Granitic Veins Research Articles

Tectonic setting of Triassic magmatic and metallogenic event in the Duobaoshan mineralization area of Heilongjiang Province, NE China

The Duobaoshan mineralization area is located in the northeast part of the Xing'an–Mongolia orogenic belt which belongs to the eastern part of the Central Asian Orogenic Belt (CAOB). This region has experienced multiple tectonic–magmatic–metallogenic events that resulted in a number of large–superlarge mineral deposits, e.g. the early Palaeozoic Duobaoshan and Tongshan porphyry Cu–Mo deposits, the early Mesozoic Sankuanggou skarn Fe–Cu deposit and Xiaoduobaoshan skarn Cu deposit and the late Mesozoic Zhengguang epithermal Au deposit. By detailed field survey, we have examined the weak alteration and mineralization in the tonalite veins intruding early Palaeozoic ore-bearing granodiorite in the Duobaoshan deposit and the veinlet-disseminated Cu–Mo mineralization in the porphyritic granite veins intruding in the ore-hosting Ordovician strata of the Duobaoshan Formation in the Tongshan deposit. Zircon grains from the Duobaoshan tonalite, the Tongshan porphyritic granite, and the metallogenic granite porphyry and the granodiorite in the Xiaoduobaoshan deposit yield LA–ICP–MS U–Pb weighted mean ages of 230.9 ± 2.3 Ma, 235.4 ± 2.7 Ma, 226.6 ± 1.7 Ma and 232.8 ± 1.7 Ma, respectively. In addition, the Re–Os model age of molybdenites from the Tongshan porphyritic granite is 229.4 ± 3.5 Ma. Those isotope dating results indicate that the Duobaoshan mineralization area experienced the Triassic magmatic–mineralization event. Considering the regional tectonic history of the area and the fact that the major and trace element characteristics of the Triassic tonalite in the Duobaoshan deposit are similar to those of island–arc type igneous rocks, it is presented that the Triassic magmatic–metallogenic event might be closely related to the subduction of the Mongolia–Okhotsk plate, and that the metallogenic elements originated from the mantle wedge metasomatized by liquid of subduction oceanic slab. Previous research and new geochemical analyses presented in this study indicate that the Duobaoshan mineralization area experienced at least four epochs of magmatic–metallogenic events including the Ordovician, the Triassic, the Early Jurassic and the Late Jurassic to Early Cretaceous. Copyright © 2015 John Wiley & Sons, Ltd.

The affinity of Archean crust on the Yilgarn—Albany–Fraser Orogen boundary: Implications for gold mineralisation in the Tropicana Zone

Craton margins can be subject to a wide array of gold genesis and redistribution processes, although high-grade terrains on craton margins are frequently viewed as less prospective than lower-grade counterparts. In contrast to this, the high-grade Tropicana Zone, a newly defined Archean crustal component on the eastern margin of the Yilgarn Craton within the Albany–Fraser Orogen (AFO), contains a significant Proterozoic gold deposit. This deposit and zone comprise mid-amphibolite to granulite-facies gneissic rocks with evidence of partial melting and granite injection. The Tropicana Zone contains significant low-Si, LILE-enriched, granites classed as sanukitoids. Along with the distinctive compositions, the rarity of these rocks within any Archean craton suggests that the granitoid protoliths represent a single suite, emplaced during one event.Due to the intense granulite-facies overprinting of the Tropicana Zone rocks, determination of the magmatic protolith age for these sanukitoids is challenging. Nonetheless, the best age estimate for magmatism is 2692±16 Ma, based on the youngest zircons preserving textural evidence of growth within a viscous silicate melt. This age is older than compositionally similar magmatism found within the Yilgarn Craton, although a sanukitoid in the Northern Foreland of the AFO has a similar age. Furthermore, the granulite-facies metamorphic zircon growth in the Tropicana Zone at 2718–2554 Ma was prolonged compared to that in the Yilgarn Craton. Nonetheless, the Hf isotopic signature of the Tropicana Zone zircon shares strong similarity to that from the Eastern Goldfields Superterrane of the Yilgarn Craton. This implies that the Tropicana Zone reflects a deeper crustal level of the Yilgarn Craton, exhumed and thrust NW an unknown distance over the craton edge. In addition, we observe that the granulite-facies zircons have a less radiogenic Hf-isotope signature than the preserved pre-metamorphic zircon cores. Based on correlations with alpha dose, U and Th content and 176Hf/177Hf we suggest this reflects the preferential destruction and release of unradiogenic Hf from inherited zircon whereas the protolith sanukitoid zircon, with lower U and Th content, was more resistant to mobilisation during high-grade metamorphism. We note this situation may be a more general response of the Hf isotopic system, in which zircon grown in a more mafic melt is less likely to contribute to the metamorphic Hf reservoir than its felsic counterpart.Juvenile granitic veins dated at c. 1780 Ma intruded into the Tropicana Zone indicate that the Tropicana Zone was structurally emplaced at or before c. 1780 Ma, given similar Proterozoic magmatic events are well documented from (para)autochthonous adjacent units. Re–Os dating of pyrite coeval with one generation of gold in these rocks indicates model ages of c. 2100 Ma, supportive of a Palaeoproterozoic age of mineralisation. This mineralisation event is distinct from major Proterozoic tectonothermal events elsewhere in the AFO. Sanukitoid magmas are well-known for gold fertility and were likely the original source of gold in the Tropicana Zone, which was subsequently concentrated into brittle structures during several episodes. Gold mineralisation post-dated peak metamorphic conditions and is significantly younger than gold mineralisation within other parts of the adjoining Yilgarn Craton.

