Genesis and geodynamic process of early Cretaceous intermediate–felsic batholith within the Chem Co zone,western Qiangtang and implications for Bangong–Nujiang Tethyan Ocean subduction

The complex evolutionary history of collision-related suture zones in the western Qiangtang is poorly understood, due to the lack of pivotal magmatic and tectonic records. The Chem Co intermediate-felsic batholith is situated in the westernmost section of Qiangtang, where it intruded into the Upper Paleozoic metamorphic sedimentary series and was cut by Late-Cenozoic active normal faults. Here, we present evidence from field observations, geochemistry, zircon UPb and Hf isotopes, and mica ⁴⁰Ar³⁹Ar analysis of these granitoid rocks. Zircon UPb ages ranging from 107.3 ± 1.4 to 123.7 ± 1.7 Ma reveal that this magmatism initiated in the Early Cretaceous. Mica ⁴⁰Ar³⁹Ar yield plateau ages from 99 to 118 Ma suggest rapid emplacement into the cold crust without disturbance by subsequent >320 °C tectono-thermal events. Major element geochemistry shows these granitic rocks are enriched in Al, depleted in Ca, lie mostly within the high–K calc–alkaline series and have peraluminous features. Mineral modes that lack amphibole but contain muscovite, together with the geochemical characteristics, indicate an S–type granite affinity. Furthermore, the rocks are strongly enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and are moderately to strongly depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE), thus indicating typical characteristics of arc-related magmatic rocks. Considering the petrogenesis, geochemical features, zircon UPb ages, negative zircon εHf(t) values, similarity with the southwestern Qiangtang intrusions, and the geological setting, we conclude that the Chem Co granite was derived from the anatexis of crustal lithosphere through a “soft” collision associated with the continuing northwards subduction of the Bangong–Nujiang Tethyan (BNT) Ocean. We propose that the final closure and termination of the western segment of the BNT Ocean was completed later than 107 Ma.     

Large Zn isotope variations in the NiMo polymetallic sulfide layer in the lower Cambrian,South China

In South China, black organic-rich shales in the lower Cambrian Niutitang Formation host a NiMo polymetallic sulfide layer that discontinuously extends over ~1600 km. Seawater and hydrothermal origins are among the many suggested hypotheses and are still under debate. In order to discriminate Zn sources, we report Zn isotopes in NiMo polymetallic sulfide layers and their host shales from the Nayong and Zunyi locations in Guizhou province and the Zhangjiajie section in Hunan province. In each section, host organic-rich shales show homogeneous Zn isotope compositions which likely resulted from quantitative scavenging of dissolved Zn from seawater under euxinic conditions. The difference in the average δ⁶⁶Zn values of organic-rich shales between the two sections in Guizhou (0.76 ± 0.09‰) and one section in Hunan (0.59 ± 0.10‰) might reflect variations of Zn isotope gradient with the depth of seawater. Therefore, the organic-rich sediments need not always represent an isotopically light Zn sink, which is dependent on Zn isotope fractionation in the local basin. However, the δ⁶⁶Zn values in the NiMo polymetallic sulfide layers are different from those of their host shales, indicating that these sulfide layers did not inherit the Zn isotope signal of seawater. Based on the regular increasing trend in δ⁶⁶Zn values from Nayong (0.54 ± 0.06‰) to Zhangjiajie (1.34 ± 0.09‰) and the presence of PbZn mineralization in the Dengying/Doushantuo Formations, we argue that hydrothermal fluids associated with PbZn mineralization could be a major source of Zn in NiMo sulfide layers, especially in the Nayong location. A possible model is that the hydrothermal fluids related to MVT-type mineralization got overprinted on a multiple-sourced synsedimentary sulfide-rich layer. We provide additional evidence that Zn isotopes have great potential as a tracer of metal source and can be applied to similar types of mineralization as e.g., the late Devonian Ni-Zn-PGE Nick deposit (Selwyn Basin, Canada) or elsewhere.     

