Granite Emplacement Research Articles

Timing and origin of skarn-, greisen-, and vein-hosted tin mineralization at Geyer, Erzgebirge (Germany)

AbstractThis contribution presents new insights into the origin and age relationships of the Geyer tin deposit in the Erzgebirge, Germany. Tin mineralization occurs in skarns, greisen, and in cassiterite-bearing fluorite-quartz veins. Skarn alteration replaces marble layers of the Cambrian Jáchymov Group and occurs in two clearly distinct stages. The first skarn stage forms skarnoid textured assemblages of clinopyroxene, garnet, and wollastonite with no tin phases recognized. Garnet U-Pb ages of this skarn stage (~322 Ma) relate the earlier skarn stage to the emplacement of the Ehrenfriedersdorf granite (~324 to 317 Ma). The second stage of skarn alteration is marked by the occurrence of malayaite and cassiterite associated with garnet recording ages of 307 to 301 Ma. Greisen- and skarn-hosted cassiterite-bearing veins provide U-Pb ages in the range of 308 to 305 Ma, relating greisenization and vein formation to the same magmatic-hydrothermal event as the second skarn stage. This suggests that tin mineralization at Geyer is related to a distinctly younger magmatic-hydrothermal event, clearly postdating the Ehrenfriedersdorf granite, which was previously assumed as the source of the tin-rich fluids. Fluid inclusions show salinities in the range of 1.0 to 31.5 % eq. w(NaCl±CaCl2) and homogenization temperatures between 255 and 340 °C. Cassiterite-associated fluid inclusions show indications for heterogeneous entrapment and dilution of hydrothermal with meteoric fluids. Dilution of high-salinity fluids with low-salinity fluids and cooling of the system was probably a decisive process in the precipitation of cassiterite in the Geyer Sn system.

Cryptic crustal growth identified through Variscan post-collisional lamprophyre-granite composite dykes, French Massif Central

The processes that control crustal growth by addition of new mantle-derived material to the crust and the recycling of crustal components that were introduced into the mantle during subduction remain poorly understood in collisional orogens. This is largely due to the fact that the sub-orogenic mantle has, in many instances, been fertilized by the introduction of crustal components during previous subduction events, and lacks the typical depleted mantle signature. Here, we present a study of lamprophyric–granitic composite dykes from the northern margin of the post-collisional Variscan Aigoual granite, French Massif Central (FMC). At Aigoual, lamprophyres and granites have gradational contacts and similar geochemical characteristics which indicate that the two were co-magmatic. UPb dating of zircon yields crystallization ages between 311 ± 3 Ma and 313 ± 3 Ma for dykes, identical within uncertainty with the ages of 312 ± 3 Ma and 313 ± 3 Ma obtained for the emplacement of the Aigoual granite. The lamprophyres are metaluminous to slightly peraluminous and display enrichment in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K, LILE, LREE) and have crustal isotopic signatures in both radiogenic (Sr, Nd and Hf) and stable (O) isotope systems. This dual crust/mantle signature of the lamprophyres can be interpreted to reflect partial melting, within the spinel stability field, of an enriched mantle that was metasomatised by the addition of subducted sediments. The granites are peraluminous, high-K and have shoshonitic affinities. Their primitive mantle-normalized trace-element patterns are similar to the lamprophyres and the isotopic compositions of the two rock types overlap. The high Cr, Ni, Fe, and Mg contents of the granites are consistent with a mantle component in the magmas. Our results suggest that between 65 and 80 wt% of material in the lamprophyres and granites was derived from the depleted mantle, and the rest from recycled crustal material. Although blurred from an (conventional) isotopic point of view, this magmatism contributed to the crustal growth via the post-collisional magmatism of the FMC. Coupled with their high preservation potential, post-collisional sites might thus represent significant sites of crustal growth.

Geochronology and detrital zircons sources from the Sierra Albarrana Domain (SW Iberian Massif)

This study compiles the U-Pb and 40Ar/39Ar geochronology of the metasedimentary, migmatitic and granitic rocks of the Sierra Albarrana Domain, based on data recently published by the authors together with other bibliographic sources. Maximum depositional ages (MDA) and magmatic and metamorphic events ages are discussed. MDAs suggest that this domain was deposited during the middle Cambrian (ca. 511) and U-Pb ages of zoned magmatic and metamorphic zircon grains indicate that the main tectono-magmatic event is late Cambrian (granite emplacement; 481 ± 2 Ma) to early Ordovician (migmatization; 478 ± 2 Ma). The 40Ar-39Ar ages evidence a Cambro-Ordovician metamorphic event (ca. 482 Ma) and another related to the Variscan Orogeny (ca. 337-392 Ma). The most important detrital source corresponds to a Cambrian magmatic event (535-515 Ma), probably developed during the initial episodes of the early Paleozoic rifting affecting the N margin of Gondwana. The remaining Mesoproterozoic, Paleoproterozoic and Archean zircon grains would have been contributed by Paleoproterozoic basement and/or older continental crust recycled in the western sections of the Gondwana margin. We suggest that this domain is probably part of the autochthonous section of the Central Iberian Zone (CIZ) and derived from the Saharan Metacraton and/or Tuareg Shield.

