Geochemistry and Sr–Nd–Pb isotopic composition of the Harrat Al-Madinah Volcanic Field,Saudi Arabia

Whole-rock geochemical and Sr, Nd and Pb isotope data are presented for the Harrat Al-Madinah volcanic field, in the north western part of the Arabian plate, aiming to understand their origin and the composition of their mantle source. This area is an active volcanic field characterized by the occurrence of two historic eruptions approximately in 641 and 1256 A.D. Field investigation of the main volcanic landforms indicates dominantly monogenetic strombolian eruptions, in addition to local phreatomagmatic eruption style. The lavas consist mainly of alkali olivine basalt, olivine transitional basalt, and hawaiite with ocean island basalt (OIB)-like characteristics. Evolved rocks, represented by mugearites, benmoreites, and trachytes, occur mainly as domes, tuff cones and occasionally as lava flows. Chemical variations in the evolved rocks indicated their evolution by low pressure crystal fractionation of olivine, plagioclase, clinopyroxene, and Fe–Ti oxides from the relatively primitive basalts. The isotopic compositions of ¹⁴³Nd/¹⁴⁴Nd (0.512954–0.512995), ⁸⁷Sr/⁸⁶Sr (0.702899 to–0.702977) and Pb (²⁰⁶Pb/²⁰⁴Pb = 18.5515–18.7446, ²⁰⁷Pb/²⁰⁴Pb = 15.5120–15.5222, ²⁰⁸Pb/²⁰⁴Pb = 38.1347–38.4468), show restricted variations suggesting only minor crustal contamination. They defined an array consistent with mixing of two geochemically distinct components of depleted MORB-mantle (DMM) and high ²³⁸U/²⁰⁴Pb ratio (HIMU). The variations in Tb/Yb, La/Yb and Sm/Yb ratios in the relatively primitive basalts (MgO > 6 wt.%) indicated garnet peridotite source. However, the positive Nb, Sr, Ba and Ti anomalies in the primitive mantle-normalized incompatible element patterns and the significant variation between Zr/Nb vs. Ce/Y and La/Yb vs. Yb suggest contribution of an amphibole-bearing spinel lherzolite source. Moreover, the negative correlations between SiO₂ vs. ⁸⁷Sr/⁸⁶Sr and Th vs. ¹⁴³Nd/¹⁴⁴Nd are interpreted as an indication of mixing melts derived from two end-members; one is garnet bearing asthenospheric source with OIB characteristic and the other is amphibole-bearing spinel lherzolite. The Harrat Al-Madinah volcanic field occurs near the Red Sea Rift System and its origin reflects a strong lithospheric control on the loci of partial melting. The dominantly NNW alignment patterns of the volcanoes, which is similar to the regional Red Sea trend, may suggest that the magmas were produced by decompression partial melting triggered by lithospheric extension related to the Red Rift.     

Volcanic eruptions in the Longgang volcanic field,northeastern China,during the past 15,000 years

The Longgang volcanic field, located in northeastern China, is volcanically active with a number of eruptions during the Quaternary but the chronology of the eruptions is poorly defined. Some tephra layers are well preserved in the annually laminated sediments of maar lakes in the region, and facilitate the construction of a much improved chronological framework for the volcanic history of the area. The results of our investigations reveal that three basaltic explosive eruptions occurred at AD 460, 11460 cal yr BP and 14000 cal yr BP, respectively. The largest explosive basaltic eruption (AD 460) produced a thick black scoria layer in the Longgang volcanic field, including lakes. The tephra distribution and chronological data suggest that this eruption is likely to be from the Jinlongdingzi volcano. Two basaltic flood eruptions occurred at Jinlongdingzi. The earlier basaltic eruption produced a lava flow that spread over a forest and encased standing trees. Two radiocarbon ages obtained from charcoal samples collected from the burned remains of these trees are 1828–1989 cal yr BP and 2164–2359 cal yr BP. In the most recent stage of volcanism, the lava flow extended only ca. 2 km, and flowed into Lake Dalongwan. From the present status of the forest ecosystem, which has not yet reached the fully mature successional stage, we estimate that this lava is very young (ca. a few hundreds years old). Jinlongdingzi is a potentially dangerous volcano. Monitoring and assessment of the potential hazards in the Longgang volcanic field should be carried out in the future.     

