Felsic Intrusions Research Articles

Interpretation of airborne and ground magnetic and gamma-ray spectrometry data in prospecting for base metals in the central-north part of the Itabuna-Salvador-Curaçá Block, Bahia, Brazil

Interpretation of airborne and ground magnetic and radiometric geophysical data is fundamental to provide support for geologic mapping and exploration of mineral resources. This is particularly important of the Itabuna-Salvador-Curaçá Block component of the São Francisco Craton in the state of Bahia, Brazil, which bears important base metal mineralization in ultramafic intrusions. The interpreted airborne data covered part of the northern segment, and the ground data comprised an area of [Formula: see text] in the northern segment of that Archean block. The radiometric interpretation was only qualitative, and the magnetic interpretation was qualitative and quantitative. The qualitative interpretation defined distinct configurations of the geophysical signatures. So, it furnished possible regional and local geologic models and also helped to infer the distribution of magnetization and radiometric content in the rocks. The quantitative interpretation started with Werner deconvolution for a preliminary identification of the magnetic sources, and 2D magnetic modeling improved the definition of the anomalous bodies in terms of geometry, depth to the top, and magnetic susceptibility. The combined qualitative and quantitative interpretation allowed mapping of the rocks that contain magnetic and metallic minerals, as well as felsic lithology, such as granites and orthogneisses, and mafic lithology, such as gabbros and amphibolites. It also suggested that most of the northwest–southeast lineaments related to felsic and mafic magnetic intrusions. The analysis may extend to other areas of the block and be integrated with field and laboratory geologic, geophysical, and geochemical studies for an adequate exploration of nickel, copper, and chromium that are potentially present in the region.

Investigation of spatially non-stationary influences of tectono-magmatic processes on Fe mineralization in eastern Tianshan, China with geographically weighted regression

The volcanic–sedimentary Fe mineralization in eastern Tianshan mineral district is controlled by a series of tectono-magmatic processes. Controlling effects of related geo-processes on Fe mineralization are varied across the space; relationships among these geo-processes are consequently spatially non-stationary. Before investigating the relationships, this paper jointly utilizes the singularity index mapping technique and principal component analysis to recognize Fe mineralization associated felsic intrusions, fault systems and the Yamansu Formation. After that, both ordinary least square and geographically weighted regression models are used to inspect the relationships that existed between Fe mineralization and these identified controlling factors. Results by geographically weighted regression quantitatively depict non-stationary influences of tectono-magmatic processes on Fe accumulation in spatial scenario, which demonstrate well the significant improvement in parameter estimation in comparison with ordinary least square regression. Examining regression coefficients, it can be inferred that Fe mineralization at different locations in eastern Tianshan mineral district was caused by interactivities of multiple geo-processes to different degrees, and these variations can be demonstrated by the results as well. Experiences derived from current study can be a useful reference to future regression analysis on geological issues.

U–Pb dating constraints on the felsic and intermediate volcanic sequence of the nickel-sulphide bearing Cosmos succession, Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia

The Cosmos succession, within the Agnew-Wiluna greenstone belt of the Yilgarn Craton, consists of ultramafic, intermediate and felsic volcanic lithologies, and contains several komatiite-hosted nickel sulphide ore bodies. The volcanic succession that forms the stratigraphic footwall to the mineralised komatiite lavas consists of an intercalated sequence of fragmental and coherent extrusive lithologies, ranging from basaltic-andesites through to rhyolites. It also contains cross cutting felsic intrusions. The stratigraphic hangingwall consists largely of reworked volcanic-derived sedimentary rocks that include polymictic conglomerates containing granitic clasts. SIMS U–Pb dating was undertaken on zircons from nine felsic lithologies spanning the established stratigraphy. The footwall succession gives emplacement ages ranging from 2736±4 to 2724±3Ma. The overlying Cosmos Ultramafic sequence consists of two units; the basal ultramafic UMu1 package and the upper ultramafic UMu2 package, which are separated by a discontinuous felsic volcanic horizon that sits approximately 30–60m above the basal contact of the Cosmos Ultramafic sequence. This discontinuous felsic horizon is dated at 2685±8Ma and represents a break in accumulation of komatiite lavas and may be associated with a major hiatus. The succession also features two felsic intrusions emplaced at 2653±7Ma and 2670±6Ma, where the latter cross-cuts the upper contact of the UMu2 package. These results bracket the age of UMu1 package between 2724±3Ma and 2685±8Ma and that of UMu2 package between 2685±8Ma and 2670±6Ma. This stratigraphic division of the Cosmos ultramafic is further strengthened by the contrasting styles of mineralisation above and below the discontinuous felsic horizon and associated unconformity; UMu1 hosts several massive nickel sulphide deposits situated at the contact with the footwall volcanic succession, while UMu2 is typified by high-tenor disseminated nickel sulphide deposits situated just above the discontinuous felsic horizon. Our data indicates that the Cosmos volcanic succession was accumulated via punctuated volcanism spanning ∼50Ma that was followed by intrusive activity lasting for at least a further 32Ma. The results also imply that construction of this relatively small, mineralised greenstone succession lasted longer than anticipated on stratigraphic considerations alone, an outcome that may have implications for the timing of emplacement of other bi-modal volcanic successions in similar greenstone terranes.