Implication of deformation fabrics of schist-migmatite-gneiss and granite in understanding regional tectonics: Eastern Dharwar Craton (EDC), India

Structural fabrics of schist belt, gneiss, migmatites and younger granitoids are different. Various morphology of migmatites is recognized for the first time in the southern part of EDC along with gneiss. The younger granite plutons are examined from northern through central to southern part of EDC. The paleosome-dominated portions of migmatite were possibly formed from low degree partial melting of older rocks and identified as metatexites. Whereas, the neosome dominated parts were developed by complete melting and marked as diatexites. There is a link between diatexite and granite in the southern part, which is chracterized as catazone segment. The four sub-types of metatexites are observed (viz, patch, dilatant, net and stromatic). The diatexites are dominated by two major structures (viz, schollen or raft and schlieren). Gneisses show lit-per-lit as well as fold and leucocratic material flow features. The litho-structural variation from north to south connotes systematic changes from epizone through mesozone to catazone. The qualitative study of mineral content and textural aspects from rocks at different localities suggest preserved portions of different parts of a pressure–temperature path. However, exact quantification should be possible after thermo-barometry and pseudo section study. The present contribution focuses on documentation of classic representative outcrops and petrographic features of gneissic and migmatitic rocks of EDC. It is observed that the six (6) major ductile deformation stages of Kenoran orogeny implies ductile signature before granite plutonism. Subsequently post granite emplacement deformation phenomenon indicates a transition from ductile to brittle regime and entrance into Hudsonian orogeny.

The coupled effect of bentonite colloid and radioactive ingrowth on the transport of actinide decay chains in a single fracture

An analytical model is used to illustrate the colloid-facilitated transport of decay chain radionuclides (RNs) in the granitic fractured porous media. The colloid source is assumed from the erosion of the bentonite used as buffer materials in a spent nuclear fuel repository. A conservative assumption that all colloids are mobile and have reached a sufficiently high concentration before RNs start to release is made in the present model. The latest reported distribution coefficient (Kd) values for both local granite and bentonite are used for several calculations. Investigating the simulation results shows that the second peaks of the breakthrough curves and profiles for daughter RNs are found using various initial concentration boundary conditions. The second peak is caused by the radioactive decay of the parent RN. Therefore, the contribution of the parents of a decay chain should not be ignored when performing a safety assessment. Moreover, the existence of colloids promotes a higher influence of the parents on the transport of their daughter RNs. A conservative colloid velocity that could reflect the effect of hydrodynamic chromatography phenomena is also investigated. Simulation results show that the Kd values of RNs on rock and bentonite colloids are the dominant factor that affects the colloid-facilitated-decay chain transport in the fractured rock system.

Timescales of ultra-high temperature metamorphism and crustal differentiation: Zircon petrochronology from granulite xenoliths of the Variscan French Massif Central

Lower crustal ultra-high temperature (UHT) metamorphism and the resulting production and transfer of granitic magmas represent key processes of intracrustal differentiation. The timescales of these phenomena are debated, due to the complexity of the granulite zircon U-Pb record and because direct links between granulites and granites are difficult to establish. To address these issues, we present the results of zircon petrochronology (coupled U-Pb dating, trace element and Lu-Hf isotopic analyses) for lower crustal felsic and mafic granulite xenoliths from the Variscan eastern French Massif Central and compare them with data from well-characterized mid-/upper crustal migmatites and granites. The felsic and mafic granulites represent pre-Variscan meta-sedimentary and meta-igneous mafic rocks, respectively; which experienced Variscan UHT peak metamorphism at 940–970°C and 8 ± 2 kbar. Zircons from the felsic granulites show U-Pb dates spreading over ∼50 Myr between ∼315 and ∼265 Ma, correlated to Ti-in-zircon temperatures decreasing from 940–970°C to 800°C and REE contents consistent with growth in equilibrium with garnet at (U)HT conditions. This is best explained by continuous crystallization of zircon upon cooling from the thermal peak owing to decreasing Zr solubility of residual melt, despite significant prograde melt loss. Zircons from the mafic granulite only record the last stage of this time-temperature evolution (295–265 Ma; 800–900°C) due to later zircon saturation. Upper crustal granite emplacement started at 340 Ma and culminated at the age of the lower crustal thermal peak of 313 ± 3 Ma (defined by the highest-temperature zircons from felsic granulites), reflecting melt extraction along the ∼27 Myr prograde path of the lower crust. In turn, the crystallization ages of mid-crustal migmatites (315–300 Ma) and the lower crustal granulites (315–265 Ma) are consistent with slow cooling in the presence of melt. These results provide a direct assessment of the timescales of melt production and residence at the crustal scale; and validate the granulite-granite connection in the framework of the melt loss theory in migmatitic systems.