Geochemical Exploration for Gold and the Association of As-Cu-Pb-Sn-Zn-Li in the Upper Proterozoic Granitoids of the Wadi Rumman Area, Southwest Jordan

The geochemical studies were conducted in an area covering about 15 km2 of the south side of the Wadi Rumman area, SW Jordan. The study area included a part of a basement of complex exposed rocks in southwestern Jordan. The complexes comprised igneous and metamorphic suites, mostly from the late Proterozoic age. A systematic geochemical sampling was conducted using rock and heavy mineral-panned concentrate of geochemical samples. Fifty rock samples were collected from the granitic rocks, simple pegmatite dyke, quartz veins and alteration zone, which covered the area. Next, 45 heavy mineral-panned concentrate samples were collected from the alluvium in the stream sediment within the catchment area, and the dray was sieved to less than 1 mm grain size. The geochemical samples were analyzed for their trace elements and gold by using Ione Conductive Coupled Plasma Emission Spectroscopy (ICP-AES) and the Atomic Absorption Spectrometer (AAS) at Natural Resources Authority (NRA) Labs. The results of the geochemical analysis indicated the presence of gold and heavy minerals in the study area, a result considered abnormal in the rock samples. A strong positive correlation was recorded of Au with As, Cu and W (r = 0.82, 0.7 and 1.0), as with Mo (r = 0.83), Cu with Pb (r = 0.83), Sn with Mo (r = 0.73), Mo with W (r = 0.97), Zn with W (r = 0.71), and Li with Bi (r = 0.7). These correlations revealed gold associated within the hydrothermal alteration, quartz veins and pegmatite dyke. Abnormal metals such as As and Bi were good path-finders to find Au. The HM samples showed low positive linear correlations among the concentrations of Au with As, Zn and Li, and negative linear correlations of Au with Pb, Sn, Bi and W. The combination of both rock and heavy mineral concentrate samples shows four geochemical anomalous areas of gold and heavy minerals. The geochemical signatures of Au and As in the Wadi Rumman appear to be in the hydrothermal alteration, quartz veins and pegmatite dyke, respectively. Heavy mineral concentration sampling delineated the Au geochemical anomaly in area 1 specificities with the rock geochemical anomaly of area 1.

From Migmatites to Plutons: Power Law Relationships in the Evolution of Magmatic Bodies

Magma is generated by partial melting from micrometre-scale droplets at the source and may accumulate to form >100 km-scale plutons. Magma accumulation thus spans well over ten orders of magnitude in scale. Here we provide measurements of migmatitic leucosomes and granitic veins in drill cores from the Estonian Proterozoic basement and outcrops at Masku in SW Finland and Montemor-o-Novo, central Portugal. Despite the differences in size and number of measured leucosomes and magmatic veins, differences in host rock types and metamorphic grades, the cumulative width distribution of the studied magmatic leucosomes/veins follows a power law with exponents usually between 0.7 and 1.8. Published maps of the SE Australian Lachlan Fold Belt were used to investigate the distribution of granitoid pluton sizes. The granites occupy ca. 22 % of the 2.6 × 105 km2 area. The cumulative pluton area distributions show good power law distributions with exponents between 0.6 and 0.8 depending on pluton area group. Using the self-affine nature of pluton shapes, it is possible to estimate the total volume of magma that was expelled from the source in the 2.6 × 105 km2 map area, giving an estimated 0.8 km3 of magma per km2. It has been suggested in the literature that magma batches in the source merge to form ever-bigger batches in a self-organized way. This leads to a power law for the cumulative distribution of magma volumes, with an exponent m V between 1 for inefficient melt extraction, and 2/3 for maximum accumulation efficiency as most of the volume resides in the largest batches that can escape from the source. If m V ≥ 1, the mass of the magma is dominated by small batches; in case m = 2/3, about 50 % of all magma in the system is placed in a single largest batch. Our observations support the model that the crust develops a self-organized critical state during magma generation. In this state, magma batches accumulate in a non-continuous, step-wise manner to form ever-larger accumulations. There is no characteristic length or time scale in the partial melting process or its products. Smallest melt segregations and >km-scale plotuns form the end members of a continuous chain of mergers of magma batches.

