that convection of this root requires a viscosity lower than 10(18) Pas, consistent with viscosity values expected for partially molten felsic rocks. Structural analysis and U-Pb geochronology of deformed granitic dikes rooting in the migmatites record the subsequent development of the Naxos dome by diapirism from ca. 16 to 13 Ma. The size of the first order migmatite dome on Naxos (5 x 12 km) requires that the unstable layer at the onset of diapirism was 5 to 10 km thick and presented a moderate viscosity contrast with its envelope. From this analysis we propose that the Naxos migmatite dome documents a two stage dynamic evolution for the partially molten root of the Aegean belt characterized by (1) crustal scale convection for at least 8 Ma and (2) diapirism for about 3 Ma during progressive thinning of the collapsing orogenic crust

Evolution of the mineralizing fluids and possible genetic links between Miduk porphyry copper and Latala vein type deposits, Kerman copper belt, South Iran

International audienceThe Cenozoic Urumieh-DokhtarMagmaticBelt (UDMB) of Iran is a major host to porphyry Cu-Mo-Au deposits (PCDs), represented by the world-class Sarcheshmeh deposit and Miduk deposit in the south and the Sungun deposit in the north. Vein type, base and/or precious metal deposits are also common and some are spatially associated with PCDs. Latala and Chahmessi are vein type, base and precious metal deposits in the north and southwest Miduk deposit. The area is covered mainly by Paleocene-Eocene volcanic and pyroclastic rocks of basaltic, basaltic-andesitic and trachy-andesitic compositions, and minor marls and limestones. The volcanic and pyroclastic rocks are intruded by Miocene shallow intrusions of quartz diorite, quartz monzonite and granodiorite compositions.The rocks are host to a set of ore-bearing quartz veins. Mineralization in both the Chahmessi and Latala deposits are controlled by faults and fractures. The role of the ring structures and faults in the distribution of hydrothermal alteration zones and mineralization is important in the Latala deposit. In these veins, euhedral quartz with sulfide mineralization occurs as open space fillings, minor replacement bodies and hydrothermal breccia. The veins consist of quartz, calcite, pyrite, chalcopyrite, galena, sphalerite, bornite and minor sulfosalts, particularly enargite. According to studies based on fluid inclusions in the Miduk porphyry, three types of fluids are responsible for mineralization. Homogenization temperatures and salinity in porphyry-type fluids vary from 566 to 162 °C and 61.3 to 1.2 wt% NaCl equiv. For the Latala vein type base and precious metals deposit, homogenization temperature and salinity vary from 380 to 131 °C and 10.6 to 0.17 wt% NaCl equiv. The gas phase in fluid inclusions of Latala is dominated by CO 2 but also shows the presence of CO and H 2 , characterizing reducing conditions associated with ore deposition. The change from lithostatic to hydrostatic regime, boiling and fluid dilution associated with the introduction of meteoric fluids provides an explanation for the widespread Th and salinity data. Calculated pressure for examples of Miduk fluid inclusions varies from 700 to 200 bars. These pressures correspond to depths of 2500 to 1500 metres for porphyry mineralization. The three-phase fluid inclusions, corresponding to magmatic fluid, show the highest pressure. The Latala base and precious metals deposit has formed at pressures between 200 and 100 bars, corresponding to a depth of less than 1 km. Sphalerite mineralization occurs in shallow parts of the sedimentary-volcanic sequence from magmatic fluids diluted by meteoric fluids and also occurs in more distal parts of the porphyry. The sulphur isotopic composition for sulfide minerals varies between-9.8 and-1.0‰, which correspond to values of magmatic sulfur. This suggests that magmatic water was responsible for transportation of metals in Latala. Epithermal mineral precipitation occurred upon dilution of the low-salinity magmatic fluid with meteoric water, which entered the hydrothermal system as it cooled and successively diluted during continued magmatic fluid ascent