Structural relationships of the younger granites of Tirmini (Damagaram south-east, Niger)

The Tirmini anorogenic ring complex is located approximately 25 km west of the town of Zinder, in the pan-African province of Damagaram-Mounio, the southeastern terminus of the Benin-Nigeria Shield. The aim of this study is to characterise the anorogenic deformation affecting the Tirmini younger granites. The petrography of Tirmini evolves from rhyolites to microgranites, granites and quartz alkaline syenite emplaced during anorogenic magmatism in the Permian (295 Ma). Very little structural data is available on the Tirmini complex. Only a cursory study of deformation has been carried out, without characterisation of the stages of anorogenic deformation or structural interpretation. The aim of this study is to characterise the structural evolution of the Tirmini younger granites. The methodology used is based on the exploitation of satellite imagery and mapping, supported by an analysis of deformation carried out in the field using Canvas software. The geological structures revealed include 360° schistosities, fracture schistosities, detachments (dexter and sinistral) anµ diaclases. The rheology of these structures has revealed two chronologically marked stages of anorogenic deformation (i) semi-ductile to brittle, Sd1, and (ii) frankly brittle, Sd2. The first stage of deformation is contemporaneous with magmatic activity, producing 360° schistosities linked to magma swelling and fracture schistosities (S1: N45°-N60° and S2: N75°-N95°) associated with the emplacement of granites and syenite. The second brittle stage, Sd2, is characterised by a system of conjugate dextral and sinistral detachments and two families of diaclases (F1: N45°-N60° and F2: N120°-N150°) associated with the emplacement of the pluton and the cooling of the magma.

Geochronology and geochemistry of the Shanagen hydrothermal vein-type Mo deposit in Derbugan metallogenic belt of the NE China and their geological significance

The Shanagen hydrothermal vein-type Mo deposit belongs to the Derbugan metallogenic belt, which is located in the Ergun block, NE China. The Mo mineralization is mainly developed in sericitized quartz siltstone around alkali-feldspar granite. In this paper, we present Molybdenite Re–Os dating, zircon U–Pb dating and geochemical data with the aim of determining metallogenic epoch and tectonic setting. Molybdenite Re–Os and zircon U–Pb dating of the alkali-feldspar granite indicate that the ore-formation and alkali-feldspar granite emplacement occurred at 143.1 ± 3.8 Ma, and 144.7 ± 0.7 Ma. Both were formed in the early Cretaceous. Chemically, the alkali-feldspar granites are enriched in L rare earth element and LILEs, depleted in H rare earth element and HFSEs, and pronounced negative Eu anomalies, are metaluminous and belong to the high-K calc-alkaline series and highly differentiated I-type granite. Geochemical features and concave upwards rare earth element pattern imply that alkali-feldspar granites were formed from magma generated by partial melting of lower crust. Combining the geochemistry, chronology, and the regional tectonic evolution, we infer that Shanagen hydrothermal vein-type Mo deposit was formed the extensional environment after the closure of the Mongol-Okhotsk Ocean.

Zircon U-Pb dating and petrogenesis of the São José do Campestre Granite Complex, NE Brazil: an example of neoarchean mantle-derived post-collisional magmatism

This article discusses the geochemical and petrological evolution of the São José do Campestre granite complex (SJCgr), the last Neoarchean plutonic event so far described in the São José do Campestre massif in NE Brazil. We report field, petrographic, zircon U-Pb dating, and whole rock and mineral chemistry for representative SJCgr samples. Laser ablation zircon U-Pb data indicate that the granite emplacement took place at 2664 ± 13 Ma. The rocks comprising the SJCgr have relatively well-preserved primary textures and fabrics and compositions varying from gabbro to syenogranite. Major and trace element contents reveal a metaluminous, calc-alkaline through transitional to alkaline signature, and LILE- and LREE-enriched series analogous to late Archean sanukitoid and modern arc granitoid. The evolution of the SJCgr is envisaged as follows: (1st) partial melting of a metasomatized mantle (2.5–3.0 GPa, ~85–102 km, 1,000–1,200°C), generating a basaltic to basaltic andesitic magma; (2nd) fractional crystallization (FC) of olivine at mantle or lower crustal depth, leading to the parental magmas of the magmatic series; and (3rd) 40–15% FC of olivine gabbro-norite and olivine monzonite cumulates (400–600 MPa, 15–23 km). The SJCgr shares similarities with post-collisional granitoids and, thus, would represent the last Neoarchean episode of mantle-derived magma in Northeastern Brazil.