The Warnie volcanic province: Jurassic intraplate volcanism in Central Australia

The Cooper and Eromanga Basins of South Australia and Queensland are the largest onshore hydrocarbon producing region in Australia. Igneous rocks have been documented infrequently within end of well reports over the past 34 years, with a late Triassic to Jurassic age determined from well data. However, the areal extent and nature of these basaltic rocks were largely unclear. Here, we integrate seismic, well, gravity, and magnetic data to clarify the extent and character of igneous rocks preserved within Eromanga Basin stratigraphy overlying the Nappamerri Trough of the Cooper Basin. We recognise mafic monogenetic volcanoes that extend into tabular basalt lava flows, igneous intrusions and, more locally, hydrothermally altered compound lava flows. The volcanic province covers ∼7500 km² and is proposed to have been active between ∼180–160 Ma. We term this Jurassic volcanic province the Warnie Volcanic Province (WVP) after the Warnie East 1 exploration well, drilled in 1985. The distribution of extrusive and intrusive igneous rocks is primarily controlled by basement structure, with extrusive and intrusive igneous rocks elongate in a NW-SE direction. Finally, we detail how the WVP fits into the record of Jurassic volcanism in eastern Australia. The WVP is interpreted as a product of extension and intraplate convective upwelling above the subducting Pacific Slab. The discovery of the WVP raises the possibility of other, yet unidentified, volcanic provinces worldwide.     

40Ar/39Ar geochronological constraints on the age and evolution of the Permo-Triassic Emeishan Volcanic Province,Southwest China

The biostratigraphically constrained Permo-Triassic Emeishan Volcanic Province (EVP), extending over wide areas in southwest China, has been recently considered as a Large Igneous Province contemporaneous with the Siberian Traps and the siliceous tuffs at the P–T boundary in South China. We report the first ⁴⁰Ar/³⁹Ar ages on this igneous province. Minimum ages have been obtained on phenocrystic plagioclase of the Emeishan basalt, which has undergone a pervasive metamorphism, most likely during subsequent tectonization as a consequence of terrane amalgamation. Comparison between the Ar–release spectra obtained on clear vs. cloudy plagioclase indicates a 40–30 Ma sericite resetting time. A minimum apparent age of 246 ± 4 Ma for plagioclase from a plagiogranite, a late-differentiate of the Panzhihua Layered Complex, and an age of 254 ± 5 Ma for phlogopite from a pyroxenite near Lake Erhai, provide the first absolute age constraint on this igneous province. Additional Ar–Ar age measurements on post-Emeishan alkaline and mafic magmatism yielded 104 ± 2 and 100 ± 2 Ma for an alkaline complex near Panzhihua, and 42 ± 1 Ma for a gabbro sill emplaced near the Ertan Dam. Further study is still needed to determine the age of the Emeishan volcanic emission accurately, and to test the validity of the assumed short duration of the eruption.     

Magmatic-hydrothermal evolution of the Cretaceous Duolong gold-rich porphyry copper deposit in the Bangongco metallogenic belt,Tibet: Evidence from U-Pb and 40Ar/39Ar geochronology

