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1.
Summary The ∼ 150 km3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last 200–300 ky. New 40Ar/39Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ± 0.11 ka, about 2 ky older than the previous best estimate. Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka. These ages, coupled with a xenocrystic sanidine component > 315 ka, extend the volcanic history of this region by over 200 ky. Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain. Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive eruptions in close proximity to the densely populated Neapolitan region. Received August 1, 2000; accepted November 2, 2000  相似文献   

2.
Summary Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents including H2O, S, Cl, F, B, and P2O5. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution well before eruption. The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO2, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P2O5, F, and many lithophile trace elements. Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas associated with explosive plinian and subplinian events younger than 3.55 ka contained more H2O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during relatively passive interplinian volcanic phenomena. Received January 10, 2000 Revised version accepted July 17, 2000  相似文献   

3.
Here we present a tephrostratigraphic record (core Co1202) recovered from the northeastern part of Lake Ohrid (Republics of Macedonia and Albania) reaching back to Marine Isotope Stage (MIS) 6. Overall ten horizons (OT0702‐1 to OT0702‐10) containing volcanic tephra have been recognised throughout the 14.94 m long sediment succession. Four tephra layers were visible at macroscopic inspection (OT0702‐4, OT0702‐6, OT0702‐8 and OT0702‐9), while the remaining six are cryptotephras (OT0702‐1, OT0702‐2, OT0702‐3, OT0702‐5, OT0702‐7 and OT0702‐10) identified from peaks in K, Zr and Sr intensities, magnetic susceptibility measurements, and washing and sieving of the sediments. Glass shards of tephra layers and cryptotephras were analysed with respect to their major element composition, and correlated to explosive eruptions of Italian volcanoes. The stratigraphy and the major element composition of tephra layers and cryptotephras allowed the correlation of OT0702‐1 to AD 472 or AD 512 eruptions of Somma‐Vesuvius, OT0702‐2 to the FL eruption of Mount Etna, OT0702‐3 to the Mercato from Somma‐Vesuvius, OT0702‐4 to SMP1‐e/Y‐3 eruption from the Campi Flegrei caldera, OT0702‐5 to the Codola eruption (Somma‐Vesuvius or Campi Flegrei), OT0702‐6 to the Campanian Ignimbrite/Y‐5 from the Campi Flegrei caldera, OT0702‐7 to the Green Tuff/Y‐6 eruption from Pantelleria Island, OT0702‐8 to the X‐5 eruption probably originating from the Campi Flegrei caldera, OT0702‐9 to the X‐6 eruption of generic Campanian origin, and OT0702‐10 to the P‐11 eruption from Pantelleria Island. The fairly well‐known ages of these tephra layers and parent eruptions provide new data on the dispersal and deposition of these tephras and, furthermore, allow the establishment of a chronological framework for core Co1202 for a first interpretation of major sedimentological changes. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

4.
Summary We present compositions of reheated melt inclusions in clinopyroxene phenocrysts from three mafic xenoliths in Breccia Museo, Campi Flegrei, Italy. Melt inclusion compositions are remarkably different from the compositions of known contemporary Campi Flegrei lavas, being significantly enriched in K2O and depleted in Na2O. Some differences are also evident in FeO* (total Fe as FeO) and TiO2 contents. The clinopyroxene phenocrysts could not have crystallised from Campi Flegrei magmas. We suggest that they originated from a volcanic system genetically very similar to, and possibly linked with, the >14 ka volcanic system of Mt. Somma, another Campanian volcano ∼ 30 km east from Campi Flegrei, from which Vesuvius subsequently developed. This result indicates a close relationship (or link) between the two volcanic systems which have until now been considered separate. We speculate that the link was established prior to eruption of the Neapolitan Yellow Tuff (NYT) (∼ 12 ka). The xenoliths were derived from a volcanic system older than the host breccias themselves. We suggest that this older volcanism had close similarities with the volcanism of the older products of Mt. Somma (∼25 ka). Received March 20, 2000; accepted November 2, 2000  相似文献   