The Bi’r Tawilah deposit, central western Saudi Arabia: Supergene enrichment of a Pan-African epithermal gold mineralization

The Bi’r Tawilah gold deposit in central western Saudi Arabia represents a Pan-African example of gold mineralization in which both hypogene and supergene ores are recorded. The sulphidic gold ore is hosted in intermediate to felsic intrusions that occur along the N–S trending thrust-fault zone within the so-called “Nabitah orogenic zone”. There are four rock units present (from oldest to youngest): serpentinites and related listwaenites, diorites, granitic rocks and porphyries. Hydrothermal alteration consists of chloritization, sericitization, carbonatization and silicification and affects all rock types. Chloritization of biotite results in abundant rutile, whereas sulphidization coincides with carbonatization. The Bi’r Tawilah ore is confined to NW-trending shears (Riedel fractures) related to N–S slip of the pre-existing Tawilah thrust due to activation within the Najd fault system. Samples from the boreholes show macro- and microscopic evidence of shearing such as micro-shear planes and strain shadows of pyrite. Sulphides and gold are present in most rock types. Paragenetically, the sulphides consist of abundant pyrite and relatively lesser amounts of arsenopyrite, in addition to very minor chalcopyrite, sphalerite and galena. In all boreholes, it was noticed that the abundance of arsenopyrite increases with depth.The elevated silver content of electrum (∼13–22wt%) at Bi’r Tawilah is typical of gold deposits and low-sulphidation epithermal deposits. The early mineralization stage took place in proximity to hydrothermally altered intermediate to felsic intrusions. The aerially restricted hydrothermal alteration by carbon-aqueous fluids led to ore remobilization in which gold amounts up to 4.3g/t. Finally, gold enrichment (up to 5.4g/t) resulted from supergene alteration that took place during weathering above the water table at a depth of ∼20–25m.

Petrology, major and trace element geochemistry, geochronology, and isotopic composition of granitic intrusions from the vicinity of the Bosumtwi impact crater, Ghana

The Bosumtwi crater is 10.5km in diameter, 1.07Ma old, well preserved impact structure located in Ghana (centered at 06°30′N, 01°25′W). It was excavated in rocks of the Early Proterozoic Birimian Supergroup, part of the West African craton. Here, we present a full and detailed characterization of the three granitoid complexes and one mafic dike in the vicinity of the Bosumtwi crater in terms of petrology, major and trace element geochemistry, geochronology, and isotopic composition. This allows us to characterize magmatic evolution of the West African Craton in this area and better understand the geological framework and target rocks of the impact.This study shows that the similar composition (strongly peraluminous muscovite granites and granodiorites) and age (between 2092±6Ma and 2098±6Ma) of all granitic intrusions in the proximity of the Bosumtwi crater suggest that they are co-genetic. Granitoids were probably formed as a result of anatexis of TTGs (or rocks derived from them) at relatively low pressure and temperature. We propose that the intrusions from the Bosumtwi area are genetically related to the Banso granite occurring to the east of the crater and can be classified as basin-type, late-stage granitoids. Also a mafic dike located to the NE of the Bosumtwi crater seems to be genetically related to those felsic intrusions. Based on those findings a revised version of the geological map of the Bosumtwi crater area is proposed.