The Zero Fault Pegmatite is Associated with Marble at Franklin, New Jersey, USA

AbstractThe Zero Fault Pegmatite is a large, unzoned pegmatite of graphic granite that is well exposed along quarry walls at Franklin, New Jersey. It is contained within the Franklin Marble and intersects the Zero Fault. New geochemical data has facilitated development of a three-step process responsible for its origin: (1) Anatectic partial fusion of a potassic metarhyolite (mapped as Microcline Gneiss) which occurred during peak-Ottawan (∼1045 Ma) granulite-facies metamorphism (∼770 °C and 600 MPa). Most of the resulting anatectic melt phase recrystallized in situ to form the leucosome of the Microcline Gneiss, but some persisted from late- to post-orogenesis and plots between the 720 and 730 °C isotherms of the 500 MPa phase diagram for the haplogranite, consistent with some relaxation and cooling from peak conditions. This residual and incompatible element enriched melt intruded along penetrating structures to form a local cluster of small, equally potassic granite plutons mapped as the Mt. Eve Granite. (2) Intrusion of the Mt. Eve Granite into marble layers, during which the anatectic melt underwent dense mineral fractionation, resulted in the formation of the Zero Fault Pegmatitic melt. (3) The melt encountered CO2 vapors, which reduced H2O activity, increased solidus temperatures, and resulted in significant undercooling. The latter led to rapid crystallization (at 720 to 740 °C), leading to development of a quenched, graphic texture. While the emplacement of graphic granite into marble is common among pegmatites, notably those in the Grenville Province, the proposed explanation provides insights into their general formation.

Petrogenesis and tectonic implications of the Early Carboniferous high‐K granites with enriched isotope signatures in the northern North China Craton

Although an extensive continental arc magmatism was recorded in the northern North China Craton (NCC) during the Carboniferous, a general geodynamic model has not been constructed, and associated crust–mantle processes are not fully understood. Here, we carried out systematic geochronological and geochemical investigations on the alkali‐feldspar granite pluton in the Zhaojinggou area, northern NCC. U–Pb dating of high‐U–Th zircons failed to obtain concordant ages due to severe metamictization, while apatite U–Pb dating yielded a perfect age of 327.6 ± 4.1 Ma, indicating the emplacement of granites in the Early Carboniferous. The granites have high SiO2 (74.99–76.88 wt%) and K2O (5.73–6.43 wt%) contents and exhibit enriched LREEs, K, Rb, Th, U, and Pb, and depleted Nb, Ta, P, and Ti contents, which are typical of highly fractionated, I‐type high‐K granites formed under a continental arc setting. Moreover, the high‐K granites show highly enriched Nd isotopic compositions (with whole‐rock and apatite εNd(t) of −33.6 to −16.3) that are distinctly lower than other Carboniferous continental arc rocks in the northern NCC (εNd(t) from −18.5 to −9.9). These geochemical characteristics indicate that the high‐K granites possibly originated by partial melting of the Archean TTG gneisses and mafic rocks in the lower crust. Involvement of mantle‐derived fluids is crucial to induce the low‐temperature melting, and sufficient fractional crystallization was reached during subsequent magmatic evolution. Previous studies suggest that the Carboniferous continental arc rocks in the northern NCC have mantle‐derived or crust–mantle mixed sources. Our study, however, indicates that there also exist coeval crust‐derived magmatic rocks in this area. In combination with previous work and this study, we support the hypothesis that the widespread Carboniferous continental arc magmatism in the northern NCC is related to the southward subduction of the Palaeo‐Asian Ocean (PAO), and the different magma sources and extensive crust–mantle interactions are responsible for the formation of diverse types of magmatic rocks with contrasting isotopic compositions. We also draw attention to the fact that phosphate minerals such as apatite may be a preferable chronometer relative to high‐U–Th zircon in a highly fractionated igneous system.