5.
Two cores were recovered in the southeastern part of Lake Shkodra (Montenegro and Albania) and sampled for identification of tephra layers. The first core (SK13, 7.8 m long) was recovered from a water depth of 7 m, while the second core (SK19, 5.8 m long) was recovered close to the present‐day shoreline (water depth of 2 m). Magnetic susceptibility investigations show generally low values with some peaks that in some cases are related to tephra layers. Naked‐eye inspection of the cores allowed the identification of four tephra layers in core SK13 and five tephra layers in core SK19. Major element analyses on glass shards and mineral phases allowed correlation of the tephra layers between the two cores, and their attribution to six different Holocene explosive eruptions of southern Italy volcanoes. Two tephra layers have under‐saturated composition of glass shards (foiditic and phonolitic) and were correlated to the AD 472 and the Avellino (ca. 3.9 cal. ka BP) eruptions of Somma‐Vesuvius. One tephra layer has benmoreitic composition and was correlated to the FL eruption of Mount Etna (ca. 3.4 cal. ka BP). The other three tephra layers have trachytic composition and were correlated to Astroni (ca. 4.2 cal. ka BP), Agnano Monte Spina (ca. 4.5 cal. ka BP) and Agnano Pomici Principali (ca. 12.3 cal. ka BP) eruptions of Campi Flegrei. The ages of tephra layers are in broad agreement with eight 14C accelerator mass spectrometric measurements carried out on plant remains and charcoal from the lake sediments at different depths along the two cores. The recognition of distal tephra layers from Italian volcanoes allowed the physical link of the Holocene archive of Lake Shkodra to other archives located in the central Mediterranean area and the Balkans (i.e. Lake Ohrid). Five of the recognised tephra layers were recognised for the first time in the Balkans area, and this has relevance for volcanic hazard assessment and for ash dispersal forecasting in case of renewed explosive activity from some of the southern Italy volcanoes. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

6.
《Quaternary Science Reviews》2004,23(23-24):2485-2500
A detailed tephrostratigraphy supported by accelerator mass spectrometry (AMS) 14C dating and isotopic and geochemical analyses has been carried out for a deep-sea core collected from the Southern Adriatic Sea, which spans the last 18 14C kyr. Fourteen ash layers have been recognized in the marine core and the origin was determined by comparing their age and geochemistry with tephra detected in terrestrial deposits. The geochemical signatures of marine ash layers indicate a prevailing Campanian and subordinate Etnean and Eolian provenances. Collectively, they are attributed to sources in the Phlegraean Fields (LAM-Lagno Amendolare; LN1/LN2-GM1; C-2/NYT-Neapolitan Yellow Tuff; C-1-Agnano Pomici Principali; AMS/PF-Agnano Monte Spina; AST-Astroni), Somma-Vesuvius (L9-Pomici di Base; L8-Greenish), Mount Etna (Et1-Y1-Biancavilla Ignimbrite), Lipari (E-1-Gabellotto-Fiumebianco) and Palinuro seamount (Pal 1). Some of these eruptions have been detected for the first time in marine cores (Astroni, LAM, L9, L8), and three were detected and dated by marine tephra (LN1, LN2, and Pal 1) providing new more precise stratigraphic markers for the last 18  kyr in the South Adriatic marine record.  相似文献   