Mafic injection as a trigger for felsic magmatism: A numerical study

AbstractThe origin of crustal‐scale silicic magmatism remains a matter of debate, and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions. A 2‐D numerical model demonstrates that injection of mantle‐derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels. The injection of mafic magma induces overpressure in the reservoir, which increases crustal stresses and triggers development of brittle/plastic shear zones, and can drive significant surface uplift. The emerging topography causes a nonuniform overpressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones. Based on systematic numerical experiments, we investigate the influence of crustal strength and injection rate. The initial upper crustal strength controls the degree of crustal faulting and surface uplift and therefore whether felsic magma ascent can be initiated or not. The final upper crustal strength influences the depth and final style of felsic intrusion. The injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage. In contrast, the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant. Our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels.

Geochemical, Sr-Nd-Pb, and Zircon Hf-O Isotopic Compositions of Eocene-Oligocene Shoshonitic and Potassic Adakite-like Felsic Intrusions in Western Yunnan, SW China: Petrogenesis and Tectonic Implications

Coeval potassic adakite-like and shoshonitic felsic intrusions in the western Yunnan province of SW China are spatially and temporally associated with Eocene^Oligocene shoshonitic mafic volcanic rocks. The shoshonitic syenite and quartz monzonite intrusions are characterized by high K2O contents (4·9^6·8 wt %) and K2O/Na2O ratios (1·1^1·7), high Y (1·7^34·8 ppm) and Yb (1·50^3·16 ppm) contents, nearly flat heavy rare earth element (HREE) patterns and moderate Eu anomalies (Eu/Eu*1⁄4 0·65^0·78).The potassic adakite-like granite and quartz monzonite intrusions are characterized by enrichment in light rare earth elements (LREE), depletion in HREE and fractionated HREE patterns, high Sr (328^1423 ppm), Sr/Y (38^243) and La/Yb (23^62), and low Y

Hydrothermal alteration related to a deep mantle source controlled by a Cambrian intracontinental strike-slip fault: Evidence for the Meruoca felsic intrusion associated with the Transbraziliano Lineament, Northeastern Brazil

One of the most prominent geological structures in Borborema Province, northeast Brazil, is the Transbraziliano Lineament that crosscuts most of the South American Platform and was active at least until the Devonian. This continental structure is responsible for the formation of rift and pull-apart basins in Northeastern Brazil, most of which filled with volcanic and continental sedimentary rocks (Parente et al., 2004). In the region of Sobral, Ceará State, this same continental structure controlled the intrusion of the Meruoca pluton and the formation of the Jaibaras Basin, which is bounded by strike-slip shear zones. Hydrothermal alterations seem to have been pervasive in Meruoca, as indicated by disturbances in both the Rb–Sr and U–Pb systems (Sial et al., 1981; Fetter, 1999) and by the large dispersion of anisotropic magnetic susceptibility (AMS) (Archanjo et al., 2009).In this paper, we address the origin of the hydrothermal fluids that affected the borders of the Meruoca batholith and their relationship with the activity of the Transbraziliano Lineament. These fluids were responsible for carbonate veins and Fe–Cu mineral concentrations that are commonly found associated with hydrothermally altered breccias. The carbon and oxygen isotope composition of these carbonate veins suggest that they may be related to CO2-bearing mantle-derived fluids that were channelized by the Transbraziliano Lineament. Based on oxygen isotopes, we argue that Fe–Cu concentrations may have formed in isotope equilibrium with the rhyolitic rocks at temperatures between 500 and 560 °C. This scenario points to magmatism as the main process in the formation of these rocks.We also report a K–Ar age of 530 ± 12 Ma for muscovite associated with the last ductile event that affected the Sobral-Pedro II Shear Zone and a U–Pb age of 540.8 ± 5.1 Ma for the Meruoca pluton. We further suggest that this granite is a late-kinematic intrusion that is most likely associated with the Parapuí volcanic rocks of the Jaibaras basin (535.6 ± 8.5 Ma, Garcia et al., 2010).

