J. M. Cantagrel

Evolution of the eastern volcanic ridge of the Canary Islands based on new KAr data

The results of 64 new KAr age determinations, together with 32 previously published ages, show that after a period of erosion of the basal complex, Miocene volcanic activity started around 20 Ma in Fuerteventura and 15 Ma in Lanzarote, forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs. This series includes five separate volcanic edifices, each one with its own eruptive history. In Fuerteventura, several Miocene eruptive cycles have been identified: in the central edifice one around 20–17 Ma, followed by two others centred around 15 and 13 Ma; in the southern edifice the maximum of activity took place around 16–14 Ma, whereas in the northern one the main activity occurred between 14 and 12 Ma. In Lanzarote a first cycle of activity took place in the southern edifice between 15.5 and 14.1 Ma, followed by another between 13.6 and 12.3 Ma. In the northern edifice three pulses occurred: 10.2–8.3, 6.6–5.3 and 3.9–3.8 Ma. An important temporal gap, greater in Fuerteventura than in Lanzarote, separates Series I from the Plio-Quaternary Series II, III and IV, formed by multi-vent basaltic emissions. In Fuerteventura the following eruptive cycles have been identified: 5, 2.9–2.4, 1.8–1.7, 0.8–0.4 and <0.1 Ma. In Lanzarote, the activity was fairly continuous from 2.7 Ma to historic times, with a maximum in the Lower Pleistocene. Eruptive rates in the Series I edifices were on the average 0.1–0.01 km3/ka, comparable but slightly smaller than in similar edifices in Tenerife and La Gomera, but much lower than in Gran Canaria. Average post-Miocene eruptive rates were about 0.013–0.027 km3/ka in Lanzarote and 0.003–0.007 km3/ka in Fuerteventura. All these volcanic edifices show a similar general sequence (fissural eruptions, erosion, multi-vent volcanism), repeated at different periods in different parts of the eastern islands of the Canaries. The model of growth of the Series I edifices is comparable to those in Tenerife and La Gomera: long periods of activity, sometimes greater than 6 m.y., with pulses separated by gaps. However, salic and intermediate differentiates, frequent in Tenerife and La Gomera, are very scarce in these islands. The Fuerteventura-Lanzarote ridge shows a decrease in volcanic activity with time, and also a certain SSW-NNE polarity in the temporal development of volcanism.

Evolution of the Cañadas edifice and its implications for the origin of the Cañadas Caldera (Tenerife, Canary Islands)

The volcano-stratigraphic and geochronologic data presented in this work show that the Tenerife central zone has been occupied during the last 3 Ma by shield or central composite volcanoes which reached more than 3000 m in height. The last volcanic system, the presently active Teide-Pico Viejo Complex began to form approximately 150 ka ago. The first Canadas Edifice CE. volcanic activity took place between about 3.5 Ma and 2.7 Ma. The CE-I is formed mainly by basalts, trachybasalts and trachytes. The remains of this phase outcrop in the Canadas Wall CW. sectors of La Angostura 3.5–3.0 Ma and 3.0–2.7 Ma., Boca de Tauce 3.0 Ma., and in the bottom of some external radial ravines 3.5 Ma.. The position of its main emission center was located in the central part of the CC. The volcano could have reached 3000 m in height. This edifice underwent a partial destruction by failure and flank collapse, forming debris-avalanches during the 2.6–2.3 Ma period. The debris-avalanche deposits can be seen in the most distal zones in the N flank of the CE-I Tigaiga Breccia.. A new volcanic phase, whose deposits overlie the remains of CE-I and the former debris-avalanche deposits, constituted a new volcanic edifice, the CE-II. The dyke directions analysis and the morphological reconstruction suggest that the CE-II center was situated somewhat westward of the CE-I, reaching some 3200 m in height. The CE-II formations are well exposed on the CW, especially at the El Cedro 2.3–2.00 Ma. sector. They are also frequent in the S flank of the edifice 2.25–1.89 Ma. in Tejina 2.5–1.87 Ma. as well as in the Tigaiga massif to the N 2.23 Ma.. During the last periods of activity of CE-II, important explosive eruptions took place forming ignimbrites, pyroclastic flows, and fall deposits of trachytic composition. Their ages vary between 1.5 and 1.6 Ma Adeje ignimbrites, to the W.. In the CW, the Upper Ucanca phonolitic Unit 1.4 Ma. could be the last main episode of the CE-II. Afterwards, the Can˜adas III phase began. It is well represented in the CW sectors of Tigaiga 1.1 Ma–0.27 Ma., Las Pilas 1.03 Ma–0.78 Ma., Diego Hernandez 0.54 Ma–0.17 Ma. and Guajara 1.1 Ma–0.7 Ma.. The materials of this edifice are also found in the SE flank. These materials are trachybasaltic lava-flows and abundant phonolitic lava and pyroclastic flows 0.6 Ma–0.5 Ma. associated with abundant plinian falls. The CE-III was essentially built between 0.9 and 0.2 Ma, a period when the volcanic activity was also intense in the ‘Dorsal Edifice’ situated in the easterly wing of Tenerife. The so called ‘valleys’ of La Orotava and Gu¨imar, transversals to the ridge axis, also formed during this period. In the central part of Tenerife, the CE-III completed its evolution with an explosive deposit resting on the top of the CE, for which ages from 0.173 to 0.13 Ma have been obtained. The CC age must be younger due to the fact that the present caldera scarp cuts these deposits. On the controversial origin of the CC central vertical collapse vs. repeated flank failure and lateral collapse of mature volcanic edifices., the data discussed in this paper favor the second hypothesis. Clearly several debris-avalanche type events exist in the history of the volcano but most of the deposits are now under the sea. The caldera wall should represent the proximal scarps of the large slides whose intermediate scarps are covered by the more recent Teide-Pico Viejo volcanoes.