First evidence for ca. 780 Ma intra-plate magmatism and its implications for Neoproterozoic rifting of the North Yili Block and tectonic origin of the continental blocks in SW of Central Asia

The North Yili Block is one of the major continental constituents of the Central Asian Orogenic Belt (CAOB), study of its basement evolution and tectonic origin is essential for the understanding of the accretionary history of the CAOB and the reconstruction of global supercontinent as well. We conducted field geological, geochronological and geochemical investigations on the mafic dikes intruding in the highly deformed Proterozoic carbonate in the North Yili Block, NW Chinese Tianshan. Zircon U–Pb dating results suggest that the gabbro and gabbroic diorite dikes and associated granitic vein formed in 776–778Ma. Element and isotope geochemical data reveal that these intrusive rocks derived from an OIB-like mantle source, and were variably influenced by fraction crystallization and important crustal contamination, thus, a continental rifting setting is proposed for these dikes. Our new data provide the first evidence for a Neoproterozoic continental intra-plate magmatism occurred in the North Yili Block, this event can be linked with the coeval continental rifting-related magmatism in northern Tarim and in other microcontinents within the SW CAOB (e.g., Central Kazakhstan, Kyrgyzstan Middle Tianshan). Based on the comparison on the Neoproterozoic to early Paleozoic stratigraphic sequences between the North Yili and Tarim blocks, it is suggested that the North Yili Block most likely originated from the Tarim Block, and is potentially one additional constituent member of the Rodinia supercontinent.

Magma oxygen fugacities of granitoids in the Xiaoqinling area, central China: implications for regional tectonic setting

The trace element compositions of zircons from four granitoids/metagranitoids in the Xiaoqinling area in the TransNorth China Orogen of the North China Craton were determined by in-situ laser ablation (LA) ICP-MS. The results were used to compute the zircon Ce anomaly, Ti-in-zircon temperature, and the magma oxygen fugacity. Generally, the Ce anomalies determined with La-Pr interpolation are prone to be underestimated due to the influence of tiny inclusions or analytical errors. This underestimation can markedly influence the data processing and generate unrealistically low oxygen fugacity values. In contrast, the lattice strain model yields more reliable results, and offers a robust tool in determining the magma oxidation state. The calculated magma oxygen fugacities are in the range of FMQ-1.41 to +3.21, FMQ-3.25 to +3.74, FMQ +3.66 to +5.27, and FMQ +2.51 to +4.67, respectively. The high average oxygen fugacities of FMQ +4.51 and FMQ +3.46 for a 1881 Ma old pegmatitic granite and 1866 Ma old granitic veins suggest these rocks were probably formed in arc setting, which further supports the ~1.8 Ga collision between the Western and Eastern Blocks of the North China Craton.

New geochemical and combined zircon U–Pb and Lu–Hf isotopic data of orthogneisses in the northern Altyn Tagh, northern margin of the Tibetan plateau: Implication for Archean evolution of the Dunhuang Block and crust formation in NW China

Abstract Dunhuang Block is previously considered to be an eastern part of the Tarim Craton, but now it is proposed to be the western extension of the Alxa Block of the NCC as a result of displacement along the Altyn Tagh fault. The oldest basement rocks of the Dunhuang Block, named Aketashitage Complex, were mainly exposed in the northern Altyn Tagh. Migmatites of the Aketashitage Complex are sodic and subalkaline in composition. Zircon U–Pb dating of the tonalitic and monzogranitic migmatites indicates that these rocks were produced by strong deformation of earlier formed tonalites with innumerous granitic veins. The tonalitic melanosome of the migmatites was generated approximately at 2.7–2.8 Ga, whereas the granitic leucosome was likely produced by partial melting of the earlier tonalitic melanosome at 2.51–2.53 Ga. Zircon SHRIMP U–Pb dating of metadiorites exposed in the Aketashitage Complex yielded a crystallization age of 2498 ± 10 Ma. The metadiorites are characterized by high Sr and Sr/Y and low Y, but their low SiO2 and high Cr, Co, and Ni indicate similar geochemical characters of modern low-Si adakites. Their high MgO, Cr, Co, Ni, and Mg# and high eHf(t) suggest that the metadiorites were probably produced by partial melting of slab-melt metasomatized mantle peridotites. In combination with previously published age data, it is concluded that the orthogneisses in the Aketashitage Complex were generated during three periods, namely at 2.83, 2.71–2.77 and 2.50–2.57 Ga. Except for the metamorphic rims, zircons from the Aketashitage tonalitic and monzogranitic orthogneisses have TDMc model ages ranging from 3.1 Ga to 3.5 Ga and suggest that their formation involved partly the recycling of Paleoarchean to Mesoarchean crust. The younger TDMc model ages of 2.7–2.9 for the metadioritic rocks suggest another period of formation of juvenile crust in the Neoarchean. The similarity of crustal formation and growth between the Dunhuang Block and the northern Tarim Craton therefore suggest that the Dunhuang Block had become a part of the Tarim Craton in the late Neoarchean. The lack of Eoarchean crust and different patterns of Hf model ages suggest that the Dunhuang Block is not a western extension of the Alxa Block of the NCC.

