Felsic Intrusions Research Articles

Origin of the post-collisional carboniferous granitoids associated with the Azhahada Cu-Bi deposit in Inner Mongolia, Northeast China and implications for regional metallogeny

The Azhahada quartz vein-type Cu-Bi deposit is located in the Erenhot-East Ujimqin polymetallic metallogenic belt, on the western part of the Great Xing’an Range, Inner Mongolia, northeast China. The Cu-Bi veins are hosted by volcanic and metasedimentary rocks in the Devonian Niqiuhe Formation and Carboniferous felsic intrusions (monzogranite, syenogranite, and granite). Three hydrothermal ore stages (I-III) for Cu-Bi formation at Azhahada have been identified. Chalcopyrite and bismuthinite are the dominant ore minerals and mainly occur in vein stage II (quartz–pyrite ± chalcopyrite ± bismuthinite veins). New LA-ICP-MS zircon U-Pb dating indicates the monzogranite, syenogranite, and granite emplaced between 313.9 ± 1.7 and 308.4 ± 1.5 Ma. The monzogranite, syenogranite, and granite show the same geochemical affinity with I-type granites, which are characterized by high SiO2, K2O, and Al2O3 contents, low MgO, TiO2, and P2O5 contents, negative Eu anomalies (EuN/EuN* = 0.04–0.57), and strong enrichment in Rb, K, Pb, and Nd. Moreover, these granitic intrusions have low Mg# values of 22 to 39, low (87Sr/86Sr)i ratios (0.7023 to 0.7053), εNd(t) values between −0.8 and +0.4, positive εHf(t) values of +1.6 to +10.4, suggesting that their parental magmas were most likely derived from partial melts from the lower crust involving some mantle materials in a post-collisional tectonic setting. Sulfide samples from Azhahada have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios similar to those of granitic intrusions. The silicon isotope compositions of the quartz samples (−0.3 to −0.1‰) are highly consistent with values of the granitic intrusions (−0.4 to −0.1‰), indicating that the silicon was derived from the granitic magma. Combined our whole-rock geochemical and Sr–Nd–Hf–Pb–Si isotopic data with previous investigation of regional tectonic evolutions, we suggest that the Azhahada granitoids genetically associated with the Cu-Bi mineralization were generated by the postorogenic lithospheric extension and magmatic underplating during the Late Carboniferous. A post-collisional extensional event coupled with the rising of hot mantle derived melts triggered partial melting of the lower crust, as well as provided metals (Cu and Bi).

Mineralogical and geochemical constraints on the origin of the variable REE enrichments in the Kangdian IOCG province, SW China

Iron oxide copper–gold (IOCG) deposits worldwide have been documented to be variably enriched in REEs in addition to Fe, Cu and Au metals, but the key factors controlling REE enrichments have long been poorly constrained. In this study, we conducted mineralogical and geochemical investigations on the timing and genesis of the REE enrichments in the Lala and Dahongshan deposits, the two largest IOCG deposits in the Kangdian metallogenic province, SW China. Both deposits have comparable nature of hosting rocks and a paragenetic sequence composed of early magnetite, accompanied with regional sodic alteration, and late Cu-sulfide mineralization, associated with K-Fe alteration, but exhibit different styles and grades of REE enrichments. The REE enrichments in Lala is close to industry-level, presenting as monazite with subordinate bastnaesite, allanite, and apatite of which apatite is mainly associated with the Fe mineralization stage, while others are co-existing with the Cu mineralization stage. In contrast, REE enrichments in Dahongshan is dominated by apatite that is mainly associated with magnetite of the Fe mineralization stage. Our new U-Pb dating of monazite from the Lala deposit yields a Neoproterozoic (~850 Ma) age. Such an age is slightly younger than previously obtained molybdenite Re-Os and allanite U-Pb ages of ~ 1.05 Ga, indicating that it should represent a fully or partially reset age that is related to regional Neoproterozoic (890–740 Ma) overprint event. On the other hand, previous zircon U-Pb dating show that the REE enrichments in the Dahongshan deposit was formed at ~ 1.66 Ga.Our in-situ Nd isotopic analyses show that the Lala monazite has εNd(t) values (calculated at 1080 Ma) ranging from −7.5 to −2.9, comparable to those of hosting rocks (7.2–0.3 at 1080 Ma) and partially overlapping contemporaneous felsic intrusions (−7.2 to 4.4), indicating hybrid sources of REEs from hosting rocks and/or coeval magmas. On the other hand, apatite grains in the Dahongshan deposit have εNd(t) values (calculated at 1660 Ma) ranging from −17.8 to −3.5, broadly similar to Archean besement rocks (−16.7 to −7.0 at 1660 Ma) in the Yangtze Block, but different from hosting rocks (2.9 to 4.6 at 1660 Ma) and contemporaneous mafic rocks (−0.2 to 4.0). In combination with textural observations (e.g., monazite-bearing apatite in Lala), this study further confirmed that different nature of Cu mineralizing fluids is the key factor controlling mobilization and leaching of REEs from the hosting rocks with similar lithologies. Available studies regarding fluid inclusion indicated that the fluids in both deposits are broadly similar, rich in Cl- and CO2 (e.g., HCO3− and CO32−). However, the presence of abundant fluorite in the Lala deposit (absent in the Dahongshan deposit) implies that the fluids in Lala should be distinctly rich in F. This conclusion is also supported by fact that the apatite in Lala has F and F/Cl values much higher than those of Dahongshan. Therefore, the relatively F-rich nature, which was generally considered to be more capable for breakdown of REE minerals, plays a key role for the extensive REE mobilization and mineralization in the Lala deposit. This study highlights that variable REE enrichments in IOCG deposits were likely mainly controlled by REE budgets of wall rocks and the nature of mineralizing fluids.