7.
After 11·5 ka of quiescence (24·5–13 ka),the Nevado de Toluca volcano started a 2500 year period of activity.This period was characterized by a dome destruction event at13 ka, a small Plinian event at 12·1 ka, and a largePlinian eruption at 10·5 ka. About 10 km3 of magma waserupted that was homogeneous in composition (63·3–65·7SiO2 wt % whole-rock) and in mineralogy. Pumice consists ofplagioclase (An30–59) > orthopyroxene (En56–59)> hornblende >> Fe–Ti oxides + rare apatite (inopx) + biotite, set in a rhyolitic matrix (72–76 SiO2wt %). 40Ar/39Ar analysis of single biotite crystals yieldedages (0·81–4·7 Ma), that do not correspondto eruption ages. The biotite represents partially assimilatedxenocrysts, which could have resided in the magma for only ashort period of time. Mineral chemical data, coupled with hydrothermalexperiments, indicate that prior to eruption the dacitic magmastagnated at a depth of 4·5–6 km below the summitat water pressures of 160–210 MPa and a temperature of824 ± 12°C on the basis of Fe–Ti oxide thermometry,and under water-saturated conditions. To stabilize a homogeneousmagma body of >10 km3 at 824°C in the upper crust, wepropose that reheating of the dacitic reservoir by hotter magmabatches was able to maintain the equilibrium between the temperatureof the magma and the assimilation of wall-rock over a periodof 2500 years. Based on similarities among the juvenile products,we suggest that the three eruptions were fed from the same magmabody. KEY WORDS: explosive volcanism; xenocrystic contamination; Nevado de Toluca, Mexico  相似文献   

8.
Summary The present review of data on the Trecase 1 well, including stratigraphy, updated 40 Ar/39Ar ages and the results of newly performed calcareous nannofossil studies, serves to resolve the chronological contradictions pointed out by Bernasconi et al. (1981) and Balducci et al. (1983) concerning the onset of volcanic activity in the area now occupied by the Somma-Vesuvius Volcanic Complex. New 40 Ar/39Ar data indicate that volcanic activity in this area started about 0.4 myr B.P. After such time, tephritic magmatic activity, distributed in small scattered centers, developed and alternated with periods of volcanic quiescence and marine sedimentation. This first phase of magmatic activity ended in the Vesuvian area about 0.3 myr B.P. and was followed by a period of marine sedimentation in a marginal environment. Complete emergence of the shoreline occurred about 37,000 yr B.P. as a result of sea level changes during the last glacial period and deposition of the 60 m thick Campanian ignimbrite (CI). Volcanic activity reappeared in the Vesuvian area only after the CI eruption. Magma rising along and at the intersection of linear and curved tectonic and volcano-tectonic elements (linked to the pre-existing Pleistocene tectonic trend and formation of the vast Phlegraean Fields caldera) formed a number of small lava and scoria edifices. One of these tephritic centers lies above the CI deposits under the Trecase 1 well area. The CI bottom in the Trecase 1 well is currently at an altitude of − 120 m a.s.l.; this allows estimating the maximum tectonic subsidence over the last 37,000 yr. by the southern sector of the Vesuvian area to be about 30 m. Received April 12, 2000; revised version accepted March 1, 2001  相似文献   

9.
Summary Vesuvius and Stromboli are two active and extensively studied volcanoes that traditionally have been considered as having different styles of eruption, rock composition and tectonic setting. Data reveal close compositional affinities between these two volcanoes. The abundant 13–15 Ka old Stromboli leucite-tephritic rocks have radiogenic isotope signatures, and abundances and ratios of incompatible elements with the exception of Rb and K, which are identical to those of Vesuvius. The Phlegraean Fields also show close affinities to these volcanoes. The similarity between Stromboli leucite-tephrites and Vesuvius rocks cannot be the result of low pressure processes, given the differences between the two volcanoes in terms of structural features, eruptive behaviour and type of basement rocks. Instead, the observed geochemical signatures are likely to represent a primary magma composition and reveal a common homogeneous source for the two suites. The higher K and Rb contents in the Vesuvius rocks suggest either selective enrichment during magma ascent or a role for phlogopite melting during mantle anatexis. The most primitive rocks from Vesuvius, Phlegraean Fields and Stromboli reveal intermediate compositions between arc and intraplate volcanics. It is suggested that the mantle sources beneath these volcanoes consist of a mixture of intraplate- and slab-derived components. Intraplate material was probably provided by inflow of asthenosheric mantle into the wedge above the subducting Ionian Sea plate, either from the Apulian plate and/or from the Tyrrhenian Sea region. Fluids or melts released from the sinking slab and associated sediments generated metasomatic modification of the intraplate material, whose melting gave rise to the Stromboli, Vesuvius and Phlegraean Fields magmas. The present study demonstrates how comparative investigations of various volcanic centres from southern Italy allow clarification of a number of problems involving magma genesis and evolution, composition of mantle sources and geodynamic significance, which have been long debated and are difficult to solve if individual volcanoes are considered in isolation. Received July 20, 2000; revised version accepted March 19, 2001  相似文献   

