U-Pb isotope dating of igneous complexes from the milogradovo gold-silver deposit (Southern Primor’e)

The Milogradovo gold-silver deposit, which is of the intracaldera structural type, was formed within the ore-magmatogenic system (OMS), which combines a set of genetically and paragenetically related subvolcanic complexes, cover volcanogenic units, diatremes, hydrothermal rocks, and ore zones of variable mineral associations. The deposit is located within the Limovskaya caldera. The first U-Pb dating of zircons from the caldera’s igneous rocks using the SHRIMP method has shown that these are related to the Eocene stage of magmatism (the Milogradovo volcanic-extrusive complex). Such types of volcanic structures are widespread in the East Sikhote Alin Belt and are the most promising in the sense of Au-Ag mineralization.     

华南花岗质潜火山杂岩与富铀矿的成矿关系及找矿前景

本文探讨了华南地区火山岩型或花岗岩型富铀矿，与岩浆旋回晚期的构造－岩浆活动的关系，火山岩型富铀矿离不开大规模火山活动的晚期次火山活动，而花岗岩型富铀矿，则与主侵入晚期的中细粒，不等粒小岩体密切相关，经过长期研究，根据岩浆作用性质，将这套独具特色并与富铀矿的形成息息相关的岩定，定为花岗质潜火山杂岩。     

华南东部中新生代火山岩型铀矿成矿规律,勘查模式及找矿远景

中新生代时期，华南东部地壳经历了挤压－松驰拉张－局部裂解的演化过程。相应地发育３期火山活动，构成初火山族回、主火山旋回和破火山旋回。破火山使回出现双峰式岩套，标志着已进入胚胎型裂谷演化阶段。火山岩型铀矿具有破火山族回成矿、破火山旋回活动带控矿、二次迭加富集特性，可称其为破火山系列新不整合脉型铀矿。划分为体型、层型和脉型３种形态类型和相应的勘查模式。华南东部破火山系列新不整合脉型铀矿具有巨大的找矿潜力，在花岗岩基底上发育的破火山旋回构造－岩浆杂岩区，是今后寻找大型铀矿的有利靶区。     

Evolution of silicic magmas in the Kos-Nisyros volcanic center,Greece: a petrological cycle associated with caldera collapse

Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center (eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures (≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content, following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units) were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions (more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO₂ rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large, potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center.     

Zircons of igneous rocks at the Erdenetuin-Obo porphyry Cu-Mo deposit (Mongolia): U-Pb dating and petrological implications

The Erdenetuin-Obo porphyry Cu-Mo deposit was formed at the final stage of development of magmatic activity occasionally manifested in the Late Permian-Early Triassic in the period of at least 40 Ma. Early plutonic (host) and late ore-bearing porphyry intrusive complexes were formed in that period. The plutonic complex is multiphase, while the porphyry complex is polyrhythmical and multiphase within rhythms. The obtained data on the U-Pb isotopic composition (SHRIMP II) of zircons from unaltered rocks of the ore field are discussed: gabbro, diorite, and granodiorite of the plutonic complex and granodiorite-porphyry I and II of the first and second rhythms of the ore-bearing complex, respectively. Zircons of different age levels and genotypes were identified in the course of performed investigations. Gabbro are dominated by postmagmatic (superimposed) zircons with the datings of 239–225 Ma. The age of xenogenic zircon brought out from the basement rocks is estimated at 1146 ± 11 Ma. Zircons occur as magmatic and postmagmatic (superimposed) minerals dated 252–247, 244–233 Ma in diorite and 244–242, 239–224 Ma in granodiorite. The ages of postmagmatic zircons from diorite are partially overlapped by datings of magmatic zircons from granodiorite and granodiorite-porphyry. In the porphyry complex, the datings of magmatic zircons are 240–234 and 222–220 Ma in granodiorite-porphyries I and II, respectively. There are also inherited zircons with datings coinciding with those of magmatic zircons from precursor intrusive rocks. Datings of such zircons are 249–241 and 257–231 Ma for granodiorite-porphyries I and II, respectively. As a whole, zircon datings in all studied igneous rocks forming a virtually uninterrupted range in the period of 257–220 Ma allow us to suggest the relation of the ore magmatic system to the long-living constantly active deep source occasionally delivering melt to the upper levels.     