Petrogenesis and tectonic significance of Early Cretaceous high-Zr rhyolite in the Dazhou uranium district, Gan-Hang Belt, Southeast China

Early Cretaceous felsic intrusions and volcanic rocks are widespread in the Gan-Hang Belt, SE China. In this study, we report a distinctive high-Zr rhyolite (802–1145ppm Zr) from the Dazhou uranium district in the eastern Gan-Hang Belt. SHRIMP zircon U–Pb dating shows that the Dazhou rhyolite erupted at 127.3±1.7Ma. Geochemical data indicate A-type characteristics for the Dazhou rhyolite, such as high contents of alkali elements, high Fe2O3∗/MgO and high Ga/Al ratios, enrichment in some LILEs, HFSEs, and REEs, and depletion in Sr, Ba, P, and Ti. Compared to other A-type granitoids reported previously in the same belt, the Dazhou high-Zr rhyolite shows only a slight enrichment in other HFSE elements and LREE, but has similar whole-rock Nd and zircon Hf isotopic compositions to those A-type granitoids with relatively lower Zr concentrations, indicating that all these A-type rocks may have similar magma sources. Hence, the difference of Zr contents in these rocks is not due to their different source rocks. We found that zircon grains in the Dazhou high-Zr rhyolite are generally inheritance-poor, and the calculated zircon saturation temperatures are extremely high at ∼1000°C. It is therefore suggested that the distinctive high-Zr characteristic of the Dazhou rhyolite was a result of high-temperature suppression of zircon crystallization by ambient mafic magmas correlative with the upwelling of the asthenospheric mantle. The fundamental difference in magma temperature and magmatic evolution mechanism between the Dazhou high-Zr rhyolite and other A-type granitoid suites in the eastern Gan-Hang Belt brought about their different Zr concentrations.

Mineralogical constraints on the origin of Neoproterozoic felsic intrusions, NW margin of the Yangtze Block, South China

Felsic intrusions in the Hannan region at the northwestern margin of the Yangtze Block mainly include the ca. 730 Ma adakitic Erliba and Wudumen plutons and the ca. 760 Ma calcic-alkali Xixiang and Tianpinghe bodies. These four intrusions were considered to have been formed by melting of the newly formed lower mafic crust. However, the two generations of granitoids have different lithologies and mineral compositions. Thermobarometry calculations reveal that the Erliba and Wudumen granitoids formed under approximately similar emplacement pressures (2.96–3.11 kbar) and temperatures (787–789°C). The Xixiang emplaced body was intruded at high pressure (˜3.54 kbar) and low temperature (˜676°C), whereas the Tianpinghe pluton solidified at low pressure (˜2.00 kbar) and high temperature (∼747°C). The four intrusions have similar oxygen fugacity ranges near the nickel-nickel oxide buffer, suggesting oxidized parental magmas. The Erliba and Wudumen are estimated to have been generated under pressures higher than 12 kbar, the Xixiang under a pressure of >10 kbar, and the Tianpinghe under a pressure of >6 kbar. Thus, the petrology and geochemical differences among these four felsic intrusions probably mainly resulted from variations of depth and degrees of partial melting. The whole-rock and mineral compositions have arc affinities, suggesting that they were formed in an active continental margin.

A Newly Discovered Swarm of Shear-Zone-Hosted Bi-As-Fe-Mg-P-Rich Aplites and Pegmatites in the Hagendorf-Pleystein Pegmatite Province, Southeastern Germany: A Step Closer to the Metamorphic Root of Pegmatites

The Miesbrunn pegmatite-aplite swarm (MPAS), part of the Hagendorf-Pleystein pegmatite province, southeastern Germany, was emplaced syn- to postkinematically relative to the Variscan tectonometamorphic processes (stage I). The MPAS developed along the contact between the autochthonous Moldanubicum and the nappes of the Bohemicum. Supercritical mobilization of REE, Zr, Mg, Zn, Li, Be and B during stage II led to a silicate-phosphate mineral association between 500 and 600 degrees C. Below 475 degrees C and a pressure of 3.8 kbar, contact-metasomatic reactions caused Mg phosphates to appear. Subsequently to the emplacement of the felsic intrusives bodies, the shear structure was reactivated and used as a conduit for fluids bearing S, As, Bi, Sb, Hg and U, from which sulfides and arsenides precipitated at 350 degrees C. Hypogene alteration during stages III and IV affected the primary phosphates at temperatures below 100 C, with fluctuating redox conditions when the MPAS was brought closer to the paleosurface as a result of strong uplift of the crystalline basement. Phosphates of Fe2+ formed during this epithermal mineralization in the pH range 5 to 10, whereas the stability field of Fe3+ phosphates extended toward lower values, below a pH of 5. The mineral assemblages of the MPAS are on one hand representative of mineralization at the deepest parts of the pegmatite, close to its root zone, and on the other, within the uppermost parts of pegmatites as they were pervasively altered by near-surface fluids. Sulfur isotopes of sulfides found in the MPAS and its wallrocks show a significant trend toward more strongly negative values away from the pegmatites and aplites. A survey of sulfides using their S isotopes may well be a useful exploration tool for blind pegmatites (i.e., not exposed).