Neoproterozoic tectonics of the Jiangnan orogen: The magmatic record of continental growth by arc and slab-failure magmatism from 1000 to 780 Ma

The Neoproterozoic NE-striking Jiangnan orogen in the South China Block (SCB) separates the Cathaysia microcontinent on the southeast from the Yangtze craton in the northwest. The origin and evolution of the Jiangnan orogen is unresolved, with individual models explaining part of the history, such as various magmatic, deformational, metamorphic, and sedimentary events including extensional, contractional, and transpressional modes. Here, we present detailed geochronological and Nd-Hf isotopic data for Neoproterozoic mafic and felsic intrusions in the Jiangnan orogen and identify three episodes of Neoproterozoic magmatism. The first stage ranging from ~1013–942 Ma is preserved predominantly in the eastern segment of the orogen, including a series of mafic rocks with ages of ~1010–952 Ma, contrasting with weak magmatic activity in the western segment with a peak at ~997 Ma. The second stage ranges from ~906 Ma to ~820 Ma, with intensive and extensive bimodal magmatism occurring in the eastern and western parts. The last stage mainly occurred in the western Jiangnan orogen in the period from ~800 to 780 Ma, associated with drastic mafic magmatism and weakly bimodal volcanic activities. The bimodal volcanism in the eastern segment exhibits a striking weak trend after ~823 Ma, distinctly different from the western segment where the magmatism lasted until ~785 Ma, indicating its syn- to post-collisional in nature. The pre-800 Ma magmatic rocks in western Jiangnan orogen have a dominant negative Nd isotopic signature but exhibit a significant positive trend after 800 Ma, different from those in east, which have a decreasing trend. Both parts display a roughly decoupled Nd-Hf isotopic character presented by a dominant positive pattern in Hf isotopes of the Neoproterozoic intrusive rocks, indicating a significant addition of previous slab-derived components or accretionary wedges in the pre-800 Ma magma evolution, and more depleted materials involved in the source in the later stages. Oceanic subduction under the northwestern Cathaysia Block occurred around ~1.0 Ga, slightly earlier than the subduction to the southeastern Yangtze. A gradual tectonic transition from post-orogenic extension to intracontinental rifting dominated the tectonics in Jiangnan orogen in the middle-late Neoproterozoic, followed by slab failure after ~790 Ma. Arc magmatism was the dominant source of crustal growth before 820 Ma in the SCB, but was exceeded in volume by later slab failure magmatism after ~790 Ma.

Vein topology, structures, and distribution during the prograde formation of an Archean gold stockwork

Abstract The Archean Cheechoo stockwork gold deposit is hosted by a felsic intrusion of tonalitic-granodioritic composition and crosscutting pegmatite dikes in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior craton). The evolution of the stockwork is characterized herein using field relationships, vein density, and connectivity measurements on drill core and outcrop zones. The statistical distribution of gold is used to highlight mechanisms of stockwork emplacement and gold mineralization and remobilization. Two statistical populations of gold concentration are present. Population A is represented by gold grades below 1 g/t with a lognormal cumulative frequency. It is widespread in the hydrothermally altered (albite and quartz) and mineralized facies of the pluton. It is controlled by the development of quartz-feldspar-diopside veins as shown by the similar lognormal distribution of grades and vein density and by the correspondence of grades with network connectivity. Diopside and actinolite porphyroblasts in deformed veins within sodic and calcsilicate alteration zones are evidence for auriferous vein emplacement prior to the amphibolite facies peak of metamorphism. Population B (>1 g/t) is erratic and exhibits a strong nugget effect. It is present throughout the mineralized portion of the pluton and in pegmatites. This population is interpreted as the result of gold remobilization during prograde metamorphism and pegmatite emplacement following the metamorphic peak. The pegmatites are interpreted to have scavenged gold emplaced prior to peak metamorphism. These results show the isotropic behavior of the investigated stockwork during regional deformation and its development during the early stages of regional prograde metamorphism.

Petrogenesis and U-Pb zircon geochronology of migmatitzation during Neo-Tethyan Jurassic magmatic arc extension: The Boroujerd example, western Iran

The migmatites in the Boroujerd region, Western Iran are spatially associated with felsic and mafic magmatism. However, the genetic relationship between partial melting and magmatism in the region is unclear. Four metapelite migmatite samples (two melanosome and two leucosome) were selected for U-Pb zircon geochronology to determine the timing of anatexis. Major and trace element compositions of leucosome in the migmatites are consistent with being derived from biotite-breakdown partial melting of metapelites and not from the injection of granites associated with the Boroujerd igneous complex. Cores of zircon from metapelitic migmatite are inferred to be detrital in origin and yield ages of 320–250 Ma. The age of the detrital zircons is compatible with the timing of Paleozoic and Mesozoic magmatism in the Central Iranian micro-plate. The pelitic protolith of the migmatites in the Boroujerd region were probably originated from the Mesozoic continental margin of the Central Iranian microplate. Zircon rims and neoblastic grains yield ages of 220–210 Ma and 170–160 Ma. The geological significance of the older age population is unclear, but the younger population is similar to that of mafic and felsic intrusions in the area and is interpreted to record the general timing of high-temperature–low-pressure contact metamorphism and partial melting. We infer that the Middle Jurassic mafic and felsic magmatism was the principal heat source for the metamorphism and partial melting of the Boroujerd migmatites in response to the subduction of Neotethyan oceanic lithosphere.