Litho-structural mapping and structural evolution of the Bocaranga pluton, northwest Adamawa-Yadé domain, Central African Republic

The Bocaranga pluton, in the northern domain of the Central African Orogenic Belt (CAOB) in the Central African Republic (CAR), is located in the Adamawa-Yadé domain, near the Mbéré Shear Zone (MBSZ). This study provides new results of the satellite image processing, the detailed structural and petrographic studies of Bocaranga pluton which helps to understand the emplacement mechanism of this pluton. Image processing of the satellite data and field showed the occurrence and spatial distribution of (1) amphibole-pyroxenites, (2) garnet-biotite rich amphibolite, (3) migmatite, (4) hornblende-biotite and hornblende-garnet gneiss, (5) hornblende-biotite granite (HBG), (6) two micas granite (TMG), (7) fine-grained biotite granite (FBG), and (8) coarse-grained biotite granite (CBG) rock types. The mapping of regional lineaments was carried out by automatic and manual lineament extraction methods. NE-SW direction corresponds to the direction of S2 and S3 foliations, faults, mineral lineation, quartz vein, dyke (aplitic granitic or pegmatitic). NW-SE direction also corresponds to the direction of S1 foliation, F1 folds, faults, quartz veins, dykes (aplitic granitic or pegmatitic). In the absence of absolute geochronological data in study area, it has been possible to propose a polyphase structural evolution, deduced from the relative chronology of the Bocaranga granitoid. Three phases of deformation can be defined in this study. D1 phase is represented by early NW-SE trending foliation (S1) that has been progressively rotated parallel to S2 overprinting foliation occurring in gneiss, amphibolites and hornblende-biotite granite (HBG) intrusion as well. D2 phase of ductile sinistral sense from NE-SW to ENE-WSW is responsible for an emplacement of Bocaranga granitic pluton. The scarcity of structure orientation in the TMG and FBG (except the contact zones), would imply that these granites would have been crystallized in the absence of any stress. D3 is a phase of brittle deformation with three main directions corresponding to the main direction of the shear zones in the Northern domain of the CAR Pan-African fold belt.The mylonitic deformation observed in study area illustrates the intensity of the deformation. The dominance of ellipsoidal structures observed in Bocaranga area suggests that the shear mechanism is the main controlling structure of Bocaranga granite emplacement, which can be correlated with the emplacement of Tamkoro-Bossangoa granitoid in CAR and the Adamawa-Yadé domain in Cameroon, Chad and NE Brazil.These results imply a syntectonic emplacement of Bocaranga pluton during sinistral simple shear regime related to deformation along the Central Cameroon Shear Zone (CCSZ).

A linked evolution for granite-greenstone terranes of the Pilbara Craton from Nd and Hf isotopes, with implications for Archean continental growth

Granite-greenstone terranes, such as in the Pilbara Craton (Western Australia), record the growth and stabilisation of Archean (>2.5 Ga) continental crust. Yet, uncertainty surrounds the formation of the silica-rich portions of these terranes, important for preservation of continental lithosphere, specifically whether this involved reprocessing of older continental nuclei over hundreds of millions of years, or rapid differentiation of juvenile mantle additions, as preserved in the greenstone belt volcanic sequences. In part, the uncertainty reflects difficulties with establishing the isotope evolution of the early Archean mantle, critical for ascertaining the time gap between the extraction of new crust and granitic emplacement in Archean cratons. To investigate the formation of silica-rich Archean crust, we report Hf and Nd isotope data for 46 mafic–ultramafic samples and zircon Hf isotope data for 83 felsic igneous rocks from the Pilbara Craton, selected to be representative of the major magmatic episodes from 3.56 to 2.72 Ga. The strikingly similar secular isotope trends defined by mafic and felsic rocks reveal a linkage between mantle and crustal processes throughout the history of the craton. From 3.56 Ga to 3.12 Ga, craton evolution was dominated by the transformation of juvenile mantle derived inputs into felsic igneous rocks on short timescales, thus recording sustained continental growth over 400 million years. Incorporation of older crust by felsic magmas was minor during this interval. Evolution toward sub-chondritic Nd and Hf after collision of the East and West Pilbara terranes at 3.1 Ga signals greater involvement of pre-existing crust in magma generation, possibly by recycling of crustal material into mantle source regions or by enhanced reworking of older, thickened cratonic lithosphere. Mafic–ultramafic rocks provide no evidence for anomalous or strongly depleted mantle domains in the Paleoarchean. Rather, the flat zircon εHf–time array indicates extraction of continental materials from mildly super-chondritic mantle from ca. 3.6 to 3.12 Ga and attests to a distinctive style of crust production that operated in the Archean. This study highlights the importance of comparing the mantle and crustal isotope records in order to reconstruct the crustal growth rates in ancient terranes.