An abrupt change in Nd isotopic composition in Australian basins at 1655 Ma: Implications for the tectonic evolution of Australia and its place in NUNA

Late Paleoproterozoic sedimentary basins across eastern Australia record a significant change in their neodymium isotopic composition at ca. 1655Ma. Prior to ca. 1655Ma, detritus was derived from comparatively evolved sources generating bulk ɛNd(1650Ma) values of generally −8 to −6. Subsequent sedimentary successions, which accumulated between ca. 1655Ma and 1600Ma, have bulk ɛNd(1650Ma) values of −2 to −1. This change is interpreted to reflect the input of sediments from a new, probable felsic volcanic source, corresponding to a fundamental change in tectonism and/or tectonic drivers for basin evolution in northern Australia. One possible driver for the change in sedimentary source is the initiation of renewed rifting from 1655Ma along the eastern margin of Paleoproterozoic Australia. As the 1655–1600Ma sedimentary successions that record the isotopic change are voluminous, a large, juvenile source of volcanic detritus must have been present, either within Paleoproterozoic Australia or in the cratonic block immediately to the east of Australia in the NUNA supercontinent. Although voluminous juvenile felsic magmatic sources are known in eastern and central Australia (e.g. 1639–1631Ma volcanics in the Warumpi Province; 1620–1610Ma granites of the St. Peter Suite in the Gawler Province), these sources are too young to have acted as a source for the juvenile detritus. Felsic intrusions of ∼1650Ma age are present in the Mount Isa Province, but the known exposed volume is very small. Of these possible sources, we favour either a buried or eroded eastern Proterozoic Australian source or an outboard Laurentian source.

Early Paleoproterozoic felsic and mafic magmatism in the Karelian province of the Fennoscandian shield

Early Paleoproterozoic (2.50Ga to 2.39Ga) mafic and felsic intrusions are widespread in the Fennoscandian shield. The bimodal magmatic event reflects an extensional phase in the Archean craton that had been tectonically quiescent for ca. 200Ma after the amalgamation of the continent. The early Paleoproterozoic felsic intrusions are mostly granitic in composition, only one syenitic pluton is known. Geochemically the felsic intrusions are A-type. The mafic layered intrusions were emplaced in the upper crust between the Archean basement and overlying early Paleoproterozoic volcanic rocks. At least two different parental magma compositions were present in the mafic magmatic event. Partial melting of the Archean lower crust due to heating effect of the hot, mafic magmas most probably generated the felsic magmas, which reflect the Nd isotope composition of the source rocks. The irregular distribution of coeval mafic and felsic intrusions within the Karelian province is still an open question.

Zircon SHRIMP U–Pb geochronology of potassic felsic intrusions in western Yunnan, SW China: Constraints on the relationship of magmatism to the Jinsha suture

The relationship between potassic felsic intrusions, cospatial and cotemporal with potassic mafic magmatism, and strike–slip displacement along the Ailao Shan-Red River shear zone in western Yunnan, SW China has been highly controversial. We report 22 new SHRIMP zircon U–Pb ages from 19 potassic felsic intrusions located as far as 150km east and 50km west of the shear zone to constrain temporal relationships. The results show that the felsic intrusions in western Yunnan were emplaced between 36.9±0.3Ma and 32.5±0.3Ma. This age range is significantly shorter than previous dating of 38Ma to 23Ma using K–Ar and Ar–Ar methods from 15 intrusions, indicating that the latter are reset or cooling ages. Potassic felsic intrusions are coeval with potassic mafic volcanic rocks (36.6Ma to 33.4Ma) in the same area, supporting a possible genetic link between the two end members. The dated intrusions occur over a 200km wide zone across the Ailao Shan-Red River shear zone; therefore they are unlikely genetically related to shearing. Rather, potassic mafic magmatism in western Yunnan may be related to delamination and melting of previously subduction-modified mantle lithosphere along the Mesozoic Jinsha suture reactivated in the Eocene–Early Oligocene, following the India-Asia continental collision; potassic felsic magmas were crustal melts hybridized with mafic liquids. Delamination in turn facilitated and localized initiation of the Ailao Shan-Red River shear zone at ca. 32Ma during continental extrusion.

