Ore-forming granites from Jurassic porphyry Mo deposits,east–central Jilin Province,China: geochemistry,geochronology, and petrogenesis

ABSTRACT

The east–central part of Jilin Province, NE China, hosts an important polymetallic metallogenic district that contains more than 10 recently discovered large-, medium-, and small-scale Mo deposits. The Mo deposits in this area include porphyry-, skarn-, and quartz vein-type mineralization, of which the porphyry-type deposits dominate. Few studies of these mineralization-related granitoids have been undertaken. Here, we present the results of a systematic regional survey of the geochemistry and geochronology of Mo mineralization-related granites in this area. Zircon U–Pb dating of the Fuanpu, Jidetun, Shuangshan, and Jiapigou granites, all of which are associated with Mo mineralization, yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 167.05 ± 0.81, 170.91 ± 0.83, 183.8 ± 1.1, and 182.3 ± 2.2 Ma, respectively, indicating that these plutons were emplaced during the Early–Middle Jurassic. They have SiO₂ = 62.59–73.5 wt.%, Al₂O₃ = 13.74–16.19 wt.%, and K₂O/Na₂O = 0.8–2.18. Chemically, they are metaluminous to peraluminous and belong to the high-K calc-alkaline to shoshonitic series. Moreover, they are enriched in large ion lithophile elements and light rare earth elements, and are depleted in high field strength elements, which are characteristics of I type granite. Whole rock Sr–Nd–Pb isotopic compositions of these granitoids are similar (initial ⁸⁷Sr/⁸⁶Sr = 0.70404 to 0.70554; εNd(t) = –0.9 to 2.4; (²⁰⁶Pb/²⁰⁴Pb)_t = 15.549–15.567, (²⁰⁷Pb/²⁰⁴Pb)_t = 18.035–18.530, ⁽²⁰⁸Pb/²⁰⁴Pb)_t = 37.966–38.229) and altogether suggest that the magmas from which the Mo deposits were generated originated from the mantle or juvenile crust. Combining our results with regional Jurassic tectonic setting, we conclude that the mineralization of these granitoids reflected Pacific plate subduction which induced magma underplating and promoted the remelting of the juvenile crust, resulting in voluminous granitic magma.     

Late Jurassic granitoids in the Xilamulun Mo belt,Northeastern China: geochronology,geochemistry, and tectonic implications

ABSTRACT

The Xilamulun Mo belt of Northeastern China, located in the southeastern segment of the Central Asia Orogenic Belt (CAOB), is composed of large deposits of porphyry Mo and quartz-vein-type Mo, which are related to Mesozoic granitoids. Previous studies led to the conclusion that all granitoids in the region formed during the Cretaceous and Triassic, but our new laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dating of magmatic zircons from five samples of four mineralized plutons (Nailingou, Longtoushan, and Hashitu granites and Erbadi and Hashitu granite porphyries) reveals that these range in age from 143.8 ± 1.2 to 149.5 ± 1.0 Ma. These granites show post-collisional (A-type) geochemical characteristics (e.g. enrichment in total alkali, LILE, and LREE and depletion in Eu, Ba, P, and Nb). The Erbadi, Longtoushan, Hashitu, and Longtoushan granitoids exhibit moderately positive Hf isotopic compositions (ε_Hf(t) = ?0.3 to 10.2), indicating that granitic magmas may reflect mixtures of mantle melts and continental crust. These mineralized granites were all emplaced along a major fault over a time span of ~6 million years during the Late Jurassic. We conclude that igneous activity and mineralization resulted from the rollback of the subducted Palaeo-Pacific plate beneath Eurasia. Confirming that the Late Jurassic granitic intrusives are related to the Mo mineralization is useful for understanding the Mesozoic tectonic evolution of the Xilamulun Mo belt and also has significant implications for the regional exploration of ores.     

Timing and tectonic significance of Paleozoic magmatism in the Sakar unit of the Sakar-Strandzha Zone,SE Bulgaria

ABSTRACT

Palaeozoic granitoids and meta-granitoids dominate the metamorphic basement of the Sakar unit of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria. In this article, we present new whole-rock geochemical data and U–Pb zircon geochronology for the Sakar unit granitoids. The igneous minerals and textures are preserved, except the meta-granitoids that experienced a weak amphibolite-facies overprint. Geochemistry reveals compositions of peraluminous high-K calc-alkaline I- to S-type granitoids of volcanic arc origin. A major group of LILE-LREE-enriched granitoids and meta-granitoids and a single HFSE-HREE-enriched meta-granitoid are distinguished. U–Pb geochronology has yielded crystallization ages between 305 and 295 Ma for the major group granitoids and a ca. 462 Ma crystallization age of HFSE-HREE-enriched meta-granitoid. Late Palaeozoic granitoids of the Sakar unit show similar compositions and a similar tectonic setting when compared to other granitoids of the SASTZ, confirming a uniform region-wide tectono-magmatic event. As the Late Carboniferous-Permian magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Palaeotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be extended into the Serbo-Macedonian-Rhodope zones to the west, where time equivalent meta-granitoids support the same geodynamic context.     