Experimental Crystallization of the Beauvoir Granite as a Model for the Evolution of Variscan Rare Metal Magmas

Abstract New experiments on the strongly peraluminous, Li-, P- and F-rich Beauvoir granite (Massif Central, France) provide a basis for understanding crystallization and chemical fractionation in Variscan rare metal magmas. Crystallization experiments were performed on two natural granite compositions under H2O-saturated conditions at 100, 200, and 300 MPa, from 540°C to 700°C and between ~NNO + 3 and NNO-1.4. Experimental charges were examined by SEM and their products (glasses and crystals) analyzed for major elements by EMPA. Trace element concentrations in selected glasses were determined by LA ICP-MS. Despite experimental durations commonly exceeding 1000 h and some up to 4000 h, kinetic problems were encountered in particular in the 100 MPa charges whereas, at 200 and 300 MPa, results consistent with previous melting experiments were obtained. Beauvoir melts crystallize quartz, plagioclase, K-feldspar and mica as major phases. At NNO-1.4, mica is a biotite, whereas it is a Li-mica between ~NNO+3 and NNO-1. Apatite, Fe-Ti oxides, either hematite or magnetite, topaz, amblygonite, cassiterite and columbite-tantalite appear as accessory phases between ~NNO + 3 and NNO + 1. Experimental plagioclases are albitic (An <4.5 mol%) and more Ca and K-rich than natural albites in the granite whereas experimental K-feldspars are more sodic (Ab <45 mol%) than the natural crystals. The less evolved starting melt crystallized Li phengites whereas the most evolved yielded Li-, F-rich micas near the polylithionite-zinnwaldite series, similar to natural micas in the granite. Equilibrium crystallization increases A/CNK, F and P and concentrates Li, Be, B, Rb, Cs, W, U in the melt. Nb and Ta are also enriched, their behavior being controlled by the solubility of columbite-tantalite in the melt. Other elements are either unchanged (Mn, Zn, Ti) or depleted (Sr, Pb) during magmatic fractionation. Sn is concentrated in Li-mica and hematite, and it behaves compatibly at high fO2. Beauvoir melts crystallize at very low temperatures, below 670°C for the two compositions studied and solidus temperatures, determined from previous melting experiments and confirmed by the new crystallization experiments, are near 550°C. The experiments demonstrate that most of the mineralogical and geochemical characteristics that make the Beauvoir granite distinctive result from magmatic rather than hydrothermal post-magmatic processes. Albitic plagioclase, Li-mica, topaz, and amblygonite are of magmatic origin. Glass major element compositions suggest that the two granite samples represent crystallized liquids. Trace element fractionations for most elements at Beauvoir can be accounted for by magmatic crystallization–differentiation processes. Implications for the mineralogy, fO2, volatile concentrations, crystallization and conditions of emplacement, fractionation mechanisms and origin of the Beauvoir granite are discussed.

Scheelite U-Pb geochronology and trace element geochemistry fingerprint W mineralization in the giant Zhuxi W deposit, South China

Abstract Skarn-type tungsten deposits are widely distributed all over the world and contribute more than 70% of the world’s W supply. The temporal relation between the W mineralization and causative intrusions and the evolution of ore-forming fluids are matters of ongoing debate. In this study, we combine in situ LA-ICP-MS U-Pb dating and trace element compositions of scheelite from Zhuxi, the world’s largest W deposit, and compare them with literature data to address the above issues. Three primary ore stages exist at Zhuxi: prograde skarn, retrograde skarn, and quartz-sulfide stages. Most scheelite occurs in the retrograde skarn stage and is further subdivided into three generations: Sch A, B, and C. The obtained LA-ICP-MS U-Pb ages for three scheelite generations in the Zhuxi deposit are 154.0 ± 2.8, 150.3 ± 3.5, and 150.4 ± 6.3 Ma, respectively, indicating that the entire W mineralization is closely related to the emplacement of the nearby Late Jurassic granites (~154 to 150 Ma, zircon U-Pb ages). In situ LA-ICP-MS trace element results demonstrate that Sch A shows the highest Mo content (mean = 1002 ppm), where those for Sch B and Sch C are 109 and 45 ppm, respectively. These, combined with the gradually increasing trend of Ce contents and δCe values, indicate a shift from oxidizing to reducing conditions for the ore-forming fluid. All three scheelite generations yield significant positive δEu anomalies, which are considered to be unrelated to the redox state, but caused by the addition of Eu (e.g., feldspar dissolution). The high-Y/Ho ratio of scheelite and a good correlation between Y/Ho ratio and δEu (R2 = 0.96) suggest that intense fluid-rock interactions between ore fluids and the Shuangqiaoshan Group metasedimentary rocks as well as earlier-formed skarns drove fluid evolution. This study demonstrates that scheelite U-Pb geochronology is a useful technique when identifying the temporal link between hydrothermal W mineralization and the causative intrusion. Our results also highlight that the reactions of the ore fluids with wall rocks and earlier-formed skarns significantly modify the primary fluid compositions.

