257 research outputs found
The origin of the Canary Island Seamount Province - New ages of old seamounts
The Canary Island Seamount Province forms a scattered hotspot track on the Atlantic ocean floor
,1300 km long and,350 km wide, perpendicular to lithospheric fractures, and parallel to theNWAfrican
continental margin. New 40Ar/39Ar datings show that seamount ages vary from 133 Ma to 0.2 Ma in the
central archipelago, and from 142 Ma to 91 Ma in the southwest. Combining 40Ar/39Ar ages with plate
tectonic reconstructions, I find that the temporal and spatial distribution of seamounts is irreconcilable with
a deep fixed mantle plume origin, or derivation from passive mantle upwelling beneath a mid-ocean ridge. I
conclude that shallow mantle upwelling beneath the Atlantic Ocean basin off the NW African continental
lithosphere flanks produced recurrent melting anomalies and seamounts from the Late Jurassic to Recent,
nominating the Canary Island Seamount Province as oldest hotspot track in the Atlantic Ocean, and most
long-lived preserved on earth
40Ar/39Ar ages of Pliocene-Pleistocene fallout tephra layers and volcaniclastic deposits in the sedimentary aprons of Gran Canaria and Tenerife (Sites 953, 954, and 956)
Six fallout tephra layers and 16 heterolithologic volcaniclastic deposits drilled at Holes 953A, 954A, 954B, and 956A, during Leg 157 in the sedimentary aprons of Gran Canaria and Tenerife, have been dated by single crystal laser 40Ar/39Ar analysis. The fallout tephra markers range in age from 0.273 ± 0.006 Ma to 2.74 ± 0.01 Ma. Maximum sedimentation ages determined for the volcaniclastic deposits range from 0.24 ± 0.01 Ma to 2.24 ±
0.02 Ma, closely matching the nannofossil and paleomagnetic chronostratigraphies of the cores. Both tephra and volcaniclastic layers represent material from PlioceneâPleistocene explosive eruptions of the Cañadas Caldera (Tenerife), mixed with Miocene volcanic debris only in the holes drilled north of Gran Canaria (954A and 954B). Two fallout layers (0.61 ± 0.02 Ma and 1.83 ± 0.02 Ma) and five volcaniclastic deposits reflect explosive eruptions, which took place during postulated dormant stages of the Cañadas edifice
Northwestern Central American Volcanic Arc: Increased contribution of enriched lithosphere to lavas along the volcanic front from Nicaragua to Guatemala and behind the volcanic front
The Central American Volcanic Arc (CAVA) has been subject of intensive research over the past decades,
leading to a large variety of different models for the origin of CAVA lavas with various source components.
Based on a comprehensive new geochemical data set (i.e. major and trace elements and Sr-Nd-Pb-Hf-O isotope
ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples
from NW CAVA (Nicaragua to Guatemala), we present a new model for the NW Central American Volcanic
Arc volcanism. Additional potential source component sample data from subducting Cocos Plate sediments,
igneous oceanic crust and Guatemalan granitic and metamorphic continental basement further contributes to our
new model.
We find systematically increasing Pb isotope ratios and decreasing Nd and Hf isotope ratios along the arc from
NW Nicaragua to Guatemala. BVF lavas generally have more radiogenic Pb and less radiogenic Nd and Hf
isotopic compositions than related VF lavas, similar to what is observed for trace element ratios going
northwards along the VF. Combined isotope and trace element data indicate the presence of three endmembers
for the volcanism in NW Central America: (1) NW Nicaraguan VF samples with very high Ba/(La, Th) and
U/Th, low La/Yb, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb, (2) NW Guatemalan VF and
Guatemalan and Honduran BVF samples with low Ba/(La, Th) and U/Th, high La/Yb, radiogenic Sr and Pb but
unradiogenic Nd and Hf, and elevated d18O, and (3) Honduran and Nicaraguan BVF samples with low Ba/(La,
Th) and U/Th, high La/Yb, unradiogenic Sr but radiogenic Nd, Hf and Pb. We interpret the NW Nicaragua VF
endmember to be dominated by a largely serpentinite-derived fluid flux from the subducting slab, possibly with
small amounts (<1 wt. %) of sediment melts, to a depleted N-MORB type of mantle wedge, resulting in large
degrees of melting of primarily peridotitic material. Based on combined Hf and Nd and Hf and Pb isotope
systematics, the isotopically enriched Guatemala VF and BVF endmember cannot be explained by the addition
of subducted pelagic sediments to the source. Instead this endmember could be derived from pyroxenitic
cumulates in the lithospheric mantle (and possibly lower crust) that were derived from parental magmas for
plutonic rocks in NW Central America, which were melted during the Quaternary subduction-related volcanism.