Thermal Convection of Fluids as a Possible Mechanism for the Formation of the Unique Streltsovka and Antei Uranium Deposits (Eastern Transbaikalia)

Geology of Ore Deposits - The hydrothermal molybdenum–uranium deposits of the Streltsovka ore field are localized in a volcanic caldera of Late Mesozoic age. The paper presents the results of...     

Development of a Continental Volcanic Field: Petrogenesis of Pre-caldera Intermediate and Silicic Rocks and Origin of the Bandelier Magmas, Jemez Mountains (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF) is the site of the Valles caldera and associated BandelierTuff. Caldera formation was preceded by > 10 Myr of volcanismdominated by intermediate composition rocks (57–70% SiO₂)that contain components derived from the lithospheric mantleand Precambrian crust. Simple mixing between crust-dominatedsilicic melts and mantle-dominated mafic magmas, fractionalcrystallization, and assimilation accompanied by fractionalcrystallization are the principal mechanisms involved in theproduction of these intermediate lavas. A variety of isotopicallydistinct crustal sources were involved in magmatism between13 and 6 Ma, but only one type (or two very similar types) ofcrust between 6 and 2 Ma. This long history constitutes a recordof accommodation of mantle-derived magma in the crust by meltingof country rock. The post-2 Ma Bandelier Tuff and associatedrhyolites were, in contrast, generated by melting of hybridizedcrust in the form of buried, warm intrusive rocks associatedwith pre-6 Ma activity. Major shifts in the location, styleand geochemical character of magmatism in the JMVF occur withina few million years after volcanic maxima and may correspondto pooling of magma at a new location in the crust followingsolidification of earlier magma chambers that acted as trapsfor basaltic replenishment. KEY WORDS: crustal anatexis; fractional crystallization; Jemez Mountain Volcanic Field; Valles Caldera; radiogenic isotopes; trace elements     

Crystal scale anatomy of a dying supervolcano: an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the Yellowstone caldera

A voluminous (>600 km³) and long-lived (~520–75 ka) phase of rhyolitic eruptions followed collapse of the Yellowstone caldera 640 ka. Whether these eruptions represent a dying cycle, or the growth of a new magma chamber, remains an important question. We use new U–Th zircon ages and δ¹⁸O values determined by ion microprobe, and sanidine Pb isotope ratios determined by laser ablation, to investigate the genesis of voluminous post-caldera rhyolites. The oldest post-caldera rhyolites, erupted between ~520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-δ¹⁸O melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from ~260 to 75 ka, with homogeneous low-δ¹⁸O zircon values of 2.7–2.8‰ that are in equilibrium with low-δ¹⁸O host melts for the majority of the youngest eruptions. New sanidine Pb isotope data define separate arrays for post-caldera rhyolites and the caldera-forming tuffs that preceded them, indicating that they were not sourced from a mushy Lava Creek Tuff batholith that remained after caldera collapse. Rather, our new age and isotopic data indicate that the post-caldera rhyolites were generated by remelting of a variety of intracaldera source rocks, consisting of pre-Lava Creek Tuff volcanic and plutonic rocks and earlier erupted post-Lava Creek Tuff rhyolites. Batch assembly of low-δ¹⁸O melts starting at ~260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption ~75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera.     

Magmatic Evolution of the Ordovician Snowdon Volcanic Centre, North Wales (UK)