An improved classification of mineralized zones using particle swarm optimization: A case study from Dagh-Dali ZnPb (±Au) prospect,Northwest Iran

Classification of mineralized areas into different geochemical classes in terms of prospectivity is crucial in the optimal management of exploration risk and costs. Machine learning (ML) algorithms can be served as appropriate alternatives for separating ore-related anomalies due to avoiding the assumptions of statistical distribution and compatibility with the multivariate nature of geochemical features. By hybridizing the ML with a metaheuristic algorithm called particle swarm optimization (PSO), this contribution aims to provide an innovative approach to optimize the classification of geochemical anomalies within the study area. The algorithm, PSO, is inspired by simulating the social behavior of flocks of birds in search of food. The Dagh-Dali ZnPb (±Au) mineral prospect in northwest Iran was subjected as a case study to examine the integrity of the proposed method. Mineralization-related features were extracted by applying principal component analysis (PCA) on metallogenic elements analyzed in soil samples as PC1 and PC2 with elemental assemblages of AgAuPbZn and PbZn, respectively. The silhouette index was employed to estimate the number of underlying geochemical clusters within the adopted feature space. To constitute a comparative analysis, two k-means clustering and PSO-based learning (PSO-L) algorithms were implemented to classify the gridded data of PC1 and PC2 within the study area. The results indicated that the use of PSO has improved the cost function of the clustering problem (up to 4%). Adapting the mineralization classes with the metallogenic evidence demonstrated by boreholes drilled in the study area indicated that PSO-L was superior to the traditional k-means method, improving the accurate estimation of subsurface mineralization classes by 7%. By overcoming the drawbacks of conventional methods for trapping at the local optima, PSO-based learning possesses the potential to highlight weak mineralization signals that are numerically located in boundary conditions. The results show that the proposed approach can serve as an effective medium for optimal modeling of geochemical classes and management of detailed exploration operations.     

Petrogenesis of the granite related to the Baishaziling Sn deposit,Dayishan ore field,Southern China

The Baishaziling greisen-type tin deposit is located in the Dayishan ore field, Nanling Range, Southern China. In this study, for the first time, we present both zircon and cassiterite UPb dating, whole-rock elements, zircon LuHf and apatite Nd isotopic compositions to better constrain the petrogenesis of granite and its genetic link with Sn mineralization. Zircon UPb ages of fine-grained granite and coarse-grained granite are 154 ± 1.8 Ma and 153 ± 2.1 Ma, respectively, which are consistent with the cassiterite UPb dating of 154 ± 5.4 Ma, implying genetic relationship between the Baishaziling granite and tin metallogenesis. The Baishaziling granites exhibit high SiO₂, K₂O + Na₂O, Zr + Nb + Ce + Y contents, low P₂O₅ and Sr contents, and high ratios of Ga/Al, ^TFeO/(^TFeO + MgO), implying A-type granite affinity with characteristics of high-K calc-alkaline and weakly peraluminous. The zircon ε_Hf(t) values and apatite ε_Nd(t) values of the granite vary from ?4.46 to ?1.81 and ?8.37 to ?7.10, with two-stage Hf and Nd model ages of 1.40 to 1.50 Ga and 1.52 to 1.64 Ga, indicating that they were generated by the partial melt of the Proterozoic basement with the involvement of mantle magma. In addition, formation of Dayishan granite was likely associated with an intraplate extensional setting caused by the subduction of the Palaeo-Pacific plate. The Baishaziling reduced granites have high stannum and boron contents, which are in favor of the tin mineralization.     

The origin of the Pailin Crystalline Complex in western Cambodia,and back-arc basin development in the Paleo-Tethys Ocean

The Pailin Crystalline Complex, which consists of amphibolites, metagabbros, and felsic igneous rocks, is located in western Cambodia. We used the geochemistry of the amphibolites and the zircon UPb ages of felsic igneous rocks to constrain the origin of the Pailin Crystalline Complex and to gain insights into the tectonic framework of Southeast Asia. Geochemical analyses indicate that the amphibolites are normal (N-type) mid-ocean ridge basalts, similar to the Sa Kaeo Back-arc Basin basalts of southeastern Thailand. The zircon UPb ages are 283 ± 2 Ma (anorthosite dike), 280 ± 2 Ma (aplite dike), and 275 ± 2 Ma (plagiogranite). The geochemistry and detrital zircon UPb ages of sandstones adjacent to the Pailin Crystalline Complex suggest that they are sediments that filled the Sa Kaeo Back-arc Basin during the Triassic. We infer that the Pailin Crystalline Complex is part of a dismembered ophiolite related to the back-arc basin, and that the igneous rocks formed during the Early Permian. Most of the metamorphism occurred during continental collision in Southeast Asia. However, the amphibolites of the Pailin Crystalline Complex also experienced ocean-floor metamorphism at a back-arc spreading center, in a tectonic setting similar to that of the mafic metamorphic rocks of the Nan Back-arc Basin, northern Thailand. We conclude that the Pailin Crystalline Complex is the southernmost extension of a back-arc basin related to subduction of Paleo-Tethys oceanic crust and the Sukhothai Arc.     