The isotopically depleted Honduras and Caribbean BA endmember could be derived from melting of young,
recycled, oceanic crust in the asthenosphere upwelling in the back-arc, based on the OIB-like major and trace
element but relatively depleted isotopic compositions of these samples. Mixing between these three endmember
types of magmas can explain the observed systematic geochemical variations along and across the NW Central
American Arc
Chronostratigraphy of Gran Canaria
A chronostratigraphy of Miocene/Pliocene volcanism on Gran Canaria (Canary Islands) has been established by single- crystal 40Ar/39Ar laser dating of feldspar crystals from 58 samples of welded ignimbrites, lava flows, fallout tephra layers, and intrusive rocks. All subaerially exposed volcanic and intrusive rocks of Gran Canaria were emplaced within the last 14.5 Ma, comprising three major magmatic/volcanic cycles. The subaerial Miocene evolution started with the rapid formation (<0.5 Ma) of the exposed, mildly alkalic shield basalts. The basaltic shield phase ended between 14.04 ± 0.10 Ma and 13.95 ± 0.02 Ma and was followed by a 0.6-m.y. magmatism of trachytic to rhyolitic composition (MogĂĄn Group). Single-crystal 40Ar/39Ar laser dating shows that the ash flows erupted at intervals of 0.03 â 0.04 m.y., with peak eruption rates as much as 2000 km3/m.y. during the initial stages of silicic magma production (Lower MogĂĄn Formation). High-precision ages have been determined for major, widespread lithostratigraphic markers of the MogĂĄn Group, such as ignimbrite P1 (13.95 ± 0.02 Ma), ignimbrite X (13.71 ± 0.02 Ma), ignimbrite D (13.44 ± 0.01 Ma), and ignimbrite E (13.37 ± 0.03 Ma).
After the rhyolitic stage, >500 km3 of silica-undersaturated nepheline trachyphonolitic ash flows, lava flows, and fallout tephra, as well as rare basanite and nephelinite dikes and lavas were erupted between 13.29 Ma and 13.04 Ma (Montaña Horno Formation) and 12.43 Ma and 9.85 Ma (Fataga Group). This stage was accompanied and followed by intrusive syenites and a large cone sheet swarm in the central caldera complex, lasting until at least 8.28 Ma. Following a major, nearly nonvolcanic hiatus lasting ~4.7 m.y. (Las Palmas Formation), eruptions resumed with the local emplacement of small volumes of nephelinites, basanites, and tholeiites at ~5 to 4.5 Ma, with peak activity and eruptions of highly evolved phonolite magma between 4.15and 3.78 Ma (Roque Nublo Group)
QuartÀr-geologisch-tephrostratigraphische Neuaufnahme und Interpretation des PleistozÀnprofils KÀrlich
Die quartĂ€re Sedimentfolge der KĂ€rlicher Tongrube ist ein SchlĂŒsselprofil fĂŒr das PleistozĂ€n Mitteleuropas: (a) Rhein- und Mosel-Terrassenablagerungen belegen die junge tektonische Hebung des palĂ€ozoischen Rheinischen Schildes, (b) LöĂ- und PalĂ€obodenschichten spiegeln spĂ€tquartĂ€re Klimaschwankungen wider, (c) Artefakt-Horizonte dokumentieren eine ausgedehnte frĂŒhmenschliche Geschichte des Mittelrheinraums, und (d) zahlreiche eingeschaltete Tephralagen â abgelagert wĂ€hrend explosiver Vulkaneruptionen im Osteifel-Vulkanfeld â bilden ideale chronostratigraphische Leithorizonte. Die chemische und mineralogische Zusammensetzung zweier phonolithischer Bims-Fallablagerungen, die bisher als âWehrer Bimse" bezeichnet wurden, unterscheidet sich drastisch von schlotnahen Tephraablagerungen am Wehr-Vulkan selber. DarĂŒber