Geochemistry of the Bayonplutonic Complex – Western Cameroon

The BayonNeoproterozoic plutonic complex located in Western Cameroon intrudes gneisses of Paleo to Neo Proterozoic age. The complex is composed of gabbro, monzogabbro and monzonites frequently crosscut by trachytic and granitic veins. The primary mineral assemblages of the gabbro and monzogabbro is plagioclase (An30 - An69), clinopyroxene (En40 – 42 Fs 12 – 18 Wo 45 - 47) hypersthene (En62 – 65 Fs34 – 37 Wo1- 4) and orthoclase (Or78-Or91) whilebiotite, magnetite, Ilmenite and apatite constitute accessory minerals. Monzonite is formed of plagioclase (An22 to An39), orthoclase (Or80-Or87),clinopyroxene (En38-39 Fs14-20 Wo45-46), biotite and quartz. Amphiboles occur as secondary minerals. Ilmenite and apatite are accessory minerals in monzonite.The rocks are mafic to intermediate in composition (41 – 61 wt % SiO2) and transalkaline with high-K and have shoshonitic features. Bayon plutonic rocks have highabundance of Ba, Sr ,V and Zr but possesses low concentrations of Rb, Sc, Y and Th.Gabbroic rocks show moderately fractioned REE patterns (LaN/LuN= 14 – 27) with none or negligible Eu anomalies. The monzonite shows also moderate fractionatedpatterns (LaN/LuN= 20 – 27) with fairly positive Eu anomalies. All the studied rocks show flat HREE features. The primitive mantle- normalized element patterns are almost homogeneous with negative anomalie in Ta, Nb, Th, P and Ti. Sm/Nd-wr-Cpx-Pl ages of the complex are 580 ± 13 Ma; 553 ± 32 Ma for the monzogabbro and 547 ± 26 Ma for the monzonite. The Nd/Sr isotopic compositions show that the Bayon plutonic rocks were generated by partial melting of subcontinental lithospheric mantle. The depleted mantle Nd model age TDM of 1.6 – 1.7 Ga indicates that the studied rocks originated by partial melting of Mesoproterozoic mantle.KEYWORDS: West Cameroon, Panafrican, Pluton, Geochemistry, partial meting

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

The 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 500Ma. 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 (1) 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.

Geochemical Characteristics and Genesis of the Metallogenic Rocks in the Chagande'ersi Molybdenum Deposit in Wulatehouqi, Inner Mongolia

AbstractGeochemical characteristics of the Chagande'ersi molybdenum deposit in Inner Mongolia and its genesis were analyzed in this study using rock mineralography and rock geochemical testing. The mineralized country rocks of the Chagande'ersi molybdenum deposit consist mainly of medium‐to fine‐grained monzogranite, medium‐to fine‐grained rich‐K granite, with minor fine‐grained K‐feldspar granite veins and quartz veins. The rocks are characterized by high silica, rich alkali, high potassium, which are favorable factors for molybdenum mineralization. The rocks have the Rittmann index ranging from 1.329 to 1.961, an average Na2O+K2O value of 7.41, and Al2O3/(CaO+Na2O+K2O) >1, suggesting that the rocks belong to the high‐K calc‐alkaline peraluminous granite. The typical rock samples are enriched in Rb, Th, K and light rare earth elements, depleted in Sr, Ba, Nb, P and Ti, and these features are similar to that of the melt granite resulting from collision of plate margins. The δEu of the rocks falls the zone between the crust granite and crust‐mantle granite, and are close to that of the crust granite; (La/Lu)N indicates the formation environment of granite is a continental margin setting. The Nb/Ta ratios are close to that of the average crust (10); the Zr/Hf ratios of monzogranite are partly below the mean mantle (34–60), while the Zr/Hf ratio of K‐feldspar granite are close to the mean value in the crust Comprehensive analyses show that the granite in this area formed during the transition period between tectonic collision and post‐collision. During the plate collision and orogeny, the crust and mantle material were mixed physically, remelting into lava and then crystal fractionation, finally gave rise to the formation of the rock body in this area. This has close spatial and temporal relation with the molybdenum mineralization.