The Ordovician Snowdon Volcanic Centre (SVC) of North Walescomprises a bimodal basalt–subalkaline/peralkaline associationemplaced around a caldera within a shallow marine environment.The tectonic setting was associated with closure of the LowerPalaeozoic Iapetus Ocean and cessation of ocean plate subduction.The SVC volcanic products include basaltic lavas and pyroclasticrocks, rhyolitic pyroclastic flow deposits, high-level intrusions,domes, and flows, together with reworked equivalents. A programmeof detailed field mapping, sampling, and chemical analysis hasbeen used to evaluate the structure and magmatic evolution ofthe SVC volcanic system. SVC basalts show a range in chemicalcharacteristics between volcanic arc type and within-plate,ocean island basalt (OIB) type. Subalkaline, silica-oversaturatedintermediate intrusions (icelandites) and five chemically distinctgroups of extrusive and intrusive subalkaline/peralkaline rhyolites(termed A1, A2, B1, B2, and B3) were emplaced during the evolutionof the SVC. This evolution was driven by material and thermalinput from basaltic magma. The SVC basaltic lavas were derivedas partial melts from a heterogeneous volcanic arc to OIB-typespinel lherzolite mantle and experienced up to 60% olivine gabbrofractionation during storage in sill networks in the sub-crustor lower crust. Some magma batches experienced further fractionalcrystallization ({small tilde}70%) and minor crustal contamination({small tilde}10%) to yield the icelandites. Trace element andNd isotope data do not favour an origin for the rhyolites bypartial or total fusion of likely crustal material, and thefive rhyolite groups are regarded as distinct homogeneous batchesof magma derived from varied basaltic magmas. The icelanditesand peralkaline rhyolites (group B3) result, respectively, from{small tilde}50% and {small tilde}80–90% zircon-free fractionalcrystallization of SVC basalts. The subalkaline rhyolites (groupsA1 and B1) result from {small tilde}80–90% fractionalcrystallization of subduction-related basalts similar to thoseof Ordovician basalts which pre-date the Lower Rhyolitic TuffFormation, and groups A2 and B2 were formed by mixing and homogenizationof A1, B1, and B3 magma batches. These data and interpretationsprovide the basis of a model for the complex evolution of asilicic magma system below the SVC caldera around the time ofcessation of Caledonian subduction in North Wales. Rhyolitemagma chambers were short lived and discontinuous; the largestwas probably disc shaped and was almost entirely evacuated duringa >60-km³ ash-flow eruption.     

The New Progress in the Study of Mesozoic Rhyolite-Trachyte Assemblage and Hydrothermal-type Uranium Mineralization in Eastern China

The most intense area of Mesozoic volcanism and main region of hydrothermal-type uranium deposits is located in Eastern China. From the northern to the southern part, it can be divided into seven volcanic belts of Great Xing’an Range, Lesser Xing’an-Zhangguangcai Ranges, Northern Hebei-Western Liaoning, the Lower Yangtze Region, Ganhang areas, Wuyi Mountain areas,the Southeast Coastal areas, five uranium metallogenic belts of Guyuan-Hongshanzi, Qinglong-Xingcheng, Luzong-Qixia, Ganhang, Wuyi Mountain, and Three uranium metallogenic perspective belts of Manzhouli-Erguna, Zhalantun, Yichun. The volcanism of all these volcanic belts can be subdivided into six stages: The Early Jurassic to early Middle Jurassic, late Middle Jurassic to early Late Jurassic, early Early Cretaceous, middle Early Cretaceous, late Early Cretaceous and early Late Cretaceous. High-K calc-alkaline rhyolite-alkali trachyte rock assemblage of the early Early Cretaceous has a close connection with the explored uranium deposits. High-K calc-alkaline rhyolites have high content of uranium, and can provide the epithermal ore forming system with uranium; Alkali trachyte associated with mantle-derived magmatism can provide alkaline ore-forming fluid of rich uranium for deep temperature mineralizing system or act as pioneers of alkaline ore-forming fluid of rich uranium.     

华南中生代花岗岩型、火山岩型、外接触带型铀矿找矿思路（I）

华南中生代铀成矿作有处于中国东部燕山期强烈火山岩浆作用、壳幔作用及流体作用时期,铀具有大规模聚集成矿的前景。因此,中生代花岗岩型、火山岩型、外接触带型铀矿找矿必须引进深源成矿理论,淡化“浅成低温活化成矿理论”,充分认识铀矿既可形成于浅源浅成条件,也能形成于深源深成高温条件及深源浅成中温条件的特点,重视早阶段（130－95Ma）铀矿化,打破以主岩类型找矿的人为界限,加强白垩纪岩浆岩体系的研究。     

The giant Imiter silver deposit: Neoproterozoic epithermal mineralization in the Anti-Atlas, Morocco

The world-class Imiter silver deposit, in the Anti-Atlas Mountains of Morocco, is a Neoproterozoic epithermal vein deposit genetically associated with a felsic volcanic event, and formed within a regional extensional tectonic regime. Rhyolitic volcanism related to ore formation has been dated at 550Dž Ma by ion-probe U/Pb on zircons. The economic silver mineralization is superimposed on an older, discrete base-metal assemblage associated with calc-alkaline granodioritic magmatism. The magmatism is dated at 572LJ Ma by ion-probe U/Pb dating on zircons, and by ⁴⁰Ar/³⁹Ar dating on hydrothermal muscovites. In the Anti-Atlas Mountains, the Precambrian-Cambrian transition appears as an important period for the formation of major, productive precious-metal deposits associated with volcanic events and extensional tectonics. The Imiter silver deposit constitutes a Precambrian analogue to modern epithermal deposits.     