The Duolong gold-rich porphyry copper deposit was recently discovered and represents a giant prospect (inferred resources of 4–5 Mt fine-Cu with a grade of 0.72% Cu; 30–50 t fine-gold with a grade of 0.23 g/t Au) in the Bangongco metallogenic belt, Tibet. Zircon SHRIMP and LA-ICP-MS U–Pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes, the first at about 121 Ma (Bolong mineralized granodiorite porphyry (BMGP) and barren granodiorite porphyry (BGP)) and the second about 116 Ma (Duobuza mineralized granodiorite porphyry (DMGP)). Moreover, the basaltic andesites also have two episodes at about 118 Ma and 106 Ma, respectively. One andesite yields an U–Pb zircon age of 111.9 ± 1.9 Ma, indicating it formed after the multiple granodiorite porphyries. By contrast, the ⁴⁰Ar/³⁹Ar age of 115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP) reveals the close temporal relationship of ore-bearing potassic alteration to the emplacement of the DMGP. The sericite from quartz-sericite vein (hosted in DMGP) yields a ⁴⁰Ar/³⁹Ar age of 115.2 ± 1.2 Ma. Therefore, the ore-forming magmatic-hydrothermal evolution probably persisted for 6 m.y. Additionally, the zircon U–Pb ages (106–121 Ma) of the volcanic rocks and the porphyries suggest that the Neo-Tethys Ocean was still subducting northward during the Early Cretaceous.     

40Ar/39Ar ages of flood basalt provinces in Russia and China and their possible link to global faunal extinction events: A cautionary tale regarding alteration and loss of 40Ar1

The hypothesis that the Permo–Triassic boundary (PTB) mass extinctions were caused by flood basalt volcanism in Russia (Siberian Traps) and/or China (the Emeishan Traps) is investigated from the point of view of time of occurrence (⁴⁰Ar/³⁹Ar ages). Numerous published ages in the literature are rejected as good estimates of the time of crystallization. The filters applied in this respect are (a) statistical reliability of plateau/isochron sections of stepheating data and (b) the alteration state of the material that was dated. Alteration appears to be ubiquitous, unsurprising since most of the material dated was used without acid leaching – a procedure that is effective in yielding fresh(er) samples. Of ∼70 ages in the literature for the main pulse of Siberian Trap volcanism, less than ten prove to be reliable ages. Similar techniques applied to ⁴⁰Ar/³⁹Ar for the Emeishan Traps, leaves only a single reliable age for the magmatic episode. These ages are compared to both published and new ⁴⁰Ar/³⁹Ar ages for the PTB as based on analysis of minerals from critical ash beds in China. There is good overlap in the ages (PTB – 250.0 ± 0.1 Ma, Siberian Trap lavas – 250.1 ± 0.4 Ma), lending credence to a genetic link between the formation of the Siberian Traps and the faunal extinction event at the PTB. A similar link for the formation of the Viluy Traps (Russia) and the Late Devonian extinction event is investigated; only a single reliable ⁴⁰Ar/³⁹Ar age is available for the Viluy Traps, and falls close to the interpolated age for the Frasnian–Fammenian boundary. The use of the unspiked K–Ar technique to yield accurate ages for such (altered) samples is questioned.A review of U–Pb data pertinent to these problems suggests a close temporal link between the formation of the Siberian Traps and the PTB. Comparison of U–Pb and ⁴⁰Ar/³⁹Ar ages for the PTB, raises questions about the accuracy of high precision sanidine ages, possibly resulting from very slow leakage of ⁴⁰Ar¹ from this mineral.     

Volcanic evolution of the back-arc Pleistocene Payun Matru volcanic field (Argentina)

