Volcanic evolution of the island of Tenerife (Canary Islands) in the light of new K-Ar data

New age determinations from Tenerife, together with those previously published (93 in all), provide a fairly comprehensive picture of the volcanic evolution of the island. The oldest volcanic series, with ages starting in the late Miocene, are formed mainly by basalts with some trachytes and phonolites which appear in Anaga, Teno and Roque del Conde massifs. In Anaga (NE), three volcanic cycles occurred: one older than 6.5 Ma, a second one between 6.5 and 4.5 Ma, with a possible gap between 5.4 and 4.8 Ma, and a late cycle around 3.6 Ma. In Teno (NW), after some undated units, the activity took place between 6.7 and 4.5 Ma, with two main series separated by a possible pause between 6.2 and 5.6 Ma. In the zone of Roque del Conde (S), the ages are scattered between 11.6 and 3.5 Ma. Between 3.3 and 1.9 Ma, the whole island underwent a period of volcanic quiescence and erosion.The large Cañadas volcano, made up of basalts, trachytes and phonolites, was built essentially between 1.9 and 0.2 Ma. To the NE of this central volcano, linking it with Anaga, is a chain of basaltic emission centers, with a peak of activity around 0.8 Ma. The Cañadas Caldera had several collapse phases, associated with large ignimbrite emissions. There were, at least, an older phase more than 1 Ma old, on the western part of the volcano, and a younger one, less than 0.6 Ma old, in the eastern side. The two large “valleys” of Guimar and la Orotava were formed by large landslides less than 0.8 Ma ago, and probably before 0.6 Ma ago. The present Cañadas caldera was formed by another landslide, less than 0.2 Ma ago. This caldera was later filled by the huge Teide volcano, which has been active even in historic times. During the same period a series of small volcanoes erupted at scattered locations throughout the island.The average eruptive rate in Tenerife was 0.3 km³/ka, with relatively small variations for the different eruptive periods. This island and La Gomera represent a model of growth by discontinuous pulses of volcanic activity, separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years. The neighbouring Gran Canaria, on the other hand, had an initial, rapid “shield-building phase” during which more than 90% of the island was built, and a series of smaller pulses at a much later period.A comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation. Several gaps are present on each island, overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the Canaries.     

87Rb-87Sr constraints on the genesis and evolution of the Cantal continental volcanic system (France)

We have investigated 24 whole rocks and mineral separates of five different rock types from the Cantal shield volcano in France, applying high-precision Rb-Sr techniques. The chemical and isotopic systematics suggest the distinction of two series throughout the different rock classes, one practically uncontaminated, the other seriously influenced by wall rock assimilation. The first group comprises basalts and intermediate rocks with⁸⁷Sr/⁸⁶Sr= 0.70340–0.70382. The second group in addition includes rhyolites and the corresponding⁸⁷Sr/⁸⁶Sr ratios vary between 0.70421 and 0.71270. The data of mineral separates support the hybridization hypothesis and possibly suggest an original⁸⁷Sr/⁸⁶Sr ratio of 0.7028 for the magma source region. Moreover they provide internal isochron ages which place a period of extensive volcanic activity at 8.1–8.8 m.y. ago in accord with K-Ar ages of volcanic rocks from the center of the Cantal volcano.     

Volcanism on Mercury

An interpretation of theMariner 10 pictures shows that volcanic plains are largely developed on Mercury, and old and young lava covers can be distinguished occupying no less than 15% of the photographed surface of this planet. Ten and a half volcanic features attributable tovolcanic domes are visible on the pictures, the largest of which being situated at the centre ofOdin Planitia and having a diameter of 7 km and a height of about 1.4 km. The domes have been mapped on a geologic-morphologic map of Mercury (Katterfeld, 1975).The study of newly processed photographs of Mercury and of stereopair pictures of its volcanic «maria» has led to the discovery of a huge volcano at the centre of «Martis Mare» with 110 km long diameter and a double top caldera, the diameters of the external and internal crater being 60 and 40 km respectively. The coordinates of this volcano are 21°S and 124°W.     

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.     