U-Pb SHRIMP ages of granitoides from the main batholith belt (North East Asia)

U-Pb SHRIMP-dating of zircons from twenty five intrusions representative of the Main granitoid batholith belt and associated dike swarms (Yano-Kolyma gold bearing province, North East of Asia) are mostly ∼150 ± 3 Ma (Kimmeridgian-Tithonian). Two less widespread impulses of magmatism dated at 160–155 Ma and 146–143 Ma representing the full range of ages present in the Main belt. Paleoproterozoic (∼1.8 Ga) inherited zircons were found in three intrusions from the south-western part of the belt where Precambrian crust of the North-Asia craton is inferred to underlie it. Published in Russian in Doklady Akademii Nauk, 2009, Vol. 426, No. 2, pp. 216–221. The article was translated by the authors.     

Pongkor (west Java, Indonesia): a Pliocene supergene-enriched epithermal Au-Ag-(Mn) deposit

The Pongkor gold-silver epithermal deposit with reserves of at least 98 tonnes of gold and 1026 tonnes of silver, average grades 16.4 g/t Au and 171.2 g/t Ag is one of the most recent and largest gold and silver discoveries in Indonesia, proven within a short period (1988–1991). ⁴⁰Ar/³⁹Ar dating on adularia samples give an age of 2.05 ± 0.05 Ma. The deposit is of the low-sulfidation epithermal type and consists of four main mineralized quartz veins located close to the internal rim of a volcano-tectonic depression (caldera). This resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks. This volcanic unit unconformably overlies Miocene subaqueous volcanic andesitic rocks with interbedded epiclastic rocks. The mineralized bodies are thick (average 4.2 m), steeply dipping, quartz-carbonate-adularia veins with a very low sulfide content (<0.5 wt.%). Their genesis is related to an extensional episode within a tectonic corridor showing NW-SE and NNE-SSW conjugate strike-slip faults, the major vein being located on the inner rim of the caldera. The vein fill reveals four successive stages of deposition marked by a specific facies: (1) carbonate-quartz breccia with dominant quartz and calcite and minor kutnahorite, rhodochrosite, and rhodonite (CQ facies), (2) a network of banded quartz and former carbonate transformed into manganese oxides through supergene alteration (MOQ facies), (3) banded opaline milky quartz (BOQ facies), and (4) grey, locally banded, sulfide-rich quartz breccia cutting all the other types (GSQ facies). Adularia was deposited at the same time as the quartz. The mineralogy and internal structures of the veins (crustiform banding, vugs, collapse breccia) clearly indicate a dilational context, which is common in low-sulfidation epithermal systems. Gold and silver grades, as well as sulfide mineral abundances, increase steadily through stages 1 to 4, locally reaching 1 kg/t in the GSQ facies. The sulfides are dominated by pyrite, accompanied by common acanthite-aguilarite, polybasite-pearceite and electrum in which the gold content ranges from 48 to 74 wt.%. Sphalerite, galena, chalcopyrite and hessite are fairly rare, although present within the CQ facies. The fluid inclusions of the four facies show homogenization temperatures ranging from 150 to 382 °C, indicating boiling of a hydrothermal fluid with an initial temperature of around 205 °C; no marked difference is seen in the GSQ facies, which has the highest gold content. Salinities are low, generally below 1 wt.% eq. NaCl. Lead isotope compositions of the associated volcanic rocks and the mineralization are very similar, ²⁰⁶Pb/²⁰⁴Pb between 18.706 and 18.814␣and between 18.744 and 18.801 respectively, demonstrating a genetic link between the Pliocene volcanism and the auriferous hydrothermal activity. The isotopic signature suggests that the source of the mineralization and associated volcanic rocks is an underlying ancient continental crust that melted and remobilized during the Pliocene volcanic and hydrothermal events. These conclusions seem applicable to the entire Bayah Dome. The existence of both a tectonic corridor and a caldera favoured channelling of the hydrothermal fluids and the deposition of primary ore in the veins. Late intense weathering of the ore deposit, to depths of 250 m below the surface, has given rise to manganese oxide layers, limonite zones, and silver micronuggets within the veins, as well as to gold enrichment. Received: 25 June 1997 / Accepted: 10 March 1998     