For the first time, about 30 volcanic formations of the back-arc Payun Matru volcanic field (Payun Matru volcanic field, Argentina, 36°S, 69°W) have been sampled for K–Ar geochronology and geochemistry in order to reconstruct the eruptive history of this key province in the Andean back-arc. The Payun Matru volcanic field has been built since final Pleistocene until present with ages ranging from 280 ± 5 to 7 ± 1 ka. Erupted lavas belong to calc-alkaline series, with characteristics of both arc and intraplate magmas. From previous studies, three main units are distinguished: (1) a basaltic field (Los Volcanes), which covers a large surface of the Payun Matru volcanic field, composed of strombolian cones and associated lava flows emitted from 300 ka to Holocene times, (2) the stratovolcano Payun, with intermediate compositions, built around 265 ka, and (3) the shield volcano Payun Matru s.s. characterized by trachytic compositions and a large summit caldera. The earlier stages of the Payun Matru volcano are not dated, but we constrain the major explosive event, related to the eruption of a widespread ignimbrite and to the formation of the caldera, between 168 ± 4 ka (internal wall of caldera) and 82 ± 1 ka (flow within the caldera). Based on the geochemical similarities of the ignimbrite and the upper lava flow of the pre-caldera cone, we suggest that the age of this event is most probably at the older end of this interval. Numerical modeling using a GIS program has been used to reconstruct the morphological evolution for Payun Matru volcano before and after the caldera collapse. The ancient edifice could be modeled as a flattened cone, 2300 m high, with a volume of about 240 km³. The ignimbrite eruption associated with the Payun Matru caldera formation could be related to the regional tectonic environment, which is characterized by multiple Plio-Pleistocene extensional stages during the last 5 Myr. The evolution of the Nazca plate subduction from a flat slab to a normal dip induced an input of fluid mobile elements and asthenosphere plume-like mantle source beneath the Patagonian lithosphere, which yields the observed intraplate signature. We also interpret this geodynamic evolution as the influence of extensive processes in the upper crust leading to caldera-forming eruptions as observed throughout this province.     

Illite authigenesis in sandstones of the Guaritas Allogroup (Early Paleozoic): Implications for the depositional age,stratigraphy and evolution of the Camaquã Basin (Southern Brazil)

Several analytical studies performed on alluvial-eolian sandstones of the Early Paleozoic Guaritas Allogroup (Camaquã Basin, southern Brazil) indicate illite to be abundant, showing different morphologies as authigenic grain rims and pore-bridging filaments. Authigenic illite separates of variable grain sizes from distinct stratigraphic intervals of the Guaritas Allogroup yielded ⁴⁰K–⁴⁰Ar ages from 521.7 ± 10.3 to 473.7 ± 9.4 Ma. These ages, interpreted to record the timing of illite authigenesis, are coincident with the age of emplacement of the Rodeio Velho andesites (470 ± 19 Ma). Moreover, field structures suggest interaction between hot, andesite lava flows and wet, poorly consolidated sediments of the Pedra Pintada Alloformation (lower strata of the Guaritas Allogroup). This set of data indicates that the Rodeio Velho volcanism could have been responsible for a widespread remobilization of interstitial fluids and consequent authigenic illite precipitation in the sandstones of the Guaritas Allogroup.     

Calibration of GA1550 biotite standard for K/Ar and 40Ar/39Ar dating

The mineral separate GA1550 biotite has become an international standard for K/Ar and ⁴⁰Ar/³⁹Ar dating studies, although it was prepared as an intralaboratory standard at ANU to monitor tracer depletion from a gas pipette. It is one of a small number of samples that has been calibrated against ³⁸Ar tracers, some of which had been mixed with known amounts of atmospheric argon, so that a so-called primary calibration has been performed. By measuring GA1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1.342 (± 0.007) × 10^? 9 mol/g, and together with the measured K content of 7.645 (± 0.050) weight percent, we derive a best estimate for the K/Ar age as 98.5 ± 0.5 Ma, where the error is derived from averaging the ages determined relative to the ³⁸Ar tracer.     

Tectonometamorphic evolution of Seram and Ambon,eastern Indonesia: Insights from 40Ar/39Ar geochronology