The Cryogenian arc formation and successive high-K calc–alkaline plutons of Socotra Island (Yemen)

The Socotra Island belongs to the southern rifted margin of the Gulf of Aden and occupied in Neoproterozoic times a key position to constrain the age and the nature of the largely hidden Neoproterozoic rocks of the Arabian plate. Our integrated field, petrographic, geochemical and geochronological study in the Neoproterozoic rocks recognises three main successive events: (a) high-temperature ductile deformation and metamorphism forming probably in a compressive or transpressive regime; (b) mafic to intermediate intrusions as vertical sheets, kilometre-scale gabbro laccoliths, mafic dike swarm and lavas which present mainly a depleted arc signature with some evidences of evolution from an enriched-arc signature; (c) felsic intrusions mainly composed of highly potassic calc–alkaline and pinkish granites dated between 840 and 780 Ma. Relationships between the various petrographic types and U–Pb data suggest that these events occurred during a relatively short time span (80 Ma at max). Earlier high-temperature–low-pressure metamorphism stage as well as geochemical signature of mafic rocks show that development of Cryogenian formations of Socotra were controlled successively by an Andean-arc and a back-arc setting. These features cannot be easily reconciled with those of the Arabian–Nubian shield to the west of Socotra and of the Mozambique Belt to the south. We propose that the Socotra basement was developed at an active margin close to the India block in Cryogenian times.

Carboniferous mantle-derived felsic intrusion in the Chinese Altai, NW China: Implications for geodynamic change of the accretionary orogenic belt

This paper reports whole-rock Sr–Nd and zircon U–Pb and Hf isotopic compositions, as well as major and trace element data, for the granodiorites and their mafic microgranular enclaves (MMEs) in the Tuokesalei area, northwestern Chinese Altai. The granodiorites are characterized by peraluminous compositions (A/CNK=1.01–1.45), with relatively high SiO2 (64.4 to 70.9wt.%) and low MgO (0.75 to 1.97wt.%) contents. These rocks possess low total rare earth element (REE) contents (67–187ppm) with weakly negative Eu anomalies and mantle-like whole-rock Nd–Sr isotopic compositions (εNd(t)=+2.5 to +4.5; (87Sr/86Sr)i=0.7038 to 0.7048) and zircon εHf(t) values (+5.93 to +12.9). The MMEs contain abundant magmatic amphiboles and needle-like apatites, have low SiO2 (51.7 to 54.3wt.%) and high MgO (2.86 to 3.75wt.%) and REE contents (162–298ppm), and are characterized by moderately negative Eu anomalies (Eu/Eu*=0.40–0.60) and slightly depleted to enriched LREE ((La/Nd)N=0.74–1.41) and flat HREE ((Gd/Yb)N=1.16–1.39) patterns. The MMEs have whole-rock Nd–Sr isotopic compositions (εNd(t)=+2.6 to +2.7, (87Sr/86Sr)i=0.7034–0.7050) similar to those of the host granodiorites. Their similarities in crystallization age, mineral assemblage and isotopic composition document that the MMEs may represent an early crystallized phase, which was broken into pieces and brought to the shallow level during the host magma ascent and emplacement. Their precursor magmas were probably derived from an oceanic lithospheric mantle, which was characterized by pronounced Nd–Ta depletions. Mineral compositions of MMEs suggest that amphibole was a major early fractional mineral, which effectively differentiated the mantle-derived basaltic liquids to silica-rich melts. Zircon U–Pb dating yielded an age of 313±5Ma for the host granodiorites and 313±13Ma for their MMEs. The geochemical data of this study records an important episode of magmatism in an environment transitional from a subduction-related regime to a post-tectonic extension. Results of this study suggest that the accretionary orogenic process did not completely terminate until the Late Carboniferous to Permian in this region.