Continental response to mid-Cretaceous global plate reorganization: Evidence from the Tan–Lu Fault Zone,eastern China

Investigation of the ~2400-km-long Tan–Lu Fault Zone (TLFZ) in eastern China is the key to understanding how the Izanagi Plate in the western Pacific Basin and the East Asian continental margin responded to global plate reorganization during the mid-Cretaceous. We present new structural and geochronological data to show that the central segment of the NNE–SSW-striking TLFZ underwent a phase of sinistral transpression after the Early Cretaceous rifting. The resultant strike-slip structures are ductile shear belts in the south of the segment and brittle faults in the north. Quartz c-axis fabrics and other microstructures indicate deformation temperatures of 350–500 °C in different parts of the shear belts. The brittle faults were associated with the formation of NE–SW-trending folds and an angular unconformity between Lower and Upper Cretaceous volcanic or sedimentary rocks. Fault-slip data indicate that sinistral faulting was the result of NS compression. UPb dating constrains the timing of sinistral faulting between 97 and 82 Ma (early Late Cretaceous). Integration of these and existing data demonstrates that the entire TLFZ underwent sinistral displacement at the beginning of the Late Cretaceous, consistent with continental-scale NS compression in eastern China. Such compression in the overriding plate was caused by rapid oblique subduction of the Izanagi Plate and reflected global plate reorganization at this time. Both the changes in the kinematics of the Izanagi Plate and the resultant variation of stress states in the continental margin around the mid-Cretaceous are ascribed to this plate reorganization.     

Monazite and cassiterite UPb dating of the Abu Dabbab rare-metal granite,Egypt: Late Cryogenian metalliferous granite magmatism in the Arabian-Nubian Shield

The Abu Dabbab rare-metal granite in the Eastern Desert of Egypt is a highly-evolved alkali-feldspar granite with transitional magmatic-hydrothermal features. Extreme geochemical fractionation and the associated significant TaSn resource make the Abu Dabbab intrusion an important feature in the metallogenic evolution of the Arabian-Nubian Shield. UPb dating by laser ablation sector field (SF)-ICPMS analysis of igneous monazite yields a Concordia age of 644.7 ± 2.3 Ma, identical within uncertainty to a lower intercept Tera-Wasserburg isochron age of 644.2 ± 2.3 Ma obtained from hydrothermal cassiterite. Both ages place tight constraints on the timing of magmatic-hydrothermal processes in the Abu Dabbab granite which represents the oldest highly-evolved granite recognized so far in the Pan-African Arabian-Nubian Shield. Thus, the new ages also date the start of a period of late-orogenic metalliferous granite magmatism, when the basement of the Eastern Desert underwent a geodynamic transition from a compressive subduction-collision regime towards orogenic collapse in the late Cryogenian.     

Contribution of recycled sediments to the mantle reservoir beneath Hainan Island: Evidence from Sr,Nd, Pb,Hf, and Mg isotopic analyses of Late Cenozoic basalts

Magnesium isotopes are a useful tool for constraining the origin of basalts with EM-like isotopic signatures in relation to ancient subducted slabs and recycled materials incorporated in mantle plumes. In this study, we present new SrNdPbHf and Mg isotopic data that were used to determine the origin of the basalt on Hainan Island and investigate the EM mantle reservoir beneath the island. Cenozoic basalts from northern Hainan Island are mainly tholeiitic, with a small amount of alkaline basalts. The Hainan basalts exhibited depleted SrNd isotopic compositions and EM2-like Pb isotopic signatures. The δ²⁶Mg values of the Hainan basalts ranged from ?0.40‰ to ?0.28‰. The origin of the low δ²⁶Mg signature can be attributed to carbonate sediments from recycled oceanic slab. Hainan basalts show a negative concave curve relationship between ⁸⁷Sr/⁸⁶Sr and ε_Nd values, a positive relationship between ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb values and exhibit an evolution trend from depleted mantle towards marine sediments. This indicates that Hainan Island basalts can be explained by the mixing between depleted mantle and marine sediments. Most Hainan basalts have higher K/U × 10^?3 and Ba/Th ratios than primitive mantle (K/U × 10^?3 ≈ 11.8, Ba/Th ≈ 83), moreover, display highly correlated K/U × 10^?3 and Ba/Th compositions with low-pressure (6–8 GPa) carbonated melt released from initial sediments. Therefore, we speculate that the primitive mantle peridotite, coupled with the low-pressure carbonated melt, ultimately became the mantle source of Hainan Island basalts.     