hinaus zeigen (40)Ar/(39)Ar-Laseranalysen von Feldspat-Kristallen, daĂ die beiden Tephraablagerungen, mit Eruptionsaltern von 452.000 ± 8.000 Jahren vor Heute (KAE-DT2) und < 618.000 ± 13.000 Jahren v. h. (KAE-DTl), bis zu doppelt so alt sind, wie bislang angenommen. Schlotnahe, strombolianisch-phreatomagmatische, mafische Tephraschichten (KAE-BT4; lokale Bezeichnung: âBrockentuff"), die einem interglazialen PalĂ€obodenhorizont im Hangenden der jĂŒngeren Bimsablagerung eingeschaltet sind (lokale Bezeichnung: âKĂ€rlicher Interglazial"), stammen von einem heute erodierten, kleinen Eruptionszentrum NW der Tongrube. (40)Ar/(39)Ar-Datierungen von Phlogopit-Ein-sprenglingen nach dem Stufenheizungs- und Laserverfahren ergeben fĂŒr KAE-BT4 ein Eruptionsalter von 396.000 ± 20.000 Jahren v. h. Unsere Untersuchung zeigt, daĂ das KĂ€rlicher Profil vor allem terrestrische PleistozĂ€nablagerungen enthĂ€lt, die zur Zeit der marinen Sauerstoffisotopenstadien 11 bis 19 abgelagert wurden. JĂŒngere Schichten sind nur unvollstĂ€ndig im Hangenden von KAE-BT4 erhalten. Das âKĂ€rlicher Interglazial locus typicus" reprĂ€sentiert eine ca. 400.000 Jahre alte interglaziale Klimaphase und wird hier mit dem marinen Sauerstoffisotopenstadium 11 korreliert. FrĂŒhmenschliche Artefakte, die in Schichten unmittelbar im Hangenden der Tephra KAE-BT4 auftreten, könnten demnach ebenfalls bis zu ca. 400.000 Jahre alt sein
Evidence for Miocene subduction beneath the Alboran Sea (Western Mediterranean) from 40Ar/39Ar age dating and the geochemistry of volcanic rocks from holes 977A and 978A
Volcanic pebbles in gravels from Sites 977 and 978 in the Alboran Sea (western Mediterranean) were dated (using the 40Ar/
39Ar single-crystal laser technique) and analyzed for their major- and trace-element compositions (determined by X-ray fluorescence
and inductively coupled plasma-mass spectrometry). The samples range from basalts to rhyolites, and belong to the
tholeiitic, calc-alkaline, and shoshonitic series. Single-crystal and step-heating laser 40Ar/39Ar analyses of plagioclase, sanidine,
biotite, and amphibole phenocrysts from basaltic to rhyolitic samples indicate that eruptions occurred between 6.1 and 12.1 Ma.
The age data conform to the stratigraphy and agree with microfossil ages, when available. The major-element and compatible
trace-element data of samples with H2O < 4 wt% show systematic variations, consistent with fractionation of the observed phenocryst
phases (plagioclase, olivine, clinopyroxene, magnetite, hornblende, quartz, and biotite). The incompatible-element patterns
formed by normalizing to primitive mantle for all samples show spiked patterns with peaks generally at mobile elements
and troughs at immobile elements, in particular Nb and Ta. The calc-alkaline affinities and the incompatible-element systematics
are characteristic of subduction zone volcanism, which indicates that subduction occurred beneath the eastern Alboran from
6 to at least 12 Ma. We propose that the change in chemistry from calc-alkaline and potassic to sodic compositions between 5â
6 Ma reflects detachment of the subducting slab. Uplift of the Strait of Gibraltar, associated with this detachment, could have
caused the Messinian Salinity Crises
The Christmas Island Seamount Province, Indian Ocean: Origin of Intraplate Volcanism by Shallow Recycling of Continental Lithosphere?