Zircon U–Pb dating and Hf isotope analysis on the Taihua Complex: Constraints on the formation and evolution of the Trans-North China Orogen

Precise age determination and source tracing of the granitic and pegmatitic gneisses of the Taihua Complex in the Xiaoqinling area are important to address the debates on the timing and processes of the amalgamation of the Western and Eastern Blocks along the Trans-North China Orogen. This paper conducted zircon dating of metamorphic rocks from the Xiaoqinling area. The results demonstrate that the collision between the Western and Eastern Blocks occurred in the Paleoproterozoic. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) zircon uranium–lead (U–Pb) analyses yielded consistent results. The Early Paleoproterozoic (the first period) age of 2346±28Ma was determined for granitic gneiss in the Taihua Complex and 2328±14Ma for the Xiaohe granite pluton that intruded into the granitic gneiss. The gneissose pegmatitic granite and granitic veins that intruded into the granitic gneiss yielded LA-ICP-MS zircon ages of 1881±24 and 1866±19Ma, respectively. These Paleoproterozoic ages strongly indicate that the ocean between the Eastern and Western Blocks were completely subducted at ∼1.85Ga. The ɛHf(t) values of the zircons from the granitic gneiss can be divided into a positive group (0.44 to 3.77) and a negative group (−3.11 to −1.27). The two-stage hafnium (Hf) model ages (TDM2(Hf)) corresponding to the positive values are between 2672 and 2887Ma. The ɛHf(t) values of the zircons from the Xiaohe granite range from −2.49 to 2.58, with the Hf isotope model ages ranging from 2742 to 3022Ma. Thus, the Xiaohe granite could have resulted from the partial melting of the Late Archean lower crust caused by subduction and the presence of juvenile mantle material. The primary magma of the granitic gneiss of the Paleoproterozoic Taihua Complex was derived from the differentiation of the depleted Late Archean mantle source with a small portion derived from the remelting of the subducted lower crust.

Molybdenite as a Rhenium Carrier: First Results of a Spectroscopic Approach Using Synchrotron Radiation

The chemical and physical properties of rhenium render it a highly demanded metal for advanced applications in important industrial fields. This very scarce element occurs mainly in ores of porphyry copper-molybdenum deposits associated with the mineral molybdenite, MoS2, but it has also been found in granite pegmatites and quartz veins as well as in volcanic gases. Molybdenite is a typical polytype mineral which crystal structure is based on the stacking of [S-Mo-S] with molybdenum in prismatic coordination by sulphide anions; however, it is not yet clearly established if rhenium ions replace Mo4+ cations in a disordered way or else, if such replacement gives rise to dispersed nanodomains of a rhenium-rich phase. As a contribution to clarify this question, an X-ray absorption spectroscopy (XANES) study using synchrotron radiation was performed at the Re L3-edge of rhenium-containing molybdenite samples. Obtained results are described and discussed supporting the generally accepted structural perspective that rhenium is mainly carried by molybdenite through the isomorphous replacement of Mo, rather than by the formation of dispersed Re-specific nanophase(s).

Comparison of the metamorphic history of the Monapo Complex, northern Mozambique and Balchenfjella and Austhameren areas, Sør Rondane, Antarctica: Implications for the Kuunga Orogeny and the amalgamation of N and S. Gondwana