High cooling and denudation rates at Kongur Shan, Eastern Pamir (Xinjiang, China) revealed by 40Ar/39Ar alkali feldspar thermochronology

Orthogneiss samples taken from the Kongur antiform show ages varying from 2 Ma to 1 Ma for 40Ar/39Ar ages of biotites and muscovites and fission tracks on apatites, leading to cooling rates of 150°C/m.y. Modeling of K-feldspars highlights the effect of a range of diffusion domains with contrasting diffusion characteristics, yielding closure temperatures from 400° to 150°C. The feldspar data document the cooling history since 5 Ma and indicate a sudden change in cooling rates of the antiform at 2 Ma. At that time, cooling increases by a factor of 5, from an average of 20°C/m.y. to a minimum of 150°C/m.y. Consideration of the regional thermal history, ongoing uplift, and erosional history of the antiform during the Quaternary suggests that denudation rates have been of the order of 5–7 km/m.y. since 2 m.y. ago and could be associated with significant upward surface movement triggered by major normal faulting. The antiform is interpreted to have formed during thrusting at the Pamir front as a result of the development of thrust ramps and normal faulting at the crustal scale. Ramp stacking is an important process of mountain building, and normal faulting in this context must be regarded as a very efficient way of building high relief.

Palaeozoic granitoids from the French Massif Central: age and origin studied by 87Rb87Sr system

Abstract According to geochronological results, granitoids of the French Massif Central fall into two main chronological groups: (1) a Cambro-Silurian group (545-410 Ma) occurring all over the Massif Central, later metamorphosed into orthogneisses during the Devonian metamorphism (400-370 Ma). (2) An Upper Devonian-Carboniferous group (360-280 Ma) including non or weakly deformed granites, the oldest (360-340 Ma) localized in the north of the Massif Central, and the youngest (340-285 Ma) outcropping all over the massif. Geochemistry of Rb, Sr and isotopic evolution diagram give the following results: Strontium initial ratios for orthogneisses increase with time from 0.704 (545 Ma) to 0.710 (410 Ma), excluding a purely mantellic origin of the magmas. Strontium initial ratios for granites show that their magmas cannot result from a remelting of orthogneisses. The isotopic evolution of the leucogranitic family defines a trend originating ∼ 370 Ma, i.e., during the main metamorphic phase. The source rocks of the early Palaeozoic granitoids contain old zircons (1.5–2.0 Ga). Probably originating from an earlier continental crust, but the evidence of this crust is not pointed out by strontium isotopes.

Magma mixing: origin of intermediate rocks and “enclaves” from volcanism to plutonism