Late Cretaceous felsic intrusions in oceanic plateau basalts in SW Ecuador: Markers of subduction initiation?

Felsic magmatic rocks (tonalites and trondhjemites) with calc-alkaline affinity were emplaced in SW Ecuador (Pascuales area; North Guayaquil) at ~87–89 Ma intruding the top of the basalts of the Caribean Large Igneous Province (CLIP), prior to the development of typical well-known island arcs from ~85 to 80 Ma onwards. A clear depletion of REE distinguishes the Pascuales felsic rocks from the more differentiated lavas of the Rio Cala Arc in Ecuador and from other Late Cretaceous arc-like felsic intrusives emplaced within the CLIP in Colombia and the Antilles. In addition, the Pascuales tonalites have very low K2O (<0.4 wt%), TiO2 (<0.4 wt%) and P2O5 (<0.15 wt%) contents compared to dacitic rocks of the Rio Cala Arc. In contrast, their geochemistry and their similar age suggest that the Pascuales tonalites are an equivalent of the Las Orquideas dacitic volcanic rocks. The Pascuales felsic rocks have the less radiogenic Nd compositions of the whole spectrum of Ecuadorian oceanic plateau rocks, nevertheless shearing similarities with some Totoras amphibolites. The geochemical characteristics of the Pascuales intrusives and Las Orquideas dacites are similar to those formed by hydrous partial melting of mafic protoliths. The dacitic magmas must have been in equilibrium with residual mineral assemblages largely dominated by amphibole + Fe–Ti oxides (no garnet). This requires two conditions: 1) low-pressure melting under high water content and 2) a source rock extremely depleted in minor incompatible elements (K, Ti, P, Y) and REE. These conditions are fulfilled by partial melting of mafic-ultramafic cumulates undergoing high temperature and shear deformation at the base of the oceanic plateau triggered by new basaltic intrusions. The Pascuales felsic rocks emplaced at the SW border of the oceanic plateau during its final stage of building; they can thus be related to asthenopheric upwelling during the initial stage of island arc magmatism.

Middle Paleozoic archipelago amalgamation and tectonic transform in the northern West Junggar, NW China: Constraints from magmatism and deformation

The West Junggar (WJ) is generally accepted as a Paleozoic archipelago-type orogenic system and records a multiple arcs amalgamation history; however, some of the associated processes require further examination. In this paper, we conducted detailed field investigations and large-scale mapping in the Saier Mountains, northern WJ, and identified several episodes of magmatism and deformation events. The felsic rocks (dioritic porphyrite and granitic porphyry), characterized by a dense cleavage, were emplaced at the Early–Middle Silurian (433–423 Ma). These rocks exhibit geochemical characteristics comparable to those of the coeval adakite-like rocks and A2-type granites in the Xiemisitai Mountains. The undeformed mafic dike swarms yielded SHRIMP zircon age of 382 ± 6 Ma and intruded the Hebukesaier ophiolitic mélange (HOM) as well as the surrounding rocks. The HOM, Silurian felsic intrusions and pyroclastic rocks (422 ± 9 Ma) in the Saier Mountains commonly display cleavage attributed north–south shortening during 422–382 Ma. Deep-sea bedded cherts and turbidites in the area also experienced deformation and exhibited cleavage, with subduction-related fold deformation partially preserved in the HOM. The fold deformation was greatly related to a subduction-accretion process of the Hongguleleng–Balkybey Ocean during the Ordovician. Considering the geochronology and geochemistry data, the extensive Silurian–Early Devonian magmatism in the northern WJ indicates a prolonged (~40 Ma) and episodic post-collisional extension in this orogen and records an obvious trend of crustal temperature increasing and thickness thinning from 435 to 410 Ma. The regional extrusion deformation was attributed to the southward subduction of the Irtysh–Zaysan Ocean during the Middle Devonian, and then the slab roll-back offered an extensional setting which favored the emplacement of mantle-derived mafic dike swarms. We propose that the northern WJ probably experienced a complicated evolution involving archipelago amalgamation and tectonic transform in the middle Paleozoic, providing a new sight to reveal the orogenic process of the Central Asian Orogenic Belt (CAOB).