Emplacement and exhumation history of Mesozoic granitic rocks in the Jiaonan uplift, eastern China

The Jiaonan uplift is one part of the Sulu orogen, which is the most prominent Phanerozoic orogenic belt on the East Asian continental margin. The exhumation history of Mesozoic intrusive rocks in the Jiaonan uplift was related to intracontinental tectonics and processes since the Mesozoic time. These tectonics and processes are of great significance for the study of the regional tectonic evolution. New U–Pb zircon data of granite samples from the Jiaonan uplift yield Early Cretaceous ages (∼114 Ma and 115 Ma). Applying apatite fission-track (AFT) and zircon (U-Th)/He geochronometers to samples from the representative intrusions from the Jiaonan uplift and its adjacent area refines the exhumation and cooling history. The new age constraints and AFT thermal inverse modeling results reveal three thermal evolutionary stages after the emplacement of the Lower Cretaceous granites. Modeling results show that the Lower Cretaceous granites rapidly cooled to the AFT partial annealing zone during the Late Cretaceous, followed by a period of relatively slow cooling from the Paleocene to Eocene, and re-entered a second cooling stage since about the Oligocene. The first rapid cooling process during the Cretaceous coincided with the late rapid exhumation of the Sulu UHP metamorphic rocks, implying that they might have been dominated by a unified deep geodynamic background. The cooling event since the Oligocene might have been influenced either by Pacific subduction and/or lateral motion during the India-Asia collision.

Zircon U-Pb Ages and Geochemistry of the Granite in the Xintianling Tungsten Deposit, SE China: Implications for Geodynamic Settings of the Regional Tungsten Mineralization

The Xintianling tungsten deposit is a super-large deposit in the Nanling tungsten–tin mineralization belt, which is genetically associated with the early-stage hornblende-biotite monzonitic granite of Qitianling pluton. The orebodies predominantly occur as veins and lenses within skarn rocks between Xintianling granite and limestone (Shidengzi group). In this work, whole-rock major and trace elements and zircon U–Pb ages of the Xintianling granite were studied in an attempt to investigate the geochronological framework, petrogenesis, tectonism, and metallogenesis with regard to the deposit. The petrographic and geochemical analyses indicated that the Xintianling granite consists of three intrusive units of medium- and coarse-grained biotite granite, fine-grained biotite granite, and granite porphyry, of which the biotite granite was strongly associated with mineralization. Biotite granite rocks are highly K-calc-alkaline and weakly peraluminous, with A/CNK ratios ranging from 0.99 to 1.05. Late-granite porphyry is aluminum-supersaturated with a high evolution degree, whose geochemical characteristics suggest that it is either an I- or S-type granite. LA-ICP-MS zircon U-Pb dating revealed that medium- and coarse-grained biotite granite (162.3 ± 1.2 Ma, MSWD = 1.3), fine-grained biotite granite (161.8 ± 1.3 Ma, MSWD = 1.8), and granite porphyry (154.3 ± 1.6 Ma, MSWD = 2.4) formed in the late Jurassic. The emplacement of the Qitianling A-type granite and associated tungsten-tin polymetallic mineralization is a continuous evolution process, and they are products of the large-scale mineralization of the Nanling in the middle–late Jurassic (150–160 Ma). Under the tectonic setting of the Mesozoic lithospheric extension, asthenosphere upwelling along deep-fault, intensive mantle–crust interaction processes probably provide not only the high heat flow, but also partly mantle-derived material for large-scale W-Sn-polymetallic mineralization in this area.

Detrital zircon U-Pb-Hf isotope record of conglomerates in the southern Aravalli orogen, NW India: Implications for stratigraphy, provenance and Archean to Paleoproterozoic crustal evolution

The inconsistent stratigraphic framework of the Proterozoic successions in the Aravalli orogen, NW India hinders the detailed reconstruction of its geodynamic evolution during the Archean to Paleoproterozoic. In this study, we present new results of combined U-Pb-Hf isotope data of detrital zircon grains of two conglomerate samples from the southern Aravalli orogen, presently assigned to the upper unit of the Aravalli Supergroup. The youngest concordant zircon grains in both the conglomerates yield concordia ages of 1735 ± 13 Ma (n = 2) and 1736 ± 15 Ma (n = 2), respectively, interpreted as maximum ages of deposition. Older zircon grains mostly reveal Neoarchean to Paleoproterozoic age clusters at 2560–2550, 1870–1860 and 1760–1740 Ma, and minor Paleoarchean ages between 3558 ± 17 Ma and 3239 ± 14 Ma, corresponding to known magmatic events in the Aravalli orogen. Most detrital zircon grains show subchondritic εHft values (+6 to −20), signifying minor reworking of late Hadean/Eoarchean crust during the Paleoarchean, and substantial reworking of Paleo-to Neoarchean crust during the late Neoarchean and Paleoproterozoic, with only minor juvenile input. The maximum depositional ages indicate that the investigated conglomerates were deposited either synchronous or after the emplacement of extension-related A-type granites, which intruded the Aravalli orogen between 1733 ± 9 Ma and 1703 ± 5 Ma. Furthermore, the data suggest that the conglomerates do not belong to the upper Aravalli unit as envisaged by previously published stratigraphic concepts. Instead, these were deposited earlier, during opening of a rift-related basin, grading from a terrestrial into a marine environment, contemporaneous with other sedimentary successions exposed in the North Delhi basin, which are presently assigned to the North Delhi Supergroup. The compiled zircon age-Hf isotope data additionally provide evidence for a series of late Hadean to Archean juvenile crust formation events in hinterland of the Aravalli orogen at 4200–3300 Ma for east Indian cratonic blocks and 3550–3100 Ma for northern and southern cratonic nuclei, and 2700–2500 Ma all over India. Substantial crustal reworking took place during the late Neoarchean at 2500 Ma, and the late Paleoproterozoic at 1850 Ma and 1730–1700 Ma.