10.
Trigger mechanisms and syn-eruptive processes of Plinian eruptions are poorly understood especially in the case of mafic powerful events. In the last decades, the combined geochemical and textural studies on volcanic rocks have proven to be fundamental tools for exploring the dynamics of magma ascent in volcanic conduits and for improving our ability to interpret volcano-monitoring signals and assess hazard. In this case study, we quantitatively investigate 2D and 3D micro-textural, geochemical, and isotopic features of pyroclastic rocks erupted during the Pomici di Base Plinian eruption (22 ka), the generally acknowledged first and most powerful event of the Somma–Vesuvius volcano. A peculiar aspect of this eruption is its high intensity that remained stable during the entire Plinian phase despite the strong magma compositional variation towards mafic terms. We infer that the transfer of magma towards the surface was intensified by the occurrence of rapid vesiculation pulses driven by limestone assimilation (skarn recycling) during magma ascent through the carbonatic bedrock. We conclude that limestone assimilation can hence be a syn-eruptive process, able to trigger further gas nucleation with deep impact on the eruption intensity, particularly crucial in the case of mafic/intermediate magma compositions.  相似文献   

11.
The Campanian Ignimbrite is a > 200 km3 trachyte–phonolitepyroclastic deposit that erupted at 39·3 ± 0·1ka within the Campi Flegrei west of Naples, Italy. Here we testthe hypothesis that Campanian Ignimbrite magma was derived byisobaric crystal fractionation of a parental basaltic trachyandesiticmelt that reacted and came into local equilibrium with smallamounts (5–10 wt%) of crustal rock (skarns and foid-syenites)during crystallization. Comparison of observed crystal and magmacompositions with results of phase equilibria assimilation–fractionationsimulations (MELTS) is generally very good. Oxygen fugacitywas approximately buffered along QFM + 1 (where QFM is the quartz–fayalite–magnetitebuffer) during isobaric fractionation at 0·15 GPa ( 6km depth). The parental melt, reconstructed from melt inclusionand host clinopyroxene compositions, is found to be basaltictrachyandesite liquid (51·1 wt% SiO2, 9·3 wt%MgO, 3 wt% H2O). A significant feature of phase equilibria simulationsis the existence of a pseudo-invariant temperature, 883 °C,at which the fraction of melt remaining in the system decreasesabruptly from 0·5 to < 0·1. Crystallizationat the pseudo-invariant point leads to abrupt changes in thecomposition, properties (density, dissolved water content),and physical state (viscosity, volume fraction fluid) of meltand magma. A dramatic decrease in melt viscosity (from 1700Pa s to 200 Pa s), coupled with a change in the volume fractionof water in magma (from 0·1 to 0·8) and a dramaticdecrease in melt and magma density acted as a destabilizingeruption trigger. Thermal models suggest a timescale of 200kyr from the beginning of fractionation until eruption, leadingto an apparent rate of evolved magma generation of about 10–3km3/year. In situ crystallization and crystal settling in density-stratifiedregions, as well as in convectively mixed, less evolved subjacentmagma, operate rapidly enough to match this apparent volumetricrate of evolved magma production. KEY WORDS: assimilation; Campanian Ignimbrite; fractional crystallization; magma dynamics; phase equilibria  相似文献   