The Origin of Felsic Intrusions Within the Mantle Section of the Samail Ophiolite: Geochemical Evidence for Three Distinct Mixing and Fractionation Trends

AbstractAn isotopically diverse suite of felsic dikes, sills, and plugs (εNd(t) = −7.8 to +7.8) intrude the uppermost mantle and lower crust in the Samail ophiolite in Oman and the United Arab Emirates (UAE). These features have been interpreted to represent amphibolite and metasediment melts from an underthrust sheet of oceanic lithosphere. As such, the intrusions provide a record of melting of oceanic crust and sediment at depth, with implications for mass transfer from the down‐going plate in young, hot subduction settings. Several studies have used geochemical data to constrain the magmatic sources of the dikes; here we build on this previous work using integrated whole rock major element, trace element and Nd isotopic data from a more geographically extensive suite of dikes. New and existing data suggest the felsic intrusions preserved within mantle peridotites in the Oman portion of the ophiolite were generated by three distinct mixing and fractionation trends: (1) three‐component mixing between sediment melt, amphibolite melt and a mantle component; (2) two component mixing between amphibolite and sediment melts, with little mantle contribution; and (3) fractional crystallization of depleted, mantle derived magmas, likely related to the ophiolite V2 volcanic series. Combined geochemical and pseudosection modeling suggest that amphibolite melting occurred at P ≤ 1.4 GPa (∼40–45 km) and T ≥ 700–750°C. Felsic intrusions in the mantle section in the UAE, including garnet‐andalusite‐cordierite leucogranites, follow similar mixing trends, but crystallized ∼0.9–4.0 Ma after the Oman intrusions.

Early Mesozoic crustal evolution in the NW segment of West Qinling, China: Evidence from diverse intermediate–felsic igneous rocks

Continental convergent margins are commonly considered as significant sites for understanding crustal reworking and growth, but the geodynamic processes involving crustal growth remain highly debatable. Triassic intermediate–felsic intrusive rocks are widespread in the northwestern segment of the West Qinling orogenic belt on the NE Tibetan Plateau. These rocks well document the evolution of one branch of the Paleo-Tethys oceans (i.e., the A'nyemaqen Ocean) in East Asia. Here we present a comprehensive study of zircon UPb and LuHf isotopic data, along with whole-rock elemental and SrNd isotopic data to decipher the origins of intermediate–felsic intrusions and crustal evolution of the West Qinling in the Triassic. Two episodes of magmatism were detected in this study, i.e., the early high-Mg diorite porphyry–granodiorite–granodiorite porphyry assemblage (Episode #1, ~244–230 Ma), followed by a later monzogranite–granodiorite–granite porphyry assemblage (Episode #2, ~228–203 Ma). These two episodes of magmatism are interpreted as products related to oceanic subduction and continental collision, respectively. In Episode #1, the mafic magmatic enclaves were mainly mantle-derived and were indicative of magma mixing. The high-Mg diorite porphyries were likely derived from partial melting of the mantle wedge metasomatized by sediment-derived melts, whereas the coeval granodiorites and granodiorite porphyries mainly originated from partial melting of amphibolites with significant addition of mantle-derived components. By contrast, the granodiorite porphyries in Episode #2 probably originated from the lower crust-sourced melts with insignificant mantle contribution. The contemporaneous granodiorites and monzogranites were generated by partial melting of the crustal amphibolites or metagreywackes. The proportion of positive zircon εHf(t) values decreased from Episode #1 to Episode #2, indicating a decreasing tendency of the mantle or juvenile component contribution. Our study suggests that the addition of mantle-derived magmas to the continental crust during oceanic subduction processes played an important role in crustal growth.

U‐Pb Zircon and Re‐Os Molybdenite Geochronology of the W‐Mo Mineralized Region of South Qinling, China, and their Tectonic Implications

AbstractA W‐Mo mineralized region is located along the northern margin of the South Qinling tectonic belt of China. W‐Mo mineralization occurs mainly in Cambrian–Ordovician clastic and carbonate rocks, and the ore bodies are structurally controlled by NW–SE‐ and NNE–SSW‐striking faults. Evidence for magmatism in the area is widespread and is dominated by intermediate–felsic intrusives or apophyses, such as the Dongjiangkou, Yanzhiba, Lanbandeng, and Sihaiping granitic bodies. Quartz‐vein‐type mineralization and fault‐controlled skarn‐type mineralization dominate the ore systems, with additional enrichment in residual deposits. At present, there are few or insufficient studies on (1) the age of mineralization, (2) the relationship between intermediate–felsic granite and W‐Mo mineralization, (3) the source of ore‐forming materials, and (4) the metallogenic and tectonic setting of the mineralized area. In this paper, we present geochronology results for numerous intrusive granitic bodies in the South Qinling tectonic belt. U‐Pb zircon geochronology of the Lanbandeng monzogranite and Wangjiaping biotite monzogranite yields ages of 222.7 ± 2.3 and 201.9 ± 1.8 Ma, respectively. In contrast to the Late Triassic age of the Lanbandeng monzogranite, the age of the newly discovered Wangjiaping biotite monzogranite places it at the Triassic–Jurassic boundary. Re‐Os molybdenite geochronology on the Qipangou W‐Mo deposit yielded a model age of 199.7 ± 3.9 Ma, indicating the deposit formed in the early Yanshanian period of the Early Jurassic. Granitoid intrusions in the mineralized area are characterized by composite granite bodies that crystallized at ca. 240–190 Ma. While there were multiple stages of intrusion, most occurred at 210–220 Ma, with waning magmatic activity at 200–190 Ma. The Re‐Os age of molybdenite in the region is ca. 200–190 Ma, which may represent a newly discovered period of W‐Mo metallogenesis that occurred during the final stages of magmatism. The heat associated with this magmatism drove ore formation and might have provided additional ore‐forming components for metallogenesis (represented by the Wangjiaping biotite monzogranite). Ore materials in the mineralized area were derived from mixed crustal and mantle sources. Enrichment of the region occurred during intracontinental orogenesis in the late Indosinian–Yanshanian, subsequent to the main Indosinian collision. At this time, the tectonic environment was dominated by extension and strike‐slip motion.