From migmatites to granites in the Pan-African Damara orogenic belt, Namibia

International audienceThe Swakop River exposes a unique structural section into the root of the Pan-African Damara orogenic belt (DOB) in Namibia formed as a result of collision between the Congo and the Kalahari cratons from ca. 550 to 500 Ma. The Central Zone of the Damara orogenic belt is characterized by amphibolite to granulite facies metamorphism accompanied by intense partial melting. Three tectonic units are defined in the Central Zone based on the proportion and distribution of the granitic fraction, namely (I) a lower unit dominated by diatexites and comprising plutons of homogeneous granites, (2) a middle unit composed by metatexites with mainly a metasedimentary protolith, and (3) an upper unit corresponding to metamorphic rocks with intrusive leucogranitic sills and laccoliths. The increase in the granitic fraction with structural depth is suggesting an increase in the degree of partial melting and implies a relative inefficiency of magma mobility from the source to higher structural levels. The transition from metatexites of the middle unit to diatexites and granites of the lower unit is interpreted as reflecting the former transition from partially molten rocks to a crustal-scale magmatic layer. Mushroom-shaped granitic plutons in the lower unit are consistent with their emplacement as diapirs and the development of gravitational instabilities within the magmatic layer. In the middle unit, granitic veins concordant and discordant to the synmigmatitic foliation localized in structurally-controlled sites (foliation, boudin's necks, shear zones, fold hinges) indicate that, within the partially molten zone, deformation plays the dominant role in melt segregation and migration at the outcrop scale. Melt migration from the partially molten zone to the intrusive zone is related to the build-up of an interconnected network of dikes and sills with diffuse contacts with the migmatitic hosts in the middle unit. In contrast, the upper unit is characterized by homogeneous leucogranitic plutons in sharp intrusive contact with genetically unrelated host rocks suggest that part of the melt fraction has migrated upward from its source to an intrusive zone

Structure of the Paleoproterozoic Kedougou - Kenieba inlier (Senegal-Mali) deduced from gravity and aeromagnetic data

The Kedougou-Kenieba Inlier (KKI) corresponds to a window through Paleoproterozoic terranes of the West African Craton (WAC). This study presents, first of all, an interpretation of a regional Bouguer gravity map (14 degrees W-9 degrees W, 11N-16 degrees N) of the KKI and then introduces a new litho-structural map of the Malian part of the KID using aeromagnetic data (12 degrees 0'W-10 degrees 80'W, 12 degrees 0'N-14 degrees 50'N). The KKI is limited to the west by a negative gravity anomaly forming a 30-100 km wide corridor, oriented N-S to NNE-SSW, and correlated to the Variscan Mauritanides orogenic belt. West of this belt, the Mesozoic to Cenozoic sedimentary deposits are marked by the highest positive anomaly of the region, attributed either to the presence of mafic rocks at intermediate to deep crustal levels or to a shallower Moho depth. Within the KKI, moderately positive to negative anomalies are correlated with exposed plutonic and mafic to intermediate metavolcanic rocks. Gravity data also reveal (i) two north-west trending lineaments attributed to crustal-scale shear zones, (ii) three north-east trending lineaments marked by gravity anomalies sub-parallel to the known Bissau-Kidira-Kayes Fault Zone, and named Kayes, Kedougou-Kenieba, and South Shear Zones, respectively, (iii) and the extension to the south of the known Mauritanides Belt Thrust. The local aeromagnetic map of the Malian part of the KKI discriminates the Mako metavolcanic belt with granitic intrusions in the west characterized by heterogeneous anomalies, from the Kofi metasedimentary series delineated by magnetic lows in the east. ESE-WNW to ENE-WSW-trending alternation of magnetic anomalies in the metavolcanic and metasedimentary rocks are attributed to pervasive structures associated with ductile deformation. The Senegalo-Malian Shear Zone is marked at the regional scale by deflection of the geophysical lineaments but is not expressed by any offset of identified structures or lithological contacts