12.
The Campi Flegrei caldera, an active volcanic field in the Campanianprovince, Italy, is a nested structure generated by the CampanianIgnimbrite (37 ka BP) and the Neapolitan Yellow Tuff (12 kaBP) eruptions. Since at least 60 ka BP Campi Flegrei has producedmagmas with variable chemical and Sr isotopic compositions.87Sr/86Sr ratios increase through time from 0·7068 to0·7086, with the highest ratios detected in the least-evolvedshoshonitic products. The origin of this progressive Sr isotopicvariability has been investigated using new Sr, Nd and Pb isotopicdata for volcanic rocks and entrained xenoliths. The data obtainedare combined and discussed with previous geochemical and Srisotope data and used to suggest a multi-stage evolution forthe magmatic system, mainly involving deeper and shallower crustalmagma storage reservoirs. The deeper reservoir is proposed tobe a magma chamber periodically refilled by primitive maficmagmas which subsequently undergo contamination by crustal material.The assimilated crustal material is represented by xenolithsrecovered in the shoshonitic pyroclastic products. Magma batchesoriginating from the deeper reservoir migrated towards the surfaceand fed a shallower complex magmatic system. The deeper chamberwas tapped during the eruption of least evolved magmas by regionalfault systems. In addition to crystal–liquid fractionation,open-system processes occurred in the shallower system. KEY WORDS: Campi Flegrei; crustal contamination; xenoliths  相似文献   

13.
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy  相似文献   

14.
Summary The evolution of the Mt. Somma caldera is reconstructed by means of volcanological, geological and geomorphologic data. The present caldera shape results from two caldera forming events: (a) break-up of the upper part of the older structure, which occurred between 17 and 8Kyr B.P. through a series of external volcaniclastic debris flow depositions and internal collapses into a cored-out vent. (b) W–SW directed sector collapse of the enlarged Mt. Somma crater, caused by excess vapour pressure generated during the Avellino eruption (3.5Kyr B.P.). At this time, the morphology of the Mt. Somma crater probably resembled the demolished crater formed in 1980 at Mt. St. Helens. Flank failure of Mt. Somma during the Avellino eruption was probably caused both by a migration of the vent to the west and a drastic change of the hydrogeological conditions at depth. Flank failure processes were extended to the S–SE sector of the Mt. Somma edifice during the 79 A.D. and 472 A.D. eruptions. After the 472 A.D. eruption interplinian activity during the Middle Ages resulted in the formation of the Vesuvius cone inside the Mt. Somma caldera.  相似文献   

15.
《Applied Geochemistry》2005,20(3):611-625
This paper presents new data on the baseline concentrations of U, Th and K in 2389 stream sediments over the whole Campania region. These data, based on systematic sampling and analysis, are compared with those obtained by gamma-ray spectrometry surveys.Variations in the U, Th and K concentration in the surficial environment of the Campania region appear to be related to bedrock lithology. Generally, high U, Th and K values in stream sediments correspond well with the occurrence of volcanic rocks in the central-western part of the region, whereas low values are found in areas characterized by silico-clastic and carbonate deposits, occurring mostly in the southern and eastern part of the region. Gamma-ray spectrometry maps show a similar pattern, although the distribution of the highest radioactivity levels define more restricted areas than the ones resulting from mapping stream sediment geochemistry. Particularly high 40K radioactivity levels delimit all the known eruptive centers (Roccamonfina, Campi Flegrei and Somma–Vesuvius), including the fissural sources of Campania Ignimbrites, much better than U and Th radioactivity. One of the concerns for human health in the Campania region is the total gamma radiation and Rn potential related mostly to alkaline volcanics of the Neapolitan volcanological province. In particular, geothermal activity occurring in all the Campanian volcanic areas represents a potential hazard for Rn gas.  相似文献   

16.
Compositions of matrix glasses from the current eruption ofSoufrière Hills Volcano indicate that decompression-drivencrystallization results in 20–70 wt % groundmass crystallizationduring eruption and variable degassing. Variations in crystallinityand volatile contents (water and chlorine) of matrix glassesare attributed to variations in extrusion rates and residencetimes in the lava dome. Residual water contents in pumice clasts(0·2–0·6 wt %) indicate minimum pressuresof 1·1–3·7 MPa in 1997 Vulcanian explosions.Residual water contents of 1·6 wt % in a ballistic blockejected in sub-Plinian explosive activity on 17 September 1996imply larger pressure drops (  相似文献   