Formation and Evolution of a Neoproterozoic Continental Magmatic Arc

Abstract Unlike many Archean diorites and granitoids that arguably formed in different geodynamic settings, their post-Archean counterparts are commonly regarded to have formed at convergent margins, although in detail their petrogenesis remains contentious. Here we present new whole-rock data and zircon Hf–O isotope analyses from dioritic (750–730 Ma), granitic (810–790 Ma) and tonalite–trondhjemite–granodiorite (TTG)-like intrusions (800–740 Ma) from the Panxi and Hannan regions, which form part of an extensive Neoproterozoic convergent margin exposed in South China. The dioritic rocks from the Panxi region exhibit high zircon εHf(t) (+10.1 to +13.1) and sub-mantle to mantle-like δ18O (3.1–6.3 ‰) values, whereas those from the Hannan region preserve low εHf(t) (+4.1 to +8.1) and high δ18O values (5.9–6.6 ‰), indicating that the dioritic melts were derived from subduction-modified lithospheric mantle sources and experienced variable degrees of lower crustal contamination. Zircons within granite and TTG from the Panxi region show a narrow range of Hf isotopic compositions generally spanning 2–4 εHf units (+3.1 to +7.9 for most felsic intrusions). By contrast, those from the Hannan region show a much wider range of zircon εHf(t) values spanning almost 10 εHf units (+1.1 to +10.9). Based on their O–Hf–Nd isotopic signatures, we propose that the granite and TTG from both areas were derived through partial melting of hydrated basaltic rocks in the arc root, and that the isotopic variability between the intrusions mirrors spatial and temporal chemical variations in these deep crustal source rocks. In both regions, the granites, along with mantle-derived mafic–ultramafic and intermediate rocks, show a coupled evolution associated with increasing εNd(t) and εHf(t) and decreasing δ18O with decreasing ages, whereas the TTGs formed during late-stage arc magmatism and preserve relatively homogeneous Nd–Hf isotopes and mantle-like δ18O values. Combined, these data record continuous crustal thickening through underplating of juvenile magmas and a progressive increase in the depth of melting, along with a decrease in the degree of interaction between the melts and basement rocks within the arc root. Our results suggest that slab melting was not required to produce post-Archean TTG signatures. Further, we suggest that the variability in the Hf–O–Nd isotopic compositions of metaluminous (I-type) granites mostly does not reflect a heterogeneity in upper mantle signatures, and that there is no conclusive evidence for the involvement of partial melts of subducted sediment based on Hf–O isotope signatures in zircon.

Petrogenesis of the 130 Ma Taolin granitic intrusion: Implications for the tectonic setting and diversity of Early Cretaceous felsic rocks in the Sulu orogenic belt, eastern China

• The parent magma of the ~130 Ma Taolin intrusion was derived from melting of the Yangtze crust. • The Early Cretaceous igneous event in the Sulu orogenic belt was related to an extensional setting. • Variable protoliths and complex magmatic processes led to the diversity of felsic rocks. Early Cretaceous felsic intrusions are widely exposed in the Sulu orogenic belt (SOB), eastern China. The Taolin granitic intrusion is an example of these intrusions and mainly comprises monzogranite, syenogranite, and granite. Here we present new geochronological and geochemical data for the intrusion. The ~130 Ma Taolin intrusion belongs to normal to fractioned I-type granite and is divided into two groups based on geochemical differences. Rocks in the Daxiezhuang and Malingzhan units belong to Group 1 and have relatively low SiO 2 (61.35–69.44 wt%), high Sr/Y ratios (26.04–60.18), low εHf(t) values (−27.2 to −21.4), and old T DM2 ages (2541–2903 Ma). They also display weak to no negative Eu anomalies (Eu/Eu*=0.65–1.29). Rocks from the Shibu and Tiqiushan units belong to Group 2 and have relatively high SiO 2 (76.38–77.68 wt%), low Sr/Y ratios (0.61–12.38), high εHf(t) values (−24.3 to −17.2) and young T DM2 ages (2271–2722 Ma). They have obviously negative Eu anomalies (Eu/Eu*=0.10–0.42). The partial melting of the Yangtze continental crust at different depths led to variable elemental and isotopic compositions of magma in Group 1 and Group 2. The SOB was in an extensional environment during the Early Cretaceous, which allowed the formation of felsic magmatic rocks with low Sr, Sr/Y, and Sr/CaO values. Early Cretaceous felsic magmatic rocks in the SOB were mainly sourced from the continental crust by crustal reworking. There are diverse rock types with variable geochemical features in the SOB, which is ascribed to the heterogeneity of the continental crust and the complex evolution of felsic magma.