Tectonic control on the spatial distribution of Sn mineralization in the Gejiu Sn district, China

Late Cretaceous granitic rocks occur in the Gejiu ore district to the east and west of the N-S striking Gejiu Fault, whereas major Sn deposits are only known to occur to the east of the Gejiu Fault. Comparison of the whole-rock chemistry, the apatite trace-element chemistry, and zircon Hf and O isotope data of the various granites demonstrates that fertile granites occur to both sides of the Gejiu fault. The results demonstrate that the c. 83 Ma old granitic intrusions (i) have similar magma sources, which are dominated by metasedimentary rocks that had experienced intense chemical weathering, resulting in reduced melts, (ii) had similar melting conditions, i.e., high temperature biotite dehydration melting, and (iii) in part had experienced large extents of fractional crystallization. The most evolved granites to both sides of the Gejiu Fault have the characteristics typical of tin granites. Therefore, the absence of major deposits to the west of the Gejiu Fault is not due to the absence of fertile granites. The areas to the east and the west of the Gejiu Fault, however, have fundamentally different fault pattern, which indicates different orientation of the stress field to both sides of the Gejiu Fault at the time of the emplacement of the Cretaceous granites. Late Cretaceous dextral movement along the Ailaoshan Fault Zone resulted in a (trans)tensional setting in areas to the east of the Gejiu Fault and in a (trans)pressional setting to the west of that fault. We speculate that the tectonic setting influences the potential for mineralization because the Sn bearing fluids need efficient pathways to transporting metals from the roof zone of the batholith into the wall rocks. In contrast to regional compression, hydraulic fracturing in an overall extensional setting has the potential to develop efficient fluid pathways and, thus, may lead to major ore deposits. To the west of the Gejiu Fault, however, granite intrusions in an overall compressional setting are likely to develop no or only small mineralization.

Geochronology and Tectonic Implications of the Nianzigou Granites and Associated Mo Deposit, Inner Mongolia

Multiple stages of igneous rocks occur in the recently discovered Nianzigou Mo deposit in Chifeng, Inner Mongolia, which can provide insights into the late Mesozoic geodynamic evolution of the southern Da Hinggan Range. The mineralization age is similar to the age of local granites, but there are few detailed studies of the tectonic setting during Cu-Mo mineralization in this area. The Nianzigou Mo deposit is located close to the northern margin of the North China Craton and in the eastern Central Asian Orogenic Belt and is a typical quartz-vein-type Mo deposit in the Xilamulun Mo ore belt. The granite in this deposit has high SiO2, Al2O3, K2O, and Na2O contents, and low MgO, CaO, and Fe2O3t contents. The granite is characterized by enrichments in large-ion lithophile elements and depletions in high-field-strength elements and, in particular, Sr, Ti, and P. The granite has high contents of rare-earth elements, is enriched in light rare-earth elements, and has marked negative Eu anomalies. The granite is an alkaline and calc-alkaline and metaluminous A-type granite. The zircon U-Pb ages of the monzogranite and granite porphyry are 157.2 ± 0.3 and 154.4 ± 0.4 Ma. The model age obtained by Re-Os isotopic dating is 154.3 ± 1.7 Ma, indicating that molybdenite mineralization also occurred during the Late Jurassic period. Given that the molybdenite Re contents are 7.8–24.9 ppm (average = 16.8 ppm), the ore-forming materials of the Nianzigou Mo deposit had a mixed crust–mantle source, but were mainly derived from the lower crust. Based on the geology and geochemistry, we propose that the Nianzigou Mo deposit formed in a postorogenic extensional tectonic setting associated with the southward subduction of the Mongol–Okhotsk oceanic plate.