17.
Radiocarbon dating of material from Late Pleistocene archaeological sites is challenging. Small amounts of modern 14C-labelled contamination will significantly affect the reliability of dates from the period, producing erroneous results. Recent developments in sample pre-treatment chemistry have shown that problems in reliable age determination during this period are surmountable. In this paper we provide an example of one such case, from the Middle to Upper Palaeolithic transitional site of the Grotta di Fumane, in northern Italy. We AMS dated two fractions of the same charcoal samples derived from a series of superimposed Mousterian, Aurignacian and Gravettian levels excavated at the site. One fraction was treated using the routine acid–base–acid (ABA) method, the other with the more rigorous acid–base-oxidation/stepped combustion (ABOx–SC) method. The latter method produced consistently older, and almost certainly more reliable, results. The eruption of the known-age Campanian Ignimbrite from the Phlegrean Fields near present-day Naples at 39.3 ka yr BP seals Ulluzzian and Proto Aurignacian levels in the south of Italy. Equivalent cultural levels are present at Fumane and the results obtained with the ABOx–SC methods are consistent with the ages inferred for sites in the south of Italy based on the presence of the Campanian Ignimbrite. New results from a sample found beneath the Campanian Ignimbrite at the Russian site of Kostenki, obtained using both the ABA and ABOx–SC, methods are also presented. They support the conclusion reached at Fumane by demonstrating that, in many cases, the ABOX–SC treatment effectively removes contamination where the ABA treatment does not. The results of the work offer a sobering examination of the problems inherent in the current radiocarbon database relating to the period, and highlight the dangers of an uncritical use of the corpus of 14C results obtained over the last few decades. Based on our results, we predict that more than 70% of the 53 previously available determinations from Fumane are erroneously young. A way forward is suggested, using these improved chemical preparation methods, applying analytical methods to characterise the material dated, and testing existing site chronologies to establish which previous determinations are liable to be inaccurate.  相似文献   

18.
Post-collisional magmatism in the southern Iberian and northwesternAfrican continental margins contains important clues for theunderstanding of a possible causal connection between movementsin the Earth's upper mantle, the uplift of continental lithosphereand the origin of circum-Mediterranean igneous activity. Systematicgeochemical and geochronological studies (major and trace element,Sr–Nd–Pb-isotope analysis and laser 40Ar/39Ar-agedating) on igneous rocks provide constraints for understandingthe post-collisional history of the southern Iberian and northwesternAfrican continental margins. Two groups of magmatic rocks canbe distinguished: (1) an Upper Miocene to Lower Pliocene (8·2–4·8Ma), Si–K-rich group including high-K (calc-alkaline)and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene(6·3–0·65 Ma), Si-poor, Na-rich group includingbasanites and alkali basalts to hawaiites and tephrites. Maficsamples from the Si–K-rich group generally show geochemicalaffinities with volcanic rocks from active subduction zones(e.g. Izu–Bonin and Aeolian island arcs), whereas maficsamples from the Si-poor, Na-rich group are geochemically similarto lavas found in intraplate volcanic settings derived fromsub-lithospheric mantle sources (e.g. Canary Islands). The transitionfrom Si-rich (subduction-related) to Si-poor (intraplate-type)magmatism between 6·3 Ma (first alkali basalt) and 4·8Ma (latest shoshonite) can be observed both on a regional scaleand in individual volcanic systems. Si–K-rich and Si-poorigneous rocks from the continental margins of southern Iberiaand northwestern Africa are, respectively, proposed to havebeen derived from metasomatized subcontinental lithosphere andsub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterraneanis presented in which subduction of oceanic lithosphere is inferredto have caused continental-edge delamination of subcontinentallithosphere associated with upwelling of plume-contaminatedsub-lithospheric mantle and lithospheric uplift. This processmay operate worldwide in areas where subduction-related andintraplate-type magmatism are spatially and temporally associated. KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways  相似文献   