The Mesoarchaean Dominion Group and the onset of intracontinental volcanism on the Kaapvaal craton – Geological,geochemical and temporal constraints

The ca. 3.07 Ga volcanic rocks of the Dominion Group, South Africa, represent the oldest example of intracontinental, rift-related volcanism on the Archaean Kaapvaal craton. The volcanic assemblage comprises a >2 km-thick succession of mafic-intermediate lavas interlayered with felsic lavas and pyroclastic rocks. Textural and geological features indicate emplacement in a subaerial environment probably in an incipient intracontinental rift. We report SHRIMP UPb zircon ages, elemental and Nd-isotope bulk-rock analyses of drill core samples and interpret their petrogenesis in the context of a Mesoarchaean continental setting. The UPb zircon ages of four felsic samples from different stratigraphic levels yielded the same dates, resulting in a pooled ²⁰⁷Pb/²⁰⁶Pb age of 3074 ± 5 Ma. Primitive mantle-normalised incompatible trace element concentrations show enriched patterns with fractionated rare earth elements over high field-strength elements and negative anomalies of Nb and Ta relative to La.Initial ε_Nd values for mafic and felsic rocks from −1.0 to −0.2 indicate melting of sources comprising time-integrated incompatible element-enriched mantle. The combined trace element and SmNd isotopic data suggest that the enrichment of incompatible elements and the low ε_Nd values in the most primitive basalt samples (Mg# of 65–67) can be explained with contamination of asthenosphere-derived melts with crustal material or melting of an incompatible element-enriched upper mantle. The chemical compositions of the Dominion Group and Pongola Supergroup represent a significant petrogenetic departure from earlier Archaean (>3.6–3.1 Ga) magmatism as recorded in the Kaapvaal craton, which was dominated by komatiite-basalt volcanism and tonalite-trondhjemite-granodiorite intrusions. This change reflects the transition from a “greenstone belt type” tectonic setting to a failed intracontinental rift setting shortly after stabilisation of the Kaapvaal craton.     

Unveiling growth histories of multi-generational garnet in a single skarn deposit via newly-developed LA-ICP-MS UPb dating of grandite

Although garnet UPb dating method has been reported recently, yet the accurate concordia ²⁰⁶Pb/²³⁸U ages and growth histories of multi generation of garnets based on ages were still lacked. LA-ICP-MS UPb dating on multi-generational grandite (grossular-andradite) garnet from the large Tonglvshan Cu-Fe-Au skarn deposit was applied in this study. Based on petrographic observation, in chronological order, three generation garnets have been distinguished, namely homogeneous Grt1-exo (in the exoskarn zone) and Grt1-endo (in the endoskarn zone), oscillatory zoning Grt2 and vein-type Grt3 cutting magnetite ores. LA-ICP-MS UPb dating on four grandite samples from the Grt1-exo, Grt1-edno, Grt2 and Grt3 yields Tera-Wasserburg lower intercept ²⁰⁶Pb/²³⁸U ages of 139.1 ± 1.0 Ma (2σ, MSWD = 0.79), 134 ± 11 Ma (2σ, MSWD = 2.5), 143.4 ± 8.3 Ma (2σ, MSWD = 2.3) and 140.3 ± 1.4 Ma (2σ, MSWD = 0.95), respectively. More importantly, two concordia ²⁰⁶Pb/²³⁸U ages of 139.2 ± 0.6 Ma (2σ, MSWD = 1.4) and 139.8 ± 1.5 Ma (2σ, MSWD = 0.13) were firstly obtained from the sample of Grt1-exo with highest U concentrations ([U]_avg > 80 ppm) contents. The precision UPb ages of 139–140 Ma from Grt1-exo and Grt3 can be considered as the timing of Cu-Fe-Au skarn mineralization, and consistent with the majority of published zircon UPb ages of the quartz dioritic stock and ⁴⁰Ar³⁹Ar plateau ages of phlogopite at Tonglvshan (142–140 Ma). The precision grandite UPb ages also indicate that the entire metasomatic hydrothermal mineralization activity in the Tonglvshan Cu-Fe-Au skarn deposit occurred within a relatively short time span of <1 (or 2.5 considering errors) Myr. In addition, we found that the grandite garnet is more easily to be enriched in U and can obtain the high-precision concordia UPb ages with higher andradite Mol%, euhedral and larger crystals, and relevant oxidized magmatic rocks or skarns.     