Reconstructions of Gondwana place Dronning Maud Land, Antarctica (DML) adjacent to N. Mozambique prior to fragmentation. The Monapo Complex outlier klippen overlying the Nampula Terrane in N. Mozambique has been correlated with rocks in eastern and central DML. Metamorphic assemblages and P–T conditions from the two areas are compared as well the timing of metamorphism. Granulite grade assemblages preserved in the Monapo Complex suggest P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of isothermal decompression, isobaric cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where near isobaric cooling and hydration at between ∼4–7kb and 550–700°C is recognized. Granulite grade assemblages from Balchenfjella in eastern Sør Rondane, DML suggest initial P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of decompression, cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where retrogression at between ∼6–7kb and 600–700°C is recognized.SHRIMP zircon data from six samples from eastern Sør Rondane show episodic zircon growth over a period from ∼630Ma to ∼530Ma with five metamorphic pulses being defined. Undeformed granitic intrusions with ages of ∼550Ma intrude Sør Rondane. SHRIMP zircon data from samples from alkaline intrusions and mafic granulites in the Monapo Complex yield crystallization and metamorphic ages of ∼635Ma and 570-590Ma respectively. The complex is intruded by granitic veins with ages of ∼550Ma.The similarities in metamorphic mineral assemblages, P–T estimates and zircon geochronology data indicate a common geological history between Sør Rondane and the Monapo Complex of northern Mozambique. This common history is interpreted to result from both areas being part of a mega-nappe structure which was emplaced in a transpressional setting involving collision between N. Gondwana (comprising rocks of the East African Orogeny) and south Gondwana (comprising the Nampula Terrane of northern Mozambique and the Maud Province of western Dronning Maud Land) along the Damara-Zambesi-Lurio-CDML collision boundary.Other erosional remnants of this nappe include the Mugeba Complex in northern Mozambique, the Kataragama klippe in Sri Lanka, the Urungwe klippe in Zimbabwe, the Masosa Suite and Mavhuradohna Complex in western Mozambique and NE Zimbabwe.

The stabilization of self-organised leucogranite networks—Implications for melt segregation and far-field melt transfer in the continental crust

Net-structured leucocratic vein systems are ubiquitous in high-grade and migmatite terrains and are interpreted to represent interconnected former magma-bearing structures that accommodated the migration of granitic magmas out of their anatectic source regions. Net-structured leucogranite vein systems in high-grade gneisses from the Damara Belt in Namibia describe a size-based hierarchy and spatial distribution that indicates self-organisation of the magma transfer system. The internal geometry and connectivity of the net-structured leucogranites are controlled by variations in melt pressure, whereas regional strains control the orientation of the leucogranites. Foliation-parallel (stromatic) and shear-band-hosted granite veins accommodate transport at relatively low melt volumes and account for the metre-scale segregation of melt close to the anatectic source. During times of higher melt supply, the formation of extensional fractures that cross-cut stromatic and shear-band-hosted melt sheets at high angles is promoted. Extensional fracturing and associated dilatancy create gradients in magmastatic head that cause melt to be drained from pervasive stromatic and shear-band-hosted vein networks into spaced dilatant fractures. This has the potential to initiate buoyancy-driven far-field melt transfer along suitably orientated extensional fractures as vertical dykes or self-propagating hydrofractures. Our results suggest that melt transfer systems are able to adjust to variations in melt supply by forming conduits that accommodate varying melt volumes and will tend to develop towards a state of self-organisation.

Degana (Rajasthan, India) and Tigrinoe (Primorye, Russia) tungsten and tin-tungsten deposits: Composition of mineral-forming fluids and conditions of wolframite deposition

Formation conditions of orebodies and conditions of wolframite deposition at the Degana tungsten deposits in Rajasthan, India and the Tigrinoe tin-tungsten deposit in the Russian Far East were studied. Differences in the composition and state of fluid systems were established by microthermometric study of fluid inclusions (FI) and thorough petrographic examination of FI. At the Degana deposit, the ore veins in granite were formed from K-Na-Ca-(Mg, Fe, etc.) chloride solutions with a salinity up to 36 wt % NaCl equiv at a temperature of >420 to 120°C and under a pressure reaching 1550 bar. The formation temperature of the orebodies hosted in breccia reached 450°C and pressure was below 400 bar. The salinity of mainly Nachloride aqueous solutions was no higher than 18 wt % NaCl equiv. At the Tigrinoe deposit, the temperature during formation of quartz-wolframite-cassiterite veins varied from 420 to 240°C and the pressure was no higher than 300 bar. The salt concentration of Na-chloride solutions was 7-3 wt %. Wolframite crystallized at the very beginning of ore deposition. Probable sources of fluids are discussed. It is suggested that the factors controlling wolframite deposition could have been different even at the same deposit.

The high-grade Tseel Terrane in SW Mongolia: An Early Paleozoic arc system or a Precambrian sliver?