Lower Pliensbachian caldera volcanism in high-obliquity rift systems in the western North Patagonian Massif,Argentina

In the Cerro Carro Quebrado and Cerro Catri Cura area, located at the border between the Neuquén Basin and the North Patagonian Massif, the Garamilla Formation is composed of four volcanic stages: 1) andesitic lava-flows related to the beginning of the volcanic system; 2) basal massive lithic breccias that represent the caldera collapse; 3) voluminous, coarse-crystal rich massive lava-like ignimbrites related to multiple, steady eruptions that represent the principal infill of the system; and, finally 4) domes, dykes, lava flows, and lava domes of rhyolitic composition indicative of a post-collapse stage.The analysis of the regional and local structures, as well as, the architectures of the volcanic facies, indicates the existence of a highly oblique rift, with its principal extensional strain in an NNE–SSW direction (∼N10°).The analyzed rocks are mainly high-potassium dacites and rhyolites with trace and RE elements contents of an intraplate signature. The age of these rocks (189 ± 0.76 Ma) agree well with other volcanic sequences of the western North Patagonian Massif, as well as, the Neuquén Basin, indicating that Pliensbachian magmatism was widespread in both regions. The age is also coincident with phase 1 of volcanism of the eastern North Patagonia Massif (188–178 Ma) represented by ignimbrites, domes, and pyroclastic rocks of the Marifil Complex, related to intraplate magmatism.     

Crustal-scale magmatic systems during intracontinental strike-slip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps

The Southern Alps host volcano-sedimentary basins that formed during post-Variscan extension and strike-slip in the Early Permian. We present U–Pb ages and initial Hf isotopic compositions of magmatic zircons from silicic tuffs and pyroclastic flows within these basins, from caldera fillings and from shallow intrusions from a 250 km long E–W transect (Bozen–Lugano–Lago Maggiore) and compare these with previously published data. Basin formation and magmatism are closely related to each other and occurred during a short time span between 285 and 275 Ma. The silicic magmatism is coeval with mafic intrusions of the Ivrea-Verbano Zone and within Austroalpine units. We conclude that deep magma generation, hybridisation and upper crustal emplacement occurred contemporaneously along the entire transect of the Southern Alps. The heat advection in the lower crust by injected mantle melts was sufficient to produce crustal partial melts in lower crustal levels. The resulting granitoid melts intruded into the upper crust or rose to the surface forming large caldera complexes. The compilation of Sr and Nd isotopic data of these rocks demonstrates that the mantle mixing endmember in the melts may not be geochemically enriched but has a depleted composition, comparable to the Adriatic subcontinental mantle exhumed to form the Tethyan sea floor during Mesozoic continental breakup and seafloor spreading. Magmatism and clastic sedimentation in the intracontinental basins was interrupted at 275 Ma for some 10–15 million years, forming a Middle Permian unconformity. This unconformity may have originated during large-scale strike-slip tectonics and erosion that was associated with crustal thinning, upwelling and partial melting of mantle, and advection of melts and heat into the crust. The unconformity indeed corresponds in time to the transition from a Pangea-B plate reconstruction for the Early Permian to the Late Permian Pangea-A plate assembly (Muttoni et al. in Earth Planet Sci Lett 215:379–394, 2003). The magmatic activity would therefore indicate the onset of >2,000 km of strike-slip movement along a continental-scale mega-shear, as their model suggests.     