Paleoarchean detrital zircon ages from the earliest tectonic basin in the Barberton Greenstone Belt, Kaapvaal craton, South Africa

Detrital zircon grains in coarse-grained sedimentary rocks of the Noisy formation, (traditionally considered as the H6 unit of the uppermost Hooggenoeg Formation) in the Paleoarchean Barberton Greenstone Belt (BGB) South Africa, are investigated by laser ablation (LA)-ICP-MS to constrain their U–Pb and 207Pb/ 206Pb ages for depositional age and provenance determination. Drill core and field observations indicate that these rocks are clastic polymictitic diamictites and sandstones. A wide range in 207Pb/ 206Pb ages between ca. 3600 and 3430 Ma is reported. The youngest detrital zircon grain identified has an age of 3432 ± 10 Ma, constraining the maximum depositional age of the sedimentary sequence and the formation of a major underlying erosional unconformity. The youngest grains correspond in age to the ca. 3460–3430 Ma tonalite–trondjemite–granodiorite (TTG) gneisses of the Stoltzburg terrane and felsic intrusions of the uppermost Hooggenoeg Formation that constitute most of the Songimvelo Block of the BGB on the Kaapvaal craton. The 207Pb/ 206Pb ages of the oldest grains identified corresponds to plutonic sources identified in older proto-continental blocks of the Kaapvaal craton, namely the ca. 3509 Ma Steynsdorp Block and ca. 3667–3223 Ma Ancient Gneiss Complex (AGC). This new data indicates provenancial linkage between the Songimvelo Block and older proto-continental blocks exposed on the Kaapvaal craton at ca. 3432 Ma. It is argued here, that the Noisy formation is a remnant of the earliest tectonic basin in the BGB, that developed during major tectonic accretion and uplift at ca. 3432 Ma that lead to formation of the erosional unconformity. In conjunction with new drill core and field observations, the geochronological constraints presented here support plate-tectonic processes operating in the proto-Kaapvaal craton that resulted in its formation as a stable continental landmass as early as ca. 3432 Ma.

Geochronological, geochemical and Sr–Nd–Hf isotopic constraints on petrogenesis of Late Mesozoic gabbro–granite complexes on the southeast coast of Fujian, South China: insights into a depleted mantle source region and crust–mantle interactions

AbstractThe Quanzhou (QZ) and Huacuo (HC) gabbro–granite complexes on the southeast coast of Fujian, South China, are important components of a Late Mesozoic calc-alkaline volcanic–plutonic belt in the region. The complexes provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas, and their interactions within the intrusive environment. The complexes are composed mainly of monzogranite and biotite granodiorite in the QZ complex, and biotite granite in the HC complex, with lesser amounts of hornblende gabbro. Zircon U–Pb dating provides consistent crystallization ages of 109 ± 1 Ma and 108 ± 1 Ma for the QZ gabbros and monzogranites, and an age of 111 ± 1 Ma for the HC gabbro, which is contemporaneous with the spatially associated HC granites. Both the mafic and felsic intrusions in these complexes are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and are depleted in high-field-strength elements (HFSEs; e.g. Nb and Ta). They show similarly homogeneous Sr–Nd isotopic compositions. All these factors indicate a close genetic relationship between the gabbroic and granitic rocks in the QZ and HC complexes. Although the enriched Sr–Nd isotopic signatures of the QZ and HC gabbros seemingly point to an enriched mantle source (EM-1), they have highly variable zircon Hf isotopic compositions, with εHf(t) values ranging from negative to positive (specifically –4.6 to +6.1 for the QZ gabbros and –4.8 to +11.6 for the HC gabbros). We interpret the parental basic magmas of these gabbros to have received contributions from a depleted mantle source and crustal components. Contributions from such a depleted mantle source resulted in the growth of juvenile basaltic lower crust, the partial melting of which generated the parental felsic magmas of the QZ and HC complexes. Furthermore, based on a synthesis of petrography, geochronology, elemental and isotopic geochemistry and tectonics, we propose that break-off and rollback of the Late Mesozoic subducted Palaeo-Pacific Plate triggered the upwelling of asthenospheric mantle below the coastal area of the South China Block, which induced extension of the overlying continental lithosphere, and finally initiated the large-scale Late Yanshanian magmatism in the study area.