The high-grade metamorphic Tseel Terrane in southwestern Mongolia is dominated by a strongly deformed sequence of migmatitic gneisses which has previously been interpreted as an early Precambrian continental sliver. However, the protolith age and timing of high-grade metamorphism remain poorly constrained. To resolve this problem, high-grade gneisses selected from the western Tseel Terrane were dated in the present study. Zircons with oscillatory zoning from three paragneisses predominantly yielded early Paleozoic ages between 450 and 550Ma, with prominent peaks at 471 and 506Ma. U–Pb dating of zircons from a granitic gneiss revealed that the protolith was emplaced at 420Ma. Zircons from a leucocratic granite vein gave a concordant age of 385±3Ma, close to ages of metamorphic zircons in the gneissic rocks, which is interpreted to record a high-temperature thermal event in the region. These results indicate that the Tseel Terrane principally consists of early Paleozoic rocks which underwent a high-grade metamorphic event in the Middle Devonian. In addition, SHRIMP dating of dominant magmatic zircons from an associated schist yielded a concordant U–Pb age of 495±5Ma, similar to those of the major zircon group in the high-grade paragneiss. A felsic tuffaceous sample yielded a U–Pb zircon age of 511±4Ma, documenting early Paleozoic volcanism which probably contributed to the deposition of the sediments in the region. Geochemically, high-grade paragneisses and low-grade schists show close similarities, and their common features suggest that their protoliths may represent significant erosion products of Cambrian arc magmatic rocks with minor old crustal components that were deposited in an active margin setting. The granitic gneiss with subduction-related chemical characteristics further favors an arc setting for the Tseel Terrane. Accordingly, our data combined with previous work suggest that the Tseel Terrane represents the constituent part of an early Paleozoic arc system situated on the southern margin of North Mongolian Domain. This arc system extended from western Mongolia to the Chinese Altai because various geological features of these two regions have close similarities.

Principle stages of granite and pegmatite formation in the southeastern part of the Main Sayan Fault

We performed U-Pb dating by the SHRIMP II method of zircon from granites and pegmatites in the southeastern part of the Main Sayan Fault (northern part of Khamar-Daban and southeastern part of the Belya-Kitoi Terrains in the Central Asian fold belt). According to the results obtained, along with the Late Ordovician granitoid magmatism, the Khamar-Daban and Belya-Kitoi Terrains were characterized by a less massive Carboniferous period at their boundary with the Siberian Craton accompanied by granite and pegmatite formation and reflecting tectonomagmatic activation of the Main Sayan Fault. Three stages of magmatic activation were noted. The earliest stage (about 350 million years ago) implies the formation of pegmatoid quartz-albite-microcline (amazonite) granite veins with Nb-Y mineralization in the fault zone. Oligoclase-microcline pegmatite veins with graphite were crystallized in the fault zone at the second stage (about 330 million years ago). The final stage of endogenetic activity (about 310 million years ago) was characterized by migmatization of the Khangarul Group of the Khamar-Daban Terrain and formation of potassium granites (Fig. 3; Table 1).

530 Ma syntectonic syenites and granites in NW Namibia — Their relation with collision along the junction of the Damara and Kaoko belts

The Lower Ugab and Goantagab structural domains are located at the junction between the N–S trending Kaoko and the E–W trending Damara belts (NW Namibia), where Neoproterozoic metavolcano-sedimentary sequences were intruded by several syenitic/granitic plutons. We present here new U–Pb ages on zircon grains from the Voetspoor and Doros plutons. Together with petrological, geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of Gondwana amalgamation.The plutons are composed of three main rock types: hornblende quartz-syenite, syenodiorite and biotite granite. The two former are predominant and show genetic correlation such as magma mingling structures and similar geochemical signatures. The biotite granite occurs in the SW parts of the intrusions and clearly cuts the syenitic rocks. Although the plutons are mainly isotropic, the structures around them demonstrate that their intrusion occurred during a second deformation phase (D2) with a component of sinistral solid state rotation with respect to the wall rocks in response to D2 transpression. Four samples were dated using U–Pb SHRIMP methodology in single zircon grains. A hornblende monzodiorite from the Voetspoor pluton yielded an age of 534±4.5Ma. A hornblende monzonite from the Doros pluton produced an age of 528±5Ma. The biotite granite facies was sampled in the Doros intrusion and yielded an age of 530±4.5Ma. In addition, a granitic vein folded by D2 close to the northeastern contact of the Doros pluton with the encasing phyllites (Amis River Formation) was also dated, yielding an age of 533±6Ma. The data show that all granite–syenite from Doros and Voetspoor intrusions are contemporaneous and crystallized in the period between 539 and 522Ma within the errors. D1–D2 deformational phases took place under greenschist facies (biotite zone) conditions and during D3 the metamorphic grade was slightly lower. We interpret that the plutons are coeval to peak metamorphism of the region (530–520Ma) and that D2 and D3 sinistral transpressional phases are due to collision in the Damara Belt. The E–W compressional event and second metamorphic episode in the Kaoko Belt occurred between 580 and 560Ma and are apparently unrelated to the thermo-tectonic evolution described here, although D1 might be partially related to this event. The sinistral transpressional D2 phase resulted probably from the position of the area considered at the junction between the belts, and not in the frontal Damara collision further to the east. This new interpretation is consistent with the Ar–Ar ages for the region (about 500Ma), interpreted to reflect cooling of the orogen. The enrichment in LREE, K, Rb, Ba and Sr, and depletion in Nb of these basic to intermediate alkalic rocks could indicate that they partially derived from melting of a subcontinental lithospheric mantle that was affected by subduction and the granitic rock types represent lower crust contamination. We interpret that they could be related to heating in the mantle caused by asthenosphere influx in a zone of slab-breakoff during collision between Kalahari and Congo cratons.