Occurrence of ice-rafted erratics and the petrology of the KR1 seamount trail from the Australian–Antarctic Ridge

ABSTRACT

A multi-disciplinary study of the KR1 segment of the Australian–Antarctic Ridge has been conducted since 2011. We present geochemical and age dating results for samples dredged from three sites on the KR1 seamount trail. The majority of the samples are alkaline ocean island basalts with subdominant enriched tholeiites. The samples from the DG05 bathymetric depression include ice-rafted erratics from Antarctica, which consist of gabbro, diabase, various granitoids, volcanic rocks such as trachyte and rhyolite and deformed or undeformed sedimentary rocks. The main provenance of glacial erratics is considered to be the Ross Sea region. However, Carboniferous to Cretaceous ages of erratics indicate that some of these may originate from the western regions of West Antarctica. Based on the size and topography of the volcanic features and geochemical characteristics of the alkaline ocean island basalts (La/Sm_N = 2.62–3.88; Tb/Yb_N = 1.54–2.67) and the enriched tholeiites, the KR1 seamount trail is interpreted to be a submarine hotspot chain that is the product of alkaline volcanic eruption and seafloor spreading.     

Geochemical and Sr–Nd isotopic characteristics of Murgul (Artvin) volcanic rocks in the Eastern Black Sea Region (Northeast Turkey)

The studied volcanic rocks are footwall and hanging wall dacites from the Murgul mine and the surrounding area. Moreover, the hanging wall dacites contain enclaves. Footwall dacite contains biotite, whereas hanging wall dacitic rocks contain hornblende as a ferromagnesian mineral. The enclaves in the hanging wall dacite have sizes that range from 1 cm to 20 cm and contain hornblende as a ferromagnesian mineral. The volcanic rocks show tholeiitic and transitional affinities. They are rich in large ion lithophile element and light rare earth element with pronounced depletion of high field strength elements. The chondrite-normalized rare earth element patterns (La_N/Lu_N = 1.47–5.12) show low to medium enrichment, which reveal that the rocks were obtained from similar sources in Murgul volcanic rocks. The initial ⁸⁷Sr/⁸⁶Sr values range from 0.70442 to 0.70525, and the initial ¹⁴³Nd/¹⁴⁴Nd values range from 0.512741 to 0.512770. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallization with minor amounts of crustal contamination and magma mixing. All geochemical data support that these rocks originated from andesitic magma, and that the parental magma of the rocks were probably derived from an enriched upper mantle, previously modified by subduction-induced metasomatism in a geodynamic setting.     

Long-lived magmatic systems and implications on the recognition of granite–pegmatite genetic relations: Characterization of the Pavia granitic pegmatites (Ossa-Morena Zone,Portugal)

It is generally accepted that pegmatites are derived from large masses of granite but, even in areas where complete mineralogical, chemical and isotopic datasets are available, the relation between pegmatites and host granitic rocks or nearby plutons is usually not simple to address. The Pavia pluton, located in the Ossa-Morena Zone (Iberian Massif), is a multiphase intrusive body constructed over ∼11 m.y. by the amalgamation of several batches of magma. At the first glance, pegmatites seem to constitute a very homogeneous pegmatite field. They are mainly “intragranitic” thin tabular dikes, unzoned, layered, or with simple internal structure and are composed by the ordinary minerals that constitute the different classes of igneous rocks. They also present identical whole rock major and trace elements geochemistry and isotopic signature [(⁸⁷Sr/⁸⁶Sr)_i = 0.70434–0.70581, ɛNd_t = −1.3 to −3.7 and δ¹⁸O = 8.2–9.6‰] but, based on previously published geochronological data, three generations of pegmatites were identified. Two of these are coeval with the emplacement of the host granites (s.l.) at 328 Ma and ca. 324 Ma. The other is related to a later magmatic event at 319–317 Ma. A similar and rather juvenile source is suggested for host granites (s.l.) and pegmatites but a simple and continuous process of intra-chamber magmatic differentiation is not supported by our data. It is suggested that pegmatites derived from slightly evolved batches of magma that interacted with fresh, newly emplaced, batches (from the same or from a similar source) with limited interaction with the crust. Therefore, the Pavia pegmatites do not represent the final products of magmatism at this level of the crust but slightly differentiated products of different batches of magma. This study demonstrates how long-lived magmatic systems can potentially affect the recognition of granite–pegmatite genetic relationships.     