The island of Seram, eastern Indonesia, experienced a complex Neogene history of multiple metamorphic and deformational events driven by Australia–SE Asia collision. Geological mapping, and structural and petrographic analysis has identified two main phases in the island's tectonic, metamorphic, and magmatic evolution: (1) an initial episode of extreme extension that exhumed hot lherzolites from the subcontinental lithospheric mantle and drove ultrahigh-temperature metamorphism and melting of adjacent continental crust; and (2) subsequent episodes of extensional detachment faulting and strike-slip faulting that further exhumed granulites and mantle rocks across Seram and Ambon. Here we present the results of sixteen ⁴⁰Ar/³⁹Ar furnace step heating experiments on white mica, biotite, and phlogopite for a suite of twelve rocks that were targeted to further unravel Seram's tectonic and metamorphic history. Despite a wide lithological and structural diversity among the samples, there is a remarkable degree of correlation between the ⁴⁰Ar/³⁹Ar ages recorded by different rock types situated in different structural settings, recording thermal events at 16 Ma, 5.7 Ma, 4.5 Ma, and 3.4 Ma. These frequently measured ages are defined, in most instances, by two or more ⁴⁰Ar/³⁹Ar ages that are identical within error. At 16 Ma, a major kyanite-grade metamorphic event affected the Tehoru Formation across western and central Seram, coincident with ultrahigh-temperature metamorphism and melting of granulite-facies rocks comprising the Kobipoto Complex, and the intrusion of lamprophyres. Later, at 5.7 Ma, Kobipoto Complex rocks were exhumed beneath extensional detachment faults on the Kaibobo Peninsula of western Seram, heating and shearing adjacent Tehoru Formation schists to form Taunusa Complex gneisses. Then, at 4.5 Ma, ⁴⁰Ar/³⁹Ar ages record deformation within the Kawa Shear Zone (central Seram) and overprinting of detachment faults in western Seram. Finally, at 3.4 Ma, Kobipoto Complex migmatites were exhumed on Ambon, at the same time as deformation within the Kawa Shear Zone and further overprinting of detachments in western Seram. These ages support there having been multiple synchronised episodes of high-temperature extension and strike-slip faulting, interpreted to be the result of Western Seram having been ripped off from SE Sulawesi, extended, and dragged east by subduction rollback of the Banda Slab.     

The volcanic succession of Baoligaomiao,central Inner Mongolia: Evidence for Carboniferous continental arc in the central Asian orogenic belt

The Baoligaomiao Formation, within the Hegenshan ophiolite-arc-accretion complex is an important segment to understand the tectonic evolution of the Central Asian Orogenic Belt (CAOB), world's largest Phanerozoic orogenic belt. In this study, we present an integrated study of zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopic data from the volcanic succession in the Baoligaomiao Formation. The volcanic succession can be divided into the lower sequence with zircon U-Pb ages in the range of 326.3 Ma–307.4 Ma and the upper sequence of 305.3 Ma. The succession belongs to two suites: calc-alkaline volcanics and high-Si rhyolites. The calc-alkaline volcanic rocks include basaltic andesite through andesite and dacite to rhyolite and their pyroclastic equivalents. These rocks exhibit a well-defined compositional trend from basaltic to rhyolitic magma, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. The calc-alkaline rocks have low initial ⁸⁷Sr/⁸⁶Sr (0.7023–0.7052), positive ɛNd(t) values (2.75–4.80), and their initial Pb isotopic compositions are 17.875–18.485 of ²⁰⁶Pb/²⁰⁴Pb, 15.481–15.520 of ²⁰⁷Pb/²⁰⁴Pb and 37.467–37.764 of ²⁰⁸Pb/²⁰⁴Pb, respectively. Geochemical and isotopic results suggest that the volcanic succession represents Carboniferous subduction-related, mature, continental arc volcanism. The outcrops of high-Si rhyolites are restricted to the northern edge of the continental arc, marking a transition zone between volcanic arc and back-arc basin, where they are interbedded with the calc-alkaline rocks in the lower sequence, and the upper sequence is composed only of high-Si rhyolites. The high-Si rhyolites have high SiO₂ (71.12–81.76 wt%) and varied total alkali contents (K₂O + Na₂O = 5.46–10.58 wt%), low TiO₂ (0.06–0.27 wt%), MgO (0.09–0.89 wt%) and CaO (0.08–0.72 wt%). Based on the presence of mafic alkali phenocrysts, such as arfvedsonite and siderophyllite, high Zr/Nb ratios (> 10) and peralkalinity index (PI) near unity, the high-Si rhyolites can be classified as peralkaline comendites. The high-Si rhyolites are characterized by unusually low Sr and Ba, and high abundance of Zr, Th, Nb, HREEs and Y. They show geochemical characteristics similar to those of typical A₂-type granites including their high K₂O + Na₂O, Nb, Zr and Y, and high ratios of FeO^T/MgO, Ga/Al and Y/Nb. Our study suggests that the high-Si rhyolites were derived from discrete trachytic parent magma with fractional crystallization within shallow magma reservoirs. Their Nd-Pb isotopic characteristics are similar to those of the calc-alkaline arc rocks and are compatible with partial melting of pre-existing juvenile continental arc crust. We observe that the widespread eruptions of A₂-rhyolitic magmas (305.3 Ma–303.4 Ma) following a short period of magmatic quiescence was temporally and spatially associated with voluminous intrusion of the bimodal magmas (304.3 Ma–299.3 Ma) in the pre-existing arc volcanic-plutonic belt (329 Ma–307 Ma). We envisage northward subduction and slab breakoff process resulting in an obvious change of the regional stress field to extensional setting within the Carboniferous continental arc running E-W for thousands of kilometers. Therefore, we propose the existence of an east-west-trending Carboniferous continental arc in the Hegenshan ophiolite-arc-accretion complex, with the slab breakoff event suggesting that the age of the upper sequence (305.3 ± 5.5 Ma) likely indicates the maximum age for the cessation of the northward subduction of the Hegenshan oceanic lithosphere.     

Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit,Eastern Tianshan Mountains,NW China: Constraints on the metallogenesis

The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4 ± 0.94 and 323.47 ± 0.95 Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO₂ (42.90–46.61 wt.%), P₂O₅ (0.08–0.12 wt.%), MnO (0.35–0.97 wt.%) and TiO₂ (0.74–0.82 wt.%), and relatively large variations in CaO (6.93–15.13 wt.%), Al₂O₃ (14.71–19.93 wt.%), total Fe₂O₃ (8.14–12.66 wt.%) and MgO (4.96–8.52 wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type.     

First documented deep submarine explosive eruptions at the Marsili Seamount (Tyrrhenian Sea,Italy): A case of historical volcanism in the Mediterranean Sea

The Marsili Seamount (MS) is an about 3200 m high volcanic complex measuring 70 × 30 km with the top at ~ 500 m b.s.l. MS is interpreted as the ridge of the 2 Ma old Marsili back-arc basin belonging to the Calabrian Arc–Ionian Sea subduction system (Southern Tyrrhenian Sea, Italy). Previous studies indicate that the MS activity developed between 1 and 0.1 Ma through effusions of lava flows. Here, new stratigraphic, textural, geochemical, and ¹⁴C geochronological data from a 95 cm long gravity core (COR02) recovered at 839 m bsl in the MS central sector are presented. COR02 contains mud and two tephras consisting of 98 to 100 area% of volcanic ash. The thickness of the upper tephra (TEPH01) is 15 cm, and that of the lower tephra (TEPH02) is 60 cm. The tephras have poor to moderate sorting, loose to partly welded levels, and erosive contacts, which imply a short distance source of the pyroclastics. ¹⁴C dating on fossils above and below TEPH01 gives an age of 3 ka BP. Calculations of the sedimentation rates from the mud sediments above and between the tephras suggest that a formation of TEPH02 at 5 ka BP MS ashes has a high-K calcalkaline affinity with 53 wt.% < SiO₂ < 68 wt.%, and their composition overlaps that of the MS lava flows. The trace element pattern is consistent with fractional crystallization from a common, OIB-like basalt. The source area of ashes is the central sector of MS and not a subaerial volcano of the Campanian and/or Aeolian Quaternary volcanic districts. Submarine, explosive eruptions occurred at MS in historical times: this is the first evidence of explosive volcanic activity at a significant (500–800 m bsl) water depth in the Mediterranean Sea. MS is still active, the monitoring and an evaluation of the different types of hazards are highly recommended.     

40Ar/39Ar age pattern associated with differential uplift along the Eastern Highlands shear zone,Cape Breton Island,Canadian Appalachians

The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende ⁴⁰Ar/³⁹Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The ⁴⁰Ar/³⁹Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The ⁴⁰Ar/³⁹Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones.     