Northern limit of Gondwana in northwestern Mexico from detrital zircon data

UPb geochronology along a north-south transect from central Sonora to northern Sinaloa in northwestern Mexico indicates several changes in provenance sources through time and space. Lower Cambrian arenites of the Proveedora Quartzite yield a single main age peak at 1075 Ma with minor Paleoproterozoic contribution. Arenites from the Sonobari Complex of southwestern Sonora-northwestern Sinaloa, purportedly assigned to the early Paleozoic, yield main peaks at 1424, 1662, and 1736 Ma, probably derived from the Yavapai and Mazatzal provinces, which are pervasively intruded by early Mesoproterozoic granites coeval to those forming the Granite-Rhyolite Province. Lower to Upper Ordovician units deposited on shelf, slope, and abyssal environments display very similar zircon age patterns, with main peaks ca. 2700 and 1850 Ma, indicating a large drainage system arising from the Archean cratons of Laurentia such as the Wyoming or Superior Provinces, and Paleoproterozoic sources similar to the Trans-Hudsonian orogen, which are older than the Yavapai Province. The Río Fuerte quartzites from southern Sonora-northern Sinaloa contain main peaks at 534, 542, and 637 Ma whose sources may be peri-Gondwanan blocks similar to Avalonia or Carolina terranes. A minor peak at 475 Ma suggests input from igneous suites reported from the Acatlán Complex in southern Mexico. UPb geochronology permit inference of a late Paleozoic collisional orogen in northwestern Mexico that was originated by the collision of Gondwanan blocks against southern Laurentia and outline the boundary between terranes related to continental blocks in the Pangaea supercontinent.     

Evolution of unconformity-related MnFeAs vein mineralization,Sailauf (Germany): Insight from major and trace elements in oxide and carbonate minerals

The Sailauf MnFeAs vein mineralization, located in the Spessart district (central Germany), is characterized by complex hydrothermal carbonate and oxide assemblages. The mineralization is hosted by a Permian rhyolite body and is structurally related to the Variscan unconformity that separates Permian sedimentary rocks from the underlying Variscan crystalline basement. The hydrothermal vein system has been studied by optical microscopy, electron-microprobe and LA-ICPMS analysis of major and trace elements (including the REE). Four distinct mineralization stages that are characterized by diagnostic carbonate-oxide assemblages are identified, which are (1) pre-ore stage, (2) ore stage 1, (3) ore stage 2, and (4) the replacement stage. Hydrothermal carbonates show complex compositional trends in CaMn (Fe + Mg) space, and comprise calcite, Mn-calcite, FeMg calcite, Ca-rhodochrosite, and Ca-kutnahorite. Oxide assemblages are dominated by braunite and hematite, with minor amounts of manganite and hausmannite. The mineralization is enriched in a distinct suite of trace elements, including As, W, Pb, Zn and Cu. Analysis of the paragenetic evolution, in conjunction with the major and trace element data, allows to reconstruct the fluid evolution of the hydrothermal system. The first order change in mineralogy between the two main ore stages (Mn oxides and calcite evolving into hematite and Mn-rich calcite) records a pronounced shift in fluid pH and silica activity of the system. This interpretation is also supported by variations in the behavior of Ce in different carbonate generations. The late stage replacement carbonates relate to destabilization of the primary ore assemblages. The distinct geological setting immediately below the Permian unconformity, in conjunction with the mineralogical and chemical data, suggests that dynamic fluid mixing processes involving basement-derived brines and more shallow groundwaters were important in the formation of the Sailauf MnFeAs mineralization. The significant enrichment in the AsWPbZnCu element suite resembles that of other MnFe deposits, and points to felsic lithologies as the main metal source of the mineralization.     

External sulphur in IOCG mineralization: Implications on definition and classification of the IOCG clan

Although the sources of the ore metals remain problematic in most Iron-oxide Cu and Au (IOCG) deposits, external sulphur, either from surficial basinal brines and seawater (e.g., Central Andean and Carajás deposits) or from formation water and metamorphic fluids (e.g., the Cloncurry deposits), or introduced by magmatic assimilation of metasedimentary units (e.g., Phalaborwa), has been documented in many major Cu-rich IOCG centres. However, only the evaporite-sourced fluids yield diagnostically high δ³⁴S values (i.e., > 10‰), while sedimentary formation water or metamorphic fluids commonly have lower values and are less clearly distinguishable from magmatic fluids, as in the Cloncurry deposits in which the involvement of external fluids is revealed by other evidence, such as noble gas isotopes. On the basis of these arguments, IOCG deposits could be redefined as a clan of Cu (AuAgU) deposits containing abundant hypogene iron oxide (magnetite and/or hematite), in which externally-derived sulphur probably plays an important role for the Cu (AuAgU) mineralization. In this definition, all “Kiruna-type” magnetite deposits, hydrothermal iron deposits (e.g., skarn Fe deposits) and magnetite-rich porphyry CuAu and skarn CuAu deposits are excluded. Two subtypes of IOCG deposits are recognized on the basis of the predominant iron oxide directly associated with the Cu (Au) mineralization, whether magnetite or hematite. Neither magnetite- nor hematite-rich IOCG deposits show any preference for specific host rocks, and both range in age from Neoarchean to Pleistocene, within a broad tectonic environment.     