19.
Summary ¶The Campanian Ignimbrite rock samples include two compositionally distinct populations of clinopyroxene phenocrysts, and the entrapped MI (melt inclusions) are also different in composition. The cores of the more MgO-enriched phenocrysts carry basaltic trachyandesite MI that contain >6wt.% MgO, whereas other phenocrysts contain MI with <4wt.% MgO. The MgO-enriched MI also contain comparatively greater abundances of F, CaO, TiO2, P2O5, SO2, and Sr and show marginally higher ratios of (CaO/Al2O3) than the low-MgO MI. Most of the high-MgO MI also contain comparatively more H2O. The MgO-enriched MI are restricted to diopsidic clinopyroxenes and show minimal compositional variability, demonstrating that they were derived from a common magmatic source or sources. We interpret these MI to represent primary, mafic magma. In contrast, the more evolved, low-MgO melt inclusions, which are restricted to salitic clinopyroxenes, span the compositional range of trachyandesite to trachyte. The low-MgO fractions of Campanian Ignimbrite magma evolved via fractional crystallization with or without mingling or mixing with more primitive, high-MgO magma.Interestingly, the MI from the Giugliano sample also cluster into low-MgO and high-MgO fractions, and the evolutionary trends for major, minor, and trace elements mirror those exhibited by the Campanian Ignimbrite MI, suggesting that both magmas were derived from similar or the same source(s) and that the processes of magma evolution were equivalent for both magmas.The MI also indicate that the Campanian Ignimbrite and Giugliano magmas did not form by evolution of Taurano magma, because the geochemical trends expressing melt evolution of the former and latter magmas are too dissimilar. Most Taurano MI show higher (CaO/Al2O3) and contain less SiO2, (Na2O+K2O), Cl, Li, Rb, Cs, Sr, Nb, Th, and U than the high-MgO and low-MgO Campanian Ignimbrite and Giugliano MI, indicating that the Taurano MI represent magmas which were much more primitive.Received July 15, 2002; revised version accepted March 27, 2003  相似文献   

20.
Young (<65 ka) explosive silicic volcanism at Taupo volcano,New Zealand, has involved the development and evacuation ofseveral crustal magmatic systems. Up to and including the 26·5ka 530 km3 Oruanui eruption, magmatic systems were contemporaneousbut geographically separated. Subsequently they have been separatedin time and have vented from geographically overlapping areas.Single-crystal (secondary ionization mass spectrometry) andmultiple-crystal (thermal ionization mass spectrometry) zirconmodel-age data are presented from nine representative eruptiondeposits from 45 to 3·5 ka. Zircon yields vary by threeorders of magnitude, correlating with the degrees of zirconsaturation in the magmas, and influencing the spectra of modelages. Two adjacent magma systems active up to 26·5 kashow wholly contrasting model-age spectra. The smaller systemshows a simple unimodal distribution. The larger system, usingdata from three eruptions, shows bimodal model-age spectra.An older 100 ka peak is interpreted to represent zircons (antecrysts)derived from older silicic mush or plutonic rocks, and a youngerpeak to represent zircons (phenocrysts) that grew in the magmabody immediately prior to eruption. Post-26·5 ka magmabatches show contrasting age spectra, consistent with a mixtureof antecrysts, phenocrysts and, in two examples, xenocrystsfrom Quaternary plutonic and Mesozoic–Palaeozoic metasedimentaryrocks. The model-age spectra, coupled with zircon-dissolutionmodelling, highlight contrasts between short-term silicic magmageneration at Taupo, by bulk remobilization of crystal mushand assimilation of metasediment and/or silicic plutonic basementrocks, and the longer-term processes of fractionation from crustallycontaminated mafic melts. Contrasts between adjacent or successivemagma systems are attributed to differences in positions ofthe source and root zones within contrasting domains in thequartzo-feldspathic (<15 km deep) crust below the volcano. KEY WORDS: zircon; U-series dating; rhyolite; Taupo Volcanic Zone; Taupo volcano  相似文献   

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