Molybdenite Re–Os and albite 40Ar/39Ar dating of Cu–Au–Mo and magnetite porphyry systems in the Yangtze River valley and metallogenic implications

The area of the Middle–Lower Yangtze River valley, Eastern China, extending from Wuhan (Hubei province) to western Zhenjiang (Jiangsu province), hosts an important belt of Cu–Au–Mo and Fe deposits. There are two styles of mineralization, i.e., skarn/porphyry/stratabound Cu–Au–Mo–(Fe) deposits and magnetite porphyry deposits in several NNE-trending Cretaceous fault-bound volcanic basins. The origin of both deposit systems is much debated. We dated 11 molybdenite samples from five skarn/porphyry Cu–Au–Mo deposits and 5 molybdenite samples from the Datuanshan stratabound Cu–Au–Mo deposit by ICP-MS Re–Os isotope analysis. Nine samples from the same set were additionally analyzed by NTIMS on Re–Os. Results from the two methods are almost identical. The Re–Os model ages of 16 molybdenite samples range from 134.7 ± 2.3 to 143.7 ± 1.6 Ma (2σ). The model ages of the five samples from the Datuanshan stratabound deposit vary from 138.0 ± 3.2 to 140.8 ± 2.0 Ma, with a mean of 139.3 ± 2.6 Ma; their isochron age is 139.1 ± 2.7 Ma with an initial Os ratio of 0.7 ± 8.1 (MSWD = 0.29). These data indicate that the porphyry/skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system. Albite ⁴⁰Ar/³⁹Ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 Ma, i.e., 10–20 Ma later. Both mineralization styles characterize transitional geodynamic regimes, i.e., the period around 140 Ma when the main NS-trending compressional regime changed to an EW-trending lithospheric extensional regime, and the period of 125–115 Ma of dramatic EW-trending lithospheric extension.     

Shallow submarine volcano group in the early stage of island arc development: Geology and petrology of small islands south off Hahajima main island,the Ogasawara Islands

Small Islands south off Hahajima, the southernmost of the Ogasawara Archipelago, consist of primitive basalts (<12 wt.% MgO) to dacite erupted during the transitional stage immediately following boninite volcanism on the incipient arc to sustained typical oceanic arc. Strombolian to Hawaiian fissure eruptions occurring on independent volcanic centers for the individual islands under a shallow sea produced magnesian basalt to dacite fall-out tephras, hyaloclastite and a small volume of pillow lava, which were intruded by NE-trending dikes. These volcanic strata are correlated to the upper part (<40 Ma) of the Hahajima main island. Volcanic rock samples have slightly lower FeO^*/MgO ratios than the present volcanic front lavas, and are divided into three types with high, medium and low La/Yb ratios. Basalt to dacite of high- and medium-La/Yb types show both tholeiitic (TH) and calc-alkaline (CA) differentiation trends. Low-La/Yb type belongs only to TH basalt. The multiple magma types are coexistence on the each island. TH basalts have phenocrysts of olivine, clinopyroxene and plagioclase, while CA basalts are free from plagioclase phenocrysts.     

40Ar/39Ar dating of the Jiehe gold deposit in the Jiaodong Peninsula,eastern North China Craton: Implications for regional gold metallogeny

The Jiehe gold deposit, containing a confirmed gold reserve of 34 tonnes (t), is a Jiaojia-type (disseminated/stockwork-style) gold deposit in Jiaodong Peninsula. Orebodies are hosted in the contact zone between the Jurassic Moshan biotite granite and the Cretaceous Shangzhuang porphyritic granodiorite, and are structurally controlled by the NNE- to NE-striking Wangershan-Hedong Fault. Sulphide minerals are composed predominantly of pyrite with lesser amounts of chalcopyrite, galena, and sphalerite. Hydrothermal alteration is strictly controlled by fracture zones, in which disseminated sulfides and native gold are spatially associated with pervasive sericitic alteration. Mineralogical, textural, and field relationships indicate four stages of alteration and mineralization, including pyrite-bearing milky and massive quartz (stage 1), light-gray granular quartz–pyrite (stage 2), quartz–polysulfide (stage 3) and quartz–carbonate (stage 4) stages. Economic gold is precipitated in stages 2 and 3.The Jiehe deposit was previously considered to form during the Eocene (46.5 ± 2.3 Ma), based on Rb-Sr dating of sericite. However, ⁴⁰Ar/³⁹Ar dating of sericite in this study yields well-defined, reproducible plateau ages between 118.8 ± 0.7 Ma and 120.7 ± 0.8 Ma. These ⁴⁰Ar/³⁹Ar ages are consistent with geochronological data from other gold deposits in the region, indicating that all gold deposits in Jiaodong formed in a short-term period around 120 Ma. The giant gold mineralization event has a tight relationship with the extensional tectonic regime, and is a shallow crustal metallogenic response of paleo-Pacific slab subduction and lithospheric destruction in the eastern NCC.     