Age,petrogenesis, and tectonic setting of granitic gneiss and amphibolite from the Weizigou gold deposit,Heilongjiang Province,NE China: Constraints from geochronology and geochemistry

The Weizigou gold deposit is located in the western Jiamusi Massif, Northeast China. Gold mineralization is hosted in the amphibolite, which intruded the granitic gneiss. Although the deposit shows similarities to iron-oxide–copper–gold deposits, the detailed ore-forming process remains uncertain. To determine the formation age, petrogenesis, and tectonic setting of the granitic gneiss and amphibolite, LA–ICP–MS zircon, titanite, and monazite UPb dating, whole-rock major- and trace-element analyses, and LA–ICP–MS in situ zircon Hf isotope analyses were conducted on samples from these rocks. The granitic gneiss yielded two age populations of 951–882 Ma, and ca. 500 Ma, with a monazite UPb concordia age of 501.5 ± 5.1 Ma. The amphibolite yielded a crystallization age of 292 Ma, consistent with the results for magmatic titanite UPb dating, and a metamorphic age of 272–258 Ma. The granitic gneiss contains typical aluminum-rich minerals, such as garnet and muscovite, mean SiO₂ = 73.31 wt%, and molar ratio Al₂O₃/(CaO + K₂O + Na₂O) values of 1.02–1.07, indicating an S-type granite protolith. The amphibolite belongs to the tholeiitic basalt series and has low SiO₂ and high MnO contents. These results, together with ε_Hf(t) values and two-stage model ages ranging from ?9.5 to 2.3 and ? 0.3 to 5.7, and from 2010 to 1659 Ma and from 1331 to 947 Ma, respectively, allow us to infer that the parental magmas of the granitic gneiss and amphibolite were derived from the partial melting of Paleoproterozoic lower crust and the partial melting of metasomatized depleted mantle, respectively. The granitic gneiss is characterized by positive Th and Hf anomalies, and negative Nb, Ta, Sr and Ti anomalies, whereas the amphibolite is enriched in K, Rb, and depleted in Ba, Nb, Ti, and Zr. These geochemical features suggest that the S-type granite was formed in an active continental margin during the Neoproterozoic and underwent granulite-facies metamorphism during the early Paleozoic. The protolith of the amphibolite was gabbro that formed in an extensional setting (e.g., a backarc basin) associated with westward subduction of the Paleo-Pacific oceanic plate beneath the eastern Jiamusi Massif during the early Permian. The gold mineralization can most likely be attributed to contact metasomatic metamorphism of gabbro during the middle–late Permian.     

Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic FeTi deposits: A case study of the Damiao titanomagnetite ore deposit,North China Craton

Geochemical and iron isotopic compositions of magnetite, ilmenite and pyrite separates from the FeTi oxide ores hosted in the Damiao anorthosite-type FeTi ore deposit were analyzed to investigate sub-solidus cooling history of the titanomagnetite. The FeTi oxides form two series of solid solutions, namely, ulvöspinel-magnetite (Usp-Mt_ss) and hematite-ilmenite (Hem-Ilm_ss) solid solutions. The magnetite separates have 14–27 mol% ulvöspinel, while the ilmenite separates have 5–8 mol% hematite. Major element compositions of the mineral separates suggest that the ilmenites were mainly exsolved from the Usp-Mt_ss by oxidation of ulvöspinel in the temperature range of ~820–600 °C and experienced inter-oxide re-equilibration with the magnetites. Associated with the exsolution is the substantial inter-mineral iron isotope fractionation. The magnetite separates are characterized by high δ⁵⁷Fe (+0.27 − +0.65‰), whereas the ilmenite separates have lower δ⁵⁷Fe (−0.65 to −0.28‰). Two types of pyrite are petrographically observed, each of which has a distinctive iron isotope fingerprint. Type I pyrite (pyrite_I) with higher δ⁵⁷Fe (δ⁵⁷Fe = +0.63 − +0.95‰) is consistent with magmatic origin, and type II pyrite (pyrite_II) with lower δ⁵⁷Fe (δ⁵⁷Fe = −0.90 to −0.11‰) was likely to have precipitated from fluids. Iron isotopic fingerprints of the pyrite_I probably indicate variations of oxygen fugacity, whereas those of the pyrite_II may result from fluid activities. The iron isotopic fractionation between the magnetite and ilmenite is the net result of sub-solidus processes (including ulvöspinel oxidation and inter-oxide re-equilibration) without needing varying oxygen fugacity albeit its presence. Although varying composition of magnetite-ilmenite pairs reflects variations of oxygen fugacity, inter-oxide iron isotopic fractionation does not.     