Chronology of cenozoic igneous and tectonic events in the central Chilean Andes — latitudes 35° 30′ to 36°S

Sixty-six K---Ar dates from igneous rocks in the central Chilean Andes between 33° and 38°S are reported in this study. From these results and observed field relations, major Cenozoic volcanic and intrusive rock units are divided into chronologic groups representing igneous events.Volcanic units of Oligocene (33.3–27.9 m.y.) and Early Miocene (20.2 m.y.) age have been dated west of the present range at 33°S but neither the magnitude nor extent of these volcanic events has yet been established. Extensive Middle to Late Miocene volcanism (15.3–6.4 m.y.) followed by regional folding is recognized in the map area between 35° 20′ and 36°S. Partly contemporaneous Middle Miocene volcanism (18.4–13.7 m.y.) also followed by regional folding is recorded in the Andes between 37° 30′ and 38°S. General volcanic quiescence from 6.4 to 2.5 m.y. is observed in the map area but whether this volcanic hiatus is of regional significance is not known.The majority of the K---Ar dates document a history of nearly continuous volcanism throughout the last 2.5 m.y. in the map area. The abundant and diverse sequences of volcanic strata formed during this time, have been divided into four successive age groups which as map units show the evolution and distribution of latest volcanic activity.Landforms preserved by this volcanic series show that topographic relief similar to the present has prevailed during this time. Deep incision of rivers into young volcanic terrain, estimated to be on the order of 1–2 m/1000 years, has produced a complex volcanic and morphologic record.Four plutons dated in this study give ages of 62.0, 41.3, 19.5, and 7.0 m.y. No spatial pattern of emplacement is observed in the map area where three of these plutons are represented.Similarities in structural style, orientation and degree of deformation of Miocene and Mesozoic strata suggest that Late Miocene regional folding may have accounted for a significant part of the observed deformation in older basement strata previously ascribed to earlier orogenies.A regional comparison of ages of recognized igneous and tectonic event at different latitudes in the central and southern Andes shows the gross chronology of Cenozoic events which can be correlated with sea-floor spreading and subduction events.     

U(Th)Pb systematics and ages of Himalayan leucogranites, South Tibet

The age and origin of five leucogranites from the High and Tethys Himalaya, and two country-rock gneisses were investigated by UPb dating of zircon fractions and single grains, and fractions of monazite. Additionally, ThU concentrations in whole rock powders and isotopic compositions of Pb in leached K-feldspars were determined. Monazites yield ages of 16.8 ± 0.6 m.y. for the Nialam migmatite-granite, 15.1 ± 0.5 m.y. for the Lhagoi Kangri granite, 14.3 ± 0.6 m.y. for a granite from Mt. Everest, and 9.8 ± 0.7 m.y. and 9.2 ± 0.9 m.y. for two varieties of the Maja granite. These data, together with monazite ages of 21.9 ± 0.2 and 24.0 ± 0.4 m.y., determined earlier on the Makalu granite [1], substantiate a period of intracontinental granite emplacements from 24 to 9 m.y. ago, i.e. from uppermost Oligocene to late Miocene times. Such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of India with Eurasia. Furthermore, the individual ages, together with structural relationships between granites and country rocks suggest that granite formation and tectono-metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m.y. Inherited lead components, present in all granite zircons point to large proportions of Precambrian material in the magma source regions, up to 2200 m.y. old.ThU systematics between monazite and country rocks indicate that U has been leached from most of the granites after crystallisation of monazite.Zircon dating of the Kangmar granite gneiss, which occurs in a window through the Tethys Himalayan sediments, shows that this pluton, transformed to a gneiss during the Alpine orogeny, crystallised in lowermost Palaeozoic times 562 ± 4 m.y. ago.     

Geochronology of Gran Canaria,Canary Islands: Age of shield building volcanism and other magmatic phases

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km³ was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (～100 km³) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km³ of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km³. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.     