Paleocene alkaline volcanism in the Nares Strait region: evidence from volcanic pebbles

Paleogene sediments in fault-bounded basins on Judge Daly Promontory, northeast Ellesmere Island, Canadian High Arctic, are rich in volcanogenic material. Volcanic pebbles within the Cape Back basin near Nares Strait were studied for their petrography, geochemistry, Sr and Nd isotopes, and geochronology to identify and characterize their parent rock. The pebbles are derived from lava flows and ignimbrites of a continental rift-related, strongly differentiated, highly incompatible element enriched, alkaline volcanic suite, the proposed Nares Strait volcanic suite, which is distinct from other alkaline volcanic suites on the northern coasts of Ellesmere Island and Greenland. ⁴⁰Ar/³⁹Ar amphibole and alkali feldspar ages indicate that volcanism was active around 61–58 Ma and was probably contemporaneous with sedimentation resulting in Middle to Late Paleocene age for deposition within the Cape Back basin and the other Paleogene basins on Judge Daly Promontory.     

40Ar–39Ar geochronology across Archean and Paleoproterozoic terranes from southeastern Guiana Shield (north of Amazonian Craton,Brazil): Evidence for contrasting cooling histories

A ⁴⁰Ar/³⁹Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma.These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new ⁴⁰Ar/³⁹Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T–t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.     

Paleo-Tethyan tectonic evolution of Lancangjiang metamorphic complex: Evidence from SHRIMP U-Pb zircon dating and 40Ar/39Ar isotope geochronology of blueschists in Xiaoheijiang-Xiayun area,Southeastern Tibetan Plateau

Blueschists are sporadically exposed as lenses within the Lancangjiang metamorphic complex, and represent unique components of the Paleo-Tethys. In this paper, we present geochemical and geochronological results of blueschists to decipher their origin and tectonic significance. The whole-rock geochemical analyses revealed strong similarities with ocean island basalt (OIB), and further discrimination diagrams confirm an affinity to a within-plate setting. Combined studies on blueschists using cathodoluminescence (CL) imaging, SHRIMP U-Pb dating of zircon domains and ⁴⁰Ar/³⁹Ar dating of phengite and glaucophane provide evidence of their magmatic origin and metamorphic evolution. Slightly oscillatory zoned or compositionally homogeneous zircon grains/domains, as well as structureless zircon rims, yield ages from 231.6 ± 3.7 to 225.3 ± 4.8 Ma, recording the blueschist facies metamorphic event. In contrast, the captured zircon grains and cores with a major age peak at ~241 Ma as well as several minor older age peaks indicate the multiple provenance of the zircons. ⁴⁰Ar/³⁹Ar step heating analyses on single grains of phengite and glaucophane separated from blueschists yield plateau ages ranging from 242.5 ± 1.4 to 228.7 ± 1.5 Ma which are interpreted to reflect high-pressure metamorphism.This study provides geochemical and geochronological constraints on the tectonic evolution of the Paleo-Tethyan ocean, which was closed and subsequently subducted as a result of the collision of the Simao and Baoshan Blocks. During subduction in Trassic (243 to 225 Ma), the protoliths of blueschists underwent blueschist facies conditions.