U-Pb SHRIMP-RG zircon ages and Nd signature of lower Paleozoic rifting-related magmatism in the Variscan basement of the Eastern Pyrenees

The ages of orthogneisses exposed in massifs of the Variscan chain can determine whether they are part of a pre-Neoproterozoic basement, a Neoproterozoic, Panafrican arc, or are, in fact, lower Paleozoic, and their isotopic compositions can be used to probe the nature of their source rocks, adding to the understanding of the types, distribution, and tectonic evolution of peri-Gondwanan crystalline basement. .Using SHRIMP U-Pb zircon geochronology and Nd isotopic analysis, pre-Variscan metaigneous rocks from the Núria massif in the Eastern Pyrenean axial zone and the Guilleries massif, 70km to the south, have been dated and their Nd signatures characterized.. All dated orthogneisses from the Núria massif have the same age within error, ~457Ma, including the Ribes granophyre, interpreted as a subvolcanic unit within Caradocian sediments contemporaneous with granitic magmas intruded into Cambro-Ordovician sediments at deeper levels. Orthogneisses in the Guilleries massif record essentially continuous magmatic activity during the Ordovician, beginning at the Cambro-Ordovician boundary (488±3Ma) and reaching a peak in the volume of magma in the early Late Ordovician (~460Ma). Metavolcanic rocks in the Guilleries massif were extruded at 452±4Ma and appear to have their intrusive equivalent in thin, deformed veins of granitic gneiss (451±7Ma) within metasedimentary rocks. In orthogneisses from both massifs, the cores of some zircons yield Neoproterozoic ages between ~520 and 900Ma. The age of deposition of a pre-Late Ordovician metapelite in the Guilleries massif is bracketed by the weighted average age of the youngest detrital zircon population, 582±11Ma, and the age of cross-cutting granitic veins, 451±7Ma. Older detrital zircons populations in this metapelite include Neoproterozoic (749–610Ma; n=10), Neo- to Mesoproterozoic (1.04–0.86Ga; n=7), Paleoproterozoic (2.02–1.59Ga; n=5), and Neoarchean (2.74–2.58Ga; n=3). Nd isotopic analyses of the Núria and Guilleries orthogneisses yielded negative εNd values ranging between −2.1 and −5.2 at 450Ma, the same as Ediacaran sediments from northwestern Iberia. We interpret these slightly negative εNd values as a signature of Panafrican arc rocks, mixed with an older crustal component as indicated by the inherited and detrital zircon ages we analyzed. The crustal level in which Ordovician melting occurred has not been recognized and the absence of pre-Neoproterozoic basement is a striking feature of the southern part of the Variscan Chain.

Geochemistry of zircons from the proterozoic weathering profile of granitoids in Northern Karelia

For many years zircon has been used in geology as the main mineral-geochronometer, since it is consid� ered to be maximally stable in secondary processes. However, the methodological base of U-Pbzir� conometry (6) postulates the possibility of transforma� tion of the isotopic system in zircon during hypergen� esis. Modern experimental investigations provide evi� dence for significant evolution of the morphology, structure, and composition of zircons during their interaction with aqueous solutions and magmatic flu� ids (3-5). At the same time, ideas about the behavior of accessory zircons during continental weathering are contradictory and not systematized. In this paper we present the first results obtained for zircons from the Paleoproterozoic weathering profile in Northern Karelia. The area of investigations is located in the eastern flange of the PanaKuolayarvinskaya structure, near Kiskinlampi Lake, where Archean granite gneiss sep� arated by the crust of weathering is overlain by sedi� mentary and volcanogenic-sedimentary Jatulian for� mations (2). The basement is composed of medium� granular pink granite gneiss intruded by veins of red microcline granite with a thickness of up to 3 m. The weathering crust developed after granitoids of the basement has a thickness of ~10 m. Its structure includes a zone of physical disintegration of the sub� strate, a zone of eluvial breccia with terrigenous-car� bonate cement, which composes the main part of the hypergene profile, and a clay zone (1).