The geological results of measurements of gravity in East Carmarthenshire

Summary Observations of gravity over the Palaeozoic rocks of East Carmarthenshire show this to be an area of positive Bouguer anomalies, and throughout the district the isogams are aligned in a «Caledonoid» direction. The regional gradient over the anthracite coalfield can largely be explained in terms of known structure, while the anomaly values in the Towy valley above Llandeilo seem to have been affected by facies variations and contemporaneous volcanic activity. Fairly large anomalies are recorded near Carmarthen, and these may be associated with similar anomalies known near Aberystwyth (Cook & Thirlaway, 1950); they are interpreted as an expression of the Welsh geosyncline. The gravity low discovered in St. George's Channel byBrowne & Cooper (1950) is re-interpreted in terms of the depth of the pre-Palaeozoic floor. There is no marked evidence to indicate the presence of a concealed «granitic» intrusion beneath the coalfield, nor to substantiate the view that there was once a great thickness of Permian or later cover.     

Age of the Hawaiian-Emperor bend

⁴⁰Ar/³⁹Ar age data on alkalic and tholeiitic basalts from Diakakuji and Kinmei Seamounts in the vicinity of the Hawaiian-Emperor bend indicate that these volcanoes are about 41 and 39 m.y. old, respectively. Combined with previously published age data on Yuryaku and Ko¯ko Seamounts, the new data indicate that the best age for the bend is 42.0 ± 1.4 m.y.Petrochemical data indicate that the volcanic rocks recovered from bend seamounts are indistinguishable from Hawaiian volcanic rocks, strengthening the hypothesis that the Hawaiian-Emperor bend is part of the Hawaiian volcanic chain.⁴⁰Ar/³⁹Ar total fusion ages on altered whole-rock basalt samples are consistent with feldspar ages and with⁴⁰Ar/³⁹Ar incremental heating data and appear to reflect the crystallization ages of the samples even though conventional K-Ar ages are significantly younger. The cause of this effect is not known but it may be due to low-temperature loss of³⁹Ar from nonretentive montmorillonite clays that have also lost⁴⁰Ar.     

Geochemistry of calc-alkaline volcanic rocks from southeastern Iran (kouh-e-shahsavaran)

The Upper Tertiary to Quaternary volcanic complex of Kouh-e-Shahsavaran in southeastern Iran is composed of calc-alkaline rocks of island are type (high-alumina basalts, basic andesites, andesites and dacites) even though it was emplaced on the continental basement. The volcanic rocks of the complex are genetically related and were probably derived by low-pressure fractional crystallization of high-Al basalts. The anomalously high content of Sr in some rocks probably reflects an accumulation of plagioclase. The trace element data are consistent with the origin of the parental magma by partial melting of an “enriched” upper mantle peridotite.     

Eastern Pacific spreading rate fluctuation and its relation to Pacific area volcanic episodes

Sea-floor spreading rates from four locations along the Nazca-Pacific plate boundary and one along the Juan de Fuca-Pacific plate boundary show variations over the past 2.4 m.y., with decreasing rates prior to the Jaramillo to Olduvai time interval (0.92–1.73 m.y. ago) and increasing rates since then. Other Pacific area volcanic phenomena in mid-plate and convergent-boundary settings also show minima about 1.3–1.5 m.y. ago and a maximum at present and another maximum about 5 m.y. ago: extrusion rates along the Hawaiian Ridge; volcanic episodes associated with calc-alkalic provinces of western Oregon and Central America; temporal variations in the SiO₂ content of Aleutian ash layers; and the number of deep-sea ash layers. These phenomena may fluctuate in response to changing spreading rates. During times of more rapid spreading increased shear and melting along lithospheric boundaries may occasion increased volcanic activity, whereas during times of less rapid spreading volcanic activity may be less intense.     

Tectonic implications of the age, composition, and orientation of lamprophyre dikes, Navajo volcanic field, Arizona

Fifty-one lamprophyre dikes of the Navajo volcanic field are randomly oriented in contrast to dikes of similar age from adjacent tectonic provinces. KAr ages obtained on nine of these dikes indicate that they were emplaced between 28 and 19 m.y. ago. Initiation of dike emplacement was approximately coeval with the initial demarcation of the Colorado Plateau in the mid-Tertiary. Chemical analyses obtained on nine dike samples show a wide range in composition. Silica contents range from 33.6% in a phlogopite katungite to 55.9% in a felsic minette. Two of the samples have Na₂O > K₂O indicating that sodic volcanism was occurring concurrently with the dominant potassic volcanism.Published results of high-pressure experimental studies of katungite suggest that this magma was derived from K-metasomatised mantle. Because this potassium metasomatism produces a positive volume change, we propose a model that relates uplift of the Colorado Plateau to metasomatism, magma genesis, and dike emplacement.     