Abstract Intermediate magmatic rocks are rarely homogeneous; a common example is the presence of magmatic “enclaves” Volcanic examples show that they are indications of magma mixing processes. Mixing events in the trachyandesitic suites from the Sancy Volcano (France). The volcanic activity from Sancy consists of several brief trachyandesitic cycles. Each of them begins with the eruption of highly heterogeneous benmoreites, followed by more homogeneous mugearites. Light porphyritic trachyandesites enclose numerous inclusions of varied darker lavas. The crenulated geometry of the contacts, the presence of chilled margins, the vesiculation of the core of the basic parts, suggest that these different rock types were magmatic at the same time although under marked thermal disequilibrium. Xenocrysts are common: partly resorbed sanidine rimmed with plagioclase in a basic matrix, reactional olivines in tridymite-bearing trachytes. This shows that the mixing occurred between partially crystallized and fractionated magmas. Chemical compositions are continuously variable from basalts to trachytes within the same eruptive cycle. All these facts might be interpreted as the result of a mechanical intermingling, more or less achieved, between two end-members of contrasting composition in the reservoir. In the Sancy Volcano this phenomenon occurred at least, 4 or 5 times during a 600 000 y span. Mixing evidence in Hercynian granodiorites from France. Similar observations may be described in granodioritic rocks. The more widespread “enclaves” exhibit the same characteristics as their volcanic equivalents. Although most of them are small, more important volumes of basic rocks may be associated in the same massif. In the neighbourhood of such large bodies, swarms of smaller enclaves recall the generation of basic pillows. The nature of the contact between small “microgranular enclaves” and their host rocks, the occurrence of xenocrysts with reaction rims (rapakivi feldspars and quartz ocelli in a basic surrounding, amphibole-pyroxene clots in a granitic matrix) and geochemical variations, again suggest thermal and mineralogical disequilibrium. Such observations indicate the comagmatic character of enclaves with regard to their host rocks. They cannot be interpreted as restites and are best described as co-igneous inclusions resulting from non achieved mixing between coexisting magmas. Photonic rocks are less suitable to demonstrate such petrogenetic processes because they result from slow cooling and crystallization during a longer residence time in a magmatic chamber. The successive mixing episodes, giving rise to different generations of co-igneous inclusions are also difficult to recognize. On the other hand, in volcanism this process is quenched and periodically sampled by eruptions. In conclusion the heterogeneity and the variability of many intermediate rocks cannot be explained only in terms of simple partial fusion and fractional crystallization processes in a closed system. Volcanic examples point to a complex evolution of magmatic reservoirs that were continuously fractionated, periodically tapped, periodically refilled and mixed.

K-Ar dating on eastern Mexican volcanic rocks — Relations between the andesitic and the alkaline provinces

Abstract Eastern Mexico is characterized by two large magmatic provinces: the trans-Mexican volcanic belt, mainly of Miocene and Quaternary andesitic rocks, and the eastern alkaline province (oversaturated alkalic and subsaturated nephelinitic-type volcanic rocks) which appears to be Oligocene to Quaternary in age. Twenty-eight K-Ar analyses show the respective positions and the relationships between the two magmatic types: 1. (1) The data indicate that the trans-Mexican andesitic zone has had various important periods of volcanic activity since the Early Miocene: the major activity seems to have taken place about 20-15 m.y. and 9-6 m.y. ago. Ages of 3-0 m.y. are concentrated in the neovolcanic belt, the southern part (high volcanoes of Quaternary age), apparently more recent than the northern part (Atotonilco lavas). 2. (2) In the eastern alkaline province, the volcanism occurred from the Late Oligocene, with various phases, until the most recent volcanic episode. Activity has moved southwards from Tamaulipas to the Veracruz region. 3. (3) Alkaline basaltic lavas (“traps”), which occur in the fault zone of the Altiplano border, show a progression of the rifting, between 9 and 3 m.y. ago, from northern Hidalgo to Veracruz. 4. (4) The periods of alkaline magmatism were not contemporaneous with andesitic phases. The two types occur independently, the andesitic one having an east-west direction and the alkaline one a north-south orientation and north-south migrations. 5. (5) Such patterns lead to a crossing and interference of the two Pliocene and Quaternary fields of magmatism. It appears that the relations between the andesitic and the alkalic provinces is not in agreement with a classical scheme recognized in the Circum-Pacific.

Eruptive history of the Colima volcanic complex (Mexico)

Abstract The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nuees ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego. The oldest caldera, C1 (7–8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3–3.5 km wide), ended the evolution of Nevado I. Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4–5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters. The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls. The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II–III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano.

K-Ar chronology of the volcanic eruptions in the Canarian archipelago: Island of La Gomera

A geochronological study of the Island of La Gomera (Canaries) has been carried out by the K-Ar method. The 26 new ages obtained, together with the 17 previous determinations, show that above the main unconformity of the island, separating the « basal complex » from the later volcanic series, there is a unit of « lower old basalts » more than 10 m.y. old. Polymictic volcanic breccias were emplaced between 10 and 9 m.y. ago. The « upper old basalts » above them were formed between 9 and 6 m.y. ago, with a peak of activity around 7 m.y. After a period of erosion (6-5 m.y.), a thick series of « young basalts » associated with trachytic and phonolitic domes and flows, were rapidly emplaced between 4.5-4 m.y. ago. Finally, local basaltic activity took place 2.8 m.y. ago. The age of the basal complex is not well known, although three ages (14.6, 15.5 and 19.3 m.y.) have been obtained for some alkaline intrusives which seem to represent the youngest events in the complex.