Permian to Triassic tectonic evolution of the Alxa Tectonic Belt, NW China: Constraints from petrogenesis and geochronology of felsic intrusions

The Alxa Tectonic Belt (ATB) is located in the southern Central Asian Orogenic Belt (CAOB), and its tectonic affinity and evolutionary history related to the closure of the Paleo-Asian Ocean are not fully understood. To refine our understanding of these issues, an investigation that includes petrographic observations, whole-rock geochemistry, zircon UPb dating and Hf isotopic analyses was carried out on four felsic intrusions in the Zongnaishan–Shalazha Zone (ZSZ) and the Nuoergong–Honggueryulin Zone (NHZ) within this region. Despite their lithological differences, the four intrusions are identified as metaluminous, calc-alkaline to high-K calc-alkaline I-type granitoids that were derived from partial melting of heterogeneous crustal basement without significant fractional crystallization and crustal contamination. Zircon LA-ICP-MS UPb dating results suggest that they were emplaced from the late Permian to the early Triassic (i.e., 247.7 ± 0.7 Ma for the Shalazha granite, 253.2 ± 0.9 Ma for the Shalazha granodiorite, 253.3 ± 0.6 Ma for the Nuoergong granite, and 251.6 ± 0.9 Ma for the Nuoergong tonalite). Geochemically, these granitoids show typical signatures of arc magma, such as enrichments in Rb, Th, K, and Pb, depletions in Zr, Nb, and Ta, and low Sr/Y and (La/Yb)N values. On tectonic discriminant diagrams, they plot in volcanic arc to post-collisional fields, indicating a tectonic transition from oceanic subduction to closure in the late Permian to early Triassic. Interestingly, granitoids from the ZSZ have positive εHf(t) values ranging from +1 to +7.9 with young two-stage crustal ages (TDMC) of 781–1209 Ma, in sharp contrast to the extremely negative εHf(t) values (−15.8 to −8.7) and old TDMC ages (1831–2272 Ma) of the granitoids in the NHZ. In addition, the zircon Hf isotopes of the ZSZ and NHZ granitoids show similarities to those of the magmatic rocks in the southern CAOB and western North China Craton, respectively, implying that the northern and southern ATB have different tectonic affinities. Thus, we propose that the Paleo-Asian Ocean lasted subduction in the late Permian and may close in the early Triassic, with subsequent accretion of the northern juvenile exotic microcontinent (ZSZ) to the southern old crustal basement (NHZ).

Evidence of the latest Paleoproterozoic (~1615 Ma) mafic magmatism the southern Siberia: Extensional environments in Nuna supercontinent

The paper presents a new geological, geochronological, geochemical and Nd isotopic data on the latest Paleoproterozoic dolerite sills, exposed in the Biryusa inlier of the southern Siberian craton among the sandstones of sedimentary sequence traditionally suggested as Neoproterozoic. One dolerite sill yielded U-Pb (ID-TIMS) baddeleyite concordia age of 1613 ± 5 Ma, which interpreted as time of their emplacement. New geochronological data allow reconsider previously reported Neoproterozoic age of sedimentary sequence hosting dolerite sills studied and suggest for them at least Paleoproterozoic age. The dolerites correspond to subalkaline and alkaline tholeiitic basalts and less often subalkaline tholeiitic basaltic andesite according to their major-element compositions with lower to moderate mg#, varying from 26 to 58. All dolerites demonstrate a similar slightly negative ɛNd(t) values range from −2.2 to −2.5. Geological, geochemical and Nd isotopic data suggest that mafic melts responsible for genesis of the dolerites were generated in an intracontinental extensional setting from homogeneous mantle source produced by interaction between OIB and ACR-like magmas. Geochemical features of the studied dolerites together with geological observations, namely – a presence of coeval rift-related sedimentary strata in the neighboring Urik-Iya graben (paleorift), suggest an overall extensional tectonic environments in the southern part of the Siberian Craton at 1.65–1.60 Ga. Synthesis of data on ca. 1.65–1.58 Ga magmatism around the world showed that the spatial distribution of various ca. 1.65–1.58 Ga mafic and felsic intrusions, including dyke swarms, anorogenic granites, as well as intracontinental basins within Nuna supercontinent suggest the overall extensional tectonic environments, probably related to the retreat of subduction slabs surrounded the supercontinent.

Geochemistry and U–Pb geochronology of the Williams Brook area, Tobique–Chaleur zone, New Brunswick: stratigraphic and geotectonic setting of gold mineralization

Gold mineralization at Williams Brook in northern New Brunswick is hosted within the Siluro-Devonian, bimodal, volcano-sedimentary rocks of the Tobique–Chaleur Zone (Wapske Formation). Gold mineralization occurs in two styles: (1) as disseminations (refractory gold) in rhyolite, and (2) in cross-cutting quartz veins (free gold). Dating of the felsic volcanic host rocks by in situ LA–ICP–MS zircon U–Pb geochronology returned ages of 422 ± 3, 409 ± 2, 408 ± 3, 405 ± 2, and 401 ± 9 Ma. Zr/Y of subvolcanic felsic intrusion (&lt;8 for syn-mineralization and &gt;8 for post-mineralization) suggests evolution from transitional to more alkalic affinities. Two mineralizing events are recognized; the first is a disseminated mineralization style formed at ∼422–416 Ma and the second consists of quartz-vein-hosted gold emplaced at 410–408 Ma. Felsic rocks from Williams Brook and elsewhere in the Tobique Group (i.e., Wapske, Costigan Mountain, and Benjamin formations), and the Coastal Volcanic Belt have similar Th/Nb ratios of ∼0.1 to 1, reflecting similar levels of crustal contamination, and similar Nb and Y content, suggesting A-type affinities. These data indicate a similar environment of formation. Regionally, mafic rocks show similar within-plate continental signatures and a E-MORB mantle source that formed from partial melts of 10%–30%. Mafic volcanic rocks from Williams Brook have a more alkaline affinity (based on Ti/V) and derivation from lower percentage partial melting (∼5%). The chemical and temporal variations in the Williams Brook rocks suggest that they were erupted in an evolving transpressional tectonic setting during the oblique convergence of Gondwana and Laurentia.

Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides

ABSTRACT Recent studies in the context of the International Continental Scientific Drilling Project “Collisional Orogeny in the Scandinavian Caledonides” have focused on the importance of the Seve Nappe Complex (SNC) for understanding the subduction history of the Baltoscandian margin during closure of the Iapetus Ocean. In the classical Åre area of western central Jämtland, granulite facies migmatites and leucogranites of the Åreskutan Nappe provide evidence of Early Silurian (c. 440 Ma) high temperature metamorphism and a previous prograde, ultra-high pressure history, with microdiamonds. New LA-ICPMS zircon isotope age investigations of the underlying amphibolite facies Lower Seve Nappes, reported here, have also identified an Early Silurian tectonothermal history with pegmatitic leucogranite (c. 443 Ma) and, at lower structural levels, another felsic intrusion of earliest Middle Ordovician age (c. 469 Ma). The latter intrudes isoclinally folded host rock amphibolites and calcareous psammitic paragneisses and is itself tightly folded. Zircons in an amphibolite proved to be highly discordant but indicate Early Silurian metamorphism during isoclinal folding. Detrital zircons in a paragneiss are dominated by Sveconorwegian populations, but also include a range of younger Neoproterozoic grains down to the Early Ediacaran (c. 600 Ma). This new evidence of early Caledonian deformation and metamorphism indicates that, as farther north in the orogen, the Seve tectonothermal history in central Jämtland probably started early in the Ordovician, or before. Subduction and accretion along the Baltoscandian outer margin occurred prior to Scandian continent-continent collision, with Siluro-Devonian emplacement of the SNC across the foreland basins onto the Baltoscandian platform.

Textural, trace elemental and sulfur isotopic signatures of arsenopyrite and pyrite from the Mandongshan gold deposit (west Junggar, NW China): Implications for the conditions of gold mineralization

The Mandongshan gold deposit, located in the west Junggar region (Xinjiang Province, NW China), is hosted by Carboniferous basalt, tuff, and diabase. The gold ores at Mandongshan gold deposit take the form of hydrothermal veins cutting and disseminations within the altered rocks that host the deposit. A characteristic feature of the Mandongshan is the presence of abundant arsenopyrite. Two gold events have been recorded at Mandongshan: (I) an initial quartz-sulfide stage, and (II) a subsequent calcite-sulfide stage. In situ LA-ICP-MS analyses suggest that arsenopyrite of stage I has higher Te and lower Pb, Bi, Au, Ag, and Cu concentrations than arsenopyrite of stage II. Nano-SIMS δ34S values of pyrite associated with arsenopyrite of stage I and II fall within two ranges of −7.6 to −1.5‰ and −7.8 to −1.6‰. The presence of titanite and lack of rutile in the gold ores were taken to imply that the Mandongshan gold deposit formed from reduced fluids. Titanite crystals recorded in the gold-bearing quartz-sulfide veins show strongly positive Eu anomalies (Eu* = 2.15–3.99) and weakly positive Ce anomalies (Ce* = 1.04–1.58). The δ34S values of pyrite provide evidence for a magmatic sulfur source, which is consistent with the geochemical data obtained from other nearby gold deposits as well as the spatial relationship in the west Junggar region between gold mineralization and widespread felsic intrusions. In addition, our analyses suggest that the native gold of stage I was deposited due to phase separation of the ore-forming fluid. Native gold deposition of stage II can be further subdivided into stages IIa and IIb. The former stage is represented by coarse-grained gold that co-precipitated with arsenopyrite, while stage IIb is represented by fine-grained gold that precipitated as a result of dissolution and re-precipitation process occurring in earlier-formed stage IIa arsenopyrite grains. This model is consistent with our phase equilibrium calculations for the As-Au-Cu-Fe-S-O system of the Mandongshan gold deposit.

Geochronological and Geochemical Constraints on the Origin of the Hutouya Polymetallic Skarn Deposit in the East Kunlun Orogenic Belt, NW China