Apollo 17 KREEPy basalt: A rock type intermediate between mare and KREEP basalts

The Apollo 17 KREEPy basalt is a unique lunar volcanic rock, observed only as clasts in the light friable breccia matrix (72275) of Boulder 1, Station 2 at Taurus-Littrow. Its status as a volcanic rock is confirmed by the absence of any meteoritic contamination, a lack of cognate inclusions or xenocrystal material, and low Ni contents in metal grains.The basalt was extruded 4.01 ± 0.04 b.y. ago, approximately contemporaneously with the high-alumina mare basalts at Fra Mauro; shortly afterwards it was disrupted, probably by the Serenitatis impact, and its fragments emplaced in the South Massif. The basalt, which is quartz-normative and aluminous, is chemically and mineralogically intermediate between the Apollo 15 KREEP basalts and the high-alumina mare basalts in most respects. It consists mainly of plagioclase and pigeonitic pyroxene in approximately equal amounts, and 10–30% of mesostatis. Minor phases outside of the mesostatis are chromite, a silica mineral, Fe-metal, and rare olivine; the mesostatis consists primarily of ilmenite, Fe-metal, troilite, and ferroaugite, set in a glassy or microcrystalline Si-rich base.Chemical and isotopic data indicate that an origin by partial melting of a distinct source region is more probable than hybridization or contamination of magmas, and is responsible for the transitional composition of the basalt. The moon did not produce two completely distinct volcanic groups, the KREEP basalts and the mare/mare-like basalts; some intermediate rock types were generated as well. A corresponding spectrum of source regions must exist in the interior of the moon.     

K-Ar geochronology of the Limestone Caribbees and Martinique,Lesser Antilles,West Indies

Twenty-seven K-Ar ages have been measured on igneous rocks from islands of the Limestone Caribbees (St. Martin, St. Bartholomew, Antigua, and Grande Terre, Guadeloupe) and Martinique in the Lesser Antilles arc. Earlier paleontological data indicated that rocks on several of these islands are Eocene in age and among the oldest known in the arc.The oldest igneous activity on the islands studied is 35–40 m.y. Bifurcation of the Lesser Antilles arc north of Guadeloupe took place between 7 and 20 m.y. ago. A distinct physical overlap of older and younger volcanic arcs exists on Martinique.Our results do not support previous suggestions that the pre-Tertiary basement of the Greater Antilles extends through the Limestone Caribbees to the anomalously old, Jurassic-Cretaceous, igneous rocks of La Desirade.     

Potassium-argon dating of igneous activity in the central Chilean Andes — latitude 33°S

Potassium-argon dating of volcanic and plutonic rocks in the Andean region of central Chile has revealed previously unrecognized episodes of igneous activity during Cretaceous and Cenozoic time. These results indicate the need to re-evaluate the classic stratigraphic subdivisions that have evolved on lithologic rather than time-stratigraphic criteria.Four radiometric age groups have been identified in the coast range volcanic belt:

1. (1) Las Chilcas Formation — Early Cretaceous continental volcanic strata (120-110 m.y.).

2. (2) Lo Valle Formation — Late Cretaceous continental volcanic strata (78-65 m.y.).

3. (3) Late Oligocene extrusive volcanics (31-28 m.y.).

4. (4) Early Miocene intrusive volcanics (20.6–19.5 m.y.).

Two radiometric age groups have also been identified in the adjacent Andean Cordillera:

1. (1) Farellones Formation — continental volcanic strata (18.5–17.3 m.y.).

2. (2) Early Pliocene extrusive volcanics (5-4 m.y.).