Le Pico de Orizaba (Mexique): Structure et evolution d’un grand volcan andesitique complexe

Volcan Pico de Orizaba, which marks the eastern end of the Trans-Mexican Volcanic Belt, is one of the largest andesitic composite volcanoes in America. It is located above a series of crustal distensive faults making the boundary of the Coast Plains of the Gulf of Mexico from theAltiplano. For this reason, the volcano shows an asymmetry: from the west, its elevation is about 3,000 m whereas on the eastern side it reaches 4,000 to 4,500 m from its base.The Pico de Orizaba is composed of a primitive stratovolcano raised by a recent summit cone. It has been built by three very distinct volcanic and magmatic phases.1.The first one, probably discontinuous effusive activity, lasted more than one million years. It is mainly composed of two pyroxenes-andesites with scarce associated basaltic and dacitic lava-flows. Amphibole is an accessory mineral in most differentiated lavas. On the eastern flank, numerous massive and autobrecciated lava-flows pass outward into thick conglomeratic formations. This effusive phase has built a primitive central volcano and a parasitic cone: the Sierra Negra.2.The second phase is of short duration — about 100,000 years or less — in comparison with the first period. It seems that this period began with the formation of a caldera followed by the extrusion of amphibole dacite domes and the overflow of viscous silica-rich (andesite to dacite) lava flows on the northern flank. An intense explosive activity develops:pelean nuées ardentes are associated with extrusion of the domes; numerous plinian eruptions leading to widespread dacitic pumiceous air-falls are produced by both the central and the adventive volcanoes. This sequence of events is interpreted as the progressive emptying of a superficial chamber containing differenciated magma. A rhyolite flow erupted during this phase.3.The age of the recent phase is better defined. It started 13,000 years B.P. with the eruption of a dacitic ash-flow containing pumice and scoria-bombs. This was such an intense event that products were found 30 km S.E. of the summit, erasing the top of the former volcano and creating a large crater (4–5 km wide). The present cone, of 1,400–1,500 m elevation, grew in this crater. During a period of 7,000 to 8,000 years, the new stratovolcano experienced various important pyroclastic eruptions with a cycle of the order of 1,000 to 1,500 years. The pyroclastic flows (ash, pumice, and bombs) associated with air-fall deposits are of Saint-Vincent type. They present an heterogeneous dacitic and andesitic magma. The dacitic component is similar to previous differenciated materials. On the other hand, the andesitic magma appears somewhat similar to lava-flows from morphologically young cones erupted outside the central vent system. This eruptive cycle can be interpreted as the result of reoccurring injections of deep basic magma within the crustal chamber. For the last 5,000 years the activity of the modern Pico de Orizaba has again been essentially effusive (andesites) with periodic plinian eruptions.

Quaternary eruptive history of Nevado del Ruiz (Colombia)

Abstract Nevado del Ruiz has a 1.8-m.y.-long eruptive record that includes alternate construction and destruction of three edifices during three main eruptive periods, termed “ancestral Ruiz”, “older Ruiz” and “Ruiz”. Nevado del Ruiz is located on a complex intersection of four groups of faults, the most significant being the N20° E Palestina strike-slip fault and the N50° W Villamaria-Termales normal fault. Ancestral Ruiz was a broad stratovolcano built by two eruptive stages of lava flows starting about 1.8 Ma ago and ending 1.0 Ma ago. A partial collapse and formation of a caldera are thought to have occurred between 1.0 and 0.8 Ma ago. Older Ruiz was a stratovolcano constructed by lava flows in three stages starting about 0.8 Ma ago and ending about 0.2 Ma ago. Extensive and voluminous welded and nonwelded pyroclastic-flow deposits that partly fill preexisting valleys record the formation of a young summit caldera between 0.2 and 0.15 Ma ago. Present Ruiz is formed by a cluster of composite lava domes that probably filled the summit caldera of older Ruiz. Present Ruiz eruptive activity is mostly explosive, but also includes dome growth, and parasitic dome activity of La Olleta and Alto La Piramide. Twelve eruptive stages occurred during the last 11,000 years, accompanied by rockslide-debris avalanches, pyroclastic flows or surges, and their subsequent interactions with the ice cap, as well as by glacial erosion and mass-wasting. Diverse processes within these twelve stages have led to a partial destruction of the summit domes. This long and complex Pleistocene and Holocene eruptive sequence helps to put the November 13, 1985, eruption into a broader perspective.