The Hutouya polymetallic skarn deposit lies in the Qimantagh area of the East Kunlun Orogenic Belt, NW China. Skarnization and mineralization at the deposit are closely associated with contemporary felsic intrusions. In this paper, zircon U-Pb ages and zircon Hf isotope as well as whole-rock geochemical and whole-rock Sr-Nd isotope data are reported for intrusive rocks and crystal tuff of the Elashan Formation in the Hutouya area. Moreover, Re-Os ages and S-Pb isotopes are also reported for the ore minerals in the Hutouya deposit. The Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U-Pb age of granodiorite and Re-Os isochron age of molybdenite suggest that mineralizations occurred at ca. 227 Ma and that the granodiorite and molybdenite are closely related petrogenetically. All the granitoids in the Hutouya deposit are high-K calc-alkaline and metaluminous to weakly peraluminous I-type granitoids. Among them, the ore-forming granitoids were derived by the mixing of crust-derived (either juvenile or ancient mature lower crust) and mantle-derived magmas, whereas the non-ore-related granite porphyry was generated by the partial melting of a single ancient mature lower crust. The magmas of all the granitoids underwent extensive fractionation–crystallization during the process of rising and emplacement. The sulfur of the analyzed samples from the northern and middle zone of Hutouya deposit (including No. II, III, IV, and VI ore belts) belongs to deep magmatic sulfur, while the sulfur of samples from the southern zone of Hutouya deposit (No. VII ore belt) includes not only deep magmatic sulfur but also a contribution of strata sulfur. All the ore mineral samples in the Hutouya deposit have similar Pb compositions that are consistently derived from a mixed source of upper crust and mantle. Tectonic discrimination diagrams indicate a post-collisional setting for all granitic rocks of the Hutouya skarn deposit, which is therefore considered a product of a the post-collision extensional system and is consistent with other porphyry-skarn deposits within the East Kunlun Orogenic Belt.

The evolution of the Sesia Zone (Western Alps) from Carboniferous to Cretaceous: insights from zircon and allanite geochronology

Microscale dating of distinct domains in minerals that contain relics of multiple metamorphic events is a key tool to characterize the polyphase evolution of complex metamorphic terranes. Zircon and allanite from five metasediments and five metaintrusive high-pressure (HP) rocks from the Eclogite Micaschist Complex of the Sesia Zone were dated by SIMS and LA-ICP-MS. In the metasediments, zircon systematically preserves detrital cores and one or two metamorphic overgrowths. An early Permian age is obtained for the first zircon rim in metasediments from the localities of Malone Valley, Chiusella Valley and Monte Mucrone (292 ± 11, 278.8 ± 3.6 and 285.9 ± 2.9 Ma, respectively). In the Malone Valley and Monte Mucrone samples, the early Permian ages are attributed to high-temperature metamorphism and coincide with the crystallization ages of associated mafic and felsic intrusions. This implies that magmatism and metamorphism were coeval and associated to the same tectono-metamorphic extensional event. In the Malone Valley, allanite from a metasediment is dated at 241.1 ± 6.1 Ma and this age is tentatively attributed to a metasomatic/metamorphic event during Permo-Triassic extension. Outer zircon rims with a late Cretaceous age (67.4 ± 1.9 Ma) are found only in the micaschist from Monte Mucrone. In metagabbro of the Ivozio Complex, zircon cores yield an intrusive age for the protolith of 340.7 ± 6.8 Ma, whereas Alpine allanite are dated at 62.9 ± 4.2 and 55.3 ± 7.3 Ma. The Cretaceous ages constrain the timing of the HP metamorphic stage. The presence of zircon overgrowth only in the central area of the Eclogite Micaschist Complex is attributed to local factors such as (1) multiple fluid pulses at HP that locally enhanced zircon dissolution and recrystallization, and (2) slightly higher temperatures reached in this area during HP metamorphism.

At the crossroads of the Lesser Caucasus and the Eastern Pontides: Late Cretaceous to early Eocene magmatic and geodynamic evolution of the Bolnisi district, Georgia

The Bolnisi district is a distinct tectonic zone of the Lesser Caucasus, which is considered to represent the eastern extremity of the Turkish Eastern Pontides. Late Cretaceous, low-K, calc-alkaline to high-K rhyolite of the Mashavera and Gasandami Suites is the predominant rock type of the district, and is accompanied by subsidiary dacite, and rare high-alumina basalt and trachyandesite of the Tandzia Suite. The Mashavera and Gasandami rhyolite and dacite have yielded U-Pb LA-ICP-MS and TIMS zircon ages between 87.14 ± 0.16 and 81.64 ± 0.94 Ma, which are in line with the Coniacan-Santonian ages of radiolarian fauna of the Mashavera Suite. The felsic rocks of the Mashavera and Gasandami Suites were deposited during a ~6.6 m.y.-long silicic magmatic flare-up event, which together with the Tandzia Suite mafic rocks, documents Late Cretaceous bimodal magmatism in an extensional tectonic setting. Trace element data indicate that high Y-Zr, low- to high-silica rhyolite and dacite, and low Y-Zr high-silica rhyolite have been erupted, respectively, from coeval deep and shallow crustal reservoirs. The rocks of the bimodal magmatic event are overlain by high-K volcanic rocks of the Campanian Shorsholeti Suite, which have been erupted during slab roll-back and steepening, from magmas produced by deep melting of a metasomatised mantle. Eocene postcollisional felsic intrusions crosscut the Late Cretaceous rock.The Coniacian to early Campanian bimodal magmatism, and the subsequent high-K magmatism of the Bolnisi district are contemporaneous and share geochemical characteristics with the Late Cretaceous magmatism of the Eastern Pontides. It documents the existence of a Late Cretaceous regional silicic magmatic province, and subsequent high-K magmatism during slab steepening. This regional magmatic evolution coincided with the opening of the Black Sea and the Adjara-Trialeti basins. This evolution was coeval with the wanning stages of northern Neotethyan subduction, after a ~40 m.y.-long magmatic lull along the southern Eurasian convergent margin. Early Eocene adakite-like magmatism affected both the Bolnisi district and the Eastern Pontides, demonstrating a common postcollisional magmatic evolution.