An older group of continental volcanic strata in the Andes represented by the Abanico Formation remains undated but is intruded by plutons dated at 19.5 and 24 m.y.Available chronologic evidence indicates that volcanic activity moved eastward from the coast range volcanic belt to the Andean Cordillera between 20 and 18 m.y. ago and remained there to the present time.     

Early Miocene to Quaternary evolution of volcanism and the basin formation in western Anatolia: a review

Western Anatolia, largely affected by extensional tectonics, witnessed widespread volcanic activity since the Early Miocene. The volcanic vents of the region are represented by epicontinental calderas, stratovolcanoes and monogenetic vents which are associated with small-scale intrusions as sills and dykes. The volcanic activity began with an explosive character producing a large ignimbritic plateau all over the region, indicating the initiation of the crustal extension event. These rhyolitic magmas are nearly contemporaneous with granitic intrusions in western Anatolia. The ignimbrites, emplaced approximately contemporaneous with alluvial fan and braided river deposits, flowed over the basement rocks prior to extensional basin formation. The lacustrine deposits overlie the ignimbrites. The potassic and ultrapotassic lavas with lamprophyric affinities were emplaced during the Late Miocene–Pliocene. The volcanic activities have continued with alkali basalts during the Quaternary.     

40Ar/39Ar dating on volcanic rocks of the Deccan Traps,India

⁴⁰Ar/³⁹Ar dating results on seven volcanic rocks from four areas of the Deccan Traps, India, suggest that volcanic activity more than 70 Ma ago might have occurred at least in limited areas.In the Igat Puri area, the uppermost flow shows an⁴⁰Ar/³⁹Ar age of 63 Ma, whereas a lower flow has an age of around 82–84 Ma.⁴⁰Ar/³⁹Ar ages of samples from the Bombay area also seem to favor the occurrence of volcanic activity more than 70 Ma ago. One rhyolite dyke from the Osam Hill in the Girnar Hill area shows a well-defined plateau age of 68 Ma, whereas two tholeiitic basalts from the Mahabaleshwar area indicate a total⁴⁰Ar/³⁹Ar age of around 63–64 Ma, though they show the effect of secondary disturbance in the age spectra.The volcanic activity(ies) more than 70 Ma ago may correspond to precursory one(s) for the main volcanic activity around 65 Ma ago in the Deccan Traps.     

Subterranean structure of collapse caldera associated with andesitic and dacitic eruptions — Structural evolution of the Miocene Kakeya Cauldron,Southwest Japan

Three stages of collapse and doming of the inner subsided block are recognized in the Miocene Kakeya cauldron. The mechanism of the first collapse is not clear, but the second and third are volcanic in origin. The second collapse was triggered by eruptions of silicic andesite lava flows and pyroclastic ejecta. The boundary fault between the subsided block and its surroundings is nearly vertical. The subsided block formed a distinct basin structure, and its marginal part was intensely deformed by faulting. The third collapse took place cylindrically, accompanied by voluminous eruptions of dacitic pyroclastic materials. The collapsed block formed a basin structure with a gently dipping marginal part. The doming of the inner subsided block was due to increase of pressure in a magma chamber.The structure formed by the second collapse is not consistent with the concept of the subterranean structure of either the so-called «Krakatau»- (funnel-shaped) type or Valles-type calderas. The second collapse is transitional between «Krakatau»- and Valles-type calderas or a new type of volcanic depression. The features of the third collapse and the resurgent doming are similar to those of Valles-type calderas, except for the size of cauldrons and composition of magmas related to collapse. The similarities indicate that the Kakeya cauldron was formed in an extensional tectonic setting similar to that for Valles-type calderas.     

Jabal an Nar: An upper miocene volcanic centre near Al Mukha (Yemen Arab Republic)

On the coastal plain in the southwestern part of the Yemen Arab Republic, near Al Mukha, stands a volcanic centre known as Jabal an Nar that had been considered an active volcano. However, our geochronological and petrographical data show that this centre has an Upper Miocene age (about 10 m.y.) and consists of basalts and andesitic basalts (tholeiitic) different in composition from the products of the Aden Volcanic Series. The magmatic activity at 10 m.y. represents the first volcanic phase after the emplacement of the Trap Series.