Opening and closure history of the Mudanjiang Ocean in the eastern Central Asian Orogenic Belt: Geochronological and geochemical constraints from early Mesozoic intrusive rocks

The tectonic evolution of the ancient Mudanjiang Ocean within the Central Asian Orogenic Belt (CAOB), is strongly debated. The ocean played an important role in the amalgamation of the Songnen and Jiamusi massifs; however, the timings of its opening and closure have remained ambiguous until now. In this study, we analyzed early Mesozoic intrusive rocks from the eastern Songnen and western Jiamusi massifs in the eastern CAOB. The new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data are used to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon UPb dating indicates that early Mesozoic magmatism in the eastern Songnen Massif occurred in three stages: Early to Middle Triassic (ca. 250 Ma), Late Triassic (ca. 211 Ma), and Early Jurassic (ca. 190 Ma). The Triassic intrusive rocks typically consist of bimodal rock suites, which include gabbros, hornblende gabbros, and granitoids. The compositional information indicates an extensional environment that was probably related to the final closure of the Paleo-Asian Ocean. We integrated the results with observations from Triassic A-type granitoids and coeval sedimentary formations in the eastern Songnen Massif, as well as depositional ages of metasedimentary rocks from Heilongjiang Complex. We conclude that the opening of the Mudanjiang Ocean took place in the Early to Middle Triassic. The Early Jurassic intrusive rocks are bimodal and include olivine gabbros, hornblendites, hornblende gabbros, gabbro diorites, and granitoids. The bimodal rock suite indicates a back-arc style extensional environment. This setting formed in relation to westward subduction of the Paleo-Pacific plate beneath the Eurasia during the Early Jurassic. Following subduction, the closure of the Mudanjiang Ocean and subsequent amalgamation of the Songnen and Jiamusi massifs happened during the late Early Jurassic to Middle Jurassic. This sequence of events is further supported by ages of metamorphism and deformation acquired from the Heilongjiang Complex. Based on these observations, we conclude that the Mudanjiang Ocean existed between the Middle Triassic and Early Jurassic, making it rather short-lived.     

Jurassic–Cretaceous granitoids and related tectono-metallogenesis in the Zapug–Duobuza arc,western Tibet

The Zapug–Duobuza magmatic arc (ZDMA), located along the southern edge of the south Qiangtang terrane in western Tibet, extends east–west for ~ 400 km. Small scattered granite and porphyry intrusions crop out in the ZDMA, but a large amount of granite may be buried by Late Cretaceous to Paleogene thrusting. Two stages of magmatism have been identified, at 170–150 Ma and 130–110 Ma. The widely distributed Middle–Late Jurassic granite intrusions in the ZDMA exhibit SrNd isotopic characteristics similar to those of ore-bearing porphyries in the Duolong giant CuAu deposit, and their εHf(t) values mostly overlap those of other porphyry CuMo deposits in the ZDMA and the Gangdese zone. The SrNdHf isotopic geochemistry suggests variable contributions of mantle and Qiangtang crustal sources, and indicates the presence of two new ore districts with potentials for CuAu, Fe, and PbZn ores, located in the Jiacuo–Liqunshan and Larelaxin–Caima areas. Except for the Duolong ore-forming porphyries, which show significant contributions of mantle components intruded into an accretionary mélange setting, the Early Cretaceous granites in other areas of the belt are of mostly crustal origin, from sources in Qiangtang felsic basement and Permo-Carboniferous strata, indicating the weak ore-forming potential of skarn-type Fe and PbZn deposits. The ephemeral but deep Bangong Co–Nujiang ocean in the Early Jurassic evolved into a shallow compressional marine basin in the Middle–Late Jurassic, possibly transitioning to northward flat subduction of oceanic crust at this time. The subducted slab broke off in the Early Cretaceous, initiating a peak in arc magmatism and metallogenesis at 125–110 Ma.     