U–Pb zircon geochronology of silicic tuffs from the Timber Mountain/Oasis Valley caldera complex, Nevada: rapid generation of large volume magmas by shallow-level remelting

Large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the SW Nevada volcanic field (SWNVF). Voluminous ash flows erupted in two paired events: Topopah Spring (TS, >1,200 km³, 12.8 Ma)–Tiva Canyon (TC, 1,000 km³, 12.7 Ma) and Rainier Mesa (RM, 1,200 km³, 11.6 Ma)–Ammonia Tanks (AT, 900 km³, 11.45 Ma; all cited ages are previously published ⁴⁰Ar/³⁹Ar sanidine ages). Within each pair, eruptions are separated by only 0.1–0.15 My and produced tuffs with contrasting isotopic values. These events represent nearly complete evacuation of sheet-like magma chambers formed in the extensional Basin and Range environment. We present ion microprobe ages from zircons in the zoned ash-flow sheets of TS, TC, RM, and AT in conjunction with δ¹⁸O values of zircons and other phenocrysts, which differ dramatically among subsequently erupted units. Bulk zircons in the low-δ¹⁸O AT cycle were earlier determined to exhibit ∼1.5‰ core-to-rim oxygen isotope zoning; and high-spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0.5–2.5‰. The following U–Pb isochron ages were calculated after correcting for the initial U–Pb disequilibria: AT (zircon rims: 11.7 ± 0.2 Ma; cores: 12.0 ± 0.1 Ma); pre-AT rhyolite lava: (12.0 ± 0.3 Ma); RM: 12.4 ± 0.3); TC: (13.2 ± 0.15 Ma); TS: (13.5 ± 0.2). Average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the Ar–Ar stratigraphy, but preserve the comparably short time gaps within each of two major eruption cycles (TS/TC, RM/AT). Notably, every sample yields average zircon ages that are 0.70–0.35 Ma older than the respective Ar–Ar eruption ages. The Th/U ratio of SWNVF zircons are 0.4–4.7, higher than typically found in igneous zircons, which correlates with elevated Th/U of the whole rocks (5–16). High Th/U could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting. For low-δ¹⁸O AT-cycle magmas, rim ages from unpolished zircons overlap within analytical uncertainties with the ⁴⁰Ar/³⁹Ar eruption age compared to core ages that are on average ∼0.2–0.3 My older than even the age of the preceding caldera forming eruption of RM tuff. This age difference, the core-to-rim oxygen isotope zoning in AT zircons, and disequilibrium quartz–zircon and melt-zircon isotopic fractionations suggest that AT magma recycled older zircons derived from the RM and older eruptive cycles. These results suggest that the low-δ¹⁸O AT magmas were generated by melting a hydrothermally-altered protolith from the same nested complex that erupted high-δ¹⁸O magmas of the RM cycle only 0.15 My prior to the eruption of the AT, the largest volume low-δ¹⁸O magma presently known.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

New chronological and geochemical constraints on the genesis and geological evolution of Ponza and Palmarola Volcanic Islands (Tyrrhenian Sea, Italy)

A new geochronological and geochemical study of the volcanic rocks of the Ponza and Palmarola Islands, Pontine Archipelago, has been carried out. This archipelago is located along the boundary between the Italian continental shelf and the opening Tyrrhenian basin. It is a key area to study volcanism related to the opening of the Tyrrhenian Sea. Ponza is the oldest felsic magmatic manifestation in the central Tyrrhenian area. Previous studies suggested that Ponza volcanic activity began before 5 Ma. Twenty-five new K–Ar ages constrain the volcanic activity (rhyolitic hyaloclastites and dykes) to the last 4.2 Ma, with two episodes of quiescence between 3.7 and 3.2 Ma and between 2.9 and 1.0 Ma. A new volcanic episode dated at 3.2–2.9 Ma has been identified on the central and southern Ponza, with emplacement of pyroclastic units. At 1.0 Ma, a trachytic episode ended the volcanic activity. The near island of Palmarola exhibits rhyolitic hyaloclastites and domes dated between 1.6 and 1.5 Ma, indicating that the island was entirely built during the Early Pleistocene in a short span of time of ca. 120 ka. Although only 6–8 km apart, the two islands display significantly different geochemical signatures. Ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction/collision zones: high-K calc-alkaline and metaluminous rhyolites (Agpaitic Index [AI] and Alumina Saturation Index [ASI] <1), high LILE/HFSE (Th/Ta=16–21) and LREE/HFSE ratios (La/Nb>3), and Nb–Ta negative anomalies. In Palmarola, the orogenic character is also present, but much less marked than in Ponza: rhyolites have a peralkaline character (AI>1), lower LILE/HFSE (Th/Ta=11–15), low LREE/HFSE ratios (La/Nb=1–2) close to those of anorogenic lavas, and the Nb–Ta negative anomalies are almost absent. Y/Nb ratios indicate different magmatic sources, one similar to island-arc or active continental margin basalts for Ponza rhyolites, and the others probably involving an OIB type component for Palmarola rhyolites and Ponza trachytes. Palmarola volcanics represent a transitional magmatism: although a preserved collisional geochemical imprint, they show geochemical features approaching those of anorogenic lavas erupted in a within-plate context. The change of magmatism evidenced in this study can be related to the tectonic evolution of the area. Indeed, Hf, Ta and Rb contents suggest that the oldest Pliocene rhyolites of Ponza would emplace in a syn- to late-collisional setting, while the younger Pleistocene rhyolites of Palmarola would be emplaced in a post-collisional setting in which the orogenic character (Th/Ta) decreases and mantle influence (Nb/Ta) increases. Geochemical modeling strongly suggests that the Palmarola rhyolites represent the waning stages of a subduction-related magmatism. The K–Ar datings allow us to estimate precisely the transition of magmatism to last less than 1.3 Ma. The transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources. This upwelling could occur during the extension of the Tyrrhenian basin, caused by the slab retreat and steepening, or during a process of slab break-off starting in the Pliocene.     