The east-west-trending Christmas Island Seamount
Province (CHRISP, 1800x600 km) in the northeastern Indian
Ocean is elongated orthogonal to present-day plate motion,
posing the question if a mantle plume formed this volcanic
belt. Here we report the first age (Ar/Ar) and geochemical (Sr-
Nd-Hf-Pb DS isotopic data) from the CHRISP seamount
chain. A crude E-W age decrease from the Argo Basin (136
Ma), to the Eastern Wharton Basin (115-94 Ma) to the
Vening-Meinesz seamounts (96-64 Ma) to the Cocos-Keeling
seamounts (56-47 Ma) suggests spatial migration of melting.
Christmas Island, however, yields much younger ages (44-4
Ma), inconsistent with an age progression. The isotopic
compositions (e.g. 206Pb/204Pb = 17.3-19.3; 207Pb/204Pb = 15.49-
15.67; 143Nd/144Nd = 0.51220-0.51295; 176Hf/177Hf = 0.28246-
0.28319) range from enriched MORB (or âCâ) to very
enriched mantle (EM1) type compositions more typical of
continental than oceanic volcanism. Lamproitic and
kimberlitic rocks from western Australia, India and other
continental areas, derived from metasomatized subcontinental
lithospheric mantle, could serve as the EM1 type endmembers.
The morphology, ages and chemical composition of the
CHRISP, combined with plate tectonic reconstructions, cannot
be easily explained within the framework of the mantle plume
hypotheses. We therefore propose that the seamounts are
derived through the recycling of continental lithosphere
(mantle ± lower crust) delaminated during the breakup of
Gondwana and brought to the surface at the former spreading
centers separating Argoland (western Burma), Greater India
and Australia
The 40Ar/39Ar age dating of the Madeira Archipelago and hotspot track (eastern North Atlantic)
The 40Ar/39Ar ages for 35 volcanic rocks and 14C ages for two charcoal samples from the Madeira Archipelago and AmpĂšre Seamount (eastern North Atlantic) are presented. The volcanic evolution of Madeira can be divided into a voluminous shield stage (>4.6â0.7 Ma) and a subsequent low-volume posterosional stage (70 Myr old hotspot which formed Porto Santo Island (11.1â14.3 Ma), Seine, AmpĂšre (31 Ma), Corral Patch and Ormond (65â67 Ma [FĂ©raud et al., 1982, 1986]) Seamounts, and the Serra de Monchique (70â72 Ma [McIntyre and Berger, 1982]) complex in southern Portugal. Age and spatial relationships result in a calculated absolute African plate motion above the hotspot of 1.2 cm/yr around a rotation pole located at 43°36âČN/ 24°33âČW
New 40Ar/39Ar age and geochemical data from seamounts in the Canary and Madeira volcanic provinces: support for the mantle plume hypothesis
The role of mantleplumes in the formation of intraplate volcanic islands and seamount chains is being increasingly questioned. Particular examples are the abundant and somewhat irregularly distributed island and seamount volcanoes off the coast of northwest Africa. New40Ar / 39Ar ages and SrâNdâPb isotope geochemistry of volcanic rocks from seamounts northeast of the Madeira Islands (Seine and Unicorn) and northeast of the Canary Islands (Dacia and Anika), however, provide support for the plumehypothesis. The oldest ages of shield stage volcanism from Canary and Madeiravolcanic provinces confirm progressions of increasing age to the northeast. Average volcanicage progression of âŒ1.2 cm/a is consistent with rotation of the African plate at an angular velocity of âŒ0.20° ± 0.05 /Ma around a common Euler pole at approximately 56° N, 45° W computed for the period of 0â35 Ma. A Euler pole at 35° N, 45° W is calculated for the time interval of 35â64 Ma. The isotope geochemistry further confirms that the Madeira and Canary provinces are derived from different sources, consistent with distinct plumes having formed each volcanic group. Conventional hotspot models, however, cannot easily explain the up to 40 m.y. long volcanic history at single volcanic centers, long gaps in volcanic activity, and the irregular distribution of islands and seamounts in the Canary province. A possible explanation could involve interaction of the Canarymantleplume with small-scale upper mantle processes such as edge-driven convection. Juxtaposition of plume and non-plume volcanism could also account for observed inconsistencies of the classical hotspot concept in other volcanic areas
- âŠ