Geochronology and geochemistry of subducted Cadomian continental basement in central Iran: Decompressional anatexis along the Jurassic Neotethys margin

Late Neoproterozoic-Early Cambrian calc-alkaline granitoids are ubiquitous in the continental basement of Iran and indicate formation within a Cadomian arc system at the northern margin of Gondwana. A basement complex comprising mainly mica schist, paragneisses, and metagranite along with metabasite and rare pegmatite is exposed in the Zayanderud region north of Shahrekord located in the hinterland of the Zagros mountain range. This complex is unique in the Neotethyan realm because it includes eclogites with Jurassic metamorphic ages implying involvement of continental crust at the onset of subduction. Ion microprobe UPb zircon dating along with trace element and oxygen isotope analyses for metagranites define two zircon age clusters of ca. 552 and 565 Ma confirming connection with the other Ediacaran age basement arc plutons in the belt. Zircon geochronology for pegmatite, by contrast, yielded a concordant age population averaging 176.5 ± 3.3 (2σ) Ma. Zircon crystals from the pegmatite also have unusually low rare earth element (REE) abundances with sharp increases towards the heavy REE. Along with an absence of a negative Eu anomaly, this indicates a high-grade metamorphic origin of zircon crystallizing from a pegmatite which was formed by melting of mica schist and possibly amphibole eclogite during decompression where incipient garnet breakdown released Zr and HREE to form zircon, and LREE were retained in stable apatite and titanite. Corresponding ⁴⁰Ar/³⁹Ar phengite dates from the pegmatite and the mica schist country-rock are overlapping with or only slightly postdate the UPb zircon ages, indicating rapid cooling after reaching maximum metamorphic pressure in the Early Jurassic. The Zayanderud basement complex is thus potentially a rare example of deep burial of continental crust and rapid exhumation due to buoyant escape during the incipient stages of subduction, well before the ultimate closing of the Neotethys ocean basin between Arabia and Eurasia in the mid-Tertiary.     

New UPb,Hf and O isotope constraints on the provenance of sediments from the Adelaide Rift Complex – Documenting the key Neoproterozoic to early Cambrian succession

The Adelaide Rift Complex is arguably one of the most complete and best studied Neoproterozoic to early Cambrian successions worldwide, preserving evidence of the breakup of Rodinia, two Cryogenian glaciations and the interglacial phase, and one of the best documented Ediacaran to early Cambrian biotic transitions. However, the complex and protracted tectono-sedimentary history of this 0.8–0.5 Ga province is still being debated. We present new and published UPb ages and Hf and O isotope data for detrital zircons from the Adelaide Rift Complex, representing the most complete assembly of such data for this succession. Deposition during initial mid-Tonia extension was largely sourced locally from rift shoulders. As the basin evolved from rift- to sag-phase following continental breakup in the Cryogenian the provenance regions extended to more distal late Mesoproterozoic terranes to west and northwest. New data from Sturtian Glacial Epoch deposits are consistent with termination of this event at 0.66 Ga, with most deposition during deglaciation. Uplift of the Musgrave region during the Ediacaran to early Cambrian Petermann Orogeny led to dominant sediment supply from that terrane at that time in the north. In the south, earliest Cambrian deposition followed local tectonism, initially revitalising local proximal basement sources. An abrupt change in provenance occurred at the base of the Cambrian Kanmantoo Group, the youngest sediment package in the south. Paleocurrent data indicates transports from the south, probably from formerly contiguous Antarctica, possibly reflecting the onset of convergent tectonics and deposition in a foreland basin, consistent with the near depositional age of the dominant detrital zircon population. Whilst several episodes of significant crustal reworking are identified in the Hf and O isotope data, many of the zircon T_DM ages lie within 0.5 Ga of the UPb ages indicating that new additions from the mantle were common in the provenance regions.     

The Earth as a multiscale quantum-mechanical system

Major features of the Earth's structure and dynamics originate in the contrast between the rigidity of SiO bonds and the softness of SiOSi linkages. Because this contrast results from orbital hybridization, a real understanding of bonding relies on ab initio quantum-mechanical principles. As investigated with first-principles interatomic potentials, the α–β transitions of SiO₂ polymorphs illustrate how soft SiOSi linkages give rise to dynamical structures at rather low temperatures and yield the low melting temperatures of SiO₂-rich minerals that are at the roots of SiO₂ enrichment in magmatic differentiation. The increasing concentration of alkalis throughout this process is another aspect that must also be studied in terms of molecular orbitals in relation with the presence of aluminum in tetrahedral coordination. Finally, calculations of noble gas solubility show that some important features can be treated with “hybrid” calculations when, in addition to quantum-mechanical effects, the energy needed to create a cavity in the silicate melt is dealt with in a classical manner.