河北沽源张麻井铀-钼矿床围岩SHRIMP锆石U-Pb定年及其地质意义

沽源-红山子铀成矿带河北沽源张麻井铀-钼矿床赋存在张家口组第三段流纹岩与流纹斑岩、石英斑岩的接触带附近,主要受隐爆角砾岩控制。研究表明,张麻井铀-钼矿床赋矿流纹岩、流纹斑岩和石英斑岩的锆石具有清晰的环带结构,Th/U比值高,属典型的岩浆成因锆石。SHRIMP锆石U-Pb测年结果显示,流纹岩12颗锆石的206Pb/238U年龄变化范围为136~144 Ma,加权平均年龄为(138.6±1.4)Ma(MSWD=2.4);流纹斑岩11颗锆石的206Pb/238U年龄变化范围为136~145 Ma,加权平均年龄为(140.2±1.6)Ma(MSWD=2.2);石英斑岩11颗锆石的206Pb/238U年龄变化范围为132~147 Ma,加权平均年龄为(136.2±2.9)Ma(MSWD=1.8)。由此可见,流纹岩、流纹斑岩和石英斑岩的同位素年龄在误差范围内一致,指示火山喷发和斑岩就位发生在早白垩世早期,与滨西太平洋成矿域以火山岩为赋矿主岩的热液型铀矿的围岩形成时代一致,沽源-红山子铀成矿带早白垩世早期次火山岩与同期火山岩的接触带附近是寻找铀矿的有利空间。     

Geochronological scale and evolution of late Cenozoic magmatism within the Caucasian segment of the alpine belt

Results of the isotope-geochronological studies of the Late Cenozoic magmatism of Caucasus have been considered. The Neogene-Quaternary volcanic activity is found to have evolved during the last 15 m. y. being most intensive in the Middle-Late Pliocene. Within separate neovolcanic areas of the Caucasus region, magmatism was of a clearly discrete character when intense eruption periods interchanged with prolonged (up to several million years) times of quiet conditions. Four stages of young magmatism of the Caucasus are recognized: the Middle Miocene (15–13 Ma), the Late Miocene (9–5 Ma), the Pliocene (4.5–1.6 Ma), and the Quaternary (less than 1.5 Ma). However, for certain areas the time limits of these stages were shifted relative to each other and overlap the whole age range from the mid-Miocene to the end of the Quaternary period. Therefore, within the collision zone, the Neogene-Quaternary magmatism evolved almost continuously during almost the last 9 m. y., but in the time interval of 13–9 Ma in the Caucasian segment, volcanic activity was possibly low. No evidence of directed lateral migration of volcanic activity within the entire Caucasus region was found. At the same time, in the Lesser Caucasus the young magmatism commenced earlier (∼15 Ma), compared to the Greater Caucasus (∼8 Ma).