Geodynamic constraints on orebody localization in the Anqing orefield, China: Computational modeling and facilitating predictive exploration of deep deposits

The Anqing Fe–Cu skarn deposit, with an age of 134 to 142 Ma and resources of 62.4 Mt at 0.906% Cu and 32.2% Fe, is one of the most important deposits in the Yangtze River Metallogenic Belt, East China. To better understand the localization of orebodies and thus facilitate predictive exploration of deep orebodies, computational modeling is used to simulate the coupled geodynamic processes during the syn-tectonic cooling of the ore-related intrusion, based on geological and geophysical investigations in the Anqing orefield. The occurrences of the ore veins and veinlets in diorite and skarn, as well as the sharp zigzag boundary of the orebody, indicate that the Cu ores were deposited after the solidification of the diorite and skarn formation, and were located in some tensional structural spaces that are unevenly distributed along the contact zone between the felsic intrusion and sedimentary carbonates. The locations of orebodies are closely associated with the contact zone shape. The computational results of two models with two typical contact-shapes show that pore fluid flow was focused into the dilation zones from different sources. All the significant dilation zones, in which the existing orebodies were located, are distributed in some specific places of the south contact zone of the intrusion. In addition, these dilation zones are closely related to the contact zone shape of the intrusion and can control the location of orebodies through the coupled mechano-thermo-hydrological processes during cooling of the intrusion in the extension setting. The skarns are not critical for controlling the localization of orebodies. This means that exploration for deep ore should target deep dilation zones close to the contact boundary of the intrusion. Such recognition may provide a useful guide in selecting exploration targets in the Anqing orefield. As a direct result of computational modeling, an orebody has been discovered in the deep dilation zone in this orefield. It demonstrates that computational modeling is a promising tool for understanding the metallogenic processes and for facilitating the deep exploration of hidden orebodies that are related to intrusions.

Gravity evidence for a mafic intrusion beneath a mineralized zone in the Bondy gneiss complex, Grenville Province, Quebec — Exploration implications

A ground gravity survey over the Bondy gneiss complex and its mineralized iron oxide- and copper-rich hydrothermal system(s) in the Grenville Province of SW Quebec was undertaken to aid mineral exploration in mapping subsurface intrusions. Several kilometric-scale positive Bouguer anomalies were identified that coincide with outcropping mafic and intermediate intrusive rocks of the post peak-metamorphic, 1.17–1.16 Ga mafic to intermediate Chevreuil suite intrusions and a 1.09–1.07 Ga Rolleau ultramafic stock. An additional 4 × 3 kilometre positive gravity anomaly indicates a mafic body underlies part of the metamorphosed hydrothermal system in the area of magnetite , pyrite , pyrrhotite , and chalcopyrite mineralization . Advanced argilic alteration associated with sulphide enrichment here is however indicative of an epithermal system with a felsic intrusion fluid source. As a felsic intrusion cannot explain the positive Bouguer gravity anomaly both felsic and mafic bodies must be present beneath the mineralized zone. Our preferred interpretation based on integrating gravity data and 2D forward gravity modelling with the results of field and geochemical studies is that this anomaly corresponds to a ca. 500 m deep mafic 1.17–1.16 Ga Chevreuil suite pluton that may have provided the source for hydrothermal fluids associated with late ductile shear- and fault-related mineralization or remobilization of early mineralization associated with a felsic pluton into late structures. This interpretation is compatible with gabbro xenoliths in the 1.07 Ga Rivard lamprophyre dyke on the NW margin of the gravity anomaly that bear significant similarities with those of the Chevreuil intrusive suite. The presence of both early felsic and late mafic intrusions beneath a group of three mineral occurrences in the Bondy gneiss complex strengthens their prospectivity in comparison to other mineral occurrences in the area. That early, pre-metamorphic mineralization was upgraded late in the tectonothermal evolution during a subsequent igneous and deformation event agrees with interpretations of other IOCG-style deposits in the Lac Manitou area of the eastern Grenville Province. ► A ground gravity survey provides new data that aids mineral exploration. ► Most gravity anomalies are associated with mafic to intermediate intrusive rocks. ► A Gravity anomaly over mineralized outcrops may be generated by a deep gabbro. ► Mafic intrusion may be the driving source for mineralization or remobilisation.