Evolution of groundwater chemistry in and around Vaniyambadi Industrial Area: Differentiating the natural and anthropogenic sources of contamination

Groundwaters in the crystalline aquifers are the major source of drinking water in Vaniyambadi area of Vellore district. Geochemical methods in collaboration with statistical methods were applied in this industrial area to understand the natural and anthropogenic influences on groundwater quality. To accomplish this objective, groundwater samples were collected and analyzed for physicochemical parameters and the results showed a dominance in the order of Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ for anions and cations, respectively. In contrast to this anion dominance were changed to Cl⁻ > HCO₃⁻ > SO₄²⁻ > NO₃⁻ in samples collected near the tannery industries. Groundwater quality evaluation using TDS and TH suggested that 57% of the total samples are hard-brackish type, indicating its unsuitability for drinking purpose. Generally the water type is Na⁺Cl⁻ to Ca²⁺Mg²⁺HCO₃⁻ type with an intermediate Ca²⁺Mg²⁺Cl⁻, suggesting the mixing of fresh groundwater with tannery effluent and cation exchange. Factor analysis and bivariate plots of major ions suggests that both natural and anthropogenic inputs are equally influencing the groundwater quality. Further investigations proved that silicate weathering is the dominant geogenic source of groundwater solute content, whereas tannery effluent is the anthropogenic source. Saline water mixing index (SWMI) and Cl⁻ vs NO₃⁻ bivariate plot were employed to differentiate the tannery contamination from the other anthropogenic inputs such as agricultural fertilizers, municipal sewages, etc. This analysis shows that samples 2, 4, 8 and 9 (located within the tannery cluster) have a SWMI value greater than 1, representing the groundwater–tannery effluent mixing. This study infers that groundwater in the Vaniyambadi area is under serious threat from both natural and anthropogenic contamination. However, the controlling discharge of untreated tannery effluents must be regulated to reduce the further deterioration of this vital resource in this part of the country.     

Avian extinction at the end of the Cretaceous: Assessing the magnitude and subsequent explosive radiation

Debate on the magnitude of Cretaceous extinctions and timing of modern bird origins has sharply coalesced over the past two decades into contested models, gradualistic or explosive. Molecular clocks, bolstered by phylogenetic, biogeographic, and vicariance models, support an Early Cretaceous origin for birds and mammals over 100 million years ago. Yet, although numerous new Chinese fossils of archaic ornithurine birds have been discovered in the Jehol Biota of the Early Cretaceous of China, none shows close affinity to modern neornithines; it is not until the latest Cretaceous when some fossils show more advanced ornithurine morphology, and are possibly Neornithes. In contrast to mass survival scenarios, most paleontological evidence appears to support an explosive radiation following the Cretaceous–Paleogene (K–Pg) mass extinction event, closely paralleling the geometry of mammal evolution. Gradualistic models ignore recent evidence of cataclysmic worldwide events following the impact event. How could mass survival of the environmentally sensitive birds have occurred following cosmopolitan environmental destruction, when other terrestrial vertebrates, particularly ectotherms, suffered dramatic loss? Given the paucity and scrappy nature of avian fossils immediately prior to and after the K–Pg boundary, it is prudent to use mammalian and other biotic evolution in the Paleogene as a guidepost for avian evolution. Our continued inability to produce a veracious phylogeny of higher avian taxa is likely related to a Paleogene explosive burst or ‘big bang’ evolution of bird and mammal evolution, resulting in short ordinal internodes.     

Axisymmetric consolidation of a poroelastic soil layer with a compressible fluid constituent due to groundwater drawdown

Axisymmetric consolidation of a poroelastic soil layer with a compressible fluid constituent induced by groundwater drawdown was studied based on Biot’s axisymmetric consolidation theory. Laplace and Hankel transforms were employed to solve the governing equation. Explicit analytical solutions are obtained in the Laplace–Hankel transform domain when groundwater drawdown is induced by a constant pumping well. Based on the solutions, numerical computations were performed to study the influences of the compressibility of the fluid constituent on the consolidation behavior of the soil layer.     

Magmatic evolution of pre-ore volcanics and porphyry intrusives associated with the Altar Cu-porphyry prospect,Argentina

Altar is a Cu-porphyry deposit related to several small plagioclase porphyry intrusions of the late Miocene formed on the margin of the Flat-Slab segment along the Andean Cordillera in north-west Argentina. New stratigraphic and structural mapping supported by geochemistry and geochronology of pre-ore volcanics at Altar has revealed that a period of ∼6–7 Ma of volcanism during the late Oligocene-early Miocene formed ∼4000 m of volcano-stratigraphic succession making up the Pachón Formation. It represents a period dominated by explosive to effusive eruption in a dynamic arc basin with local ash fall and flow deposition in lacustrine and fluvial sites. Volcanism is typified by medium- to high-K calc-alkaline arc magmatism with a shift from mafic compositions at the base to felsic rocks at the top of the formation containing zircons aged 21.9 ± 0.2 Ma (2 Std.Dev, U–Pb). A clear geochemical separation exists between early Miocene pre-ore volcanics that show signatures akin to non-adakitic, normal arc, extensional tectonic settings conducive of chemical differentiation at shallow crustal levels and correlate with intra-regional Abanico and Farellones Formations; and the middle to late-Miocene Cu-mineralised porphyry intrusions. After a break of ∼9 Ma in the geological record at Altar, these Cu-fertile bodies are emplaced entirely within the Pachón Rhyolite and represent adakite-like magmas with fractionation trends evolving from a lower crustal MASH zone. This distinction is controlled by a change from an extensional to compressive tectonic regime in the region during the middle Miocene in which magmas were stalled in the lower crust for an extended period, subsequently became enriched in metals and then formed several Cu-porphyry bodies which were emplaced during a relatively short period towards the late Miocene.